energies

Article Assessment of the Environmental Impact of Using Methane Fuels to Supply Internal Combustion Engines

Krzysztof Biernat 1,* , Izabela Samson-Br˛ek 2 , Zdzisław Chłopek 3, Marlena Owczuk 1 and Anna Matuszewska 1

1 Łukasiewicz Research Network—Automotive Industry Institute, 55 Jagiello´nskaStr., 03-301 Warsaw, Poland; [email protected] (M.O.); [email protected] (A.M.) 2 Department of Environmental Chemistry and Risk Assessment, Institute of Environmental Protection—National Research Institute, 55/11D Krucza Str., 00-548 Warsaw, Poland; [email protected] 3 Faculty of Automotive and Construction Machinery Engineering, Warsaw University of Technology, Narbutta 84, 02-524 Warsaw, Poland; [email protected] * Correspondence: [email protected]

Abstract: This research paper studied the environmental impact of using methane fuels for supplying internal combustion engines. Methane fuel types and the methods of their use in internal combustion engines were systematized. The knowledge regarding the environmental impact of using methane fuels for supplying internal combustion engines was analyzed. The authors studied the properties of various internal combustion engines used for different applications (specialized engines of power generators—Liebherr G9512 and MAN E3262 LE212, powered by biogas, engine for road and off-road



 vehicles—Cummins 6C8.3, in self-ignition, original version powered by diesel fuel, and its modified version—a spark-ignition engine powered by methane fuel) under various operating conditions in Citation: Biernat, K.; Samson-Br˛ek,I.; approval tests. The sensitivity of the engine properties, especially pollutant emissions, to its operating Chłopek, Z.; Owczuk, M.; states were studied. In the case of a Cummins 6C8.3 modified engine, a significant reduction Matuszewska, A. Assessment of the in the pollutant emission owing to the use of methane fuel, relative to the original self-ignition Environmental Impact of Using engine, was found. The emission of carbon oxide decreased by approximately 30%, hydrocarbons Methane Fuels to Supply Internal Combustion Engines. Energies 2021, by approximately 70% and nitrogen oxide by approximately 50%, as well as a particulate matter 14, 3356. https://doi.org/10.3390/ emission was also eliminated. Specific brake emission of carbon oxide is the most sensitive to the en14113356 operating states of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with the least sensitive being specific brake emission of nitrogen oxide: 0.121 for a self-ignition Academic Editor: Frede Blaabjerg engine and 0.097 for a spark-ignition engine. The specific brake emission of carbon monoxide and hydrocarbons for stationary engines was higher in comparison with both versions of Cummins Received: 6 May 2021 6C8.3 engine. However, the emission of nitrogen oxide for stationary engines was lower than for Accepted: 2 June 2021 Cummins engines. Published: 7 June 2021 Keywords: alternative fuels; methane fuels; biogas; biomethane; approval tests; emission of Publisher’s Note: MDPI stays neutral pollutants; sensitivity of engines with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction The observed growth of the alternative fuel market is associated with priorities in the field of environmental protection and the tendency to implement the idea of sustainable Copyright: © 2021 by the authors. development and circular economy in all branches of the economy. They are driven Licensee MDPI, Basel, Switzerland. by global trends that have quantified targets for progressively reducing greenhouse gas This article is an open access article distributed under the terms and emissions and energy intensity, as well as increasing the share of renewable energy sources conditions of the Creative Commons and their use in transport [1,2]. Such idea additionally enables a partial independence on Attribution (CC BY) license (https:// fossil fuels due to replacement of part of fossil fuels by alternative ones. For this reason, creativecommons.org/licenses/by/ the development of engine fuels obtained from renewable sources is very important. There 4.0/). are many different types of compounds which can be used as a substitute for conventional

Energies 2021, 14, 3356. https://doi.org/10.3390/en14113356 https://www.mdpi.com/journal/energies Energies 2021, 14, 3356 2 of 19

fuels, e.g., bioethanol [3,4], dimethyl ether (DME) [5,6], fatty acid methyl ester (FAME) [7,8], methane [9,10] and hydrogen [11–13]. These compounds can be produced with the use of various methods and raw materials. Recently, methane fuels have become very popular and are promoted as low-emission fuels. These fuels include: • Fuels from a fossil raw material—natural gas, purified to a standard required for various applications, i.e., as engine fuel—the name natural gas, the same as the raw material they originate from, has been adopted for this type of fuels. • Biogas fuel—biogas cleaned of specific pollutants, primarily hydrogen sulfide, water and silicon compounds, excluding, i.e., carbon dioxide. • Fuel obtained from biogas purified to a natural gas fuel standard—so-called biomethane. Biogas is a product derived from biomass (a renewable energy source), produced in the course of a process of anaerobic organic substance decomposition, by a diverse popula- tion of microorganisms. The physical and chemical properties of biogas and its availability are the key reasons behind its application as an alternative engine fuel, reducing the de- pendence on fossil fuels and diversifying energy sources. It can be used for producing thermal or electrical energy or combined cycle energy production (CHP—combined heat and power) in cogeneration [14] or trigeneration [15] systems. After relevant purification to biomethane, it can be pumped into a natural gas distribution network or be used as an internal combustion engine fuel [16]. It is also possible to use biogas for producing syn- gas [17], which is a semi-finished product in the processes of producing, e.g., methanol [18], hydrogen [19] and in the BTL (biogas-to-liquid) process of hydrogen synthesis [20]. Biomethane and natural gas are used in [21–23]: • Compressed form (compressed natural gas—CNG, and compressed biomethane —BCNG). • Liquefied form (liquefied natural gas—LNG, and liquefied biomethane—BLNG). • Adsorbed form (adsorbed natural gas—ANG, and adsorbed biomethane—BANG). The feasibility of using methane fuels for powering internal combustion engines is determined by several parameters, where the most important ones include: chemical com- position (methane concentration, pollutant content, etc.), resistance to knock combustion (determined based on the methane number and octane number), ignition and self-ignition temperatures, fuel stoichiometric constant, combustion rate of the fuel–air mix and energy value measured by the calorific value or the Wobbe index.

