CFD 3D Analysis of Charge Motion and Combustion in a Spark-Ignition Internal Combustion Engine under Close-to-Idle Condition

The increasingly stringent limitations on noxious missions of transport sector highly affect the development of new engines. The operating conditions of the engine at low-load and idle play a relevant role along the regulatory homologation cycles, contributing to overall emissions. In this work, the effectiveness of some solutions to improve the behaviour under close-to-idle operation of a Spark-Ignition motorcycle engine are compared by 3D CFD analyses. Specifically, the effects of two designs of the intake port and of the opening direction of the throttle valve, either clockwise or counterclockwise, are investigated. Multi-cycle simulations are carried out, under motored and fired conditions, for a single close-to-idle operating point. The various designs are compared in terms of capability to generate a stable tumble vortex during the intake phase and to produce an adequate turbulence level at the beginning of the combustion process. The analyses revealed that a clockwise throttle opening can produce enhanced turbulence levels at the end of the compression stroke, especially in a close-to-spark region (increase of about 5% and 27 % at the TDC at a global and local level, respectively, compared to the base configuration). Additional limited improvements are obtained with the high tumbling design, where, however, a penalty on the maximum power output could emerge. The flow and turbulence motion differences among the tested geometries reflect on combustion development in its early stage, and on its degree of completeness at the exhaust valve opening. A clockwise opening of the throttle valve leads to an increase of the mass fraction burned of 5 percent points, compared to the base configuration.