A Study On The Mixture Formation And Air Flow In The "Bump" Combustion Chamber Of A Diesel Engine

The new concept of combustion in I. C. engines, HCCI combustion mode, becomes the highlight of current researches for its outstanding advantages of low NOX, PM emissions and high thermal efficiency.The control of auto-ignition time and heat release rate is the most difficult technical question in realizing HCCI combustion. The compound combustion technology was presented in the state key laboratory of engines (Tianjin University) for the purpose of realizing the concept of HCCI combustion in a D I diesel engine. The mechanism and role of the BUMP combustion chamber in mixture formation of lean diffusion combustion and the main parameters that influence diesel secondary impingement jet were the major objective of the investigation in this paper in order to lay the foundation of improving the design of the BUMP combustion chamber.The author investigated diesel spray wall-impingement mixing process with the help of both experiments and numerical simulation. The experimental investigations were first conducted on diesel spray wall-impingement mixing process with or without a BUMP on a flat wall and in a ω combustion chamber by means of PLIF method in the self-made constant volume combustion spray simulation set-up which can reproduce the typical working conditions and high-pressure fuel injection of modern diesel engines. The simulation computation of a CFD code about them was then carried out. The simulation results were in good agreement with the experimental ones. The BUMP height and secondary wall-impingement distance were discovered to play an important role in producing a secondary space jet, reducing the quantity of fuel stuck on the wall and forming lean mixture, and their suitable values being 1.0-1.5mm and 0-2.0mm respectively. Moreover, the secondary wall-impingement distance should be properly reduced in order that the BUMP play a part in the formation of lean mixture with the increase of environmental pressure or density owing to both the increase of air entrainment, momentum losses and the decrease of the spray tip velocity and penetration.The experimental and simulative results showed that a wall-impingement jet could strip off the wall when it encountered the BUMP which was appropriately set up on the wall of a combustion chamber, and a secondary space jet was produced. The space mixing volume of the wall-impingement jet and air was enormously enlarged, a flash mixing phenomenon occurred. The shape, height and secondary wall-impingement distance of the BUMP and the incident angle of the spray must be optimized and matched rationally because of the distinct effects of different BUMP on mixture formation.A two-vortex structure near the BUMP or three-vortex structure in the BUMP combustion chamber which could promote the formation of lean mixture and an effective approach to lean mixture formation and further to HCCI combustion assisted by other countermeasures in diesel engines were found through the investigation on the in-cylinder gas flow , fuel concentration distribution, velocity and pressure field etc. with a CFD code.