Lean Operating Limit Research Articles

Fundamental study on lean operation limit of super lean-burn spark ignition engines: MIE transition and limit prediction

Fuel-lean combustion is challenging because of the difficulty in successful ignition-to-flame propagation transition in intense turbulence conditions. This study aims to elucidate the governing factor of fuel dependence on the lean limit through fundamental ignition experiments and numerical simulations. Previous scaling analysis has reported strong correlations between lean engine operation limit and Minimum Ignition Energy (MIE) transitions. Additionally, the temporal evolution of turbulent intensity in the engine cylinder plotted on Peters diagram suggested that the flame kernel growth occurs only in relatively weak turbulent intensity, u′, the condition under which u′ is lower than the MIE transition. To investigate the behavior of flame kernel growth in the vicinity of the MIE transition condition, we conducted ignition experiments under both laminar and turbulent conditions utilizing a constant volume chamber with counter-rotating fans. Flame initiation was achieved by spark discharge at various turbulent intensities. The results showed notable distinctions in flame kernel growth processes between below and above the MIE transition condition. For u′< MIE transition, flame kernel development is governed by molecular transports showing an apparent Lewis number effect, whereas for u′> MIE transition, the effect seems to disappear. Subsequently, experiments and numerical simulations on spherically propagating flames in quiescent mixtures with various blended fuels were conducted. The results indicated that fuels facilitating rapid flame kernel growth generally exhibited leaner engine operation limits, regardless of engine specifications. The present study successfully demonstrated that the fuels suitable for lean combustion could be predicted by investigation of spherically propagating flames in quiescent mixtures.

Jet Characteristics of a Narrow Throat Pre-Chamber and Influence on the Main-Chamber Combustion

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Lean combustion is one of the most applied methods to increase engine efficiency and maintain a good trade-off with engine emissions. The pre-chamber combustion (PCC) is one of the most promising combustion concepts to extend the lean operating limits of the engine. The Narrow throat pre-chamber has shown better lean limit extension compared to other ignition sources. The pre-chamber jets and the main-chamber combustion were studied in a Heavy-Duty optical engine using methane fuel. The tested conditions covered global excess air ratios (λ), between 1.9 to 2.3. The combustion process was recorded using three collection systems: (a) Natural Flame Luminosity (NFL) with a temporal resolution of 0.1 CAD; (b) OH* Chemiluminescence, and (c) CH* Chemiluminescence with a temporal resolution of 0.2 CAD for both. The propagating velocity of the reacting jets was studied using Combustion Image Velocimetry (CIV) based on bottom view images of the main chamber. After the pre-chamber jets ignite the main-chamber charge, a two-stage heat release rate (HRR) was observed for all the tested conditions. None of the three combustion visualizations exhibited a linear correlation with the HRR. The CIV reported high-speed jets penetrating around 400 m/s into the main chamber. The richest case of λ 1.9 presented a higher axial and radial propagation speed than the other cases. The individual vector fields have significant differences with the ensemble average fields, behavior characteristic of a high turbulent process. Finally, the ultra-lean case showed higher vorticity during the pre-chamber discharge, a synonym of high air-fuel entrainment that generates heavy waviness on the flame front. This heavy waviness generates local/partial or total extinction, resulting in abnormal combustion cycles.&lt;/div&gt;&lt;/div&gt;

Ultra-Lean Premixed Turbulent Combustion: Challenges of RANS Modelling

The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type “A” of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH4-air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap.

