Flow field assessment in a fired spray-guided spark-ignition direct-injection engine based on UV particle image velocimetry with sub crank angle resolution

摘要

A new, high-speed, UV particle image velocimetry (PIV) technique has been developed and applied to a fired, spray-guided spark-ignition direct-injection (SIDI) engine operated at 600 rpm in stratified mode. The technique involves the use of a single, frequency-tripled Nd-YAG laser operated at 16 kHz. A single CMOS camera, synchronized with the laser, was focused onto an 11×11 mm~2 field-of-view between the spark plug and the injector. Raw PIV images were recorded just before fuel injection (45 °BTDC) through the early stages of flame propagation (25 °BTDC). With this temporal resolution, approximately four instantaneous velocity fields per crank angle were obtained. The use of 355 nm laser light, combined with short exposure time (i.e., 50 μs), successfully eliminated detection of combustion product luminosity, making the flame front easily identifiable. The deformation of the plasma channel 250 μs after spark timing was also captured. The velocity fields provided information about the flow characteristics in this portion of the engine cycle. Specifically, the spatially averaged velocity magnitude in the vicinity of the spark plug increased by a factor of three over the crank angle range considered. The quasi two-dimensionality of the flow before fuel injection was assessed via continuity. An increase in the shear strain rate was noted in 50% of the engine cycles. The increase in shear strain rate, combined with high stratification at the time of ignition and extension of the plasma channel, could prevent flame kernel development in SIDI engines.