Sufficient Condition on Valve Timing for Robust Load Transients in HCCI Engines

Homogeneous Charge Compression Ignition (HCCI) combustion is known for its significant fuel economy benefit with near-zero NOx and particulate emissions. Stable HCCI combustion relies on a well-controlled temperature and composition of the cylinder charge at the intake valve closing that in turn requires a precise coordination of all engine inputs. In this paper, the HCCI combustion is realized by retaining hot residual from the previous combustion event using the recompression valve strategy. The recompression valve strategy closes the exhaust valves before the top dead center and opens the intake valves at an angle symmetric to the exhaust valve closing. Depending on the engine load, different valve open/close timings with respect to the crank position are used to trap different amounts of residual gases. It is critical to coordinate the change in the valve open/close timings with the change in the injected fuel quantity during load transients in order to maintain stable combustion. In this paper, a control-oriented engine model is used to establish a sufficient condition for valve timing change relative to the fuel change that guarantees combustion stability during load transients in HCCI engines. The sufficient condition is then experimentally validated in a single-cylinder HCCI engine during transients with a step change in load demand.