Numerical investigation of a fueled pre-chamber spark-ignition natural gas engine

Pre-chamber spark-ignition (PCSI) is a leading advanced ignition concept for internal combustion engines with the potential to enable diesel-like efficiency in medium-duty/heavy-duty (MD/HD) natural gas (NG) engines. By leveraging distributed ignition sources from multiple turbulent jets, the PCSI technology can deliver extremely short combustion duration in ultra-lean mixtures and significantly improve the engine thermal efficiency. However, in the automotive industry there is a lack of adequate science base and predictive simulation tools required for commercial development of PCSI engines. In this study, Reynolds-Average Navier-Stokes simulations are carried out to describe the combustion process in lean-burn NG engines, focusing on the combustion modeling approach. Two combustion models, multi-zone well-stirred reactor (MZ-WSR) and G-equation, are used to simulate the combustion process in an MD NG engine equipped with a fueled-PCSI system for four operating conditions close to the lean operating limit. A skeletal chemical mechanism and a laminar flame speed tabulation are used to compute the combustion accurately. Simulation results are compared with experimental data regarding measured cylinder pressure, heat release rate, and combustion duration. By dividing the PCSI combustion process into four distinct phases, the difference between the two models’ results for each phase is analyzed in detail. The MZ-WSR model overestimates the combustion duration for early flame kernel growth in the pre-chamber due to the lack of a specific formulation to take turbulence-chemistry interaction into account. Despite the prolonged combustion duration and low pressure built-up inside the pre-chamber, the model matches the combustion rate in the main-chamber. In contrast, the G-equation model delivers good agreements for the pre-chamber combustion and turbulent jet-driven combustion processes. However, the model starts to underestimate the combustion rate in the main-chamber, especially under ultra-lean mixture conditions. Finally, improvements are needed for both models to simulate the later combustion stage that occurred in the near-wall regions.

Assessment of Turbulent Combustion Models for Simulating Prechamber Ignition in a Natural Gas Engine

Abstract Lean combustion is a promising strategy to increase thermal efficiency in an internal combustion engine, by exploiting a favorable specific heat ratio of the fresh mixture while simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition and slow flame propagation at fuel-lean conditions lead to large cycle-to-cycle variability and limit the high-efficiency engine operating range. Prechamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling a faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling standpoint to date science base and available simulation tools are inadequate to properly understand and predict the combustion processes in prechamber ignited engines. In this paper, conceptually different Reynolds-averaged Navier–Stokes (RANS) combustion models widely adopted in the engine modeling community are used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a prechamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multizone well-stirred reactor model are employed for this modeling study. Simulation results are compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two evaluated approaches.

Improvement of performance and emission in a lean‐burn gas fueled spark ignition engine by using a new pre‐chamber

AbstractLean burn combustion of natural gas in spark‐ignited engines is a promising approach to improving engine performance and efficiency by reducing pollutant emissions and better fuel consumption. However, inducing excessive lean mixture to the engine makes the combustion unstable, and misfire happens in a significant number of cycles. The pre‐chamber ignition technology is a useful solution that can make spark‐ignition engines more efficient. In this study, the lean mixture's combustion was empirically investigated with a new pre‐chamber spark plug in a one‐cylinder engine fueled with natural gas. Tests were performed at 11.5, 13.5, and 16 compression ratios and a wide range of air‐fuel ratios. With the pre‐chamber spark plug usage, the lean operating limit is enlarged by 10%–45% and gives 4% and 3% the average lower in‐cylinder pressure and exhaust gas temperature than the conventional spark ignition system. The brake‐specific fuel consumption was improved by 4%–11% using the pre‐chamber spark ignition. The HC, CO, and NOx formation is about 11%, 16.3%, and 10% lower than the conventional spark ignition usage. Outcomes confirmed that the new design pre‐chamber plug gives 1% improved COVIMEP than the standard spark plug. The use of pre‐chamber due to the lean‐burn combustion causes larger ignition delay and increases brake thermal efficiency by up to 9%. It can be concluded that the pre‐chamber increases the ignition ability, and the lean mixture limit is extended.

