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Direct injection stratified charge gasoline engines are becoming increasingly popular due to their potential for improved fuel economy and emissions. However, the benefits are restricted to low speed and load conditions due to the large air requirements during stratified operation. With boost, the air flow can be increased, extending the stratified operating regime and potentially the fuel economy and emissions benefits as well. This study assesses the feasibility of this technology using a variable geometry turbocharger and a supercharger as boost devices. The effect of boost on fuel economy, delivery of recirculated exhaust gas, and exhaust gas temperature are considered.

Low-Pressure Exhaust Gas Recirculation (LP-EGR) has been shown to be an effective means of improving fuel economy and suppressing knock in downsized, boosted, spark ignition engines. LP-EGR is particularly beneficial at low-speed, high-load conditions, but can lead to combustion instability at lower loads. The transport delays inherent in LP-EGR systems slow the reduction of intake manifold EGR concentrations during tip-out events, which may lead to excessive EGR concentrations at low load. This paper explores leveraging Variable Valve Timing (VVT) as a means of improving the rate of reduction of intake manifold EGR concentration prior to tip-out. At higher boost levels, high valve overlap may result in intake manifold gas passing directly to the exhaust manifold. This short-circuiting behaviour could potentially improve EGR evacuation rates.