Low Emission Concepts for Diesel Engines

Doctoral thesis, 2006

In the recent past, the Diesel engine has been improved considerably in the areas of performance, fuel efficiency and exhaust emissions. In Europe, these improvements have resulted in a strong increase in the sales of passenger cars equipped with a Diesel engine. Since the emission of the green house gas CO2 is directly linked to engine efficiency, the Diesel engine is also attractive from an environmental point of view. However, despite the emission reductions achieved by improvements to the combustion process, the stringent emission legislations of the near future will be difficult to meet without exhaust gas after-treatment systems. Disadvantages of after-treatment systems are the considerable cost as well as durability and servicing issues, which is a motivation to continue studying possibilities to reduce the engine-out emissions. This thesis discusses an experimental study of some of these possibilities, including modifications to conventional Diesel combustion and Homogeneous Charge Compression Ignition (HCCI). An experimental high speed DI Common Rail engine was used in all of the experiments. The effects of post injection and swirl ratio on emission formation have been studied by optical diagnostics of the combustion process. Processing the obtained data with the two- colour pyrometry method provided temporally resolved soot concentration and flame temperature data, which proved to be beneficial in answering the raised questions. Adding a third, post-injection after the main injection showed significant reductions of soot emissions with only minor changes of NOx emissions and fuel consumption. The reason is believed to be the increased turbulence caused by the extra injection and to some extent the increased flame temperature during the second half of the combustion process, both of which contribute positively to the soot oxidation rate. Increasing the swirl ratio of the intake charge also showed significant reductions of soot emissions. Similar to post injection, the increased turbulence increased the soot oxidation rate late in the combustion process. HCCI combustion is known for its potential to significantly reduce NOx and soot emissions. The experiments involving HCCI combustion of conventional Diesel fuel were focussed on investigating the modifications to the engine hardware and the injection strategy that are necessary for operation in HCCI mode. A major difficulty of HCCI operation with Diesel fuel was found to be preventing over-advanced ignition and excessive combustion rates. The compression ratio was therefore reduced to delay ignition and high loads of EGR were used to further delay ignition and to reduce the combustion rate. Fuel was injected during the compression stroke in five subsequent short duration injections, so that a homogeneous mixture was created before start of combustion. Initially, compression ratios of 11.5 and 13.4 were used, while later in the development, the compression ratio was raised to 15. The higher compression ratio was used in combination with retarded intake valve closing timing, which reduced the effective compression ratio. Both NOx and soot emissions were reduced to near-zero levels, while the HC and CO emissions increased significantly. The increase in HC and CO emissions reduced the combustion efficiency, which was the main cause of the significant increase of fuel consumption. With the use of more accurate piezo injector and with modifications to the injection strategy, HC and CO emissions as well as fuel consumption could be reduced significantly, while maintaining low NOx and soot emission levels for most operating loads of up to 0.5 MPa IMEP.