Local mixture injection to extend the lean limit of spark-ignition engines

The effect of charge stratification by direct, in-cylinder injection of a s mall quantity of propane-air mixture in a single-cylinder, propane-fuelled, spart-ignition engine, has been determined in terms of in-cylinder pressur e, flame visualisation and exhaust emissions. The operating conditions rang ed from low load to wide open throttle at 1000 and 1500 rpm, and with quies cent and swirling in-cylinder flows. The effects of injection-driven flow a nd turbulence on combustion have been considered independently of mixture s trength by assessing the consequences of injecting a local mixture of an eq uivalence ratio equal to that of the port-induced charge. The results provide further evidence to support the concept of direct in-cy linder mixture injection. At 1000 rpm and low load, faster development of t he flame kernel and subsequent enflamed area were observed, together with a 60% increase in peak cylinder combustion pressure at an overall equivalenc e ratio of 0.7 (A/F of 22.5). The relative effect of local-charge stratific ation increased with reduction in equivalence ratio at this engine speed an d without increase in emissions; at 1500 rpm, stable combustion was achieve d even at an equivalence ratio of 0.61 (A/F of 25.8), which was below the f lammability limit of the homogeneous port-only propane/ air mixture. Unthro ttled engine operation produced similar results so that at an equivalence r atio of 0.55 (A/F of 28.5), for example, the Coefficient of variation of th e indicated mean effective pressure was reduced from 0.4 to less than 0.1 b y mixture local injection. With a shrouded inlet valve generating mean gas velocities of 6 m/s at the time of ignition in the vicinity of the spark pl ug, control of rich mixture injection both with and against the bulk in-cyl inder flow, resulted in stable combustion with Coefficient of variation of the indicated mean effective pressure of less than 0.1 at an equivalence ra tio of 0.55 (A/F of 28.5).