Lean Operating Limit Research Articles

LPG gaseous phase electronic port injection on performance, emission and combustion characteristics of Lean Burn SI Engine

Gaseous fuels have always been established as an assuring way to lessen emissions in Spark Ignition engines. In particular, LPG resolved to be an affirmative fuel for SI engines because of their efficient combustion properties, lower emissions and higher knock resistance. This paper investigates performance, emission and combustion characteristics of a microcontroller based electronic LPG gaseous phase port injection system. Experiments were carried out in a single cylinder diesel engine altered to behave as SI engine with LPG as fuel at a compression ratio of 10.5:1. The engine was regulated at 1500 rpm at a throttle position of 20% at diverse equivalence ratios. The test results were compared with that of the carburetion system. The results showed that there was an increase in brake power output and brake thermal efficiency with LPG gas phase injection. There was an appreciable extension in the lean limit of operation and maximum brake power output under lean conditions. LPG injection technique significantly reduces hydrocarbon and carbon monoxide emissions. Also, it extremely enhances the rate of combustion and helps in extending the lean limit of LPG. There was a minimal increase of NOx emissions over the lean operating range due to higher temperature. On the whole it is concluded that port injection of LPG is best suitable in terms of performance and emission for LPG fuelled lean burn SI engine.

Extending the lean operation limit of a gasoline Wankel rotary engine using hydrogen enrichment

In this paper, an experimental investigation of the effect of hydrogen enrichment on a gasoline Wankel engine's Lean Operation Limit (LOL) was performed. A monorotor Wankel engine operating at 3000 rpm and wide open throttle was modified to run on gasoline and hydrogen blends. Excess air ratio varying from stoichiometric mixtures to the engine lean burn limit with 0%, 3% and 6% of hydrogen energy fractions in the intake were investigated. The experimental results proved that adding hydrogen to gasoline extends the original engine LOL while improving the engine stability. An increased thermal efficiency and reduced burn duration, brake specific energy consumption, and HC, CO and CO2 emissions were observed with hydrogen enrichment. Furthermore, the extension of the engine LOL with hydrogen addition could potentially control the NOx emissions of a gasoline Wankel engine to very low levels without using a three-way catalytic converter.

Performance and NOx emissions of a biogas-fueled turbocharged internal combustion engine

The performance and NOx (nitrous oxide) emissions of a biogas-fueled turbocharged internal combustion engine were investigated using one-dimensional cycle simulation. Analyses were carried out using the design of experiment method, and the results were verified by comparison with experimentally measured data. The combustion behaviors were improved as the CH4 content in the biogas increased. The brake power, brake thermal efficiency, and NOx emissions increased as the CH4 content or the boost pressure increased. Appropriate boost pressures to produce the same brake power at a given relative air/fuel ratio of 1.1 without boost were determined for each relative air/fuel ratio or each biogas composition considering brake power. In general, the lean operation limit was extended up to a relative air/fuel ratio of 1.5 with various biogas compositions and up to a relative air/fuel ratio of 1.7 for CH4:CO2 volume ratios of 65%:35% and 70%:30% without knocking. The maximum brake thermal efficiency was 35.9%, which was observed with a relative air/fuel ratio of 1.7 and a boost pressure of 1.44 bar at CH4:CO2 = 70%:30%. The NOx emissions were reduced by more than 90% via a reduction in the combustion temperature, which was achieved as a result of the lean combustion.

