Development and Optimization of a Small-Displacement Spark-Ignition Direct-Injection Engine - Stratified Operation

Superior fuel economy was achieved for a small-displacement spark-ignition direct-injection (SIDI) engine by optimizing the stratified combustion operation. The optimization was performed using computational analyses and subsequently testing the most promising configurations experimentally. The fuel economy savings are achieved by the use of a multihole injector with novel spray shape, which allows ultra-lean stratification for a wide range of part-load operating conditions without compromising smoke and hydrocarbon emissions. In this regard, a key challenge for wall-controlled SIDI engines is the minimization of wall wetting to prevent smoke, which may require advanced injection timings, while at the same time minimizing hydrocarbon emissions, which may require retarding injection and thereby preventing over-mixing of the fuel vapor. These conflicting requirements are heightened in a small-displacement engine, which has short path lengths between the injector and piston and is therefore prone to increased wall wetting. However, a side-injection system also requires sufficient spray penetration to reliably transport fuel to the centrally mounted spark plug at suitable injection timings. The multihole injector can simultaneously satisfy these different requirements because of enhanced vaporization, resulting in a shortened liquid length. This is attributable to increased air entrainment available because the spray does not collapse under elevated cylinder pressures typical of late injection. The piston bowl was optimized with respect to bowl depth and bowl volume to ensure sufficient mixing and air utilization at higher part-loads, minimization of wall wetting, and containment of the fuel-air mixture at light loads. The use of a variable swirl-control valve allows the air-motion to be optimized depending on the condition. It was found that the multihole injector configuration requires higher swirl compared to a fan-type injector. The combination of in-cylinder swirl, a suitable piston bowl shape and multi-hole injectors made it possible to obtain stable stratified combustion throughout a wide operating range with reduced NOX emissions under a large EGR rate.