Combustion Thermodynamics of Ethanol, N-Heptane, and N-Butanol in a Rapid Compression Machine with a Dual Direct Injection (DDI) Supply System

energies

Article Combustion Thermodynamics of Ethanol, n-Heptane, and n-Butanol in a Rapid Compression Machine with a Dual Direct Injection (DDI) Supply System

Ireneusz Pielecha 1,* , Sławomir Wierzbicki 2,* , Maciej Sidorowicz 3 and Dariusz Pietras 4

1 Department of Combustion Engines and Transport, Faculty of Civil and Transport Engineering, Poznan University of Technology, pl. M. Sklodowskiej-Curie 5, 60-965 Poznan, Poland 2 Department of Mechatronics, Faculty of Technical Sciences, University of Warmia and Mazury in Olsztyn, ul. Oczapowskiego 11, 10-710 Olsztyn, Poland 3 AC S.A., ul. 42 Pulku Piechoty 50, 15-181 Białystok, Poland; [email protected] 4 Department of Combustion Engines and Vehicles, Faculty of Mechanical Engineering and Computer Science, University of Bielsko-Biala, ul. Willowa 2, 43-309 Bielsko-Biała, Poland; [email protected] * Correspondence: [email protected] (I.P.); [email protected] (S.W.)

Abstract: The development of internal combustion engines involves various new solutions, one of which is the use of dual-fuel systems. The diversity of technological solutions being developed determines the efficiency of such systems, as well as the possibility of reducing the emission of carbon dioxide and exhaust components into the atmosphere. An innovative double direct injection system was used as a method for forming a mixture in the combustion chamber. The tests were carried out with the use of gasoline, ethanol, n-heptane, and n-butanol during combustion in a model test engine—the rapid compression machine (RCM). The analyzed combustion process indicators included the cylinder pressure, pressure increase rate, heat release rate, and heat release value. Citation: Pielecha, I.; Wierzbicki, S.; Optical tests of the combustion process made it possible to analyze the flame development in the Sidorowicz, M.; Pietras, D. Combustion Thermodynamics of observed area of the combustion chamber. The conducted research and analyses resulted in the Ethanol, n-Heptane, and n-Butanol in observation that it is possible to control the excess air ratio in the direct vicinity of the spark plug a Rapid Compression Machine with a just before ignition. Such possibilities occur as a result of the properties of the injected fuels, which Dual Direct Injection (DDI) Supply include different amounts of air required for their stoichiometric combustion. The studies of the System. Energies 2021, 14, 2729. combustion process have shown that the combustible mixtures consisting of gasoline with another https://doi.org/10.3390/en14092729 fuel are characterized by greater combustion efficiency than the mixtures composed of only a single fuel type, and that the influence of the type of fuel used is significant for the combustion process and Academic Editor: Jamie W.G. Turner its indicator values.

Received: 20 April 2021 Keywords: combustion engines; dual direct injection; alternative fuels; combustion; heat release Accepted: 6 May 2021 Published: 10 May 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional afﬁl- The contemporary development of propulsion systems concerns mostly the search iations. for new technologies in the ﬁeld of vehicle propulsion sources. Over the hundred years of its history, motorization has always faced challenges related to the user demands and the ongoing economic situation. Internal combustion engines powered by hydrocarbon fuels obtained from fossil sources have become the basic power units in the global automotive industry. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Lively discussions are regularly taking place at different levels regarding the environ- This article is an open access article mental impacts of transport. This aspect cannot be overlooked, nor can its importance; it distributed under the terms and is central various sides of the discussion. The social awareness of this problem continues conditions of the Creative Commons to grow, impacting how the future of transport is being shaped today. According to the Attribution (CC BY) license (https:// European Environment Agency, transport in 2017 was responsible for 27% of all green- creativecommons.org/licenses/by/ house gas emissions in Europe—2.2% more than in the year prior [1]. Other reasons for the 4.0/). development of modern propulsion sources include striving to reduce energy consumption

Energies 2021, 14, 2729. https://doi.org/10.3390/en14092729 https://www.mdpi.com/journal/energies Energies 2021, 14, 2729 2 of 20

for vehicle propulsion, as well as the increasingly stringent exhaust emission norms and regulations [2]. Dual indirect injection (PFI-PFI) systems are generally well known; however, they are mainly used to deliver a single fuel type simultaneously (mainly in motorcycle engines) or in the classic configuration of an engine powered by unleaded gasoline with an additional LPG fueling system. There are many fuel injection solutions implemented in the PFI-DI system. The most common are diesel (DI) and natural gas (PFI) injection systems [3–7]. By comparing the observed impacts of using diesel oil (DI) and natural gas (PFI), the overall efficiency of the engine was found to have increased by 5–8% [3]. In [4], it was found that the gas fuel does not combust completely in dual-fuel engines. This is mainly due to the fact that the combustion reaction does not involve the entire dose of this fuel. The best combustion conditions were obtained with an excess air coefficient λ of about 2. In the studies on exhaust emissions [5,6], it was found that the number of solid particles in the average diameter range was approximately 2 times smaller in the dual-fuel supply system than in the case of diesel fuel alone. Such results were obtained regardless of the engine load value. Work carried out by You et al. [7] confirmed that the fuel mixture quality has a significant impact on the combustion quality and the emissions of exhaust components (including the timing of the gas injection in relation to the crankshaft angle and the injection direction in relation to the inlet duct). Multiparameter research results on methane and diesel fuel combustion were analyzed in [8]. In that paper, the effects of the premixed ratio, injection pressure, and fuel doses in relation to the engine efficiency were assessed. The experimental results were compared with the simulation results. Additionally, the exhaust emission assessment was presented in relation to the energy distribution in the engine, taking into account the range of losses in the combustion and exhaust. It was found that with appropriate system settings (especially in terms of injection timing settings), it was possible to obtain a 2–3.5% increase in the indicated efficiency of the engine. A study of dual-fuel systems for the combustion of diesel oil and ethanol was conducted by Beatrice et al. [9]. The obtained results indicated a significant reduction in the exhaust emissions of PM and CO2 from a dual-fuel engine compared to a regular engine running on diesel fuel (despite the acceptable higher HC and CO emission values). A number of dual-fuel solutions also exist for spark ignition engines, including gasoline (DI) and ethanol (PFI) [10], or in reverse configuration [11], gasoline (PFI) and diesel oil (DI) [12], gasoline (DI) and LPG (PFI) [13], gasoline (DI or PFI) and hydrogen (PFI or DI) [2], and methanol (DI) and hydrogen (PFI) [14]. Work has also been carried out using other fuels with the above-mentioned injection systems for these fuels [15,16]. The existing solutions for the injection of gasoline and ethanol mainly involve mixtures of E85 (85% ethanol and 15% gasoline) and E15 (85% gasoline and 15% ethanol), as used by Benajes et al. [17]. Information from research on dual-fuel direct injection systems for gasoline and ethanol is also available in the literature, such as in the research conducted by Kang et al. [18]; these analyses, however, include fuel supply with a stoichiometric mixture. Studies have shown that increasing the proportion of ethanol reduces CoV(IMEP). In the absence of ethanol, this ratio was approximately 4%. Increasing the proportion of ethanol to 10% resulted in a 50% reduction in CoV(IMEP). A further increase in the share of ethanol allowed for this indicator to be reduced down to the level of 1–1.5% (with the permissible limit of 1.5%). Dual-fuel direct injection systems currently involve a number of combinations, among which are the injection of methanol and diesel fuel [19], as well as n-butanol in combination with biodiesel [20]. In studies conducted by Dong et al. [19], analyses of methanol injection (with the injector located centrally in the cylinder) and diesel fuel (in a non-axial position) were performed. Changes in the excess air coefficient were not analyzed, however. The research investigated the notion of replacing diesel fuel with methanol in the dose range of 45% to 95%. Energies 2021, 14, 2729 3 of 20

