Investigations on Supercharging Stratified Part Load in a Spray-Guided DI SI Engine

Given the fact that, in an endeavor to achieve the goals of engineering for a trade-off between cleaning up exhaust emissions and maximizing fuel economy, two main paths are being followed in advancing and optimizing SI-engine operating strategy in the upper part-load range. On the one hand, homogenization and operation in the compression ignition mode seem to offer a promising means of minimizing NOx emission by keeping the combustion temperature below the formation borderline and accepting a high cylinder-pressure gradient to obtain benefits in fuel economy. On the other hand, there are ambitions to widen the range of stratified operation using a supercharger or turbocharger. This way, efficiency of the engine cycle can be improved by operating at a higher global air-fuel ratio and, with this, a higher polytropic exponent, thereby taking the efficiency chain to a higher level. As seen in wall-guided stratified combustion systems in the past, there is a trade-off between NO x mass flow / PM emission and fuel economy. However, it has not been possible to achieve efficient homogeneous lean-burn operation with this type of engine. Now, the spray-guided engine is seen to provide a much greater potential for optimizing mixture formation by using an advanced injection system that offers the greatest scope for avoiding wall impingement on the piston surface while delivering well-atomized fuel spray, even under high backpressure conditions.