A Visualization Study of Liquid Fuel Distribution and Combustion Inside a Port-Injected Gasoline Engine Under Different Start Conditions

High-speed video of combustion processes and cylinder pressure traces were obtained from a single-cylinder optical-accessible engine with a production four-valve cylinder head to study the mixture formation and flame propagation characteristics at near-stoichiometric start condition. Laser-sheet Mie-scattering images were collected for liquid droplet distributions inside the cylinder to correlate the mixture formation process with the combustion results. A dual-stream (DS) injector and a quad-stream (QS) injector were used to study the spray dispersion effect on engine starting, under different injection timings, throttle valve positions, engine speeds, and intake temperatures.

It was found that most of the fuel under open-valve injection (OVI) conditions entered the cylinder as droplet mist. A significant part of the fuel droplets hit the far end of the cylinder wall at the exhaust-valve side. Some of the droplets were found to travel toward the squish area at the short side of the intake valves. Images taken just before spark timing showed that significant fraction of the fuel was still in the liquid phase. In contrast, Mie-scattering images did not show significant fuel droplets entering the imaging plane under close-valve injection (CVI) conditions.

The combustion video showed that combustion during the early cycles of cold start could be categorized into three phases. The first phase was at those early cycles when most of the injected fuel ended up as liquid film on the chamber walls, with insufficiently vaporized fuel in the mixture to sustain combustion. The second phase started when visible weak flame fronts appeared due to improvement in liquid vaporization process. Diffusion-controlled pockets became obvious at this phase and the in-cylinder pressure increase was also significant. The third phase started when the heat of combustion evaporated most of the liquid fuel and produced an overall-rich mixture that combusted at a higher rate and produced a higher cylinder pressure.