In-cylinder Flow with Negative Valve Overlapping - Characterised by PIV Measurement

Negative valve overlapping is widely used for trapping residual burned gas within the cylinder to enable controlled Homogeneous Charge Compression Ignition (HCCI). HCCI has been shown as a promising combustion technology to improve the fuel economy and NOx emissions of gasoline engines. While the importance of in-cylinder flow in the fuel and air mixing process is recognised, the characteristics of air motion with specially designed valve events having reduced valve lift and durations associated with HCCI engines and their effect on subsequent combustion are not yet fully understood. This paper presents an investigation in an optical engine designed for HCCI combustion using EGR trapping. PIV techniques have been used to measure the in-cylinder flow field under motored conditions and a quantitative analysis has been carried out for the flow characterisation with comparison made against the flow in the same engine with conventional valve strategies for SI combustion. It is found that the short duration, negative valve overlap, valve events result in in-cylinder flows up to 3 times faster than corresponding SI valve events; however, the SI flow structures are more stable with a more obvious tumble motion. It may also be observed that with short duration, negative valve overlap valve events the early flow is largely valve event and induction driven (flows > 20m/s) which degrade to a mainly piston driven upward flow with reduced RMS fluctuation later in the compression stroke (flows < 4m/s)