In the work by Ning et al. [21] from 2020, there were 10 additional publications on direct injection of two fuels: diesel oil and methanol, indicating a high potential of these fuels for use in dual-fuel systems with direct injection. Despite considerable design challenges, Long et al. [22] used a dual direct injection system in a compression–ignition engine. This solution uses a premixed dose system prepared by the main preinjection blended fuels of diesel and ethanol–gasoline—the system was called the jet controlled compression ignition (JCCI). The typical compression chamber of the diesel engine was replaced with a flat chamber to fit the premixed combustion mode. The fuel used was a mixture of gasoline and ethanol with diesel fuel constituting 15% (D15) or 30% (D30) share by mass. The injection system was equipped with two independently driven fuel injection pumps. The fuel injection pressure in the pre and main injection systems was maintained at 60 MPa (in the case of an engine load rising above 50%, the fuel injection pressure was increased to its maximum value of 90 MPa). Diesel oil was supplied by the central injector (the fuel injection direction corresponded to the piston movement axis). Mixture D15 or D30 was injected into the chamber with an injector placed at a 10 degree angle. The experimental results and analysis demonstrated that the fuel JCCI mode with the dual direct injection strategy could effectively and robustly control the combustion and emissions in premixed combustion mode. Similar studies were also conducted by Huang et al. [23]. Direct injection of two fuels was used in their study, namely methanol and biodiesel. The tests were carried out in a combustion system called an intelligent charge compression ignition (ICCI) system. This combustion mode was proposed to allow flexible stratification of the concentration and reactivity with the best gradient according to the engine operating conditions. In ICCI mode, most of the low-reactivity fuel is directly injected during the intake stroke with single or multiple stage split injection. Then, the rest of the low-reactivity and high-reactivity fuels are directly injected in succession to establish crossed stratification of the equivalence ratio and reactivity in the cylinder. An internal combustion engine was fueled with hydrogen and diesel oil in a hydrogen– diesel dual direct injection (H2DDI) system was presented by Liu et al. [24]. Hydrogen was injected into the cylinder at a pressure of 20 MPa and diesel fuel at 100 MPa. The research resulted in the following conclusions: • Direct injection of hydrogen into the cylinder induced up to a 10% increase in the end-of-compression pressure, which is associated with additional compression work. At later injection timings, this effect is less pronounced; • Under the same conditions of this study, the shape of the apparent heat release rate (aHRR) resembles that of the baseline diesel combustion, except when hydrogen is injected late, resulting in insufficient time for mixing, in which case slower aHRR indicative of hydrogen mixing and controlled combustion will be observed. The dual direct fuel injection systems presented so far do not include gasoline together with other light fuels besides ethanol. For this reason, the authors extended the above systems to include the combustion of other fuels that serve as substitutes for gasoline. The new knowledge presented by the authors in this article is conceptof the system, which shows different aspects from the systems used in the literature. Importantly, it contains precisely defined angular positions for the injectors. These result from the shape of the flowing stream, which moves towards the spark plug. Such a concept in a spark ignition engine, in addition to dual-fuel combustion, also allows the excess air ratio in the vicinity of the spark plug to be effectively controlled.

2. Experiment Method 2.1. Combustion System Concept The dual-fuel direct injection combustion system was based on the concept of creating stratiﬁed mixtures in the combustion chamber. As is the case in single-fuel systems with spark ignition, efforts were made to ensure its stoichiometric composition in the vicinity of the spark plug [25]. Scientiﬁc analyzes of these phenomena, discussed in the previous Energies 2021, 14, 2729 4 of 20

chapter, help validate this direction of development of mixture formation systems. The use of a second fuel, supplied directly into the cylinder using a separate injector, plays into the concept of the active formation of a stoichiometric mixture in the vicinity of the spark plug (where the global value of this coefficient in the combustion chamber may be much greater than one). The use of dual direct injection allows the selection of any possible composition of the fuel mixture for each engine cycle. Controlling the dosages of both fuels allows an individual mixture composition to be created in the combustion chamber each time. The resultant value of the excess air coefficient λ depends on the specific fuel doses and the type of fuels used. As a result of the above procedures, it is possible to control the ignitability of the mixture by adjusting its chemical composition—the quantitative ratio of the supplied fuels. The variable value of the coefficient λ in the vicinity of the spark plug also extends the possibility of controlling the reactivity of the resulting mixture. It also allows selection of the dose depending on the expected effective pressure, exhaust emissions, resistance to knocking, and other effects related to combustion.

2.2. Fuel Selection It was assumed that liquid (light) hydrocarbon fuels would be used in this research. Due to the growing interest in unconventional fuels (heptane, butanol), a more arbitrary selection of fuels was made, also taking into account their use in research, as shown in the previous chapter. Petrol was the reference fuel and alternative fuels included ethanol (C2H6O), n-heptane (C7H16), and n-butanol (C4H10O). The physical properties of the analyzed fuels are listed in Table1.

Table 1. Selected physical properties of fuels used in the research.

Property Gasoline Ethanol N-Heptane N-Butanol Density [kg/m3] 744.6 790.9 684.0 810.0 Enthalpy of evaporation [kJ/kg] 373 840 317 592 Isobaric speciﬁc heat [kJ/(kg·K)] 2.22 2.44 2.24 2.43 Stoichiometric air/fuel value [kg/kg] 14.6 9.0 15.3 11.2 Caloriﬁc value [MJ/kg] 43.50–44.00 26.90–29.70 44.50–44.60 33.08–33.10

Each fuel has different values of air for the stoichiometric mixture, as well as different values of the mass of carbon dioxide resulting from the complete combustion of a given volume of fuel (Table2).

Table 2. Values of the stoichiometric excess air coefﬁcient and CO2 mass resulting from combustion for selected fuels [26].

Stoichiometric Excess The Mass of CO2 Resulting Percentage Difference in CO2 Fuel Type Air Coefﬁcient from the Combustion Emissions Compared to Gasoline [kg air/kg Fuel] [kg] [%] gasoline 14.6 2.32 – ethanol 9.0 1.52 –34% n-heptane 15.3 0.79 –66% n-butanol 11.2 1.93 –17%

According to the Spindt formula, extended by Bresenham et al. [27] to enable calculations for fuels containing oxygen in terms of their chemical composition, the stoichiometric excess air coefﬁcient is calculated as follows: 34.56(4 + y − 2z) λ = , (1) st 12.011 + 1.008y + 16z Energies 2021, 14, 2729 5 of 20

where y and z are indices of the generalized chemical composition of the fuel (or fuel mixture), the summary formula of which can be denoted as CHyOz. Daniel et al. [28] proposed calculating the excess air coefﬁcient for two different fuels using the weighted average (where the weight is the mass of the fuel dose):

ma λmix = , (2) mp1·Lst_p1 + mp2·Lst_p2

where ma is the air mass for fuel doses for complete combustion, mp1 is the dose mass of the first fuel type, Lst_p1 is the value of the air required for the combustion of 1 kg of the first fuel, mp2 is the mass of the second fuel dose, and Lst_p2 is the value of air required for the combustion of 1 kg of the second fuel. The relation (2) shows that the excess air coefficient can assume different values for the same air mass ma by appropriately selecting fuel doses 1 and 2 with different stoichiometric excess air coefficients. The share of energy Ep of each fuel can be determined using:

mp1·Wop_p1 Ep = , (3) mp1·Wop_p1 + mp2·Wop_p2

where mp1 and mp2 are the respective masses of each fuel, while Wop is their calorific value. The above considerations lead to the conclusion that it is possible to shape the global excess air coefficient in the combustion chamber in a so-called qualitatively extended way, i.e., the change in λ is possible to obtain by changing the fuel dose or by changing the proportions of fuels supplied directly to the combustion chamber. This approach means changing the global excess air coefficient, while at the same time also changing its local values. This tendency to alter the combustion process was the basis for further experimental work. The experimental work was carried out in terms of assessing the effects of such modifications, as it was not possible to determine the local values of the excess air coefficient in the chamber.

2.3. Research Method The combustion process was tested with the use of (i) indicator methods and (ii) optical methods. Indication of the engine consisted of simultaneous registration of the pressure curve inside the combustion chamber along with the position of the piston in the cylinder. The recording of these characteristics enables quantiﬁcation of the energy values of the combustion process. These values are the combustion pressure, the rate of pressure build-up after ignition, and the heat release rate. The fuel combustion process has a very direct and speciﬁc impact on the process of heat release (its release rate—dQ), which was determined based on the following relationship (without taking into account the heat transfer and with the constant polytropic index value): dQ κ Pn + Pn+1 1 Vn + Vn+1 = (V + − V ) + (P + − P ) (4) dt κ − 1 2 n 1 n κ − 1 2 n 1 n

where the average value of the polytropic index was assumed as κ = 1.32 and the indices n and n + 1 denote the current and the next pressure value in the cylinder (P) or the corresponding cylinder volume (V). The fuel combustion process in a rapid compression machine was simultaneously subjected to optical and indicator measurements. The observation of the flame development made it possible to quantify the overall volume taken up by the flame field in the observed projection of the combustion chamber. The dynamics of the flame development observed with the optical method can be correlated with the thermodynamic indicators concerning combustion recorded with the indicator methods. Energies 2021, 14, 2729 6 of 20

