Effects of spray behavior and wall impingement on particulate matter emissions in a direct injection spark ignition engine equipped with a high pressure injection system

The effects of injection pressure on spray behaviors governing particulate matter emissions were investigated for various injection timings and engine loads in an optically accessible single cylinder engine equipped with gasoline direct injection system. In a spray visualization and particle number (PN) measurement experiment, injection timing and engine load were varied to examine their effects on the spray behavior including cylinder wetting that causes the emission of particulate matter. The analysis was based on the time-averaged spray images, spray variations between the cycles, combustion, and PN emission characteristics. The spray structure in the cylinder under the engine operating conditions used in the present study was a collapsed spray that was evident at high pressure injection under atmospheric conditions. For an injection timing of before top death center (BTDC) 300 deg, at the initial stage of the intake process, the magnitude of spray deflection formed by the in-cylinder flow interacting with the spray in the reverse direction increased owing to the high pressure injection promoting fuel atomization and increasing the effects of the in-cylinder flow. The spray variations between the cycles caused by the in-cylinder flow in the region of spray deflection were also increased by high pressure injection. When the weak in-cylinder flow was formed in the late intake process (BTDC 210 deg), increasing the injection pressure accelerated the initial droplet velocity and increased the amount of fuel reaching the cylinder wall opposite the injector, thus increasing the wall film and PN emissions. In contrast, for the conditions under which the wall film was formed on the piston top by fuel interaction, the large droplet momentum corresponding to high pressure injection increased in the amount of rebounded droplets at collision, thereby reducing the amount of wall film and PN emissions.