Experimental investigation on the combustion process in a spark ignition optically accessible engine fueled with methane/hydrogen blends

The combustion process of a four-stroke optically accessible single cylinder Port Fuel Injection spark ignition (PFI SI) engine was experimentally investigated. It was fueled with two methane/hydrogen blends. The in-cylinder pressure and the related data were analyzed as indicators of the combustion quality. 2D-digital imaging measurements were performed to evaluate the flame propagation. UV–visible spectroscopy allows to characterize the combustion by means of the detection of OH* and CH*. The exhaust was characterized using conventional analyzers. For the methane/hydrogen blends the indicated data suggests an increase of the thermal efficiency and a decrease of the combustion duration with the increase of the hydrogen fraction. The optical results highlight a more homogeneous mixture that increases the combustion reaction rate and provides a more uniform and rapid flame propagation. On the other hand, high NO_x emissions were measured likely because of the higher combustion temperature due to hydrogen addition.     

Combustion analysis of a spark ignition engine fueled with gaseous blends containing hydrogen

This paper presents the combustion characteristics of a naturally aspirated spark ignition engine, intended for installation in vehicles, fueled with different hydrogen and methane blends. The experimental tests were carried out in a wide range of speeds at equivalence ratios of 1, 0.8 and 0.7 and at full load. The ignition timing was maintained for each speed, independently of the equivalence ratio and blend used as fuel. Four methane-hydrogen blends were used. In-cylinder pressure, mass fraction burned, heat released and cycle-by-cycle variations were analyzed as representative indicators of the combustion quality. It was observed that hydrogen enrichment of the blend improve combustion for the ignition timing chosen. This improvement is more appreciable at low speeds, because at high speeds hydrogen effect is attenuated by the high turbulence. Also, hydrogen addition allowed the extension of the LOL, enabling the engine to run stable in points where methane could not be tested. The main inconvenience detected was the high NO_x emissions measured, especially at stoichiometric conditions, due mainly to the increment in the combustion temperature that hydrogen produces.     

Effects of ignition advance on a dual sequential ignition engine at lean mixture for hydrogen enriched butane usage

In this study, the effects of ignition advance on dual sequential ignition engine characteristics and exhaust gas emissions for hydrogen enriched butane usage and lean mixture were investigated numerically and experimentally. The main purpose of this study is to reveal the effects of h-butane application in a commercial spark ignition gasoline engine. One cylinder of the commercially dual sequential spark ignition engine was modeled in the Star-CD software, taking into account all the components of the combustion chamber (intake-exhaust manifold connections, intake-exhaust valves, cylinder, cylinder head, piston, spark plugs). Angelberger wall approximation, k-ε RNG turbulence model and G-equation combustion model were used for analysis. In the dual sequential spark ignition, the difference between the spark plugs was defined as 5° CAD. At the numerical analysis; 10.8:1 compression ratio, 1.3 air-fuel ratio, 2800 rpm engine speed, 0.0010 m the flame radius and 0.0001 m the flame thickness were kept constant. The hydrogen-butane mixture was defined as 4%–96% by mass. In the analysis, the optimal ignition advance was determined by the working conditions. In addition, the effects of changes in ignition advance were examined in detail at lean mixture. For engine operating conditions under investigation, it has been determined that the 50° CAD ignition advance from the top dead center is the optimal ignition advance in terms of engine performance and emission balance. It has also been found that the NO_x formation rises up as the ignition advance increases. The BTE values were approximately 12.01% higher than butane experimental results. The experimental BTE values for h-butane were overall 3.01% lower than h-butane numerical results.     

Influence of laser ignition on characteristics of an engine for hydrogen enriched CNG and iso-octane usage

The study has focused on determining the laser plug effects on engine characteristics and the laser plug usage results have compared with spark plug usage. The laser ignition technique is a type of new ignition technique and an important solution that can make combustion systems more efficient. The testing of an engine with a laser plug is the novelty of the study and the tests were carried out with reference to equivalence ratio and plug power ranges. The behaviors of the engine at full load were examined so experimentally for both ignition techniques at hydrogen enriched CNG and iso-octane mixture usage. The tests were carried out for variations of 0.4–2.0 equivalence ratio and 20–120 W plug power. A mixture that 90% iso-octane and 10% HCNG in mass was used at two ignition modes in tests for 3300 rpm maximum engine torque speed. Also, the flame formation and propagation for both ignition techniques were detected via a high-speed camera. The tests have shown the laser ignition leads to more energy consumption in the rich mixture conditions and also, less energy is required in the lean conditions. The laser ignition discharge has extended the engine's lean combustion limits via a small energy input at the tests. The high-speed camera images have shown that the laser ignition reduces the Kernel flame formation and propagation time. The laser ignition technique was produced less NO_x than the conventional spark ignition method.     