2. Directions of Research in the Field of Using Methane Fuels for Supplying Internal Combustion Engines Based on the conducted overview of the available source literature, the authors concluded that due to the widespread availability of natural gas on the market, most of the conducted studies involved this type of methane fuel. The studies concerned, among others, the application of natural gas in the automotive industry, in particular: • Impact of fuel on the performance of engines, in particular, the operating parameters of spark-ignition (SI) engines (torque, exhaust gas temperature, etc.), depending on the mixture composition and ignition advance angle, compared to the case of supplying self-ignition (CI) engines with conventional fuel—diesel fuel [24]. • Evaluation of fuel combustion in a spark-ignition engine or a self-ignition dual-fuel engine, with the addition of other gases, e.g., hydrogen [25]. • Impact of changing the fuel initiating mixture ignition, from diesel fuel to, e.g., biodiesel [26–28], biodiesel with ethanol [29,30] or diesel fuel with ethanol or with methanol [31–33]. • Emission of toxic components of exhaust gases [34–36] and the environmental impact throughout the entire life cycle of gaseous fuel (from raw material acquisition, through manufacturing, until its use within an engine [37,38]). • Economic effects of fueling an engine with CNG [39,40]. Energies 2021, 14, 3356 3 of 18

• Emission of toxic components of exhaust gases [34–36] and the environmental impact throughout the entire life cycle of gaseous fuel (from raw material acquisition, through manufacturing, until its use within an engine [37,38]). Energies 2021, 14, 3356 3 of 19 • Economic effects of fueling an engine with CNG [39,40]. Research papers on biomethane are significantly scarcer. This is due to the fact that in many countries, due to the lack of infrastructure for upgrading biogas to biomethane, Research papers on biomethane are significantly scarcer. This is due to the fact that in this fuel ismany still countries,unavailable. due toThe the conduct lack of infrastructureed research for work upgrading primarily biogas concentrated to biomethane, on this the impact offuel biomethane is still unavailable. on the The environment conducted research (the workemission primarily of particulates concentrated onand the impactgaseous substances,of biomethanein particular), on the relative environment to fuel (the emissionwith natural of particulates gas [41,42]. and gaseous Authors substances, of [43] concludedin that particular), the use relative of methane to fuel fuels with significantly natural gas [41 reduces,42]. Authors emissions of [43] concludedcompared that to fossil the fuels. Theuse GHG of methane emissions fuels for significantly gasoline, reduces diesel emissionsfuel andcompared LPG in the to fossil whole fuels. life The cycle GHG are emissions for gasoline, diesel fuel and LPG in the whole life cycle are estimated at 164, 156 estimated at 164, 156 and 141 g CO2-eq/km, respectively (Figure 1). For biomethane, it is and 141 g CO2-eq/km, respectively (Figure1). For biomethane, it is only from 33 to 66 g 2 only fromCO 332 -eq/kmto 66 g (depending CO -eq/km on (depending the raw material on used),the raw and ma forterial natural used), gas, 124 and g CO for2/km natural in gas, 124 gthe CO whole2/km lifein the cycle. whole life cycle.

Figure 1. ComparisonFigure 1. Comparison of GHG emissions of GHG emissions from passenger from passenger cars fueled cars fueled with with different different fuels fuels [43]. [43].

The reduction of greenhouse gas emissions by biomethane fuel throughout the life The reductioncycle compared of greenhouse to the use of conventionalgas emissions fuels by is biomethane due not only to fuel the combustionthroughout process the life cycle comparedin the engine to the but use also of to conventional the technology fuels of its is production due not [only44,45 ].to The the type combustion of substrate process used in the enginefor methane but also fermentation to the technology has the greatest of its impact production on the reduction [44,45]. ofThe GHG type emission of substrate in the used for methanewhole life fermentation cycle. For biomethane has the producedgreatest impact from arable on the crops reduction such as maize, of GHG greenhouse emission in the wholegas specific life cycle. distance For emission, biomethane expressed produced as CO2-eq, from is 66 g/km,arable of crops which such more thanas maize, 50% is attributed to cultivation (nitrogen fertilization) and harvesting of raw materials (fuel greenhouse gas specific distance emission, expressed as CO2-eq, is 66 g/km, of which more combustion), and 28% to the process of purification and upgrading of biogas to natural gas than 50%quality. is attributed Greenhouse to gascultivation emission, thanks(nitrogen to the fertilization) use of biomethane and produced harvesting from edibleof raw materials raw(fuel materials, combustion), can therefore and be28% reduced to the by process approximately of purification 60% compared and to upgrading gasoline [43]. of biogas to Thenatural use of gas manure quality. (waste Greenhouse material) as gas input emission, for the production thanks to of the biogas use reducesof biomethane green- producedhouse from gas edible emission raw by materials, approximately can 80% th comparederefore be to gasoline,reduced and by theapproximately use of biomethane 60% comparedfrom to gasoline municipal [43]. waste The reduces use of greenhouse manure (waste gas emission material) by approximately as input for 73%the production compared to fossil fuels. For biomethane from municipal waste, the standard assumption is that of biogas reduces greenhouse gas emission by approximately 80% compared to gasoline, the emission factor expressed in CO2-eq is 83.8 g/MJ, according to the Renewable Energy and the useDirective of biomethane (RED) [46]. from This means municipal a reduction waste in reduces greenhouse greenhouse gas emission gas per emission kilometer by approximatelydriven 73% by around compared 70% compared to fossil to fuels. a petrol-powered For biomethane passenger from car. municipal If, on the other waste, hand, the standard duringassumption the production is that of the biomethane, emission the factor places ofexpressed uncontrolled in methaneCO2-eq emission, is 83.8 suchg/MJ, according to the Renewable Energy Directive (RED) [46]. This means a reduction in greenhouse gas emission per kilometer driven by around 70% compared to a petrol-

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as the method of storing raw materials and digestate (storage in closed tanks) are delimited to a minimum, and the emissions associated with the process of methane fermentation of animal manure (slurry and manure) are taken into account, then we can talk about GHG emission in the entire life cycle close to zero or below this value. Apart from research on the use of natural gas and biomethane for road vehicles, work is also carried out on its use for off-road vehicles [47,48]. Researchers study their application in the engines of: trucks [49–51], water vessels [52,53], traction locomotives [18,54] or tractors and agriculture machines (AGCO Valtra [55], Same Deutz-Fahr [56], New Holland [57] and others [58,59]). The main factor determining the possibility of using biogas as a raw material for the production of various energy carriers is the high content of methane (over 60%). Table1 presents the differences in the composition of raw biogas, biogas after purification (biomethane) and natural gas.

Table 1. Differences in the composition of raw biogas, biomethane and natural gas [60,61].

Gas Component. Raw Biogas Biomethane Natural Gas Methane, (%) 45–70 94.0–99.9 93–98 Carbon dioxide, (%) 25–45 0.1–4.0 1 Nitrogen, (%) <3 <3 1 Oxygen, (%) <2 <1 - Hydrogen, (%) <1 traces - Hydrogen sulfide, (ppm) 20–20,000 <10 - Ammonia, (%) traces traces - Ethane, (%) - - <3 Propane, (%) - - <2 Siloxane, (%) traces - - Water, (%) 2–7 - -

