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In an effort to shed additional light on the effect of insulation on engine performance. further analysis was conducted on the experiment of Moore [15]. Shown in Table 3 is an estimate of the distribution of heat rejection for the baseline and insulated configurations. Looking first at the left column. total heat rejection of the baseline configuration was measured, as were finite element models, giving a predicted reduction of in-cylinder heat rejection of 28.6 percent. This resulted in a calculated specific heat rejection of .403 kW/kW. However, referring now to the final column in Table 3, the measured specific heat rejection was .448 kW/kW. Assuming the friction and exhaust port conditions not to have changed appreciably, the reduction of in-cylinder heat rejection was only 7.9 percent - less than one-third that of the predictions made with detailed finite element models. These and several other experimental results using different engines, and different insulation strategies, yet giving similar results, have led to questions concerning the fundamental assumptions about in-cylinder heat transfer processes.

Heat Flux Measurements

Recently several, more fundamental measurements of the impact of increased wall temperature on in-cylinder heat flux have been undertaken. A Cummins study, aimed at further understanding the unexplained results described in the previous section, was conducted on the cylinder head of a V-903 single cylinder engine [16]. The intent of these measurements was to assess the impact of increased metal wall temperature on instantaneous heat flux during the combustion process, and throughout the cycle. In virtually every measurement taken. the increased wall temperature had no discernable impact on heat transfer during the combustion process where such reductions would most favorably impact performance.

Several additional interesting findings resulted from this work. As has been demonstrated by several previous researchers, there was evidence of the heat transfer reversing directions late in the expansion stroke - at a time when the gas temperature would still have been significantly higher than the wall temperature. Of further interest was the fact that heat transfer during the compression process reversed directions well in advance of the gas temperature reaching the level of the wall temperature. Neither of these phenomena can be explained by classical in-cylinder heat transfer formulations, and both suggest the potential importance of energy storage in the thermal boundary layer.

Several researchers have recently undertaken instantaneous heat flux measurements under various insulated conditions. Enomoto [17] reported measurements in which the use of an insulated piston caused a significant increase in heat flux during the combustion process. Huang

[18] demonstrated a significant reduction in peak heat flux with zirconia insulation, while similar increases in metal wall temperature resulted in an increase in heat flux. Finally, Woschni

[19] has recently demonstrated a significant increase in .combustion heat flux with hot metal walls. Each of these experiments have raised new questions about the fundamentals of the in-cylinder heat transfer processes.

Computational Analysis

In addition to fundamental experimental work, a couple of detailed computational studies, focusing on the heat transfer process, have recently been undertaken. Figure 1 is taken from the computations of Ikegami

[20], where the compression and expansion processes, under motoring operation, are assessed. The compression process is shown on the left, where each curve represents the gas temperature versus position from the wall. at a given crank angle. Note that as the compression process continues, the temperature of the gas immediately adjacent to the wall was calculated to increase above that of the bulk gas temperature. Recognizing that the instantaneous heat flux is determined by the temperature profile immediately adjacent to the wall, the phenomena calculated here would cause heat transfer into the wall during the compression process prior to the bulk gas temperature attaining the temperature of the wall. A similar phenomena is seen in the expansion process, shown at the right of Figure 1. where the temperature adjacent to the wall falls below the bulk gas temperature. causing a reversal in the direction of heat flux.

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