Excess Air Ratio Research Articles

Multi-objective optimisation of the emission parameters and efficiency of pellet stove at different supply airflow positions based on machine learning approach

The main aim of this study is to optimise the effect of burner pot design by changing supply airflow position in a pellet stove. For this purpose, the calibration model based on Gaussian Process Regression (GPR) with optimised hyper-parameters was proposed to determine the relationship between the input and output parameters of the pellet stove. The designed model was validated by the experimental data and was measured by several statistical tools. The modelling findings showed that the developed model was an efficient technique and has the potential to predict pellet stove output characteristics with high accuracy. Subsequently, a multi-objective optimisation (MOO) by genetic algorithm using this developed GPR model was implemented to design the burner pot. The optimisation process was repeated for different excess air ratios with manufactured different burner pots which have four different supply airflow distances and the performance of the burner pots with optimised geometries was compared to the average of experimental data in three total thermal power input cases. The results showed that the significant improvements on emission and efficiency performance of the pellet stove was achieved with the optimised inputs in both each thermal power inputs and global cases.

Energy and exergy analyses of hydrogen-fueled spark ignition engine with various air excess ratios and ignition timings

Many studies have been conducted to determine the efficiency and emission of hydrogen internal combustion engines. However, only a few of these investigations have performed exergy analysis (especially when important parameters, such as excess air ratio, vary) based on the second law of thermodynamics. Accordingly, in this study, experiments were conducted on a commercial automotive spark ignition engine. The engine is modified to run on hydrogen as fuel at 1500 rpm with various spark timings and excess air ratios under a wide-open throttle. In addition, zero-dimensional two-zone modeling was conducted by matching and examining the experimental results considering both energy (first law of thermodynamics) and exergy (second law of thermodynamics) analyses.The results indicate that with the same excess air ratio, the trend exergy analysis results, such as increasing exhaust exergy loss and decreasing heat transfer exergy loss by retarding ignition timing, are similar to the energy analysis results. Additionally, irreversibility increased as the spark timing retarded owing to the reduction in combustion temperature caused by prolonged combustion duration. In considering various excess air ratios, the gap between the energy and exergy analysis results was evaluated: the heat transfer exergy loss increased from 27% to 31%, the exhaust exergy loss decreased from 20% to 15%, and irreversibility was maintained at approximately 10%. Irreversibility was maintained because of the offset between the irreversibility increment (due to the decrease in combustion temperature) and decrement (due to the decrease in fuel amount). Moreover, the relationship between the thermo-mechanical exergy rise rate and behavior of each exergy component was examined.

Combustion of Lean Methane/Propane Mixtures with an Active Prechamber Engine in Terms of Various Fuel Distribution

The possibilities for reducing the fuel consumption of internal combustion engines focus mainly on developing combustion systems, as one such solution is a two-stage combustion system using jet ignition. The combustion of gaseous mixtures with a high excess air ratio leads to an increase in overall efficiency and a reduction in the emissions of selected exhaust components. In such a convention, gas combustion studies were conducted in the methane/propane configuration. Using an active prechamber where spark plugs were placed and direct injection through a check valve, the fuel dose was minimized into the prechamber. The tests were conducted for a constant center of combustion (CoC). The combustion process in both the prechamber and main chamber was analyzed using a test stand equipped with a 0.5 dm3 single-cylinder engine. The engine was controlled by varying the fuel supply to the prechamber and main chamber in excess air ratio λ = 1.3–1.8. The study analyzed thermodynamic indices such as the combustion pressure in both chambers, based on which the SoC in both chambers, the rate and amount of heat released, AI05, AI90 and, consequently, the indicated efficiency were determined. Based on the results, it was found that the use of CH4/C3H8 combination degraded the thermodynamic indicators of combustion more than using only the base gas (methane). In addition, the stability of the engine’s operation was decreased. The advantage of using propane for the prechamber is to obtain more beneficial ecological indicators. For the single-fuel system, a maximum indicated efficiency of more than 40% was obtained, while with the use of propane for the prechamber, a maximum of 39.3% was achieved.