A comprehensive analysis of the effect of ethanol, methane and methane-hydrogen blend on the combustion process in a PFI (port fuel injection) engine

The energy crisis and environmental issues make the alternative fuels, both liquid and gaseous, even more attractive because of their potentiality in reducing the fuel consumption and the pollutant emissions. Ethanol is the most promising alternative liquid fuel for spark ignition engines. It has a higher octane number, which provides good anti-knock characteristics and in the possibility to work with higher compression ratios, so improving the engine efficiency. The higher heat of vaporization compared to gasoline leads to an increased power output. Moreover, the larger oxygen content provides a more complete combustion and therefore reduced emissions. Among gaseous fuels, methane is considered one of the most interesting. It has wider flammable limits and better anti-knock properties than gasoline. Moreover, it is characterized by lower CO2 emissions. On the other hand, the slow flame propagation speed and its poor lean-burn capability produce lower engine power output with respect to gasoline. The addition of a high burning velocity fuel, such as hydrogen, allows to improve the combustion process in terms of burning velocity and extend the lean operation limit. The objective of this paper is the analysis of the effect of different fuels on the engine performance and emissions. Experimental investigations were carried out in an optically accessible small single-cylinder, spark ignition four-stroke engine. It was equipped with the cylinder head of a Port Fuel Injection commercial 244 cc engine. The engine was fueled with gasoline, ethanol, methane and a blend of hydrogen in methane. Optical measurements were performed to analyze the combustion process with high spatial and temporal resolution. In particular, the optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV–visible spectroscopy allows the detection of chemical markers of the combustion process such as the radicals OH* and CH*. The exhaust emissions were characterized by means of gaseous analyzers. The measurements were performed at steady state conditions.

Experimental results of hydrogen enrichment of ethanol in an ultra-lean internal combustion engine

An investigation was made to determine the effects of hydrogen enrichment of ethanol at ultra-lean operating regimes utilizing an experimental method. A 0.745 L 2-cylinder SI engine was modified to operate on both hydrogen and ethanol fuels. The study looked at part throttle, fixed RPM operation of 0%, 15%, and 30% hydrogen fuel mixtures operating in ultra-lean operating regimes. Data was collected to calculate NO and HC emissions, power, exhaust gas temperature, thermal efficiency, volumetric efficiency, brake-specific fuel consumption, and Wiebe burn fraction curves.It was shown that hydrogen enrichment of ethanol demonstrated an ability to reduce NOx and stabilize and accelerate the combustion process. The experiments showed that operating near the LOL at both 15% and 30% hydrogen by volume reduced engine out NOx emissions by more than 95% as compared to stoichiometric gasoline operation. This reduction is comparable to the efficiency of modern three-way catalyst and could offer an alternative to current NOx reduction technologies. Power, thermal efficiency, and volumetric efficiency were not affected by the hydrogen mixture at a given equivalence ratio. However, hydrogen addition allowed an increase in the lean operating limit which helped further reduce NOx emissions, but also at reduced power and thermal efficiency.

Coal-derived syngas MILD combustion in parallel jet forward flow combustor

The effect of air-fuel momentum flux ratio on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using non-reactive numerical simulations and on OH∗ radicals distribution and exhaust emissions using experiments. The predicted gas recirculation ratios in the combustor are high enough to establish the reaction condition of MILD scheme. Lower air-fuel momentum flux ratio associated with higher heat load benefits the drop of peak flame temperature and the increase of reaction zone volume. The critical air-fuel momentum flux ratios below which MILD combustion occurred were identified for three MILD configurations. The MILD configuration equipped with larger air nozzles and smaller fuel nozzles was observed to achieve MILD combustion at leaner condition. The MILD regime was established for syngas fuel with lean operational limit and ultra-low NOx and CO emissions.

Effect of air preheat temperature on the MILD combustion of syngas

The effect of air preheat temperature on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using numerical simulations and on global flame signatures, OH∗ radicals distribution and exhaust emissions using experiments. The discrete and high speed air/fuel injections into the combustor is necessary for the establishment of MILD conditions, because they cause strong gas recirculation and form large mixing region between the air and fuel jets. The critical equivalence ratio above which MILD combustion occurred was identified. The MILD regime was established for syngas fuel under air preheating conditions with lean operational limit and suppressed NOx and CO emissions. In the MILD combustion regime, the air preheating resulted in higher NOx but lower CO emissions, while the increase of equivalence ratio led to the increase of NOx and the decrease of CO emissions.