Owing to its chemical composition, as well as the need for refining, biogas is only used as a biogas fuel for producing electricity and/or thermal energy, i.e., to supply stationary self-ignition or spark-ignition engines, which are a component of a cogeneration plant [62]. These engines differ from vehicle engines in terms of design, adapted to constant operating conditions. The ongoing studies in the field of using biogas fuel to supply these engines concern, in particular, the impact of biogas composition on the performance of the engine [63–65] and the exhaust gas component emissions [66–70]. Moreover, the influence of the concentration of hydrogen added to biogas fuel on the ecological and energy parameters of an engine [71] and the application of ignition-initiating heavy fuel other than diesel fuel, e.g., biodiesel [72], were also studied. Methane fuels, due to their chemical composition (carbon and hydrogen content), enable greater reduction of carbon dioxide emission relative to conventional fuels. Fur- thermore, since they are gaseous fuels, they enable the combustion of more homogeneous mixtures than in the case of liquid fuels. This allows limiting the emission of gaseous pollutants and particulates [73,74]. Owing to the decreased heat generated in the kinetic phase of combustion, emission of nitrogen oxides from an engine dual-fueled by natural gas and diesel fuel can be limited compared to an engine supplied with diesel fuel, depending on the composition of the fuel mixture, excess air coefficient and the adjustment of the fuel pilot dose injection angle [75,76]. Due to the lack of self-ignition ability in gaseous fuels, they are mostly used in spark-ignition engines. On the market are vehicles with modern gas-fuel engines, which are factory-adapted to run solely on gaseous fuel (so-called NGVs—Natural Gas Vehicles). In case of vehicles without a gas-fuel engine, they can be adapted to run on gaseous fuel by [18,75,77–80]: • Adapting a gasoline-fueled spark-ignition engine to run on gaseous fuel (a relatively simple procedure involving the installation of an additional gas supply system, as well as controllers for the engine operating processes, especially the fuel dose). Energies 2021, 14, 3356 5 of 19

• Converting a self-ignition engine to a spark-ignition engine—a costly and complicated engine modification, which involves replacing the injector with a spark plug, imple- menting a gas fuel ignition system, controllers for the engine operating processes (fuel mixture composition, fuel dose, ignition angle of advance, etc.), throttle in the suction system, the catalytic reactor in the outlet system and decreasing the compression ratio. The implemented modifications enable a vehicle to run only on gaseous fuel. • Changing a self-ignition engine’s supply system from single-fuel to dual-fuel (the engine will then be supplied with gaseous fuel and a pilot dose of heavy fuel, primarily diesel fuel). In some cases, for example for large fleets of vehicles, the adaptation of currently used vehicles (with conventional self-ignition engines) to run on gaseous fuels is more economically justified than replacing the entire fleet with new vehicles equipped with gas engines. Conversion of a self-ignition engine into a spark-ignition engine is a solution which has been especially applied in buses and trucks. After modifications in ignition systems (installation of spark ignition equipment) and fuel injection, the vehicle can be bi-fueled (with methane fuel and gasoline) or fueled only with methane fuel. Studies on such engine modification can be found in the literature [81,82], where the performance of engine and harmful gas emissions were investigated. However, the studies conducted so far do not cover the topic of the sensitivity of the pollutant emission to the operating states of the engines, determined by the operating conditions of the device powered by the engine. The measure of the sensitivity of a physical quantity to engine operating states is the coefficient of variation (ratio of the mean standard deviation to the mean value) of that quantity for various engine operating states. By the operating states of engines in thermally stabilized conditions (heated engines), one should understand the engine speed and the value characterizing the load—most often the torque. The largest set of information on the emission of pollutants from combustion engines supplied with methane fuels concerns the approval test results. The operating states of internal combustion engines in the course of approval tests often vary greatly from their real operating conditions. For example, the operating states of combustion engines in city buses are characterized by significantly lower values of the average engine speed and load, and a higher share of idling time, than in the case of approval in static and dynamic tests [83,84]. Similarly, there are large differences between the actual operating states of combustion engines of construction machinery and the operating states defined in approval tests [85]. The authors have not found a comparison of empirical assignments of engines in different operating states corresponding to different conditions of engine application in the world’s literature so far. Moreover, to the best of the authors’ knowledge, the results of comparative studies of self-ignition engines converted to spark-ignition engines supplied with methane fuel, in various operating states, corresponding to actual operating conditions of these engines in vehicles and other applications, have not been reported elsewhere. For this reason, the authors conducted such tests and compared the obtained results. It was decided to investigate the sensitivity of operational properties to various states of engine operation, e.g., in motor vehicles and working machines. The functional properties of internal combustion engines for driving power generators in an agricultural biogas plant were also investigated. The authors decided to fill this research gap by empirically testing the engine, before and after such conversion, in four different tests. It allowed to assess the sensitivity of the engine properties, especially pollutant emissions, to its operating states. The significance of this lies in the fact that the conversion to methane fuel is usually performed for engines used in heavy-duty applications: buses, trucks, stationary machines and power generators that differ largely in operating conditions. The results presented in this paper broaden the knowledge of the real benefits of converting engines to run on methane fuels, precisely in specific applications, and not only under the conditions defined in the law. Energies 2021, 14, 3356 6 of 19

3. Materials and Methods 3.1. Introduction to Experiments The research was conducted with the use of the conventional fuel (diesel fuel) and two methane fuels with different methane content (natural gas and biogas containing approximately 97% and 60% of methane, respectively). It was planned to conduct empirical research on biogas and biomethane. Due to the lack of availability of biomethane in the Polish market, its fossil-based counterpart—natural gas—was used for the research. The results obtained for natural gas can be treated as representative for biomethane, because both fuels have similar physicochemical properties and are used interchangeably as engine fuel. Different content of methane in tested fuels enabled to evaluate the performance properties of these fuels. The research methodology used in the work is an original conception of the authors. The idea of this methodology is to study the properties of various internal combustion engines used for different applications (specialized engines of power generators, engines for road and off-road vehicles) under various operating conditions, including in the conditions of approval tests. Two cogeneration engines (Liebherr G9512 and MAN E3262 LE212) and one universal engine used in road and off-road vehicles as well as machines, aggregates, etc. (Cummins 6C8.3), were selected for approval tests. Research tests were carried out in two stages. Empirical tests of internal combustion engines were performed in certified laboratories. The apparatus used met the requirements of: Directive 1999/96/EC of the European Parliament and of the Council of 13 December 1999, Regulation (EC) No. 715/2007 of the European Parliament and of the Council of 20 June 2007 and Regulation (EC) No. 692/2008 of the European Parliament and of the Council of 18 July 2008, and ISO 3046-1: 2002, ISO 3046-3: 2006, ISO 8178-3: 1994 and ISO 8178-10: 2002.

3.2. Stage One In the first stage, comparative tests of the original Cummins 6C8.3 engine and the same engine after its conversion were performed. An original Cummins 6C8.3, turbocharged, 6-cylinder engine with a displacement volume of 8.3 dm3 has a rated effective power of 153 kW and an engine speed of 1950 min–1. The idle engine speed is 750 min–1. The Cummins 6C8.3 engine is a self-ignition, diesel-fueled engine. In its modified versions, it was a spark-engine fueled with natural gas, set to run on a stoichiometric mixture. The conversion of the original diesel engine into a spark-ignition one included: change of the gas fuel supply system, change in the engine control system (concerning fuel mixture composition, fuel dose, ignition timing, etc.), installation of a spark-ignition system and use of a multi-functional catalytic reactor in the exhaust system. The Cummins 6C8.3 engine is originally intended for the drive of motor vehicles, and in the modified configuration for the drive of working machines. These are the engine operating conditions, characterized by a relatively low relative load (in relation to the maximum engine load). The internal combustion engine was studied on a test bench (Figure2) equipped, among others, with: • An AFA-E 460/4,4-9 EU electric brake by AVL, • A T10F digital torque and engine speed meter by HBM, • A CEB II exhaust gas analyzer assembly by AVL, • A system for measuring particulate emission comprised of a Smart Sammler SPC partial flow dilution tunnel by AVL and a MT5 microscale by Mettler Toledo, • A 735-type system for measuring fuel consumption with a device for stabilizing fuel temperature by AVL, • A Sensyflow P system for measuring air consumption. Energies 2021, 14, 3356 7 of 18

