13th International Colloquium Fuels - September 2021 143 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Dipl.-Ing. Jure Galović*, Assoc. Prof. Dr. Peter Hofmann Institute of Powertrains and Automotive Technology, TU Wien, Vienna, Austria Dipl.-Ing. Tom Popov, Dipl.-Ing. Dr. techn. Stefan Müller Institute of Chemical, Environmental and Bioscience Engineering, TU Wien, Vienna, Austria Dipl.-Ing. Dr. techn. Christof Weinländer GLOCK Ecoenergy Bengerstraße 1, 9112 Griffen, Austria Dominik Frieling, M.Sc. GLOCK Technology Gaston-GLOCK-Park 1, 9170 Ferlach, Austria *Corresponding author e-mail address: jure.galovic@ifa.tuwien.ac.at (J. Galović) Summary A significant contribution to reach the goals of Paris’ climate agreement and to become greenhouse-gas-free by 2050 can be provided by utilizing alternative fuels in internal combustion engines. More precisely, the conversion of CO 2 neutral wood gas from air gasification in stationary internal combustion engines enables sustainable power generation. However, due to its variable composition, especially H 2 and H 2 O content, as well as low heating value, the engine operation with wood gas is challenging. As a part of the research projects, the wood gas production process and its utilization in the internal combustion engine of a wood gas Combined Heat and Power plant (P el <100kW el ) have been analyzed. In addition, the engine operation with various wood gas compositions has been investigated on the adaptive engine test bench with a gas mixer at TU Wien. Based on these investigations, a joint experimental approach to optimize the wood gasification process and the engine operation is defined. Regardless of the adverse properties of wood gas for engine operation, high efficiency and low emissions can be realized with selective optimization measures of the engine and combustion process. 1. Introduction Power generation from renewable energy sources is becoming increasingly important to reduce greenhouse gas (GHG) emissions and their adverse impact on climate change. To achieve the ambitious EU energy and climate goals, which envisage decarbonization by 2050 [1], the transformation of energy systems from centralized to decentralized structures coupled with smart grids shall be strived. In addition to volatile renewable sources such as wind and photovoltaic, the wood gas Combined Heat and Power plants (wgCHP) have a crucial role in power generation and grid stability during the volatile energy demand. For instance, the main advantage of wgCHPs is decentralized power and heat generation by utilizing regenerative and sustainable biomass such as wood chips as feedstock. As shown in Fig. 1, the wood captures CO 2 from the atmosphere for its growth. During its utilization as the feedstock within the wgCHP, CO 2 is returned to the atmosphere. Therefore, wood can be considered a CO 2 neutral energy source [2] since no additional CO 2 from fossil fuels is released. Figure 1: CO 2 recycling by the use of wgCHP 144 13th International Colloquium Fuels - September 2021 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions The GLOCK-Group has also specialized in the production of innovative, highly efficient, and environmentally friendly wood gas CHPs, which offer the possibility to produce self-sufficient and CO 2 -neutral heat and electricity from the renewable raw material wood. With the decentralized and constantly available energy supply of its wgCHP, the GLOCK-Group wants to actively shape the current and future energy supply to achieve climate protection, pioneer in the decentralized energy supply, and preserve resources for the future generation. GLOCK-wgCHP systems are based on the autothermal gasification of wood chips or wood pellets and internal combustion engine (ICE), which utilizes the gasification product wood gas. In particular, the developed wgCHP systems have power ranges from 10-50kW el and 35- 110kW th and can cover power demand for homes or small hotels. However, constant development is necessary to increase the efficiency and thus the viability of the plant. Though the most challenging aspect is to ensure high system efficiency and long-term durability under various operating conditions that affect the gasification process and engine operation. The product of biomass gasification is a gas mixture that consists of combustible gases carbon monoxide (CO), hydrogen (H 2 ), methane (CH 4 ) as well as high molecular hydrocarbon compounds (C x H y ). It also contains specific incombustible gases, carbon dioxide (CO 2 ), water vapor (H 2 O) and, when air is used as a gasification agent, high amounts of nitrogen (N 2 ). These gases reduce the calorific value of the mixture. Moreover, tars and dust in product gas are seen as impurities. The product gas composition is depending