A numerical study of a methane-fueled gas engine generator with addition of hydrogen using cycle simulation and DOE method

Conventional fossil fuels for combustion systems, such as gasoline and diesel, have a number of problems related to energy security and emissions. Alternative fuels, such as methane, hydrogen, and mixtures of these two gases, are being promoted as clean energy substitutes for primary fossil fuels. Natural gas (which consists mainly of methane) is one of the most promising of these fuels, providing lower cost, cleaner emissions and is direct applicable to existing combustion systems. However, the use of natural gas as fuel can adversely affect engine performance. Therefore, hydrogen is sometimes used as an additive, as its higher burning rate often leads to enhanced combustion. In this study, cycle simulation was used to numerically investigate the performance and emission characteristics of an engine employed primarily to power a generator, and fueled with methane and methane - hydrogen blends. Dominant parameters such as excess air ratio, spark timing, and volume percent of hydrogen content, were investigated as independent variables. The fundamental effect of hydrogen on methane combustion was investigated for a fixed excess air ratio of 1.2 and a spark timing of 14CA(Crank Angle) BTDC (Before Top Dead Center), with an accompanying reduction in ignition delay. By varying the excess air ratio, hydrogen was demonstrated to play an important role in extending the lean operating limit. The DOE (Design of Experiment) method was applied to study MBT (Maximum Brake Torque) spark timing for various excess air ratios and hydrogen contents. When MBT spark timing was employed, maximum brake torque could be achieved under leaner burning conditions by increasing the hydrogen content.     

Efficiency and emissions in a vehicle spark ignition engine fueled with hydrogen and methane blends

This paper shows the results of the tests carried out in a naturally aspirated vehicle spark ignition engine fueled with different hydrogen and methane blends. The percentage of hydrogen tested was up to 50% by volume in methane. The tests were carried out in a wide range of speeds with the original ignition timing of the engine. Also, lean equivalence ratios were proved. Just the fuel injection map was modified for each fuel blend and equivalence ratio tested. In this paper, the results of thermal efficiency and pollutant emissions achieved at full load have been compared with the corresponding gasoline test results. The best balance between thermal efficiency and pollutant emissions was observed with the 30% hydrogen and 70% methane fuel blend.     

Experimental study of combustion performances and emissions of a spark ignition cogeneration engine operating in lean conditions using different fuels

This work presents an experimental study describing a six-cylinder spark ignition engine running with a lean equivalence ratio, high compression ratio, ignition delay and used in a cogeneration system (heat and electricity production). Three types of fuels; natural gas, pure methane and methane/hydrogen blend (85% CH₄ and 15% H₂ by volume), were used for comparison purposes. Each fuel has been investigated at 1500 rpm and for various engine loads fixed by electrical power output conditions. CO, CO₂, HC, and NOx emissions values, and exhaust gas temperature were measured. The effect of fuel composition on engine characteristics has been studied. The results show, that the hydrogen addition increased HC emissions (around 18%), as well as performance, whilst it reduced NO_x (around 31%), exhaust gas temperature, CO and CO₂.     

Advancing lean combustion of hydrogen-air mixtures by laser-induced spark ignition

We investigate how ignition through laser-induced plasma can improve the application of lean combustion, in particular in environmental conditions relevant to hydrogen internal combustion engines (ICE). Major design goals when developing combustion engines are increasing thermal efficiency and decreasing combustion emissions. High compression ratios, lean combustion and precise ignition timings are contributing factors in ICE optimization. In our studies, several gains from laser spark ignition are investigated. The high energy content of laser-induced ignition kernels are shown to speed up the development of the early flame kernels. These extended ignition kernels transfer into self propagating flames even in lean mixtures. Leaner mixtures are ignited in our experiments using laser spark ignition in comparison to conventional electrical spark plugs. Precise ignition timing is realized. Multi-point ignitions are synchronized on the timescale of microseconds to enhance the progress of combustion. We modified the locus of ignition in a mixture flow to decrease the temporal extent of flame contact with the wall. Therefore, burning duration and heat loss can be reduced.     

