Investigation on Blending Effects of Gasoline Fuel with N-Butanol, DMF

energies

Article Investigation on Blending Eﬀects of Gasoline Fuel with N-Butanol, DMF, and Ethanol on the Fuel Consumption and Harmful Emissions in a GDI Vehicle

Haifeng Liu 1 , Xichang Wang 1, Diping Zhang 1, Fang Dong 2, Xinlu Liu 2, Yong Yang 2, Haozhong Huang 3, Yang Wang 1, Qianlong Wang 1,* and Zunqing Zheng 1,*

1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China; [email protected] (H.L.); [email protected] (X.W.); [email protected] (D.Z.); [email protected] (Y.W.) 2 China Petrochemical Sales Limited Oil Technology Research Institute, Tianjin 300170, China; [email protected] (F.D.); [email protected] (X.L.); [email protected] (Y.Y.) 3 College of Mechanical Engineering, Guangxi University, Nanning 530004, China; [email protected] * Correspondence: [email protected] (Q.W.); [email protected] (Z.Z.); Tel.: +86-17521183846 (Q.W.); +86-22-27406842 (Z.Z.) Received: 21 March 2019; Accepted: 13 May 2019; Published: 15 May 2019

Abstract: The effects of three kinds of oxygenated fuel blends—i.e., ethanol-gasoline, n-butanol-gasoline, and 2,5-dimethylfuran (DMF)-gasoline-on fuel consumption, emissions, and acceleration performance were investigated in a passenger car with a chassis dynamometer. The engine mounted in the vehicle was a four-cylinder, four-stroke, turbocharging gasoline direct injection (GDI) engine with a displacement of 1.395 L. The test fuels include ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline with four blending ratios of 20%, 50%, 75%, and 100%, and pure gasoline was also tested for comparison. The original contribution of this article is to systemically study the steady-state, transient-state, cold-start, and acceleration performance of the tested fuels under a wide range of blending ratios, especially at high blending ratios. It provides new insight and knowledge of the emission alleviation technique in terms of tailoring the biofuels in GDI turbocharged engines. The results of our works showed that operation with ethanol–gasoline, n-butanol–gasoline, and DMF–gasoline at high blending ratios could be realized in the GDI vehicle without any modification to its engine and the control system at the steady state. At steady-state operation, as compared with pure gasoline, the results indicated that blending n-butanol could reduce CO2, CO, total hydrocarbon (THC), and NOX emissions, which were also decreased by employing a higher blending ratio of n-butanol. However, a high fraction of n-butanol increased the volumetric fuel consumption, and so did the DMF–gasoline and ethanol–gasoline blends. A large fraction of DMF reduced THC emissions, but increased CO2 and NOX emissions. Blending n-butanol can improve the equivalent fuel consumption. Moreover, the particle number (PN) emissions were significantly decreased when using the high blending ratios of the three kinds of oxygenated fuels. According to the results of the New European Drive Cycle (NEDC) cycle, blending 20% of n-butanol with gasoline decreased CO2 emissions by 5.7% compared with pure gasoline and simultaneously reduced CO, THC, NOX emissions, while blending ethanol only reduced NOX emissions. PN and particulate matter (PM) emissions decreased significantly in all stages of the NEDC cycle with the oxygenated fuel blends; the highest reduction ratio in PN was 72.87% upon blending 20% ethanol at the NEDC cycle. The high proportion of n-butanol and DMF improved the acceleration performance of the vehicle.

Keywords: oxygenated fuels; emissions; energy consumption; GDI engine

Energies 2019, 12, 1845; doi:10.3390/en12101845 www.mdpi.com/journal/energies Energies 2019, 12, 1845 2 of 21

1. Introduction In recent years, in order to alleviate environmental pollution, reduce dependence on petroleum resources, and meet increasingly stringent emission regulations, biofuels have been investigated widely [1] and considered as attractive alternative fuels for gasoline. Ethanol, as a representative biofuel, has the properties of a high octane number, large latent heat of vaporization, and less soot formation tendency in the engine combustion process [2]. Ethanol is the main alternative fuel for SI (spark ignition) engines [3]. Unlike a co-solvent being required when ethanol is blended with diesel [4], ethanol and gasoline are mutually soluble. As a result, there is no requirement for a co-solvent, which means that the production and storage of ethanol–gasoline blended fuel is very convenient and low cost. Compared with ethanol, n-butanol is one of four isomers of butanol, which has a straight-chain structure with the OH at the terminal carbon [5,6]. N-butanol has higher energy density, lower vapor pressure, better compatibility, and lower corrosion to the engine system than that of ethanol [7–9]. Similar to n-butanol, the heating value of DMF (2,5-dimethylfuran) is about 15% higher than that of ethanol. The characteristics of DMF include a high boiling point, insolubility in water, good storage stability, etc. [10–12]. Meanwhile, DMF has a high research octane number (RON) and consequently a low knock tendency in engines [13,14]. Therefore, both n-butanol and DMF also have been considered as new alternative fuels or fuel additions in SI engines. Furthermore, all the three biofuels-ethanol, n-butanol, and DMF—are relatively easy to obtain and can be processed from waste biomass; thus, the production process has less pollution to the environment. Ethanol has been already widely used in spark ignition engines along with gasoline, which is due to its high octane number and high volatility. Some scholars have studied the effect of ethanol blending into gasoline on the performance of GDI (gasoline direct injection) engines. In recent years, GDI has been an important innovation of the automotive industry that has been rapidly developed and applied [15]. With respect to the PFI (port fuel injection) engine, the GDI engine has apparent advantages such as better fuel economy, transient response, cold-start emissions due to the precise control and fast response of fuel injection, better mixing with high injection pressure and direct injection in the cylinder, etc. [16,17]. Costa et al. [18] compared the engine performance and emissions of a production four-stroke engine fueled with hydrous ethanol and a gasoline-ethanol fuel blend. The results showed that the BMEP (brake mean effective pressure) was higher when the gasoline-ethanol was fueled at lower engine speeds. Palmer [19] indicated that adding ethanol to gasoline increased the octane number. The results showed that ethanol–gasoline fuel blends containing 10% ethanol improved the engine power by 5%. However, Hsieh et al. [20] pointed out that mixing 5%, 10%, 20%, and 30% ethanol by volume into gasoline increased the fuel consumption due to the low heat value of ethanol. Chen et al. [21] investigated ethanol-gasoline blends of E5, E10, E20, E30, and E40. As far as emissions were concerned, E5 and E10 showed an indistinguishable trend with gasoline (E0), while E20 to E40 obviously decreased HC, CO, and NOX emissions. Costagliola et al. [22] studied the influence of four bioethanol–gasoline blends prepared with 10%, 20%, 30%, and 85% ethanol by volume in gasoline, and CO2 was reduced by 7% when fueled with E85 compared to the gasoline fuel. St˛epieńet al. [23] investigated a flex-fuel direct-injection vehicle with ethanol-gasoline blend fuels containing 10% or 85% ethanol with special consideration of nanoparticle and non-legislated gaseous emissions. It indicated that nanoparticle emissions from ethanol–gasoline blends of E85 showed the lowest PN (particle number) concentrations. In addition, the emissions of nitrogen dioxide (NO2) and nitrous oxide (N2O) could be found in only negligible amounts. Ericsson et al. [24] also found that an ethanol-gasoline fuel blend with 85% ethanol content (E85) could simultaneously reduce the quantity and quality of particulate emissions in a direct injection engine, and the particulate quality emissions during the cold-start stage would also be significantly reduced. Butanol has been widely regarded as an alternative fuel to gasoline in recent years. Many scholars have studied the effects of butanol on the performance and emissions of gasoline engines. Karavalakis et al. [25] conducted comparative tests of ethanol–gasoline and n-butanol-gasoline blends at different blending ratios in a direct-injection gasoline engine, and found that high-oxygen fuels could significantly Energies 2019, 12, 1845 3 of 21 reduce particulate quality emissions. The concentration of particulate quantity generally decreased when the proportion of ethanol and butanol increased. Other researchers observed that the addition of n-butanol could result in shorter ignition delay, faster combustion, better combustion stability, and better emissions, but advancing the spark timing will increase HC and NOX emissions in spite of decreasing CO emissions [26–28]. Compared with gasoline, the concentrations of nitrogen oxides either decreased or increased slightly when operated with gasoline–butanol blends and pure n-butanol [29,30]. He et al. [31,32] investigated the combustion and emission characteristics of a HCCI (Homogeneous Charge Compression Ignition) engine fueled with n-butanol-gasoline blends in a single cylinder port fuel injection engine, and found that the indicated mean effective pressure (IMEP) decreased as the n-butanol amount in the blends increased. The presence of n-butanol increased both formaldehyde and acetaldehyde emissions. Venugopal et al. [33] investigated the emission characteristics of gasoline and n-butanol under a dual fuel mode employing a dual-injection system. The results showed that with the proper selection of the fuel ratio, a significant reduction in HC emissions could be achieved compared to operation on neat gasoline. Singh et al. [34] analyzed the technical feasibility of a multi-point port fuel injection SI engine with butanol-gasoline blends of 5%, 10%, 20%, 50%, and 75% n-butanol by volume. The research found that butanol-gasoline blends have slightly higher BSFC (brake-specific fuel consumption) than gasoline. Bata et al. [35] investigated the effects of butanol on the performance of a four-cylinder spark-ignition engine, and found that the thermal efficiency of the fuel with 20% butanol in gasoline was reduced by 2.5%, and the effective fuel consumption rate was increased by 6.5%. Chen et al. [36] also investigated the combustion and performance under stoichiometric combustion conditions with n-butanol-gasoline blends of 15%, 30%, and 50% n-butanol by volume on a turbocharged GDI engine. It indicated that n-butanol-gasoline blends increased the combustion pressure and pressure rise rate, and decreased the ignition delay and combustion duration compared with pure gasoline. A higher n-butanol ratio in fuel blends increased the combustion temperature and brake thermal efficiency, decreased the temperature in the later stage of the expansion stroke, and showed a slightly higher knock possibility in high-load conditions. Furthermore, Chen et al. [37] investigated the thermodynamic process and engine emissions of n-butanol-gasoline blends with 30% and 50% n-butanol by volume. The results showed that the maximum BMEP of the engine was increased to 1.99 MPa with B50. A high ratio of n-butanol increased unburned hydrocarbon (UHC) and CO emissions, but decreased NOX and CO2 emissions. Besides, the group of Yao et al. [38–41] investigated the effects of the addition of n-butanol by experimental study on diesel engine and laser diagnostics. It can be seen that blending n-butanol can reduce soot emissions, because the oxygenated structure of n-butanol has a higher ability to reduce PAHs (polycyclic aromatic hydrocarbons) and soot. As a new oxygenated fuel, DMF has also been widely studied worldwide. Christensen et al. [42] investigated the chemical and physical properties of fuel mixtures with the addition of DMF for a spark-ignited engine to determine their feasibility as gasoline alternative fuels. The results showed that blending DMF decreased the vapor pressure and density of the blends, but increased the viscosity. Daniel et al. [43] investigated PFI, GDI, and dual-injected modes in a single-cylinder, four-stroke SI research engine; the results indicated that 25% DMF in gasoline offered higher thermal efficiencies and lower fuel consumption, but also produced higher NO emissions. Rothamer et al. [14] investigated DMF/gasoline blends of 5%, 10%, and 15% DMF by volume. The results showed that slight knocking occurred when an engine was fueled with DMF5. There was no distinctive difference in knock suppression capabilities between DMF10 and DMF15. E10 (ethanol/gasoline blend with 10% ethanol by volume) had a better anti-knock property than DMF10 and DMF15. It was found that DMF exhibited less sensitivity to engine parameters such as combustion phasing, injection timing, and variable valve timing, which meant allowing a wider window for emissions optimization. Daniel et al. [44] investigated the effects of combustion phasing, injection timing, relative air-fuel ratio, and variable valve timing on the performance and emissions of a SI engine using 2,5-dimethylfuran. The results showed that emissions were inherently low, except for NO emissions. Energies 2019, 12, 1845 4 of 21

Based on the above discussion, it can be found that blending three biofuels, i.e., ethanol, n-butanol, and DMF into gasoline can improve the performance and emissions of gasoline engines to some extent. However, the existing investigations mainly focused on small blending proportions of ethanol, n-butanol, and DMF into gasoline in spark ignition (SI) engines or vehicles. Therefore, this paper mainly highlighted the effect of high blending ratios of ethanol, n-butanol, and DMF on the fuel consumption, emissions at different vehicle speeds, and acceleration performance. Meanwhile, the steady-state, transient-state, cold-start, and acceleration performance of the tested fuels have been firstly systematically studied in this paper. Considering that the downsizing is the main development direction in SI engines, the tested vehicle is equipped with a direct-injection turbocharged engine. Through comparison and analysis, the research aims to produce knowledge regarding which biofuels can offer better characteristics on combustion and emissions in GDI turbocharged engines.