The reactivity of hydrogen enriched turbulent flames

The use of hydrogen enriched fuel blends, e.g. syngas, offers great potential in the decarbonisation of gas turbine technologies by substitution and expansion of the lean operating limit. Studies assessing explosion risks or laminar flame properties of such fuels are common. However, there is a lack of experimental data that quantifies the impact of hydrogen addition on turbulent flame parameters including burning velocities and scalar fluxes. Such properties are here determined for aerodynamically stabilised flames in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Ret=314±19) using binary H2/CH4 and H2/CO fuel blends. The binary H2/CH4 fuel blend is varied from α=XH2/(XH2+XF)= 0.0, 0.2 and 0.4–1.0, in steps on 0.1, and the binary H2/CO fuel blend from α=0.3−1.0 also in steps of 0.1. The equivalence ratio is adjusted between the mixture specific lower limit of local flame extinction and the upper limit of flashback. The flames are characterised using PIV measurements combined with a flame front detection algorithm. The study quantifies the impact of hydrogen enrichment on (i) turbulent burning velocity (ST), (ii) turbulent transport and (iii) the rate of strain acting on flame fronts. Scaling relations (iv) that correlate ST with laminar flame properties are evaluated and (v) flow field data that permits validation of computational models is provided. It is shown that CH4 results in a stronger inhibiting effect on the reaction chemistry of H2 compared to CO, that turbulent transport and burning velocities are strongly correlated with the rate of compressive strain and that scaling relationships can provide reasonable agreement with experiments.

Reactive computational fluid dynamics modelling of methane–hydrogen admixtures in internal combustion engines: Part I – RANS

The effects of hydrogen addition to internal combustion engines operated by natural gas/methane has been widely demonstrated experimentally in the literature. Already small hydrogen contents in the fuel show promising benefits with respect to increased engine efficiency, lower CO2 emissions, extended lean operating limits and a higher exhaust gas recirculation tolerance while maintaining the knock resistance of methane. In this article, the influence of hydrogen addition to methane on a spark ignited single cylinder engine is investigated. This article proposes a modelling approach to consider hydrogen addition within three-dimensional reactive computational fluid dynamics in order to establish a framework to gain further insights into the involved processes. Experiments have been performed on a single-cylinder spark-ignition engine situated at a test bed and cater as reference data for validating the proposed reactive computational fluid dynamics modelling approach based around the G-Equation combustion model. Within the course of the first part, crucial aspects relevant to the modelling of the mean engine cycle are highlighted. In this article, a simplified early combustion phase model which considers the transition towards a fully developed turbulent flame following ignition is introduced, along with a second submodel considering combined effects of the walls. The sensitivity of the combustion process towards the modelling approach is presented. The submodels were calibrated for a reference operating point, and a sweep in hydrogen content in the fuel as well as stoichiometric and lean operation has been considered. It is shown that the flame speed coefficient A appearing in the used turbulent flame speed closure, weighting the influence of the turbulent fluctuating speed [Formula: see text], has to be adjusted for different hydrogen contents. The introduced submodels allowed for significant improvement of the in-cylinder pressure and heat release rate evolution throughout all considered operating conditions.

Experimental investigation on lean burn spark ignition engine using alcohol-gasoline blends

This study discusses the effects of addition of small amount of alcohols on performance and emission behaviour of a gasoline fuelled lean burn SI engine. Experiments were carried out in a single cylinder diesel engine, which is made to operate as SI engine on lean condition with gasoline as a fuel. The engine was operated at wide open throttle at a compression ratio of 10.5:1 and 1,500 rpm at diverse equivalent ratios by injecting the fuel into manifold. The test outcomes stipulate that use of ethanol-gasoline blend (10% by volume) was better compared with methanol-gasoline blend (10% by volume) and pure gasoline. The test results also revealed that there was 2.5% and 7.5% improvement in brake thermal efficiency with M10 and E10 blends and apparent extension in the lean limit of operation with alcohol-gasoline blends. In emission front, reduction in hydrocarbon and carbon monoxide emissions were observed. However, carbon dioxide and nitric oxide emissions were increased due to better combustion and high in-cylinder temperature. On the whole it is concluded that small alcohol addition to gasoline improves performance and reduce emissions for lean operating SI engine.

