Quick viewing(Text Mode)

Assessment of Direct Injected Liquefied Petroleum Gas-Diesel Blends for Ultra-Low Soot Combustion Engine Application

Assessment of Direct Injected Liquefied Petroleum Gas-Diesel Blends for Ultra-Low Soot Combustion Engine Application

applied sciences

Article Assessment of Direct Injected Liquefied Gas-Diesel Blends for Ultra-Low Application

Roberto Ianniello 1,* , Gabriele Di Blasio 1 , Renato Marialto 1, Carlo Beatrice 1 and Massimo Cardone 2

1 CNR–Istituto Motori, Viale Marconi, 4, 80125 Naples, Italy; [email protected] (G.D.B.); [email protected] (R.M.); [email protected] (C.B.) 2 Department of Chemical, Materials and Production Engineering, University of Naples Federico II, via Claudio, 21, 80125 Naples, Italy; [email protected] * Correspondence: [email protected]

 Received: 19 June 2020; Accepted: 15 July 2020; Published: 18 July 2020 

Abstract: Technological and economic concerns correlated to fulfilling future emissions and CO2 standards require great research efforts to define an alternative solution for low emissions and highly efficient propulsion systems. Alternative fuel formulation could contribute to this aim. Liquefied petroleum gas (LPG) with lower content than other fossil fuels and which is easily vaporized at ambient conditions has the advantage of lowering CO2 emissions and optimizing the combustion process. Liquefied petroleum gas characteristics and availability makes the fuel a promising alternative for internal combustion . The possible combination of using it in high-efficiency compression ignition engines makes it worth analyzing the innovative method of using LPG as a blend component in diesel. Few relevant studies are detectable in literature in this regard. In this study, two blends containing diesel and LPG, in volume ratios 20/80 and 35/65, respectively, were formulated and utilized. Their effects on combustion and emissions performance were assessed by performing proper experimental tests on a modern light-duty single- engine test rig. Reference operating points at conventional engine calibration settings were examined. A specific recirculation (EGR) parametrization was performed evaluating the LPG blends’ potential in reducing the emissions at standard engine-out NOx levels. The results confirm excellent NOx-smoke trade-off improvements with smoke reductions up to 95% at similar NOx and efficiency. Unburnt emissions slightly increase, and to acceptable levels. Improvements, in terms of indicated specific fuel consumption (ISFC), are detected in the range of 1–3%, as well as the CO2 decrease proportionally to the mixing ratio.

Keywords: DI diesel-LPG blends; ultra-low emissions; outstanding NOx-soot trade-off; high efficiency; ; alternative fuels

Highlights:

Innovative solution to fuel a compression ignition (CI) diesel engine with low-sooting fuels; • Alternative fuels for diesel engines: new frontiers to exploit high-efficiency CI engines; • Liquefied petroleum gas (LPG)–diesel blends effective approach for ultra-low soot-NOx • engine application; Stable and efficient combustion for LPG–diesel blends in CI engines; • LPG–diesel blend storage modification for CI engine application. •

Appl. Sci. 2020, 10, 4949; doi:10.3390/app10144949 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4949 2 of 15

1. Introduction

The concentration of CO2 is increasing due to combustion, and one of the most significant contributors to worldwide CO2 emissions is the transport sector. Transportation is responsible for almost 30% of the total CO2 emissions in Europe, of which 72% are attributed to road transportation. As part of the effort to reduce CO2 emissions, the 2050 target is to reach 60% less compared to the 1990 level [1]. Within this context, the use of alternatives (e.g., liquefied petroleum gas (LPG), , and alcohols, etc.) to liquid fossil fuels, combined with new combustion concepts [2] and engine technology paths (turbocharging, right-sizing, electrification, etc.) is a feasible way to achieve better efficiency and meet emissions targets. Advanced combustion approaches in a compression ignition (CI) engine have the potential of higher efficiency and face the high NOx-soot emission dilemma, as one of the main drawbacks for diesel engines [3]. However, the use of alternative fuels will further limit the soot production during the combustion process by replacing, for example, a portion of the diesel with a less-sooty fuel like LPG [4,5]. LPG, as a by-product of crude oil refining processes, is composed mainly of propane (80%) and butane (20%). In particular, the propane is like . It can be liquefied under 760–1030 kPa at ambient temperature and mixed with diesel in any proportion. Theoretically, the high-octane number (correspondent to low ) makes the fuel suitable for operating in high CI engines. LPG fuel, in both phases, can be employed in a diesel engine. In dual-fuel combustion mode, the LPG is taken into the cylinder through a port fuel (PFI) into the manifold, and the air-LPG mixture induced into a . A small diesel quantity is injected for the ignition of the LPG-air mixture [6]. The LPG used in a diesel engine has been widely studied and ensures better engine performance, low particles, and smoke emissions according to the results in the literature [7]. The LPG-diesel is injected into the combustion chamber at high pressure, therefore rapidly passing to the gas phase due to the low boiling point. The quick LPG evaporation improves the atomization of the blend spray [8]. Increasing the LPG percentage will reduce the cetane number (CN), as well as the lower heating value (LHV) and the latent heat of evaporation, which causes an increase in the ignition delay [4]. However, the LPG high percentage could bring combustion noise and engine knock [9,10]. Cao et al. [9] investigated the use of LPG and diesel mixture in CI engines. The test campaign was performed using blends at various LPG ratios (10% and 30%). No variations were observed in terms of engine performance for the used fuels at a constant speed. The best results for the emissions have been obtained using the 30% LPG ratio with a penalty of (HC) emission. Ma et al. [10] studied the effect of the diesel-propane blend in the CI engine. At different propane percentages, heat release rate, combustion phase, peak firing pressure, and NOx emissions increased at the same engine speed-load, while the combustion duration, smoke, CO, and HC emissions were reduced. The experimental activity was dedicated to the evaluation of the LPG-diesel potential in terms of both emissions and thermal efficiency compared to the conventional diesel one. Two diesel-LPGs at different mass ratios, 20% and 35% in mass named “DLPG20” and “DLPG35”, respectively, were tested. Also, a hydraulic characterization was carried out in order to provide useful information for evaluating the LPG influence on injector performance. The hydraulic characterization tests were performed at the same injection strategies to have a direct comparison with the test on the single-cylinder engine (SCE). The activity was part of a research theme that aims to use the LPG–diesel mixture in a CI engine through the adaptation of the conventional injection system with few modifications. The main purpose of this study is to assess the applicability of the proposed concept and the potential in the real operating conditions. The present study, with its methodology and analysis, aims to cover the information and the scientific gap with what is present in the literature. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 15 Appl. Sci. 2020, 10, 4949 3 of 15 2. Materials and Methods