2. Experimental Apparatus and Methods

2.1. Experimental Equipment The test was conducted on a commercial GDI vehicle (manual model of a FAW-Volkswagen BORA 2016, FAW-Volkswagen Automotive Company Ltd, Changchun, Jilin, China). The main technical parameters of the engine of the tested vehicle are shown in Table1. The accumulated mileage of the GDI vehicle before the test was 500 km, and the vehicle was in good operation condition. The experiment was performed on a chassis dynamometer test bench, and the schematic diagram of the test bench is shown in Figure1. The car simulated the actual road operating conditions on the chassis dynamometer. The exhaust gas passed through the dilution channel, which introduced air to dilute the exhaust gas so as to simulate the condition of the vehicle exhaust into the atmosphere. The concentration of the diluted exhaust gas was continuously measured with the emission analyzer during the test. Meanwhile, a part of the diluted exhaust gas with a fixed flow rate was collected in the gasbag, and was analyzed to quantify the concentration of the components in the exhaust gas after the end of the test cycle. The fuel consumption was also calculated with the exhaust components and their concentrations in the gasbag. The emission analyzer employed in current study was a Horiba MEXA-7200H (HORIBA Ltd, Kyoto, Japan); the full-flow dilution sampling system was a Horiba CVS-7200T (HORIBA Ltd, Kyoto, Japan), and the dilution channel was a Horiba DLS-7100E (HORIBA Ltd, Kyoto, Japan). CO2, total hydrocarbon (THC), CO, and NOX emissions were tested by an emission analyzer. PN emissions were measured by a MEXA-2000SPCS (HORIBA Ltd, Kyoto, Japan) particle counter. The measuring principle of the MEXA-2000SPCS is laser scattering condensation particle counting. The measured solid particles were between 23 nm and 2.5 um. The counting efficiency of 23-nm solid particles was 50% 12%, and the ± counting efficiency of 41-nm solid particles was 90% or above. The particulate matter (PM) were collected by filter paper and then weighed by precision electronic balance (MSE6.6S-000-DF, Sartorius Ltd, Gottingen, Germany). Table2 shows the major equipment used in this study. Table3 shows the measurement errors of the major instruments and equipment.

Table 1. Detailed engine parameters. GDI: gasoline direct injection.

Parameter GDI Car Engine type Four cylinders, four valves per cylinder Displacement 1395 mL Maximum power 96 kW Air intake system Turbocharged Compression ratio 10.5 Fuel supply method Direct injection in cylinder Exhaust treatment No Knock sensor Exists Emission standards China V Gasoline recommended 95# Maximum design total mass 1765 kg Energies 2019, 12, x 5 of 23

Energies 2019, 12, 1845 Gasoline recommended 95# 5 of 21 Maximum design total mass 1765 kg

188 189 FigureFigure 1.1. SchematicSchematic ofof thethe experimentalexperimental setup.setup. HEPA: HighHigh EEfficiencyﬃciency ParticulateParticulate AirAir ﬁlter;filter; OVN:OVN: 190 OvenOven TypeType Heated Heated Analyzer; Analyzer; CFV: CFV: Critical Critical Flow Flow Venturi. Venturi. Table 2. Major equipment. 191 Table 2. Major equipment. Name Type Manufacturer Name Type Manufacturer Chassis dynamometer Roadsim48"compact AVL Chassis dynamometer Roadsim48"compact AVL Emission analyzer MEXA-7200H Horiba DilutionEmission sampling analyzer system CVS-7200T MEXA-7200H Horiba Horiba DilutionDilution sampling channel system DLS-7100E CVS-7200T Horiba Horiba DilutionParticle counter channel MEXA-2000SPCS DLS-7100E Horiba Horiba High precisionParticle electronic counter balance MSE6.6S-000-DF MEXA-2000SPCS Sartorius Horiba High precision electronic balance MSE6.6S-000-DF Sartorius Table 3. Measurement errors of major instruments and equipment. PN: particle number, PM: 192 particulateTable 3. Measurement matter. errors of major instruments and equipment. PN: particle number, PM: 193 particulate matter. Equipment Test Items Measurement Error

Equipment Test itemsCO2 Measurement error COCO2 1% Full scale or 2% Emission analyzer ≤ ≤1% Full scale or 2% COTHC measurements, take the minimum Emission analyzer measurements，take the THCNO X minimum Particle counterNO PNX <10% Particle counter Speed constant PN adjustment diﬀerence <0.05% < 10% Full scale Chassis dynamometer Speed constantTime measurement adjustment tolerance difference < 0.05% 0.00005% Full scale Chassis Time measurement tolerance 0.00005% dynamometer Traction constant tolerance <0.2% Full scale Precision electronic balanceTraction Filter constant paper quality tolerance (PM) < 0.2%1 µg (5%Full load) scale ± Precision electronic Filter paper quality (PM) ± 1 µg（5% load） 2.2. Experimentalbalance Methods

194 2.2. ExperimentalThe New European methods Drive Cycle (NEDC) was chosen as the test cycle in this study. The entire NEDC cycle is divided into an Urban Drive Cycle (UDC) and Extra-Urban Drive Cycle (EUDC). 195 The speedThe New of the European UDC operating Drive Cycle condition (NEDC) is was lower chosen compared as the withtest cycle that ofin thethis EUDCstudy. operatingThe entire 196 condition,NEDC cycle and is adivided similar into trend an isUrban shown Drive in the Cycle engine (UDC) temperature. and Extra-Urban The vehicle Drive speed Cycle of(EUDC). the NEDC The 197 cyclespeed is shownof the inUDC Figure operating2. To evaluate condition the emissions is lower and compared fuel economy with ofthat various of the fuels EUDC under operating di fferent 198 operatingcondition, conditions, and a similar six trend steady-state is shown vehicle in the speeds, engine includingtemperature. 0 km The/h, 15vehicle km/h, speed 40 km of/h, the 65 NEDC km/h, 199 90cycle km /ish, shown and 120 in km Figure/h were 2. tested.To evaluate The emission the emissions analyzer and sampled fuel economy exhaust of gas various for 100 fuels seconds under at 200 eachdifferent vehicle operating speed. Beforeconditions, measurement, six steady-state the vehicle vehicle speed speeds, was allowedincluding to 0 stabilize km/h, 15 for km/h, 20 seconds 40 km/h, to 201 ensure65 km/h, stable 90 km/h, operation. and In120 order km/h to were avoid tested. the influence The emission of the after-treatment analyzer sampled device exhaust and highlight gas for the100 impact of different fuels on emissions, the three-way catalytic converter of a GDI car was disabled Energies 2019, 12, x 6 of 23

202 secondsEnergies 2019 at each, 12, 1845 vehicle speed. Before measurement, the vehicle speed was allowed to stabilize for6of 20 21 203 seconds to ensure stable operation. In order to avoid the influence of the after-treatment device and 204 highlight the impact of different fuels on emissions, the three-way catalytic converter of a GDI car 205 wasbefore disabled the test before without the atestﬀecting without the exhaustaffecting pressure the exhaust of the pressure vehicle. of The the air–fuel vehicle. ratio The wasair–fuel controlled ratio 206 wasby thecontrolled lambda by sensor, the lambda and the calibrationsensor, and data the ofcalibra this productiontion data ofvehicle this production did not change, vehicle which did not the 207 change,vehicle ranwhich naturally the vehicle with diranﬀerent natura experimentallly with different fuels in experimental order to evaluate fuels the in adaptabilityorder to evaluate of various the 208 adaptabilitytested oxygenated of various fuels tested to the oxygena existingted vehicle. fuels to the existing vehicle.

120 100 EUDC 80 UDC 60 40 20 0 0 200 400 600 800 1000 1200 Theoretical speed (km/h) Time (s) 209 Figure 2. Vehicle speed of the New European Drive Cycle (NEDC) cycle. UDC: Urban Drive Cycle. 210 Figure 2. Vehicle speed of the New European Drive Cycle (NEDC) cycle. UDC: Urban Drive Cycle. The car acceleration performance was used to evaluate the impact of different fuels on the power 211 The car acceleration performance was used to evaluate the impact of different fuels on the performance of the vehicle. Since the GDI vehicle used in this test adopts a manual transmission, 212 power performance of the vehicle. Since the GDI vehicle used in this test adopts a manual in order to eliminate the error caused by the shift process, each gear was measured separately. 213 transmission, in order to eliminate the error caused by the shift process, each gear was measured The starting speed of each gear shift lever referred to the minimum speed at which the engine operated 214 separately. The starting speed of each gear shift lever referred to the minimum speed at which the stably, while the maximum speed referred to the highest speed and the extreme point of the acceleration 215 engine operated stably, while the maximum speed referred to the highest speed and the extreme curve of the vehicle at that gear shift lever. Accordingly, the speed of the third gear was 30–65 km/h, 216 point of the acceleration curve of the vehicle at that gear shift lever. Accordingly, the speed of the that of the fourth gear was 40–80 km/h, and that of the fifth gear was 60–110 km/h. Each acceleration 217 third gear was 30–65 km/h, that of the fourth gear was 40–80 km/h, and that of the fifth gear was 60– test was performed at least four times. 218 110 km/h. Each acceleration test was performed at least four times. To ensure the reliability of the data, the tire pressure during the test was maintained at 2.25–2.3 bar, 219 To ensure the reliability of the data, the tire pressure during the test was maintained at 2.25–2.3 and the lubricant, air filter, and oil filter of the vehicle were replaced periodically. Once the tested fuel 220 bar, and the lubricant, air filter, and oil filter of the vehicle were replaced periodically. Once the was changed, the fuel tank and pipe needed to be drained and cleaned thoroughly with the next set of 221 tested fuel was changed, the fuel tank and pipe needed to be drained and cleaned thoroughly with fuel, and a new fuel filter was replaced. Then, the vehicle was operated at a speed of 70–90 km/h for 222 the next set of fuel, and a new fuel filter was replaced. Then, the vehicle was operated at a speed of more than 20 min with the new test fuel to eliminate the interference of the previous test fuel on the 223 70–90 km/h for more than 20 minutes with the new test fuel to eliminate the interference of the test results. 224 previous test fuel on the test results. 2.3. Test Fuels 225 2.3. Test fuels The test fuels were fuel blends of gasoline and oxygenated fuels including ethanol, n-butanol, 226 andThe 2,5-dimethylfuran, test fuels were i.e.,fuel gasoline blends of was gasoline blended an withd oxygenated each oxygenated fuels including fuel in a certainethanol, volume n-butanol, ratio. 227 andThe 2, gasoline 5-dimethylfuran, was bought i.e., from gasoline a Shell was gas blended station with theeach research oxygenated octane fuel number in a certain (RON) volume of 93.1; 228 ratio.there The was gasoline no ethanol was addition, bought andfrom oxygen a Shell content gas station was belowwith the 0.82%. research Three octane oxygenated number fuels-ethanol, (RON) of 229 93.1;n-butanol, there was and DMF—wereno ethanol addition, bought from and Tianjinoxygen Yuanli content Chemical was below Engineering 0.82%. Three Limited oxygenated Company, 230 fuels-ethanol, n-butanol, and DMF—were bought from Tianjin Yuanli Chemical Engineering and their purities were all higher than 99%. The volume percentage of oxygenated fuels in the fuel 231 Limited Company, and their purities were all higher than 99%. The volume percentage of blends was set to 20%, 50%, 75%, and 100%. Table4 shows the physicochemical properties of the 232 oxygenated fuels in the fuel blends was set to 20%, 50%, 75%, and 100%. Table 4 shows the three oxygenated fuels as well as gasoline. Table5 shows the key fuel blends’ properties. The octane 233 physicochemical properties of the three oxygenated fuels as well as gasoline. Table 5 shows the key number, density, and latent heat of vaporization of each oxygenated fuel were higher than those of 234 fuel blends’ properties. The octane number, density, and latent heat of vaporization of each gasoline. However, the lower heating value of each oxygenated fuel was lower than that of gasoline. 235 oxygenated fuel were higher than those of gasoline. However, the lower heating value of each In the following, simple signs will be used to designate various types of blended fuels. The form of 236 oxygenated fuel was lower than that of gasoline. In the following, simple signs will be used to sign was oxygenated fuel abbreviation and volumetric percentage, among which the abbreviation of 237 designate various types of blended fuels. The form of sign was oxygenated fuel abbreviation and ethanol, n-butanol, and 2,5-dimethylfuran were E, B, and D, respectively. Following such a rule, E20 238 volumetric percentage, among which the abbreviation of ethanol, n-butanol, and 2,5-dimethylfuran represented the ethanol-gasoline blends with an ethanol volume percentage of 20%, and so on. 239 were E, B, and D, respectively. Following such a rule, E20 represented the ethanol-gasoline blends 240 with an ethanol volume percentage of 20%, and so on.