development made it possible to quantify the overall volume taken up by the flame field in the observed projection of the combustion chamber. The dynamics of the flame development observed with the optical method can be correlated with the thermodynamic indicators concerning combustion recorded with the indicator methods. Due to the specific nature of the research in which the RCM was used, the engine indicators characterizing the operation of a typical internal combustion engine were not analyzed. The scope of work performed should be considered fundamental research. The Energies 2021, 14, 2729 6 of 20 combustion process was analyzed in the context of the initial phase, involving the formation of a combustible mixture, ignition and the initial development of the flame. For this Due reason, to the no specific tests nature related of the to research heat transfer, in which energy the RCM balance was used,, or the measurement engine of exhaustindicators emissions characterizing (which theare operationdifficult ofto a obtain typical internalwith a combustionrapid compression engine were machin not e) were performed.analyzed. The The works scope of on work energy performed and shouldexergy be analysis considered carried fundamental out by research. Kul and The Kahraman [29] werecombustion based process on an was internal analyzed combustion in the context ofengine. the initial Similarly, phase, involving the determination the formation of heat lossesof applies a combustible mainly mixture, to engine ignition tests and the (e.g. initial, [8]). development The use of of the RCM flame. with For this only reason, partial work no tests related to heat transfer, energy balance, or measurement of exhaust emissions strokes(which does are not difficult allow to results obtain withto be a obtained rapid compression that would machine) be qualitatively were performed. comparable. The The aboveworks considerations on energy and limited exergy analysis this type carried of research. out by Kul and Kahraman [29] were based on an internal combustion engine. Similarly, the determination of heat losses applies mainly 2.4. Testto engine Objects tests (e.g., [8]). The use of RCM with only partial work strokes does not allow results to be obtained that would be qualitatively comparable. The above considerations limitedThe literature this type of analysis research. of the design solutions for injector placement in the combustion chamber [30–34] showed that the position of the injector in relation to the 2.4. Test Objects spark plug depends on the injector type: The literature analysis of the design solutions for injector placement in the combustion 1. chamberSmaller [angle30–34] (10 showed–11°) that—when the position using ofmulti the- injectorhole injectors in relation; to the spark plug 2. dependsLarger angle on the injector(18–22° type:)—when using outward-opening injectors. 1. Smaller angle (10–11◦)—when using multi-hole injectors; In the present test cases,◦ the angular position of the injector in relation to the spark plug 2.was Larger assumed angle (18–22to be at)—when 22 degrees using outward-opening (due to using injectors. outward-opening injectors in the tests). In the present test cases, the angular position of the injector in relation to the spark plug was assumed to be at 22 degrees (due to using outward-opening injectors in the tests). The Thelayout layout of ofthe the finished finished headhead of of a rapida rapid compression compression machine machine with two with mounted two mounted injectorsinjectors is shown is shown in in Figure Figure 11..

Injectors Injector holder Spark plug with combustion pressure sensor

(a) (b) Figure 1. FigureRapid 1. compressionRapid compression machine machine head: ( a) (a view) view of the of engine the head engine with thehead injectors; with ( b the) model injectors; of the head (b cross-section.) model of the head cross-section. Piezoelectric injectors with a hollow cone-shaped stream of injected fuel were used in the research. The injector needle allows fuel ﬂow by extending slightly outside the injector (outward-openingPiezoelectric injectors design). with The technicala hollow data cone for-shaped the injector stream are presented of injected in Table fuel3. were used in the research. The injector needle allows fuel flow by extending slightly outside the injector (outward-opening design). The technical data for the injector are presented in Table 3.

Energies 2021, 14, 2729 7 of 20

Table 3. Technical data for the Siemens VDO piezoelectric injector [35].

Energies 2021, 14, 2729 Parameter Value 7 of 20 Dynamic flow 14.5 mg/inj ± 10% Static flow 35 g/s Table 3. TechnicalMinimum data for dose the Siemens VDO piezoelectric injector [35]. <2 mg/inj

SprayParameter angle Value90° ± 3° DropletDynamic size flowSMD 14.5 mg/inj ±~1510% μm Minimum openingStatic flow duration 35 g/s>150 μs Minimum dose <2 mg/inj Operating temperatureSpray angle range 90◦–±303 ◦to + 140 °C MaxDroplet voltage size SMD ~15 µm 190 V Minimum opening duration >150 µs OperatingInjection temperature pressure range –30 to + 1405–30◦C MPa MaxFuel voltage 190 Vgasoline Injection pressure 5–30 MPa Fuel gasoline Achieving the research objective required the use of a fuel supply system for two different fuels.Achieving Hence the, researchit was necessary objective required to build the such use of a asystem. fuel supply The system injection for twosystem used two highdifferent-pressure fuels. Hence, injection it was pumps necessary from to build the suchBMW a system. M4 GTS The vehicle injection systemmodel. used The original systemtwo was high-pressure modified injectionby disconnecting pumps from the the pipe BMW connecting M4 GTS vehicle the two model. pumps, The original which made it possiblesystem to use was both modified pumps by disconnecting independently, the pipe and connecting thus to theuse two two pumps, different which fuels. made it possible to use both pumps independently, and thus to use two different fuels. The The test test stand stand (Figure (Figure2) was 2) equipped was equipped with one drive with system one for drivethe two high-pressure system for the two high-pressureinjection pumps, injection each pumps, of which each can generate of which a different can generate fuel pressure a different value. fuel pressure value.

Front side

Back side

Pressure regulator (2x) Dual HP fuel pump Motor LP regulator (2x) Supply (380 V) Fuel tank (2x) LP – low pressure LP fuel pump (2x) HP – high pressure

(a) (b) Figure 2. FigureDual- 2.fuelDual-fuel injection injection station: station: (a) ( a) the the fuel fuel pressure pressure regulation regulation system; system; (b) the independently (b) the independently operated high- operated high-pressurepressure pumps pumps.. A rapid compression machine was used to test the combustion process (Figure3). The Atest rapid represented compression a physical machine model of awas combustion used to chamber test the in combustion a reciprocating process engine, in(Figure 3). The testwhich represent phenomenaed a occur physical under model transient of thermodynamic a combustion conditions. chamber Thein a moving reciprocating piston engine, in whichof the phenomenamachine had a pneumatic occur under drive. transient The piston thermodynamic of the machine was translucent, conditions. which The moving pistonenabled of the opticalmachine observation had a pneumatic and registration drive. of phenomena The piston occurring of the machine in the combustion was translucent, chamber. Technical speciﬁcations of the RCM are presented in Table4. which enabled optical observation and registration of phenomena occurring in the combustion chamber. Technical specifications of the RCM are presented in Table 4.

Energies 2021, 14, 2729 8 of 20 Energies 2021, 14, 2729 8 of 20

A C

FigureFigure 3.3. Rapid compression machine: machine: (A (A) mirror) mirror directing directing the the image image from from under under the thepiston piston to the to the recording camera; (B) piston; (C) engine head. recording camera; (B) piston; (C) engine head. A LaVision High Speed Star 5 camera was used to record the images of the Table 4. Technical data for the rapid compression machine used for the combustion tests. combustion process. Image recording was performed at a frequency of f = 10 kHz with an AF Nikkor 50 mmParameter with a 1:14D fixed focal length lens. Value Indicator tests consisted of simultaneous recording of the pressure inside the RCM Piston stroke 81 mm combustion chamber and the position of the piston. The combustion pressure signal was Cylinder diameter 80 mm recorded using an AVL GM11D sensor (0–25 MPa; 18.84 pC/bar) integrated with the M12 Cylinder volume 407 cm3 spark plug.Combustion The piston chamber position type signal was read from hemispherical a linear position + chamber sensor in the (Megatron piston LSR 150 STCombustion R5k SR with chamber a range volume of 150 mm and a resolution of 0.0116.5 mm) cm 3coupled with the pistonThe of total the rapid combustion compression chamber machine. volume 55.9 cm3 SynchronizCompressionation of the ratio injection of various fuels, the piston movement, 8.94:1 the opening and closing of thePiston machine seal’s solenoid valves, and the starting piston rings, of the teﬂon camera seal and the indicators systemAir supply was ensured method by the HardSoft HSD V711 electromagnetic sequencer system. valves Exhaust release method electromagnetic valves Table 4. TechnicalPiston data drive for the method rapid compression machine used for the pneumatic combustion tests. Optical access quartz glass window φ50 mm in the piston Parameter Value Piston stroke 81 mm A LaVision High Speed Star 5 camera was used to record the images of the combustion process. ImageCylinder recording diameter was performed at a frequency of f =80 10 mm kHz with an AF Nikkor 3 50 mm with aCylinder 1:14D ﬁxed volume focal length lens. 407 cm IndicatorCombustion tests consisted chamber oftype simultaneous hemispherical recording of the+ chamber pressure in insidethe piston the RCM 3 combustionCombustion chamber chamber and the volume position of the piston. The combustion16.5 cm pressure signal was recordedThe total using combustion an AVL GM11D chamber sensor volume (0–25 MPa; 18.84 pC/bar)55.9 integratedcm3 with the M12 spark plug. TheCompression piston position ratio signal was read from a linear8.94:1 position sensor (Megatron LSR 150 ST R5k SRPiston with seal a range of 150 mm and a resolutionpiston ofrings, 0.01 teflon mm) coupledseal with the piston of theAir rapid supply compression method machine. electromagnetic valves SynchronizationExhaust release of the method injection of various fuels,electromagnetic the piston movement, valves the opening and closingPiston of the drive machine’s method solenoid valves, and the startingpneumatic of the camera and the indicators systemOptical was ensuredaccess by the HardSoftquartz HSD glass V711 window sequencer 50 mm system. in the piston

3.3. FuelFuel Combustion in in a a Dual Dual-Fuel-Fuel System System 3.1.3.1. TestTest ConditionsConditions ExperimentalExperimental combustion combustion studies studies were were aimed aimed at conﬁrming at confirming the possibility the possibility of control- of lingcontrolling the mixture the mixture preparation preparation and combustion and combustion processes processes for various for various fuel mixtures.fuel mixtures. CombustionCombustion tests were were carried carried out out in in two two research research groups: groups: 1.1. FForor gasoline supplied supplied w withith at at least least one one injector: injector: • GGasolineasoline + + gasoline; gasoline; • GGasolineasoline + + ethanol; ethanol; • GGasolineasoline + + n n-heptane;-heptane; • Gasoline + n-butanol.