on the design of the gasifier, the gasification agent as well as the type and quality of the biomass feedstock. Fixed bed gasifiers with air as gasification agent lead to product gases with low heating values and high amounts of nitrogen (N 2 ). Mixtures of air and steam as gasification agent could lead to higher amounts of hydrogen (H 2 ) in product gas [3]. By using only steam as a gasification agent in allothermal gasification, a hydrogen rich (H 2 -rich) and nitrogen free (N 2 -free) product gas can be obtained. This concept is used in the dual fluidized bed steam gasifier for the gasification of a wide range of different feedstocks [4] and has been successfully demonstrated in a pilot CHP in Güssing, Austria, for over 100.000 operating hours until 2016 [5]. The concept of “heat pipe reformer” reported in [6] also supplies a gas mixture with a high fraction of H 2 and a low fraction of inert gases. An overview of different gasifier technologies as well as a ranking of gasifiers for CHP applications according to key performance indicators is given in [7]. Several investigations of engine operation and the combustion process with different gas compositions have been reported in [8], [9], and [10]. Among the gas constitutes, H 2 favors a fast combustion process within the engine and ensures flame propagation, especially when high fractions of inert gases are present [11]. Due to its high laminar flame velocity, combustion of highly diluted mixtures in extreme lean operation with high efficiency and low emissions is possible [10]. The wgCHP development aims to attain high efficiency with low pollutant emissions and minor influence on air quality by applying optimal operational strategy combined with modern engine technology. In the framework of the present work, the investigation of the GLOCKwgCHP plant and operation of the wood gas engine is presented. In the first part, the operating principle of GLOCK-wgCHP is described. In the second part, the methodology for wood gas analysis and investigation of the wgCHP operation is presented. In addition, the developed engine test bench, as well as methodology for laboratory investigations of the combustion process, are described. With these methods, the wgCHP operation was analyzed and the developed test engine could be verified for further investigations. In particular, it was investigated to what extent the combustible mixture can be diluted for lean operation. Thereby, several wood gas mixtures that could be supplied from different gasification processes were utilized by the test engine, and the engine efficiency and exhaust gas emissions were analyzed. Further, the results obtained from conducted measurements of operating wgCHP and during investigations on the engine test bench at the Institute of Powertrains and Automotive Technology (IFA) at TU Wien are discussed. Finally, an outlook for an optimal operating strategy with high engine efficiency and low exhaust gas emissions as well as an approach for further development are given. 2. GLOCK-wgCHP and Wood Gas Engine 2.1 Configuration of GLOCK-wgCHP The configuration of GLOCK-wgCHP is schematically presented in Fig. 2. Throughout the process, the wood chips are fed to the gasifier, where they undergo the gasification process. The produced wood gas is extracted from the gasifier and then passed to a hot dry gas filter where char and dust are separated. Before entering the engine, the wood gas is cooled down in a heat exchanger, filtered through an additional safety wet filter, and mixed in a venturi nozzle with air. Thereby, the airflow is controlled using a throttle valve. The applied wood gas engine is based on the geometry of an industrial diesel engine, which has been modified for spark ignition (SI) operational mode. The compression ratio (ɛ) has been reduced due to different fuel characteristics compared to diesel. A big displacement volume of the engine ensures feasible power output during operation with wood gas. Since the engine is naturally aspirated, the intake manifold pressure is below the atmospheric pressure because the suction of the wood gas is driven by the engine. The engine drives the generator at constant engine speed, which converts the mechanical energy into the electrical energy. Prior to releasing the engine exhaust gas in the at- 13th International Colloquium Fuels - September 2021 145 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions mosphere, it is cooled down in an exhaust gas cooler. The exhaust gas and wood gas heat exchangers, as well as the engine coolant circuit, are connected to the heating grid. Figure 2: Schematic view of GLOCK-wgCHP 2.2 Wood Gasification and Wood Gas Properties For GLOCK-wgCHP, fixed bed downdraft gasifiers are used. Downdraft means that wood chips and gas are moving