This study is an examination of the effects of air fuel ratio and ignition timing on in-cylinder thermal loading of the non-moving parts of a spark ignited air-cooled engine. Also, the study examined the effects of air fuel ratio and ignition timing on several engin performance parameters including power, IMEP, COV of IMEP, cylinder pressure and emission production. To understand thermal loading, two distinct approaches were taken. The first approach involved the use of a heat flux micro-sensor that was mounted directly in the combustion chamber to measure heat transfer from cylinder gasses to the engine head. The second approach used computer engine cycle simulation and a finite element model of the research engine to predict and analyze thermal flows.

A sensitivity study was performed in order to determine which boundary conditions applied to surfaces of the finite element model have the greatest impact on the resulting solution. It was determined that in-cylinder surface temperatures were significantly affected by the spatial variation of heat flux to the cylinder head. However, the spatial variation of in-cylinder heat flux did not affect external-cylinder considerations, such as fin geometry or the amount of heat rejected through the fins. Also, the magnitude of heat flux from the piston and rings to the cylinder liner can have a strong influence on cylinder head temperatures. When varying heat transfer though the range possible by air as the cooling medium, in-cylinder maximum temperatures were not significantly affected.

From the sensitivity study, a need for better understanding of the spatial variation of in-cylinder heat flux was clear. Experiments were conducted mounting the heat flux micro-sensor in three different locations of the cylinder head and the engine was run at various air fuel ratios, ignition timings and engine loads. The resulting data were reduced into a simple quadratic model that can predict the spatial variation of integrated heat flux per cycle to any point on the cylinder head surface. Using a constant heat flux model and the spatially dependent model that was developed, a comparison was made with the finite element model. The results showed that the constant heat flux model under-predicted the solution by a significant amount when compared to the spatially dependent model.

In-cylinder heat flux measurements were collected for a range of air fuel ratios including regimes rich of stoichiometric, stoichiometric and lean of stoichiometric. Ignition timing was advanced in increments from the stock timing of 22 (BTDC to 40 (BTDC. The measured instantaneous heat flux was integrated over the 720( to arrive at an integrated heat flux per cycle, which then could be averaged to produce at a "steady state" heat flux to the combustion surface. The integrated heat flux per cycle peaked at the stoichiometric air fuel ratio. Advancing the ignition timing increased the integrated heat flux. Peak cylinder pressures also increased with ignition timing advance; however, peak cylinder pressures occurred at air fuel ratios rich of stoichiometric. When normalizing the integrated heat flux per cycle by engine power, the effect of air fuel ratio and ignition timing became less significant.

Along with heat flux measurements for the different operating conditions, engine power, IMEP and COV of IMEP data were collected. Engine power and IMEP showed vary similar trends, with peak values occurring slightly rich of stoichiometric and decreasing with leaner air fuel ratios at stock ignition timing. However, at the lean air fuel ratio condition advancing the ignition timing recovered 87% of the lost power. The COV of IMEP exhibited the lowest values at timings that produced the maximum power for the respective air fuel ratios tested.

Emission measurements were collected for the range of air fuel ratios and ignition timings under investigation at an engine load of 78% of full load. Increasing the ignition timing produced an increase of unburned hydrocarbons and oxides of nitrogen due to the higher cylinder pressures and temperatures. Carbon monoxide, carbon dioxide and oxygen emissions were a strong function of air fuel ratio. Very little to no effect of ignition timing could be seen.

A complete in-cylinder heat flux model was developed that could predict the integrated heat flux per cycle to the cylinder head for any operating condition tested. This model was used as a boundary condition to the finite element model and was compared with the solution obtained using the cycle simulation prediction of boundary conditions. The two resulting solutions were also compared with temperature measurements that were made with thermocouples mounted near the combustion surface. The model based on actual heat flux measurements was in good agreement with measured values; however, the cycle simulation model over predicted the combustion chamber surface temperatures.