Energies 2021, 14, 3356 • A Sensyflow P system for measuring air consumption. 7 of 19

FigureFigure 2.2.Scheme Scheme ofof thethe testtest bench,bench, where:where: 1—exhaust1—exhaust outlet,outlet, 2—air2—air inlet,inlet, 3—turbocharger,3—turbocharger, 4—crankshaft4—crankshaft angleangle sensor,sensor, 5—gas fuel injector, 6—boost pressure sensor, 7—fuel electronic dosage control module, 8—cylinder pressure sensor, 9— 5—gas fuel injector, 6—boost pressure sensor, 7—fuel electronic dosage control module, 8—cylinder pressure sensor, engine, 10—thermocouple, 11—knocking combustion sensor, 12—heating pipes supplying exhaust gas to the analyzers, 9—engine, 10—thermocouple, 11—knocking combustion sensor, 12—heating pipes supplying exhaust gas to the ana- 13—AVL electric brake, 14—liquid fuel tank, 15—methane fuel tanks, 16—computer, 17—exhaust gas analyzers, 18— lyzers,reference 13—AVL gases. electric brake, 14—liquid fuel tank, 15—methane fuel tanks, 16—computer, 17—exhaust gas analyzers, 18—reference gases. The instrumentations used for the tests complied with the requirements of Directive The instrumentations used for the tests complied with the requirements of Directive 97/68/EC. The tests of the natural gas-fueled engine and the self-ignition engine involved 97/68/EC. The tests of the natural gas-fueled engine and the self-ignition engine involved the determination of: the determination of: • External speed characteristics, • External speed characteristics, • Specific brake emission within test studies. • Specific brake emission within test studies. The test studies, which concerned the pollutant emission (carbon oxide—CO, The test studies, which concerned the pollutant emission (carbon oxide—CO, hydrocarbons hydrocarbons—HC, particulate matter—PM and nitrogen oxides—NOx), corresponded to —HC, particulate matter—PM and nitrogen oxides—NOx), corresponded to the static the static operating states of the engine. The basis test is the D2 test for studying the operating states of the engine. The basis test is the D2 test for studying the emission of emission of pollutants from internal combustion engines used to drive power generators pollutants from internal combustion engines used to drive power generators with variable load,with asvariable per the load, ISO as 8178-4 per the standard ISO 8178-4 [86]. standard For comparative [86]. For purposes,comparative the purposes, authors also the conductedauthors also studies conducted within studies other staticwithin tests other [84 static,87]: tests [84,87]: • • ECEECE RR 49.02—the 49.02—the so-called so-called thirteen-phase thirteen-phase test, te inst,accordance in accordance with with regulation regulation No. ECE No. RECE 49, forR 49, testing for testing car engines. car engines. • • ESCESC (European(European StationaryStationary Cycle),Cycle), inin accordanceaccordance withwith regulationregulation No.No. ECEECE RR 49,49, forfor testingtesting carcar engines.engines. • • NRSCNRSC (Non-Road(Non-Road StationaryStationary Cycle),Cycle), identicalidentical toto thethe C1C1 testtest accordingaccording toto ISOISO 8178-4,8178-4, forfor testingtesting self-ignitionself-ignition enginesengines inin machinery.machinery. Schematic representations of the engines’ test bench are shown in Figure3.

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Schematic representations of the engines’ test bench are shown in Figure 3.

(a) (b)

Figure 3.Figure Scheme 3. Scheme of the test of the bench test bench with with the thetested tested engines: engines: (a (a)) original original CumminsCummins 6C8.3 6C8.3 engine, engine, and (andb) modified (b) modified Cummins Cummins 6C8.3 engine.6C8.3 engine. 3.3. Stage Two 3.3. Stage Two In the second stage, the Cummins 6C8.3 engine (which is a universal engine that canIn the beused second as a stage, drive for the vehicles Cummins and machinery6C8.3 engi andne power(which generators) is a universal was compared engine that can be usedwith as two a drive cogeneration for vehicles engines and used machinery in biogas plants.and power The Cumminsgenerators) 6C8.3 was engine compared was with two poweredcogeneration by diesel engines fuel (self-ignition used in biogas version) plants. and The natural Cummins gas (version 6C8.3 after engine modification was powered by dieselfor spark-ignition). fuel (self-ignition The mainversion) reason and for natural the conversion gas (version of a self-ignitionafter modification engine to for a spark- spark-ignition engine to methane fuel was the need to reduce the emission of pollutants, ignition).especially Theparticulate main reason matter for and the nitrogen conversion oxides. of a self-ignition engine to a spark-ignition engine toTwo methane variants fuel of this was engine the wereneed compared to reduce with the Liebherr emission G9512 of andpollutants, MAN E3262 especially particulateLE212 engines. matter Theand tested nitrogen Liebherr oxides. and MAN are turbocharged, spark-ignition engines, highlyTwo variants specialized of withthis aengine large displacement were compared volume, with adapted Liebherr to work G9512 on aand very MAN lean E3262 LE212mixture. engines. They The are usedtested to driveLiebherr power and generators MAN inare agricultural turbocharged, biogas plants.spark-ignition In the CHP engines, highlycase, specialized alternative dieselwith enginesa large are displacement not used. Moreover, volume, CHP adapted engines are to not work supplied on a with very lean a natural gas, so during the tests, they were fueled with biogas (containing approximately mixture.60% ofThey methane). are used Table to2 driveshows power the technical generators data of in tested agricultural turbocharged biogas Liebherr plants. G9512 In the CHP case,and alternative MAN E3262 diesel LE212 engines stationary are CHP not engines. used. Moreover, CHP engines are not supplied with a natural gas, so during the tests, they were fueled with biogas (containing approximatelyTable 2. Technical 60% data of methane). of tested Liebherr Table G9512 2 shows and MAN the technical E3262 LE212 data stationary of tested CHP turbocharged engines Liebherrused inG9512 agricultural and biogasMAN plants. E3262 LE212 stationary CHP engines. Title 1 Unit Liebherr G9512 MAN E3262 LE212 Table 2. Technical data of tested Liebherr G9512 and MAN E3262 LE212 stationary CHP engines used in agricultural Cylinder arrangement – 12 V 12 V biogas plants. Displacement volume dm3 25.0 25.8 Compression ratio – 13.3 13.6 Title 1 Rated engine speed min−1 Unit Liebherr1500 G9512 MAN 1500 E3262 LE212 Cylinder arrangementRated effective power kW – 515 12 V 550 12 V Brake mean effective MPa3 1.65 1.71 Displacementpressure volume in nominal rating dm 25.0 25.8 Compression Excessratio air coefficient – – 1.6 13.3 1.6 13.6 Rated engine speed min−1 1500 1500 Rated effective Inpower order to compare engines used for operationkW in different 515 conditions and fueled 550 Brake mean effective pressureby different in nominal fuels, the rating D2 test in line with ISO MPa 8178-4 was chosen. 1.65 This test is a basic 1.71 test for studying the overall efficiency of the emission of pollutants from internal combustion Excess air enginescoefficient used to drive power generators with – variable load. The 1.6 instrumentation used 1.6 in the tests complied with the requirements of approval tests. The tests were conducted in the laboratoriesIn order to of compare engine manufacturers. engines used Schematic for operat representationsion in different of the conditions engine test and bench fueled by differentare shown fuels, in the Figure D24 test. in line with ISO 8178-4 was chosen. This test is a basic test for studying the overall efficiency of the emission of pollutants from internal combustion engines used to drive power generators with variable load. The instrumentation used in the tests complied with the requirements of approval tests. The tests were conducted in the laboratories of engine manufacturers. Schematic representations of the engine test bench are shown in Figure 4.

