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

Full-load operation of a small-displacement spark-ignition direct-injection (SIDI) engine was thoroughly investigated by means of computational analysis and engine measurements. The performance is affected by many different factors, which can be grouped as those pertaining to volumetric efficiency, to mixing and stratification, and to system issues, respectively. Volumetric efficiency is affected by flow losses, tuning and charge cooling. Charge cooling due to spray vaporization is often touted as the most significant benefit of direct-injection on full-load performance. However, if wall wetting occurs, this benefit may be completely negated or even reversed. The fuel-air mixing is strongly affected by the injection timing and characteristics at lower engine speeds, while at higher engine speeds the intake flow dominates the transport of fuel particles and resultant vapor distribution. A higher injector flow rate enhances mixing especially at higher engine speeds. The design of the intake ports is critical not only to optimize the through-flow but also to create a suitable in-cylinder flow that optimizes mixing. At lower speeds swirl may be used to enhance mixing and increase the torque. Using a flat piston provided little or no benefit relative to a bowl-in-piston except at high speeds, even though the mixture distribution showed differences at lower speeds. The combustion characteristics are significantly affected not only by the global mixture distribution but also by the local equivalence ratio in the vicinity of the spark plug. Global stratification adversely affects the knock propensity of the combustion system, which may be exacerbated if the charge cooling benefit of direct-injection is negated by wall impingement. By optimizing the various components of direct-injection full-load performance, a significant improvement was obtained relative to the initial baseline configuration, and the expected gains in torque and power over an equivalent PFI engine achieved.