Influence of Cavitation in the Injection Nozzle on Combustion in Diesel Engines

Abstract

This paper is concerned with the influence of cavitation in the injection nozzle on combustion in diesel engines. After an overview of the fundamental definitions to characterize nozzles — where above all the injection pressure, the back pressure, the injection mass flow, and the spray momentum through the nozzle as well as the geometry play a role — the difference between a cavitating and a non-cavitating nozzle will be clarified both theoretically and based on engine measurements. To observe the influence of cavitation on combustion in isolation, a cavitating and a non-cavitating nozzle were designed in such a way that they possessed the same mass flow and the same nozzle discharge velocity. In addition to the manufacturer's measurement, the nozzles were measured using a combined flowrate—spray momentum device at levels of injection pressure and back pressure close to those in an engine. A single-cylinder research engine with a modern common rail injection system served as the test engine for the experiments. The experiments revealed striking differences in emission levels. Especially notable are the differences in the soot values. To explore in more detail these differences between the cavitating and non-cavitating nozzle, optical investigations were conducted in an injection chamber. CCD high-speed imaging was used to visualize mixture formation of the two different nozzles.