black smoke

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metgreentd
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black smoke

Post by metgreentd »

hi guys my 95 td xantia blows black smoke from the exhaust when i give her a bit of wellie cars done 160000 has jus had new filters and oil any ideas on the possible cause?maybe injectors or possibly turbo seals?car uses minimal oil between services.thanx
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Post by CitroJim »

Black smoke is partially burned diesel, either because too much diesel has been injected for the available air or too little air is available for the amount of diesel injected.

Turbo Diesels are designed to smoke a little on full boost (i.e. when given a bit of wellie) due to deliberate over-fuelling. This is the purpose of the boost-sense pipe going from the turbo hot pipe to the UFO-shaped housing on the injector pump. This ensures adequate fuel is always available and helps in part, with cooling, and guards against damaging detonation.

Black smoke emissions can be particularly bad when you have been poodling through a town for a while and then put your foot hard down when conditions permit. It is possible to make a real smoke-screen this way :lol: It should quickly clear though and even at full power/boost the exhaust should soon clear and remain clear.

Give the car a good "Italian Decoke" :lol: and see how it is then. I doubt you have a real problem.

If you continue to have continuous black smoke under hard running then check for air restriction (turbo hoses collapsing/squashed etc.) first. The injectors could possibly be the cause but that is not too likely at your mileage. The timing being a fraction out could be a cause but the turbo will not be. They're very reliable and the most likely failure would be failed oil seals which would give copious blue smoke and high oil consumption.
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Post by jeremy »

Turbo Diesels are designed to smoke a little on full boost (i.e. when given a bit of wellie) due to deliberate over-fuelling. This is the purpose of the boost-sense pipe going from the turbo hot pipe to the UFO-shaped housing on the injector pump. This ensures adequate fuel is always available and helps in part, with cooling, and guards against damaging detonation.


I'm intrigued about diesel fuel helping with cooling and guarding against detonation. Fuel is only introduced into the cylinder at the injection point and starts burning immediately. There's only clean air there before this point and this won't suffer from pre-ignition and the fuel can't do much cooling when its exploding.

Black smoke is indeed partially burnt fuel - often due to too much fuel or not enough air. While in some respects they are the same thing excess fuel may be a pump adjustment problem and too little air an intake restriction problem. It can also be due to poor atomisation generally due to worn or dirty injectors.

What are you seeing and where from? Sometimes even our ZX 1.9D will chuck out some smoke if its been driven gently for a time. Some Citroens have huge silencers which don't seem to get very hot during gentle running and soe hard acceleration will throw out a considerable amount of condensation and soot - which clears quite quickly. This is much more noticeable and looks much more dramatic in the headlights of a following car.
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Post by CitroJim »

jeremy wrote: I'm intrigued about diesel fuel helping with cooling and guarding against detonation. Fuel is only introduced into the cylinder at the injection point and starts burning immediately. There's only clean air there before this point and this won't suffer from pre-ignition and the fuel can't do much cooling when its exploding.
I know it sounds a bit bizarre Jeremy, that a diesel can suffer detonation when the whole principle of detonation is used to run the thnng in the first place but yes, it can happen. Although diesel combustion takes place in conditions of excess oxygen, it is nevertheless the case that actual combustion of diesel fuel can only take place at or near stochiometric. Excess air is used to ensure there is always enough air about to ensure combustion under stochiometric conditions. the theroy is very deep and complex but there can be conditions where extreme heat and pressure in the combustion chambers can cause detonation in the same way as a petrol engine, the kind of detonation that results in two colliding flame fronts. Squirting in a bit more diesel than is strictly required does have a cooling effect, as does ensuring plenty of cool air is available. Diesel, even though it is hot when injected is still very cool compared to the combustion chamber temperature. This works in the same way as enriching the mixture in a petrol turbo to guard against detonation by using petrol as a coolant. That is how I understand it :wink:

I have some very interesting pictures of damage in big diesel engines resulting from detonation :)

Diesel combustion is a very deep subject and apparantly still not fully understood due to the difficulty of observing and measuring precisely what is happening in a diesel combustion chamber.
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Post by Kowalski »

I agree with Jeremy here about detonation.....

The damage you get on diesel engines that you're attributing to detonation is the sort of damage you get if your injector drips instead of having a nice spray pattern. Diesel doesn't evapourate in the same way that petrol does, it has to be atomised by the injector into a fine spray and should burn as it comes out of the injector, like a flame thrower.

If the injector doesn't make a nice mist, you can get a load of fuel going into the cylinder and not burning, then it all goes up at once, i.e. rather than gradually burning the spray as it goes in, all of the fuel goes BANG. Bangs mean big temperatures and big pressures and can mean new engine time.

