ID 353

Investigation of Shot-to-Shot Variability during Short Injections

 

Abstract:

Cycle-to-cycle variability (CCV) in internal combustion engines is undesirable, as it is responsible for devia-tions from the expected operating conditions, which in turn affect the engine efficiency and performance. Shot-to-shot variability of the fuel injection process is one strong candidate among CCV’s many sources. Previous numerical work employing a diesel injector geometry measured with x-ray diagnostics has shown that: (1) manufacturing tolerances and needle radial motion are responsible for orifice-to-orifice mass flow rate differ-ences, and (2) needle motion variability is responsible for shot-to-shot inconsistencies. Furthermore, needle mo-tion variability tends to be greatest during the transient phase of the injection, motivating an investigation effort toward short injection durations. 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, under typical diesel operating conditions, these high-volatility fuels generally exhibit considerable propensity for cavitation in the orifices and at the needle seat, which has been shown to correlate strongly with radial needle motion. Cavitation within the orifices might increase shot-to-shot variability by reducing the orifice cross-sectional areas and discharge coefficients. Furthermore, the systematic presence of cavitation might lead to local erosion of the nozzle internal geometry, leading to even higher variability with time. This might result in further CCV, as the combined effects of cavitation, needle radial motion, and orifice-to-orifice differences are expected to have a strong influence on shot-to-shot variations. This study focuses on the evaluation of orifice-to-orifice variability of gasoline-like fuel injection under diesel operating conditions and short, transient injec-tion durations. High-resolution x-ray geometry characterization of an eight-hole heavy-duty diesel injector was combined with measurements of the needle motion. A well-validated computational setup using the CON-VERGE code was employed to perform a series of simulations and quantify the shot-to-shot variability of the in-nozzle flow.