Contributors to the Fuel Economy Advantage of DISI Engines Over PFI Engines

A methodology was developed, based on engine-simulation analysis and experiments, to evaluate quantitatively the contributions of the various factors on the fuel-economy advantage of direct-injection, spark-ignition (DISI) engines over corresponding port-fuel injection (PFI) engines. The fuel-economy comparison was based on a set of seven, steady-state test points, which simulate a 2400-kg vehicle powered by a 5.3-L V8 engine over the Federal Test Procedure (city cycle). The results show that the DISI engine has a 15% fuel-economy advantage over the corresponding PFI engine operating without EGR. The biggest positive contributor to this gain is the reduced pumping losses, which account for a 10% gain, followed by: favorable mixture properties due to lean/dilute operation with about a 7.5% gain, lower heat losses with a 2% gain, and higher compression ratio with a 3% gain. In contrast, the negative contributors (i.e., factors causing a decrease in fuel economy) were: lower combustion efficiency and combustion phasing losses which account for a 4% loss, and higher friction due to higher peak pressures, which yields a 4% loss. With further reduction of pumping losses, minimization of the surface area-to-volume ratio, and improvements in combustion through optimization of the phasing and improvements in the combustion efficiency, it may be possible for the DISI engine to achieve a 26% fuel economy improvement relative to a corresponding PFI engine.