An Investigation of Air-Assist Injection Timings and Durations in a GDI CAI Engine 2007-01-1857
A single-cylinder, 4-stroke, 4-valve research engine was operated in CAI mode by using low lift and short duration valve lift profiles with a negative valve overlap to trap exhaust gases. The engine was fuelled with super-unleaded 98 gasoline and was tested in normally aspirated mode and with a boost of 0.18bar applied to the intake system. A centrally-mounted Orbital gasoline direct-injection system was used. The injection timing, as defined by the start of the air-assist injection (SOA), was varied, as was the air-assist injection duration (Adur).
The SOA sweeps in normally aspirated mode showed that the IMEP varied with injection timing and highest IMEP occurred with early SOA. When the engine was boosted, the variation in IMEP over the SOA sweep was significantly less than the naturally aspirated test. In boosted mode, the lowest ISFC, ISCO and ISNOx emissions occurred at a SOA of 10°CA after the intake valve opened. The lowest ISHC emissions were recorded at early injection timings.
The Adur was varied in normally aspirated mode. By increasing the duration, the combustion timing could be retarded by as much as 3°CA. CAI timing could therefore be controlled through Adur. By retarding the combustion timing, higher IMEP values could be obtained. Finally, at late injection timings, an increased duration improved in-cylinder mixing and both ISCO and ISNOx emissions were reduced.