In-cylinder heat flux and temperature measurements were obtained in an air-cooled four-stroke utility engine for a range of air-fuel ratios. For these measurements, the magnitude of the integrated heat flux peaked at the stoichiometric air-fuel ratio, with an approximately linear decrease on either side of stoichiometric. Advancing the spark generally increased the magnitude of the integrated heat flux. Evaluation of the Brake Specific Integrated Heat Flux (BSIHF) mitigated these trends, and, the effects of changes in timing were eliminated for some operating conditions
Examination of the BSIHF from the compression and expansion stroke showed behavior mimicking the full cycle BSIHF. However, the fraction of the total flux contributed by this portion of the cycle varied greatly from approximately 98% of the total to approximately 75% of the total. In addition, BSIHF for the gas exchange portion of the cycle exhibited much different trends than the full cycle BSIHF, with the peak values occurring with mixtures lean of stoichiometric and retarded timings.
Steady-state values of cylinder head temperatures near the surface were also obtained to observe how air-fuel ratio and spark timing would affect temperatures. Different trends were noticed depending on the location of the thermocouple measurement. For example, measurements of surface temperature near the spark plug had trends similar to that observed for the BSIHF. Temperatures varied by more than 70°C at this location, depending on air-fuel ratio and timing. In contrast, temperature measurements in the valve bridge were constant for air-fuel ratios richer than stoichiometric. Cylinder head temperatures near the cylinder wall displayed yet different behavior, with no apparent effect from ignition timing changes.