Tests of Thick Flame Combustion Estimates in a Single-Cylinder Engine

Abstract

Estimates of turbulent burning speed, burning zone thickness, and randomness of ignition delay have been incorporated in a model of spark-ignited engine combustion. The estimates have been made on the assumption that turbulence during combustion is homogeneous, isotropic, and has Tennekes’ small-scale structure with integral length scale proportional to chamber height. Flame propagation rate has been assumed to depend on turbulence intensity in accord with Chomiak’s vortex-bursting hypothesis. The resulting method of calculating combustion has been tested with cylinder pressure data from a Ricardo single-cylinder engine over a wide range of rpm and equivalence ratio and operating with natural gas fuel. Hot-wire measurements of turbulence intensity were made in the motored engine, but window-averaged estimates of intensity were a factor of two lower than ensemble-averaged estimates. Given a factor of two uncertainty in the turbulence intensity measurements it can be said that estimates of combustion duration and pressure agreed well with experimental data over the range of speed and equivalence ratio. The sensitivity of the calculation method to alterations in assumed parameters has been tested. The most important uncertainty appears to be the turbulence intensity.