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Diluents have an essential effect during combustion. Discovering the influence of CO2 and H2O as diluents on laminar burning velocity (LBV) is helpful for combustion control and optimization. In this study, CH4/air/CO2/H2O mixtures were investigated and validated using the FFCM‐Mech 1.0 over extensive boundary conditions. The chemical effects of the diluents CO2 and H2O were separated using a decoupling method. It was found that an increase in initial temperature promotes the chemical effects, while an increase in initial pressure does the opposite. In addition, the inhibiting effect of CO2 on LBV is stronger than that of H2O. Sensitivity, mole fraction, and rate of production (ROP) analyses were used to reveal that the sum of the chemical effects of adding CO2 and H2O separately was greater than the chemical effects of adding equal amounts of CO2 and H2O simultaneously. This paper not only investigates the effect of CO2 and H2O on the LBV under wide boundary conditions, but also offers a valuable guide for studying the operating conditions and intensity settings of exhaust gas recirculation (EGR) and theoretical guidance for further research on the combination of EGR and in‐cylinder water injection technology.

At partial load, traditional automotive gasoline engines have high pumping losses due to the throttling of the intake charge for load control. Variable Valve Timing (VVT) and the introduction of externally cooled EGR could reduce the pumping losses but only with a very limited effect. On the other hand, in the medium to full load range, the engine cannot utilize a high compression ratio due to limitations in knocking. A variable displacement, variable compression ratio device which utilizes an asymmetric camshaft to realize the different closure times of the two intake valves is discussed in this paper. The large-scale change in the intake valve timing leads to the large-scale change in the effective cylinder volume at the intake valve closure, which realizes a variable cylinder volume and a variable effective compression ratio. The device is utilized to reduce the pumping losses and to increase the in-cylinder thermal efficiency at the same time. Engine dyno test results indicate that, in the low to medium load range, a later closure of the intake valve could reduce the effective cylinder volume, and the intake pressure could be significantly increased, and therefore pumping losses reduced. However, the reduced effective cylinder volume due to a later intake valve closure would lead to reduction in the effective compression ratio (ECR) and a drop in in-cylinder thermal efficiency. Therefore, there is a balance point between the pumping loss reduction and the drop in in-cylinder thermal efficiency. On the other side, in the medium to full load range, when avoiding knocking becomes the major controlling factor of the combustion phasing (degree of constant-volume combustion) and the effective expansion ratio (EER), too high of an effective compression ratio would lead to significant drop in the effective expansion ratio EER and also the in-cylinder thermal efficiency. Therefore, there exists a best compromise between the ECR and EER, and the best system would be one with a moderate ECR but an EER as high as possible. The quantitative equations which include both ECR and EER in the thermal efficiency calculations captured the above observations pretty well and can be utilized to optimize for the best compromise of IVC, EVO, ECR, EER and engine performances during the concept stage and/or the calibration stage of an engine.

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