Exhaust smoke and unburnt fuel

[huldah]

Re: Exhaust smoke and unburnt fuel. Re DogWatch

A friend had problems with unburnt fuel on one of his twin Volvo engines. The engineer had the right puller to remove the injectors, without loosening the inserts. Expensive, replacement injectors were fitted, without any improvement. Had the injectors been swapped between engines, a lot of expense would have been spared.

Philip

 

[ShipsWoofy]

Re: Exhaust smoke and unburnt fuel. Re DogWatch

That will be a plan, i.e. swap them over and see what happens.

I was more interested in what a stuck injector meant, I know the spray can go askew and cause problems; which needs servicing, there is a diesel service centre near me, handy!.. But better if there was some maintenance I can carry out to the injectors before taking it in.

This of course following confirmation.

 

[scottie]

Re: Exhaust smoke and unburnt fuel. Re DogWatch

a conventional injector is only a form of spring loaded valve that opens at a set pressure so if the the spring is weak then the efect is to drible fuel the engine relyes on have an aerosol like spray delivered at the exact time to work
it is a precise but basic tedhnology
there nothing apart from keeping your fuel clean that a user can do with injectors

serviceing injectors is a specialist job now and many modern injectors with double chamforded ? seats have been ruined by being lapped in in correctly

on a multi cylinder engine a rough check can be to losen the feed pipe slightly on each cylinder in turn and seeing the the drop in revs is the same for each cylinder at tickover
injectors shoul NEVER be sprayed out of an engine without screening the spray as it can have major damages if diesel penetrates human skin

 

[oldharry]

[ QUOTE ]

From various readings here and on the moody Owners forum, I thought unburnt fuel produced white smoke.... cant remember what black smoke was.

i may be wrong!

[/ QUOTE ] Not quite! White smoke is atomised diesel straight from the injectors, and has never ignited in the first place. Black smoke is incomplete combustion caused by incorrect air/fuel ratio, commonly when an overloaded diesel is getting more fuel delivered to the injectors than it can use at reduced RPM. Blocked air inlets can cause the same problem.

Usually though, complaints of white smoke are actually steam from reduced coolant flow in a water cooled exhaust as fitted to most boats.

 

[oldharry]

<<What is a stuck injector>>

Injectors have a spring loaded valve which is set to open when the injection pulse in the fuel line reaches a certain pressure. If this sticks open or leaks, then fuel is delivered to the cylinder too soon, and will not be sprayed correctly into the combustion chamber. This results in partial non-combustion, or even complete misfire, with neat fuel passing into the exhaust unburnt.

Diesel is so much less volatile than petrol that even if the cylinder fires, it may not burn all the fuel present, particularly if it is still in liquid form, and not properly atomised by the injector.

 

[huldah]

My friends boat engine continues to smell of unburnt fuel, even with the new injectors. The other engine is fine. The only reason I can think of, is lack of compression. I will suggest to him that he slackens off the fuel pipe to the injectors, one at a time as suggested, to see if there is a weak cylinder. I have done this on Pendragon's engine, when tracing problems.

Philip

 

[oldharry]

Nothing worse than a smelly engine in the confines of a boat! Are you sure there is not a fuel leak somewhere - maybe in the return lines? Low compression from piston wear is more likely to produce fumes from the breather which have distinctive lube oil smell, and if only one cylinder is suspect on a multi the engine wil have an irregular 'beat'.

 

[huldah]

Re: Exhaust smoke and unburnt fuel, ref oldharry

The fuel smell can only be smelt by passengers sat in the open, at the rear of the boat, and they are not very happy! It is definitely coming from the exhaust. Assuming that the new injector is firing at the correct time, and that the air intake is correct, then the only thing that I can think of, is that the cylinder temperature is not high enough to burn all the fuel. I was thinking of possible valve leakage, as the engine breather is fine.

There is no noticeable irregular beat, the engine starts fine, and the problem surfaces as the boat reaches cruising speed.

I will be interested to see what his mechanic finds, after he has fixed the engine bearer, that broke on his last trip.

Philip

 

[TheBoatman]

[ QUOTE ]
This results in partial non-combustion, or even complete misfire, with neat fuel passing into the exhaust unburnt.


[/ QUOTE ]

It also normally results in a serious "knocking" from the engine ~ funny enough called a diesel knock /forums/images/graemlins/wink.gif

Peter.