Black smoke from an overloaded diesel

[FlyingDutchman]

No, I don't have problems with my engine but I was just wondering why a diesel engine should emit black smoke when overloaded.

Let us assume there is weed or a rope in the prop so the engine can not rev up.
The amount of air sucked in the cylinders per revolution remains constant, in fact at lower revs the breathing is even better (non turbo diesel).
This leaves the question why black smoke is produced when opening the throttle as the amount of fuel injected per stroke is the same as with a free prop and high revs.

What do I oversee?

 

[DAVIDO]

As i understand it from asking the question before the black smoke is unburnt fuel. The engine under load is being given more fuel than it can burn.

 

[spannerman]

You oversee how the throttle works on a diesel, throttling is done by the pump not a throttle plate as in petrol engines, to increase revs you deliver more fuel, ( thats why diesels 'run away if the governor in the pump fails) the engine speeds up accordingly as a diesel will run as fast as it can on a given amount of fuel so its the ultimate 'lean burn' motor.
Say your 'overloaded engine' which can't rev up is being supplied with the fuel to make it run at say 3500rpm, and you have something in the prop or even too high a pitched prop and it can only manage 3000rpm, then you have a surplus of fuel which will not be burnt as you have already used up all of the air being supplied at that rpm, once the oxygen is used up there can be no more combustion or power no matter how much fuel you throw into the cylinder. This then comes out as black smoke. a diesel which is running correctly will be running very lean i.e. 20-25>1 air/fuel ratio and have no smoke as all of the fuel has been consumed and there will actually be some unused oxygen in the exhaust.
Hope this clarifies the smoke... /forums/images/graemlins/grin.gif

 

[Micky]

Yes, black smoke = unburnt fuel, much the same as pulling the choke out on a petrol engine.
Normal load and a warm engine at correct RPM should result in a reasonably clean exhaust.
Pushing hard flat out through strong tides, head winds etc = a bit like going up a steep hill, and there could be a little black smoke appear, but that's life.

 

[FlyingDutchman]

Maybe I am just plain stupid, but I still don't understand it.
The fuel is injected once in every 2 rotations of the crankshaft (4 stroke).
The amount of fuel that can be burnt in the given amount of air is limited. At low amounts of fuel, the engine is running lean, at the maximum output, the maximum amount of fuel that can be burnt in the given amount of air is injected and the combustion will be near the stoichiometric point.
This is also the idea behind a turbocharger: more air in the cylinders so more fuel can be burnt giving the extra power.
The RPM of the engine has nothing to do with this all and that is why I don't understand the black smoke.

 

[Its_Only_Money]

[ QUOTE ]
...at the maximum output, the maximum amount of fuel that can be burnt in the given amount of air is injected

[/ QUOTE ]

No, this is where you go wrong, diesels can inject more fuel than can be burnt, the excess comes out as black smoke. At WOT and max rpm the excess that can be injected is limited by the capacity of the injector pump, at lower rpm though the excess can be considerable and lead to smoke in large quantities on some motors.

 

[cliff]

The volume of fuel injected is variable NOT constant. opening the "throttle" allows the injector pump to inject more fuel on each stroke until the governor kicks in, the fuel is then cut back to maintain that speed. If the engine cannot reach its "throttle" speed you get black smoke as the pump is injecting excess fuel trying to reach a higher speed. Simply put of course /forums/images/graemlins/grin.gif
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[spannerman]

[The RPM of the engine has nothing to do with this all and that is why I don't understand the black smoke.

[/ QUOTE ]

Quite correct, the rpm where max power is developed is determined by the camshaft profile, also the size of the engine, small car diesels can rev to 5000 +
large freight ships develop their max output at 800-1000 rpm, because you can't have so much weight thrashing around at high rpms.
If you understand the concept of the stoichiametric point then its not a long leap to understanding that for a given amount of fuel and air a given amount of power will be developed, I think you are confused by the fact that at low rpm a diesel isn't using all of the air in the cylinder to develop power and there will be some left over when the fuel is burnt away. To obtain higher rpms we introduce more fuel for a bigger bang and ideally no air left over, if we inject even more fuel we don't get a bigger bang as there is no oxygen remaining for it to react with so we just throw it out of the exhaust as smoke. You are correct in that turbocharging allows us to push more air into the cylinder and thats why the injection pump usually has a diaphragm linked to the inlet manifold to sense boost and this makes the pump inject that extra diesel that can now be burned by the extra air, so its like increasing the capacity of the engine.

 

[FlyingDutchman]

Thanks guys, I had to sleep over this one but now I understand it.

The governor is trying to rev up the engine untill the desired rpm is reached.
If this speed can not be reached due to overload, the governor/injection pump injects more fuel to reach this speed regardless of the amount of oxygen available.

Simple really!

 

[oldharry]

If you set fire to diesel the fuel will burn more quickly than air can reach the centre of the fire - so large amounts of black smoke (soot) are given off as the fuel only partly combusts.

Technically 'unburnt' diesel is the white smoke that elderly or faulty engines give off when the full fuel charge in the cylinder fails to ignite fully.

Black smoke occurs when the fuel burns but there is insufficient oxygen present for efficient combustion - so an overloaded engine is getting a full charge of fuel, but because of reduced revs is not able to balance it with the correct amount of oxygen. The unused fuel converts to airborn black soot particles, which generate a lot of extra revenue for the giovernment on the pretext diesel is 'dirty'!

 

[essexboy]

I still think that if the pump is set up properly, you wont get black smoke. its wrong setting of the pump thats the root cause, Automotive reg BS AU 145a (if memory serves) says no visible smoke at any revs / any load

 

[FlyingDutchman]

Interesting point!
But I think an automotive diesel engine does not have a governer.
If I drive uphill with my (diesel) car, I have to push the accelerator pedal to maintain the same speed. If it would have a governor this would not be necessary.

 

[tico]

<pedant mode on>
BS Au 145a relates to number plates, BTW its been replaced by BS Au 145d.
<pedant mode off>
/forums/images/graemlins/smile.gif

 

[oldharry]

[ QUOTE ]
I still think that if the pump is set up properly, you wont get black smoke. its wrong setting of the pump thats the root cause, Automotive reg BS AU 145a (if memory serves) says no visible smoke at any revs / any load

[/ QUOTE ]

Diesel always produces some smoke: there may not be enough to be visisble, but it is there and the 'no visible smoke' MOTregulation only applies to diesel vehicles pre around 1990. Anything newer has to have a particulate emission test, which is well below the 'visible smoke' level.

Any diesel engine can be made to emit visible smoke momentarily, and continuously in heavy load conditions.