Homogeneous Charge Compression Ignition (HCCI) combustion is being considered as a practical solution for diesel engines due to its high efficiency and low NOx and PM emissions. However, for diesel HCCI operation, there are still several problems that need to be solved. One is the spay-wall impingement issue associated with early injection, and a further problem is the extension of HCCI operation from low load to higher engine loads. In this study, a combination of Adaptive Injection Strategies (AIS) and a Two-Stage Combustion (TSC) strategy are proposed to solve the aforementioned problems.
A multi-dimensional Computational Fluid Dynamics (CFD) code with detailed chemistry, the KIVA-CHEMKIN-GA code, was employed in this study, where Genetic Algorithms (GA) were used to optimize heavy-duty diesel engine operating parameters. The TSC concept was applied to optimize the combustion process at high speed (1737 rev/min) and medium load (57% load). Two combustion modes are combined in this concept. The first stage is ideally HCCI combustion and the second stage is diffusion combustion under high temperature and low oxygen concentration conditions. AIS were proposed to achieve and optimize the TSC. AIS incorporate the use of multiple injections with different spray geometries and injection pressures. Part of the fuel is injected into the cylinder at a low injection pressure (10MPa) and a narrow spray angle (90°) to avoid spray-wall impingement and to prepare a homogeneous mixture for the HCCI combustion. The remaining fuel is injected into the cylinder at a high injection pressure (150MPa) and a wide spray angle (127°) to experience diffusion combustion and to provide extended load capability. An Adaptive Injection Strategy, which uses one injector with single included angle but two different injection pressure levels, was also evaluated in this study.
Several engine operating parameters were optimized: Intake Valve Closure (IVC) timing, Exhaust Gas Recirculation (EGR) ratio, start of low-pressure and high-pressure injection timings and the fraction of fuel participating in the HCCI combustion (HCCI ratio). The results show that by combining the AIS with a late IVC timing, a medium EGR level, and a high HCCI ratio, TSC is able to achieve low engine-out emissions with spray-wall impingement avoided at a medium engine load. The TSC concept shows great potential to meet future ultra-low emission standards.