Theoretical Research On Mixture Formation For A Lean-burn Direct Injection LPG Engine

LPG (Liquefied Petroleum Gas) mainly consists of C3H8 and C4H10. Compared with gasoline, it has lower carbon hydrogen ratio, higher octane number and vapor pressure, and lower UBHC, NOx pollutants emissions. LPG has almost the same calorific value as gasoline but much cheaper in per Joule. LPG becomes an excellent alternative fuel with a good fuel economy and low emission. But current LPG engine goes against with a pre-mixing method, but a DI (Direct Injection) LPG engine can evidently improve the power performance, fuel economy and emissions, especially employing lean-burn direct injection can efficiently improve fuel economy at part load. Different thermo-physical properties between LPG and gasoline makes mixture formation of LPG differ from that in GDI (gasoline direct injection) engine, and mixture formation makes a crucial effect on the organization of combustion. Therefore study on mixture formation of LPG for a direct injection engine goes for an important theory and application significances, especially on LPG engine's optimization of combustion and emissions.Flash boiling have a very important affect on fuel's atomization and evaporation, taking place with LPG's liquid phase injection. However, the existing CFD software FIRE has no LPG properties date and equations describing flash boiling, then this thesis modified evaporation model and fuel properties files to describe spray motion properly. And the accordance of LPG spray between simulation results and visualization validate the accuracy of models and feasibility of calculation.And mixture formation at different loads in a wall guided system is investigated. Results show that:homogeneous power mixture achieved employing early intake injection with intake valves all opening at full load, stratified mixture obtains using late compression injection (60℃A BTDC) at part load, with a closed intake valve brings about deflective swirl enhancing turbulences in late compression stroke.Furthermore, a combined guided system (spray guided system and wall guided system) employing a multi-holes injector was proposed and initially simulated in this thesis. And results show that:a part of fuel injected directly to the spark plug and achieves a combustible mixture utilizing a spray guided method; the other part moves upward to spark plug under the guide of the mixture cloud and cavity wall, achieves a stratified mixture around spark plug at 15℃A BTDC. Combined guided system can solve the problem of mixture too thick to increase UBHC, NOx emissions around spark plug at part load, and could also steadily achieve lean stratified combustion at a large range of speed.