Numerical investigation on the effects of natural gas and hydrogen blends on engine combustion

The use of hydrogen blended with natural gas is a viable alternative to pure fossil fuels because of the expected reduction of the total pollutant emissions and increase of efficiency. These blends offer a valid opportunity for tackling sustainable transportation, in view of the future stringent emission limits for road vehicles. The aim of the present paper is the investigation of the performance of internal combustion engines fuelled by such blends. A numerical investigation on the characteristics of natural gas–hydrogen blends as well as their effect on engine performance is carried out. The activity is focused on the influence of such blends on flame propagation speed. Combustion pattern modelling allows the comparison of engine brake efficiency and power output using different fuels. Results showed that there is an increase in engine efficiency only if Maximum Brake Torque (MBT) spark advance is used for each fuel. Moreover, an economic analysis has been carried out to determine the over cost of hydrogen in such blends, showing percent increments by using these fuels about between 10 and 34%.     

Efficiency and emissions of a spark ignition engine fueled with synthetic gases obtained from catalytic decomposition of biogas

This paper presents the results of the tests developed in a naturally aspirated spark ignition engine, intended for installation in vehicles, fueled with synthetic gases obtained from catalytic decomposition of biogas. The experimental tests were carried out at three equivalence ratios and different speeds and loads. Two synthetic blends were used and the results were compared with those of gasoline and methane. Efficiency and emissions were calculated for the different fuels under the same operation conditions and it was found that at lean equivalence ratios, brake thermal efficiency with synthetic gases approached to the traditional fuels and even improved it at Φ = 0.7. BSCO₂ emissions increased due to the CO₂ content of the gaseous blends. While CO increased at stoichiometric conditions, it decreased at lean conditions because the H₂ contained in synthetic gases improved combustion at these conditions. BSHC measured were very low with synthetic gases because of the low content of methane in blends. The change in the fraction of H₂ and CO₂ of the synthetic blends led to quite different results in BSNOx. Syngas 1 BSNOx emissions were the lowest of all fuels, while syngas 2 BSNOx were the highest because of its high H₂ fraction.     

Comparative study on air dilution and hydrogen-enriched air dilution employed in a SI engine fueled with iso-butanol-gasoline

Hydrogen and iso-butanol are notable potential alternative fuels. Hydrogen addition under air dilution conditions was investigated in this study in an attempt to enhance the thermal efficiency of spark ignition (SI) engines fueled with iso-butanol-gasoline (B33) at partial load. Hydrogen appears to have positive effect on combustion progress that is prolonged during air dilution. Under lean hydrogen-enriched mixture conditions, the brake thermal efficiency was increased by about 4% and combustion instability was reduced; the lean burn limit migrated from 1.4 to 1.8 for B33 engine after hydrogen addition. Under lean burn conditions, the durations of initial flame development and rapid burning were shortened markedly by hydrogen; both were extended by air dilution. After hydrogen addition, the unburnt HC emissions of iso-butanol-gasoline decreased markedly and carbon monoxide (CO) emissions decreased slightly. NO_x emissions from hydrogen-enriched iso-butanol-gasoline increased as lambda grew near to 1.0, at a significant reduction with increasing excess air rate regardless of fuel type. The combination of hydrogen addition and air dilution exhibited a positive inhibition on particle matter (PM) emissions regardless particle in nucleation or the accumulation mode, and the particle surface concentration was reduced significantly. Finally, an improved combustion progress was observed after hydrogen addition during air dilution, as well as a higher brake thermal efficiency and wider lean burn limit with acceptable combustion stability.     