Investigation of peripheral vortex reverse flow (PVRF) combustor for gas turbine engines

In a reverse flow combustor air is injected from the exit side so that the bulk gas flow reverses its direction while exiting the combustor. In the combustor investigated here, a dominant peripheral vortex away from the exit is stabilized by carefully choosing the location of the air jet. Combustion is stabilized due to internally recirculated hot product gases in this peripheral vortex. The fuel is injected in the peripheral vortex region at high velocity and rapid mixing of injected fuel with the oxidizer results in avoidance of hot spot regions and leads to overall lean combustion and low pollutant emissions. Due to reverse flow geometry, along with the presence of the peripheral vortex, favourable residence time distribution of gases is achieved (shown numerically). This resulted in lower CO emissions (confirmed experimentally). In the non-premixed mode, fuel injection port is either placed on the exit/air port side (named as RS configuration) or on the opposite side of the exit/air port (named as RO configuration). The combustor operating in RO configuration displayed wider lean operational limit, and lower CO emissions as compared to the RS configuration, while NOx emissions were almost same in both the cases.

Influence of spark discharge characteristics on ignition and combustion process and the lean operation limit in a spark ignition engine

Lean combustion technologies have been investigated to decrease the heat loss of spark ignition engines. However, as the excess air ratio approaches the lean operation limit, the cycle-to-cycle variation of combustion becomes an obstacle to improving thermal efficiency. This paper discusses the influences of spark discharge characteristics, such as discharge current and spark-shortening phenomena, on the ignition and combustion process under lean conditions (excess air ratio (λ) of 1.8–2.3) to suppress the cycle-to-cycle variation of combustion and extend the lean operation limit. In this study, a customized inductive ignition system equipped with 20 conventional ignition coils was applied to enhance the ignition energy. The discharge interval between each coil unit was controlled to change the discharge current and duration. The results show that the in-cylinder discharged energy increased with the discharge interval. The cycle-to-cycle variation of combustion was minimized when the discharge interval was 0.4 ms, and consequently, the lean operation limit was extended to an excess air ratio (λ) of 2.1. The discharge waveforms indicated that the longer discharge interval could promote spark-shortening phenomena such as re-strike due to the lower discharge current. Finally, in-cylinder photographs of ignition and combustion process showed that the flame kernel formation could be promoted by the repetition of spark-shortening phenomena such as re-strike, as well as by the high elongation of the spark channel.

Investigations on effects of ethanol blending on performance and combustion characteristics of gasoline fuelled lean burn SI engine

This study discusses the effects of addition of small amount of ethanol on performance and emission behaviour of a gasoline fuelled lean burn SI engine. Experiments were carried out in a single cylinder diesel engine which is made to operate as SI engine on lean condition with gasoline as a fuel. The engine was operated at wide open throttle at a compression ratio of 10.5:1 and 1500 rpm at diverse equivalent ratios by injecting the fuel into manifold. The test outcomes stipulated that use of ethanol-gasoline blend (10% by volume) was better compared with pure gasoline. The test results also revealed that there was an accountable increase in brake power output, brake thermal efficiency and an apparent extension in the lean limit of operation with ethanol-gasoline blend. In emission front, reduction in hydrocarbon and carbon monoxide emissions were observed. There was an increase in carbon dioxide and NO emissions due to better combustion and high in-cylinder temperature. On the whole it is concluded that small ethanol addition to gasoline improves performance and reduces emission for lean operating SI engine.

저온 플라즈마가 인가된 예혼합화염의 희박 운전 및 소화 한계 확장

저온 플라즈마가 인가된 예혼합화염의 희박 운전 및 소화 한계 확장

An experimental analysis of hydrogen enrichment on combustion characteristics of a gasoline Wankel engine at full load and lean burn regime

In this paper, a gasoline Wankel engine was modified and equipped with self-developed hybrid electronic control unit to experimentally investigate the effect of hydrogen-enrichment level on combustion characteristics of a gasoline Wankel engine at wild open throttle position and lean burn regime. Testing were carried out under constant engine speed of 3000 rpm and the lean operating limit of the original gasoline engine. The spark timing was set at 15 °BTDC. The hydrogen energy fraction in the intake was gradually increased from 0% to 10%. The results showed that hydrogen enrichment was effective on improving the combustion process through the shortened of the flame development and the flame propagation periods, advancing the central heat release, increasing the HRRmax and reducing the cyclic variation proportionally to the amount of hydrogen added to the air fuel mixture. Furthermore, increasing hydrogen fraction in the intake improves the engine economy by reducing the cooling loss.

