No abstract

Fuels that are more resistant to autoignition allow more time for mixing before combustion occurs and help to reduce nitrogen oxides (NO x) and smoke in a diesel engine. However, hydrocarbon (HC) and carbon monoxide (CO) emissions are high at low loads because combustion is more likely to take place in lean mixture packets with better mixing caused by longer ignition delays. These problems can be significantly alleviated by managing the mixture strength by changing the injection pressure and the nozzle geometries. A singlecylinder diesel engine is run on a mixture of gasoline and diesel with a research octane number of 91, at different speeds, loads, and exhaust gas recirculation levels using three different nozzles. It is much easier to obtain low NO x and low smoke emissions with this fuel than with a European diesel fuel using the standard nozzle. Larger injector holes and lower injection pressures help to reduce the HC and CO emissions at low loads and also enable the gasolinelike fuel to run at a higher speed of 4000 r/min at a reasonably high load (indicated mean effective pressure) of 10 bar.

If fuels that are more resistant to autoignition are injected near top dead centre in compression ignition engines, they ignite much later than diesel fuel does, and combustion occurs when the fuel and air have had more chance to mix. This helps to reduce nitrogen oxide and smoke emissions. Moreover, this can be achieved at much lower injection pressures than for a diesel fuel. However, it is preferable to have fuels with a lower research octane number than those of commonly available gasolines, because this makes low-load operation easier while retaining the advantages at higher loads. A practical approach to making such fuels is to blend the gasoline and diesel fuel available in the market. Such fuel blends have a wide volatility range since they contain high-boiling-point components from the diesel but have a lower research octane number than that of the gasoline used but have a much longer ignition delay than that of the diesel fuel. This work describes the results of running a single-cylinder diesel engine on three such fuel blends. The engine could be run on such blends with extremely low smoke and low nitrogen oxide emissions at speeds of up to 4000 r/min and loads (indicated mean effective pressures) of up to 10 bar with an injection pressure of only 400 bar. The smoke levels at comparable nitrogen oxide levels were extremely high with diesel fuel in these conditions, even with an injection pressure of 1100 bar. The engine could also be run at near-idle conditions on these blends but with higher hydrocarbon and carbon monoxide emissions but much lower nitrogen oxide emissions and maximum pressure rise rate compared with those of the diesel fuel. The wider volatility range might be of benefit in avoiding over-mixing and over-leaning, which could lead to poor combustion stability. The paper also considers the trade-offs between the nitrogen oxide, smoke, hydrocarbon and carbon monoxide emissions and the maximum pressure rise rate and discusses approaches to optimise this type of combustion.