Fig. 3 Fuel-air mixture forming model of free spray(3)
In the compact high speed engine, the fuel injected from the fuel nozzle is impinged with the wall surface of the combustion chamber before forming the stoichiometric combustion of fuel-air mixture of λ=1, and forms the wall surface flame, while in the medium speed engine, the nozzle hole diameter is relatively small due to many holes, and actually large, and the distance from the nozzle to the wall surface is long, and it is estimated that the flame in the radial direction is constantly formed while forming the stoichiometric combustion of fuel-air mixture of λ=1 during the injection period. Compared with the high speed engine, the residence time becomes longer in the medium speed engine low in engine speed, and the NOx emission is increased, and it is also estimated that the difference in the mode of combustion constitutes the factor of much NOx emission in the medium speed engine.
It is thus considered that the NOx emission can be suppressed by reducing the combustion portion of the stoichiometric combustion of fuel-air mixture of λ=1 if the fuel injected from the fuel nozzle is impinged with the wall surface of the combustion chamber before forming the stoichiometric combustion of fuel-air mixture of λ=1.
2.2.2 Promotion of mixing combustion portion with air
Some air flow is required to promote the mixing of the combustion portion with air. In the conventional medium speed engines of this class, no swirl which has been naturally employed for the combustion technology in the compact high speed engine, has ever been employed. It is however considered that the mixing of the combustion portion with air is promoted and the NOx emission can be reduced by also employing the swirl in the medium speed engine.
The low NOx technology based on two concepts described above, can be summarized as follows.
(a) Impingement of the fuel with the wall surface of the combustion chamber before forming the fuel-air mixture of λ=1
(b) Necessity of appropriate swirl to promote the mixing of the combustion portion with air
