Development of a Fuel Spray Impulse Measurement Device and Correlation with Time Resolved Mass Flow 2009-01-1880
The fuel injection process plays an important role in the combustion and emission formation processes of the DI diesel engine. One important fuel spray characteristic is the spray impulse. The most commonly used method to measure fuel spray impulse is the impingement method where the fuel spray impinges perpendicularly on the surface of a force transducer. This work deals with the theoretical background of such measurements as well as with developing and testing some different impulse measurement setups. The measured impulse is compared to measurements of the instantaneous mass flow and theoretical flow calculations. When measuring the impulse by impingement on the transducer membrane a fuel temperature related measurement error was encountered. This problem was solved by gluing a strike plate to the transducer membrane. The plate shielded the membrane from direct contact with the fuel. Initially plates made out of aluminum were used, they were however found to be sensitive to erosion. After a number of injections a small pit was formed and this led to an overestimation of the impulse as the fuel more effectively was reflected back towards the direction where it came from. It is crucial for the accuracy of the method that the spent fuel exits the plate perpendicularly, if some of the fuel bounces back towards the direction where it comes from the spray impulse is overestimated. With a flat strike plate it is difficult to be sure that all the spent fuel exits the plate perpendicularly. Therefore a plate with a rotationally symmetrical curvature which allows a gradual and thus more controlled direction change was manufactured and evaluated. When the injection rate of an injector is characterized using a conventional rate tube a number of problems are caused by pressure fluctuations in the fuel volume inside the rate tube. The measurements are disturbed by superimposed fluctuations which are especially problematic when small post injections are to be evaluated. The post injection rate can be disturbed by fluctuations introduced by the main injection, such fluctuations does not occur with impulse measurements. The new impulse measurement device produces measurements with high precision in both rate shape and absolute value. Because of this it is well suited for injection rate evaluation and when a high precision value of fuel spray impulse is required, for instance when calculating nozzle flow loss factors. Flow calculations based on the instantaneous mass flow and the fuel spray impulse are made.