Optimizing local charge stratification in a lean-burn spark ignition engine

Gas pressure and local gas velocities have been measured in a single-cylinder spark ignition engine operating at low load and 1000 r/min and the results have characterized the extent to which combustion was enhanced by the injection of a small quantity of a mixture of propane vapour and air towards the spark plug in an otherwise quiescent chamber filled with a homogeneous lean propane/air charge. The effects of the locally generated mean flow/turbulence and equivalence ratio on combustion were examined separately by first injecting a mixture of equivalence ratio identical to that of the homogeneous charge and then a slightly rich mixture into homogeneous charges of lower equivalence ratios. The results show the advantageous effect of jet-induced local turbulence for overall air-fuel ratios between 17 and 24 with a maximum gain in peak pressure of 55 per cent at an air-fuel ratio of 20. The local injection of a rich mixture, in addition to increasing the gain in peak pressure from 30 to 50 per cent at an air-fuel ratio of 24, has extended the lean limit of the engine to 29. The timing of ignition relative to the end of injection, which varied as a function of the injection pressure, was found to have a strong effect on the peak combustion pressure so that, for example, a reduction of 8 degrees(CA) in the time between the spark and the end of injection resulted in a 25 per cent reduction in combustion pressure at an air-fuel ratio of 22. The average flame speed was increased by local injection at all equivalence ratios; for example, a value of 7 m/s was obtained with local injection at an equivalence ratio of 0.7 which is equivalent to the flame speed measured with a homogeneous charge at the much higher equivalence ratio of 0.9.