Numerical analysis of the combustion process in a compressed natural gas direct injection engine

The design, development and optimization of an internal combustion engine require the application of a modern sophisticated analysis tool. In addition to experimental work, numerical calculations are now necessary to provide an insight into the complex in-cylinder process. The combustion process and its emission characteristics in a compressed natural gas direct injection engine were analyzed and investigated. The numerical studies were performed on a single cylinder of a 1.6-liter engine running at wide open throttle. The grid generation was established through an embedded algorithm for moving mesh and boundary in order to provide a more accurate transient condition. The combustion process was modelled with the eddy break-up model of Magnussen for unpremixed or diffusion reaction with three global reaction scheme. The computational fluid dynamics (CFD) simulations at two baseline conditions are carried out to examine the fluid flow, air-fuel mixing formation, combustion process, carbon monoxide emission distribution as well as NO emission formation occurred inside engine cylinder. The CFD results were compared with the experimental data and showed a very good agreement for two baseline conditions. A set of parametric studies were carried out by varying the timings of start of injection (SOI) and start of ignition (SI). The examined engine performance is in-cylinder pressure, while the considered emissions to be minimised are CO and NO levels. In order to study the effect of injection timing, the SOI timing was varied from 120º -140º with fixed ignition timing at 19º bTDC. On the other hand, SI timing was positioned from 15º-23º bTDC with fixed SOI timing for studying its influences. The CFD results indicated that slightly retarded SOI and SI timing can be chosen to reduce CO and NO levels while increasing engine performance.