in the same downwards direction inside the gasifier. This type of gasifier consists of four different reaction zones, as shown in Fig. 3. Wood chips are fed on the top. Without the presence of oxygen, the wood chips are heated up, and at the first stage, water is being vaporized and the chips are being dried. At higher temperatures, the pyrolysis takes place where the volatiles of the wood are going into the gas phase. In the combustion or oxidation zone, the gasification agent air is being introduced and the wood chips are partially burned under the presence of oxygen. In the reduction zone, the main gasification reactions as stated in [12] and [13] take place: ΔH R,850 °C C + H 2 O → CO + H 2 +135,7 kJ/ mol (1) C + CO 2 → 2CO +169,4 kJ/ mol (2) C + 2H 2 → CH 4 -89,8 kJ/ mol (3) CO + H 2 O ↔ CO 2 + H 2 -33,6 kJ/ mol (4) CH 4 + H 2 O ↔ CO + 3H 2 +226,8 kJ/ mol (5) CH 4 + CO 2 ↔ 2CO + 2H 2 +260,4 kJ/ mol (6) As reactions in the reduction zone are mostly endothermic, heat is required for them to take place. This heat is being generated in the oxidation zone, defining the autothermal gasification character of this type of gasifier. After the reduction zone, wood gas and solid particles char and ash leave the gasifier. The advantage of downdraft gasifiers is the potentially low tar content in the wood gas. Tars are mainly released during pyrolysis and, while passing the oxidation zone, are being thermally cracked. Making this kind of gasifier suitable for combined heat and power generation with gas engines without the need for extensive gas cleaning steps [13]. Though, the downdraft gasifiers require specific feedstock properties such as a defined particular size to maintain a specific pressure drop throughout the bed and moisture contents of less than 20-25%. High pressure drops or moisture contents can negatively affect the temperatures in the different zones and the gasification process, leading to potentially higher tar contents [13], [14]. Figure 3: Reaction zones in downdraft gasifier [15] 3. Methodology In the framework of the present work, the investigations were conducted on the operating GLOCK-wgCHP and the developed engine test bench at IFA-TU Wien. 3.1 Wood Gas Analysis and Operation of GLOCKwgCHPs The focus of measurements conducted on the wgCHP was placed on the analysis of wood gas composition and wood gas utilization in engine. For investigations, the wgCHP is equipped with measurement instruments for gas analysis, air mass flow measurement, pressure gauges, thermocouples, engine indicating instruments, λ-sensor, and data acquisition systems. A measuring spark plug with integrated cylinder pressure sensor type 6113C Kistler is used to indicate cylinder pressure. Fig. 4 shows the GLOCK-wgCHP with measurement instruments. 146 13th International Colloquium Fuels - September 2021 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Figure 4: GLOCK-wgCHP with the measurement instruments For the analysis of the wood gas composition, online and offline measurements were conducted at sampling point 1 in Fig. 2. Fig. 5 shows the gas cleaning setup for the online and offline measurements. For the online measurements, the wood gas was filtered with washing bottles filled with toluol and placed in a cryo-cooler at - 8°C in order to remove tar and water from the wood gas. After filtration the components H 2 , CO, CO 2 and CH 4 have been measured continuously with a Rosemount NGA 2000 analyser. C 2 H 4 , C 2 H 6 , C 3 H 8 and N 2 have been measured with a gas chromatograph (GC). Every 20 minutes the GC pulled a gas sample for the analysis leading to a discontinued recording of the higher carbon hydrogens and nitrogen. Offline measurements were conducted for tar and water content in accordance to DIN CEN/ TS 15439 [16]. The principle for the tar and water removal from the wood gas is the same as for the online measurements. The only difference is the precise recording of the taken gas volume during the offline measurement in order to have a reference for the amount of tar and water in the wood gas stream. The taken offline samples were than analyzed at the TU Wien Test Laboratory for Combustion Systems by means of gravimetric and gas chromatographic/ mass spectrometric (GC/ MS) analyses. Figure 5: Gas cleaning setup for online and offline measurements. Modified from [17] Further, air mass flow (ṁ Air ) for the engine operation was measured at the intake of air supply pipe (sampling point 2, see Fig. 2) by the flowmeter, based on the ultrasonic transit-time differential technique. To measure the oxygen proportion in the exhaust gas, the λ-sensor was applied in the exhaust gas pipe. Due to the composition of wood gas, a calibration factor determined on the engine test bench was used to calculate the actual relative air fuel