Fig. 1 Low NOx diesel combustion concept (1)
It is thus considered the combustion temperature is rapidly dropped and NOx can be reduced if the combustion portion is rapidly mixed with air at low temperature.
Finally, in the final stage of combustion (3), the space of the combustion chamber is occupied by the already burnt part and the air not used in the combustion, and the combustion temperature is low. Thus, the NOx emission is small in this stage.
The concept on the diesel combustion for reducing NOx emission focusing on the initial stage and middle stage of combustion where the combustion temperature is high, can be summarized as follows.
(a) Reduction of the zone of the stoichiometric combustion of fuel-air mixture (λ=1)
(b) Promotion of mixing of the burnt part with air
2.2 Low NOx Technology
The more specific low NOx technology is introduced here based on the above-described two concepts (a) and (b).
2.2.1 Reduction of zone of theoretical fuel-air mixture (λ=1)
It is firstly examined where the stoichiometric combustion of fuel-air mixture (λ=1) is present in the combustion chamber of the medium speed engine. Fig. 2 shows the comparison of the photographs of combustion of the compact high speed engine with that of the medium speed engine(2). In the compact high speed engine (at the left of the figure). the distance from the nozzle to the wall surface of the combustion chamber is short, and thus, the flame in the initial stage of combustion is formed of the wall surface of the combustion chamber. On the other hand, in the medium speed engine (at the right of the figure), it is shown that the distance from the nozzle to the wall surface of the combustion chamber is long, and thus, the flame in the initial stage of combustion is formed around the developed spray. It was further analyzed how the difference in the mode of combustion affects NOx.
Fig. 3 shows the model and the formula for estimation to express at which distance from the nozzle, the injected fuel from one hole in the fuel nozzle reaches the condition of the stoichiometric combustion of fuel-air mixture (λ=1) and burned (3). The distance from the nozzle for λ=1 can be obtained by substituting the left side of the formula (1) with λ=1. This distance X is given by X=200･d for the nozzle hole diameter d when the physical value or typical characteristic value is substituted (for example, under the assumption of the density of fuel ρf=850 kg/m3, the density ρa of the compressed air=15.6 kg/m3, the theoretical air volume necessary for combustion Lth=14.6 kg/kg, the angle of spray θ=25deg., and the coefficient of discharge C=0.7). That is, the stoichiometric combustion of fuel-air mixture of λ=1 is performed at the position of approximately 200 times the nozzle hole diameter, and it is estimated that the NOx emission is active at this position. In addition, Fig. 4 shows the relationship between the diameter of one nozzle hole and the distance from the nozzle to the wall surface of the combustion chamber in the compact high speed engine and the medium speed engine, and also shows the distance from the nozzle for λ=1 (X=200･d for the nozzle hole diameter d).
