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In-Nozzle Cavitation-Induced Orifice-to-Orifice Variations Using Real Injector Geometry and Gasoline-Like Fuels

Excerpt

Many concurrent sources contribute to cycle-to-cycle variability (CCV) in internal combustion engines. This variability is undesirable, as it is responsible for deviations from the expected operating conditions, which in turn affect the engine efficiency and performance. Shot-to-shot variability connected to the fuel injection process is one strong candidate among CCV’s many sources. Previous numerical work employing diesel fuel in a nozzle geometry measured with x-ray diagnostics has shown that manufacturing tolerances and needle radial motion are responsible for orifice-to-orifice variations in total injected mass. These variations are significant in the presence of shot-to-shot variability in the needle radial motion. The growing interest for low temperature combustion modes such as gasoline compression ignition has made the use of gasoline-like fuels in compression ignited engines desirable. However, when operating under typical diesel conditions, these fuels generally show considerable propensity for cavitation in the orifices and at the needle seat, due to their high volatility. In previous work, in-nozzle cavitation has been shown to correlate strongly with needle radial motion. This might constitute another source of CCV, as cavitation within the orifices might enhance the already present shot-to-shot variability by reducing the available cross-sectional areas and the relative orifice discharge coefficients. In addition, the systematic presence of cavitation might also result in the local erosion of the nozzle internal geometry, leading to even higher variability with time. This study focuses on the evaluation of orifice-to-orifice variability of gasoline-like fuel injection under diesel operating conditions. High-resolution x-ray geometry characterization of an eight-hole heavy-duty diesel injector has been combined with new measurements of the needle motion using a straight-run gasoline. A well-validated computational setup using the CONVERGE code was employed to perform a series of simulations, which highlighted the influence of cavitation on the internal nozzle flow. This combined effect of cavitation, needle radial motion, and orifice-to-orifice differences found is expected to have a strong influence on shot-to-shot variations.

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