ratio (λ). The mass flow of wood gas (ṁ Fuel ) is calculated according to equation (7) [18]: The air required for complete combustion of wood gas, i.e., the stoichiometric air/ fuel ration (A min ), is determined by solving the elementary combustion reactions for each combustible component as stated in [19]: The composition of raw exhaust gas was measured continuously, whereby the volumetric concentration of CO, CO 2 , O 2 , and NO were logged. The exhaust gas was collected on the sampling point 3 (see Fig. 2) for the analysis. The wgCHP engine was operated lightly leaner than stoichiometric, at the highest possible load and constant engine speed during the conducted measurements. Thereby, the spark advance variations on the maximum load at constant engine speed have been performed. The gas analysis was conducted for 30 minutes. During the gas analysis, the engine cylinder pressure was indicated in 200 consecutive cycles for several times. Thereby the measurements occurred in the middle engine cylinder. The operating parameters of the gasification process have not been changed in the framework of this investigation. 3.2 Setup of Engine Test Bench with Gas Mixer The test bench architecture and methodology is adopted from [20] and modified for the investigation of the wgCHP engine and operation with mixed wood gas. The in-line gas engine has been modified to operate as a single-cylinder engine, as presented in Fig. 6. 13th International Colloquium Fuels - September 2021 147 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Figure 6: Schematic view of the adapted single-cylinder engine equipped with measurement instruments at the engine test bench The high demand of the gas components makes single-cylinder operation reasonable since the investigation focuses on combustion process development. Only cylinder one is fired; the other three are deactivated by separated intake and exhaust manifolds. The fired one is equipped with a cooled piezoresistive pressure transducer (quartz) for cylinder pressure measurement. The intake and exhaust pipes of the fired cylinder are equipped with thermocouples, pressure gauges, and piezoelectric pressure transducers. The mixed wood gas is injected into the intake pipe, whereby the air mass flow and the intake pressure are controlled by an air throttle valve. Further, air mass flow (ṁ Air ) for the engine operation is measured at the intake of air supply pipe. For gas analysis, sampling points in intake and exhaust pipes are adapted. The composition of raw exhaust gas is measured continuously based on the infra-red principle (CO and CO 2 ), paramagnetic principle (O 2 ), as well as by flame ionization detector (CH 4 ) and chemiluminescence analyzer for nitric oxide (NO) and nitrogen dioxide (NO 2 ). NO and NO 2 are usually grouped together as nitrogen oxides (NO X ) [21]. Additionally, a pipe that connects the exhaust and intake manifold for exhaust gas recycled (EGR) was designed. The EGR mass flow is controlled by an electrically adjustable EGR valve. To obtain it, the CO 2 volumetric fraction is also measured in the intake pipe based on the infra-red principle. The test bench automation system comprises the engine control (by wgCHP engine control unit) and data acquisition. Engine indicating occurs by AVL Indicom. Due to the long warm-up phase, as well as the high volatile gas composition, utilizing a gasifier for the engine test bench is not suitable. Instead, a gas mixer for the supply of wood gas composition has been developed for laboratory experiments. As schematically presented in Fig. 7, the gas mixer consists of controlling software and hardware components. The calculation and control of the mixing process are based on the required wood gas composition and mass flow. The hardware involves mass flow sensors, solenoid valve coils, and gas analyzer. The gas mixing process occurs in the engine intake manifold, whereby each individual component of the gas mixture can be injected with a defined mass flow. The single gas components are supplied from the gas pressure cylinders (combustible gases and part of inert gases) and recycled from exhaust gas (water vapor and part of other inert gas components). Thereby, the mass flow of EGR (ṁ EGR ) is determined from the measured CO 2 concentration in the fresh charge. With the information of exhaust gas composition, the mass flow of each component supplied from EGR can be calculated. Figure 7: Schematic view of developed gas mixer for gas supply of test engine For simplification of the mixed wood gas composition, volume fractions of higher HC compounds are substituted by an additional volume fraction of CH 4 equivalent to the initial lower heating value (LHV). At the test engine, λ is determined from the measured mass flow of each wood gas constitute (ṁ Fuel ) and air (ṁ Air ) supplied to the engine according to equation (7). Thereby A min is calculated by solving the elementary combustion reactions for each combustible component. In addition, λ is also calculated from the measured exhaust gas and wood gas composition according to [22] and [23] to check the conformity of determined values. The same operating points analyzed on the operating wg- CHP engine are measured again on the engine test bench. The results are compared to validate the experimental approach and check the accordance between the wgCHP engine and the test engine. The measurements occurred at the defined constant load (intake manifold pressure), engine speed, gas composition, λ and temperature of the fresh charge corresponding to the conditions during wg- CHP operation. In order to investigate the effects of wood gas composition on efficiency and exhaust gas emissions, the test engine is fuelled with different mixtures. These mixtures represent the possible wood gas composition that different concepts of biomass gasification could supply. Thereby, the H 2 mole fraction in investigated mixtures is increased, and the fractions of other components are reduced, respectively. The measurements of engine operation are carried out at several operating points, all defined with a constant load (intake manifold pressure) and engine speed, which correspond to wgCHP operation. 148 13th International Colloquium Fuels - September 2021 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Thereby, the parameter λ is varied. The engine spark timing for measured operating points is adjusted to give the maximum indicated mean effective pressure (imep) for a given engine operating parameters. 4. Results 4.1 Wood Gas Analysis Fig. 8 depicts measured wood gas composition during the online gas analysis at the wgCHP. N 2 is not shown in the Fig.8. One can see that the volume fraction of the wood gas constitutes significantly fluctuates with time. The observed fluctuations seems to be characteristic for fixed bed gasifiers and have also been reported in [24]. Figure 8: Fluctuations of wood gas composition (dry) during online gas analysis The average wood gas composition is presented in Fig. 9. The highest volume fraction of the combustible components in wood gas has CO, followed by H 2 and CH 4 . The C 2 H 4 , C 2 H 6 , C 3 H 8 have low volume fractions. The analyzed wood gas contains a high amount of inert gases CO 2 , N 2 , and H 2 O (the latest is not shown in Fig. 9). Besides, the wood gas contains tar compounds, S-compounds, N-compounds, Cl-compounds, and dust. Although they have adverse effects on the engine operation and exhaust gas after-treatment system, as reported in [25], they do not significantly influence the combustion process due to low content. Therefore, they are neglected for investigations on the engine test bench. Figure 9: Wood gas composition (dry) at the wgCHP Respective to the volume fractions of combustible components, CO and H 2 have the highest contribution to the LHV of wood gas. Although the hydrocarbons are present with a low amount, their contribution to the LHV of wood gas is significant (see Fig. 10). In addition, the stoichiometric air-fuel ratio is low due to the high content of inert gases in wood gas composition. Therefore, a high mass flow of wood gas compared to the mass flow of air is needed for the engine operation. Figure 10: Contribution of wood gas constitutes to the LHV The results obtained during the online gas analysis show that the gas composition fluctuated over the time of measurements. Due to the observed fluctuations of gas composition, an engine test bench with precisely adjustable operating conditions and gas composition offers a viable solution for investigating combustion and engine development. The results gathered during the measurements of the operating wgCHP present a data basis for validation of the developed test engine. 4.2 Comparison of wgCHP Engine and Test Engine Operation The results of the measurements of the wgCHP engine and the single-cylinder test engine are compared to va- 13th International Colloquium Fuels - September 2021 149 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions lidate the operation of the developed single-cylinder test engine and gas mixer. The measurements on the engine test bench occurred at the same operating conditions as on the wgCHP engine - constant load (intake manifold pressure), engine speed, fresh charge composition, as well as the pressure level and temperature of the fresh charge. Thereby, the spark advance variations were performed. The focus of comparison is placed on the combustion process and exhaust gas emissions. Combustion Process Fig. 11 shows the indicated mean effective pressure (imep), as well as the coefficient