Effect of hydrogen addition on combustion and emissions performance of a spark ignition gasoline engine at lean conditions

Hydrogen has many excellent combustion properties that can be used for improving combustion and emissions performance of gasoline-fueled spark ignition (SI) engines. In this paper, an experimental study was carried out on a four-cylinder 1.6 L engine to explore the effect of hydrogen addition on enhancing the engine lean operating performance. The engine was modified to realize hydrogen port injection by installing four hydrogen injectors in the intake manifolds. The injection timings and durations of hydrogen and gasoline were governed by a self-developed electronic control unit (DECU) according to the commands from a calibration computer. The engine was run at 1400 rpm, a manifold absolute pressure (MAP) of 61.5 kPa and various excess air ratios. Two hydrogen volume fractions in the total intake of 3% and 6% were applied to check the effect of hydrogen addition fraction on engine combustion. The test results showed that brake thermal efficiency was improved and kept roughly constant in a wide range of excess air ratio after hydrogen addition, the maximum brake thermal efficiency was increased from 26.37% of the original engine to 31.56% of the engine with a 6% hydrogen blending level. However, brake mean effective pressure (Bmep) was decreased by hydrogen addition at stoichiometric conditions, but when the engine was further leaned out Bmep increased with the increase of hydrogen addition fraction. The flame development and propagation durations, cyclic variation, HC and CO₂ emissions were reduced with hydrogen addition. When excess air ratio was approaching stoichiometric conditions, CO emission tended to increase with the addition of hydrogen. However, when the engine was gradually leaned out, CO emission from the hydrogen-enriched engine was lower than the original one. NO_x emissions increased with the increase of hydrogen addition due to the raised cylinder temperature.     

Combustion and emission characteristics of an Acetone-Butanol-Ethanol (ABE) spark ignition engine with hydrogen direct injection

Butanol could reduce emissions and alleviate the energy crisis as a bio-fuel used on engines, but the production cost problem limits the application of butanol. During the butanol production, ABE (Acetone-Butanol-Ethanol) is a critical intermediate product. Many studies researched the direct application of ABE on engines instead of butanol to solve the production cost problem of butanol. ABE has the defects of large ignition energy and vaporization heat. Hydrogen is a gaseous fuel with small ignition energy and high flame temperature. In this research, ABE port injection combines with hydrogen direct injection, forming a stratified state of the hydrogen-rich mixture around the spark plug. The engine speed is 1500 rpm, and λ is 1. Five αH₂ (hydrogen blending fractions: 0, 5%, 10%, 15%, 20%) and five spark timings (5°, 10°, 15°, 20°, 25° CA BTDC) are studied to observe the effects of them on combustion and emissions of the test engine. The results show that hydrogen addition increases the maximum cylinder pressure and maximum heat release rate, increases the maximum cylinder temperature and IMEP, but the exhaust temperature decreases. The flame development period and flame propagation period shorten after adding hydrogen. Hydrogen addition improves HC and CO emissions but increases NO_x emissions. Particle emissions decrease distinctly after hydrogen addition. Hydrogen changes the combustion properties of ABE and improves the test engine's power and emissions. The combustion in the cylinder becomes better with the increase of αH₂, but a further increase in αH₂ beyond 5% brings minor improvements on combustion.     

Combining experimental and numerical investigations to explore the potential of downsized engines operating with methane/hydrogen blends

Due to the shortening of oil reserves, many research efforts are currently performed to promote alternative fuels for transport. Among them, natural gas, which is mainly composed of methane, offers one of the most promising potential as its large scale production can today be envisaged from biomass or shale. An other advantage of methane is that its high octane rating allows the use of increased compression ratios compared to gasoline, then improving the thermal efficiency of spark ignition engines. This property, combined with its low carbon content makes natural gas one of the best candidates to drastically reduce CO₂ emissions from piston engines. However, methane exhibits a low burning velocity, leading to high cycle-to-cycle variations and, in some cases, to increased CH₄ emissions, these latter having a huge impact in terms of greenhouse effect. One solution then consists in blending natural gas with hydrogen, a component easily available in refineries. H₂ enrichment indeed allows to reach high flame velocities and to limit quenching effects at the combustion chamber walls. Nevertheless, for the specific case of downsized engines, hydrogen may also lead to increase the knock sensitivity. A compromise in terms of blending rate, compression ratio and boost level should then be necessary to reach an optimal configuration. The objective of the present work is to combine experimental and numerical investigations to explore the influence and limits of hydrogen addition in highly downsized engines. The impact of the fuel composition on the combustion velocity and knock occurrence is studied for three compression ratios (9.5, 11.5 and 13). Experiments are conducted with a single-cylinder engine for a wide range of operating conditions in the stoichiometric mode and hydrogen blending rates from 0 to 40%. 3D CFD simulations are then performed using the Extended Coherent Flame Model (Colin et al, Oil & Gas Sci. & Tech., 2003) to describe the turbulent flame propagation, in combination with the Tabulated Kinetics for Ignition model (Colin et al., Proc. Combust. Inst., 2005) for knock prediction. These models require flame velocities as well as auto-ignition delays and reaction rates data, which have to account for the fuel composition. These data are provided using a priori computations of premixed flames and homogenous reactors with the GRI 3.0 and Curran mechanisms. A very good agreement is obtained between engine simulations and experiments, allowing to use CFD to improve the understanding of the observed engine behavior on specific operating points. It is first shown that the effect of hydrogen addition on the combustion velocity is almost linear for the considered blending levels, and that knock can be hardly found even for high load and high compression ratio cases. It is also demonstrated that optimizing an engine for CH₄–H₂ blends combustion is a challenging task and that a dedicated engine design should be chosen.     