Energies 2019, 12, 1845 7 of 21

Table 4. Physical and chemical properties of three oxygenated fuels and gasoline [36,45,46].

Fuel Gasoline Ethanol n-Butanol 2,5-Dimethylfuran

Molecular formula C2–C14 C2H5OH C4H9OH C6H8O Molecular weight (g/mol) 110.8 46.07 74.12 96.1 Research octane number 93.1 107 96 119 Density at 20 ◦C (g/mL) 0.745 0.789 0.81 0.89 Lower heating value (MJ/L) 32.9 21.3 26.9 29.3 Laminar ﬂame burning speed at 1 bar, 390K (cm/s) 52 63 57 50 Viscosity at 20 ◦C (cSt) 0.4–0.8 1.52 3.35 0.57 Surface Tension at 20 ◦C (mN/m) 20–25 22.39 24. 6 25.9 Vapor Pressure (kPa) 55–103 18 4.08 1.253 Boiling point (◦C) 35–210 78 117 92 Latent heat of vaporization (kJ/kg) 180–373 840 546 332 H/C ratio 1.8 3 2.5 1.3 O/C ratio 0 0.5 0.25 0.167 Stoichiometric A/F ratio 14.56 8.95 11.13 10.72 Stoichiometric CO2 (kg/L, fuel) 2.38 1.51 1.93 2.45

Table 5. Key fuel blend properties. Fuel: ethanol–gasoline blends (E), n-butanol–gasoline blends (B), and 2,5-dimethylfuran–gasoline blends (D) with an E/B/D volume percentage of 20%, and so on.

Density at Lower Heating Stoichiometric Fuel H/C Ratio O/C Ratio 20 ◦C (g/mL) Value (MJ/L) A/F Ratio E20 0.754 30.6 2.0 0.07 13.27 E50 0.767 27.1 2.3 0.19 11.61 E75 0.778 24.2 2.6 0.33 10.26 B20 0.758 31.7 1.9 0.04 13.72 B50 0.778 29.9 2.1 0.11 12.71 B75 0.794 28.4 2.3 0.18 11.91 D20 0.774 32.2 1.7 0.03 13.57 D50 0.818 31.1 1.6 0.08 12.41 D75 0.854 30.2 1.5 0.13 11.53

3. Results and Discussions

3.1. Ignition Timing Comparison Figure3 shows the ignition timing of three kinds of blended fuels-ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline, at the blending ratios of 20% and 75%; the results of pure gasoline are presented for comparison as well. The ignition timing has a direct influence on combustion in the cylinder, which consequently shows a great influence on the performance of the engine. As the vehicle speed increases from 15 km/h to 120 km/h, the ignition timing first advances; then, it stays basically the same, and finally retards. The retardation of the ignition timing may be to ensure the high catalytic efficiency of the three-way catalytic converter and control the emissions at the low speed of 15 km/h; meanwhile, the low turbulent intensity restrains the reactivity of fuels and the laminar flame speed at a low speed of 15 km/h, which may make the ignition timing retard. The combustion speed is faster at a high speed of 120 km/h, and the accumulation of ignition delay will be much less than that at a lower engine speed, so the ignition timing should be retarded. The ignition timings of ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline all retard compared to that of pure gasoline at the low speed of 15 km/h; in addition, as the blending ratios of ethanol, n-butanol, and DMF increase, the ignition timings of them retard more. Studies have shown that the combustion durations of DMF, ethanol, and n-butanol are shorter than those of gasoline under the same ignition timing; in addition, the higher in-cylinder temperature of DMF increases the burning speed rate [46], so the ignition timings of ethanol-gasoline, n-butanol-gasoline, and DMF—gasoline retard may be to ensure a similar combustion phase with gasoline. However, the ignition timings of ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline slightly advance compared to that of gasoline at high speed. Energies 2019, 12, x 8 of 23

261 same ignition timing; in addition, the higher in-cylinder temperature of DMF increases the burning

262 Energiesspeed 2019rate, 12[46],, 1845 so the ignition timings of ethanol-gasoline, n-butanol-gasoline, and DMF—gasoline8 of 21 263 retard may be to ensure a similar combustion phase with gasoline. However, the ignition timings of 264 ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline slightly advance compared to that of 265 Thisgasoline may at be high due speed. to the This high may octane be numberdue to the of high DMF, octane ethanol, number and n-butanol,of DMF, ethanol, and the and high n-butanol, laminar 266 ﬂameand the burning high laminar speed offlame ethanol burning and n-butanol.speed of ethanol and n-butanol.

35 E20 35 E75 B20 B75 30 D20 30 D75 Gasoline Gasoline 25 25 20 20 15 15 10 10 5 5 0 20 40 60 80 100 120 0 20 40 60 80 100 120

gnitiontiming (BTDC/deg) Speed (km/h) gnitiontiming (BTDC/deg) Speed (km/h) I I (a) (b)

267 FigureFigure 3.3.Comparison Comparison of of ignition ignition timing timing of di ofﬀ erentdiffere fuelsnt atfuels diﬀ erentat different blending blending ratios: ( aratios:) 20% blending (a) 20% 268 ratio;blending (b) 75%ratio; blending (b) 75% ratio.blending ratio.

269 3.2.3.2. Energy Consumptionconsumption comparison Comparison 270 FigureFigure4 4 showsshows volumetricvolumetric fuel consumption consumption of of ethanol-gasoline, ethanol-gasoline, n-butanol-gasoline, n-butanol-gasoline, and 271 andDMF-gasoline DMF-gasoline at the at blending the blending ratios ratios of 20%, of 20%, 50%, 50%, 75%, 75%, and and 100%. 100%. Similar Similar to CO to2 CO emissions,2 emissions, the 272 thecommonality commonality of the of the four four curves curves is isthat that the the volumetric volumetric fuel fuel consumption consumption is is higher higher at at a low vehiclevehicle 273 speedspeed ofof 1515 kmkm/h./h. AsAs thethe vehiclevehicle speedspeed increases,increases, thethe volumetricvolumetric fuelfuel consumption decreases firstly;firstly; 274 then,then, itit increasesincreases and and remains remains at at the the lowest lowest level level when when the the vehicle vehicle speed speed increases increases from from 40 to 6540 kmto /65h 275 forkm/h each for fuel. each This fuel. takes This place takes because place thebecause weak the airflow weak at airflow low speed at resultslow speed in a lowresults flame in propagationa low flame 276 speedpropagation and high speed heat and loss, high which heat contributes loss, which to thecont lowributes thermal to the effi lowciency thermal and high efficiency volumetric and high fuel 277 consumption.volumetric fuel At consumption. high speed, theAt high low degreespeed, ofthe constant low degree volume of constant combustion volume due combustion to the increasing due to 278 crankthe increasing angle of combustioncrank angle andof combustion sometimes and incomplete sometimes combustion incomplete cause combustion low thermal cause effi lowciency thermal and 279 highefficiency volumetric and high fuel consumption.volumetric fuel Meanwhile, consumption. the air Meanwhile, resistance is the greater air resistance at high speed, is greater which at creates high 280 highspeed, volumetric which creates fuel consumption. high volumetric The volumetricfuel consumption. fuel consumptions The volumetric of the fuel ethanol-gasoline consumptions blended of the 281 fuelsethanol-gasoline are higher than blended that of fuels pure are gasoline higher at than all the that four of pure blending gasoline ratios. at Asall thethe proportionfour blending of ethanol ratios. 282 increases,As the proportion the volumetric of ethanol fuel increases, consumption the volumetric also increases fuel accordingly, consumption which also isincreases mainly attributedaccordingly, to 283 thewhich lower is heatingmainly valueattributed of ethanol. to the For lower n-butanol-gasoline, heating value theof volumetricethanol. For fuel n-butanol-gasoline, consumption is almost the 284 atvolumetric the same levelfuel consumption of gasoline with is almost 20% and at 50%the same blending level ratios. of gasoline However, with when 20% and the blending50% blending ratio 285 isratios. increased However, further, when the volumetricthe blending fuel ratio consumption is increased increases further, gradually the volumetric compared fuel withconsumption gasoline. 286 Theincreases lower gradually heating value compared of n-butanol with gasoline. is lower The than lower that heating of gasoline. value of Therefore, n-butanol more is lower n-butanol than that is 287 requiredof gasoline. to releaseTherefore, the samemore amountn-butanol of is heat, required which to is release the main the reason same amount for the increased of heat, which volumetric is the 288 fuelmain consumption reason for the of increased n-butanol–gasoline. volumetric Whilefuel consumption in the case of of the n-butanol–gasoline. smaller blending ratiosWhile of in 20% the andcase 289 50%,of the since smaller the latentblending heat ratios of vaporization of 20% and of 50%, n-butanol since isthe higher latent than heat that of vaporization of gasoline, the of temperaturen-butanol is 290 inhigher the cylinder than that is lower,of gasoline, which the reduces temperature the heat in loss. the cylinder Meanwhile, is lower, the oxygen which in reduces n-butanol the promotesheat loss. 291 combustionMeanwhile, andthe oxygen improves in n-butanol the efficiency. promotes These comb twoustion factors and are improves beneficial the for theefficiency. reduction These in fueltwo 292 consumptionfactors are beneficial and compensate for the reduction for the impact in fuel of theconsumption reduction inand the compensate lower heating for value the impact of n-butanol, of the 293 soreduction the fuel in consumptions the lower heating of n-butanol–gasoline value of n-butanol, at low so blendingthe fuel consumptions ratios are similar of n-butanol–gasoline to that of gasoline. 294 Regardingat low blending DMF-gasoline, ratios are theresimilar is onlyto that a small of gasoline. effect on Regarding volumetric DMF-gasoline, fuel consumption there compared is only a small with 295 gasolineeffect on atvolumetric various blending fuel consumption ratios. The compared main reason wi isththat gasoline the lower at various heating blending value of ratios. DMF isThe slightly main 296 lowerreason than is that that the of gasoline,lower heating while value it is the of highest DMF is among slightly the lower three than oxygenated that of additions.gasoline, while When it taking is the 297 allhighest the test among fuels the into three consideration, oxygenated the additions. volumetric When fuel consumptiontaking all the oftest ethanol-gasoline fuels into consideration, is the highest the 298 amongvolumetric the testfuel fuels. consumption The volumetric of ethanol-gasoline fuel consumption is the highest of n-butanol-gasoline among the test and fuels. DMF-gasoline The volumetric are 299 similarfuel consumption to that of pure of n-butanol-gasoline gasoline at both 20% and and DMF-gasoline 50% blending are ratios, similar but to higherthat ofthan pure that gasoline of pure at 300 gasolineboth 20% at and 75% 50% and 100%blending blending ratios, ratios. but higher The addition than that of DMFof pure has gasoline better volumetric at 75% and fuel 100% consumption blending than that of the addition of ethanol and n-butanol.