of variance (COV imep ), measured at the engine test bench and on the operating wgCHP engine. It can be seen that the observed data are in good agreement over the investigated operating points. The shaded area presents the spark timing, which gives the maximum imep in the absence of irregular combustion phenomena. Since only indicated values are observed, the spark timing that gives maximum imep is considered as optimal in the framework of this study. According to [21], this timing also gives the minimum specific fuel consumption. As spark was advanced from optimal settings, imep slightly decreased; as the spark was retarded from optimal settings, the imep decreased. Considering the combustion stability, the COV imep increased exponentially, as the spark was retarded from initially advanced timing. Figure 11: Combustion process of wood gas engines - indicated mean effective pressure (imep, on the top), coefficient of variance (COV imep , on the bottom) Fig. 12 shows the combustion duration (MFB 5-90%), ignition delay, and center of combustion (MFB50%) measured at the engine test bench and on the operating wgCHP engine. The combustion duration linearly increased as the spark was retarded from initially advanced spark time. The center of combustion was linearly shifted toward the optimum and then further toward the TDC, as the spark was advanced from initially retarded settings. The ignition delay remained constant over the variations of spark advance since the composition of fresh charge was not changed during the conducted measurements. Figure 12: Combustion process of wood gas engines - combustion duration (MFB5-90%, on the top), ignition delay (middle) and combustion center (on the bottom) 150 13th International Colloquium Fuels - September 2021 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Figure 13: Combustion process of wood gas engines - exhaust gas temperature (T exhaust ) Fig. 13 shows the exhaust gas temperature (T exhaust ) measured at the engine test bench and the operating wgCHP engine. As the spark was retarded from initially advanced settings, the exhaust temperature increased. That results in reduced engine efficiency and heat loss to the combustion chamber [21]. Emissions Fig. 14 shows the raw exhaust gas emissions of CO and NO measured at the engine test bench and the operating wgCHP engine. Due to the limitations of measurement instruments, the exhaust emissions of the wgCHP engine could not be measured over all operating points. However, it can be seen that the CO and NO linearly increased as the spark was advanced from initially retarded settings. The increase in CO is less intensive than in NO over investigated operating points for both engines. Retarding of the spark from the initially advanced settings reduces the peak cylinder pressure and temperature, and thereby hinders the NO formation. This approach could sometimes be used for NO X emission control [21]. Further, high cylinder pressures during advanced spark are favorable for filling the crevice volumes with an unburned mixture that does not participate in the combustion process and contributes to a slight increase of the CO emissions. Discussion The same trends observed in both engines proved the accordance of single-cylinder engine operation on the test bench with wgCHP engine operation. Minor discrepancies in results could be attributed to fluctuations of wood gas composition supplied by the gasifier (see Fig. 8). For example, an alteration of gas composition during logging of 200 consecutive engine cycles could significantly affect cylinder pressure measurements and combustion analysis results. In addition, the exhaust gas emissions could be affected in the same way. On the engine test bench, the operating conditions were held constant during the conducted measurements. Furthermore, the observed long combustion duration and ignition delay indicated that the combustion process occurred moderately and negatively affected the engine efficiency. Figure 14: Raw exhaust gas emissions of wood gas engines - carbon monoxide (CO, on the top) and nitric oxide (NO, on the bottom) 4.3 Effects of H 2 in Wood Gas Composition on Engine Efficiency and Emissions The engine operation with different wood gas mixtures was investigated on the engine test bench. For this purpose, the percentage of H 2 in gas mixtures was increased, whereby the percentage of other components was reduced respectively. These mixtures represent the possible wood gas composition supplied from different gasification concepts. Thereby, the effects of H 2 O content were not investigated in the scope of this research work. The composition and properties of wood gas mixtures are listed in Table 1. Table 1: Composition and properties of wood gas mixtures for investigation on the