Methods to achieve HCCI/CAI combustion at idle operation in a 4VVAS gasoline engine

Homogeneous Charge Compression Ignition (HCCI) combustion, also known as controlled auto-ignition (CAI), has been shown to reduce substantially NOx emissions and fuel consumption in gasoline engines. However, it is still confronted with the problem of its limited operation range. One of the focuses of the current HCCI combustion research is to expand its low load limit to the idle operation. The main obstacle in developing HCCI idle combustion is to avoid misfire due to high dilution and low charge temperature. In this work, research was carried out on a single cylinder engine equipped with fully variable valve lifts and timing devices in order to identify the appropriate engine control strategies to extend the HCCI low load limit. Negative valve overlap (NVO) with port fuel injection and direct gasoline injection were investigated for achieving the appropriate environment for auto-ignition at idle and the optimal tradeoff between the combustion stability and fuel consumption. The result shows that the early intake valve opening (EIVO) strategy is most conductive to produce stable combustion at low load conditions. HCCI operation range was successfully extended to idle condition by employing EIVO strategy at the stoichiometric air-fuel ratio. Good combustion stability can be achieved at 800 RPM, 85 kPa NIMEP condition. In addition, the fuel consumption could be improved further by introducing direct injection together with the EIVO strategy. The analyses demonstrate that the idle HCCI operation was enabled by the stratified residual gas and temperature distribution in the cylinder associated with the EIVO strategy. Furthermore, fuel-reforming processes from the direct gasoline injection into the hot residual gas could reduce dependence on the stratification at low loads. (C) 2013 Elsevier Ltd. All rights reserved.