Experimental investigation and modelling of the in-cylinder heat transfer during ringing combustion in an HCCI engine

Homogeneous Charge Compression Ignition (HCCI) engines can achieve both a high thermal efficiency and near-zero emissions of NOx and soot. However, their maximum attainable load is limited by the occurrence of a ringing combustion. At high loads, the fast combustion rate gives rise to pressure oscillations in the combustion chamber accompanied by a ringing or knocking sound. In this work, it is investigated how these pressure oscillations affect the in-cylinder heat transfer and what the best approach is to model the heat transfer during ringing combustion. The heat transfer is measured with a thermopile heat flux sensor inside a CFR engine converted to HCCI operation. A variation of the mass fuel rate at different compression ratios is performed to measure the heat transfer during three different operating conditions: no, light and severe ringing. The occurrence of ringing increases both the peak heat flux and the total heat loss. This effect should be accounted for in the heat transfer models by increasing the convection coefficient. It is shown that the heat transfer correlations of Annand and Woschni are not able to accurately model the heat transfer during ringing combustion. Two modifications to Annand's model are proposed that take into account the additional heat transfer during ringing combustion.

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