Engines operating in Homogeneous Charge Compression Ignition (HCCI) combustion mode offer significant benefits of high fuel economy and low engine-out NO
x
emissions over the conventional spark ignition (SI) combustion mode. However, due to the nature of HCCI combustion, traditional HCCI combustion can be realized only in a limited operating range. High load is limited by the trade-off between ringing (combustion noise) and stability (COV of IMEP). Low load is restricted by the trade-off between NO
x
emissions and combustion stability (standard deviation of IMEP).
The present research is focused on the extension of lo w load limit of HCCI combustion by developing HCCI idle operation. The main obstacle in developing HCCI idle combustion is lack of available thermal energy necessary for successful auto-ignition. The current paper relates to a method for achieving the approriate environment for auto-ignition at idle and the optimal trade-off between combustion stability and NO
x
emissions by employing multiple injection and multiple ignition (MIMI) strategy.
HCCI operating range was successfully extended to idle condition. Very low NO
x
emissions ( < 1.0 g/kg-fuel) and good stability ( < 10 kPa stdv of IMEP) can be achieved at 800 rpm, 85 kPa NMEP condition. Compared to throttled homogeneous SIDI operation, over 25% fuel consumption benefit can be achieved.
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