Spark-ignition engine speed profile optimization for maximizing the net indicated efficiency and quantitative analysis of the optimal speed profile

Abstract

To meet the tightened fuel economy or greenhouse gas emission standards in the transportation sector, many studies focus on the vehicle powertrain. Because hydrocarbon-based fuel is mainly used, an increase in the internal combustion engine efficiency is essential to meet the future regulations. This study discusses the improvement of spark-ignition engine efficiency through intracycle speed profile modulation. An intracycle speed profile modulating strategy could be adopted in the series hybrid electric vehicle powertrain architecture because the internal combustion engine shaft is not directly connected to the vehicle driveshaft. Under the wide-open throttle condition, the net indicated efficiency with the optimized profile increases by 1.3 %p compared to the peak efficiency achievable in conventional operation over 1,000–5,000 rpm and increases by 5 %p compared to the conventional operation with the same cycle average speed. This leads to the conclusion that the engine speed modulation in a spark-ignition engine would be largely effective near knock-limited operating conditions of a conventional engine. The efficiency gain is achieved by improving various performance parameters, conventionally in a trade-off relation. Combustion phasing is improved to the level of knock-free operating condition. In the optimal speed profile, the intake and compression process speed profiles are modulated to suppress knocking. The expansion process speed profile is mainly influenced by heat loss, and the gas exchange process speed profile is related to the pumping work. These propositions lay the foundational knowledge for further investigation into the topic in the future, including the hardware implementation.