engine test bench Mixture 1 Mixture 2 Mixture 3 CO [% vol.dry ] 24 21 18 CO 2 [% vol.dry ] 9 8 7 CH 4 [% vol.dry ] 5 5 4 H 2 [% vol.dry ] 14 25 35 N 2 [% vol.dry ] 48 41 36 LHV [kJ/ kg] 5,7 7 8,6 A min [kg/ kg] 1,6 2 2,4 Fig. 15 shows that the combustion occurs faster and ignition delay is reduced as the H 2 percentage in mixture was higher over measured operating points. Correspondingly, the spark timing was also set closer to the TDC during 13th International Colloquium Fuels - September 2021 151 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions engine operation with hydrogen rich mixtures in order to get the maximum imep for a given engine operating parameters. Thereby, the COV imep during engine operation with hydrogen rich mixtures increases less intensive with increasing λ, indicating higher combustion stability. This trend is significant at very lean engine operation (see Fig. 16). Figure 15: Effects of H 2 in wood gas on lean operation - combustion duration (MFB5-90%, on the top), ignition delay (middle) and spark timing (on the bottom) The achieved indicated engine efficiency (η i ) increased significantly as the H 2 percentage in the wood gas mixtures was higher, especially in the lean operating points (see Fig. 16). As a result of H 2 enrichment, the lean operation could be extended to over 30%. Irregular combustion and self-ignition have not been observed during the investigation. The observed trends of CO and NO X are presented in Fig. 17. On the one hand, the CO emissions increased less intensive as the λ increased by engine operation with H 2 rich wood gas mixtures. On the other hand, the NO X was higher at the engine operation with H 2 rich mixtures. However, the NO X decreased intensively as the λ increased, and therefore, due to the extended lean operation, low NO X emissions at the operating points with high efficiency can be achieved. Discussion The lean engine operation can be extended for 30% with H 2 rich wood gas mixtures. Due to the high laminar flame velocity of H 2 , stabile combustion of mixtures with higher dilution is possible. In addition, the combustion occurs faster as the percentage of H 2 in mixtures is higher over the measured point in the lean operation. As a result, higher η i can be achieved. Moreover, reduced share of the unburned fuel (CO, C x H y and H 2 from wood gas) over the measured operating points in lean operation indicate the increase of combustion efficiency as the H 2 percentage in gas mixtures is increased. Further, higher NOx emissions were observed at engine operation with H 2 rich gas mixtures over the measured points. As the H 2 favors the combustion process, higher combustion temperatures could be reached. Since NO X formation is mainly driven by the peak temperature, the NO X emissions increased as its percentage in wood gas mixtures was higher. However, the NO X decreased, as engine was operated leaner. Thus, the lean operation can be considered as the approach for reducing NO X emissions. Figure 16: Effects of H 2 in wood gas on lean operation - coefficient of variance (COV imep ), and indicated efficiency (η i ) 152 13th International Colloquium Fuels - September 2021 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions Figure 17: Effects of H 2 in wood gas on lean operation - exhaust gas emissions 5. Summary and Outlook Summary This paper presents the methodology for investigating the GLOCK wgCHP and the wood gas engine on an engine test bench at IFA-TU Wien. In addition, engine operation with different wood gas mixtures was investigated. The operating wgCHP plant has been equipped with measurement instruments for investigations of engine operation and gas analysis. In addition, a wgCHP engine on the engine test bench has been modified to operate as a single-cylinder engine and equipped with measuring instruments as well. For the supply of the test engine with mixed wood gas, a gas mixer has been developed. The gas mixer enables the supply of different wood gas mixtures with high accuracy. The conducted gas analysis on the operating wgCHP shows that wood gas composition fluctuates over time due to complex processes within the gasifier. The produced wood gas contains a high amount of inert gases N 2 and CO 2 . The highest volume fraction of the combustible components has CO, followed by H 2 and C x H y . The results obtained during the investigations of operating wgCHP present a data basis for investigations on the test engine. Due to the observed fluctuations in gas compositions, an engine test bench with precisely adjustable operating conditions and gas composition offers a viable solution for investigating the combustion process and engine development. By comparing the wgCHP engine and test engine operation, the same trends in imep, COVimep, combustion