A comparative study of effect of valve overlap on HCCI engines

One of the suggested methods, to overcome drawbacks of HCCI engines, is using of the variable valve timing technology (the VVT system). The VVT system can affect HCCI engines characteristics due to the EGR's main role in controlling the HCCI combustion. The auto-ignition in HCCI engines can be facilitated by adjusting the timing system of exhaust-valve-closing and, to some extent, the timing system of intake-valve-opening in order to capture a proportion of hot exhaust gases in the engine cylinder during the gas exchange process. In the present paper, a comparative analysis is performed to investigate the effect of the VVT system on the combustion, the performance and the emission characteristic of HCCI engines. For this purpose, a multi-zone model was developed and coupled with a gas exchange model. Results were compared to experimental results, obtained from a single-cylinder engine. Finally, it can be concluded that if the duration of opening of the intake and exhaust valves were kept constant, positive-valve-overlapping was an reliable strategy for the HCCI combustion as it could improve combustion characteristics such as the IMEP, the work and engines efficiencies and reduce exhaust emissions.