Energies 2019, 12, x 9 of 23

301Energies ratios.2019, 12The, 1845 addition of DMF has better volumetric fuel consumption than that of the addition of9 of 21 302 ethanol and n-butanol.

14 14 E50 E20 B50 12 B20 12 D50 D20 Gasoline 10 Gasoline 10 8 8 6 6 FC (L/100km) FC FC (L/100km) FC 4 4 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (a) (b) 14 14 E75 E100 12 B75 12 B100 D75 D100 10 Gasoline 10 Gasoline 8 8 6 6

FC (L/100km) FC 4 (L/100km) FC 4 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (c) (d)

303 FigureFigure 4. Comparison 4. Comparison of volumetric of volumetric fuel fuel consumption consumption (FC) (FC) of of di ffdifferenterent fuels fuels at at di ffdifferenterent blending blending ratios: 304 ratios: (a) 20% blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending (a) 20% blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. 305 ratio. Figure5 shows the equivalent fuel consumption of three kinds of blended fuels-ethanol-gasoline, 306 Figure 5 shows the equivalent fuel consumption of three kinds of blended 307n-butanol–gasoline, fuels-ethanol-gasoline, and DMF-gasoline n-butanol–gasoline, at blending and DMF-gasoline ratios of 20%, at blending 50%, 75%, ratios and of 20%, 100%, 50%, respectively. 75%, 308The equivalentand 100%, fuelrespectively. consumption The equivalent is defined fuel as theconsumption equivalent is gasolinedefined as consumption the equivalent calculated gasoline with 309the lowerconsumption heating calculated value of eachwith fuel.the lower It is calculatedheating value by of the each following fuel. It is Formula calculated (1): by the following 310 Formula (1): EFCEFCfuel fuel= FC=fuelFC × LHVfuel fuelLHV/LHVfuelG /LHVG (1) (1) Energies 2019, 12, x × 10 of 23 311 where EFCfuel, FCfuel, and LHVfuel are the equivalent fuel consumption, volumetric fuel consumption, 312where and EFC lowerfuel, heating FCfuel, andvalue LHV of a fuelblendedare the fuel; equivalent LHVG is the fuel lower consumption, heating value volumetric of gasoline. fuel consumption, 331 effects on the equivalent fuel consumption, meaning that the thermal efficiency of the different fuels 313and lowerThe heating definition value of of equivalent a blended fu fuel;el consumption LHV is the can lower provide heating an insight value into of gasoline. the efficiency of 332 are similar. G 314 combustion by eliminating the effect of the variation in the heating values of different fuels on the 315 fuel consumption.10 It can be seen clearly that the effect10 of blending various proportions of E20 E50 316 oxygenated9 fuels on the equivalent B20 fuel consumption is fairly9 small. Similar to the above-mentioned B50 317 trend (Figure 4), for all the test D20fuels, the equivalent fuel consumption is higher at low D50 speed. As the 8 Gasoline 8 Gasoline 318 vehicle speed7 increases, the equivalent fuel consumption decreases7 firstly, and then increases. The 319 high equivalent6 fuel consumption at low speed is mainly6 due to the severe throttling loss. The 320 equivalent5 fuel consumption of ethanol–gasoline is the same5 as that of pure gasoline, which may be 321 due to the4 increase of flame propagation speed improving4 the combustion efficiency for ethanol; 322 however, 0the higher 20406080100120 latent heat of vaporization of ethanol0 reduces 20406080100120 the combustion efficiency. equivalent FC (L/100km) equivalent equivalent FC (L/100km) 323 Oxygen and the lowerspeed latent (km/h heat) of vaporization of butanol improvesp eedthe ( km/hcombustion) process, 324 resulting in the higher (combustiona) efficiency of butanol compared to that(b of) pure gasoline. When 10 10 325 the blending ratios are up to E7575% and 100%, DMF exhibited a slightly better equivalent fuel 9 9 E100 326 consumption. Although the laminar B75 flame burning speed of DMF is slightly lowerB100 than that of 8 D75 8 D100 327 gasoline, the high-octane value Gasoline of DMF allows slightly advanced ignition and Gasoline combustion at 65 7 7 328 km/h and6 90 km/h, as shown in Figure 3, which can make combustion6 closer to the top dead center 329 and increase5 the degree of constant volume combustion.5 Similar results can also be found in 330 previous4 studies [14,47]. In summary, at the same speed, different4 fuel components show very small 0 20406080100120 0 20406080100120 (L/100km) FC equivalent equivalent FC (L/100km) equivalent speed (km/h) speed (km/h) (c) (d)

333 FigureFigure 5. Comparison 5. Comparison of equivalentof equivalent fuel fuel consumption consumption of of different diﬀerent fuels fuels at atdifferent diﬀerent blending blending ratios: ratios: 334 (a) 20% blending ratio; (b) 50% blending ratio; ; and (a) 20% blending ratio; (b) 50% blending ratio; (c) 75%(c) 75% blending blending ratio; ratio and (d) 100%(d) 100% blending blending ratio. 335 ratio.

336 Figure 6 shows the CO2 emissions of three kinds of blended fuels-ethanol-gasoline, 337 n-butanol-gasoline, and DMF-gasoline, at the blending ratios of 20%, 50%, 75%, and 100%; the 338 results of pure gasoline are also presented for comparison. The commonality trend for all the test 339 fuels is that the CO2 emissions are the highest at the low speed of 15 km/h. As the vehicle speed 340 increases, CO2 emissions decrease firstly, and then increase. The CO2 emissions are the lowest when 341 the vehicle speed is in the range of 40 to 65 km/h. The CO2 emissions at a vehicle speed of 120 km/h 342 are smaller than those at a vehicle speed of 15 km/h. This can be explained by the fuel consumption, 343 because the CO2 emissions are positively correlated with it. For n-butanol-gasoline blends, the CO2 344 emissions at various n-butanol blending ratios are lower than those of gasoline. One reason is that 345 the ratio of carbon atoms to the energy of n-butanol is lower; releasing the same amount of heat 346 produces less CO2. In addition, as shown in Figure 5, the equivalent fuel consumption of n-butanol is 347 lower than that of gasoline, because the oxygen content in n-butanol can promote combustion, 348 enhance the combustion efficiency, and improve the thermal efficiency, which results in lower 349 equivalent fuel consumption and less CO2 formation. The CO2 emissions of DMF-gasoline with a 20% 350 DMF fraction remain almost at the same level as that of gasoline. However, at larger blending 351 proportions, the CO2 emissions of DMF-gasoline become higher than gasoline due to the larger ratio 352 of carbon atoms to the energy of DMF. For ethanol-gasoline fuel blends, the CO2 emissions are 353 slightly lower those of pure gasoline. The theoretical calculation of ethanol can reduce CO2 354 emissions due to its lower ratio of carbon atoms to the energy compared with that of gasoline. 355 However, meanwhile, the larger latent heat of vaporization and the surface tension of ethanol may 356 reduce the in-cylinder temperature, deteriorate the fuel atomization and evaporation, and have 357 negative impacts on the combustion and fuel efficiency. Therefore, combining the influence of the 358 two aspects mentioned, the differences in CO2 emissions between ethanol–gasoline blends and pure 359 gasoline are very small. Taking all the test fuels into consideration, the DMF-gasoline fuel blends 360 show the highest CO2 emissions, and the higher the blending ratio of DMF is, the greater the 361 presented difference in CO2 emissions. The CO2 emissions of n-butanol-gasoline and 362 ethanol-gasoline are slightly lower than those of pure gasoline.

Energies 2019, 12, 1845 10 of 21

The definition of equivalent fuel consumption can provide an insight into the efficiency of combustion by eliminating the effect of the variation in the heating values of different fuels on the fuel consumption. It can be seen clearly that the effect of blending various proportions of oxygenated fuels on the equivalent fuel consumption is fairly small. Similar to the above-mentioned trend (Figure4), for all the test fuels, the equivalent fuel consumption is higher at low speed. As the vehicle speed increases, the equivalent fuel consumption decreases firstly, and then increases. The high equivalent fuel consumption at low speed is mainly due to the severe throttling loss. The equivalent fuel consumption of ethanol–gasoline is the same as that of pure gasoline, which may be due to the increase of flame propagation speed improving the combustion efficiency for ethanol; however, the higher latent heat of vaporization of ethanol reduces the combustion efficiency. Oxygen and the lower latent heat of vaporization of butanol improve the combustion process, resulting in the higher combustion efficiency of butanol compared to that of pure gasoline. When the blending ratios are up to 75% and 100%, DMF exhibited a slightly better equivalent fuel consumption. Although the laminar flame burning speed of DMF is slightly lower than that of gasoline, the high-octane value of DMF allows slightly advanced ignition and combustion at 65 km/h and 90 km/h, as shown in Figure3, which can make combustion closer to the top dead center and increase the degree of constant volume combustion. Similar results can also be found in previous studies [14,47]. In summary, at the same speed, different fuel components show very small effects on the equivalent fuel consumption, meaning that the thermal efficiency of the different fuels are similar. Figure6 shows the CO 2 emissions of three kinds of blended fuels-ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline, at the blending ratios of 20%, 50%, 75%, and 100%; the results of pure gasoline are also presented for comparison. The commonality trend for all the test fuels is that the CO2 emissions are the highest at the low speed of 15 km/h. As the vehicle speed increases, CO2 emissions decrease firstly, and then increase. The CO2 emissions are the lowest when the vehicle speed is in the range of 40 to 65 km/h. The CO2 emissions at a vehicle speed of 120 km/h are smaller than those at a vehicle speed of 15 km/h. This can be explained by the fuel consumption, because the CO2 emissions are positively correlated with it. For n-butanol-gasoline blends, the CO2 emissions at various n-butanol blending ratios are lower than those of gasoline. One reason is that the ratio of carbon atoms to the energy of n-butanol is lower; releasing the same amount of heat produces less CO2. In addition, as shown in Figure5, the equivalent fuel consumption of n-butanol is lower than that of gasoline, because the oxygen content in n-butanol can promote combustion, enhance the combustion efficiency, and improve the thermal efficiency, which results in lower equivalent fuel consumption and less CO2 formation. The CO2 emissions of DMF-gasoline with a 20% DMF fraction remain almost at the same level as that of gasoline. However, at larger blending proportions, the CO2 emissions of DMF-gasoline become higher than gasoline due to the larger ratio of carbon atoms to the energy of DMF. For ethanol-gasoline fuel blends, the CO2 emissions are slightly lower those of pure gasoline. The theoretical calculation of ethanol can reduce CO2 emissions due to its lower ratio of carbon atoms to the energy compared with that of gasoline. However, meanwhile, the larger latent heat of vaporization and the surface tension of ethanol may reduce the in-cylinder temperature, deteriorate the fuel atomization and evaporation, and have negative impacts on the combustion and fuel efficiency. Therefore, combining the influence of the two aspects mentioned, the differences in CO2 emissions between ethanol–gasoline blends and pure gasoline are very small. Taking all the test fuels into consideration, the DMF-gasoline fuel blends show the highest CO2 emissions, and the higher the blending ratio of DMF is, the greater the presented difference in CO2 emissions. The CO2 emissions of n-butanol-gasoline and ethanol-gasoline are slightly lower than those of pure gasoline. Energies 2019, 12, 1845 11 of 21 Energies 2019, 12, x 11 of 23

250 250 E20 E50 B20 B50 200 D20 200 D50 Gasoline Gasoline 150 150 (g/km) (g/km)

2 100 2 100

CO 50 CO 50 0 0 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (a) (b) 250 250 E75 E100 200 B75 200 B100 D75 D100 Gasoline Gasoline 150 150 (g/km) (g/km)

2 100 2 100

CO 50 CO 50 0 0 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (c) (d)

2 363 FigureFigure 6. 6.Comparison Comparison of COof 2COemissions emissions of di ofﬀerent different fuels atfuels diﬀ erentat different blending blending ratios: ( aratios:) 20% blending(a) 20% 364 ratio;blending (b) 50% ratio; blending (b) 50% ratio; blending (c) 75% ratio; blending (c) 75% ratio; blending and (ratio;d) 100% and blending (d) 100% ratio.blending ratio.

