Experimental Study of Combustion Characteristics and Emissions of Pre-Chamber Induced HCCI Combustion

It is a well-known fact that HCCI combustion offers the possibility of achieving high efficiency with low emissions, but with the challenges in combustion control and ability to adjust to changing environmental conditions. To resolve the aforementioned challenges, a pre-chamber induced homogeneous charge compression ignition (PC-HCCI) combustion mode was experimentally tested with aim of providing initial operating boundaries in terms of combustion stability and obtaining initial performance results. The single cylinder engine equipped with active pre-chamber and compression ratio (CR) of 17.5 was fueled by gasoline. The initial experiments were performed at the engine speed of 1600 rpm with intake air temperatures varied from 33°C to 100°C to verify the possibility of achieving the PC-HCCI combustion mode and to compare the achieved engine performance and emission results with both PCSI and pure HCCI combustion modes used as reference cases. The results showed that PC-HCCI combustion mode ensures stable operation at low loads, extending the lean limit while maintaining higher efficiency compared to PCSI combustion mode. When compared to the pure HCCI combustion mode, lower efficiency is obtained, possibly only due to non-optimized operating parameters. The results however confirm that a significant reduction and narrowing of the required intake temperature range is feasible along with the direct control of ignition timing and thus much improved resilience to the changing of boundary conditions. The analysis further showed that pre-chamber fuel mass has a major impact not only on ignition and combustion stability but is also directly corelated with total emissions of NO_X, confirming that at such diluted mixtures most of the engine-out emissions come from the pre-chamber.