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Figure 4. Scheme of the test bench with the two tested CHP internal combustion engines. FigureFigure 4. 4.Scheme Scheme of the of testthe bench test bench with the with two testedthe two CHP tested internal CHP combustion internal engines. combustion engines. 4. Results4. Results 4.1. Stage4.1.4. Results Stage One One Figure4.1. FigureStage 5 shows 5One shows the the external external speedspeed characteristics characteristics of the of torquethe torque and effective and effective power power for a self-ignition Cummins 6C 8.3 engine running on diesel fuel—designated DF—and for a self-ignitionFigure 5Cummins shows the 6C external 8.3 engine speed running characte on dieselristics fuel—designated of the torque and DF—and effective a power a Cummins 6C 8.3 engine, modified as a spark-ignition methane-fueled engine—natural Cumminsgas—designatedfor a self-ignition6C 8.3 engine, MF. Cummins modified 6C as 8.3 a spark-ignitionengine running methane-fueled on diesel fuel—designated engine—natural DF—and a gas—designatedCummins 6C MF. 8.3 engine, modified as a spark-ignition methane-fueled engine—natural gas—designated MF.

(a) (b)

Figure 5. FigureExternal 5. External speed characteristics speed characteristics(a) of Cummins of Cummins 6C 6C 8.3 8.3 engine: engine: (aa)) torquetorque and and (b )( effectiveb) effective power, (power,b) for a self-ignition, for a self-ignition, diesel-fueleddiesel-fueled engine (DF), engine and (DF), a andspark-ignition, a spark-ignition, methane methane fuel—natural fuel—natural gas—supplied gas—supplied engine engine (MF). (MF). Figure 5. External speed characteristics of Cummins 6C 8.3 engine: (a) torque and (b) effective power, for a self-ignition, diesel-fueled engine (DF), and aThe spark-ignition, average engine methane torque and fuel—natural effective power gas—supplied difference in engine the studied (MF). engines were The average engine torque and effective power difference in the studied engines were approximately 0.04, and higher torque and effective power values were recorded for the approximately 0.04, and higher torque and effective power values were recorded for the self-ignition,The average diesel-fueled engine engine. torque This and results effective primarily power from difference the better in filling the studied of the engines were self-ignition, diesel-fueled engine. This results primarily from the better filling of the cylinders,approximately since the 0.04, upgrading and higher of the engine torque to a and spark-ignition effective engine power did values not involve were the recorded for the cylinders,modification since the of the upgrading engine charging of the control engine algorithm. to a spark-ignition engine did not involve the self-ignition, diesel-fueled engine. This results primarily from the better filling of the modificationFigure of6 theshows engine the overall charging efficiency control of the algorithm. tested engines. cylinders, since the upgrading of the engine to a spark-ignition engine did not involve the Figure 6 shows the overall efficiency of the tested engines. modification of the engine charging control algorithm. In all tests, the overall efficiency was higher for a self-ignition engine fueled with Figure 6 shows the overall efficiency of the tested engines. diesel. This is also due to the better filling of the cylinders in a self-ignition engine. The In all tests, the overall efficiency was higher for a self-ignition engine fueled with fact of the relatively low sensitiveness of the engine in overall efficiency of their operating diesel. This is also due to the better filling of the cylinders in a self-ignition engine. The states varying within various tests is also significant. The measure of the sensitivity of a quantityfact toof theengine relatively operating low sensitivenessstates is the ofcoefficient the engine of in variation overall efficiency(ratio of ofthe their mean operating standardstates deviation varying to within the value) various of thattests quantity is also significant. for various Theengine measure states. ofThe the variation sensitivity of a coefficientquantity for theto overallengine efficiencyoperating in states all te stsis thewas coefficient0.026 for the of self-ignition variation (ratio engine of and the mean 0.029 standardfor the spark-ignition deviation to engine. the value) of that quantity for various engine states. The variation coefficient for the overall efficiency in all tests was 0.026 for the self-ignition engine and 0.029 for the spark-ignition engine.

Energies 2021, 14, 3356 10 of 18 Energies 2021, 14, 3356 10 of 19

D2

NRSC Test ESC

MF ECE R 49.02 DF

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 η e FigureFigure 6. 6. General efficiency efficiency in a self-ignitionin a self-ignition Cummins Cummins 6C 8.3 engine 6C running 8.3 engine on diesel running fuel (DF), on diesel fuel (DF), andand a aspark-ignition spark-ignition Cummins Cummins 6C 8.3 6C engine 8.3 runningengine onrunning methane on fuel—natural methane gasfuel—natural (MF). gas (MF).

In all tests, the overall efficiency was higher for a self-ignition engine fueled with diesel.Figure This is7 shows also due the to the specific better fillingbrake of emission the cylinders of pollutants in a self-ignition for a engine. self-ignition The Cummins 6C 8.3fact engine of the relativelyrunning low on sensitivenessdiesel fuel, of and the enginea spark-ignition in overall efficiency Cumm ofins their 6C operating 8.3 engine running on methanestates varying fuel—natural within various gas. tests The is particulate also significant. matte Ther measure emission of the from sensitivity the engine of a supplied with quantity to engine operating states is the coefficient of variation (ratio of the mean standard naturaldeviation gas to thewas value) on ofthe that boundary quantity for of various measurement engine states. accuracy, The variation therefore, coefficient these tests were discarded,for the overall assuming efficiency for in allcomparative tests was 0.026 purposes for the self-ignition that the specific engine and brake 0.029 emission for the of particulate matterspark-ignition from the engine. engine supplied with natural gas was 0. Although there is no practical possibilityFigure 7of shows comparing the specific the brake particulate emission ofmatter pollutants emissions for a self-ignition from the Cummins tested engines, it was 6C 8.3 engine running on diesel fuel, and a spark-ignition Cummins 6C 8.3 engine running decidedon methane to present fuel—natural the results gas. The fo particulater a self-ignition matter emissionengine to from show the enginethe absolute supplied benefit of using a modifiedwith natural positive-ignition gas was on the boundarynatural gas of measurementengine instead accuracy, of the therefore, original these self-ignition tests engine. were discarded, assuming for comparative purposes that the specific brake emission of particulate matter from the engine supplied with natural gas was 0. Although there is no practical possibility of comparingMF the particulate matter emissions from the tested MF DF DF D2 engines, it was decided to present the results for aD2 self-ignition engine to show the absolute benefit of using a modified positive-ignition natural gas engine instead of the original self-ignition engine. NRSC NRSC Decreased specific brake emission of pollutants was owing to replacing a self-ignition Test engine with a spark-ignition engine suppliedTest with natural gas. ESC The emissions of carbon monoxide, hydrocarbons,ESC nitrogen oxides and particulate matter for driving and stationary tests of the engine have been compared. The test was conducted for a self-ignition Cummins 6C 8.3 engine fueled with natural gas and for an ECE R 49.02 ECE R 49.02 original self-ignition Cummins 6C 8.3 engine supplied with diesel fuel. Figure8 shows a

