Fig. 5 Shroud valve position
4.2 Test results with low NOx particulars
For the specific low NOx technology, the constant intake swirl ratio (=0.8) was given, and the nozzle hole angle φ was reduced as illustrated in Fig. 7 in order to reduce the distance from the fuel nozzle to the wall surface aiming at the impingement of the fuel with the wall surface of the combustion chamber before forming the fuel-air mixture of λ=1.
Fig. 8 shows the relationship between the fuel consumption, NOx emission and the exhaust smoke at the rated output when the nozzle hole angle was changed from 130° to 150°. In a case of the nozzle hole angle of 150' where the distance from the nozzle to the wall surface is long with high-pressure injection, the atomization of the spray seems excellent, but the exhaust smoke is remarkably degraded when the swirl is given. The cause seems attributable to the fact that a very lean fuel-air mixture is formed by the atomization of the high-pressure spray and the air flow by the swirl, and the mixture is discharged in the unburnt condition since it is leaner than that in the inflammable limit range. Thus, in the conventional large engine, the swirl is unnecessary so long as the fuel consumption and the exhaust smoke are taken into consideration, while only the exhaust smoke is remarkably degraded.
However, the smaller the nozzle hole angle is (φ=150°→140°→130°), the more excellent the exhaust smoke is, and the effect of improvement is remarkable especially in the case of the nozzle hole angle of 130°. In the figure, the case where the fuel injection timing is advanced at the nozzle injection angle of 130° is shown, and the trade-off relationship between the fuel consumption and the NOx emission is greatly improved compared with the case of changing the injection timing with the conventional engine.
Fig. 6 Test results of changing injection timing with standard specifications