Improvement of thermal efficiency and reduction of NOx emissions by burning a controlled jet plume in high-pressure direct-injection hydrogen engines

A new combustion process called the Plume Ignition Combustion Concept (PCC), in which the plume tail of the hydrogen jet is spark-ignited immediately after the completion of fuel injection to accomplish combustion of a rich mixture has been proposed by the authors. This PCC combustion process markedly reduces nitrogen oxides (NOx) emissions in the high-output region while maintaining high levels of thermal efficiency and power. On the other hand, as burning lean mixture of fuel and air is the conventional way to improve thermal efficiency and reduce NOx, a high λ premixed mixture of hydrogen and air formed by injecting hydrogen in the early stage of the compression stroke has been used in direct-injection hydrogen engines. It was recently reported, however, that this mixture condition does not always offer expected improved thermal efficiency under even lean mixture conditions by increasing unburned hydrogen emissions caused by incomplete flame propagation in the non-uniform and extremely lean portion of the mixture. In this study, the effect of retarding the injection timing to late in the compression stroke but slightly advanced from original PCC was examined as a way of reducing unburned hydrogen emissions and improving thermal efficiency. These effects result from a centroidal axially stratified mixture that positions a fairly rich charge near the spark plug. This stratified mixture is presumably effective in reducing incomplete flame propagation thought to be the cause of unburned hydrogen emissions and also promoting increasing burning velocity of the mixture that improve thermal efficiency. Finally, this research is characterized by measuring the hydrogen fuel concentration at the point and the time of spark ignition quantitatively by spark-induced breakdown spectroscopy in order to identify the changes in mixture ratio mentioned above caused by the parameters involved.     

电喷LPG发动机快速燃烧过程的应用研究

介绍了应用于高速单燃料LPG电喷发动机的高能双火花塞快速燃烧系统的组成及其在发动机稳态运行工况的稀燃研究。开发了发动机多通道瞬态燃烧分析系统用于LPG快速燃烧过程的研究，快速燃烧系统的同步、异步点火通过ECU及其控制策略的控制实现。试验结果表明：LPG混合气的火焰传播速度得到提高，LPG的燃烧稀限由过量空气系数1．25—1．4拓展为1．4—1．5；结合燃烧室和火花塞位置的优化，火焰传播距离被缩短以实现LPG稀混合气的快速燃烧。     

Improving burning speed by using hydrogen enrichment and turbulent jet ignition system in a rotary engine

Low flame speed restrains engine efficiency and increases HC emissions in rotary engines. Hydrogen addition and turbulent jet ignition have a great potential in increasing engine performance as they increase fuel burning speed. In this study, the classical R13b-Renesis Wankel engine and a modified one with a turbulent jet ignition configuration are numerically investigated by using hydrogen as a supplement. Eccentric motion of the rotor was generated by using User Defined Function in ANSYS-Fluent software. Pure methane and methane blended with 3% and 6% hydrogen energy fractions were used as fuels in the calculations. Combustion was modeled by using reduced mechanism of hydrogen-methane combustion having 22 species and 104 reactions. The Wankel engine was simulated at 2000 rpm speed and partial load conditions. At first, classical engine configuration having two spark plugs was simulated with pure methane. Then, hydrogen blended methane simulations were conducted to investigate the benefits of the hydrogen addition. Similar procedure was applied for the turbulent jet ignition application. The results show that both approaches are effective on increasing the burning speed of the fuel. It is revealed that hydrogen addition increases the indicated mean effective pressure (IMEP) by 1.8% and 5.2% for 3% and 6% hydrogen fraction cases respectively in the classical engine. Turbulent jet ignition with pure methane increases IMEP by 4.7% compared to the classical engine. Hydrogen addition only in pre-chamber is effective as much as 6% hydrogen fraction of classical engine. As the burning speed is increased by the application of these methods, CO and HC emissions are reduced and NO emission is increased. It is concluded that benefits of hydrogen addition and turbulent jet ignition applications can be optimized for both reducing harmful emissions and increasing engine performance.     