0 0.2relative 0.4 decrease 0.6 in the 0.8 specific 1 brake emission of pollutants0 0.1 0.2 owing 0.3 tothe 0.4 application. 0.5 0.6 0.7 0.8 0.9 ⋅ e [g/kW⋅h)] eCO [g/kW h)] HC

(a) (b)

D2 D2

NRSC NRSC Test Test ESC ESC

MF MF ECE R 49.02 DF ECE R 49.02 DF

024681012 0 0.05 0.1 0.15 0.2 0.25 ⋅ e [g/kW⋅h)] eNOx [g/kW h)] PM

(c) (d) Figure 7. Specific brake emission: (a) carbon oxide, (b) hydrocarbons, (c) nitrogen oxides and (d) particulates, in tests for a self-ignition Cummins 6C 8.3 engine running on diesel fuel (DF), and a spark-ignition Cummins 6C 8.3 engine running on methane fuel—natural gas (MF).

Energies 2021, 14, 3356 10 of 18

D2

NRSC Test ESC

MF ECE R 49.02 DF

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 η e Figure 6. General efficiency in a self-ignition Cummins 6C 8.3 engine running on diesel fuel (DF), and a spark-ignition Cummins 6C 8.3 engine running on methane fuel—natural gas (MF).

Figure 7 shows the specific brake emission of pollutants for a self-ignition Cummins 6C 8.3 engine running on diesel fuel, and a spark-ignition Cummins 6C 8.3 engine running on methane fuel—natural gas. The particulate matter emission from the engine supplied with natural gas was on the boundary of measurement accuracy, therefore, these tests were discarded, assuming for comparative purposes that the specific brake emission of particulate matter from the engine supplied with natural gas was 0. Although there is no practical possibility of comparing the particulate matter emissions from the tested engines, it was Energies 2021, 14, 3356 decided to present the results for a self-ignition engine to show the absolute benefit11 ofof 19 using a modified positive-ignition natural gas engine instead of the original self-ignition engine.

MF MF DF DF D2 D2

NRSC NRSC Test Test ESC ESC

ECE R 49.02 ECE R 49.02

0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ⋅ e [g/kW⋅h)] eCO [g/kW h)] HC

(a) (b)

D2 D2 Energies 2021, 14, 3356 11 of 18

NRSC NRSC Test Test ESC ESC Decreased specific brake emission of pollutants was owing to replacing a self-ignition MF MF ECE R 49.02 engine with a spark-ignitionDF ECEengine R 49.02 supplied with natural gas. DF

024681012The emissions of carbon monoxide,0 0.05 hydroc 0.1arbons, 0.15 nitrogen 0.2 0.25 oxides and particulate ⋅ e [g/kW⋅h)] eNOx [g/kW h)] PM matter for driving and stationary tests of the engine have been compared. The test was (conductedc) for a self-ignition Cummins 6C 8.3(d engine) fueled with natural gas and for an

Figure 7.Figure Specific 7. Specific brake brakeemission: emission:original (a) carbon (a) carbonself-ignition oxide, oxide, (b (b) )hydrocarbons, hydrocarbons, Cummins ( c( )c6C nitrogen) nitrogen 8.3 oxidesengine oxides and suppliedand (d) particulates, (d) particulates, with in tests diesel in for tests a fuel. for Figure 8 shows a a self-ignitionself-ignition Cummins Cummins 6C 6C8.3relative 8.3engine engine ru runningdecreasenning on dieseldiesel in the fuel fuel (DF),specific (DF), and and a spark-ignitionbrake a spark- emissionignition Cummins Cummins of 6C pollutants 8.3 engine6C 8.3 runningengine owing onrunning to the application on methanemethane fuel—natural fuel—natural gas gas (MF). (MF).

D2

NRSC Test ESC

ECE R 49.02 CO HC NOx PM

00.20.40.60.81 δ FigureFigure 8.8.The The relative relative decrease decrease in the specific in the brake specific emission brake of pollutants emission due of to replacingpollutants diesel due to replacing diesel fuelfuel with with natural natural gas gas as the as Cummins the Cummins 6C 8.3 engine 6C fuel.8.3 engine fuel. The average relative decrease of the specific brake pollutant emission owing to sup- plyingThe the engineaverage with relative natural gas decrease instead of dieselof the fuel spec is, therefore:ific brake pollutant emission owing to supplying• Carbon the oxide: engine 0.29, with natural gas instead of diesel fuel is, therefore: • Hydrocarbons: 0.69, • • CarbonNitrogen oxides:oxide: 0.48, 0.29, • Hydrocarbons: 0.69, • Nitrogen oxides: 0.48, • Emission of particulate matter: 1. The higher sensitivity of specific brake pollutant emission to the operating states in individual tests is visible. Figure 9 shows the variation coefficient for the specific brake pollutant emission in all tests for both a self-ignition and spark-ignition Cummins 6C 8.3 engine.

0.4 DF MF

0.3

W 0.2

0.1

0 CO HC NOx

Figure 9. The variation coefficient for the specific brake pollutant emission in all tests for a self- ignition engine running on diesel fuel (DF), and a spark-ignition Cummins 6C8.3 engine running on methane fuel—natural gas (MF).

Specific brake emission of carbon oxide is the most sensitive to the operating states of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with

Energies 2021, 14, 3356 11 of 18

Decreased specific brake emission of pollutants was owing to replacing a self-ignition engine with a spark-ignition engine supplied with natural gas. The emissions of carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter for driving and stationary tests of the engine have been compared. The test was conducted for a self-ignition Cummins 6C 8.3 engine fueled with natural gas and for an original self-ignition Cummins 6C 8.3 engine supplied with diesel fuel. Figure 8 shows a relative decrease in the specific brake emission of pollutants owing to the application

D2

NRSC Test ESC

ECE R 49.02 CO HC NOx PM

00.20.40.60.81 δ Figure 8. The relative decrease in the specific brake emission of pollutants due to replacing diesel fuel with natural gas as the Cummins 6C 8.3 engine fuel.

The average relative decrease of the specific brake pollutant emission owing to supplying the engine with natural gas instead of diesel fuel is, therefore: • Carbon oxide: 0.29, • Hydrocarbons: 0.69, Energies 2021, 14, 3356 • Nitrogen oxides: 0.48, 12 of 19 • Emission of particulate matter: 1. The higher sensitivity of specific brake pollutant emission to the operating states in • Emission of particulate matter: 1. individualThe higher tests sensitivity is visible. of specific Figure brake pollutant9 shows emission the variation to the operating coefficient states in for the specific brake pollutantindividual tests emission is visible. in Figure all tests9 shows for the both variation a se coefficientlf-ignition for theand specific spark-ignition brake Cummins 6C 8.3 engine.pollutant emission in all tests for both a self-ignition and spark-ignition Cummins 6C 8.3 engine.

