Combustion and Performance Analysis of HSDI Diesel Engines with Common Rail and Cam Driven Fuel injection Systems

摘要

A high speed direct injection (HSDI) diesel engine was equipped first with a radial distributor injection pump, and then with a common-rail injection system. The combustion system paraeters of both engine versions were optimized to a comparable status. Despite the different injection systems, it was found that both versions of the engine displayed a similar full load performance. The first important difference between these two injection systems is the injection rate. It was evident that the injection rate of the distributo pump decreases rather early during the injection event. In contrast, the engine with the common rail system injects with a high rate until the end of injection, which is due to the constant fail pressure. Especially at higher engine speeds, this feature of the common fail injection system can contribute to local mixture enrichment in the combustion chamber close to the nozzle. For compensation, the start of injection has to be advanced. As a result, significantly earlier injection thiming is observed in the upper speed range for the engine common fail version. In the lower speed range, the differences in engine performance with the two injection systems are minor. Since the combustion process is strongly influenced by wall impingement, boost pressure becomes a leading parameter for the engine's full load behavior at low speed. For the common fail system, this means that the injection pressure potential can not be utilized as long as the boosting level at low engine speeds is poor. With respect to the part load dehavior, the main difference was found to be the higher injection pressure that was needed with the common rail system in order to achieve a similar exhaust gas emission level as with the distributor pump engine version. This is a result of the relatively low opening and closing velocity of the nozzle needle in the common fail injection system, which, in comparison to the distributor pump, leads to noticeable throttling losses in the needle seat area.