2.2.1. Materials Experimental and Setup Methods 2.1. ExperimentalThe experimental Setup test rig employed a prototype SCE integrating the main combustion system components of a modern CI ignition engine. The fuel system was refurbished to permit the use of The experimental test rig employed a prototype SCE integrating the main combustion system LPG/diesel blends. Some critical aspects were evidenced during the experimental test campaign, such components of a modern CI ignition engine. The fuel system was refurbished to permit the use of as the blend temperature increase during the engine run and the need to modify the fuel system to LPG/diesel blends. Some critical aspects were evidenced during the experimental test campaign, such ensure the liquid phase on supply and return fuel lines. It is worth underlining that the petroleum as the blend temperature increase during the engine run and the need to modify the fuel system to gas was liquid, which is LPG, at a pressure above the 5 bar pressure at ambient temperature. To this ensure the liquid phase on supply and return fuel lines. It is worth underlining that the petroleum gas aim, the injection return line was modified to ensure sealing at higher . For this reason, LPG was liquid, which is LPG, at a pressure above the 5 bar pressure at ambient temperature. To this aim, could not be directly mixed in the diesel fuel , and for the storage of the mixture, a proper the injection return line was modified to ensure sealing at higher pressures. For this reason, LPG could pressurized LPG- was used. The tank made of anti-corrosion metal material must guarantee not be directly mixed in the diesel fuel tank, and for the storage of the mixture, a proper pressurized the absence of fluid leaks, and must also withstand pressures of 20 bar, and above the nominal LPG-fuel tank was used. The tank made of anti-corrosion metal material must guarantee the absence working pressure of 8 bar, for safety reasons. In the LPG configuration, as illustrated in Figure 1, the of fluid leaks, and must also withstand pressures of 20 bar, and above the nominal working pressure of was not installed to prevent the fuel phase separation (liquid to gaseous) due to the 8 bar, for safety reasons. In the LPG configuration, as illustrated in Figure1, the fuel filter was not concentrated pressure losses. The fuel and system described needed to be filled every time installed to prevent the fuel phase separation (liquid to gaseous) due to the concentrated pressure before starting the test campaign. However, to have a more flexible system for real engine losses. The fuel tanks and system described needed to be filled every time before starting the test applications, further developments of the storage and fuel lines are under consideration. The injection campaign. However, to have a more flexible system for real engine applications, further developments system, fuelled by diesel-LPG, a closed pressure circuit (8 bar), was realized to avoid vaporization by of the storage and fuel lines are under consideration. The injection system, fuelled by diesel-LPG, reducing the risk of at the inlet of the high-pressure [4]. The fuelling system was a closed pressure circuit (8 bar), was realized to avoid vaporization by reducing the risk of cavitation at modified in a way it could exploit the original diesel injection system composed of a conventional the inlet of the high-pressure pump [4]. The fuelling system was modified in a way it could exploit injector with a 7-hole nozzle characterized by a 0.14 mm diameter and a cone the original diesel injection system composed of a conventional common rail solenoid injector with a angle of 148 degrees. 7-hole nozzle characterized by a 0.14 mm diameter and a cone angle of 148 degrees.

HIGH PRESSURE PUMP

PRESSURE REGULATOR

DIESEL SOLENOID INJECTOR FILTER

ELECTRIC PUMP

DIESEL TANK FUEL BLEND TANK Figure 1. Detailed scheme of the fuel fuel-injection-injection system [4].

The other engine components were properly designed,designed, manufactured, and assembled at the CNR-IstitutoCNR-Istituto MotoriMotori laboratories. A system capable of monitoring and controlling the engine control parameters,parameters, pressure, temperature signals, and and pollutant pollutant emissions emissions was was developed developed on on Labview. Labview. Combustion metrics (indicating) werewere measuredmeasured andand calculatedcalculated onon thethe base of a piezo quartz pressure sensor. TheThe averageaverage cycle cycle was was estimated estimated by by 128 128 consecutive consecutive cycles cycles at theat the sampling sampling rate rate of 0.1 of degree0.1 degree and usedand forused detailed for thermodynamicdetailed thermodynamic analysis. For analysis. a comprehensive For a thermodynamiccomprehensive characterization thermodynamic of Appl. Sci. 20202020,, 10,, x 4949 FOR PEER REVIEW 4 of 15 characterization of the engine system, the entire test rig was equipped with pressure and temperature sensors.the engine An system, exhaust the gas entire-analyzer test rig model was equipped AVL CEB with II and pressure AVL315s and temperature were used to sensors. measure An gaseous exhaust andgas-analyzer smoke emission model composition AVL CEB II andat the AVL315s exhaust, were respectively. used to A measure detailed gaseous scheme andof the smoke test cell emission layout iscomposition depicted in at Figure the exhaust, 2. respectively. A detailed scheme of the test cell layout is depicted in Figure2.

FUEL BLEND TANK LOW PRESSURE EGR COOLER EGR PUMP

HIGH RAIL PRESSURE PUMP BACKPRESSURE INTAKE VALVE HEATER INJECTOR

INTAKE EXHAUST VESSEL VESSEL HIGH LOW AMF AMF SCREW EXHAUST (THC,MHC, CO, AIR CO2,O2,NOx,PM) RESERVOIR FILTER PACK Figure 2. SingleSingle-cylinder-cylinder engine ( (SCE)SCE) test cell layout.