0.4 DF MF

0.3

W 0.2

0.1

0 CO HC NOx

Figure 9. The variation coefficient for the specific brake pollutant emission in all tests for a self- Figureignition engine 9. The running variation on diesel coefficient fuel (DF), and for a spark-ignition the specific Cummins brake 6C8.3 pollutant engine running emission on in all tests for a self- ignitionmethane fuel—natural engine running gas (MF). on diesel fuel (DF), and a spark-ignition Cummins 6C8.3 engine running on methaneSpecific brake fuel—natural emission of gas carbon (MF). oxide is the most sensitive to the operating states of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with the leastSpecific sensitive brake being specific emission brake emissionof carbon of nitrogen oxide oxide: is the 0.121 most for a sensitive self-ignition to the operating states engine and 0.097 for a spark-ignition engine. of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with 4.2. Stage Two Figures 10 and 11 show the specific brake emission of carbon monoxide, hydrocar- bons and nitrogen oxides within the D2 tests of the studied engines in the conditions of the homologation test of engines used for driving power generators: MAN E362 LE212, Liebherr G9512, fueled with biogas, and the Cummins 6C8.3 engine: in the original version of the self-ignition engine, fueled with diesel, and in the modified version as an ignition engine spark plug fueled with methane fuel (natural gas). The specific brake emissions of carbon oxide and hydrocarbons are similar and rel- atively high for the Liebherr G9512 and MAN E3262 LE212 stationary engines supplied with the biogas. Specific brake emission of carbon monoxide is over 0.9 g/(kW·h) and specific brake emission of hydrocarbons is over 0.5 g/(kW·h) for both engines. However, the emission of these substances does not pose a significant threat in the operating condi- tions of power generator engines. On the other hand, a clear benefit in terms of nitrogen oxide emission has been achieved (about 1 g/(kW·h) for MAN E3262 LE212 and about 3 g/(kW·h) for Liebherr G9512), let alone particulate matter emission. Energies 2021, 14, 3356 12 of 18 Energies 2021, 14, 3356 12 of 18

the least sensitive being specific brake emission of nitrogen oxide: 0.121 for a self-ignition theengine least and sensitive 0.097 forbeing a spark-ignition specific brake engine. emission of nitrogen oxide: 0.121 for a self-ignition engine and 0.097 for a spark-ignition engine. 4.2. Stage Two 4.2. Stage Two Figures 10 and 11 show the specific brake emission of carbon monoxide, hydrocarbonsFigures 10 and and nitrogen 11 show oxides the within specific the brakeD2 tests emission of the studiedof carbon engines monoxide, in the hydrocarbonsconditions of theand homologation nitrogen oxides test ofwithin engines the used D2 testsfor driving of the powerstudied generators: engines inMAN the conditionsE362 LE212, of Liebherrthe homologation G9512, fueled test ofwith engines biogas, used and for the driving Cummins power 6C8.3 generators: engine: MANin the E362original LE212, version Liebherr of the G9512,self-ignition fueled engine, with bi fuogas,eled withand thediesel, Cummins and in the6C8.3 modified engine: version in the original version of the self-ignition engine, fueled with diesel, and in the modified version Energies 2021, 14, 3356 as an ignition engine spark plug fueled with methane fuel (natural gas).13 of 19 as an ignition engine spark plug fueled with methane fuel (natural gas).

MAN E3262 LE212 MAN -E3262 SI - BF LE212 - SI - BF Liebherr G9512 - LiebherrSI - G9512BF - SI - BF Cummins 6C8.3 - CumminsSI - MF 6C8.3 - SI - MF Cummins 6C8.3 - CumminsCI - DF6C8.3 - CO HC CI - DF CO HC 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 e [g/(kW∙h)] e [g/(kW∙h)]

Figure 10. Specific brake emission of carbon oxide and hydrocarbons in the D2 test for the studied Figure 10. Specific brake emission of carbon oxide and hydrocarbons in the D2 test for the studied Figureengines 10. running Specific on brake diesel emission fuel (DF), of methancarbon eoxide fuel—natural and hydrocarbons gas (MF) and in the biogas D2 test (BF). for the studied engines running on diesel fuel (DF), methane fuel—natural gas (MF) and biogas (BF). engines running on diesel fuel (DF), methane fuel—natural gas (MF) and biogas (BF).

MAN E3262 LE212 MAN -E3262 SI - BF LE212 - SI - BF Liebherr G9512 - LiebherrSI - G9512BF - SI - BF Cummins 6C8.3 - CumminsSI - MF 6C8.3 - SI - MF Cummins 6C8.3 - CumminsCI - DF6C8.3 - CI - DF 024681012 024681012 e [g/(kW∙h)] e [g/(kW∙h)]

FigureFigure 11.11.Specific Specific brake brake emission emission of nitrogen of nitrogen oxides in oxides the D2 testin the for theD2 studied test for engines the studied running engines running Figureonon diesel 11. fuel fuel Specific (DF), (DF), methane brake methane fuel—naturalemission fuel—n of gasatural nitrogen (MF) gas and oxides (MF) biogas and in (BF). the biogas D2 test (BF). for the studied engines running on diesel fuel (DF), methane fuel—natural gas (MF) and biogas (BF). Significantly, the specific emission of carbon monoxide and hydrocarbons for the CumminsThe 6C8.3specific engine brake in both emissions versions (fueledof carbon with the oxide diesel and and naturalhydrocarbons gas as a sub- are similar and relativelystituteThe for biomethane)specifichigh for brakethe is Liebherr relatively emissions G9512 low (aboveof andcarbon 0.4 MAN g/(kW oxide E3262·h) and of LE212 carbon hydrocarbons stationary monoxide and enginesare similar supplied and relativelywithabove the 0.2 g/(kW biogas.high for·h) Specific ofthe hydrocarbons Liebherr brake G9512 foremission SI engineand MANof and carbon respectively E3262 monoxide LE212 above stationary 0.5is over g/(kW 0.9engines·h) g/(kW supplied·h) and of carbon monoxide and above 0.6 g/(kW·h) of hydrocarbons for CI engine), while the with the biogas. Specific brake emission of carbon monoxide· is over 0.9 g/(kW·h) and specificspecific emission brake emission of nitrogen of oxides hydrocarbons is significantly is higher over than0.5 g/(kW for typicalh) enginesfor both for engines. CHP However, specific brake emission of hydrocarbons is over 0.5 g/(kW·h) for both engines. However, theaggregates emission supplied of these with the substances biogas. This isdoes due tonot the factpose that a thesignificant Cummins 6C8.3threat engine in the operating theconditionswas emission originally of intended ofpower these for generator thesubstances drive ofengines. motor does vehicles, notOn posethe and other ina thesignificant modifiedhand, a configuration, clearthreat benefit in the in operating terms of conditionsnitrogenfor the drive oxide ofof working power emission machines. generator has b Theseeen engines. areachieved the engine On (about the operating other 1 g/(kW conditions, hand,·h) afor characterizedclear MAN benefit E3262 in LE212 terms and of nitrogenaboutby a relatively 3 g/(kW oxide low· h)emission relative for Liebherr load has (in b relation eenG9512), achieved to thelet maximumalon (aboute particulate engine1 g/(kW load), ·matterh) which for MANemission. is particu- E3262 LE212 and aboutlarly visible 3 g/(kW for the·h) averagefor Liebherr relative G9512), power use let in alon thee research particulate tests for matter ECE R emission. 49.02 and ESC, as well as NRSC in relation to the D2 test. In addition, the algorithms for controlling the work processes of Liebherr G9512 and MAN E3262 LE212 engines are adapted only to work in the driving conditions of power generators with the priority of reducing the specific emission of nitrogen oxides, because it is one of the most important problems of