Energies 2021, 14, 2729 9 of 20

2. For one fuel type supplied with both injectors: • Gasoline + gasoline; • Ethanol + ethanol; • N-heptane + n-heptane; • N-butanol + n-butanol. In each research variant, the same dose of energy in the form of fuel was supplied to the combustion chamber as determined using the caloriﬁc value of the fuels used. The injector opening time was calculated based on the fuel density and the ﬂow characteristics of the injectors. For the gasoline dose, the basic duration taken to one injector was set at t = 300 µs, and this data point was treated as a reference value for other fuels. The resulting injection times for the analyzed fuels are presented in Table5.

Table 5. Doses and injection times for fuels delivered during one research process by one injector.

Fuel Caloriﬁc Value Density Fuel Dose Mass Fuel Dose Volume Injection Duration [MJ/kg] [kg/m3] [mg] [ml] [µs] Gasoline 43.5 744.6 22.34 0.030 300 Ethanol 29.7 790.9 32.72 0.041 522 N-heptane 44.6 684.0 21.79 0.032 446 N-butanol 33.1 810.0 29.36 0.036 537

Each time, a dose of chemical energy of about 1943 J was delivered to the combustion chamber (the equivalent of two single doses from both injectors). This value was used to further evaluate the combustion process efficiency. Having determined doses for each fuel, the values of the excess air coefficient during combustion were also determined. The air mass m delivered to the cylinder was determined using the Clapeyron equation: PV m = , (5) RT where P is the cylinder pressure at volume V,R is the gas constant, and T is the dose temperature. 3 Taking the values for the time t = 0 (Vcyl = 462.9 cm ) as the baseline for the calculations, the value of the air pressure (P = 0.9 bar) was determined (based on the data from the cylinder pressure curve, analyzed later in the article), which was compared to the absolute values (the pressure sensor shows overpressure values in relation to the ambient conditions). Assuming R = 287 J/(kg K) and T = 20 ◦C and using Equation (5), the air mass m = 1011 mg was determined. Using Equation (2) describing the excess air coefficient for the fuel mixture and the data from Table5, the global values of the excess air coefficient were determined for the analyzed mixtures. The results of these calculations are included in Table6.

Table 6. Determining the global excess air coefﬁcients of the analyzed fuels.

λ q◦ Lt Gasoline Ethanol N-Heptane N-Butanol [[–] [mg] [kg Air/kg Fuel] q◦ [mg] 22.34 32.71 29.35 21.78 Lt [kg air/kg fuel] 14.56 8.96 11.14 15.11 Gasoline 22.34 14.56 1.55 1.64 1.55 1.55 Ethanol 32.71 8.96 1.64 1.73 1.63 1.63 n-heptane 29.35 11.14 1.55 1.63 1.55 1.54 n-butanol 21.78 15.11 1.55 1.63 1.54 1.54

Combustion tests were carried out for all conﬁgurations listed at the beginning of Section 3.1. A full presentation is provided in Figure4. Energies 2021, 14, 2729 10 of 20

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Fuel Gasoline (B) N-butanol (BU) Ethanol (E) N-heptane (H)

B1_B1_300_300_01 BU1_B 1_537_300_01 E1_B1_522_300_01 H1_B1_446_300_01 B1_B1_300_300_02 BU1_B 1_537_300_02 E1_B1_522_300_04 H1_B1_446_300_02 B1_B1_300_300_03 BU1_B 1_537_300_03 E1_B1_522_300_03 H1_B1_446_300_03 60 60 60 60 bar 37.761 bar 42.010 bar 35.125 bar 37.659 50 bar 34.178 50 bar 41.754 50 bar 40.628 50 bar 40.551 bar 35.841 bar 36.328 bar 38.196 bar 38.554 Gasoline 40 40 40 40 30 30 30 30

20 20 20 20

P_cyl [bar] P_cyl [bar] P_cyl [bar] (b) P_cyl [bar] 10 10 10 10 0 0 0 0 11880 80 11890 90 11900 100 11880 80 11890 90 11900 100 11880 80 11890 90 11900 100 11880 80 11890 90 11900 100 Time [ms] Time [ms] Time [ms] Time [ms]

BU1_B U1_537_537_01 BU1_E 1_537_522_04 BU1_H1_537_446_01 BU1_B U1_537_537_02 BU1_E 1_537_522_02 BU1_H1_537_446_02 BU1_B U1_537_537_04 BU1_E 1_537_522_03 BU1_H1_537_446_03

60 bar 40.525 60 bar 40.167 60 bar 37.531 50 bar 40.397 50 bar 43.750 50 bar 35.227 bar 41.780 bar 40.295 bar 34.459 N-butanol 40 40 40 30 30 30

(bu) 20 20 20

P_cyl [bar] P_cyl [bar] P_cyl [bar] 10 10 10 0 0 0 11880 80 11890 90 11900100 11880 11890 11900 11880 11890 11900 Time [ms] Time [ms] Time [ms]

E1_E1_522_522_01 E1_H1_522_446_01 E1_E1_522_522_02 E1_H1_522_446_02 E1_E1_522_522_03 E1_H1_522_446_04

60 bar 38.887 60 bar 39.962 50 bar 38.580 50 bar 38.734 bar 40.346 bar 37.889 Ethanol 40 40 30 30

(e) 20 20

P_cyl [bar] P_cyl [bar] 10 10 0 0 11880 80 11890 90 11900 100 11880 11890 11900 Time [ms] Time [ms]

H1_H1_446_446_01 H1_H1_446_446_02 H1_H1_446_446_03 60 bar 38.682 50 bar 38.580 bar 38.862 N-heptane 40 30 20

(h) P_cyl [bar] 10 0 11880 80 11890 90 11900 100 Time [ms]

Figure 4. Summary of indicator tests (pressure in the RCM cylinder) concerning the combustion of two fuelsCreated (tests with were Concerto Student Edition. Licensed for: TU Posen Figurerepeated three4. Summary times). of indicator tests (pressure in the RCM cylinder) concerning the combustion of two fuels (tests were repeated three times). Each experiment was repeated three times. A single representative variant was selected for the analysis, for which the maximum combustion pressure value was obtained. Each experimentThese values was and their repeated greatest percentage three deviations times. are A listed single in Table 7 representative. variant was selected for theTable analysis, 7. Maximum for combustion which pressure the values maximum for representative characteristics,combustion including pressure the value was obtained. These greatestvalues relative and deviation. their greatest percentage deviations are listed in Table 7. Fuels Maximum Pressure [bar] Relative Deviation [%] Gasoline + gasoline 37.76 10.5 Table 7. Maximum combustionGasoline + ethanol pressure values 40.63 for representative characteristics, 15.7 including the Gasoline + n-heptane 40.55 7.7 greatest relative deviationGasoline. + n-butanol 42.01 15.6 Ethanol + ethanol 40.35 4.6 N-heptane + n-heptane 38.86 0.7 Fuels N-butanol + n-butanolMaximum 41.78Pressure [bar] 3.4Relative Deviation [%] Gasoline + gasoline 37.76 10.5 In three cases, the deviation of the relative maximum pressure values exceeded 10%, Gasoline + howeverethanol the quality of the results was considered40.63 to be sufﬁcient for further analysis. 15.7 Gasoline + n-heptane 40.55 7.7 Gasoline + n-butanol 42.01 15.6 Ethanol + ethanol 40.35 4.6 N-heptane + n-heptane 38.86 0.7 N-butanol + n-butanol 41.78 3.4

In three cases, the deviation of the relative maximum pressure values exceeded 10%, however the quality of the results was considered to be sufficient for further analysis.

3.2. Analysis of the Indicator Values (for the Combustion of Gasoline with Other Fuels) Indicator tests carried out with the use of a rapid compression machine included the simultaneous measurement of the pressure curve in the combustion chamber and the piston position during the combustion process. Based on these values, it was possible to analyze the value of the maximum pressure, the rate of pressure increase after ignition, the heat release rate, and the total released heat. The time characteristics of pressure inside the combustion chamber during the experiments for gasoline with various fuels are given in Figure 5.

Energies 2021, 14, 2729 11 of 20

3.2. Analysis of the Indicator Values (for the Combustion of Gasoline with Other Fuels) Indicator tests carried out with the use of a rapid compression machine included the simultaneous measurement of the pressure curve in the combustion chamber and the piston position during the combustion process. Based on these values, it was possible to analyze the value of the maximum pressure, the rate of pressure increase after ignition, the Energies 2021, 14, 2729 heat release rate, and the total released heat. 11 of 20

The time characteristics of pressure inside the combustion chamber during the experiments for gasoline with various fuels are given in Figure5.

