The DISI (Direct-Injection Spark Ignition) spray characteristics, in-cylinder airflow motion, and engine combustion were investigated experimentally. The engine combustion chamber had the injector and spark plug at the center of the pent-roof. Spray-guided air-fuel mixing scheme was applied.
For spray characterization, Light Extinction technique was used. Spray penetration, cone angle, and overall SMD (Sauter Mean Diameter) were found, by utilizing Nd:YAG laser as a light source. For in-cylinder airflow experiment, a one-dimensional hot-wire probe was used. Airflow velocities at 19 locations in the combustion chamber were measured, and the 3-dimensional mean velocity as well as the turbulent intensity were calculated. The engine combustion test includes obtaining COV of imep and measuring NOx, CO2, CO emissions. Both early injection case (start of injection being 270?CA BTDC) and late injection case (start of injection being 120?CA BTDC) were tested.
By comparing the results from the three stages of experiments, the effect of spray and in-cylinder flow on the engine combustion was studied. It was shown that for the early injection case in which the start of injection was at 270?CA BTDC, the most of the droplets from the injection were vaporized before the spray reached to the top of the piston head, and the combustion chamber had homogenous charged air-fuel mixture, with help of a strong turbulence. For the late injection case in which the start of injection was at 120?CA BTDC, poor engine combustion due to the misfire was shown. Because there was not a swirl motion in the combustion chamber, the rich region of the air-fuel mixture was not confined to the center of the cylinder where the spark plug was located at. To improve combustion performance of this type of engine, a suggestion was made.