FFueluel consumption evaluation, as the LPG-dieselLPG-diesel blend system was designed, was not possible through aa conventional conventional gravimetric gravimetric balance. balance. Therefore, Therefore, to determine to determine the fuel the consumption, fuel consumption, an indirect an indirectestimate estimate of fuel consumption of fuel consumption was obtained was obtained by equating by equating the carbon the masscarbon in mass the exhaust in the exhaust to the carbon to the concentration of the fuel, estimated from the exhaust emissions (CO , CO, and HC). carbon concentration of the fuel, estimated from the exhaust emissions2 (CO2, CO, and HC). The detailed procedure for the indirect calculation of fuel consumption is is given given in in Appendix Appendix AA.. The applied method was preliminarily validated by comparing the fuel consumption of diesel, using the conventionalconventional open-circuit open-circuit fuelling fuelling system, system, measured measured by the by gravimetric the gravimetric balance andbalance the calculation and the calculationmethods. The methods. maximum The detected maximum error wasdetected around error 3%.The was employed around nozzle3%. The characteristics employed are nozzle listed characteristicsin Table1. are listed in Table 1. Table 1. SCE characteristics. Table 1. SCE characteristics. Parameters Units Specifications Parameters Units Specifications Specific displacementSpecific displacement l/cyll/cyl 0.5 0.5 mm 82 90.4 × × CompressionBore Ratio × Stroke -mm 82 × 90.4 16.5 perCompression cylinder Ratio -- 16.5 4 Nozzle holeValves number per cylinder -- 4 7 Nozzle coneNozzle opening hole angle number deg- 7 155 ActuatingNozzle cone type opening angle - deg 155 Solenoid Actuating type - Solenoid It is well known that the combustion process and its associated emissions are significantly affected by theIt injectionis well known process, that as well the ascombustion the fuel properties. process Toand better its associated identify their emissions effects on are the significantly combustion process,affected aby specific the injection injector process, test campaign as well hydraulic as the fuel characterization properties. To was better associated identify with their the effects engine on tests. the Thecombustion injection process, flow was a specific affected, injector among test other campaign factors, hyd mainlyraulic by characterization the , density, was associated and especially with the engine boiling tests. point. The Therefore, injection the flow injector, was affected, fuelled among with di otherfferent factors, fuels, wasmainly hydraulically by the viscosity, characterized density, andby a especially the boiling rate meter point. working Therefore, on the the Bosch injector, tube fuelled principle with [11 different]. The injected fuels, was fuel hydraulically amount was characterizedobtained by the by relativea fuel injection pressure rate signal meter [12 working] and was on proportionalthe Bosch tube to principle the injection [11]. rateThe injected through fuel the pipes’amount geometry was obtained and the by fuel the chemical-physicalrelative pressure signal properties [12] and [11]. was The proportional results were averagedto the injection over more rate throughthan 100 strokes,the pipes and’ geometry the standard and deviation the fuel was chemical around-physical 2%. The properties diesel quantity [11]. was The compared results were with averaged over more than 100 strokes, and the standard deviation was around 2%. The diesel quantity Appl. Sci. 20202020,, 10,, x 4949 FOR PEER REVIEW 5 of 15 was compared with those obtained at the discharge pipe by a precision balance while for the LPG blends,those obtained being the at theoutlet discharge at atmospheric pipe by a backpressure, precision balance and while in such for theconditions, LPG blends, the LPG being was the in outlet the gaseousat atmospheric state, and backpressure, it was not possible and in such to compare conditions, the theresults. LPG was in the gaseous state, and it was not possible to compare the results. 2.2. Test Methodology and Fuel Characteristics 2.2. Test Methodology and Fuel Characteristics A set of four-engine partial load points were chosen for the setup of the experimental campaign. TheyA were set ofinside four-engine the Worldwide partial load Harmonized points were Light chosen Vehicles for theTest setup Procedure of the experimental(WLTP) operating campaign. area, Theyand therefore were inside usable the Worldwide as an indicator Harmonized for efficiency Light Vehicles and emission Test Procedure performance (WLTP) at operating partial area,load conditions.and therefore In usablethis regard, as an indicatorthey were for performed, efficiency andapplying emission a reference performance engine at partialparameter load calibration conditions. (railIn this pressure, regard, intake they were pressure, performed, swirl, applyingexhaust gas a reference recirculation engine (EGR parameter), etc.) derived calibration from (rail a real pressure, four- cylinderintake pressure, engine swirl,with the exhaust same recirculation architecture (EGR), of etc.)the derivedSCE. The from key a-points real four-cylinder are schematically engine withreported thesame in Figure engine 3 architectureand named ofsynthetically the SCE. The as key-points engine speed are schematically per brake m reportedean effective in Figure pressure3 and named synthetically as engine speed per brake mean effective pressure (BMEP), e.g., 1500 5 indicates (BMEP), e.g., 1500 × 5 indicates 1500 rpm per 5 bar BMEP. For this work, four operating ×points were 1500selected, rpm two per speeds 5 bar BMEP. (1500 Forand this 2000 work, rpm), four and operating two loads points (2 and were 5 bar selected, of BMEP) two points speeds at (1500 constant and engine2000 rpm), out NOx and twolevel. loads To better (2 and explore 5 bar ofand BMEP) analyze points the LPG at constant blends potential, engine out a conventional NOx level. To pattern better characterizedexplore and analyze by a pilot the-main LPG blendsinjection potential, was employed. a conventional The start pattern of injection characterized (SOI) of bythe a pulses pilot-main and theinjection main wasenergizing employed. time The(ET) start were of adjusted injection to (SOI) achieve of the the pulses reference and thediesel main combustion energizing barycentre time (ET) were(MBF50) adjusted and load to achieve values; the also, reference the EGR diesel rate was combustion controlled barycentre to achieve (MBF50) the constant and load NOx values; target. also, All the additional EGR rate was injection controlled parameters to achieve (ET the pilot, constant dwell NOxtime, target. rail pressure, All the additionaletc.) were maintained injection parameters constant (ETand pilot,equal dwell to the time, reference rail pressure, values. etc.)As regards were maintained the hydraulic constant characterization, and equal to both the reference pilot and values. main Asinjection, regards the the mass hydraulic flow characterization,profiles for different both pilotfuels and were main performed injection, under the mass theflow same profiles injection for conditionsdifferent fuels tested were on performed the SCE. under the same injection conditions tested on the SCE.

Figure 3. EngineEngine operating operating points in the area of interest for this test campaign.

In this r regard,egard, the following fuels were consideredconsidered inin thisthis study:study:

• Reference EN590 European commercialcommercial diesel;diesel; • • Blend of 20%wt of LPG in diesel (DLPG20); • • Blend of 35%wt of LPG in diesel (DLPG35). • Table 2 reports the main chemical and physical characteristics of the reference diesel and LPG, Table2 reports the main chemical and physical characteristics of the reference diesel and LPG, and the roughly estimated ones, obtained by linear interpolation. and the roughly estimated ones, obtained by linear interpolation.

Appl. Sci. 2020, 10, 4949 6 of 15

Table 2. Fuels proprieties of diesel–liquefied petroleum gas (LPG) blends [4].

Fuel Name DIESEL DLPG20 DLPG35 LPG LPG content w/w [%] 0 20 35 100 Liquid density [kg/m3] 836 772 723 514 Mass LHV [MJ/kg] 42.5 43.2 43.8 46.1 Heat of evaporation [kJ/kg] 260 294 320 430 Stoichiometric /F [-] 14.67 15.4 15.6 16.7 Boiling point [ C] 362 - - 42 ◦ − Autoignition Temperature [◦C] 250 - - 365–470 H/C [-] 1.9 2.3 2.5 3.4 Kinematic viscosity [m2/s] 2.5 10 6 5.0 10 6 6.9 10 6 1.5 10 5 × − × − × − × −

For the sake of completeness, the EGR sweep results are presented in terms of the NOx-particulate matter (PM) trade-offs. The analysis was very effective and is a general approach employed for evaluating the potential of fuel formulations, technologies, etc. fuel–engine interaction performance and, in particular, for the critical engine control conditions in terms of NOx and PM.