Experimental study on combustion and emission characteristics of ethanol-gasoline blends in a high compression ratio SI engine

The utilization of spark induced compression ignition (SICI) is a proven method to extend the operation load range, improve thermal efficiency as well as ensure robust control of the combustion process of homogeneous charge compression ignition (HCCI). However, negative valve overlap (NVO), intake-charge heating, and other methods of achieving SICI and the knock occurrence limit its load range and industrialization. In this paper, the effects of ethanol blending with different excess air ratios (λ) on the knock and combustion characteristics, output power, indicated thermal efficiency (ITE), and emissions were investigated in a high compression ratio (CR), single-cylinder, four-stroke SICI engine. The experimental results show that weak stratification increases the knock intensity, while strong stratification suppresses knock tendency at the stoichiometric ratio. Ethanol addition can suppress knock tendency, reduce the randomness of knock and improve the combustion stability. In comparison with pure gasoline, the IMEP and ITE of E100 increase by 6.3–15.1% and 6.8–9.1% at different λ, respectively. For conditions with low ethanol blending ratio (such as 0% or 10%) and λ with 1.0 or 1.2, knock is the limitation to further improve its thermal efficiency, while fuel economy is more affected by the overall combustion phase with an increase in ethanol blending ratio and λ. There exists a non-monotonic correlation between the NOx, THC, and CO emissions and the ethanol blending ratio due to the competing effects of oxygen-containing properties, higher latent heat of evaporation (LHE), intake-charge cooling effect, and higher combustion temperature. Ethanol blending, except for E100, does not exhibit the PN reduction tendency. Knock deteriorates PN emissions, especially in nucleation mode, and ethanol blending shifts its size distribution towards nucleation mode.

Effect of excess air ratio and ignition timing on performance, emission and combustion characteristics of high speed hydrogen engine

The main goal of automobile researchers is to develop internal combustion engines that are fuel efficient and emit zero pollutants. It can be inferred from prior research publications that lean burn conditions can significantly reduce emissions while improving engine efficiency. The lean-burn engine combustion temperatures are lower hence harmful emissions like NO are reduced. Gasoline fuels have a narrow equivalence ratio window hence it was necessary to evaluate the other alternative fuels with a wider equivalence ratio for using it in IC engines for better performance and fewer emissions. This experiment is conducted on a single-cylinder digital three-spark ignited electronic fuel injected (DTSI-EFI) single-cylinder, 4 stroke high-speed SI engine fuelled by hydrogen. The excess air ratios are changed and MBT timing was also optimized. Hydrogen has delivered the lowest emissions under lean conditions. This data gives guidelines for developing SI engines with hydrogen port fuel injection for meeting future emissions norms. This experimental attempt is to protect the environment from greenhouse gas (GHG) emissions. The highest Brake Thermal Efficiency (BTE) is recorded at the leaner condition (λ = 4) as 37.53%, the highest power output is 7.02 kW at λ=1.5. CO and THC emissions are absent in hydrogen fuel and NO emissions reduces towards lean combustion.

Effects of the Secondary Air Excess Ratio on the Self-Preheating Combustion Characteristics and NOx Emission of Semi-Coke

Effects of the Secondary Air Excess Ratio on the Self-Preheating Combustion Characteristics and NOx Emission of Semi-Coke

NO Emission and Burnout Characteristic in Combustion of Coal Gasification Fine Slag with Preheating Technology

ABSTRACT Medium-temperature (850 ~ 1000°C) preheating technology has proven to burn coal gasification fine slag (CGFS) steadily. However, the potential of further de-NO x and improving burnout at high preheating temperature (>1000°C) has not been reported. Likewise, the feasibility of preheating treatment at low temperature (<800°C) has also not been verified. Aiming to better understand the influence of preheating treatment on disposing CGFS, the preheating technology with a wider range of preheating temperature (600 ~ 1400°C) was tested on a two-stage drop-tube furnace system in this study. The effects of preheating temperature (Tp), O2 concentration at the outlet of system (O2outlet) and the excess air ratio in preheating zone (λ1) on NO emission and burnout were investigated. Our results showed that by increasing Tp above 1000°C and maintaining O2outlet concentration above 4%, the combustion of CGFS could achieve both low NO emission and high combustion efficiency. A critical preheating temperature of CGFS was obtained to be 600°C. Under this condition, the conversion ratios of C and N were extremely low in the preheating process, and the final NO emission hardly changed with λ1. Under pyrolysis condition (λ1 = 0), Tp should increase to 1400°C for achieving satisfying reduction of NO emission. λ1 had a positive effect on the reduction of NO emission as Tp was above 800°C. By controlling Tp to 1200°C and λ1 to 0.6, 39.2% of fuel nitrogen could be converted to gaseous nitrogen, in which the portion of N2 was 56.1%. Compared with coal, N-containing species in syngas of CGFS were mainly composed of NO, NO2 and N2. Finally, application prospect in the combustion of CGFS using a burner based on the idea of preheating is provided.