Emissions and fuel consumption characteristics of an HCNG-fueled heavy-duty engine at idle

The idle performance of an 11-L, 6-cylinder engine equipped with a turbocharger and an intercooler was investigated for both compressed natural gas (CNG) and hydrogen-blended CNG (HCNG) fuels. HCNG, composed of 70% CNG and 30% hydrogen in volume, was used not only because it ensured a sufficient travel distance for each fueling, but also because it was the optimal blending rate to satisfy EURO-6 emission regulation according to the authors' previous studies. The engine test results demonstrate that the use of HCNG enhanced idle combustion stability and extended the lean operational limit from excess air ratio (λ) = 1.5 (CNG) to 1.6. A decrease of more than 25% in the fuel consumption rate was achieved in HCNG idle operations compared to CNG. Total hydrocarbon and carbon monoxide emissions decreased when fueled with HCNG at idle because of the low carbon content and enhanced combustion characteristics. In particular, despite hydrogen enrichment, less nitrogen oxides (NOx) were emitted with HCNG operations because the amount of fuel supplied for a stable idle was lower than with CNG operations, which eventually induced lower peak in-cylinder combustion temperature. This low HCNG fuel quantity in idle condition also induced a continuous decrease in NOx emissions with an increase in λ. The idle engine test results also indicate that cold-start performance can deteriorate owing to low exhaust gas temperature, when fueled with HCNG. Therefore, potential solutions were discussed, including combustion strategies such as retardation of spark ignition timing combined with leaner air/fuel ratios.

Effects of H2 Addition to Compressed Natural Gas Blends on Cycle-to-Cycle and Cylinder-to-Cylinder Combustion Variation in a Spark-Ignition Engine

An experimental investigation and a burning-rate analysis have been performed on a production 1.4 liter compressed natural gas (CNG) engine fueled with methane-hydrogen blends. The engine features a pent-roof combustion chamber, four valves per cylinder, and a centrally located spark plug. The experimental tests have been carried out in order to quantify the cycle-to-cycle and the cylinder-to-cylinder combustion variation. Therefore, the engine has been equipped with four dedicated piezoelectric pressure transducers placed on each cylinder and located by the spark plug. At each test point, in-cylinder pressure, fuel consumption, induced air mass flow rate, pressure, and temperature at different locations on the engine intake and exhaust systems as well as “engine-out” pollutant emissions have been measured. The signals related to engine operation have been acquired by means of a National Instruments PXI-DAQ system and software developed in house. The acquired data have then been processed through a combustion diagnostic tool resulting from the integration of an original multizone thermodynamic model with a computer-aided design (CAD) procedure for the evaluation of the burned-gas front geometry. The diagnostic tool allows the burning velocities to be computed. The tests have been performed over a wide range of engine speeds, loads, and relative air-fuel ratios (up to the lean operation limit (LOL)). For stoichiometric operation, the addition of hydrogen to CNG has produced a brake-specific fuel combustion (bsfc) reduction ranging between 2% and 7% and a brake-specific total unburned hydrocarbons (bsTHCs) decrease up to 40%. These benefits have appeared to be even higher for lean mixtures. Hydrogen has shown to significantly enhance the combustion process, thus leading to a sensibly lower cycle-to-cycle variability. Hydrogen addition has generally resulted in extended operation up to relative air-to-fuel ratio (RAFR) = 1.8. Still, the LOL consistently varies depending on the considered cylinder.

Effect of swirl on the performance and combustion of a biogas fuelled spark ignition engine

The influence of swirl on the performance, emissions and combustion in a constant speed Spark Ignition (SI) engine was studied experimentally. A single cylinder diesel engine was modified to operate as a biogas operated spark ignition engine. The engine was operated at 1500rpm at throttle opening of 25% and 100% at various equivalence ratios. The tests covered a range of equivalence ratios from rich to lean operating limits and also at an optimum compression ratio of 13:1 with normal and masked intake valve to enhance swirl. The spark timing was set to MBT (Minimum advance for Best Torque). It was found that masked valve configuration enhanced the power output and brake thermal efficiency at full throttle. The lean limit of combustion also got extended. Heat release rates indicated enhanced combustion rates with masked valve, which are mainly responsible for the improvement in thermal efficiency. NO level increased with masked valve as compared to normal configuration. The spark timings were to be retarded by about 6°CA and 4°CA when compared to normal configuration at 25% and 100% throttle respectively.