Effects of compression ratio and hydrogen addition on lean combustion characteristics and emission formation in a Compressed Natural Gas fuelled spark ignition engine

A single cylinder diesel engine was modified to operate as a Compressed Natural Gas (CNG) fuelled lean burn Spark Ignition (SI) engine. The engine was tested at 1500rpm under wide open throttle condition at different compression ratios over varying equivalence ratios. The optimized compression ratio for CNG operation was found to be 12.5:1 which was further investigated for hydrogen substitution at 5% and 10% on energy basis to study and compare the performance, emission and combustion behavior of CNG fuelled lean burn SI engine. The brake thermal efficiency and brake power output increases with rise in compression ratio and achieved a peak brake thermal efficiency of 30.2% with 12.5:1 compression ratio and above a critical value of 12.5:1, the improvement was small when compared to the increase in emissions. Wide flammability limits of hydrogen enable ultra-lean combustion thereby extends the lean limit of operation to an equivalence ratio of 0.42 with 10% hydrogen addition as compared to 0.50 with neat CNG operation and its anti-knock enhancement makes it advantageous compared to increasing the compression ratio under low load conditions. Hydrogen addition also enhanced the combustion rate, heat release rate and reduced cyclic variations. On the emissions front, hydrogen addition showed reduction in hydrocarbon emissions from 65g/kWh to 6.9g/kWh at an equivalence ratio of 0.5 alongside carbon monoxide and carbon dioxide reduction. Due to retarded ignition timing to avoid knock, the Oxides of Nitrogen (NOx) emission increase was not significant.

Numerical prediction of effects of CO2 or H2 content on combustion characteristics and generation efficiency of biogas-fueled engine generator

The fundamental effect of additives and the lean burn capability of methane combustion are investigated using cycle simulation and Latin hypercube sampling. The target engine is a spark-ignition engine fueled by methane and biogas mixed with hydrogen. The dominant variables were CO2 and H2 content, spark timing, and excess air ratio (EAR). Varying the amount of additives (CO2 and H2), spark timing, and EAR demonstrated that hydrogen plays an important role in extending the lean operating limit. The fractional factorial design of the experiment, Latin hypercube sampling, was applied to obtain the maximum brake torque (MBT) spark timing with variants in excess air ratios and additive content. When MBT spark timing is employed, the maximum mass fraction burned can be enhanced in the lean-burn region by increasing the hydrogen content with improved generating efficiencies under lean operating conditions.

Thermal and Acoustic Performance of Al2O3, MgO–ZrO2, and SiC Porous Media in a Flow-Stabilized Heterogeneous Combustor

A comparative analysis of performance of three different porous ceramic materials used as a flame-contained media within a flow-stabilized combustor is presented within this work. The experiments were performed at a constant air flow rate and variable methane flow rates. α-Al2O3, MgO–ZrO2, and SiC highly porous ceramics were used as the porous media. All three porous media used in the study had equal dimensions and porosity and were located in the same fixed position within the combustion chamber. Characterization of the structure of porous media before experimentation was performed using multiple methods. During combustor operation, temperature profiles were collected along with optical and acoustic emissions and correlated with the composition of the gaseous methane/air mixtures. It was found that, while SiC enables the highest temperatures within the combustion chamber at higher equivalence ratios, the ionically conducting MgO–ZrO2 porous media greatly expands the lean limit of combustor operation and ...