365 3.3.3.3. Comparison Comparison of of Harmful harmful Gaseousgaseous emissions Emissions 366 FigureFigure7 shows7 shows the the CO CO emissions emissions of of ethanol–gasoline, ethanol–gasoline, n-butanol–gasoline, n-butanol–gasoline, andand DMF–gasolineDMF–gasoline 367 withwith blending blending ratios ratios of of 20%, 20%, 50%, 50%, 75%, 75%, and and 100%, 100%, respectively. respectively. In In general, general, CO CO emissions emissions are are higher higher at 368 lowerat lower speeds, speeds, and and as the as the vehicle vehicle speed speed increases, increases, CO CO emissions emissions decrease decrease firstly, firstly, and and then then increase increase as 369 theas vehiclethe vehicle speed speed furtherly furtherly increases. increases. At low At speeds, low speeds, the combustion the combustion temperature temperature is low is as low well, as which well, 370 causeswhich incomplete causes incomplete combustion combustion and low combustionand low combustion efficiency; efficiency; as a result, as the a result, CO emissions the CO areemissions higher. 371 Theare combustion higher. The temperaturecombustion temperature increases when increases the vehicle whenspeed the vehicle becomes speed higher, becomes so the higher, combustion so the 372 efficombustionciency is improved efficiency and is improved CO emissions and CO decrease. emission Whens decrease. the vehicle When runs the at vehicle high speed runs at and high at aspeed high 373 gear,and theat enginea high alsogear, working the engine in high also speed, working and thein combustionhigh speed, durationand the iscombustion shortened significantly.duration is 374 Asshortened a result, theresignificantly. is not enough As a result, time available there is not for theenough complete time oxidizationavailable for of the CO, complete which results oxidization in the 375 increaseof CO, inwhich CO emissions. results in Blendingthe increase oxygenated in CO emi fuelssions. can decreaseBlending CO oxygenated emissions fuel at low can speed. decrease On oneCO 376 hand,emissions the oxygen at low in speed. fuels isOn helpful one hand, to oxidize the oxygen CO. On in the fuels other is helpful hand, the to eoxidizeffect of volatilityCO. On the becomes other 377 significanthand, the wheneffect theof volatility mixing betweenbecomes airsignificant and fuel wh isen uneven the mixing at low between speed. Theair and volatility fuel is ofuneven ethanol at 378 islow better speed. than The those volatility of gasoline, of ethanol n-butanol, is better and than DMF, those so theof gasoline, CO emissions n-butanol, of E50, and E75, DMF, and so E100 the CO are 379 lowestemissions at low of speed. E50, E75, In the and case E100 of are large lowest blending at low proportions, speed. In the COcase emissions of large blending of n-butanol-gasoline proportions, 380 arethe the CO lowest emissions at medium of n-butanol-gasoline to high vehicle speeds.are the Thelowest possible at medium reasons to mayhigh be vehicle as follows. speeds. Firstly, The 381 thepossible oxygen reasons in n-butanol may be is beneficialas follows. for Firstly, the oxidization the oxygen of in CO. n-butanol Secondly, is beneficial OH radical for form the oxidization more easily 382 withof CO. the longerSecondly, molecular OH radical chain form of n-butanol, more easily and with OH the is alonger strong molecular oxidant tochain oxidize of n-butanol, CO. Blending and 383 ethanolOH is anda strong DMF oxidant in gasoline to oxidize has a lessCO. significant Blending ethanol effect on and the DMF reduction in gasoline of CO emissionshas a less atsignificant medium 384 toeffect high on vehicle the reduction speeds. Onof CO one emissions hand, due at medium to the oxygenated to high vehicle nature speeds. and higher On one octane hand, numberdue to the of 385 ethanoloxygenated and DMF, nature the and blended higher fuels octane attain number better combustionof ethanol and than DMF, gasoline. the blended On the otherfuels hand,attain duebetter to 386 thecombustion high latent than heat gasoline. of vaporization On the other of these hand, two due fuels, to therethe high is a latent large numberheat of vaporization of CO formed of inthese the 387 low-temperaturetwo fuels, there region,is a large where number oxidation of CO formed is not easy. in the The low-temperature simultaneous e region,ffects of where these twooxidation aspects is 388 not easy. The simultaneous effects of these two aspects lead to a smaller effect of CO emissions on lead to a smaller effect of CO emissions on the blending ratio of ethanol and DMF. 389 the blending ratio of ethanol and DMF. Figure8 shows THC emissions of ethanol-gasoline, n-butanol-gasoline, and DMF–gasoline with blending ratios of 20%, 50%, 75%, and 100%, respectively. The high THC emissions are mainly concentrated at the low speed of 15 km/h due to the low combustion temperature and consequent incomplete combustion. With the increase of speed, the THC emissions decrease quickly, and then

Energies 2019, 12, 1845 12 of 21

remain at a low level with small changes with speed; at the same time, the difference in THC emissions among different kinds of blended fuels decreases. This is because as the speed increases, the increased gas flow in the cylinder enhances the homogeneity and the combustion rate increases; so, the THC emissions decrease. However, with the increase of blending proportion, the difference in THC emissions among different fuels increases. Under all the operating conditions, the THC emissions of oxygenated fuel blends are lower than those of pure gasoline, since the oxygen atoms in the oxygenated fuel promote combustion. As the blending ratio increases, the amount of THC emissions decreases. This is probably due to the fuel containing more oxygen, which makes the combustion more complete. Ethanol has lower THC emissions at various blending ratios than gasoline, which is attributed to the highest oxygen content of ethanol promoting full combustion, and thus reducing THC emissions. However, when n-butanol was blended in a large proportion, the THC emissions were higher than those of ethanol and DMF. This is probably because the addition of n-butanol is not conducive to atomization due to its higher viscosity and boiling point, making it is easy to cause impinging [48]. Energies 2019, 12, x 12 of 23

7000 7000 E20 E50 6000 B20 6000 B50 5000 D20 5000 D50 Gasoline Gasoline 4000 4000 3000 3000 2000 2000

CO (mg/km) CO 1000 (mg/km) CO 1000 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 speed (km/h) speed (km/h) (a) (b) 7000 7000 E75 E100 6000 B75 6000 B100 5000 D75 5000 D100 Gasoline Gasoline 4000 4000 3000 3000 2000 2000

CO (mg/km) CO 1000 (mg/km) CO 1000 0 0 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (c) (d)

390 FigureFigure 7. Comparison 7. Comparison of CO of emissions CO emissions of diff oferent different fuels atfuels diff erentat different blending blending ratios: ratios: (a) 20% (a) blending 20% 391 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. 392 Figure 8 shows THC emissions of ethanol-gasoline, n-butanol-gasoline, and DMF–gasoline Figure9 shows NO X emissions of ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline with 393 with blending ratios of 20%, 50%, 75%, and 100%, respectively. The high THC emissions are mainly blending ratios of 20%, 50%, 75%, and 100%, respectively. It can be seen that NO emissions are low 394 concentrated at the low speed of 15 km/h due to the low combustion temperatureX and consequent 395and changeincomplete little combustion. at low to medium With the speeds increase (15–65 of speed km,/ h).the However,THC emissions when decrease the vehicle quickly, speed and isthen higher 396than 65remain km/h, at NO a Xlowemissions level with increase small rapidly.changes Thiswith isspeed; because at the the same combustion time, the temperature difference in is higherTHC at 397the higheremissions vehicle among speed, different so the kinds increase of blended in combustion fuels decreases. temperature This is because leads as to the more speed NO increases,X formation. 398At mediumthe increased and high gas flow speeds, in the blending cylinder ethanolenhancesand the homogeneity n-butanol result and the in combustion lower NO Xrateemissions increases; than 399those ofso, gasoline.the THC emissions This is mainly decrease. due However, to the high with latent the increase heat of of vaporization blending proportion, of ethanol the and difference n-butanol, 400which in results THC emissions in a lower among in-cylinder different temperature fuels increase ands. Under combustion all the temperatureoperating conditions, as well the [26 ].THC This is 401the dominantemissions factor of oxygenated influencing fuel NOx blends formation. are lower So,than with those an of increase pure gasoline in the, since blending the oxygen ratio ofatoms ethanol 402 in the oxygenated fuel promote combustion. As the blending ratio increases, the amount of THC and n-butanol, NOx emissions decrease accordingly, and ethanol-gasoline shows the lowest NOx 403 emissions decreases. This is probably due to the fuel containing more oxygen, which makes the because of this blend having the highest latent heat of vaporization of ethanol. Such a trend is more 404 combustion more complete. Ethanol has lower THC emissions at various blending ratios than 405obvious gasoline, at high which blending is attributed ratios, e.g.,to the 75% highest and 100%.oxygen However, content ofthe ethanol large promoting proportion full of combustion, DMF blending 406resulted and in thus higher reducing NOX THCemissions emissions. than However, gasoline, when and NO n-butanolX emissions was blended increased in a as large the DMFproportion, blending 407ratio increased,the THC emissions which is oppositewere higher to thethan trends those ofof blendingethanol and ethanol DMF. and This n-butanol. is probably According because the to the 408theoretical addition air-fuel of n-butanol ratio and is not low conducive heating valueto atomization of DMF due and to gasoline, its higher calculating viscosity and the boiling homogeneous point, 409mixture making with it an is equivalenteasy to cause ratio impinging of one [48]. under complete combustion conditions, it can be concluded

2000 E20 2000 E50 B20 B50 1600 D20 1600 D50 Gasoline Gasoline 1200 1200 800 800 400 400 THC (mg/km) THC (mg/km) THC 0 0 0 20 40 60 80 100 120 0 20406080100120 speed (km/h) speed (km/h) (a) (b)

Energies 2019, 12, x 12 of 23

7000 7000 E20 E50 6000 B20 6000 B50 5000 D20 5000 D50 Gasoline Gasoline 4000 4000 3000 3000 2000 2000

CO (mg/km) CO 1000 (mg/km) CO 1000 0 0 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (c) (d)

390 Figure 7. Comparison of CO emissions of different fuels at different blending ratios: (a) 20% 391 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio.

