Quantification of Efficiency Gains for Dilute IC Engines due to Increases of the Ratio of Specific Heats

Recent engine developments have demonstrated significant thermal efficiency gains for IC engines employing lean mixtures and high levels of exhaust gas recirculation (EGR). These efficiency gains have often been attributed to reduced heat losses and increases of the ratio of specific heats. No previous publication, however, has provided the quantitative contributions from these two items. This lack of information, therefore, motivated the current work. An automotive engine was selected for this study, and a thermodynamic engine cycle simulation was used for the evaluation. Engine conditions included a range of loads and speeds. For each engine condition, three cases were considered. These cases varied the equivalence ratio from stoichiometric to 0.7, and varied the EGR from zero to 45%. Depending on the engine conditions, the net indicated thermal efficiency increased between 4.2% and 8.9% (absolute) for the engine with the lean mixture (ϕ = 0.7) and EGR (45%). The lower gas temperatures and lean mixtures resulted in reduced heat losses and increases of the ratio of specific heats. For all conditions examined, the majority of the thermal efficiency gains were due to the increases of the ratio of specific heats. The contributions from the increases of the ratio of specific heats toward the efficiency gains ranged between about 46% and 82% for the conditions examined. The rest of the gains were from the reduced heat losses.