Abstract
Losses related to pumping and knocking are the most detrimental to the efficiency of SI engines. That is why concepts offering variable displacement, extended expansion and variable compression ratio capabilities are regularly investigated.
In this context, this article aims at analyzing the impacts of piston kinematics on thermal efficiency and knocking combustion. This work is not based on any particular technological concept. Therefore, the reported results are not limited to existing architectures. Contrariwise, this study offers a pure thermodynamic analysis which points out the technological levers that should be optimized to improve the knocking resistance and the efficiency of future breakthrough concepts. Thanks to 0D simulations, this analysis clearly highlights the known but often underestimated link between the piston kinematics and the efficiency.
Simulations of various piston kinematics at different loads and for different spark timings have been performed with a two-zone combustion model. The Woschni correlation has been adapted to non-conventional kinematics and tabulated auto-ignition delay times have been used to determine the knock onset according to the Livengood-Wu integral.
Simulation results show that extended expansion is beneficial for improving both the thermal efficiency at low loads and the knocking resistance. At high loads, high expansion ratio can indeed improve the knocking resistance thanks to a fast volume increase and therefore a fast temperature decrease after top dead centre. For the same reason, a short and fast expansion after top dead centre can also decrease the heat loss to the walls and improve the efficiency. These characteristics can also help in improving the knocking resistance at high loads by limiting the temperature increase during combustion, and thus by controlling the auto-ignition delay times.
KEYWORDS – Spark-ignition engine, knocking, efficiency, piston kinematics, variable strokes, extended expansion, variable compression ratio