3. Results and Discussion This section is divided into three parts. The first part analyses the effects of the factors on the global injector response to different fuels through a hydraulic characterization. Next, the tested points comparison is presented in terms of performance, combustion noise, and emissions. Finally, a trade-off emission analysis carried out through the EGR sweep is shown for all the operating points using different blends comparing to a diesel one.

3.1. Hydraulic Characterization In this section, the main results obtained in the study are analyzed. The focus is pointed to the description of the hydraulic results, showing the injection rate shape characteristics of each tested fuels. Figure4 shows the rate of injection (ROI) traces for all the fuels used in the test campaign, respectively. The injection rate, therefore, the fuel injection quantity, as discussed above, is obtained by the pressure signal, the pipes’ geometric characteristics, and the fuel chemical-physical characteristics, reported in Table2[ 4]. ROI profiles with two different energizing times, which correspond to a pilot and main injections for the 1500 2 test case at the injection pressures of 640 bar, are shown in Figure4 for × all tested fuels. Both tests were carried out in the engine-like backpressure conditions (pback), at 30, and 50 bar for the pilot and main injection, respectively. Regarding the injector performance, there is a hydraulic delay defined as the time between the electric command and the fuel injection; more details are available in the literature [13]. The different viscosity, as well as the compressibility of the blends (due to the presence of LPG) in comparison to diesel fuel, justifies the differences in the dynamic and injection timing. The hydraulic delay is reduced proportionally to the LPG percentage, in the range 0.35–0.4 ms, while for the conventional diesel is higher by around 15%. Similar considerations also for the closing phase, with greater durations in the range 0.1–0.12 ms for the blends, keeping constant the energizing time (ET). Therefore, different amounts of delivered fuel have been registered at the same ET; the values are inserted in Figure4c,d. The trends observed for this operating point can be extended to the other points tested and are not reported here for brevity. In general, the hydraulic characterization has confirmed the blends do not alter the injector performance except for the different opening and closing delays of the mixture, which can be solved through an appropriate calibration of the engine map. Appl. Sci. 2020, 10, 4949 7 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15

FigureFigure 4. 4.Mass Mass flow flow rate rate( (cc,d,d),), energizingenergizing current ( (a,ba,b),), and and fuel fuel amount amount (table) (table) for for all all fuels. fuels.

3.2. CombustionRegarding Analysis the injector performance, there is a hydraulic delay defined as the time between the electric command and the fuel injection; more details are available in the literature [13]. The different In this section, the assessment of the fuel blends’ performance on combustion characteristics and viscosity, as well as the compressibility of the blends (due to the presence of LPG) in comparison to emissionsdiesel fuel, is regarded justifies atthe 1500 differences rpm. For in each the DLPGdynamic blend, and andinjection each operatingtiming. The point, hydraulic the EGR delay and is the SOIreduced levels wereproportionally varied to to achieve the LPG the percent NOx andage, combustionin the range center0.35–0.4 targets. ms, while for the conventional dieselFigure is higher5 shows by the around in-cylinder 15%. Similar pressure, considerations rate of heat also release for (HRR),the closing and phase, the injection with greater pattern for operating 1500 2 and 1500 5 at constant MBF50 and NOx emissions. Advancement of the durations in the ×range 0.1–0.12× ms for the blends, keeping constant the energizing time (ET). injectionTherefore, pattern different is needed amounts for the of blendsdelivered to achievefuel have the been MBF50 registered reference at value,the same increasing ET; the thevalues premixed are combustioninserted in phase,Figure combustion4(c, d). The trends peaks observed and burning for this rates operating as illustrated point can in Figurebe extended5. At to higher the other loads, thepoints LPG tested effect isand even are morenot reported evident. here Indeed, for brevity. the lower In general, cetane the rating hydraulic of the characterization blends increases has the chemicalconfirmed delay the and blends confirms do not an alter overall the longer injecto ignitionr performance delay of except 3-4 degrees for the and different 5-8 degrees opening (Figure and 6), forclosing DLPG20 delays and of DLPG35, the mixture, respectively. which can be solved through an appropriate calibration of the engine map.Figure 5 provides a further representation of the combustion duration (MBF10-90) for diesel and LPG blends. Moreover, the effect of varying the engine load on the angular ignition delay can also be drawn.3.2. Combustion It increases Analysis with the load due to the cooling effect of LPG, which in turn lowers the in-cylinder chargeIn temperature this section, proportional the assessment tothe of the fuel fuel amount. blends The’ performance LPG cooling on combustion effect is detectable characteristics in the interval and 10emissions to 5 CA, is asregarded also demonstrated at 1500 rpm. For in Figureeach DLPG6, which blend, distinguishes and each operating a reduction point, ofthe the EGR in-cylinder and the − temperatureSOI levels were up to varied a maximum to achieve of 30 the and NOx 60 and◦C for combustion DLPG20 andcenter DLPG35, targets. respectively. In the case, the low-temperatureFigure 5 shows heat the release in-cylinder phase related pressure, to therate pilotof heat injection, release practically(HRR), and disappear,the injection being pattern involved for almostoperating entirely 1500 into × 2 and the 150 main0 × combustion5 at constant MBF50 phase. and As theNOx engine emissions load. Advancement increases, the of higher the inje ignitionction delaypattern and is then needed the premixedfor the blends combustion to achieve phase the reduceMBF50 thereference combustion value, durationincreasing with the premixed advantages incombustion terms of cycle phase, conversion combustion effi ciency,peaks and as shownburning in rates Figure as 6illustrated. The e ffi inciency Figure benefits 5. At higher are confirmed loads, withthe an LPG indicated effect is specific even more fuel consumptionevident. Indeed, (ISFC) the reduction,lower cetane as shownrating inof Figurethe blends6, of aboutincreases 1–2% the for chemical delay and confirms an overall longer ignition delay of 3-4 degrees and 5-8 degrees (Figure DLPG20 and 3% for DLPG35. Slight differences in terms of combustion are observed at 2 bar of load, 6), for DLPG20 and DLPG35, respectively. while at 5 bar, the prominent premixed phase, in comparison to only diesel case, causes an increase of the maximum pressure rise rate (MPRR) and noise combustion. For both blends, the combustion noise values are over the acceptable “comfort” limit of 90 dBA [10], and to reduce it, a more complex Appl. Sci. 2020, 10, 4949 8 of 15 multi-injection pattern [4], out of the scope of this study, is required. The covariance of indicated mean effective pressure (COVIMEP), as an indicator of the cycle-to-cycle combustion process stability, shows Appl.any significantSci. 2020, 10, x drift FOR inPEER comparison REVIEW to diesel. 8 of 15

(a) (b)