Energies 2021, 14, 3356 14 of 19

this type of engines, while the original algorithms controlling the work processes in the engine Cummins 6C8.3 are adapted to the operating conditions for typical conditions of use, i.e., for a lower relative load for the use in cars and working machines in relation to the relative load in the application for driving power generators. The presented research has shown that the use of methane fuels (biogas, natural gas and biomethane, which can be used as a substitute for natural gas) causes beneficial environmental effects in comparison to conventional fuel. The use of methane fuels enables the reduction of emissions of pollutants: carbon monoxide, organic compounds, especially cyclic hydrocarbons, and their derivatives, especially polycyclic aromatic hydrocarbons, nitrogen oxides and solid particles. This is largely due to the combustion of highly homo- geneous air–fuel mixtures in the engine cylinders and, therefore, the possibility of burning very lean mixtures. It should be mentioned that biogas and biomethane can be produced from various biodegradable raw materials, including bio-waste. The use of bio-waste provides additional environmental benefits, because for the calculation of GHG emissions throughout the life cycle of these fuels, the stage of obtaining the raw material is zero- emission. It results directly from the provisions of Directive 2009/28/EC. It is important from the perspective of decreasing the impact of our civilization on the intensification of the greenhouse effect, thus counteracting climate changes. Using fuels produced from wastes is also an element of rationalizing the application of natural energy resources, which is particularly advantageous when using local biogas sources, among others, originating from municipal resources of urban agglomerations.

5. Conclusions The paper presented the results of research on the supply of methane fuels to internal combustion engines, especially in terms of their environmental impact. The authors con- verted the universal Cummins 6C8.3 diesel engine into a spark-ignition one (supplied with natural gas) and carried out various approval tests of both engines’ versions. Additionally, both Cummins 6C8.3 engine versions were compared with two cogeneration engines used in biogas plants (Liebherr G9512 and MAN E3262 LE212 supplied with biogas). The tests were carried out for various operating states of the engine, resulting from various operating conditions, e.g., in motor vehicles, working machines and power generators. As a result of the conversion of the Cummins motor from diesel fuel to methane fuel, the reduction of specific brake emission of carbon monoxide was seen in the range of 26–34%, hydrocarbons in the range of 60–75%, depending on the type of approval test, the reduction of specific brake emission of nitrogen oxides by approximately 50% and complete elimination of particulate matter was seen in all tests. Moreover, the engine conversion caused lower sensitivity of pollutant emissions to various operating states of internal combustion engines for the use of natural gas, which is a methane fuel, compared to the use of diesel fuel in a self-ignition engine. Specific brake emission of carbon oxide is the most sensitive to the operating states of the engine: 0.324 for a self-ignition engine and 0.264 for a spark-ignition engine, with the least sensitive being specific brake emission of nitrogen oxide: 0.121 for a self-ignition engine and 0.097 for a spark-ignition engine. Conversion of the engine from self-ignition into spark-ignition (and therefore changing diesel fuel into methane fuel) was characterized by the environmental advantages. In the second stage of the work, it was found that the specific brake emissions of carbon monoxide and hydrocarbons were similar and relatively high for the stationary CHP engines. Specific brake emission was over 0.9 g/(kW·h) and over 0.5 g/(kW·h) for carbon monoxide and hydrocarbons respectively, for both engines. Significantly, the specific emission of carbon monoxide and hydrocarbons for the Cummins engine in both versions (fueled with the diesel fuel and natural gas as a substitute for biomethane) was relatively low (above 0.4 g/(kW·h) of carbon monoxide and above 0.2 g/(kW·h) of hydrocarbons for the SI engine and respectively above 0.5 g/(kW·h) of carbon monoxide and above 0.6 g/(kW·h) of hydrocarbons for the CI engine). The specific brake emission of nitrogen Energies 2021, 14, 3356 15 of 19

oxides was achieved at about 1 g/(kW·h) and 3 g/(kW·h) for MAN E3262 LE212 and Liebherr G9512 respectively, while for Cummins engines, it was significantly higher. The application of biogas for supplying CHP spark-ignition engines provided tangible ecological advantages only in terms of nitrogen oxides emission in comparison with using diesel-fueled self-ignition engines. Suppling the Cummins engine with natural gas caused a significant reduction of hydrocarbons and carbon monoxide levels of emission. Generally, it can be stated that the change of conventional fuel into methane fuels causes a significant decrease in the emission of pollutants particularly harmful to the health of living organisms, and others that are hard to eliminate, primarily particulates and nitrogen oxides, as well as carbon oxide, under conditions simulating the operation of an internal combustion engine for driving a power generating set.

Author Contributions: Conceptualization, Z.C. and I.S.-B.; methodology, Z.C. and I.S.-B.; validation, M.O., A.M. and K.B.; formal analysis, Z.C., I.S.-B., M.O., A.M. and K.B.; investigation, Z.C., I.S.-B., M.O., A.M. and K.B.; resources, Z.C., I.S.-B. and K.B.; data curation, Z.C., I.S.-B. and M.O.; writing— original draft preparation, Z.C., I.S.-B., M.O., A.M. and K.B.; writing—review and editing, Z.C., I.S.-B., M.O., A.M. and K.B. 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: MDPI Research Data Policies. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

ANG Adsorbed Natural Gas BANG Adsorbed Biomethane BCNG Compressed Biomethane BF Biogas Fuel BLNG Liquefied Biomethane CHP Combined Heat and Power CI Compression Ignition CNG Compressed Natural Gas CO carbon oxide CO2 carbon dioxide CO2-eq carbon dioxide equivalent DF Diesel Fuel e specific brake emission ECE Economic Commission for Europe ESC European Stationary Cycle GHC Greenhouse Gas HC hydrocarbons ISO International Organization for Standardization LNG Liquefied Natural Gas LPG Liquefied petroleum gas Me torque MF Methane Fuel NGVs Natural Gas Vehicles Energies 2021, 14, 3356 16 of 19

NOx nitrogen oxides NRSC Non-Road Stationary Cycle Pe effective power PM particulate matter RED Renewable Energy Directive SI Spark Ignition W variation coefficient δ relative decrease ηe general efficiency

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