湍流射流点火对天然气发动机燃烧特性影响的试验研究

湍流射流点火（Turbulent Jet Ignition,TJI）是一种有效的燃烧增强技术,可提供更高的点火能量,使发动机稳定着火,且可以提高燃烧压力和燃烧速率,缩短燃烧持续期,是实现发动机稀薄燃烧的有效手段。基于一台带有预燃室的点燃式单缸试验机,开展了TJI模式下天然气发动机性能的试验研究。首先,研究了不同过量空气系数下TJI对天然气发动机动力性能、排放性能及燃烧特性的影响,并与火花塞点火（Spark Ignition,SI）模式进行对比;其次,在稀燃条件下分别探究了进气增压和预燃室喷氢对天然气发动机动力性、经济性及燃烧过程的优化作用。结果表明：TJI的使用可有效拓展天然气发动机的稀燃极限,且燃烧滞燃期和燃烧持续期均更短,放热率更高;过量空气系数1.5为甲烷TJI最佳稀燃工况,此时燃油消耗率最低,且可实现氮氧化物近零排放;此外,采用进气增压的方式可以提高TJI发动机在高负荷下的经济性;TJI模式下,相较于预燃室喷甲烷,预燃室喷氢气可进一步缩短滞燃期和燃烧持续期,提高放热率,达到提升TJI性能的效果。     

Comparison of the effects of EGR and lean burn on an SI engine fueled by hydrogen-enriched low calorific gas

A naturally aspirated spark ignition (SI) engine fueled by hydrogen-blended low calorific gas (LCG) was tested in both exhaust gas recirculation (EGR) and lean burn modes. The “dilution ratio” was introduced to compare their effects on engine performance and emissions under identical levels of dilution. LCG composed of 40% natural gas and 60% nitrogen was used as a main fuel, and hydrogen was blended with the LCG in volumes ranging from 0 to 20%. The engine test results demonstrated that EGR operations at stoichiometry showed a narrower dilution range, inferior combustion characteristics, lower brake thermal efficiency, faster nitrogen oxides (NO_x) suppression, and higher total hydrocarbon (THC) emissions for all hydrogen blending rates compared to lean burn. These trends were mainly due to the increased oxygen deficiency as a result of using EGR in LCG/air mixtures. Hydrogen enrichment of the LCG improved combustion stability and reduced THC emissions while increasing NO_x. In terms of efficiency, hydrogen addition induced a competition between combustion enhancement and increases in the cooling loss, so that the peak thermal efficiency occurred at 10% H₂ with excess air ratio of 1.5. The engine test results also indicated that a close-to-linear NO_x-efficiency relationship occurred for all hydrogen blending rates in both operations as long as stable combustion was achieved. NO_x versus combustion duration analysis showed that adding H₂ reduced combustion duration while maintaining the same level of NO_x. The methane fraction contained in the THC emissions decreased slightly with an increase in hydrogen enrichment at low EGR or excess air dilution ratios, but this tendency was diminished at higher dilution ratios because of the combined dilution effects from the inert gas in the LCG and the diluents (EGR or excess air).     

Experimental study of a single-cylinder engine fueled with natural gas–hydrogen mixtures

The objective of this study is to evaluate the power, efficiency and emissions of an electronic-controlled single-cylinder engine fueled with pure natural gas and natural gas–hydrogen blends, respectively. Replacing the nature gas with hydrogen/methane blend fuels was found to have a significant influence on engine performance. The effects of excess air ratio and spark timing were discussed. The results show that under certain engine conditions the maximum cylinder gas pressure, maximum heat release rate increased with the increase of hydrogen fraction. The increase of hydrogen fraction in the blends contributed to the increase of NO_x and the decrease of HC and CO. The brake specific fuel consumption decreased with the increase of hydrogen fraction. Using HCNG at relatively leaner fuel–air mixtures and retarded spark timing totally improved the engine emissions without incurring the performance penalty.