A comparative study of the port geometrical effects on sharp corners’ jet triple flames

The performance of sharp corners’ jet triple flames was modulated by compromising the increase in both the number of sharp corners between 3 and 16 and the overall perimeter up to 218% with the reduction in vertex angle to 30°. While the turbulent kinetic energy peak value correlated with the vertex angle and its average value depended on the number of corners, the entrainment rate gain was pronounced in accordance with the perimeter. By having each port stabilizing the inner flame wing such that the sharp corners lay on the diffusion flame envelope at its base, the flow strain due to corners increased the triple flame stability limit to a maximum of 9.2m/s and minimized the NOx emissions to 14ppm. Upon incorporating the swirl impact, the highest firing rate was reached at a jet velocity of 26.3m/s at which excessive entrainment of reactants into corners provided a compact flame with a firing intensity of 50.3MJ/m3. While the flame length was related to the extent of jet deformation from the circular shape, a flame shortening by 67% was verified by an enhancement of 192% over the circular port entrainment rate. Improved combustion efficiencies with respective decrease in CO and HC emissions to 61ppm and 0.04% were reported by maximizing the effect of sharp edged appendices. The swirl introduced a favorable advection transport, whereby the turbulence peaks at corners merged to provide a relative increase of 11.8 times in the average turbulence energy. By effectively combining the active and passive control techniques via providing swirl and modulating the port geometry, the lean operation limit extended by 19% from that of the circular port whereby NOx emissions for triple flames case II had a minimum of 4ppm.

Hydrogen addition effects on high intensity distributed combustion

Distributed combustion provides significant improvements under high intensity conditions characteristic of gas turbine to provide uniform thermal field (improved pattern factor), ultra-low pollution, enhanced stability and higher efficiency. Mixing between fresh air/fuel stream with hot reactive species is critical to result in distributed reactions and spontaneous ignition. Hydrogen enrichment of fuel is examined with emphasis on combustion stability and emissions under swirling flow conditions. Results are presented on the role of hydrogen enrichment to methane (4–15% by mass, 25–58.5% by volume) on the combustion characteristics under fuel-lean conditions. CO emission was substantially reduced with hydrogen enrichment, with minimal effect on NO emission under premixed combustion. Hydrogen addition extended the lean operational limits of the combustor with stable combustion and no flame fluctuations or flashback. Results obtained on pollutants emission and flame marking via OH* chemiluminescence revealed near volume distributed high intensity combustion with ultra-low emission (<3PPM NO and <9PPM CO) and high performance at lower equivalence ratio. Hybrid numerical–experimental approach can provide more realistic prediction of NO emission from hydrogen enriched methane combustion.

Characterization of Lean Misfire Limits of Mixture Alternative Gaseous Fuels Used for Spark Ignition Engines

Increasing on gaseous fuels as clean, economical and abundant fuels encourages the search for optimum conditions of gas-fueled internal combustion engines. This paper presents the experimental results on the lean operational limits of Recardo E6 engine using gasoline, LPG, NG and hydrogen as fuels. The first appearance of almost motoring cycle was used to define the engine lean limit after the fuel flow was reduced gradually. The effects of compression ratio, engine speed and spark timing on the engine operational limits are presented and discussed in detailed. Increasing compression ratio (CR) extend the lean limits, this appears obviously with hydrogen, which has a wide range of equivalence ratios, while for hydrocarbon fuel octane number affect gasoline, so it can' t work above CR=9:1, and for LPG it reaches CR=12:1, NG reaches CR=15:1 at lean limit operation. Movement from low speeds to medium speeds extended lean misfire limits, while moving from medium to high speeds contracted the lean misfiring limits. NOx, CO and UBHC concentrations increased with CR increase for all fuels, while CO 2 concentrations reduced with this increment. NOx concentration increased for medium speeds and reduced for high speeds, but the resulted concentrations were inconcedrable for these lean limits. CO and CO 2 increased with engine speed increase, while UBHC reduced with this increment. The hydrogen engine runs with zero CO, CO 2 and UNHC concentrations, and altra low levels of NOx concentrations at studied lean misfire limits

