Combustion control for waste-heat recovery system in internal combustion engine vehicles: Increase in exhaust-gas heat by combustion phasing and its effect on thermal efficiency factors

Utilizing exhaust-gas heat can be effective to improve the overall thermal efficiency of engine vehicles. However, the exhaust-gas heat and temperature at light loads may be insufficient for some exhaust-gas heat recovery systems. Exhaust-gas heat is largely influenced by the combustion phase, and a retardation of spark timing could increase exhaust-gas heat considerably. The current work evaluates the effect of a large retardation of spark-ignition timing on the exhaust-gas temperature and heat at steady operating conditions and at vehicle driving mode conditions. The relationships between the exhaust heat and thermal efficiency factors such as cooling-loss fraction, degree of constant volume, and pumping-loss fraction are also analysed in detail. The results show that a combination of spark retardation and intake throttle opening can increase exhaust-gas heat with minimum deteriorations in engine efficiency. The minimum deterioration in efficiency is attributed to reductions in cooling and pumping losses, while the degree of constant volume is decreased by the retarded combustion phase.