50 B1_B1_300_300_01 E1_B1_522_300_04 H1_B1_446_300_02 40 11.3% BU1_B1_537_300_01

P_cyl_max bar 37.863 P_cyl_max bar 40.622 30 P_cyl_max bar 40.739 P_cyl_max bar 42.121

P_cyl [bar] P_cyl 20

0 70 80 90 100 110 120 t [ms]

Figure 5. Compilation of the pressure values in the combustion chamber of a rapid compression machine during the Figure 5. Compilation of the pressure values in the combustion chamber of a rapid compression combustion of gasoline with other fuels along with the maximum obtained values. machine during the combustion of gasoline with other fuels along with the maximum obtained values. The cylinder pressure characteristics during combustion show that the type of alternative fuel supplied into the chamber for combustion with gasoline affected the resulting maximum pressure value; each alternative fuel increased the pressure value compared to The cylinder pressurepure gasoline characteristics combustion. Additionally, during the combustion use of n-butanol show delays the that moment the when type the of alternative fuel suppliedmaximum int pressureo the chamber is reached by for the burning combustion mixture. withCreated with gasoline Concerto Student affected Edition. Licensed the for: TU Posen resulting maximum pressureThe indicated value combustion; each alternative pressure curves fuel point increased to a similar the combustion pressure rate value (similar values of the curve slope after ignition), however they also indicate different com- compared to pure gasolinebustion conditions. combustion. Despite Additionally, providing the same the amount use of of energy n-butanol in the injecteddelays fuel the moment when the maximumdoses, different pressur Pmaxe values is reached were observed. by the Theburning maximum mixture. difference was found to be The indicated combustion∆Pmax = 13%. pressure curves point to a similar combustion rate The heat release rate for the four tested experimental cases was shown in Figure6a. (similar values of theAdditionally, curve slope in the after case of ignition), the heat release however rate, there they was a also noticeable indicate tendency different of the combustion conditions.maximum Despite value providing of this parameter the same to occur amou later fornt theof variantenergy of combustionin the injected of gasoline fuel with n-butanol. The combustion of gasoline alone results in the slowest process of heat doses, different Pmaxrelease values compared were to observed. all other tested The variants. maximum difference was found to be Pmax = 13%. The heat release rate for the four tested experimental cases was shown in Figure 6a. Additionally, in the case of the heat release rate, there was a noticeable tendency of the maximum value of this parameter to occur later for the variant of combustion of gasoline with n-butanol. The combustion of gasoline alone results in the slowest process of heat release compared to all other tested variants. In addition, the dQ/dt analysis showed much greater divergence in combustion than the Pcyl analysis presented previously. The maximum values of dQ/dt (from the smallest ones) were higher in relation to gasoline combustion—n-heptane–butanol by 18% and ethanol–n-butanol by 19.4%. The maximum changes were recorded for n-butanol– gasoline combustion at dQ/dt = 28.4%. Taking into account the occurrence time of dQ/dt, it was determined that the fastest combustion fuel configuration was the n-heptane–gasoline (time to achieve dQ/dt was faster by 0.15 ms compared to the combustion of gasoline alone), then ethanol and n-heptane with gasoline, while the slowest process was butanol and gasoline, for which the delay was measured at 0.25 ms compared to gasoline combustion. The characteristics of the heat released from the combustion of fuel mixtures are shown in Figure 6b. The declining shape of the heat release curve resulted from the nature of the rapid compression machine operation and the gas expansion conditions (significant leakage of gases) when the piston was at a certain distance from the TDC. The total amount of heat released from the combustion process allowed determination of the combustion process efficiency. Assuming that a constant value of the chemical energy was supplied to the cylinder in the form of fuels, the relative values of combustion efficiency were calculated for the tested fuel configurations. The highest value of total released heat was taken as the reference value equal to 100%:

Energies 2021, 14, 2729 12 of 20

• Gasoline + gasoline: 78.1%; • Gasoline + ethanol: 87.7%; • Gasoline + n-heptane: 92.4%; • Gasoline + n-butanol: 100%. The heat release analysis (Figure 6b) showed the same slopes of the heat release curves for all cases except for gasoline—for this case the heat gain was the slowest and overall the lowest. The decrease in the amount of heat after reaching the maximum value was due to the specificity of the RCM’s operation, mostly as a result of not having a crank 600 mechanism. B1_B1_300_300_01 E1_B1_522_300_04 500 From the obtained resultH1_B1_446_300_02s it could be concluded that alternative fuels delivered by 28.4% 18.7% 12.4% BU1_B1_537_300_01 t Q Q Q Q N° meanst dQ of directdQ dQinjectiondQ into the combustionms chamberJ J in parallelJ J with gasoline increase ms J/ms J/ms J/ms J/ms 400 Q_maxEnergiesJ 2021421.28, 14, 2729 88.850 420.177 471.845 500.070 488.257 12 of 20 Q_max J 473.62 1 88.200the 480.626overall573.841 efficiency532.694 511.723 of the combustion process. 89.200 407.557 461.219 486.801 540.896 Q_max J 500.07 2 88.050 444.711 521.348 567.329 391.178 0.350 -12.620 -10.626 -13.269 52.639 300 Q_max J 540.90 2-1 -0.150 -35.916 -52.493 34.635 -120.545

3 88.450 380.477 448.763 324.838 616.948 Q [J] Q 200 800 base 800 dQ/dt_max J/ms 480.63 B1_B1_300_300_01 +18% dQ/dt_max J/ms 573.84 E1_B1_522_300_04 +19.4% dQ/dt_max J/ms 567.33 H1_B1_446_300_02 100 +28.4% dQ/dt_max J/ms 616.95 BU1_B1_537_300_01 600 600 0 t = 0.35 ms

-100 80 400 85 90 95 400 100

t [ms] Q [J] Q

dQ/dt [J/ms] dQ/dt 200 200

t Q Q Q Q ms J J J J 88.900 421.279 473.448 499.974 503.253 89.200 407.557 461.219 486.801 540.896 base 0.300 -13.7220 -12.229 -13.173 37.643 0 Q_max J 421.28 +12.4% Created with Concerto Student Edition. Licensed for: TU Posen Q_max J 473.62 +18.7% Q_max J 500.07 –0.15 ms +0.35 ms +28.4% +0.25 ms Q_max J 540.90 -200 -200 86 87 88 89 88 89 90 91 92 t [ms] t [ms] (a) (b) Figure 6. CombustionFigure 6. processCombustion indicators process indicators (for the (for injection the injection of of variousvarious fuels): fuels): (a) comparison (a) comparison of the time of characteristics the time ofcharacteristics of the heat release therate heat in release the RCM rate in combustion the RCM combustion chamber chamber during during thethe combustion combustion of gasoline of gasoline with other with fuels, other along with fuels, the along with the maximum values obtained; (b) summary of time characteristics of the heat generated in the combustion chamber of a rapid maximum valuescompression obtained; machine (b) duringsummary the combustion of time of characte gasoline withristics other fuels,of the with heat the maximum generated values in of parametersthe combustion for each chamber of a rapid compressionof the machine tested cases. during the combustion of gasoline with other fuels, with the maximum values of parameters for each of the tested cases. In addition, the dQ/dt analysis showed much greater divergenceCreated with inConcerto combustion Student Edition. than Licensed for: TU Posen the Pcyl analysis presented previously. The maximum values of dQ/dt (from the smallest Basedones) on were the higher combustion in relation toprocesses gasoline combustion—n-heptane–butanol analyzed for the dual-fuel by 18% system, and it was found that theethanol–n-butanol combustion by rates 19.4%. for The maximumgasoline changes and were n-butanol recorded for were n-butanol–gasoline the highest. The worst combustion at ∆dQ/dt = 28.4%. Taking into account the occurrence time of dQ/dt, it was indicatorsdetermined were found that the fastestfor gasoline combustion combustion. fuel configuration The was thevalues n-heptane–gasoline of the combustion (time indicators of all configurationsto achieve dQ/dt a wasre summarized faster by 0.15 ms in compared Figure to 7. the combustion of gasoline alone), then ethanol and n-heptane with gasoline, while the slowest process was butanol and gasoline, for which the delay was measured at 0.25 ms compared to gasoline combustion. 105 The characteristics of the heat released from the combustion of fuel mixtures are shown10% in Figure6b. The declining17% shape of the heat release22% curve resulted from22% the nature 100 of the rapid compression machine operation and the gas expansion conditions (significant leakage of gases) when the piston was at a certain distance from the TDC. 95 The total amount of heat released from the combustion process allowed determination of the combustion process efficiency. Assuming that a constant value of the chemical 90 energy was supplied to the cylinder in the form of fuels, the relative values of combustion efficiency were calculated for the tested fuel configurations. The highest value of total 85 released heat was taken as the reference value equal to 100%: • Gasoline + gasoline: 78.1%; 80 • Gasoline + ethanol: 87.7%;

Change of indicator [%] Change of indicator • Gasoline + n-heptane: 92.4%; 75 • Gasoline + n-butanol: 100%. Gasoline + gasoline Gasoline + ethanol Gasoline + n-heptane Gasoline + n-butanol The heat release analysis (Figure6b) showed the same slopes of the heat release curves 70 for all cases except for gasoline—for this case the heat gain was the slowest and overall the Pcyl dP/da dQ Q Figure 7. Summary of combustion indicator values when burning gasoline with other fuels.