Characterization of Lean Misfire Limits of Mixture Alternative Gaseous Fuels Used for Spark Ignition Engines

Increasing on gaseous fuels as clean, economical and abundant fuels encourages the search for optimum conditions of gas-fueled internal combustion engines. This paper presents the experimental results on the lean operational limits of Recardo E6 engine using gasoline, LPG, NG and hydrogen as fuels. The first appearance of almost motoring cycle was used to define the engine lean limit after the fuel flow was reduced gradually. The effects of compression ratio, engine speed and spark timing on the engine operational limits are presented and discussed in detailed. Increasing compression ratio (CR) extend the lean limits, this appears obviously with hydrogen, which has a wide range of equivalence ratios, while for hydrocarbon fuel octane number affect gasoline, so it can' t work above CR=9:1, and for LPG it reaches CR=12:1, NG reaches CR=15:1 at lean limit operation. Movement from low speeds to medium speeds extended lean misfire limits, while moving from medium to high speeds contracted the lean misfiring limits. NOx, CO and UBHC concentrations increased with CR increase for all fuels, while CO2 concentrations reduced with this increment. NOx concentration increased for medium speeds and reduced for high speeds, but the resulted concentrations were inconcedrable for these lean limits. CO and CO2 increased with engine speed increase, while UBHC reduced with this increment. The hydrogen engine runs with zero CO, CO2 and UNHC concentrations, and altra low levels of NOx concentrations at studied lean misfire limits.

Extending Lean Operating Limit and Reducing Emissions of Methane Spark-Ignited Engines Using a Microwave-Assisted Spark Plug

A microwave-assisted spark plug was used to extend the lean operating limit (lean limit) and reduce emissions of an engine burning methane-air. In-cylinder pressure data were collected at normalized air-fuel ratios ofλ=1.46,λ=1.51,λ=1.57,λ=1.68, andλ=1.75. For eachλ, microwave energy (power supplied to the magnetron per engine cycle) was varied from 0 mJ (spark discharge alone) to 1600 mJ. At lean conditions, the results showed adding microwave energy to a standard spark plug discharge increased the number of complete combustion cycles, improving engine stability as compared to spark-only operation. Addition of microwave energy also increased the indicated thermal efficiency by 4% atλ=1.68. Atλ=1.75, the spark discharge alone was unable to consistently ignite the air-fuel mixture, resulting in frequent misfires. Although microwave energy produced more consistent ignition than spark discharge alone atλ=1.75, 59% of the cycles only partially burned. Overall, the microwave-assisted spark plug increased engine performance under lean operating conditions(λ=1.68)but did not affect operation at conditions closer to stoichiometric.

Effect of compression ratio on the performance and combustion of a biogas fuelled spark ignition engine

A single cylinder diesel engine was modified to operate as a biogas operated spark ignition engine. The engine was operated at 1500rpm at throttle opening of 25% and 100% at various equivalence ratios. The tests were covered a range of equivalence ratios from rich to the lean operating limit and a number of compression ratios. The spark timing was set to MBT (Minimum advance for Best Torque). The performance, emission and combustion characteristics with different compression ratios are compared. It has been found from the results that the higher the compression ratio, the higher the brake thermal efficiency. When the compression ratio was above a critical value of 13:1, brake power and thermal efficiency increased little. At higher compression ratios above 13:1, increased NOx, HC, and CO emissions were measured. Power and thermal efficiency reached their highest values with the compression ratio between 13:1 and 15:1 and the equivalence ratio between 1.08 and 0.95. Under these conditions, HC and CO emissions were low but the NOx values were high. Power and thermal efficiency reduced for leaner mixtures. The MBT spark timing is retarded with increase in compression ratio. The peak pressure decreases, as the mixture becomes lean at all the compression ratios. The peak pressure is higher with higher compression ratio. Increase in compression ratio leads to high heat release rate.

