Scaling fuel sprays for different size diesel engines

摘要

Diesel engines are widely used not only for heavy-duty vehicles such as ships and trucks but also for light-duty passenger cars, with a wide range of bore diameters. Since development of a new diesel engine is usually expensive and time consuming, the ability to accurately predict engine performance from existing models can reduce the resources and time required in the engine development procedure. However, so far knowledge on scaling diesel engines is far from adequacy, particularly for spray combustion processes. In this paper, two injectors with nozzle hole diameters of 0.11 mm and 0.14 mm are adopted for scaling fuel sprays under the non evaporating conditions based on the three similarly rules. Firstly, the existence of similarly is theoretically analyzed in diesel fuel sprays of both free injection and impinging upon a wall. Then, the similarly of diesel injection rate is experimentally verified by using the Bosch long tube method. Finally, the similarly of spray angle, tip penetration length, excess air ratio and impinging spray characteristics are investigated under various injection pressures and ambient densities in an optically accessible constant volume vessel by the high-speed shadowgraphy. The theoretical analysis and experimental results reveal that the similarly rule keeping the injection pressure constant is more preferable for scaling the spray angle, tip penetration length and excess air ratio, while the other two similarly rules with reduced injection pressure result in narrowed spray angle and increased spray tip penetration length. With respect to the post-impingement behavior, although the spray from the large-hole nozzle impinges upon the wall later than those of the small-hole nozzles with the speed rule and the lift-off rule, the rebound height of the large-hole nozzle is higher than those by the small holes with the speed and the lift-off rule. The mechanism of the above phenomena is discussed in depth.