Investigation On The Mechanism Of Diesel TR Combustion System Based Spray Impingement

Diesel engine is used widely for its potential of promotion of power and lowing fuel cost. But the combustion mode of diesel engine embodies the conflicts between improving combustion and emission as well as PM and NO_x. Efforts of the researchers contribute to find the tradeoff between them. As a novel combustion mode, HCCI focuses on realizing highly efficient and clean combustion. The challenging problem it faces is how to control the ignition timing and how to get full-load performance. Another popular combustion mode is spray impingement combustion. Especially, impinging spray onto the fixed location, boosting the mixture and combustion with bounced fuel, was developed in recent few years.Reviewing current diesel combustion system with spray impingement, four strategies are adopted: reflex edge is fixed on the chamber wall, forming secondary jet; the raised plane is provided in the cavity, forming discal fuel spray; structure near nozzle can form diffused fuel spray; contracted chamber and sharp convexity can enhance air flow and promote mixture.Based on the design theoretics of diesel engine combustion system sumed up in the paper, TR combustion system with fuel impingement was set up and studied. It is modified from ω combustion chamber with an oriented arc at the chamber wall, specially and perfectly fit in a nozzle with 4× Φ0.36 orifices in conic section plus 1× Φ0.20 orifice in center. In the test study TR demonstrated an excellent performance with smoke emission. Optical investigation and CFD were used to study the mechanism of TR mixture formation and performance of combustion and emission. The main work is covered as the following:First a digital measurement was set up and spark photography of spray field was taken. The concentration field, cone angle and penetration were automatically got. The image processing software was developed to analyse the picture and obtain the digital results. The comparison between TR combustion chamber and the conventional ω combustion chamber show that, in the TR chamber, fuel was peeled off from the wall after impinging and turning to the center. So there is less fuel in the chamber bottom than in the ω chamber.The air flow, combustion, emission and fuel film of diesel was studied. It is found that contracted chamber has preferable mixture and combustion as well as smoke emission. Naturally, the much thicker film and richer mixture are formed after fuel impinging on the wall, which will deteriorate the combustion. On the other hand, fuel impingement is beneficial to droplet breakup and vapor. According to these, the theory of diesel combustion system with spray impingement is brought forward: contracted chamber and sharp convexity should be adopted to enhance air flow and promote mixture. Fuel impingement is used to accelerate fuel atomization. The film thickness should be reduced and have a thin distribution widely. The ignition time is reasonable. Fuel is injected into cylinder before ignition to avoid fuel surrounding fire.In TR combustion system, the oriented arc changes the flow field in the cylinder. The air movement near the wall is enhanced and eddy distribution is more reasonable, improving mixture formation. Central orifice spray and oriented arc make fuel distribute equably, which reduce the amount of rich fuel-air mixture near the wall. Then the air is sued effectively and unburned mixture cut down, which is useful to combustion. TR chamber has the high temperature in the center and the low temperature near the wall. The mixture formed by central orifice spray is ignited first and in turn ignites other mixture. The diameter of the central orifice could not be too large.The engine study of TR shows that it has very low smoke emission. At the low and part load, NOx emission of TR combustion system is less than original diesel, but more at high load conditions. When the speed decreases, ignition delay period is longer, temperature becomes higher,NOx emission increases and smoke reduces. The intake port swirl is not demanded by TR. In the four valves diesel, the intake port swirl is appropriate, so the intake port needn't change. When EGR is applied, TR improves NOx emission obviously with fuel consumption changed little, but smoke emission increases. It is needed to make reasonable EGR ratio. With fuel supply advance angle delaying, the NOx emission of TR decreases and smoke emission increases. In die BTDC 5 0 CA, NOX reduces less than before. The BTDC 11° CA is the trade-off of fuel supply advance angle. The application of supercharge and intercooling in the TR is forecasted. The results show that supercharge reduces smoke by 6% but increases NO by 75%. However intercooling reduces smoke by 15% but increases NO only by 16%. Supercharge makes cylinder pressure enhanced, which means more output power. Split injection used on TR was studied. Comparing with conventional single injection, the split injection and its period as well as quantity can achieve better combustion in the same condition of fuel supply advance angle. But the fuel quantity of each injection needs to be optimized in order to get the tradeoff between PM and NOx. TR combustion system has great potential to improve emission.