Figure 5. Pressure, rate of heatheat releaserelease (HRR),(HRR), injection injection energizing energizing pattern, pattern, and and combustion combustion duration duration at Appl. 1500atSci. 150 20200 2×, 10(2a ),( ax and) FOR and 1500 PEER 1500 REVIEW5× (5b ()b at) at constant constant engine-out engine-out NOx. NOx. 9 of 15 × × Figure 5 provides a further representation of the combustion duration (MBF10-90) for diesel and LPG blends. Moreover, the effect of varying the engine load on the angular ignition delay can also be drawn. It increases with the load due to the cooling effect of LPG, which in turn lowers the in-cylinder charge temperature proportional to the fuel amount. The LPG cooling effect is detectable in the interval −10 to 5 CA, as also demonstrated in Figure 6, which distinguishes a reduction of the in- cylinder temperature up to a maximum of 30 and 60 °C for DLPG20 and DLPG35, respectively. In the case, the low-temperature heat release phase related to the pilot injection, practically disappear, being involved almost entirely into the main combustion phase. As the engine load increases, the higher ignition delay and then the premixed combustion phase reduce the combustion duration with advantages in terms of cycle conversion efficiency, as shown in Figure 6. The efficiency benefits are confirmed with an indicated specific fuel consumption (ISFC) reduction, as shown in Figure 6, of about 1–2% for DLPG20 and 3% for DLPG35. Slight differences in terms of combustion are observed at 2 bar of load, while at 5 bar, the prominent premixed phase, in comparison to only diesel case, causes an increase of the maximum pressure rise rate (MPRR) and noise combustion. For both blends, the combustion noise values are over the acceptable “comfort” limit of 90 dBA [10], and to reduce it, a more complex multi-injection pattern [4], out of the scope of this study, is required. The covariance of indicated mean effective pressure (COVIMEP), as an indicator of the cycle-to-cycle combustion process stability, shows any significant drift in comparison to diesel.

FigureFigure 6.6. CovarianceCovariance ofof indicatedindicated meanmean eeffectiveffective pressure pressure (COV (COVIMEPIMEP), maximummaximum pressurepressure riserise raterate (MPPR),(MPPR), ignitionignition delay,delay, and and noise noise ( left(left)) and and eefficiencyfficiency performances performances ( right(right)) for for all all test test points. points.

FigureFigure6 6also also displays displays combustion combustion e efficienciesfficiencies ( η(ηcomb)comb). Despite Despite the consistent didifferencesfferences atat lowerlower loads,loads, atat higherhigher loads,loads, theythey are are not not significant, significant, and and in in the the range range of of variation variation of of 0.5%. 0.5%. The The lower lowerη combηcomb is a consequence of the higher CO and HC emissions explicable by the worsening of the air–fuel spray interaction inside the combustion chamber and linked to the presence of over-lean regions and fuel trapped into the crevices [14]. HC and CO emissions will be further discussed in the following paragraphs. Indeed, at higher loads, the higher adiabatic in-cylinder conditions temperatures convert more CO into CO2 while comprehensively oxidizing the fuel.

Figure 7. LPG cooling effect at 1500 × 5.

The , ηtherm, being a function of, among other parameters, the exhaust temperature shows a trend in which the blends have an advantage over diesel [15]. In particular, at 2 bar of BMEP, the ηtherm increase of about 0.6% and 2.1% for DLPG20 and DLPG35, respectively. The increase, as explained before, relates to the blends cooling effect on the in-cylinder charge temperature before the combustion, as illustrated in Figure 7, making the combustion more premixed [16]. The global efficiency (ηfuel) shows approximately the same trend as ηtherm. The relationships for the calculation of efficiencies are reported in Appendix B. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 15

Appl. Sci. 2020, 10, 4949 9 of 15 is a consequence of the higher CO and HC emissions explicable by the worsening of the air–fuel spray interaction inside the combustion chamber and linked to the presence of over-lean regions and fuel trapped into the crevices [14]. HC and CO emissions will be further discussed in the following paragraphs.Figure 6. Indeed, Covariance at higher of indicated loads, mean the higher effective adiabatic pressure in-cylinder (COVIMEP), conditions maximum temperaturespressure rise rate convert (MPPR), ignition delay, and noise (left) and efficiency performances (right) for all test points. more CO into CO2 while comprehensively oxidizing the fuel. The thermal efficiency, ηtherm, being a function of, among other parameters, the exhaust Figure 6 also displays combustion efficiencies (ηcomb). Despite the consistent differences at lower temperature shows a trend in which the blends have an advantage over diesel [15]. In particular, at 2 bar loads, at higher loads, they are not significant, and in the range of variation of 0.5%. The lower ηcomb of BMEP,the η increase of about 0.6% and 2.1% for DLPG20 and DLPG35, respectively. The increase, is a consequencetherm of the higher CO and HC emissions explicable by the worsening of the air–fuel spray as explained before, relates to the blends cooling effect on the in-cylinder charge temperature before interaction inside the combustion chamber and linked to the presence of over-lean regions and fuel the combustion, as illustrated in Figure7, making the combustion more premixed [ 16]. The global trapped into the crevices [14]. HC and CO emissions will be further discussed in the following efficiency (η ) shows approximately the same trend as η . The relationships for the calculation of paragraphs.fuel Indeed, at higher loads, the higher adiabatic intherm-cylinder conditions temperatures convert efficiencies are reported in AppendixB. more CO into CO2 while comprehensively oxidizing the fuel.

Figure 7 7.. LPG cooling effe effectct at 1500 × 5.5. ×

The followingthermal efficiency analysis discusses, ηtherm, being the LPG a function effects on of, the among overall performancesother parameters, in terms the of exhaust specific temperatureemissions at shows constant a trend engine-out in which NOx. the Forblends an easierhave an comparison advantage among over diesel the blends, [15]. In the particular, results are at 2reported bar of BMEP, in Figure the8 η astherm radar increase plots. of DLPG20 about 0.6% and and DLPG35 2.1% arefor comparedDLPG20 and with DLPG35, diesel at respectively. the same engine The increase,calibration as and explained the NOx engine-outbefore, relates emissions. to the blends cooling effect on the in-cylinder charge temperaFigureture8 beforedisplays the the combustion, specific emissions as illustrated for all in test Figure points 7, making and fuels. the Thecombustion blends show more a premixed relevant [increase16]. The ofglobal HC efficiency emission, (η andfuel) inshows particular approx usingimately DLPG35. the same At trend engine as η loadtherm. 2The bar, relationships they are about for thetwo calculation times higher of efficiencies than diesel, are while reported at the in highest Appendix load, B. DLPG20 has values as diesel, and only for DGPL35, the values are higher by about 50%. This is attributed to its ignition-related properties, evaporation behavior, and blends composition. The higher CO emissions related to the LPG mass ratio is observed for both tested points because of the longer ignition delay of LPG blends, which causes overmixing due to the evaporation characteristics combined with the relatively high engine swirl number [7]. The smoke emissions reduction is significant compared to diesel and in the range 94–99%. The reduced in-cylinder fuel-rich zones or any exposure to high-temperature combustion products are a precondition for the reduction of soot. Due to the high volatility of LPG, which in the vapor phase reaches high specific volumes, it probably inhibits the formation of fuel-rich areas, soot precursor. Therefore, based on the results obtained, it can be observed that the spray formation using a diesel-LPG mixing is sufficiently inclined to resist the formation of smoke [7]. Indeed, a higher /carbon ratio characterizes the tested LPG blends compared to the diesel one (Table2), with benefits on CO 2 emissions that combined with the higher thermal efficiencies lead to higher efficiencies, proportional to the LPG amount, and up to 10%, for the blends. The EGR rates do not vary regardless of the fuel used for all tested points in the range 35–40%. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 15

