It has been over a century since the compression ignition (c.i.) engine, also known as the diesel engine, was invented. Since then, the diesel engine has experienced significant changes in terms of continuous improvement in engine quality. Nowadays the transportation of goods is almost entirely carried out by diesel engine powered vehicles in the developed countries. Over the last three decades there have been marked increases in the number of diesel engines used in light duty trucks and passenger cars. This trend has been mainly influenced by sharply increasing oil prices post 1980 and the superior fuel economy of diesel engines over spark ignition (s.i.) engines. Current forecasts of the European fuel demand show a continuing decrease in petrol demand and a steady increase in the diesel fuel demand, indicating a promising future for diesel engines. Besides its use in diesel and gasoline engines, crude oil derivatives are widely found in power stations and space heating.
While all oil users face stringent emissions legislation, engine manufacturers in particular are facing some strong challenges, presented by diesel particulates emissions, and also a deterioration in fuel quality as a result of increasing the amounts of non-oil based materials used in transport fuels as well as in heating[1].
In general, diesel combustion is often said to be diffusion-controlled, which means that the combustion is mainly controlled by the rate of mixing of fuel and air similar to that in diffusional spray flames. Due to this heterogeneous combustion nature, diesel engines and furnaces emit far more particulates than spark ignition engines, which depend on so called premixed combustion (the fuel and air are “well mixed” before being charged into engine cylinders).
Diesel Particulates
Diesel particulates are composed of combustion-generated carbonaceous soot particles and the absorbed liquid phase hydrocarbons. Although soot particles have not been found to be significant as a health hazard, some hydrocarbons, particularly some polycyclic hydrocarbons (PAH), in the absorbed liquid phase are considered to be carcinogenic. In some developed countries legislation has been introduced on particulates emission from diesel engine powered vehicles. And more importantly, this legislation has become more and more stringent in the last few years and will likely keep on doing so in the future. For instance, the Air Resouce Board in California has lowered the standard from 0.4 g/mile for 1985 to 0.2 g/mile for 1986 and 0.08 g/mile for 1989 for both light duty diesel trucks and passenger cars.
One way to reduce particulates emission from diesel engines so as to meet the future standard is by developing some type of regenerative exhaust filters to trap particulates in the engine exhaust. Such research has been underway for 25 years. However reducing particulates from within the engine by developing more advanced combustion systems for new engines seems to be more desirable. Therefore understanding of the in-cylinder processes associated with particulates is crucially important for such development.
The major pathways leading to particulates formation in diesel engines can be clearly demonstrated. One important point is that particulates formation and combustion are closely associated with the combustion of diesel fuel itself, which is characterised mainly by the vaporization of liquid fuel, mixing of fuel vapour and air, and chemical reaction.
The understanding of diesel engine combustion has certainly increased significantly by the work carried out over the last 40 years. But there is still some way to go before a full understanding can be achieved. This is because of the complexity of the process. Firstly the sub-processes involved, e.g. the multi-phase turbulence flow and the multi-step chemical reactions, are complicated and far from being fully understood. Secondly the interaction between these processes is very complicated and often presents serious problems in computer modelling. The work reported here (Chen, 1990, Bush 1976, 2003) has been carried out with a view to making a mechanism-based contribution to modelling this complexity in a way which can be employed in the design of combustion systems.
References
[1] The main non-oil based fuels are biofuels such as ethanol in gasoline, made from sugar-beet, and assorted fats in diesel fuels. Wood chips are being used instead of coal in some electricity power stations.