The Effect of Fuel Injection on the Velocity Fluctuations in the Bowl of a DISI Engine 2005-01-2102

Swirl plane Particle Image Velocimetry (PIV) measurements were performed in a single-cylinder optically accessible gasoline direct injection (DISI) engine using a borescope introduced through the spark plug hole. This allowed the use of a contoured piston and the visualization of the flow field in and around the piston bowl. The manifold absolute pressure (MAP) was fixed at 90 kPa and the engine speed was varied in increments of 250 rpm from 750 rpm to 2000 rpm. Images were taken from 270° to 320° bTDC of compression at 10° intervals to study the evolution of the velocity fluctuations. Measurements were performed with and without fuel injection to study its effect on the in-cylinder flow fields. Fuel was injected at 10 MPa and 5 MPa. The 2-D spatial mean velocities of individual flow fields and their decompositions were averaged over 100 cycles and used to investigate the effects of engine speed and image timing on the flow field.

The results showed that characterizing the in-cylinder flows using the mean velocity of an ensemble average yields very different results, both in trends and in velocity magnitudes, when compared to the average of the mean velocity of 100 individual cycles. The results of the flow field decomposition showed that the dominant component was small-scale structures. Fuel injection disrupted the swirl structure, increased flow velocities and the contribution of large-scale structures. The measurements showed a delay in the arrival of the fuel jet and that the increase in velocity fluctuations caused by the fuel jet was temporary. The delay in the fuel jet reaching the measurement plane was a function of spray velocity and time after End-of Injection (EOI) and appeared independent of engine speed, as did the decay rate of the increase in the fluctuation velocities.