Extending the Load Range of a Natural Gas HCCI Engine using Direct Injected Pilot Charge and External EGR

Natural gas is a challenging fuel for HCCI engines because its single-stage ignition and rapid combustion make it difficult to optimize combustion timing over a significant load range. This study investigates direct injection of a pilot quantity of high-cetane fuel near TDC as a range extension and combustion control mechanism for natural gas HCCI engines.

The EGR and load range is studied in a supercharged natural gas HCCI engine equipped with external EGR, intake heating and a direct injection system for n-heptane pilot fuel. The operating range and emissions are of primary interest and are compared between both the baseline HCCI engine with variable intake temperature and the direct injected HCCI (DI-HCCI) engine with constant intake temperature.

Test results show the EGR and load range at fixed intake temperature can be extended using pilot direct injection. However, it must be noted that the emissions are higher than pure HCCI and the load range is still narrower than that available with variable intake temperature. One advantage of pilot direct injected HCCI is that variations in engine load can be more readily managed with reduced emissions when compared to a conventional compression ignition strategy. Increasing EGR reduces the NOx emissions generated by the direct injection within the range that requires a sufficiently small pilot injection.