Study Of The Injection And Mixture Process And The Combustion Simulation Of Natural Gas/Diesel Dual Fuel

With the depletion of fossil energy and increasingly stringent regulation of vehicle emissions, the development and utilization of natural gas engines have attracted enormous attention from many research institutes and engine companies throughout the world. According to some former researches, the conventional natural gas engines in which natural gas is inducted or injected in the intake manifold to mix uniformly with air suffered from some problems including the reduction of power performance, the increase of unburnt mixtures, cold start difficulties et. al. On the contrast, natural gas/diesel dualfuel engine using high pressure direct injection(HPDI) technology can maintain the power performance comparing with the same displacement diesel engine, optimize the combustion process and reduce the emissions like CO2,NOx,PM substantially. The injection and mixture process of the natural gas and diesel and the natural gas substitution have quite important impact on the combustion process and emissions of the HPDI natural gas and diesel dual fuel engine.In order to carry out visualization studies of the injection and mixture process of natural gas and diesel, a new kind of dual fuel HPDI coaxial injector was established by the Auto CAD software and some key parameters were determined based on a real diesel injector. However, a combined dual fuel injector was built due to the difficulties in manufacturing HPDI coaxial injector.The Schlieren technique was assembled with a constant volume chamber to detect the natural gas jet and the diesel spray and the effect of ambient pressure, natural gas injection pressure, diesel injection pressure and the time differences between the start of natural gas and diesel on the injection and mixture process were studied in this paper. The results obtained from the experiments indicated that:(1) the penetration of natural gas jet and diesel spray decreased with the increasing ambient pressure, the cone angle of the two fuels also increased with higher ambient pressure;(2) the penetration and cone angle of natural gas jet increased with the higher natural gas injection pressure at the same time and the mixture among the natural gas, diesel and air were improved under higher natural gas injection;(3) the penetration and cone angle of diesel spray also went up by enhancing the diesel injection pressure and the momentum transference from diesel spray to natural gas jet was observed by comparing the penetration under dual fuel injection and single diesel injection condition;(4) mixing of the vaporized diesel with the ambient air in the chamber as well as with the natural gas were likely to be inhibited if the SOI(start of injection) of natural gas was-1ms earlier than diesel, while the liquid tip velocities were high, stripped or highly-atomized liquid was left well behind and the gas front easily penetrates into these potential ignition sources if there was a natural gas injection delay of 0.5 ms after the SOI of diesel and 1.0 ms delay was too great as the diesel jet had moved too far from the tip of natural gas jet.Furthermore, a ω model combustion chamber was installed inside the constant volume chamber to study the flow structure and mixture formation of natural gas and diesel under wall impinging circumstance. There were four stages of wall impinging process: transient free jet stage, initial stage of wall-impinging jet, development stage of wall-impinging jet and end stage of wall impinging jet existed in both natural gas and diesel development. The vortex structure and interaction of the two fuels declined under longer distance between the ω model combustion chamber and the diesel orifice.At last, Numerical simulation of the combustion and emissions of HPDI engine under different natural gas substitution rate was conducted using AVL FIRE. The effects of natural gas substitution rate on cylinder average pressure, temperature, heat release rate, NO and Soot were analyzed under four typical conditions. Cylinder average pressure and temperature decreased with higher natural gas substitution rate and the NO emission was reduced significantly comparing with only diesel fueled engine under the same circumstances.The results obtained from the studies about the injection and mixture of natural gas and diesel will contribute to the researches about combustion characteristics on dual fuel engines and the optimization of the micro pilot diesel ignited HPDI natural gas dual fuel engine.