The following analysis discusses the LPG effects on the overall performances in terms of specific emissions at constant engine-out NOx. For an easier comparison among the blends, the results are reported in Figure 8 as radar plots. DLPG20 and DLPG35 are compared with diesel at the same engine Appl. Sci. 2020, 10, 4949 10 of 15 calibration and the NOx engine-out emissions.

Figure 8. SpecificSpecific emissions comparison among all fuels and operating points.

3.3. Trade-OFigure ff8s displays by Exhaust the Gas spe Recirculationcific emissions (EGR) for all Sweep test points and fuels. The blends show a relevant increaseThe of eff ectHC of emission, diesel-LPG and blends in particular on the PM using reduction DLPG35. effectiveness At engine was load carried 2 bar, outthey employing are aboutEGR two sweepstimes higher at diff erentthan enginediesel, loadswhile andat the speeds. highest The load, PM emissionsDLPG20 ofhas all values key-points as diesel, plotted an againstd only thefor NOxDGPL35, values the are values shown are in higher Figure by9. Theabout engine 50%. operatingThis is attributed conditions to (intakeits ignition pressure,-related rail properties, pressure, exhaustevaporation backpressure, behavior, intake and blends temperature) composition. were the The same higher during CO the emissions tests. Dashed related lines, to the representative LPG mass ofratio the is post-Euro6 observed for NOx both target, tested are point displayeds because and of usedthe longer as a reference ignition delay for the of specific LPG blends, comparison. which Ascauses shown overmixing in Figure 9due, the to trade-o the evaporationff based on characteristics diesel, showing combined high levels with of PMthe andrelatively NOx forhigh all engine tested points,swirl number is drastically [7]. The improved, smoke emissions employing reduction the LPG is blend significant fuel, in compared the range 90–95%to diesel in and PM, in depending the range on94– the99%. load. The Byreduced adopting in-cylinder the highest fuel EGR-rich ratezones in or the any case exposure of blends, to characterizedhigh-temperature by lower combustion cetane numbersproducts andare a lower precondition evaporation for the temperatures, reduction of both soot. NOx Due and to PMthe werehigh volatility reduced simultaneously of LPG, which belowin the referencevapor phase targets. reaches high specific volumes, it probably inhibits the formation of fuel-rich areas, soot precursor.In particular, Therefore, significant based on benefits the results are obtained, shown at it a can higher be observed load, of that about the 95% spray in formation terms of PMusing is a diesel-LPG mixing is sufficiently inclined to resist the formation of smoke [7]. Indeed, a higher observed. This result is significant considering that, these points are particularly critical for PM engine hydrogen/carbon ratio characterizes the tested LPG blends compared to the diesel one (Table 2), with control, and is one of the operating points with the greatest contributing factor in applying an emission benefits on CO2 estimation procedure emissions of the test that emission combined cycle with [17,18 the]. Therefore, higher thermal the PM emissionefficiencies index lead improvement to higher inefficiencie these pointss, proportional appear significantly to the LPG favorable amount, forand the up engine to 10%, performance for the blends. over The the EGR WLTP rates procedure do not vary regardless of the fuel used for all tested points in the range 35–40%. when the LPG blend is used. In deeper detail, Figure 10 shows the detailed trend in the post-Euro6 region, as well as the zoom of the area highlighted in the previous figure. The PM benefit by adopting the highest EGR ratio is explored, and corresponds to the last trade-off point of Figure9. The test points of 1500 5 and × 2000 2 are reported. Starting from the diesel reference, adopting the DPLG20 a PM reduction of 70% × Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 Appl. Sci. 2020, 10, 4949 11 of 15 3.3. Trade-Offs by Exhaust Gas Recirculation (EGR) Sweep in 1500The5 effect and 93% of diesel for 2000-LPG blends2 is observed on the PM at the reduction same engine effectiveness calibration was setup.carried Byout increasing employing the × × LPGEGR percentage sweeps at updifferent to 35%, engine a further loads reduction and speeds. of 93%The PM and emissions 50% is obtained of all key for-points 1500 plotted5 and against 2000 2, × × respectively.the NOx values In general, are shown soot-less in Figure levels can9. The be achievedengine operating without deteriorationconditions (intake of thermal pressure, effi ciency,rail indicatedpressure, in exhaust the box backpressure, (Figure 10) for intake each testedtemperature) point, thanks were the to thesame promotion during the of tests. the premixing Dashed lines, thanks torepresentative the LPG blends. of the The post eff-ectivenessEuro6 NOx shown target, allowsare displayed reaching and very used low as a PM reference emissions for the in thespecific whole operatingcomparison. range. As shown The LPG in Figure blend 9 guarantees, the trade-off significant based on benefitsdiesel, showing even in high the l engineevels of operatingPM and NOx areas for all tested points, is drastically improved, employing the LPG blend fuel, in the range 90–95% in where the NOx values are very low, and soot control particularly sensitive. It is proper to specify PM, depending on the load. By adopting the highest EGR rate in the case of blends, characterized by the PM results obtained by using the LPG blends are close to the limit-sensitive thresholding of the lower cetane numbers and lower evaporation temperatures, both NOx and PM were reduced instrument used. For this reason, future characterization with appropriate tools will be performed, simultaneously below reference targets. such as a fast particle analyzer or micro-soot sensor.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15 indicated in the box (Figure 10) for each tested point, thanks to the promotion of the premixing thanks to the LPG blends. The effectiveness shown allows reaching very low PM emissions in the whole operating range. The LPG blend guarantees significant benefits even in the engine operating areas where the NOx values are very low, and soot control particularly sensitive. It is proper to specify the PM results obtained by using the LPG blends are close to the limit-sensitive thresholding of the instrumentFigureFigure 9.used. 9.Specific Specific For trade-o this trade reason,-ffoffss NOx-particulate NOx future-particulate characterization matter matter (PM) (PM) by by with exhaust exhaust appropriate gas gas recirculation recirculation tools (EGR) will(EGR be)sweep sweep performed, for such allasfor operatinga fallast operating particle points pointsanalyzer and fuels.and fuels.or The m icroThe dashed -dashedsoot lines sensor. lines are are indicative indicative of of post-Euro6 post-Euro6 levels. levels.

