Introduction

The spark-ignition engine finds wide applicability in light-duty vehicles for its low weight, relative simplicity, and more importantly, its ability to run on high energy–density liquid fuel. However, losses due to throttling and fixed cam timings affect its performance. Variable valve timing and variable valve lift with mechanical cams have successfully ameliorated this problem to a large extent. Downsizing by turbocharging reduces friction and improves the engine performance by running at lean conditions. Direct injection of fuel in the cylinder is currently actively pursued to improve fuel consumption by reducing the pumping losses and introducing lean burn over wide operating conditions. The active and passive prechamber concepts are continuously explored to further lean out combustion and burn new generation fuels including hydrogen gas, with the help of turbocharging even for heavy-duty applications. The emissions from lean burn and stratified charge engines need solutions different from a three-way catalyst which is suitable for only stoichiometric combustion engines. Lean NOx catalyst, oxidation catalyst, selective catalytic reduction, and particulate trap are some of the solutions. Thus, the frantic development of spark-ignition engines needs the support of simulation and models to understand the interaction of combustion and complex airflow, corroborated by experiments that are expensive and somewhat limited in scope. Classical thermodynamics and fluid mechanics are used in zero- and one-dimensional models. To understand the details of flow in the cylinder and the manifolds, 3D CFD is used. The CFD is well-matured and validated to apply with confidence by the designers.