A Numerical Study of the Influence of Different Operating Conditions on the Combustion Development in an Automotive-Size Diesel Engine

In this paper, numerical simulations of an automotive-size optical diesel engine have been conducted employing the Reynolds-Averaged Navier-Stokes (RANS) equations with the standard k-ε turbulence model and a reduced n-heptane chemical mechanism implemented in OpenFOAM. The current paper builds on a previous work where the model has been validated for the same engine using optical diagnostic data. The present study investigates numerically the influence of different operating conditions - relevant for modern diesel engines - on the mixture formation development under non-reactive conditions as well as low- and high-temperature ignition behaviour and flame evolution in the presence of strong jet-wall interactions typically encountered in automotive-size diesel engines. Also, emissions of CO and unburned hydrocarbons (UHC) are considered. This has been systematically studied by varying four different engine parameters: 1) Engine swirl: zero, nominal (baseline) and double of the swirl; 2) Fuel injection pressure: 1000 and 1600 (baseline) bar; 3) Intake pressure: 1 (baseline) and 2 bar; 4) Ambient oxygen mole fraction: 21 (baseline) and 15%.

The swirl was found to have a minor influence on the inert fuel spatial distribution. On the other hand, the swirl has a large impact on the high-temperature rather than on the low-temperature ignition and this effect is more pronounced for the low fuel injection pressure case where high-temperature combustion on the up-swirl side is suppressed for the high swirl case. This effect is not observed for the 1600 bar injection pressure case and it can potentially have a strong impact on the unburned hydrocarbons (UHC) emissions.