Influence of Pre-Chamber Volume, Orifice Diameter and Orifice Number on Performance of Pre-Chamber SI Engine—An Experimental and Numerical Study

This paper presented an experimental and numerical study of pre-chamber volume, number of orifices and orifice diameter influence on engine performance and emissions. All the measurements were performed on a single cylinder test engine at fixed engine speed of 1600 rpm, while engine load was varied by a change of the excess air ratio in the main chamber from a stochiometric mixture to a lean limit. The total of nine pre-chamber variants comprised three different pre-chamber volumes, two orifice number combinations (six and four orifices) and nine different orifice diameters. It was observed that the pre-chamber volume affects the indicated efficiency in a trend which is mostly independent of excess air ratio, with the efficiency gain between the best and worst results ranging from 1 to 4.4%. While keeping the same pre-chamber volume and the total cross-sectional area of the orifices, the larger number of orifices show better performance on two out of three investigated pre-chamber volumes, with the efficiency gains more pronounced at higher excess air ratios. Finally, on a fixed pre-chamber volume and number of orifices, the variation of orifice diameter leads to a trend in efficiency gains which favor larger orifice diameter. The comparison of the obtained efficiencies between all pre-chamber variants identified two pre-chambers, differing in each of the varied geometrical parameters, that show the best performance depending on excess air ratio range. On the other hand, a single variant which showed the worst performance on each excess ratio was identified. An additional investigation was performed by the application of the cycle-simulation model to quantify the share of emissions which are formed in the pre-chamber. The presented results showed that when PC volume is lowered, PC emission shares of NOX and CO grow larger. The influence of orifice number and size has a minor effect on the pre-chamber emissions shares. The maximum PC emission shares of 54.8% and 80.6% are achieved at lean limit (λ = 2.2) for NOX and CO, respectively. THC emission share, on the other hand, is not affected in a significant manner by either the pre-chamber geometry or operating conditions.

Study of n-heptane combustion atomized with superheated steam and at different excess air ratios in the gas generation chamber

Study of n-heptane combustion atomized with superheated steam and at different excess air ratios in the gas generation chamber

Comparison of the Engine Performance of Soybean Oil Biodiesel Emulsions Prepared by Phase Inversion Temperature and Mechanical Homogenization Methods

The engine performance and emission characteristics of burning emulsions of soybean oil biodiesel in a compression-ignition diesel engine prepared through the phase inversion temperature method were compared with those of neat soybean oil biodiesel and the emulsion prepared by the mechanical homogenization method. The engine torque was set constantly at 98 N·m with varying engine speeds. The experimental results show that the emulsion prepared by the method of phase inversion temperature had higher O2 and NOx emissions, a higher excess air ratio, a higher exhaust gas temperature, and a higher brake fuel conversion efficiency than the emulsion prepared by the mechanical homogenization method, which had lower CO and CO2 emissions, a lower equivalence ratio, and lower brake-specific fuel consumption. While the neat soybean oil biodiesel was found to have the lowest fuel consumption rate, brake-specific fuel consumption, and CO and CO2 emissions, it had the highest exhaust gas temperature and brake fuel conversion efficiency, NOx and O2 emissions, and excess air ratio among those three fuels. Therefore, the phase inversion temperature method is considered promising for preparing fuel emulsions as an alternative to petro-derived diesel for compression-ignition engines.

Analysis of Performance of Passive Pre-Chamber on a Lean-Burn Natural Gas Engine under Low Load

Stable ultra-lean combustion is an effective way for natural gas engines to reduce NOx emissions, but it also has higher requirements for ignition stability. The passive pre-chamber can effectively increase the ignition energy and extend the lean-burn limit of natural gas engines. However, the scavenging capacity of the pre-chamber limits its performance under low engine load. In this paper, several passive pre-chambers are designed, and the CFD simulation is carried out to investigate the influence of direction, number and diameter of the nozzle on the scavenging process. The discharge of residual gas in the pre-chamber is mainly affected by the nozzle diameter, which varies from 5.9% to 1% when the nozzle diameter increases from 0.8 mm to 1.6 mm. At the same time, claw-shaped electrodes are applied in the pre-chamber to increase the combustion rate, which is validated in a cylinder combustion simulation. Finally, the passive pre-chamber is tested on a 6-cylinder natural gas engine under 25% load. The results show that, under the proper ignition timing, the excess air ratio (λ) can reach 1.9 under 25% load of propulsion characteristic with a low level of cyclic variation. At this time, the NOx emission can be as low as 0.34 g/kWh without aftertreatment equipment.

