Thermal Boundary Layer Modelling in ‘Motored’ Spark Ignition Engines

A newly developed piece-wise method for calculating the effects of near-wall turbulence on the transport of enthalpy and hence the thermal boundary layer temperature profile in “motored” spark ignition engines has been compared with methods that have previously been employed in the development of expressions for the gas-wall interface heat flux. Near-wall temperature profiles resulting from the inclusion of the respective expressions in a “quasi-dimensional” thermodynamic engine simulation have been compared and in one case show considerable differences throughout the compression and expansion strokes of the “motored” engine cycle. However, the corresponding heat fluxes calculated from the simulated temperature profiles all show good agreement with measured results. It is postulated that gas-wall interface heat flux is largely controlled by the boundary layer behaviour close to the combustion chamber surfaces and the temperature profile in the outer regions of the boundary layer has considerably less influence. Comparisons have also been made between measured TDC near-wall temperature data (expressed in dimensionless form) and the wall function approach when modified for use in the cylinders of reciprocating engines. The high swirl case shows good agreement with the wall functions, but the low swirl case does not. This is attributed to the changing nature of the hydrodynamic boundary layer in the two instances with the low swirl case being more consistent with laminar behaviour. Engine simulations incorporating the new method for the evaluation of near-wall turbulence effects have been used to demonstrate the thermal boundary layer behaviour throughout the compression and expansion strokes of a “motored” cycle. Piston induced work effects are observed to have a significant influence.