392 Figure 8 shows THC emissions of ethanol-gasoline, n-butanol-gasoline, and DMF–gasoline 393 with blending ratios of 20%, 50%, 75%, and 100%, respectively. The high THC emissions are mainly 394 concentrated at the low speed of 15 km/h due to the low combustion temperature and consequent 395 incomplete combustion. With the increase of speed, the THC emissions decrease quickly, and then 396 remainEnergies 2019 at ,a 12 low, x level with small changes with speed; at the same time, the difference in13 THC of 23 397 emissions among different kinds of blended fuels decreases. This is because as the speed increases, 398 the increased gas flow in the cylinder enhances the homogeneity and the combustion rate increases; 2000 E75 2000 E100 399 so, the THC emissions decrease. However, B75 with the increase of blending proportion, B100 the difference 1600 D75 1600 D100 400 in THC emissions among different fuelsGasoline increases. Under all the operating conditions, Gasoline the THC 1200 1200 401 emissions of oxygenated fuel blends are lower than those of pure gasoline, since the oxygen atoms Energies 2019, 12, 1845 13 of 21 402 in the 800oxygenated fuel promote combustion. As the blending800 ratio increases, the amount of THC 400 400

403 emissions (mg/km) THC decreases. This is probably due to the fuel (mg/km) THC containing more oxygen, which makes the 404 combustion0 more complete. Ethanol has lower THC emissions0 at various blending ratios than that the combustion temperature of DMF is higher than that of gasoline. Studies have also shown that 405 gasoline, which0 is 20406080100120 attributed to the highest oxygen content of ethanol0 20406080100120 promoting full combustion, the adiabatic ﬂame temperaturespeed (km/h of) DMF is higher than that of gasolinespeed [46], (km/h which) can explain the 406 and thus reducing THC(c )emissions. However, when n-butanol was blended(d) in a large proportion, 407increase the in THC NO Xemissionsemissions were as the higher DMF than blending those ratioof ethanol increases. and DMF. In addition, This is the probably higher because octane numberthe 408410of DMFaddition resultsFigure of in n-butanol8. aComparison slightly is not advanced of conduciveTHC emissions combustion to atomization of different phase due fuels and to at its high different higher constant viscosityblending volume ratios:and boiling of(a) combustion,20% point, 411 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. 409which making increase it is the easy NO toX causeemissions impinging to some [48]. extent.

412 Figure 9 shows NOX emissions of ethanol-gasoline, n-butanol-gasoline, and DMF-gasoline with 2000 E20 2000 E50 413 blending ratios of 20%, 50%, 75%, B20and 100%, respectively. It can be seen that NOX emissions B50 are low 1600 D20 1600 D50 414 and change little at low to medium Gasoline speeds (15–65 km/h). However, when the vehicle Gasoline speed is 415 higher1200 than 65 km/h, NOX emissions increase rapidly. This1200 is because the combustion temperature 416 is higher800 at the higher vehicle speed, so the increase in combustion800 temperature leads to more NOX 417 formation.400 At medium and high speeds, blending ethanol400 and n-butanol result in lower NOX THC (mg/km) THC (mg/km) THC 418 emissions0 than those of gasoline. This is mainly due to 0the high latent heat of vaporization of 419 ethanol and0 n-butanol, 20 40 60which 80 results 100 120 in a lower in-cylinder0 20406080100120temperature and combustion speed (km/h) speed (km/h) 420 Energiestemperature 2019, 12 ,as x well [26]. This is the dominant factor influencing NOx formation. So, with13 of an23 (a) (b) 421 increase in the blending ratio of ethanol and n-butanol, NOx emissions decrease accordingly, and 422 ethanol-gasoline shows the lowest NOx because of this blend having the highest latent heat of 2000 E75 2000 E100 423 vaporization of ethanol. Such a trend B75 is more obvious at high blending ratios, e.g., B100 75% and 100%. 1600 D75 1600 D100 424 However, the large proportion of DM GasolineF blending resulted in higher NOX emissions Gasoline than gasoline, 1200 1200 425 and NOX emissions increased as the DMF blending ratio increased, which is opposite to the trends 426 of blending800 ethanol and n-butanol. According to the theoretical800 air-fuel ratio and low heating value 400 400

427 of DMF (mg/km) THC and gasoline, calculating the homogeneous mixture (mg/km) THC with an equivalent ratio of one under 428 complete0 combustion conditions, it can be concluded that 0the combustion temperature of DMF is 429 higher than 0that 20406080100120of gasoline. Studies have also shown that the adiabatic0 20406080100120 flame temperature of DMF speed (km/h) speed (km/h) 430 is higher than that of gasoline(c) [46], which can explain the increase in NO(dX) emissions as the DMF 431 blending ratio increases. In addition, the higher octane number of DMF results in a slightly 410 Figure 8. Comparison of THC emissions of different fuels at different blending ratios: (a) 20% 432 Figureadvanced 8. Comparison combustion of THCphase emissions and high of constant diﬀerent fuelsvolume at diofﬀ erentcombustion, blending which ratios: increase (a) 20% blendingthe NOX 411 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. 433 ratio;emissions (b) 50% to blendingsome extent. ratio; (c) 75% blending ratio; and (d) 100% blending ratio.

412 Figure3000 9 shows NOX emissions of ethanol-gasoline, 3000n-butanol-gasoline, and DMF-gasoline with E20 E50 413 blending2500 ratios B20 of 20%, 50%, 75%, and 100%, respectively.2500 It can be B50 seen that NOX emissions are low D20 D50 414 and 2000change little Gasoline at low to medium speeds (15–65 km/h).2000 However, Gasoline when the vehicle speed is 415 higher1500 than 65 km/h, NOX emissions increase rapidly. This1500 is because the combustion temperature 416 is higher1000 at the higher vehicle speed, so the increase in combustion1000 temperature leads to more NOX 417 formation.500 At medium and high speeds, blending ethanol500 and n-butanol result in lower NOX NOx (mg/km) NOx 418 emissions0 than those of gasoline. This is mainly dueNOx (mg/km) to 0the high latent heat of vaporization of 419 ethanol and0 n-butanol, 20406080100120 which results in a lower in-cylinder0 20406080100120temperature and combustion speed (km/h) speed (km/h) 420 Energiestemperature 2019, 12 ,as x well [26]. This is the dominant factor influencing NOx formation. So, with14 of an23 421 increase in the blending(a )ratio of ethanol and n-butanol, NOx emissions decrease(b) accordingly, and 422 ethanol-gasoline shows the lowest NOx because of this blend having the highest latent heat of 3000 E75 3000 E100 423 vaporization of B75 ethanol. Such a trend is more obvious at high blending B100 ratios, e.g., 75% and 100%. 2500 D75 2500 D100 424 However,2000 the Gasolinelarge proportion of DMF blending resulted2000 in higher Gasoline NOX emissions than gasoline, 425 and NO1500X emissions increased as the DMF blending ratio1500 increased, which is opposite to the trends 426 of blending1000 ethanol and n-butanol. According to the theoretical1000 air-fuel ratio and low heating value 427 of DMF500 and gasoline, calculating the homogeneous mixture500 with an equivalent ratio of one under NOx (mg/km) NOx 0 (mg/km) NOx 0 428 complete combustion conditions, it can be concluded that the combustion temperature of DMF is 429 higher than 0that 20406080100120of gasoline. Studies have also shown that the adiabatic0 20406080100120 flame temperature of DMF speed (km/h) speed (km/h)

430 is higher than that of gasoline [46], which can explain the increase in NOX emissions as the DMF (c) (d) 431 blending ratio increases. In addition, the higher octane number of DMF results in a slightly 434 Figure 9. Comparison of NOX emissions of different fuels at different blending ratios: (a) 20% 432 Figureadvanced 9. Comparison combustion of NOphaseX emissions and high of constant diﬀerent fuelsvolume at diofﬀ erentcombustion, blending which ratios: increase (a) 20% blendingthe NOX 435 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio. 433 ratio;emissions (b) 50% to blendingsome extent. ratio; (c) 75% blending ratio; and (d) 100% blending ratio.

436 3.4. Particle3000 Number emissions 3000 3.4. Particle Number E20 Emissions E50 2500 B20 2500 B50 437 Figure 10 D20 shows the PN (particle number) emissions of ethanol-gasoline, D50 n-butanol-gasoline, Figure2000 10 shows the PN (particle number) emissions2000 of ethanol-gasoline, n-butanol-gasoline, 438 and DMF-gasoline Gasoline with blending ratios of 20%, 50%, 75%, and Gasoline 100%, respectively. At low and and DMF-gasoline1500 with blending ratios of 20%, 50%, 75%,1500 and 100%, respectively. At low and medium 439 medium vehicle speeds, the PN emissions of oxygenated blended fuels at various blending ratios vehicle speeds,1000 the PN emissions of oxygenated blended fuels1000 at various blending ratios are kept at a 440 are kept at a low level; however, PN emissions are higher at high speed (≥ 90 km/h). This is mainly 500 500 NOx (mg/km) NOx 441 because 0the fuel mass is increased at high speed, andNOx (mg/km) more0 fuel will impinge into the wall; the 442 mixing timing is also shortened, which results in poor fuel-air mixing, and the PN increases 443 0 20406080100120 0 20406080100120 accordingly. At the sfourpeed blending(km/h) ratios, the PN emissions of the oxygenatedspeed (km/h blended) fuels are 444 generally lower than those(a) of gasoline. This is because blending oxygenated(b) fuels can provide a 445 certain amount of oxygen for combustion, improving the mixing between fuel and air and reducing 446 the content of aromatic in the blending fuel, which reduces the soot formation. When the blending 447 ratios are 75% and 100%, the PN emissions of the blended fuels are decreased remarkably, and both 448 n-butanol-gasoline and DMF-gasoline exhibit lower PN emissions than ethanol–gasoline. This may 449 be mainly because the combustion temperatures of n-butanol-gasoline and DMF-gasoline are 450 higher than that of ethanol-gasoline, which enhances the oxidation of soot.

50 E20 50 E50 B20 B50 40 D20 40 D50 Gasoline Gasoline 30 30 20 20 10 10 PN (1e10#/km) PN (1e10#/km) PN 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 speed (km/h) speed (km/h) (a) (b)

50 E75 50 E100 B75 B100 40 D75 40 D100 Gasoline Gasoline 30 30 20 20 10 10 PN (1e10#/km) PN (1e10#/km) PN 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 speed (km/h) speed (km/h) (c) (d)

451 Figure 10. Comparison of particle number (PN) emissions of different fuels at different blending 452 ratios: (a) 20% blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending 453 ratio.

Energies 2019, 12, x 14 of 23

3000 E75 3000 E100 B75 B100 2500 D75 2500 D100 2000 Gasoline 2000 Gasoline 1500 1500 1000 1000 500 500 NOx (mg/km) NOx 0 (mg/km) NOx 0 0 20406080100120 0 20406080100120 speed (km/h) speed (km/h) (c) (d)

434 Figure 9. Comparison of NOX emissions of different fuels at different blending ratios: (a) 20% 435 blending ratio; (b) 50% blending ratio; (c) 75% blending ratio; and (d) 100% blending ratio.

436 3.4. Particle Number emissions 437 Energies 2019Figure, 12, 184510 shows the PN (particle number) emissions of ethanol-gasoline, n-butanol-gasoline,14 of 21 438 and DMF-gasoline with blending ratios of 20%, 50%, 75%, and 100%, respectively. At low and 439 medium vehicle speeds, the PN emissions of oxygenated blended fuels at various blending ratios low level; however, PN emissions are higher at high speed ( 90 km/h). This is mainly because the fuel 440 are kept at a low level; however, PN emissions are higher ≥at high speed (≥ 90 km/h). This is mainly mass is increased at high speed, and more fuel will impinge into the wall; the mixing timing is also 441 because the fuel mass is increased at high speed, and more fuel will impinge into the wall; the 442 shortened,mixing timing which resultsis also inshortened, poor fuel-air which mixing, results and in the poor PN increasesfuel-air mixing, accordingly. and Atthe the PN four increases blending 443 ratios,accordingly. the PN emissionsAt the four of theblending oxygenated ratios, blendedthe PN emissions fuels are generallyof the oxygenated lower than blended those offuels gasoline. are 444 Thisgenerally is because lower blending than those oxygenated of gasoline. fuels This can is provide because a blending certain amount oxygenated of oxygen fuels can for combustion,provide a 445 improvingcertain amount the mixing of oxygen between for combustion, fuel and air andimprovin reducingg the themixing content between of aromatic fuel and in air the and blending reducing fuel, 446 whichthe content reduces of the aromatic soot formation. in the blending When fuel, the blendingwhich reduces ratios the are soot 75% formation. and 100%, When the PN the emissions blending of 447 theratios blended are 75% fuels and are 100%, decreased the PN remarkably, emissions of and the both blended n-butanol-gasoline fuels are decreased and remarkably, DMF-gasoline and exhibitboth 448 lowern-butanol-gasoline PN emissions than and ethanol–gasoline.DMF-gasoline exhibit This lower may bePN mainly emissions because than theethanol–gasoline. combustion temperatures This may 449 of n-butanol-gasolinebe mainly because andthe DMF-gasolinecombustion temperatures are higher than of n-butanol-gasoline that of ethanol-gasoline, and DMF-gasoline which enhances are the 450 oxidationhigher than of soot. that of ethanol-gasoline, which enhances the oxidation of soot.