Energies 2021, 14, 2729 12 of 20

• Gasoline + gasoline: 78.1%; • Gasoline + ethanol: 87.7%; • Gasoline + n-heptane: 92.4%; • Gasoline + n-butanol: 100%. The heat release analysis (Figure 6b) showed the same slopes of the heat release curves for all cases except for gasoline—for this case the heat gain was the slowest and overall the lowest. The decrease in the amount of heat after reaching the maximum value was due to the specificity of the RCM’s operation, mostly as a result of not having a crank 600 mechanism. B1_B1_300_300_01 E1_B1_522_300_04 500 From the obtained resultH1_B1_446_300_02s it could be concluded that alternative fuels delivered by 28.4% 18.7% 12.4% BU1_B1_537_300_01 t Q Q Q Q N° meanst dQ of directdQ dQinjectiondQ into the combustionms chamberJ J in parallelJ J with gasoline increase ms J/ms J/ms J/ms J/ms 400 Q_max J 421.28 88.850 420.177 471.845 500.070 488.257 Q_max J 473.62 1 88.200the 480.626overall573.841 efficiency532.694 511.723 of the combustion process. 89.200 407.557 461.219 486.801 540.896 Q_max J 500.07 2 88.050 444.711 521.348 567.329 391.178 0.350 -12.620 -10.626 -13.269 52.639 300 Q_max J 540.90 2-1 -0.150 -35.916 -52.493 34.635 -120.545

-100 80 400 85 90 95 400 100

t [ms] Q [J] Q

dQ/dt [J/ms] dQ/dt 200 200

105

10% 17% 22% 22% 100

80 Change of indicator [%] Change of indicator 75 Gasoline + gasoline Gasoline + ethanol Gasoline + n-heptane Gasoline + n-butanol 70 Pcyl dP/da dQ Q FiFiguregure 7.7. SummarySummary of of combustion combustion indicator indicator values values when when burning burning gasoline gasoline with other with fuels.other fuels.

The combustion of gasoline and n-butanol in relation to the combustion of gasoline alone could be described by stating that: • The maximum combustion pressure in the cylinder was 10% greater—changes in relation to other fuel mixtures reached values up to 4%; • The pressure increase was 17% greater—combustion of the remaining mixtures indicated changes in the pressure increase value of up to 7%; • The maximum heat release rate was measured to be 22% greater—combustion of other fuels showed smaller differences of up to 8%; • The maximum amount of heat released was 22% greater—changes of this ratio for other fuels were 13% (gasoline and ethanol) and 8% (gasoline and n-heptane). Summarizing the research results discussed above, it was found that the configuration containing n-butanol and gasoline achieved the highest indicator values for the combustion process. The highest pressure in the cylinder enabled the greatest derivative values to be obtained, up to and including the amount of released heat. These benefits were additionally confirmed by the reduced amount of CO2 produced during combustion (reduced by 17% compared to gasoline, as shown in Table2). Optical tests for the combustion process were carried out in parallel with the indicator tests. They allowed for a more complete identification of the phenomena occurring in the engine during the combustion of various fuels and their mixtures.

3.3. Optical Analysis (Combustion of Gasoline with Other Fuels) The images recorded from the optical tests of the combustion of selected fuels are shown in Figure8. The differences in the flame development for different fuel mixture configurations were significant and indicated slight differences in the combustion process for the individual tested mixtures. Energies 2021, 14, 2729 13 of 20

The combustion of gasoline and n-butanol in relation to the combustion of gasoline alone could be described by stating that: • The maximum combustion pressure in the cylinder was 10% greater—changes in relation to other fuel mixtures reached values up to 4%; • The pressure increase was 17% greater—combustion of the remaining mixtures indicated changes in the pressure increase value of up to 7%; • The maximum heat release rate was measured to be 22% greater—combustion of other fuels showed smaller differences of up to 8%; • The maximum amount of heat released was 22% greater—changes of this ratio for other fuels were 13% (gasoline and ethanol) and 8% (gasoline and n-heptane). Summarizing the research results discussed above, it was found that the configuration containing n-butanol and gasoline achieved the highest indicator values for the combustion process. The highest pressure in the cylinder enabled the greatest derivative values to be obtained, up to and including the amount of released heat. These benefits were additionally confirmed by the reduced amount of CO2 produced during combustion (reduced by 17% compared to gasoline, as shown in Table 2). Optical tests for the combustion process were carried out in parallel with the indicator tests. They allowed for a more complete identification of the phenomena occurring in the engine during the combustion of various fuels and their mixtures.

3.3. Optical Analysis (Combustion of Gasoline with Other Fuels) The images recorded from the optical tests of the combustion of selected fuels are Energies 2021, 14, 2729 shown in Figure 8. The differences in the flame development for different fuel mixture 14 of 20 configurations were significant and indicated slight differences in the combustion process for the individual tested mixtures.

t t [ms] B+B B+E B+H B+Bu [ms] B+B B+E B+H B+Bu

0 14

2 16

4 18

6 20

Energies 2021, 14, 2729 14 of 20 8 22

Thanks10 to the recorded images, 24 it was possible to quantify the stages of flame

development in the cross-section view of the combustion chamber. The percentages of the combustion chamber volume covered by the flame are shown in Figure 9a. The12 analysis of Figure 9a concerning26 the surface flame area allows for two different types of flame development process to be distinguished. The first of the se, in the case of

gasolineFigure 8. + The gasoline flame development and gasoline process recorded + ethanol, during optical was experimental characterized tests for various by an intense flame Figurefuel combustion 8. The flame configurations. development process recorded during optical experimental tests for various fuel developmentcombustion configurations. in the initial combustion stage and its rapid extinction. In the second, namely gasoline + n-heptane and gasoline + n-butanol, slower flame development can be Thanks to the recorded images, it was possible to quantify the stages of flame de- observed in the initial combustion stage, however this flame development lasts longer thanvelopment for the previously in the cross-section discussed view fuel of mixtures. the combustion chamber. The percentages of the combustion chamber volume covered by the flame are shown in Figure9a.

100 25 –-14% –23-23%%

80 20

60 15

[mm]

A[%] z 40 r 10

20 Gasoline + gasoline 5 Gasoline + gasoline Gasoline + ethanol Gasoline + ethanol Gasoline + n-heptane Gasoline + n-heptane gasoline + n-butanol Gasoline + n-butanol 0 0 0 10 20 30 0 200 400 600 800 t [ms] dQ/dt [J/ms]

(a) (b) Figure Figure 9. Combustion 9. Combustion process process indicators: indicators: (a ()a comparison) comparison of the of time the characteristics time characteristics of the percentage of the share percentage of the combustion share of the combustionchamber chamber cross-sectional cross-sectional area occupied area byoccupied the ﬂame by for the the flame combustion for the of gasolinecombustion with otherof gasoline fuels supplied with other with separate fuels supplied with separateinjectors; injectors; (b) relationships (b) relationship between thes between equivalent the ﬂame equivalent area radius flame and thearea heat radius release and rate the during heat the release growth rate of these during the growth quantitiesof these quantities for the combustion for the ofcombustion fuel mixtures of supplied fuel mixtures with separate supplied direct with injectors. separate direct injectors.

Additionally, it can be observed that the combustion of the mixture with ethanol lasted about 10 ms less than in the case where only gasoline was used. In [36], it was found that for the combustion of lean mixtures created by means of direct injection, there was a linear relationship between the heat release rate and the equivalent radius of the flame area in the phase of the heat release rate increase. Therefore, we checked whether this relationship held true for the performed experiments. The equivalent flame radius rz was determined from the flame area of the circle by assuming that the flame formed a circular shape. The correlation analysis results for the heat release rate and the equivalent flame radius during the combustion of gasoline with various fuels are shown in Figure 9b. This showed that the linear relation occurred only for the combustion of gasoline alone and gasoline with n-heptane. In other fuel mixture cases, a linear relationship was not observed. The differing nature of the relationship between these two values was an important observation in the research on the dual-fuel system and in the creation of a stratified mixture in the combustion chamber. The occurrence of a flame in the area of the combustion chamber does not necessarily define the intensity of the combustion pressure increase as a result of this combustion. This may be due to non-uniformity in calorific value and ignitability of the fuels used to form the mixture. The final confirmation of these hypotheses can be made after analyzing the combustion of one type of fuel delivered by means of direct injection with two injectors.

