Investigation into the Optimized Heat Release Rate and Corresponding Variation of In-Cylinder Specific Heat Ratio for the Improvement in Thermal Efficiency by Utilizing Two-Zone Combustion Model Analysis

Improvement in heat loss could be an important factor to increase the brake thermal efficiency (BTE) of an internal combustion engine; however, the heat energy saved isn’t all converted to brake work. Theoretically, to increase the conversion efficiency of heat energy into indicated work, the compression (or expansion) ratio and specific heat ratio (γ) are important. Nevertheless, γ has not been well-studied thus far, since it can’t be easily controlled. This study utilized a two-zone model to calculate the time-resolved γ and local excess air ratio of the burned gas (λ_b), which varied with the heat release rate. The two-zone combustion model, in which the cylinder volume is simply separated into burned and unburned zones to simulate the overall diesel combustion phenomena, was developed to investigate the current status of heterogeneous (diesel) combustion compared to ideal homogeneous combustion. The study focused on the instantaneous variation in γ and resulting indicated thermal efficiency. The model results were also directly applied to analysis of experimental results through λ_b estimation. Engine experiments were carried out utilizing a single cylinder engine with multiple injectors. Initial zero-dimensional simulation with various combinations of excess air ratio for isochoric and subsequent isobaric burned zone suggested the potential of additional thermal efficiency improvement by further diluting and homogenizing the combustion zone, particularly during the isobaric combustion phase. Then by application of the model to experimental results, λ_b was estimated and a potential improvement in BTE (due to improved mixture formation during isobaric combustion) was identified.