Similarity of split-injected fuel sprays for different size diesel engines

Abstract

Establishment of a scaling relationship between small and large bore diesel engines is useful for saving energy, cost and time in a new engine development. Split-injection strategies, which are deemed effective to reduce simultaneously the engine noise and fuel consumption as well as soot and NOx emissions, have been commonly used in modern diesel engines. Thus, far knowledge about scaling the split-injected sprays is however still absent. In this article, the similarity of spray characteristics including the spray tip and tail penetrations as well as the averaged equivalence ratio is theoretically analyzed. With two nozzles of 0.11 and 0.14 mm hole diameters, the similarity of fuel injection rate and non-evaporating spray mixture formation processes with the main-post and pilot-main injection strategies is studied for the three scaling rules, via the Bosch long tube method and high-speed shadowgraphy, respectively. The experimental results agree well with the theoretical analyses. All the three scaling rules can scale the injection rate and injection mass very well with the split-injection strategies under the varied injection pressures. The pressure rule is better to scale the spray tip and tail penetrations, while the lift-off rule and speed rule lead to relatively longer spray tip and tail penetration lengths. The spray lengths of the small-type nozzles with the lift-off rule and speed rule are longer than those of the small-type nozzle with the pressure rule and the large-type nozzle, resulting in a decreased averaged equivalence ratio for the former cases. These results are valuable for evaluating the scaling rules for different size diesel engines with a wide range of operation conditions and guiding new diesel engine development.