Spray Characteristics in the Port and Cylinder of a Four-Valve Spark-Ignition Engine
Measurements of fuel droplet velocity, size and distribution have been obtained inside the cylinder and the inlet port of a four-valve, four-cylinder spark-ignition engine. The local measurements in the port were complemented by photography and were obtained mainly with the engine motored at 1200 rpm. The fuel was added as a double jet from a production injector with a single injection period of constant duration and with the onset of injection as variable. A shaft encoder and computer provided the constant rotational speed, and a motor acted as a driver for the motored experiments and as a dynamometer for normal operation.
Operation with low engine load and injection with the valves closed at crank angles between 150 and 690 degrees led to nearly-constant combustion performance which suggested good homogeneity of the mixture at the time of ignition. The number of droplets detected in the cylinder was small and constituted a small fraction of injected fuel due to the residence time in the port which allowed evaporation of droplets and of liquid films. The evaporation was also assisted by the back flow of hot residual gases from the cylinder to the inlet port which occurred with low load at the early stages of intake.
The photographs and the measurements of the temporal velocity characteristics of fuel droplets in the inlet port showed that the magnitude of the back flow of gases during valves overlap was sufficient to cause reatomisation of the spray and to reverse its trajectory so that no droplets from the injector reached the cylinder in the crank angle interval between 10 to 30 degrees, with only few droplets detected afterwards. The back flow decreased as the throttle was opened and high load conditions led to droplets entering the port from the valve gap a short time before the closure of the inlet valves, that is around 210 CAD after the top-dead-centre of intake.
KeywordsSI engine Inlet valve Fuel Spray Injection Atomization Impingement Liquid films Droplets Combustion
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