A Fundamental Investigation On Clean And High-Efficiency Combustion Modes Of Diesel Engines

Many researchers in the world focus on the advanced combustion modes of diesel engines which can avoid NOx and soot forming regions. In this study, combustion mechanisms of three advanced combustion modes were investigated: Homogenous charge compression ignition (HCCI), Charge stratification compression ignition (SCCI) and Low temperature combustion (LTC). The potential of clean and high-efficiency of the three combustion modes was investigated using fully coupled multi-dimensional CFD and reduced chemical kinetics model combined with experiments.Based on the analysis of detailed mechanism, a reduced mechanism of n-heptane HCCI combustion is developed. The simulation results with CFD coupled the reduced mechanism model indicate that low temperature reaction begins from the vicinity of the cylinder wall and the bottom of the piston bowl, while the high temperature reaction takes place around the center of the bowl region and is widely distributed in space. UHC emissions mainly reside in the piston-ring crevice region. The majority of CO emissions are located in the region near the top surface of the piston. The Experimental study on n-heptane HCCI combustion shows that the increase of fuel/air equivalence ratio increases the maximum cylinder pressure and the maximum rate of heat release of combustion. With the increase of fuel/air equivalence ratio, the combustion efficiency increases; the indicated thermal efficiency increases first and then decreases; the UHC emissions decreases first and then increases; and the CO emissions decreases.A reduced chemical kinetics model of n-heptane SCCI combustion which consists of 42 species and 58 elementary reactions is developed. Applying CFD coupled the reduced mechanism model, seven different kinds of imposed stratification have been introduced according to the position of the maximal local fuel/air equivalence ratio in the cylinder at intake valve close. The results show that: The former four kinds of stratification, whose maximal local equivalence ratios locate between the cylinder center and half of the cylinder radius, advance ignition timing, reduce the pressure-rise rate, and retard combustion-phasing. All kinds of stratification can reduce UHC emissions. The last two kinds of stratification can reduce UHC and CO emissions while maintain low NOx emissions simultaneously. For premixed/direct-injected fuel combustion, the combustion process by fuel injection at the timing near the start of low temperature reaction is similar to the imposed stratification cases whose maximal local equivalence ratios locate between the cylinder center and half of the cylinder radius; the combustion process by early fuel injection before -65 deg. ATDC is similar to the imposed stratification cases whose maximal local equivalence ratios appear between half of the cylinder radius.Effects of oxygen concentration on combustion process and emissions of diesel engine are investigated by engine experiments and numerical simulation. The results indicate: Soot emissions increase first then decrease. It is the essential reason of low soot emissions at low oxygen concentration that the low in-cylinder combustion temperature leads to the inhibition of soot formation. The decrease of in-cylinder temperature leads to rapidly decrease of NOx emissions and the increase of CO emissions. HC emissions increase decrease first then rapidly increase at very low oxygen concentration. With the increase of injection pressure, the flow velocity and turbulent kinetic energy in the cylinder increases which are benefit to improving the mixing of fuel and air. The enhancement of intake pressure improves the mixing of fuel and air in the condition of oxygen lack in local regions at lower oxygen concentration. They are significant measures to reduce soot emissions in LTC combustion.