Energies 2021, 14, 2729 15 of 20

The analysis of Figure9a concerning the surface flame area allows for two different types of flame development process to be distinguished. The first of these, in the case of gasoline + gasoline and gasoline + ethanol, was characterized by an intense flame development in the initial combustion stage and its rapid extinction. In the second, namely gasoline + n-heptane and gasoline + n-butanol, slower flame development can be observed in the initial combustion stage, however this flame development lasts longer than for the previously discussed fuel mixtures. Additionally, it can be observed that the combustion of the mixture with ethanol lasted about 10 ms less than in the case where only gasoline was used. In [36], it was found that for the combustion of lean mixtures created by means of direct injection, there was a linear relationship between the heat release rate and the equivalent radius of the flame area in the phase of the heat release rate increase. Therefore, we checked whether this relationship held true for the performed experiments. The equivalent flame radius rz was determined from the flame area of the circle by assuming that the flame formed a circular shape. The correlation analysis results for the heat release rate and the equivalent flame radius during the combustion of gasoline with various fuels are shown in Figure9b. This showed that the linear relation occurred only for the combustion of gasoline alone and gasoline with n-heptane. In other fuel mixture cases, a linear relationship was not observed. The differing nature of the relationship between these two values was an important observation in the research on the dual-fuel system and in the creation of a stratified mixture in the combustion chamber. The occurrence of a flame in the area of the combustion chamber does not necessarily define the intensity of the combustion pressure increase as a result of this combustion. This may be due to non-uniformity in calorific value and Energies 2021, 14, 2729 ignitability of the fuels used to form the mixture. The final confirmation of these hypotheses15 of 20 can be made after analyzing the combustion of one type of fuel delivered by means of direct injection with two injectors.

3.4. Analysis of Operation Indicators for the Combustion Process of One Fuel Type SuppliedSupplied withwith Two InjectorsInjectors In this this section, section, the the impacts impact ofs one of fuel one type fuel on type the combustion on the combustion results in a results two-injector in a systemtwo-injector were system assessed. were Figure assessed 10 shows. Figure the 10 combustion shows the pressurecombustion analysis pressure for the analysis four fuel for typesthe four supplied—each fuel types supplied time one—each fuel time type one was fuel supplied type was using supplied both injectors. using both injectors.

50 B1_B1_300_300_01 E1_E1_522_522_03 H1_H1_446_446_03 40 10.6% BU1_BU1_537_537_04

P_cyl_max bar 37.863 P_cyl_max bar 40.386 30 P_cyl_max bar 39.117 P_cyl_max bar 41.917

P_cyl [bar] P_cyl 20

10 t = 0.9 ms

0 70 80 90 100 110 120 t [ms]

Figure 10. ComparisonComparison of of time time characteristics characteristics for for the the combustion combustion chamber chamber pressure pressure in a rapid in a rapid compression machine during the combustion of one type of fuel injected injected using using two two injectors injectors along along

with the maximumt P_c yl_1 parameterP_c yl_1 P_c yl_1 valuesvaluesP_c yl_1 forfor eacheach ofof thethe testedtested cases.cases. ms bar bar bar bar 88.850 37.836 40.352 39.117 24.400 89.700 34.644 36.732 36.698 41.833 The cylinder 0.850 -3.193 pressure -3.619 -2.419 curves 17.432 show that any alternative fuel replacing gasoline

increases the resulting maximum pressure value. Additionally,Created with the Concerto use Student of Edition. n-butanol Licensed for: TU Posen delays the moment of reaching that maximum value. The trend for maximum combustion pressure values is similar for partial and complete replacement of gasoline with another fuel. The heat release rate characteristics for the four tested experimental cases are presented in Figure 11a. Unlike in the case of using a fuel mixture, the use of n-butanol alone does not result in the maximum value for the heat release rate among the four tested fuels. This means that for n-butanol, the addition of gasoline increased the reactivity of the mixture. However, using only gasoline by itself resulted in the lowest value of the heat release rate among the analyzed fuels. Figure 11b shows the characteristics of heat released in time during fuel combustion. The descending shape of the heat release rate was a result of the nature and operating conditions of the rapid compression machine when the piston was near the bottom dead center. The combustion process efficiency was determined in the analysis of single fuel type combustion cases. The relative combustion efficiency values for the fuels used were calculated. The results are presented relative to the base fuel (gasoline): • Gasoline: reference value; • Ethanol: 0.9% more than for combustion of the gasoline + ethanol mixture; • N-heptane: 2.1% less than for the combustion of the gasoline + n-heptane mixture; • N-butanol: 1.7% less than for the combustion of the gasoline + n-butanol mixture. The combustion of the same fuels makes the high density of n-butanol even more prominent. It did not show such different properties when mixed with gasoline (point 3.3). The combustion of homonymous fuels also corresponds to the high initial heat release rates seen for n-hexane and ethanol. Such combustion process characteristics were also repeated for the analysis of heat release.

Energies 2021, 14, 2729 16 of 20

The cylinder pressure curves show that any alternative fuel replacing gasoline increases the resulting maximum pressure value. Additionally, the use of n-butanol delays the moment of reaching that maximum value. The trend for maximum combustion pressure values is similar for partial and complete replacement of gasoline with another fuel. The heat release rate characteristics for the four tested experimental cases are presented in Figure 11a. Unlike in the case of using a fuel mixture, the use of n-butanol alone does not result in the maximum value for the heat release rate among the four tested fuels. This means that for n-butanol, the addition of gasoline increased the reactivity of the mixture. However, using only gasoline by itself resulted in the lowest value of the heat release rate t Q Q Q Q t dQ dQamongdQ the analyzeddQ fuels. Figure 11b shows the characteristics of heat released in time Energies 2021, 14, 2729 ms J J J J 16 of 20 ms J/ms J/ms J/ms J/ms during fuel combustion. The descending 88.950 shape421.056 of the489.494 heat release459.252 rate217.233 was a result of the 88.200 480.626 564.427 528.844 128.259 89.800 363.099 435.434 424.827 506.708 89.050 -51.617 -60.693 -29.395 556.610 nature and operating conditions of the rapid 0.850 compression-57.957 -54.060 machine-34.425 289.475 when the piston was 0.850 -532.243 -625.120 -558.240 428.351 800 near the bottom dead center. B1_B1_300_300_01 800 E1_E1_522_522_03 900 baseH1_H1_446_446_03 dQ/dt_max J/ms 480.63 BU1_BU1_537_537_04 B1_B1_300_300_01 600 +20.6% dQ/dt_max J/ms 579.80 E1_E1_522_522_03 15.8% 20.6% 800 dQ/dt_max J/ms 12541.67.7% +12.7% H1_H1_446_446_03 dQ/dt_max J/ms 480.63 +15.8% dQ/dt_max J/ms 573.84 dQ/dt_max J/ms 556.61 700 BU1_BU1_537_537_04 400 dQ/dt_max J/ms 567.33 600 dQ/dt_max J/ms 616.95 600

500

dQ/dt [J/ms] dQ/dt 200 400 400

0 [J] Q 300

dQ/dt [J/ms] dQ/dt 200 t = 0.85 ms 200 -200 base 100 Q_max J 421.28 84 86 88 90 92 94 +16.7% 0 t [ms] Q_max J 491.43 0 Q_max J 459.31 +9.0% Q_max J 506.80 +20.3% 0.85 ms -100 0.85 ms