Effect of compression ratio and hydrogen addition on part throttle performance of a LPG fuelled lean burn spark ignition engine

A single cylinder CI engine was modified to operate as a LPG fuelled lean burn SI engine. The engine was tested at 1500rpm and 20% throttle opening at compression ratios 9:1, 10:1, 10.5:1 and 11:1 by varying equivalence ratios. The influence of compression ratio and 15% hydrogen substitution on energy basis at the optimal compression ratio of 10.5:1 on performance, emission and combustion behavior were studied and compared. The brake thermal efficiency and brake power output increases with rise in compression ratio, and above a critical value of 10.5:1 the improvement was small when compared to the increase in emissions. The advantage of brake thermal efficiency from higher compression ratio is narrow under low load conditions. The ever increasing load due to the auxiliaries which are likely to grow more require better low load performance and emissions. Wide flammable limits of hydrogen enable ultra-lean combustion and its anti-knock enhancement makes it advantageous compared to increasing the compression ratio at part throttle condition. Hydrogen addition enhanced the combustion rate and heat release rate, reduced cyclic variations, and extended the lean limit of operation. There was perceptible improvement in brake thermal efficiency, brake power and considerable reductions in hydrocarbon, carbon monoxide and carbon dioxide levels. Due to retarded ignition timing the NOx emission increase was not significant.

Combustion of a hydrogen jet normal to multiple pairs of opposing methane–air mixtures

The combustion performance of a cylindrical burner accommodating up to six multiple pairs of opposing methane–air mixtures with a cross-flow of hydrogen was addressed. The cross-flow initially duplicated the stagnation impact and enriched the vortical structures. Aided by the resulting flow strain, the transport of heat and active species from the hydrogen oxidation zone to the methane reaction zones accelerated the combustion across the opposing premixed flames and reduced the peak temperature across the outer diffusion flame. Increasing the cross-flow/opposing jets’ velocity ratio to 0.89 merged the two stagnation centers and maximized the shearing stress. By the slight increase in the velocity ratio to 1.07, the H and OH pools provided for methane combustion became closer to the ports such that a hydrogen/methane mass percent of 10.3% extended the stoichiometric blowout velocity from 28.3 to 35.7 m/s. Since the turbulent kinetic energy thus increased to 8.4 m2/s2, the firing intensity reached values as high as 48.2 MW/m3. Not only was there a reduction in the residence time for NOx formation, but also the blowout velocity relative gain overrode the relative increase in the NOx formation rates such that the NOx emission index decreased to 17 g/MWhr. By the excessive increase in velocity ratio, the vortical structures shrank such that the NOx exponential increase became dominant above 21 ppm. With fuel-lean mixtures, the hydrogen was partially combusted by the excess air from the opposing flames but the blowout velocity decreased to 13.1 m/s at Φ = 0.50. The hydrogen flame NOx emissions decreased by providing the excess air at larger jets’ diameter/separation ratios, thus reducing the residence times for thermal NOx formation and simultaneously interrupting the prompt NOx formation. At the lean operational limit, tripling the number of opposing jets decreased the hydrogen flame length by 54% such that the NOx emissions decreased by 38.4%.

Research on cycle-by-cycle variations of an SI engine with hydrogen direct injection under lean burn conditions

Considering the improvement on combustion stability for hydrogen addition under lean burn conditions, cycle-by-cycle variations of a spark ignition gasoline engine with hydrogen direct injection were studied. Effect of hydrogen fraction of 10% on cycle-by-cycle variations for lean burn with various excess air ratio and throttle position was analyzed. The results showed that coefficient of variation in indicated mean effective pressure decreased firstly and increased afterward with the retard in ignition timing, confirming there existed a minimal cycle-by-cycle variations ignition timing for a specified excess air ratio and throttle position. It was also found that the peak cylinder pressure, the maximum rate of pressure rise and the indicated mean effective pressure increased, and their corresponding cyclic variations decreased after hydrogen addition. Interdependency between the combustion pressure parameters and their corresponding angles tended to become strongly correlated after hydrogen addition. Moreover, the pressure-derived combustion parameters and the indicated mean effective pressure decreased with the increase of excess air ratio, and coefficient of variation in both the gross indicated mean effective pressure and peak cylinder pressure increased under leaner conditions. This effect would be weakened with the enlargement of throttle position. The engine lean operating limit could be extended after hydrogen addition.