Improvement in the combustion mode transition for a spark ignition engine capable of homogeneous charge compression ignition

Abstract

It is fairly challenging to achieve a smooth mode transition between spark ignition combustion and homogeneous charge compression ignition combustion for a spark ignition engine capable of homogeneous charge compression ignition, because their in-cylinder thermal and charge mixture properties are quite different owing to the distinct combustion characteristics. In this paper, a mode transition strategy between spark ignition combustion and homogeneous charge compression ignition combustion was developed over the entire transition boundary between the spark ignition mode and the homogeneous charge compression ignition mode, where the engine speed is between 1100 r/min and 2000 r/min and the corresponding load (the indicated mean effective pressure) is between 3.8 bar and 5.0 bar. During the combustion mode transition and steady-state homogeneous charge compression ignition operation the normalized air-to-fuel ratio of the engine was maintained between 1.0 and 1.3; the charge air was heated by a manifold charge cooler heated by the engine coolant, and a test gasoline fuel with a research octane number of 85 was used for all the experiments conducted. The mode transition strategy was experimentally validated on a spark ignition engine capable of homogeneous charge compression ignition equipped with electric variable-valve-timing systems, dual-lift valves and an electronic throttle control system. Because of the limitation on the electric variable-valve-timing response time, it takes 5–10 cycles of the engine, depending on the engine speed, to complete the mode transition. The throttle was opened step by step to its wide-open position for the transition from spark ignition to homogeneous charge compression ignition or closed directly to the target position for the transition from homogeneous charge compression ignition to spark ignition, where the hybrid combustion mode was employed with spark assistance to change gradually to the desired homogeneous charge compression ignition mode or spark ignition combustion mode respectively, by increasing or decreasing the percentage of homogeneous charge compression ignition combustion during the hybrid combustion operation. The experimental results show that the developed strategy is able to achieve a smooth combustion mode transition with the net mean effective pressure and the combustion phase fluctuations at the level of stable spark ignition and homogeneous charge compression ignition combustion.