Diesel combustion study at full load using CFD and Design of Experiments 2003-01-1858

As the environment protection is a key issue for the 21^st century, the world-wide auto manufacturers are now considering the pollutant emission reduction as a major task for the future. Thanks to a combustion occurring in a lean well premixed charge, the HCCI (Homogeneous Charge Compression Ignition) concept offers the possibility to almost reach the near zero-NOx and particulate emission ambitious target. Nevertheless, the customer application of such a system must clearly preserve the well known advantages of Diesel engines such as high specific power and torque and low fuel consumption. For this reason the full load application of such an engine still uses conventional combustion whilst the homogeneous charge compression ignition is used at part load.

This work presents how the pre-optimization of a combustion chamber, dedicated to HCCI Diesel combustion, is carried out at full load thanks to the use of design of experiments and a CFD code. Consequently the part load optimization under homogeneous conditions is not discussed here.

The bowl shape was investigated by the mean of width to depth ratio, the injector nozzle number of holes as well as protrusion and spray angle were optimized as regard to the cylinder head swirl ratio.

Besides its interest in reducing the number of calculation configurations, the design of experiments allows us to point out the most important interactions between the above parameters.

Main results show that the specific power at maximum speed is not sensitive to the bowl shape as far as the other parameters are optimized in order to ensure good mixing conditions and long ignition delays. Early injection together with adapted injection configuration - eg spray angle, number of holes and swirl ratio, are found to be of uppermost importance to reach good performance with low smoke level. Optimization at the specific torque shows that an appropriate bowl width to depth ratio allows the best use of oxygen as well as an efficient soot post-oxidation.