Due to increasingly stringent emission and fuel consumption regulations, diesel engines for vehicle are facing more and more technical challenges. Engine downsizing technology is the most promising measures to deal with these challenges at present. With the enhancement of power density, a small engine displacement with a high turbocharging technique becomes popular. In order to increase the intake mass flow rate on a downsizing diesel engine, the tilting axis of intake valve was chosen to enlarge the intake valve diameter and decrease the arc radius of intake ports. Thus cylinder head had to be redesigned to meet this demand. Geometry of cylinder head made a notable effect in organization of in-cylinder flow, fuel-air mixing quality and further combustion characteristics.
3-D CFD was a convenient and economical tool to explore effects of geometry of cylinder head on the combustion process. In this study, two cases of combustion chamber structure formed by one-cut and four-cut processing methods respectively were studied. This work was done on the ESE Engine module of commercial CFD software AVL FIRE and operated by comparisons of these two cases on the development of swirl ratio, equivalence ratio distribution and combustion characteristics. The results show that, in term of swirl ratio, two cases reflect a similar growing trend, but a weaker swirl (down by 9.1%) is formed in the one-cut case. Resulting from its weaker swirl level, this case delivers a 0.6°CA longer in CA5 and a 0.5°CA later in CA50 than the four-cut case under the same injection timing. However, under the same injector protrusion height, a better diffusion of mixture can be found in the one-cut case contributed by its dome shape, which enhances the increase of portion of heat release of premixing phase and the decrease by 0.9°CA in combustion duration, but the combustion efficiency of these two cases are almost the same.