Effects of hydrogen addition under lean and diluted conditions on combustion characteristics and emissions in a spark-ignition engine

Experimental investigations on the effects of hydrogen addition to spark-ignition (SI) engines running under lean and diluted conditions are presented in this paper. Experiments were carried out in a mono-cylinder SI engine with different rates of nitrogen dilution (0 to 20 per cent by volume in the total mixture) and hydrogen/iso-octane blends (from 0 to 80 per cent by volume in the fuel). The study of the impact of hydrogen addition on combustion characteristics and emissions was performed for two different engine speeds and loads. The equivalence ratio, the rate of dilution, and the intake pressure were varied either separately or simultaneously in order to maintain a constant engine load at a fixed hydrogen fraction in the fuel. The lean and dilution operating limits were also determined for all the iso-octane/hydrogen/air/nitrogen mixtures investigated, and results show that these limits are extended only when the hydrogen percentage in the fuel is higher than 40 per cent by volume. At a fixed engine load, hydrocarbon (HC) and CO emissions decrease with an increase in the hydrogen fraction in the intake mixture, while NO x emissions are mainly affected by the equivalence ratio and by the amount of dilution. Pumping losses, combustion efficiency and indicated efficiency are also improved with the addition of hydrogen. High values of indicated engine efficiency with low values of HC, NO x, and CO emissions can be achieved by combining hydrogen addition with lean and/or diluted conditions.

Performance and emission characteristics of a SI engine fueled by low calorific biogas blended with hydrogen

In this study, an experimental investigation on a naturally aspirated (NA), 8-L spark ignition engine fueled by biogas with various methane concentrations – which we called the N 2 dilution test – was performed in terms of its thermal efficiency, combustion characteristics and emissions. The engine was operated at a constant engine rotational speed of 1800 rpm under a 60 kW power output condition and simulated biogas was employed to realize a wide range of changes in heating value and gas composition. The N 2 dilution test results show that an increase of inert gas in biogas was beneficial to thermal efficiency enhancement and NO x emission reduction, while exacerbating THC emissions and cyclic variations. Then, as a way to achieve stable combustion for the lowest quality biogas, H 2 addition tests were carried out in various excess air ratios. H 2 fractions ranging from 5 to 30% were blended to the biogas and the effects of hydrogen addition on engine behavior were evaluated. The engine test results indicated that the addition of hydrogen improved in-cylinder combustion characteristics, extending lean operating limit as well as reducing THC emissions while elevating NO x generation. In terms of efficiency, however, a competition between enhanced combustion stability and increased cooling energy loss was observed with a rise in H 2 concentration, maximizing engine efficiency at 5–10% H 2 concentration. Moreover, based on the peak efficiency operating point, a set of optimum operating conditions for minimum emissions with the least amount of efficiency loss was suggested in terms of excess air ratio, spark ignition timing, and hydrogen addition rate as one of the main results.

Hydrogen addition effects on methane–air colorless distributed combustion flames

In this investigation the role of hydrogen addition in a reverse flow configuration, consisting of both non-premixed and premixed combustion modes, have been examined for the CDC flames. In the non-premixed configuration the air injection port is positioned at combustor exit end while the fuel injection port is positioned on the side so that the fuel is injected in cross-flow with respect to air injection. The thermal intensity of the flames investigated is 85 MW/m 3 atm to simulate high thermal intensity gas turbine combustion conditions. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra low NO x emissions are found for both the premixed and non-premixed combustion modes. Addition of hydrogen to methane fuel resulted in only a slight increase of NO emission, significant decrease of CO emission and extended the lean operational limit of the combustor.

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.