Marine Engine -Question and answer-Injector

With reference to fuel injectors:

a.sketch a fuel injector, labelling the parts which influence atomisation of the fuel and the spray pattern;
b.explain how a fuel injector may deteriorate in service resulting in poor atomisation;
c.explain how defective fuel atomisation may be detected during engine service.

a.


b.
Repeated operation of the fuel injector will lead to weakening of the spring. this will allow the injector to open early, and at a lower pressure correct atomisation and penetration into the combustion space will not occur as there is not sufficient pressure energy for conversion in the atomisation holes. Repeated operation will lead to erosion and enlarging of the atomisation holes, accelerated by using fuel contaminated with water or catalytic fines. The holes will be of the wrong shape and size to allow efficient atomisation. Wear of the needle and nozzle seating will allow dribbling to take place, again interfering with the pressure rise required to allow correct atomisation to take place.

c.
The first signs of defective atomisation may be noticing a drop in power from the engine. The colour of the exhaust from the funnel will darken indicating poor combustion. Because atomisation and penetration is not correct and the fuel droplets larger than they should be, afterburning may occur causing higher than normal exhaust temperatures. If allowed to continue over a period of time, fouling of the turbocharger will occur, leading to a drop off in performance. If defective fuel injection is suspected, then an out of phase card can be used to show faults with the fuel injection. If confirmed, the injector should be changed as soon as possible.