t dQ dQ -200dQ dQ -200 ms J/ms J/ms J/ms J/ms 88.200 480.626 564.427 528.844 128.25985 86 87 88 89 90 91 84 86 88 90 92 94 96 89.050 -51.617 -60.693 -29.395 556.610 t [ms] t [ms] 0.850 -532.243 -625.120 -558.240 428.351 Created with Concerto Student Edition. Licensed for: TU Posen (a) (b) Figure 11. CombustionFigure 11. Combustion process process indicators indicators (for the the injection injection of various of fuels): various (a) summary fuels): of heat(a) releasesummary rate characteristics of heat release rate characteristicsin in the the combustion combustion chamber chamber of a rapid of compression a rapid machinecompression during the machine combustion during of one typethe ofcombustion fuel supplied byof twoone type of fuel injectors along with the maximum values for each of the test cases; (b) summary of heat release characteristics in the supplied by two injectors along with the maximum values for each of the test cases; (b) summary of heat release combustion chamber of a rapid compression machine during the combustion of one type of fuel supplied by two injectors characteristicsalong in the with combustion the maximum valueschamber for each of ofa therapid test cases.compression machine during the combustion of one type of fuel supplied by two injectors along with the maximum values for each of the test casesCreated with. Concerto Student Edition. Licensed for: TU Posen The combustion process efﬁciency was determined in the analysis of single fuel type combustion cases. The relative combustion efﬁciency values for the fuels used were Basedcalculated. on the The above results areresearch, presented it relative was found to the base that fuel alternative (gasoline): fuels supplied instead of gasoline• intoGasoline: the reference combustion value; chamber by means of direct injection increase the combustion• Ethanol: process 0.9% efficiency.more than for combustion However, of thewhen gasoline using + ethanol n-heptane mixture; and n-butanol, the combustion• N-heptane: efficiency 2.1% does less thannot forincrease the combustion monotonically of the gasoline with + n-heptane the increase mixture; of their share in • N-butanol: 1.7% less than for the combustion of the gasoline + n-butanol mixture. the fuel mixture. There is a certain non-zero ratio of gasoline content to the content of The combustion of the same fuels makes the high density of n-butanol even more each oneprominent. of these It fuels did not in show the such mixture different for properties which whenthe highest mixed with combustion gasoline (point efficiency 3.3). value is obtainedThe. combustion of homonymous fuels also corresponds to the high initial heat release rates seen for n-hexane and ethanol. Such combustion process characteristics were also repeated for the analysis of heat release. 3.5. Optical Analysis of the Combustion Process of One Fuel Type Supplied with Two Injectors Optical tests of the combustion process were carried out in parallel with the indicator tests. Images from optical tests of fuel combustion are shown in Figure 12. These images indicate significant differences in the flame development depending on the fuel used. The quantitative analysis of the flame development was carried out in the cross-sectional view of the combustion chamber. Figure 13a presents the change in time of the percentage share of the area occupied by the flame in the observed cross-section image of the combustion chamber. The analysis of the presented characteristics of the flame surface area for the stratified, homonymous fuel mixtures showed that the nature of combustion was similar. This was due to the uniform reactivity of the fuel in the combustion chamber. Another observation made was the significantly reduced flame proportion in the combustion of n-butanol compared to the other tested fuels. In order to better understand the combustion process in these cases, image analysis and the results of indicator tests were used. The relationship between the equivalent radius and the heat release rate was assessed. In most typical fuel mixtures, this relation is linear [36]. The correlation results are shown in Figure 13b.

Energies 2021, 14, 2729 17 of 20

Based on the above research, it was found that alternative fuels supplied instead of gasoline into the combustion chamber by means of direct injection increase the combustion process efficiency. However, when using n-heptane and n-butanol, the combustion efficiency does not increase monotonically with the increase of their share in the fuel mixture. There is a certain non-zero ratio of gasoline content to the content of each one of these fuels in the mixture for which the highest combustion efficiency value is obtained.

3.5. Optical Analysis of the Combustion Process of One Fuel Type Supplied with Two Injectors Optical tests of the combustion process were carried out in parallel with the indicator tests. Images from optical tests of fuel combustion are shown in Figure 12. These images indicate significant differences in the flame development depending on the fuel used. The quantitative analysis of the flame development was carried out in the cross-sectional view of the combustion chamber. Figure 13a presents the change in time of the percent- Energies 2021, 14, 2729 17 of 20 age share of the area occupied by the flame in the observed cross-section image of the combustion chamber.

t t [ms] B+B E+E H+H Bu+Bu [ms] B+B E+E H+H Bu+Bu

0 14

2 16

4 18

6 20

8 22

10 24

12 26

FigureFigure 12.12. The flameflame development process process recorded recorded during during optical optical experimental experimental tests tests for for single single fuel fuel type combustion configurations. type combustion configurations. The analysis of the presented characteristics of the flame surface area for the stratified, 100 homonymous fuel mixtures showed25 that the nature of combustion was similar. This was –1-13%3% due to the uniform reactivity of the fuel in the combustion chamber. 80 Another observation made was20 the significantly reduced flame proportion in the combustion–4-40%0% of n-butanol compared to the other tested fuels. In order to better understand 60 the combustion process in these cases,15 image analysis and the results of indicator tests

were used. The relationship between[mm] the equivalent radius and the heat release rate was

z A [%] A 40 assessed. In most typical fuel mixtures,r 10 this relation is linear [36]. The correlation results are shown in Figure 13b. Gasoline + gasoline Gasoline + gasoline 20 5 Ethanol + ethanol Ethanol + ethanol N-heptane + n-heptane N-heptane + n-heptane N-butanol + N-butanol N-butanol + N-butanol 0 0 0 10 20 30 0 200 400 600 800 dQ/dt [J/ms] t [ms] (a) (b) Figure 13. Combustion process indicators for the same fuels: (a) comparison of the time characteristics of the percentage share of the combustion chamber cross-sectional area occupied by the flame for the combustion of a single fuel type supplied with both injectors; (b) the relationship between the equivalent radius of the flame area and the heat release rate during the combustion of single fuel types supplied with both injectors.

According to the analysis of the relationship between the equivalent radius and the heat release rate for one type of fuel, a linear relationship was found for gasoline and n-heptane. For the remaining fuels, these values were not directly proportional to each other.

Energies 2021, 14, 2729 17 of 20

t t [ms] B+B E+E H+H Bu+Bu [ms] B+B E+E H+H Bu+Bu

0 14

2 16

4 18

6 20

8 22

10 24

12 26

Energies 2021, 14, 2729 Figure 12. The flame development process recorded during optical experimental18 oftests 20 for single fuel type combustion configurations.

100 25 –1-13%3%

80 20 –4-40%0%

60 15

[mm]

z A [%] A 40 r 10

Gasoline + gasoline Gasoline + gasoline 20 5 Ethanol + ethanol Ethanol + ethanol N-heptane + n-heptane N-heptane + n-heptane N-butanol + N-butanol N-butanol + N-butanol 0 0 0 10 20 30 0 200 400 600 800 dQ/dt [J/ms] t [ms] (a) (b) Figure 13. CombustionFigure 13. Combustion process indicators process indicators for the for thesame same fuels: fuels: ((aa)) comparison comparison of the of time the characteristics time characteristics of the percentage of the percentage share of the combustionshare of the combustion chamber chamber cross cross-sectional-sectional area area occupied occupied by the by flame the for flame the combustion for the of a combustion single fuel type of supplied a single fuel type with both injectors; (b) the relationship between the equivalent radius of the flame area and the heat release rate during the supplied with both injectors; (b) the relationship between the equivalent radius of the flame area and the heat release rate combustion of single fuel types supplied with both injectors. during the combustion of single fuel types supplied with both injectors. According to the analysis of the relationship between the equivalent radius and the Accordingheat release to rate the for analysis one type of of fuel,the relationship a linear relationship between was foundthe equivalent for gasoline radius and n- and the heat heptane. For the remaining fuels, these values were not directly proportional to each other. release rate for one type of fuel, a linear relationship was found for gasoline and n-heptane. For the 4.remaining Conclusions fuels, these values were not directly proportional to each other. The results of the combustion process analysis made it possible to unequivocally confirm the combustion system using direct injection with two injectors as a valid method for selecting the air–fuel mixture. The performer analysis led to the following conclusions: 1. The use of different fuels and their mixtures formed using two injectors has a significant impact on the combustion process indicators. It was considered that differences in the physicochemical properties of fuels determine the reactivity of the resulting mixture, which translates into differences in the flame development and the resulting thermodynamic values of the combustion process; 2. Mixtures consisting of gasoline and other fuel types were characterized by a greater combustion efficiency than mixtures composed of only one fuel type. The only observed exception was for the injection of ethanol—in this system the combustion efficiency was greater than for the combustion of gasoline with ethanol, as well as for gasoline only; 3. The combustion of gasoline and n-butanol could be described in relation to the combustion of gasoline alone as follows: • The maximum cylinder combustion pressure was 10% greater; changes in relation to other fuel mixtures reached values of up to 4%; • The pressure increase was 17% greater; combustion of the remaining mixtures indicated changes in this value of up to 7%; • The maximum heat release rate was 22% faster; combustion of other fuels showed smaller differences of up to 8%; Energies 2021, 14, 2729 19 of 20

• The maximum total amount of heat released was 22% greater; changes of this ratio for other fuels were 13% (gasoline and ethanol) and 8% (gasoline and n-heptane); 4. Among the tested fuel configurations, the gasoline + n-butanol configuration was the most efficient; the efficiency value for this test case was measured at 27.8% and was 6.1% greater than for the base configuration, i.e., with the use of gasoline only; 5. Using a mixture formed directly in the combustion chamber increased the possibility for quality control of the fuel mixture; however, this indicated the need for technical development of fuel supply systems and their control solutions; 6. The use of two types of fuel with different physicochemical properties supplied using two separate injectors could be another method for selecting the qualitative fuel mixture during engine supply (mainly in the cases of partial engine load conditions).

Author Contributions: Conceptualization, I.P., S.W., M.S. and D.P.; methodology, I.P. and M.S.; validation, I.P., S.W. and D.P.; formal analysis, I.P., M.S. and D.P.; investigation, I.P., S.W., M.S., and D.P.; resources, M.S. and D.P.; data curation, I.P. and M.S.; writing—original draft preparation, I.P., S.W., M.S. and D.P.; writing—review and editing, I.P., S.W., M.S. and D.P.; visualization, I.P., S.W. and M.S.; supervision, S.W. and D.P.; project administration, I.P., S.W., M.S. and D.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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