In particular, significant benefits are shown at a higher load, of about 95% in terms of PM is observed. This result is significant considering that, these points are particularly critical for PM engine control, and is one of the operating points with the greatest contributing factor in applying an emission estimation procedure of the test emission cycle [17,18]. Therefore, the PM emission index improvement in these points appear significantly favorable for the engine performance over the WLTP procedure when the LPG blend is used. In deeper detail, Figure 10 shows the detailed trend in the post-Euro6 region, as well as the zoom of the area highlighted in the previous figure. The PM benefit by adopting the highest EGR ratio is explored, and corresponds to the last trade-off point of Figure 9. The test points of 1500 × 5 and 2000 × 2 are reported. Starting from the diesel reference, adopting the DPLG20 a PM reduction of 70% in 1500 × 5 and 93% for 2000 × 2 is observed at the same engine calibration setup. By increasing the LPG percentage up to 35%, a further reduction of 93% and 50% is obtained for 1500 × 5 and 2000 × 2, respectively.Figure In 10.general, NOxNOx-PM- PMsoot by-less varying levels the can fuel be atachieved the highest without EGR ratedeterioration at 1500 × 5 ofand thermal 2000 × 2.efficiency,2. × × 4. Conclusions A comprehensive engine test investigation using LPG–diesel blends was carried out. The approach and the tests were designed and aimed at demonstrating the feasibility and the potential of the application of a new concept using LPG–diesel blends with any injection system modifications. • The first part of the activity was devoted to analyzing the effects of using LPG diesel blends with a conventional common rail diesel injector. The hydraulic characterization assessed the effects of the different compressibility and viscosity of the blends, causing an increase in the injection duration, estimated to be approximately 0.1 ms for both tests carried out. On the other hand, a reduction of the hydraulic delay of 0.35–0.4 ms, and proportionally to the LPG content in the blend, was noted. Therefore, fuel-injected values for almost double the diesel amounts are observed, at the same ET, by using the LPG blend. In general, the blends do not alter the injector performance except for the different opening and closing delays, which can be solved through an appropriate calibration of the engine map. • Concerning the engine test campaign, the results confirmed the need for optimizing the conventional injection strategy (pilot-main) to be appropriate for the DLPG application. Indeed, the lower CN of the DLPG seems critical for HC emissions and combustion efficiency. Optimization should be oriented to improve the charge reactivity during the low-temperature heat release phase and shape the premixed combustion phase for combustion noise control. • It is worth highlighting that LPG blends are very effective in PM emission suppression that is about 90–95% at very low NOx targets. Acceptable penalties on HC and CO are detected. Based on the presented results, DLPG20 is the best fuel compromise between engine performance and emissions at a constant engine setting. • In general, soot-less levels can be achieved without deterioration of the thermal efficiency and test-to-test repeatability (COVIMEP), thanks to the promotion of the premixing thanks to the LPG blends. Future activities will be addressed to develop the fuelling system, with reference to the design and realization of a proper fuel mixing system able to vary in real-time the fuel blending ratio. Then, further activities will be oriented to optimize the engine calibration setting in the whole engine operating area to evaluate the full potential of applying this new concept.

Appl. Sci. 2020, 10, 4949 12 of 15

4. Conclusions A comprehensive engine test investigation using LPG–diesel blends was carried out. The approach and the tests were designed and aimed at demonstrating the feasibility and the potential of the application of a new concept using LPG–diesel blends with any injection system modifications.

The first part of the activity was devoted to analyzing the effects of using LPG diesel blends with a • conventional common rail diesel injector. The hydraulic characterization assessed the effects of the different compressibility and viscosity of the blends, causing an increase in the injection duration, estimated to be approximately 0.1 ms for both tests carried out. On the other hand, a reduction of the hydraulic delay of 0.35–0.4 ms, and proportionally to the LPG content in the blend, was noted. Therefore, fuel-injected values for almost double the diesel amounts are observed, at the same ET, by using the LPG blend. In general, the blends do not alter the injector performance except for the different opening and closing delays, which can be solved through an appropriate calibration of the engine map. Concerning the engine test campaign, the results confirmed the need for optimizing the • conventional injection strategy (pilot-main) to be appropriate for the DLPG application. Indeed, the lower CN of the DLPG seems critical for HC emissions and combustion efficiency. Optimization should be oriented to improve the charge reactivity during the low-temperature heat release phase and shape the premixed combustion phase for combustion noise control. It is worth highlighting that LPG blends are very effective in PM emission suppression that is • about 90–95% at very low NOx targets. Acceptable penalties on HC and CO are detected. Based on the presented results, DLPG20 is the best fuel compromise between engine performance and emissions at a constant engine setting. In general, soot-less levels can be achieved without deterioration of the thermal efficiency and • test-to-test repeatability (COVIMEP), thanks to the promotion of the premixing thanks to the LPG blends.

Future activities will be addressed to develop the fuelling system, with reference to the design and realization of a proper fuel mixing system able to vary in real-time the fuel blending ratio. Then, further activities will be oriented to optimize the engine calibration setting in the whole engine operating area to evaluate the full potential of applying this new concept.

Author Contributions: Data curation, R.I.; investigation, R.I. and G.D.B.; methodology, M.C.; supervision, C.B.; writing—original draft, R.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge C. Rossi, B. Sgammato and A. Schiavone for their technical support. Conflicts of Interest: The authors declare no conflict of interest.

Definitions/Abbreviations

BMEP Brake Mean Effective Pressure CA Angle Degrees CI Compression Ignition CO CO2 cyl cylinder EC Energizing Current EGR Exhaust Gas Recirculation ET Energizing Time deg degree Appl. Sci. 2020, 10, 4949 13 of 15

HC Hydrocarbons HRR Heat Release Rate IMEP Indicated Mean Effective Pressure IMEPCOV Covariance of IMEP isX Indicated Specific Emissions ISFC Indicated Specific Fuel Consumption LHV Lower Heating Value LPG Liquefied Petroleum Gas MBF Mass Burning Fraction MBF10-90 Combustion Duration MBF50 50% Mass Burning Fraction MPRR Maximum Pressure Rise Rate NOx Oxide pback Backpressure PFI Port Fuel Injection PM Particulate Matter prail Rail Pressure Pcyl In-Cylinder Pressure Q Injected amount ROI Rate of Injection SCE Single-Cylinder Engine SOI Start of Injection TDC Top Dead Centre WLTP Worldwide Harmonized Light Vehicles Test Procedure ηcomb Combustion efficiency ηfuel Global efficiency ηthermal Thermal efficiency