duration, ignition delay, exhaust temperature, and CO and NO emissions are observed in both engines. These results prove the accordance of the wgCHP engine and single-cylinder engine operation on the test bench. Minor discrepancies observed between several measurements could be attributed to the fluctuations of wood gas composition on the operating wgCHP. The observed combustion duration and ignition delay indicate that the combustion process occurs moderately and negatively affects engine efficiency. In order to investigate the effects of different wood gas compositions on the engine operation, experiments have been conducted on the engine test bench. For this purpose, the percentage of H 2 in the investigated wood gas mixtures was increased, and the percentage of other components was decreased respectively. Over the investigated lean engine operating points, higher efficiency was achieved by utilizing hydrogen rich wood gas mixtures. Due to the high laminar flame velocity of H 2 , faster and stabile combustion of mixtures with higher dilution was possible. Moreover, reduced share of the unburned fuel (CO, C x H y and H 2 from wood gas) over the measured operating points in lean operation indicate the increase of combustion efficiency as the H 2 percentage in gas mixtures has been increased. Higher NO X emissions over the measured points were achieved at engine operation with hydrogen rich gas mixtures due to higher combustion temperatures. As the dilution of the combustible mixture was higher (high λ), NO X emissions significantly decreased. The results in this paper demonstrate the potential for improvement of the combustion process within the wood gas engines. Furthermore, the accordance of the developed test engine with the wgCHP engine has been proved. With the presented methodology and developed hardware, different engine operating strategies and modern engine technology can be investigated further. Outlook The activities within the project go beyond the investigations presented in this paper. A joint experimental approach to optimize the engine operation for wood gas can be defined based on conducted measurements. An optimal engine design, coupled with modern technology, would ensure the lowest possible exhaust emissions and high durability with maximized electrical efficiency under given conditions. In particular, optimizing the combustion chamber for high charge motion could reduce combustion duration and improve combustion stability even at lean operation. A possible approach to increasing the power output and overcoming the adverse effects of lean operation would be applying a turbocharger. In that way, the efficiency could benefit even more from exhaust heat recovery and extreme lean operation. However, more effort is needed to overcome the adverse effects of tars and other condensables in wood gas composition. Besides the measures, such as cleaning of wood gas, maintaining the gasification parameters, or utilizing 13th International Colloquium Fuels - September 2021 153 Application of Wood Gas in Internal Combustion Engines - Efficiency and Emissions higher quality wood, a modification of mixture formation could contribute to avoiding tar condensation within the system. Further, an adaptive combustion control would ensure optimal engine operation despite permanent fluctuations of wood gas composition. In that way, possible irregular combustion and knock would be avoided. 6. Nomenclature A min = stoichiometric air/ fuel ration COV imep = coefficient of variation (COV) in indicated mean effective pressure EU = European Union FFG = Die Österreichische Forschungsförderungsgesellschaft (eng. The Austrian Research Promotion Agency) EGR = exhaust gas recycled GHG = greenhouse gas GC = gas chromatograph HC = hydrocarbon IFA = Institut für Fahrzeugantriebe und Automobiltechnik (eng. Institute of Powertrains and Automotive Technology) imep = indicated mean effective pressure LHV = lower heating value ṁ Air = air mass flow ṁ EGR = mass flow of EGR ṁ Fuel = fuel mass flow MS = mass spectrometer MFB5-90% = combustion duration MFB50% = center of combustion P el = electric power SI = spark ignition TDC = top dead center T exhaust = temperature of exhaust gas ΔH R = reaction enthalpy ɛ = compression ratio λ = relative air fuel ratio η i = indicated engine efficiency 7. Acknowledgment The achievements presented in this paper were obtained in the framework of the research projects “holzgasBHKW+” (Grant Nos. 871725, eCall 22011585) commissioned by the Austrian Research Promotion Agency (FFG) and the GLOCK ResearchLab, founded by GLOCK Privatstiftung. The authors would like to thank the Austrian Research Promotion Agency and GLOCK Privatstiftung for founding these projects. 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