Jet and spray characteristics in advanced compression ignition engines

Abstract

This thesis investigates jet, spray and resulting combustion characteristics in the context of advanced compression ignition engines. Measurements are reported of mixing, scalar dissipation rate (SDR) and entrainment in a decaying non-reacting jet by planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV). For the first time, joint statistics of mole fraction and SDR are reported and are closer to a Gumbel, rather than a lognormal, distribution. After the end of injection the SDR for the most reactive mixture fraction at the area near the nozzle exit implies that autoignition is unlikely occur there and contributes to increased unburned hydrocarbons and CO emissions. There is indirect evidence of an “entertainment wav”. A novel method to describe the transient jet boundary provides direct evidence of this process, although peak entrainment is only 1.2 that of the steady jet, while theory suggests a factor of 2. A high pressure and temperature constant volume cell was used to study decaying liquid spray from commercial diesel injectors on the liquid penetration and dispersion rates. Measurements were made by Mie scattering of the liquid phase. The effects of conicity factor (CF), hydro-erosive (HE) rounding, hole length and hole outer diameter of the injector nozzle were investigated. The measured liquid penetration rate shows a transition away from linear at around a dimensionless value of time around unity: the rates are lower than the theory of Naber and Siebers. Higher CF leads to longer spray penetration and smaller cone angles, higher HE Rounding leads to wider sprays and shorter penetration distance , suggesting that the mixing and evaporation is promoted by high HE rounding. Computationally, a multi-zone computer program is developed to simulate a homogeneous charge compression ignition engine, with heat loss effects calibrated by using the motoring indicator diagram and measured in-cylinder temperature profiles. A detailed n-heptane mechanism and 10 zones provide good prediction of the measured indicator diagram. Agreeing with experiments, the simulation shows that intermediate or heavy internal exhaust gas recirlcuation (iEGR) allows operation at leaner conditions or lower inlet air temperature without misfire. The amount of NOx is negligibly small using iEGR at the fuel lean condition but there is little beneficial effect on the high level of CO and UHC emissions. This program is extended to partially premixed compression ignition engines, coupling an inter-zone mixing model in mixture fraction space and three ways are explored to extract temporal profiles of the SDR calculated from a stochastic code. The heat release rate (HRR) from simulations is much larger than measurements. Although indicated mean effective pressure (IMEP) is over-predicted, both simulations and engine tests show that the IMEP is insensitive to injection pressure or injection timing but, as is expected, is increased by increased fuelling. The tendency that later injection leads to lower maximum PRR is found by both engine tests and simulation. However, the maximum PRR from simulations is much larger because of the higher HRR.