During most of the operating conditions occurring on a vehicle driving cycle, a reciprocating IC engine works at low load and low speed, with poor fuel efficiency. In this regard downsizing appears as a major way of improving fuel consumption of Spark Ignition Engines. In fact, downsized engines have smaller friction surfaces and can work on the same vehicle and on the same driving cycle with higher mean effective pressure and higher efficiency. In this paper the main technical trends and problems related to SI engine downsizing are reviewed and discussed. Assuming a stoichiometric boosting, a simulation code is used to outline a strategy to improve low end torque of a downsized DISI engine. In the numerical experiments volumetric efficiency is enhanced by an optimal configuration of the inlet system. For the same objective, assuming a Variable Valve Timing, a proper selection of maximum lift and opening duration of the inlet valve allows a reduction of the reverse flow of fresh mixture. The optimization of the exhaust system and of the lift diagram of the exhaust valve leads both to the enhancement of volumetric efficiency and to the reduction of residual exhaust gas, with beneficial effects on knock phenomenon. An evaluation of fuel consumption gains resulting from downsizing is made as well, with reference to a New European Driving Cycle.

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