High Load HCCI Operation Using Different Valving Strategies in a Naturally-Aspirated Gasoline HCCI Engine

Homogenous Charge Compression Ignition (HCCI) combustion offers significant efficiency improvements compared to conventional spark ignition engines. However, due to the nature of HCCI combustion, traditional HCCI combustion can be realized only in a limited operating range. In order to maximize fuel economy benefits the HCCI operating range needs to be extended to higher loads. One immediate benefit is to maximize the portion of the standard driving cycles (NEDC, FTP, etc) that can be run with HCCI combustion, so that transitions between SI operation and HCCI operation can be avoided. The HCCI operation at high load range is typically limited by a trade-off between combustion noise and combustion stability. In a previous research, we showed how to improve this trade-off using spark-assisted HCCI combustion strategy, and concluded that the HCCI high load operation is limited by the air availability due to a low lift cam when spark-assisted HCCI combustion was applied. In this research different valving configurations were investigated to maximize the extension of high load limit of HCCI operation. Then two different valving strategies, such as Negative Valve Overlap (NVO) and Positive Valve Overlap (PVO), were compared to improve the efficiency of high load HCCI operation by controlling the amount of internal residual fraction and pumping loss. When the NVO strategy was employed, as engine load increases, combustion phasing must be retarded to reduce combustion noise due to high amount of hot internal residuals. Retarded combustion phasing at high load HCCI operation is a key solution for the NVO strategy. On the other hand, when the PVO strategy was applied, optimal residual fraction can be achieved resulting in efficiency improvement due to optimal combustion phasing, lower pumping loss and less heat transfer loss. The high load limit was successfully extended to 10 bar IMEP (g) (Gross Indicated Mean Effective Pressure) while maintaining good efficiency and complying with emissions requirements.