50 E20 50 E50 B20 B50 40 D20 40 D50 Gasoline Gasoline 30 30 20 20 10 10 PN (1e10#/km) PN (1e10#/km) PN 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 speed (km/h) speed (km/h) (a) (b)

50 E75 50 E100 B75 B100 40 D75 40 D100 Gasoline Gasoline 30 30 20 20 10 10 PN (1e10#/km) PN (1e10#/km) PN 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 speed (km/h) speed (km/h) (c) (d)

451 FigureFigure 10. 10.Comparison Comparison of particleof particle number number (PN) (PN) emissions emission ofs diofff erentdifferent fuels fuels at di atff erentdifferent blending blending ratios: 452 (a)ratios: 20% blending (a) 20% blending ratio; (b) ratio; 50% blending(b) 50% blending ratio; (c )ratio; 75% (c) blending 75% blending ratio; and ratio; (d )and 100% (d) blending 100% blending ratio. 453 ratio. 3.5. Effect of Blending Small Fraction Oxygenated Fuel in Gasoline on Fuel Consumption and Emissions of NEDC Cycle In the sections above, the fuel consumptions and emissions of three kinds of oxygenated blended fuels have been compared at different blending ratios under steady-state operating conditions. However, considering that transient and cold-start operations are important in real driving cycles, the effects of oxygenated fuels on fuel efficiency and emissions also need to be investigated. The NEDC cycle, which is a standard test cycle of European Union emission regulation, include transient and cold-start operations, and can be employed in the evaluation for different fuels concerning such operating conditions. However, for oxygenated blended fuels with blending ratios over 30%, there are certain technical obstacles related to running in the NEDC cycle because of the difficulties regarding cold starts without the changes in the vehicle engine. Therefore, only the small blending ratio of 20% is selected for the comparison of different fuels in the NEDC cycle.

3.5.1. Comparison of Energy Consumption Figure 11 shows the energy consumption of E20, B20, and D20 in the NEDC cycle. It can be seen that the CO2 emission of B20 is lower than that of gasoline in the NEDC cycle. Meanwhile, the CO2 emissions of B20 are reduced both in the UDC and EUDC stages, which may be due to the dehydrogenation of n-butanol in the early stage of combustion generating a large amount of active Energies 2019, 12, x 15 of 23

454 3.5. Effect of blending small fraction oxygenated fuel in gasoline on fuel consumption and emissions of NEDC 455 cycle 456 In the sections above, the fuel consumptions and emissions of three kinds of oxygenated 457 blended fuels have been compared at different blending ratios under steady-state operating 458 conditions. However, considering that transient and cold-start operations are important in real 459 driving cycles, the effects of oxygenated fuels on fuel efficiency and emissions also need to be 460 investigated. The NEDC cycle, which is a standard test cycle of European Union emission 461 regulation, include transient and cold-start operations, and can be employed in the evaluation for 462 different fuels concerning such operating conditions. However, for oxygenated blended fuels with 463 blending ratios over 30%, there are certain technical obstacles related to running in the NEDC cycle 464 because of the difficulties regarding cold starts without the changes in the vehicle engine. Therefore, 465 only the small blending ratio of 20% is selected for the comparison of different fuels in the NEDC 466 cycle. Energies 2019, 12, 1845 15 of 21 467 3.5.1. Comparison of energy consumption free468 radicals. InFigure turn, 11 the shows hydroxyl the energy group consum promotesption of theE20, low-temperatureB20, and D20 in the oxidationNEDC cycle. process It can be of gasoline 469 seen that the CO2 emission of B20 is lower than that of gasoline in the NEDC cycle. Meanwhile, the and improves the combustion chemical reaction rate [49,50]. The CO emissions of E20 and D20 both 470 CO2 emissions of B20 are reduced both in the UDC and EUDC stages,2 which may be due to the increase471 indehydrogenation the NEDC cycle of n-butanol compared in the to gasoline,early stage andof combustion the increase generating mainly a large comes amount from of theactive UDC stage, while472 the COfree2 emissionsradicals. In inturn, the the EUDC hydroxyl stage group is basically promotes equivalent the low-temperature to gasoline. oxidation The latterprocess is mainlyof due 473 2 to the lowgasoline combustion and improves temperature the combustion and incomplete chemical reaction combustion rate [49,50]. in The the CO UDC emissions stage of resulting E20 and in high 474 D20 both increase in the NEDC cycle compared to gasoline, and the increase mainly comes from the equivalent fuel consumption. The dominant factor affecting volumetric fuel consumption is the lower 475 UDC stage, while the CO2 emissions in the EUDC stage is basically equivalent to gasoline. The heating476 value.latter Equivalentis mainly due fuel to the consumption low combustion is temperature the fuel consumption and incomplete after combustion removing in the the UDC eff ect of the lower477 heating stage value.resulting The in high equivalent equivalent fuel fuel consumption. consumption The and dominant volume factor fuel affecting consumption volumetric offuel B20 in the 478 whole NEDCconsumption cycle are is the lower lower than heating that value. of gasoline, Equivalent whichfuel consumption may be becauseis the fuel theconsumption oxygen after in n-butanol 479 removing the effect of the lower heating value. The equivalent fuel consumption and volume fuel promotes480 combustionconsumption of and B20 improvesin the whole the NEDC fuel ecyclefficiency. are lower In addition,than that of the gasoline, faster which laminar may flamebe speed of n-butanol481 because results the inoxygen a shorter in n-butanol combustion promotes durationcombustion andand improves high constant the fuel efficiency. volume ofIn addition, combustion [51]. The482 energy the consumption faster laminar of flame E20 isspeed higher of n-butanol than that resu oflts gasoline in a shorter at all combustion stages in duration the NEDC and cycle, high and D20 483 shows a slightlyconstant lower volume equivalent of combustion fuel [51]. consumption The energy consumption than gasoline of E20 in is the higher whole than cycle.that of Thegasoline main reason 484 at all stages in the NEDC cycle, and D20 shows a slightly lower equivalent fuel consumption than is same485 asgasoline the steady in the state whole for cycle. energy The main consumption. reason is same as the steady state for energy consumption.

240 gasoline ethanol 220 n-butanol 200 DMF 4.26% 04% . 1.65% 180 -6 /(g/km)

2 160 % 2.39% -5.7%1.18% 140 % CO 92% 64 -5.46 120 0. 0. 100 UDC EUDC NEDC Energies 2019, 12, x 16 of 23 (a)

14 gasoline ethanol 12 n-butanol .18% 15 2% DMF 10 4 1. 87% -2.47% . 4% 8 1% % % 11 9 0.72% 9.1 -1. 6 -1.53-0.04 4 FC (L/100km) 2 0 UDC EUDC NEDC

(b)

14 gasoline ethanol 12 n-butanol 10 DMF 7.27% -0.62% 8 -5.93% 3% 3% 5% 4.19% -5.4 -1.3% 6 1.62%-5.0 -2.0 4 2 0

equivalent FC (L/100km) UDC EUDC NEDC

(c)

486 Figure 11. Comparison of CO2, volumetric fuel consumption (FC), and equivalent fuel consumption Figure 11. Comparison of CO2, volumetric fuel consumption (FC), and equivalent fuel consumption of 487 of E20, B20, D20, and gasoline: (a) CO2 emissions; (b) Volumetric FC; and (c) Equivalent FC. E20, B20, D20, and gasoline: (a) CO2 emissions; (b) Volumetric FC; and (c) Equivalent FC. 488 3.5.2. Comparison of harmful emissions 489 Figure 12 shows the harmful emissions emitted during the NEDC cycles of E20, B20, and D20. 490 The CO and THC emissions in the UDC stage are higher than those of the EUDC stage, which is 491 mainly because during the start-up of the SI engine, fuel-rich injection is required to ensure ignition 492 in the first 30 s, and excessive fuel supply due to low temperature and poor mixing in the cylinder 493 produces large amounts of THC and CO emissions during cold starts [21]. The CO, THC, and NOX 494 emissions of B20 in the whole NEDC cycle are lower than those of gasoline. The oxygenated effect 495 and fast combustion of n-butanol reasonably reduced CO and THC formation, especially in the 496 UDC stage, which features low speed and frequent cold starts. The harmful emissions of DMF in all 497 stages of the NEDC cycle are generally slightly higher than those of gasoline, which may be because 498 the low H/C and O/C of DMF are not conducive to oxidizing THC and CO. The small increase in 499 combustion temperature resulting from the oxygenated property of DMF causes slightly high NOX 500 emissions. The CO and THC emissions of E20 are higher than those of gasoline, but NOX emissions 501 are lower than those of gasoline. The latter is mainly attributed to the higher latent heat of 502 vaporization of ethanol resulting in a lower combustion temperature in the cylinder, which 503 decrease the combustion efficiency and NOx formation simultaneously. The high latent heat of 504 vaporization of ethanol increases the combustion loss, resulting in higher CO emissions.

Energies 2019, 12, 1845 16 of 21

3.5.2. Comparison of Harmful Emissions Figure 12 shows the harmful emissions emitted during the NEDC cycles of E20, B20, and D20. The CO and THC emissions in the UDC stage are higher than those of the EUDC stage, which is mainly because during the start-up of the SI engine, fuel-rich injection is required to ensure ignition in the first 30 s, and excessive fuel supply due to low temperature and poor mixing in the cylinder produces large amounts of THC and CO emissions during cold starts [21]. The CO, THC, and NOX emissions of B20 in the whole NEDC cycle are lower than those of gasoline. The oxygenated effect and fast combustion of n-butanol reasonably reduced CO and THC formation, especially in the UDC stage, which features low speed and frequent cold starts. The harmful emissions of DMF in all stages of the NEDC cycle are generally slightly higher than those of gasoline, which may be because the low H/C and O/C of DMF are not conducive to oxidizing THC and CO. The small increase in combustion temperature resulting from the oxygenated property of DMF causes slightly high NOX emissions. The CO and THC emissions of E20 are higher than those of gasoline, but NOX emissions are lower than those of gasoline. The latter is mainly attributed to the higher latent heat of vaporization of ethanol resulting in a lower combustion temperature in the cylinder, which decrease the combustion efficiency and NOx formation simultaneously. The high latent heat of vaporization of ethanol increases the combustion loss, resulting in higher CO emissions. Energies 2019, 12, x 17 of 23

15000 gasoline % 1 ethanol 12000 .1 3 5 n-butanol

% DMF 9000 7 % % 1 9 . 8 6 5 .8 . 8 2 % % - 3 9 7 % 5 6000 9 % % .1 . 3 4 1 5 2 . 8 1 - 8 . . 0 2 CO (mg/km) 3000 - - 0 UDC EUDC NEDC

(a)

2100 3% gasoline 11% % .0 5. 3 5 .7 ethanol 1800 -3 n-butanol 1500 DMF % % 1 5 1200 2.33% .9 3. -2 900 % % % 2 6 .22 .6 -2 -1 0.0 THC (mg/km) 600 300 UDC EUDC NEDC

(b)

gasoline % 1500 % % .7 ethanol 1 1 2 4 . .9 2 % n-butanol 3 % 9 1 1 % 1200 - - 5 1 0 1 . DMF . .2 3 2 3 1 1 900 - - % % % 3 8 4 5 0 . . .5 1 0 5 600 1 1 - -

NOx (mg/km) 300 0 UDC EUDC NEDC

(c)

