(151c) Clean Diesel Combustion Technology Using Supercritical Fluids

One major problem associated with conventional diesel engines is incomplete, inhomogeneous fuel air mixing during the combustion process. This contributes to significant engine emissions and limits energy efficiency. A novel concept is proposed to increase energy efficiency and to reduce both PM and NOx emissions using the supercritical fluid technology. This process involves preparation of diesel fuel/EGR mixtures, heating of the mixtures to the supercritical state, injection of the supercritical fuel mixture, complete fuel/air mixing, and clean combustion. EGR is used as diluent to prevent diesel fuel coking. In this presentation, the conceptual design of this new process will be introduced and compared with conventional diesel engines, and results from preliminary experimental studies on supercritical fuel injection and fuel stability will be presented.

A flow rig was constructed to test injectors and to study spray behavior. A high speed camera was used to take shadowgraph images of sprays. Experiments were conducted in both liquid and supercritical conditions of the fuel-diluent mixtures. Results showed that the spray cone angle and the level of mixing with air improved as both pressure and temperature increased. Significant improvements were achieved when the fluid was injected under the supercritical conditions.

Fuel stability is one major issue that needs to be addressed. In this work, the effects of temperature, residence time and CO₂ on thermal stability of diesel fuel were investigated by both batch and continuous thermal stressing experiments. CO₂ was used as a surrogate of EGR. Results showed that thermal stability of diesel fuel decreased as both temperature and residence time increased and that the presence of 10 wt% CO₂ reduced accumulation of solid deposits. It was further concluded that 400-420 ^oC would be an optimal temperature range where supercritical fuel injection and combustion can work.