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

Abstract

 The effect of charge stratification by direct, in-cylinder injection of a small quantity of propane-air mixture in a single-cylinder, propane-fuelled, spart-ignition engine, has been determined in terms of in-cylinder pressure, flame visualisation and exhaust emissions. The operating conditions ranged from low load to wide open throttle at 1000 and 1500 rpm, and with quiescent and swirling in-cylinder flows. The effects of injection-driven flow and turbulence on combustion have been considered independently of mixture strength by assessing the consequences of injecting a local mixture of an equivalence ratio equal to that of the port-induced charge.

The results provide further evidence to support the concept of direct in-cylinder mixture injection. At 1000 rpm and low load, faster development of the flame kernel and subsequent enflamed area were observed, together with a 60% increase in peak cylinder combustion pressure at an overall equivalence ratio of 0.7 (A/F of 22.5). The relative effect of local-charge stratification increased with reduction in equivalence ratio at this engine speed and without increase in emissions; at 1500 rpm, stable combustion was achieved even at an equivalence ratio of 0.61 (A/F of 25.8), which was below the flammability limit of the homogeneous port-only propane/ air mixture. Unthrottled engine operation produced similar results so that at an equivalence ratio of 0.55 (A/F of 28.5), for example, the Coefficient of variation of the indicated mean effective pressure was reduced from 0.4 to less than 0.1 by 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 plug, control of rich mixture injection both with and against the bulk in-cylinder flow, resulted in stable combustion with Coefficient of variation of the indicated mean effective pressure of less than 0.1 at an equivalence ratio of 0.55 (A/F of 28.5).