505 Figure 12. Comparison of CO, THC, and NOX emissions of E20, B20, D20, and gasoline: (a) CO Figure 12. Comparison of CO, THC, and NO emissions of E20, B20, D20, and gasoline: (a) CO emissions; 506 emissions; (b) THC emissions; and (c) NOX X emissions. (b) THC emissions; and (c) NOX emissions. 507 3.5.3. Comparison of particle emissions 508 Figure 13 shows particulate emissions of E20, B20, and D20 in the NEDC cycle. In all the stages 509 of the NEDC cycle, the PM and PN of the oxygenated blended fuels decrease significantly 510 compared with gasoline, and the largest reduction is obtained with D20. It is generally believed that 511 the oxygenated fuel contains oxygen atoms in its molecule, which decrease the formation of the 512 soot precursor and is favorable for the later oxidation of soot, and consequently reduces soot 513 emissions compared with gasoline. For D20, the combustion temperature is somewhat higher than 514 gasoline, which can further promote the oxidation of soot. Therefore, a significant reduction in

Energies 2019, 12, 1845 17 of 21

3.5.3. Comparison of Particle Emissions Figure 13 shows particulate emissions of E20, B20, and D20 in the NEDC cycle. In all the stages of the NEDC cycle, the PM and PN of the oxygenated blended fuels decrease significantly compared with gasoline, and the largest reduction is obtained with D20. It is generally believed that the oxygenated fuel contains oxygen atoms in its molecule, which decrease the formation of the soot precursor and is favorable for the later oxidation of soot, and consequently reduces soot emissions compared with gasoline. For D20, the combustion temperature is somewhat higher than gasoline, which can further promoteEnergies the 2019 oxidation, 12, x of soot. Therefore, a significant reduction in particulate emissions18 of 23 can be achieved when D20 is fueled to the vehicle engine. The oxygen content of E20 is higher than that of B20, 515which particulate is beneficial emissions to suppress can be the achieved formation when of D20 soot, is fueled resulting to the in lowervehicle emissions engine. The of oxygen particles content compared 516 of E20 is higher than that of B20, which is beneficial to suppress the formation of soot, resulting in with B20. Particle emissions of B20 are the highest among the three oxygenated blended fuels because 517 lower emissions of particles compared with B20. Particle emissions of B20 are the highest among of the high viscosity (poor mixing) and fairly higher latent heat of vaporization (low combustion 518 the three oxygenated blended fuels because of the high viscosity (poor mixing) and fairly higher 519temperature) latent heat compared of vaporization to D20. (low combustion temperature) compared to D20.

30 gasoline 25 ethanol n-butanol

DMF % % 9 3 - % 20 % 3 .2 9 .4 9 5 1 . 3 % - 1 % % 9 - 9 1 6 7 - 15 .6 8 % . .6 6 0 3 2 % 5 .4 - 2 4 - 6 - 6 0 4 PM (mg/km) . - 10 8 4 - 5 UDC EUDC NEDC

(a) 35 gasoline 30 ethanol 25 n-butanol % DMF 9 20 .6 2 % 5 % 6 - 4 15 4 3 % . . 8 1 9 % 8 % 7 6 . 8 - - 7 2 10 % 8 8 % 3 % . 5 . - 5 6 2 3 2 . . 6 9 7 - . 3 6 - 5 6 5 5 PN (10^11#/km) 7 - - - 0 UDC EUDC NEDC

(b)

520 FigureFigure 13. Comparison13. Comparison of of particulate particulate mattermatter (PM) and and particle particle number number (PN) (PN) emissions emissions of E20, of B20, E20, B20, 521 D20,D20, and and gasoline: gasoline: (a) (a) PM PM emissions; emissions; ( b(b)) PN PN emissions.emissions.

5223.6. Comparison3.6. Comparison of Acceleration of acceleration Performance performance 523 FigureFigure 14 shows14 shows the the acceleration acceleration time time of ofeach each gear gear of ethanol-gasoline, of ethanol-gasoline, n-butanol-gasoline, n-butanol-gasoline, and 524 DMF-gasoline with blend ratios of 20%, 50%, 75%, and 100%, respectively. Here, the acceleration and DMF-gasoline with blend ratios of 20%, 50%, 75%, and 100%, respectively. Here, the acceleration 525 time is employed to represent the dynamic performance of the vehicle. It can be seen that the time is employed to represent the dynamic performance of the vehicle. It can be seen that the blending 526 blending of the oxygenated fuels reduces the acceleration time at each gear. The high octane 527of thenumber oxygenated of the fuelsoxygenated reduces fuel the and acceleration the consequently time atstrong each anti-knock gear. The properties high octane contribute number to of the 528oxygenated improving fuel the and engine the consequently thermal efficiency strong anti-knockby advancing properties the ignition contribute and combustion to improving phasing, the engine 529 especially in the accelerated operating condition. At each gear, as the blending ratio of DMF 530 increases, the acceleration time decreases. This indicates that the engine exhibits a higher power 531 during the acceleration process. The reason is that although the lower heating value of DMF is 532 relatively low, the theoretical air–fuel ratio of DMF is small. This results in the fuel energy of D20 533 that is injected into the cylinder being higher than that of gasoline at the stoichiometric ratio 534 condition under the same intake air amount; therefore, the power output of D20 is increased. When

Energies 2019, 12, 1845 18 of 21

thermal eﬃciency by advancing the ignition and combustion phasing, especially in the accelerated operating condition. At each gear, as the blending ratio of DMF increases, the acceleration time decreases. This indicates that the engine exhibits a higher power during the acceleration process. The reason is that although the lower heating value of DMF is relatively low, the theoretical air–fuel ratio of DMF is small. This results in the fuel energy of D20 that is injected into the cylinder being higher than that of gasoline at the stoichiometric ratio condition under the same intake air amount; Energies 2019, 12, x 19 of 23 therefore, the power output of D20 is increased. When the blending ratios of n-butanol are 75% and 535100%, the the blending acceleration ratios time of n-butanol in each gear are position75% and is100%, smaller the thanacceleration that of gasoline.time in each It can gear be position seen from is the 536above smaller analysis than that that n-butanol of gasoline. has It acan signiﬁcant be seen from improvement the above analysis in combustion. that n-butanol Overall, has thea significant acceleration 537performance improvement of blended in combustion. ethanolis Overall, better thanthe accelera the othertion two performance blended oxygenatedof blended ethanol fuels except is better E100 in 538three than and fourthe other gears. two The blended reason oxygenated is the high fuels latent except heat E1 of00 vaporization in three and offour ethanol, gears. The which reason promotes is the the 539charge high cooling latent for heat an of increased vaporization power of density.ethanol, which Meanwhile, promotes at thethe samecharge amount cooling offor intake an increased air, ethanol 540 power density. Meanwhile, at the same amount of intake air, ethanol allows 11.5% more energy to allows 11.5% more energy to be injected than gasoline due to the lowest stoichiometric air-fuel (A/F) 541 be injected than gasoline due to the lowest stoichiometric air-fuel (A/F) ratio of ethanol, which leads ratio of ethanol, which leads to a shorter acceleration time. 542 to a shorter acceleration time.

5.0 gasoline ethanol 4.8 n-butanol DMF 4.6

4.4

4.2 acceleration time (s) 20% 50% 75% 100%

(a) 8.2 gasoline 8.0 ethanol N-butanol 7.8 DMF 7.6 7.4 7.2

acceleration time (s) 7.0 20% 50% 75% 100%

(b)

gasoline ethanol 13.2 n-butanol DMF 12.8 12.4 12.0 11.6 acceleration time (s) 20% 50% 75% 100%

(c)

543 FigureFigure 14. Acceleration14. Acceleration time time of of three,three, four, and and five ﬁve gears: gears: (a) Acceleration (a) Acceleration time of time three of gears; three (b) gears; 544 (b) AccelerationAcceleration time time ofof fourfour gears; (c) (c )Acceleration Acceleration time time of five of ﬁve gears. gears.

5454. Conclusions4. Conclusion 546 In thisIn paper, this paper, the e ﬀtheects effects of n-butanol-gasoline, of n-butanol-gasoline, DMF-gasoline, DMF-gasoline, and and ethanol-gasoline ethanol-gasoline on on the the energy 547 consumption,energy consumption, harmful emissions, harmful emissions, and acceleration and acceleration performance performance of a GDIof a vehicleGDI vehicle with with a chassis a 548 chassis dynamometer were investigated. The experimental study was carried out at a steady-state dynamometer were investigated. The experimental study was carried out at a steady-state cycle and an 549 cycle and an NEDC cycle with the oxygenated blending ratios of 20%, 50%, 75%, and 100% by 550NEDC volume. cycle with The results the oxygenated were compared blending with ratios pure ofgasoline. 20%, 50%, The main 75%, conclusions and 100%by are volume. summarized The resultsas 551were comparedfollows: with pure gasoline. The main conclusions are summarized as follows:

552 (1) At steady state, CO2 emissions and equivalent fuel consumption were reduced when n-butanol 553 was blended into gasoline. The emissions of CO, THC, NOX, and PN also decreased as a large 554 proportion of n-butanol was used. Blending DMF into gasoline has little effect on CO2 555 emissions. A large proportion of DMF resulted in the decreased THC and PN emissions and 556 increased NOX emissions. When a large proportion of ethanol was blended into gasoline, THC,

Energies 2019, 12, 1845 19 of 21

(1) At steady state, CO2 emissions and equivalent fuel consumption were reduced when n-butanol was blended into gasoline. The emissions of CO, THC, NOX, and PN also decreased as a large proportion of n-butanol was used. Blending DMF into gasoline has little effect on CO2 emissions. A large proportion of DMF resulted in the decreased THC and PN emissions and increased NOX emissions. When a large proportion of ethanol was blended into gasoline, THC, NOX, and PN emissions simultaneously decreased. Among the three oxygenated blended fuels, n-butanol–gasoline is the best for improving harmful emissions and CO2 emissions. (2) According to the results of the NEDC cycle, when blending 20% n-butanol into gasoline, the CO2 emissions, volumetric fuel consumption, and equivalent fuel consumption are reduced by 5.7%, 1.94%, and 5.434%, respectively, compared with gasoline. Meanwhile, CO, THC, and NOX emissions decreased. However, when blending 20% DMF into gasoline, the CO2 emissions and volumetric fuel consumption were higher than those of gasoline in the NEDC cycle, and CO, THC, and NOX emissions increased, while the equivalent fuel consumption was lower than that of gasoline. When blending 20% ethanol into gasoline, the CO2 emissions, volumetric fuel consumption, and equivalent fuel consumption were higher than that of gasoline. Ethanol–gasoline blends increased CO and THC emissions in all stages in the cycle, but NOX emissions decreased. Blending 20% of the three oxygenated fuels significantly reduced PN and PM emissions in the NEDC cycle. (3) The acceleration performance of three, four, and five gears is improved by blending oxygenated fuel into gasoline. As the DMF blending ratio increases, the acceleration time decreases gradually. Blending a large proportion of n-butanol also improved the acceleration performance. Overall, the acceleration performance of ethanol–gasoline was the best among the three kinds of blended fuels at five gears, although its acceleration time at three and four gears under 100% blending was slightly longer than that of the other two oxygenated fuels. (4) The impacts of the addition of ethanol, n-butanol, and DMF on the steady-state performance of the vehicle were positive. However, when the blending ratio was over 30%, the cold start performance of the vehicle will be challenging without changing the engine. So, the coupling of fuel and the engine needs to be further studied.

Author Contributions: All the authors have worked on this manuscript together, and all the authors have read and approved the final manuscript. Conceptualization, H.L., F.D., H.H. and Z.Z.; Data curation, X.L.; Formal analysis, X.W.; Investigation, D.Z. and Q.W.; Project administration, Y.Y. and Y.W.; Writing—original draft, X.W.; Writing—review & editing, Q.W., H.L. and Z.Z. Funding: This research was funded by [the National Natural Science Foundation of China (NSFC)] grant number [51576138] and by [the National Natural Science Foundation of China (NSFC)] grant number [91541205]. Conflicts of Interest: The authors declare no conflict of interest.

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