Reduction in CO Emission from Small Reciprocating Engine Operated with Wood Gasifier by Mixture LHV Changing

In order to exchange the wood biomass energy for electric power with small capacity and high efficiency, it is most effective to use a reciprocating engine operated with a wood gasifier. On the other hand, such a small-capacity system is often installed in urban areas. Therefore, strict emission regulation should be observed. Normally, as the low heating value (LHV) of bio-syngas is small, the engine should be operated with a stoichiometric mixture to achieve a maximum power density. However, the emission with a stoichiometric mixture contains much unburned CO. This means that a stoichiometric mixture operation shows low efficiency and can’t observe the regulations. In this report, a mechanism of the unburned CO is considered, and a method to reduce the unburned CO ratio is shown with experimental results. In the experiment, a commercial reciprocating engine (4-stroke, modified single cylinder) is used. The bio-syngas, a producer gas from a fixed bed gasifier, is produced by a self-made wood pellet gasifier (fixed bed, auto thermal down-draft). The bio-syngas flow rate is calculated with the nitrogen ratio between input air and bio-syngas. The LHV is adjusted with the city gas (as an alternative to methane) and hydrogen. The CO volume ratio of the exhaust from the engine is more than 3 v% when the excess air ratio of bio-syngas/air mixture is 1.3, as the LHV of bio-syngas is less than 5.0 MJ/m3-LHV. On the other hand, the CO volume ratio of the exhaust under operation of the mixture, the bio-syngas, and methane with more than 7.0 MJ/m3-LHV was less than 0.2 v%. The CO in the exhaust with low LHV fuel means that the combustion is not finished in the chamber. The unburned ratio could be predicted in consideration of the gap/clearance as crevice, the temperature boundary layer, and the quenching distance.

Analysis on influencing factors and improvement of thermal efficiency of bagasse boilers based on performance test data

Based on the performance test data of the 136 samples for bagasse boiler in Guangxi Zhuang Autonomous Region, China, the energy analysis of bagasse boilers is performed. The results show that the average thermal efficiency was estimated to be 86.75%, and 39.71% of samples did not meet the limited value requirements, only 1.47% in quantity met the target value, and the energy saving potential is very large. The heat loss of bagasse boilers is dominated by the heat loss of flue gases. Therefore, the effects of bagasse moisture content, excess air ratio and oxygen concentration on flow rate of flue gas and thermal efficiency were further simulated. The average thermal efficiency of bagasse sample boiler will increase obviously, when the moisture content of bagasse and excess air ratio decrease and oxygen concentration increases. In addition, the average value of carbon dioxide emission was 131 kg/GJ. If the thermal efficiency of the substandard boilers reaches the target level, bagasse consumption and carbon dioxide emissions will be reduced by 526 kt and 466 kt per year, respectively. This will be of great significance to energy conservation and emission reduction.

Experimental study on the effect of hydrogen substitution rate on combustion and emission characteristics of ammonia internal combustion engine under different excess air ratio

Reducing carbon emissions in the transport sector requires technology upgrades to engines and the use of carbon–neutral fuels. Ammonia is a zero-carbon renewable fuel that complements hydrogen. This study is based on a Miller cycle spark ignition engine using ammonia-hydrogen fuel. The effects of the mixture ratio of ammonia and hydrogen under stoichiometric ratio and lean burn condition on the performance of the Miller cycle engine are focused. First, a commercial Miller-cycle ICE was modified to allow a dual fuel supply of ammonia and hydrogen. The engine was run steadily at 1500 rpm with the manifold absolute pressure (MAP) maintained at 60 kPa, and the ammonia and hydrogen supplies were adjusted to achieve different mixing ratios and excess air ratios. The test results showed that an increased ammonia percentage under stoichiometric ratio conditions significantly improved the Braking mean effective pressure (BMEP) and Braking thermal efficiency (BTE) of the ICE. Under lean burn conditions, the changes of BMEP and BTE are small with the increase of the ammonia ratio. The exhaust temperature of the ammonia-hydrogen engine is relatively low, so it is necessary to consider matching a small inertia turbocharger. Ammonia ratios above 80 % by volume resulted in erratic engine operation and even misfires. Ammonia-hydrogen fuel blends lead to higher Nitrogen oxide (NOx) emissions.

