Hideyuki Ogawa

Highly oxygenated fuels can effectively reduce afterburning in diesel diffusion combustion and improve the degree of constant volume heat release in spite of increases in injection duration due to smaller heating values. The mechanism of afterburning reduction and the structural differences in the diesel fuel spray with changing the oxygen content in the fuel were investigated in a constant volume vessel and analyzed with 3D CFD simulation. The result showed that the equibalance ratio inside the spray decreased with increases in the oxygen content due to lower theoretical air-fuel ratios, promoting the spray combustion after the end of injection. Further, the combustion characteristics and the exhaust gas emissions of the oxygenated fuels were investigated in an experimental single cylinder diesel engine with modern specifications. With increasing oxygen contents, the degree of constant volume heat release increased with reductions in the afterburning, resulting in higher indicated thermal efficiencies. However, the indicated thermal efficiency showed a maximum at around 27 mass% of the oxygen in fuel and further increases in the oxygen contents resulted in lower indicated thermal efficiencies due to larger cooling losses.

A combination of a fuel injector with staggered nozzle hole arrangement with a combustion chamber divided into upper and lower layers by placing a lip at the middle of the side wall is proposed to improve diesel combustion with suppression of interactions among fuel spray flames. The fuel injector has twelve alternately arranged spray holes ( ϕ0.09 mm), staggered at two injection angles, 79° and 55° from the central axis of the injector. Two ordinary injectors with eight ( ϕ0.113 mm) and twelve ( ϕ0.092 mm) holes with an injection angle of 78° from the central axis with a conventional re-entrant combustion chamber were also examined as references. The experimental results showed that the improvements in thermal efficiencies and the reduction in smoke emissions can be established over a wide IMEP range with the proposed combination of the staggered nozzle hole injector and the divided combustion chamber. The exhaust loss is reduced with the combination of the injector and the combustion chamber while the cooling loss increases slightly. The rate of heat release from the combination of the injector and the combustion chamber has a more active main combustion and smaller afterburning than conventional systems. The three-dimensional CFD simulations showed that the combination of the injector and the combustion chamber can efficiently suppress the interactions among fuel spray flames at the combustion chamber wall after the impingement of fuel spray flames and also suitably distribute the fuel mixture, resulting in reductions in the afterburning and the smoke emissions. The slight increase in cooling loss with the proposed combination may be due to the increase in contact area of hot burned gas on the cylinder head; further improvements in thermal efficiency can be expected with optimization of the design factors related to the fuel injection and the combustion chamber configuration.