Unsteady in-cylinder heat transfer in a spark ignition engine: experimentsand modelling

Instantaneous heat flux measurements have shown that, in the expansion stro ke, heat can flow from the wall into the combustion chamber, even though th e bulk gas temperature is higher than the wall temperature. This unexpected result has been explained by modelling of the unsteady flows and heat cond uction within the gas side thermal boundary layer. This modelling has shown that these unsteady effects change the phasing of the heat flux, compared with that which would be predicted by a simple convective correlation based on the bulk gas properties. Twelve fast response thermocouples have been i nstalled throughout the combustion chamber of a pent roof, four-valve, sing le-cylinder spark ignition engine. Instantaneous surface temperatures and t he adjacent steady reference temperatures were measured, and the surface he at fluxes were calculated for motoring and firing at different speeds, thro ttle settings and ignition timings. To make comparisons with these measurem ents, the combustion system was modelled with computational fluid dynamics (CFD). This was found to give very poor agreement with the experimental mea surements, so this led to a review of the assumptions used in boundary laye r modelling. The discrepancies were attributed to assumptions in the law of the wall and Reynolds analogy, so instead the energy equation was solved w ithin the boundary layer. The one-dimensional energy conservation equation has been linearized and normalized and solved in the gas side boundary laye r for a motored case. The results have been used for a parametric study, an d the individual terms of the energy equation are evaluated for their contr ibution to the surface heat flux. It was clearly shown that the cylinder pr essure changes cause a phase shift of the heat flux forward in time.