Studying the effects of manifold pressure boosting and EGR on combustion and NOx emission of hydrogen-fueled SI engine

Aiming to the global energy insecurity due to extensive dependence on fossil fuels, hydrogen is supposed to provide relief by researchers. However, employed to combustion engines, this green fuel leaves hazardous NOx and limits output due to low volumetric energy content. Hence, improving operating parameters like compression ratio and manifold pressure, and NOx mitigating technique like EGR may prove a boon. Therefore, in the present study, combustion and NOx emission behavior of the SI-engine is studied under variable manifold air pressure (MAP) (up to 130 kPa) and EGR (up to 15%) at compression ratio 14:1. The outcomes shows the flame development increased by 1.24%, while flame propagation is increased by 1.63% by MAP boosting due to increased air and fuel supply. Cylinder pressure and heat-release rate (HRR) are also surged due to the increased fuel supply to maintain excess-air ratio (λ). However, peak cylinder pressure is retarded due to elongated combustion. The exhaust gas temperature (EGT) is increased by 19.4%, while elevated Tmax is observed to shoot up NOx by 40%. Combustion in improved at low EGR due to reduced λ, and also the specific NOx. At higher EGR rate deteriorated combustion due to increased heterogeneity by high dilution, increasing combustion duration, cooling losses and reducing cylinder pressure, Tmax and EGT. A 9.33% and 5.67% increase in CA10 and CA10–90 respectively is observed for 130 kPa at 15% EGR than no EGR. The rapid reduction in oxygen and higher heterogeneity reducing residence time, resulted in rapid drop (33% at N/A to 42.68% at 130 kPa for 15% EGR) in NOx by EGR. The coefficient variation is reduced by boosting MAP but increased severely at higher EGR, which restricts operating EGR rate.

Assessment of the application of oxygen enrichment in the hydrogen-fueled Wankel rotary engine

Hydrogen-fueled Wankel rotary engines (WREs) with excellent dynamic and emission characteristics have the potential to become a substitute for current fossil-fueled reciprocating piston engines. Oxygen enrichment can be employed to solve the lower power of hydrogen-fueled internal combustion engines, however, it has not been applied in hydrogen-fueled WRE at present. Hence, the goal of this work is to experimentally assess the application of oxygen enrichment in the hydrogen-fueled WRE. This work is conducted at 1500 r/min, wide-open throttle and 2.0 excess air ratio. The work found that within the test range, oxygen enrichment can effectively improve the brake torque of hydrogen-fueled WRE, however, penalize the brake thermal efficiency and NO emission. When the oxygen fraction in air increases from 21% to 29.1%, the brake torque is increased by about 45% while the brake thermal efficiency is lowered by about 8.5% and NO emission deteriorates by three orders of magnitude. Oxygen enrichment can accelerate flame development and propagation process. Besides, as the oxygen-enriched level is increased, abnormal combustion, including knock, backfire and pre-ignition, began to occur and affected the normal operation of hydrogen-fueled WRE. In general, compared with the high oxygen-enriched level, which has severe penalties on performance except for power output, low oxygen-enriched level has higher application potential.

Part-load performance prediction of a novel diluted ammonia-fueled solid oxide fuel cell and engine combined system with hydrogen regeneration via data-driven model

This paper proposes a novel diluted ammonia-fueled solid oxide fuel cell and X-type rotary engine (SOFC-XRE) combined system with hydrogen regeneration sub-system to realize high energy conversion efficiency. The combined system key design parameters and geometric parameters are optimized. The influences of SOFC inlet temperature, stack fuel utilization ratio, stack excess air ratio and pressure ratio across membrane for permeating hydrogen on system off-design performance are investigated. Meanwhile, the improved off-design performance of combined system is revealed by comparing with standalone SOFC. The data-driven neural network model (NNM) is established for predicting system part-load performance based on the off-design performance datasets. The optimal system operation parameter combinations and performances in a wide power load range are obtained by combining data-driven NNM with particle swarm optimization (PSO) algorithm. The results show that the cell stack temperature gradient decreases with decreasing fuel flow rate coefficient. The stack fuel utilization ratio has a larger effect on combined system energy efficiency than the fuel flow rate coefficient. The optimal membrane pressure ratio and fuel flow rate coefficient decrease with decreasing power load. The combined system energy efficiency is increased by 5.94% as the power load is decreased by 75%, while it is decreased by 2.1% as the power load is increased by 20%. The combined system energy efficiency is 10.9–12.2% higher than standalone SOFC in a wide combined system power load range of 120–25% without carbon emission.

