OPTIMIZING LOCAL CHARGE STRATIFICATION IN A LEAN-BURN SPARK-IGNITION ENGINE

Gas pressure and local gas velocities have been measured in a single-c ylinder spark ignition engine operating at low load and 1000 r/min and the results have characterized the extent to which combustion was enh anced by the injection of a small quantity of a mixture of propane vap our and air towards the spark plug in an otherwise quiescent chamber f illed with a homogeneous lean propane/air charge. The effects of the l ocally generated mean flow/turbulence and equivalence ratio on combust ion were examined separately by first injecting a mixture of equivalen ce ratio identical to that of the homogeneous charge and then a slight ly rich mixture into homogeneous charges of lower equivalence ratios. The results show the advantageous effect of jet-induced local turbulen ce for overall air-fuel ratios between 17 and 24 with a maximum gain i n peak pressure of 55 per cent at an air-fuel ratio of 20. The local i njection 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 exten ded the lean limit of the engine to 29. The timing of ignition relativ e 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 pr essure 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 r eduction in combustion pressure at an air-fuel ratio of 22. The averag e flame speed was increased by local injection at all equivalence rati os; 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 me asured with a homogeneous charge at the much higher equivalence ratio of 0.9.