Development of a spark plug fuel injector for direct injection of natural gas in spark ignition engine

摘要

The use of methane in spark ignition engines is mainly due to its cleaner emissions and relatively low price. However, when methane replaces gasoline in the externally mixing carburettor or port injection engine, power is reduced and upper speed is limited. These are because the burning velocity of methane is slower than of gasoline, and some air is displaced in the intake manifold in order to compensate the low density methane. The problem can be mitigated when fuel is directly injected into the combustion chamber after the intake valve closes. This results in an increased volumetric efficiency, a higher absolute heating value of mixture and a faster burning rate. The work presented in this thesis aims to develop a conversion system that enables methane to be directly injected into the combustion chamber of a spark ignition engine without modifying the original structure of the engine. The system, named as Spark Plug Fuel Injector (SPFI) combines a fuel injector with a spark plug. A fuel path is drawn along the periphery of the spark plug body to deliver the injected fuel to the combustion chamber. The system was installed and tested on a Ricardo E6 single cylinder engine with compression ratio of 10.5: 1. Cylinder pressures were taken as the main indicator of the engine performance and selected indicated performance were presented. A set of port injected data for the engine running on methane was also taken in order to provide a comparison of performance with SPFI direct injection. Results show that the indicated performance of the SPFI methane direct injection at the tested speed was lower than the optimised methane port injection operation. This was mainly due to the quality of air-fuel mixing, which is a result of spatial and temporal limitation of direct injection operation. Flow visualization using the PLIF method shows that even though sufficient gas jet penetration from SPFI injection nozzle was achieved, the cone angle was very narrow. The conclusion from imaging experiments implies poor mixing, hence the performance suffers drawback. However, with direct injection, volumetric efficiency is increased ands combustion duration is faster. These two factors are desirable for engine performance improvement. SPFI has proven to be a practical and low cost conversion to methane. Even though the performance is lower than port injection, its benefits are significant. As the SPFI design is simple and requires no modification to the original structure of the converted engine.