Reducing Diesel Engine Emissions - An Experimental Investigation

Licentiate thesis, 2004

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 interesting from an environmental point of view. However, despite the emission reductions achieved by improvements of the combustion process, the stringent emission legislations of the near future will be difficult to meet without exhaust gas aftertreatment systems. Disadvantages of after-treatment systems are the considerably costs as well as durability and servicing issues, which is a motivation to continue studying possibilities to reduce the engine-out emissions. This thesis discusses the experimental investigations of some of these possibilities, including post injection, increase of swirl ratio and homogeneous charge compression ignition (HCCI). A 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 measurements of the combustion process. Processing the obtained data with the two colour method provided temporally resolved data on the soot concentration and the flame temperature, 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 by closing off one of the intake ports also showed significant reductions of soot emissions. NOx emissions increased, but to a lesser extent. The soot concentration and temperature distribution calculations derived from the combustion images, showed slightly increased flame temperatures and more significantly, a higher soot oxidation rate, later in the cycle. HCCI is an alternative combustion type in which a homogeneous mixture is ignited by the heat of compression. Known benefits are near-zero NOx and soot emissions. An experimental investigation was carried out to explore the possibilities of this type of combustion for a high speed DI Diesel engine, using conventional Diesel fuel. As a measure to prevent the fuel from igniting too early, the compression ratio of the engine was reduced to 13.4:1 and 11.5:1. The fuel was injected during the compression stroke in five subsequent stages, so that a homogeneous mixture was obtained before start of ignition. Due to the low compression ratio and high EGR rates, the start of the high temperature reactions was delayed to close to top dead centre. Operation was possible in the load range from 0.2 to 0.9 MPa IMEP. The high EGR rates proved to crucial to control the combustion rate. Both NOx and soot emissions were reduced to near zero levels, while the HC and CO emissions increased dramatically. The increase in HC and CO emissions reduced the combustion efficiency, which was the main cause of the significant increase of fuel consumption.