Imaoka, Y., Shouji, K., Inoue, T., and Noda, T., "A Study of a Multistage Injection Mechanism for Improving the Combustion of Direct-Injection Gasoline Engines," SAE Int. J. Engines 8(3):1080-1087, 2015, doi:10.4271/2015-01-0883.
Technologies for improving the fuel economy of gasoline engines have been vigorously developed in recent years for the purpose of reducing CO2 emissions. Increasing the compression ratio for improving thermal efficiency and downsizing the engine based on fuel-efficient operating conditions are good examples of technologies for enhancing gasoline engine fuel economy. A direct-injection system is adopted for most of these engines. Direct injection can prevent knocking by lowering the in-cylinder temperature through fuel evaporation in the cylinder. Therefore, direct injection is highly compatible with downsized engines that frequently operate under severe supercharging conditions for improving fuel economy as well as with high compression ratio engines for which susceptibility to knocking is a disadvantage. On the other hand, direct-injection engines have certain issues such as the need to reduce particulate matter (PM) emissions, and technical measures must be developed for that purpose. Multistage injection is one method of improving direct-injection engines and has both advantages and disadvantages. One benefit of multistage injection is lower PM emissions caused by liquid fuel impinging on the cylinder wall. That is because multistage injections reduce the momentum of each fuel injection, and spray tip penetration is shorter than for a single injection. A second benefit is that the homogeneous quality of the fuel-air mixture is improved by the dispersion of the fuel spray. This report presents a detailed analysis of the following two new effects. One is increased air charging in the cylinder by the pressure oscillations generated by multiple fuel injections. The other is lower fuel consumption by improving the trade-off between mixture homogeneity and the cooling effect. By using computational fluid dynamics, in-cylinder visualization and tests of an actual engine, the mechanism involved in these four effects was analyzed, including consideration of the fuel injection state, fuel-air mixture formation and the effect of the in-cylinder temperature.