Study on Cycle-To-Cycle Variations of CAI Combustion Using NVO Strategy

Based on the application of self-developed electro-hydraulic variable valve train (VVT), Controlled Auto-Ignition(CAI) was achieved with Negative Valve Overlap (NVO) strategy. This VVT system that replaced the original camshaft was mounted on the head of a modified single cylinder engine. The cycle-to-cycle variation of CAI Combustion was investigated under several different operating points with 100 consecutive cycles sampled and stored. The experimental results indicated that as the excess air coefficient increases, with fixed engine speed and valve event, the coefficient of variation of indicated mean effective pressure (CoV_IMEP) dropped firstly and then increased. The CoV_IMEP had the minimum value when the excess air coefficient ranges from 1.1 to 1.3. The engine speed and the valve lift showed the potential to influence CoV_IMEP. Increased engine speed led to a more intense mixing progress, which introduced high intensity turbulence into the cylinder and resulted in a higher level of charge homogeneity at the end of compression stroke, and finally a steady CAI combustion. Meanwhile, decreasing the valve lift had a relatively minor effect. The variation of IMEP was consequently suppressed to some extend.