The effects of enhanced flame diffusion surface on thermal efficiency of small-bore HPD diesel

In the context of energy conservation and emission reduction, high power density(HPD) and low fuel consumption are the consistent pursuit of diesel engine development. Among the small-bore diesel, the limited space in the cylinder poses higher challenges and requirements for the arrangement of sprays.The high injection pressure results in a greater impulse when the spray impinges chamber, which allows the combustibles to develop along the chamber wall. Based on these characteristics of small-bore HPD diesel, a reasonable injection scheme is proposed to help flame diffusion surface increasing and thermal efficiency enhancing. This work proposes an optimization path to increase the flame diffusion surface, then improve thermal efficiency. It can be achieved with matching between the injector extension length and the spray spray angle. The combustion process of the small-bore is divided into surface and suction combustion, the effect mechanism of the research variables on different combustion stages were analyzed. The results reveal that the low temperature oxidation products develops with little suction along the chamber wall after impinging, and releasing heat appears on envelope until it impinges the cylinder head. Shortening the extension length of injector can increase the flame diffusion surface. The impingement position is closer to the cylinder head, which will exacerbate inadequate combustion at suction combustion. Reducing the spray angle can improve this phenomenon. The combination of smaller spray angle and shorter injector extension length results in improved thermal efficiency by 1.325% and reduced brake specific fuel consumption by 8.4g/kWh. This work has certain guiding significance for the design and manufacture of small bore diesel.