Effect of Excessive Air Ratio on Hydrogen-fueled Spark Ignition Engine with High Compression Ratio Using Direct Injection System toward Higher Brake Power and Thermal Efficiency

Effect of Excessive Air Ratio on Hydrogen-fueled Spark Ignition Engine with High Compression Ratio Using Direct Injection System toward Higher Brake Power and Thermal Efficiency

Combined impact of excess air ratio and injection strategy on performances of a spark-ignition port- plus direct-injection dual-injection gasoline engine at half load

Spark-ignition direct-injection gasoline engine is one of the main power of passenger cars. In this study, combined impact of excess air ratio (λ) and injection strategy on performances of a spark-ignition port- plus direct- injection (PI + DI) dual-injection gasoline engine at half load was investigated experimentally. Experimental results show that the brake mean effective pressure decreased with increasing λ at half load; the brake thermal efficiency first increased, then reached the highest value at approximately λ = 1.1 or 1.2 for different injection strategies, and finally decreased with increasing λ; the maximum cylinder pressure, maximum heat release rate and maximum cylinder temperature decreased with increasing λ, and their corresponding crank angles delayed; the starting of combustion (CA05) increased with increasing λ, the CA05 at PI0DI100 (0 % PI and 100 % DI per cycle), late DI mode and any λ values was the closest to top dead center; for early DI mode, combustion center (CA50) of PI0DI100 was greater or lower than that of PI50DI50 (50 % PI and 50 % DI per cycle) at roughly λ < 1.15 or λ > 1.15, respectively; combustion stability (COVimep) of early DI mode can be controlled within 2 %; the λ had a significant effect on brake-specific carbon monoxide (BSCO) emission at rich mixture and a little effect on BSCO emission at lean mixture; the brake-specific hydrocarbon (BSHC) emission of early DI mode was much higher than that of late DI mode, the injection strategy had an important effect on HC (hydrocarbon) emission; for early DI mode, the brake-specific nitrogen oxides (BSNOX) emissions first increased with increasing λ, then reached the highest value at λ = 1.1, and finally rapidly decreased with further increasing λ; for late DI mode, the BSNOX emissions decreased with increasing λ; the soot emission decreased with increasing λ; the soot emission of late DI mode was much higher than that of early DI mode; at a constant λ, sootPI50DI50,early DI mode ≈ sootPI0DI100, early DI mode ≪ sootPI50DI100, late DI mode < sootPI0DI100, late DI mode.

Energy and exergy characteristics of an ethanol-fueled heavy-duty SI engine at high-load operation using lean-burn combustion

Ethanol, as the most produced renewable biofuel, is considered a promising low-carbon alternative to petroleum-based fuels in the transport sector due to its high energy density and auto-ignition resistance. The lean-burn combustion in spark-ignition (SI) engines has the potential to further improve thermal efficiency in regard to knock mitigation and the reduction of combustion temperature. However, the characteristics of lean-burn combustion in an ethanol-fueled engine in relation to the combustion losses and the gas-exchange process remain unclear, especially for high-load operation. This study contributes with a deeper understanding of the high-load performance of an ethanol-fueled heavy-duty SI engine using lean-burn combustion. Based on the experimental results from a single-cylinder engine test, a 6-cylinder engine model is built by integrating a validated predictive combustion model to characterize the lean-burn combustion process. The engine’s thermal efficiency and combustion phasing are evaluated for knock limited operation and then compared to the theoretical optimum which is regardless of knock. The energy and exergy balances are applied to evaluate the effect of dilution with excess air ratios up to 1.8. Losses through heat transfer, exhaust flow, and incomplete combustion are quantified. In addition, entropy generated through combustion is discussed to identify the relationship between exergy destruction and different operating conditions. In the context of lean-burn combustion, the thermal efficiency at high-load operation incrementally increases from 40.4% at stoichiometric condition to 47.3% at an excess air ratio of 1.8. At the same time, the exergy destruction through combustion increases by 3.3 percentage points across the selected dilution range. Furthermore, the challenging requirements to realize lean-burn combustion with lower exhaust gas temperatures and higher intake boost pressures is assessed through an exergy analysis of the turbocharging system.