Appendix A The fuel consumptions for both tested fuels are calculated by the carbon balance method using measured emissions of carbon dioxide (CO2) and other carbon-related emissions (HC, CO). The equation is shown below:

2nO2 ∅ =    (A1) npexH O + np 1 exH O ex∗ + 2ex∗ + 2ex∗ + ex∗ + 2ex∗ r 2 − 2 CO CO2 O2 NO NO2 − where the dry and wet mole fraction are linked by:   exi = 1 exH O ex∗ (A2) − 2 i and n np =    (A3) x + x x + x eCH b 1 eH2O eCO∗ eCO∗ a − 2

x∗ + x∗ m eCO eCO2 exH2O =   (A4) 2n     exCO∗ m  1 +   + ex∗ + ex∗   2n CO CO2   Kex∗  CO2 x x eH2O eCO∗ exH2 = (A5) Kex∗ CO2   x∗ + x∗ 3.773 nO2   eNO eNO2 exN2 = 1 exH2O (A6) ∅ np − − 2 i x x x For species , the wet molar fraction is indicated as ei, while ei∗, the dry molar fraction; eCHb/a is the wet molar fraction of HC concentration. K varies in the range 1.5–5.5 for lean mixtures, while for the stoichiometric one from 2.5–4.5. It is possible to calculate the fuel amount based on the experimental airflow data. Appl. Sci. 2020, 10, 4949 14 of 15

Appendix B

The relationship for the calculation of the global (ηfuel) and combustion (ηcomb) efficiencies are:

mmix LHVmix ηcomb = 1 · (A7) − Qlost(HC,CO)

1 η = (A8) f uel ISFC LHV · mix where: " # kJ LHV = x LHV + (1 x) LHV (A9) mix · LPG − · D kg m  g  ISFC = mix (A10) Pind kWh The percentage of the LPG in the blend has been quantified on mass basis (x) in according to the following equation: m x = LPG (A11) mLPG + md

References

1. CO2 Emissions from : Facts and Figures. Available online: https://www.europarl.europa.eu/pdfs/news/ expert/2019/3/story/20190313STO31218/20190313STO31218_en.pdf (accessed on 18 April 2019). 2. Martin, J.; Boehman, A.; Topkar, R.; Chopra, S.; Subramaniam, U.; Chen, H. Intermediate combustion modes between conventional diesel and RCCI. SAE Int. J. Engines 2018, 11, 835–860. [CrossRef] 3. Di Blasio, G.; Belgiorno, G.; Beatrice, C. Effects on performances, emissions and particle size distributions of a dual fuel (methane-diesel) light-duty engine varying the compression ratio. Appl. Energy 2017, 204, 726–740. [CrossRef] 4. Marialto, R.; Sequino, L.; Di Blasio, G.; Cardone, M.; Beatrice, C.; Ianniello, R.; Fontana, G. Combustion and Emission Characteristics of a Diesel Engine Fuelled with Diesel-LPG Blends. SAE Tech. Pap. 2019.[CrossRef] 5. Cardone, M.; Mancaruso, E.; Marialto, R.; Sequino, L.; Vaglieco, B.M. Characterization of Combustion and Emissions of a Propane-Diesel Blend in a Research Diesel Engine. SAE Tech. Pap. 2016.[CrossRef] 6. Aydin, M.; Irgin, A.; Çelik, M.B. The impact of diesel/LPG dual fuel on performance and emissions in a single cylinder . Appl. Sci. 2018, 8, 825. [CrossRef] 7. Alam, M.; Goto, S.; Sugiyama, K.; Kajiwara, M.; Mori, M.; Konno, M.; Motohashi, M.; Oyama, K. Performance and Emissions of a DI Diesel Engine Operated with LPG and Ignition Improving Additives. SAE Tech. Pap. 2001.[CrossRef] 8. Mancaruso, E.; Marialto, R.; Sequino, L.; Vaglieco, B.M.; Cardone, M. Investigation of the Injection Process in a Research CR Diesel Engine using Different Blends of Propane-Diesel Fuel. SAE Tech. Pap. 2015.[CrossRef] 9. Cao, J.; Bian, Y.Z.; Qi, D.H.; Cheng, Q.; Wu, T. Comparative investigation of diesel and mixed liquefied petroleum gas/diesel injection engines. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2004, 218, 557–565. [CrossRef] 10. Ma, Z.; Huang, Z.; Li, C.; Wang, X.; Miao, H. Effects of fuel injection timing on combustion and emission characteristics of a diesel engine fueled with diesel- propane blends. Energy Fuels 2007, 21, 1504–1510. [CrossRef] 11. Bosch, W. The Fuel Rate Indicator: A New Measuring Instrument for Display of the Characteristics of Individual Injection. SAE Tech. Pap. 1966, 660749. [CrossRef] 12. Montanaro, A.; Allocca, L.; Beatrice, C.; Ianniello, R. Outwardly Opening Hollow-Cone Diesel Spray Characterization under Different Ambient Conditions. SAE Tech. Pap. 2018.[CrossRef] 13. Litzinger, T.; Stoner, M.; Hess, H.; Boehman, A. Effects of oxygenated blending compounds on emissions from a turbocharged direct injection diesel engine. Int. J. Engine Res. 2000, 1, 57–70. [CrossRef] 14. Rakopoulos, D.C.; Rakopoulos, C.D.; Kakaras, E.C.; Giakoumis, E.G. Effects of –diesel fuel blends on the performance and exhaust emissions of heavy duty DI diesel engine. Energy Convers. Manag. 2008, 49, 3155–3162. [CrossRef] Appl. Sci. 2020, 10, 4949 15 of 15

15. Sanli, A.; Ozsezen, A.N.; Kilicaslan, I.; Canakci, M. The influence of engine speed and load on the heat transfer between gases and in-cylinder walls at fired and motored conditions of an IDI diesel engine. Appl. Therm. Eng. 2008, 28, 1395–1404. [CrossRef] 16. Al-Hasan, M. Effect of ethanol–unleaded blends on engine performance and exhaust emission. Energy Convers. Manag. 2003, 44, 1547–1561. [CrossRef] 17. Di Blasio, G.; Belgiorno, G.; Beatrice, C. Parametric Analysis of Compression Ratio Variation Effects on Thermodynamic, Gaseous Pollutant and Particle Emissions of a Dual-Fuel CH4-Diesel Light Duty Engine. SAE Tech. Pap. 2017.[CrossRef] 18. Di Blasio, G.; Bonura, G.; Frusteri, F.; Beatrice, C.; Cannilla, C.; Viscardi, M. Experimental Characterization of Diesel Combustion Using Glycerol Derived Mixtures. SAE Int. J. Fuels Lubr. 2013, 6, 940–950. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Top View