Effects of Fuel Injection Strategy during Negative Valve Overlap on HCCI Combustion and Ion Current Characteristics

Homogeneous Charge Compression Ignition (HCCI) combustion has the characteristics of high efficiency and low emissions. However, the lack of a decisive control method of combustion timing hinders its application. In this paper, the effects of fuel injection strategies on HCCI combustion and ion current (IC) characteristics were studied experimentally and numerically. The results show that by advancing the fuel injection timing or increasing the fuel injection ratio during negative valve overlap (NVO), the combustion phase and the start of ion current in the main combustion are both advanced, and the strength of the IC signal during NVO is enhanced as well, indicating that more active species will lead to earlier combustion. Additionally, HCCI combustion is determined not only by the reforming of fuel composition in NVO but also by the distribution of mixture and the gradient of concentration in the cylinder. The IC signal during NVO has a strong correlation with the concentration distribution of the mixture in the cylinder. From the simulation results, the equivalence ratio (φ) in the range of 0.9-1.1 is more conducive to the generation of electrons. Hence, the HCCI combustion phase could be controlled effectively by changing the φ during both NVO and main combustion. Therefore, by reflecting the local concentration of active species, the IC signal can be utilized to extract combustion information in the cylinder. By adjusting the cycle-based injection strategy, the combustion phase can be optimized, thereby improving the thermal efficiency and the stability of HCCI combustion.