To get a diesel engine to be efficient, quiet and not do damage to itself you rely on the injected diesel starting to burn rather than than accumulating and detonating. Modern common rail engines do something called pilot injection, where the first part of the injection involves a small amount of fuel so that if it doesn't start to burn immediatly you've only got a small amount of fuel to detonate, it also means that the cylinder is hotter for when the main injection starts, so thats more likely to burn straight off the injector.
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Post by CitroJim »

Thanks Kowalski, I'd never looked at from that angle but yes, I see and can understand that :) It just goes to demonstate the extreme value of these forums and the danger of drawing a (duff) conclusion after reading a tract of very heavy text late at night :twisted: :lol:

So why then, is the TD pump set to deliberately overfuel under high load/boost conditions? I thought I had it neatly answered but my theroy is now well out of the window :?
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Post by Kowalski »

citrojim wrote: So why then, is the TD pump set to deliberately overfuel under high load/boost conditions? I thought I had it neatly answered but my theroy is now well out of the window :?
The truth is that it isn't set to do it deliberately, but its mechanical and it can't compensate for the air temperature / humidity and several other factors. The pumps wear, the injectors get dirty, the quality of fuel changes etc.

There is a fine line between good power and bad emissions, the engine is set up so that generally it gives good power and that generally no bad smoke!
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Post by CitroJim »

Excellent Kowalski :D That makes perfect sense.
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Post by jeremy »

I think the answer is 'what is overfuelling?'

Maximum power comes when the maximum amount of fuel is fully burnt. The limiting factor is the amount of oxygen available - so provide more oxygen - burn more fuel - produce more power.

On a naturally aspirated engine this is simply controlled by the pump maximum fuel setting. Provide more air (ie boost) you need more fuel. You could sense the airflow with some nice device like a hot wire as used for measuring volume on a petrol engine - but simply measuring the boost pressure will do and the additional fuel device allows more fuel to be injected.

If you set it a couple of % below max power no-one will ever notice and you'll get complete combustion.

This assumes that the droplets are small enough to burn completely. If they are larger possibly due to poor injectors leading to a poor spray pattern - then smoke will be produced.

The 1.9 TD engine would seem to be on the limit so far as gasflow is concerned even under boost. I say this as it doesn't produce much more power than the 1.7 and when it was enlarged to 2.1 it was necessary to fit a second inlet valve.
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Post by CitroJim »

jeremy wrote:The 1.9 TD engine would seem to be on the limit so far as gasflow is concerned even under boost. I say this as it doesn't produce much more power than the 1.7 and when it was enlarged to 2.1 it was necessary to fit a second inlet valve.
And yes, being on the limit of airflow would be another explanation of why a TD smokes under heavy load/boost, nothing to do with overfuelling :)

So on that basis then Jeremy, is it reasonable to conclude that the power gains that can obtained by adjusting the wastegate and max. fuelling on a 1.9TD are not all they're claimed to be and will not achieve much apart from more smoke :wink:

And if your pump and/or wastegate has been fiddled with in the past, metgreentd, this could well be the reason. Are the little splodges of yellow paint all intact around the pump adjustments?
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Post by bxbodger »

And let's not forget that the smoke test allows for turbo diesels being more smoky than n/a diesels; from the tester's manual-
2. After 6 free accelerations, the mean of the last 3 smoke levels is:

a. for a non-turbocharged engine, more than 2.50m-1

b. for turbocharged engines more than 3.00m-1
- so a turbo is allowed to be dirtier!
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Post by jeremy »

To return to the original post the likely causes of black smoking probably relatively simple - ie restricted airflow due to dirty air filter or a collapsed hose. If it only does it with the turbo running - induction leaks are unlikely - as it simply wouldn't attain enough pressure for the extra fuel device to operate.

Dirty or worn injectors will produce black smoke but this will probably be accompanied by a generally smelly exhaust at idle and poor power and flexibility, poor starting and generally rough running. To be fair most of these will probably only be noticed by the improvement with new or clean injectors.

Its possible someone has been fiddling with the pump - particularly near London I always see 4WD's belching out black smoke as they endeavour to keep up with traffic and expect that is due to fiddling. There isn't much history of fiddling with XUD pumps - but I recall some 4WD sites like Land Rover ones are full of instructions on how to do it - and the pumps are basically the same.

I think poor timing will tend to produce white smoke - and either a very quiet (retarded) or very noisy (over advanced) engine.
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Post by davek-uk »

Another reason for smoke clouds is driving practices of course. If you steadily depress the accelerator as you accelerate you get less smoke and usually better acceleration. Unfortunately most of us are poor judges to how much foot power is needed (me included) and stamp down to go really fast. The manual pump doesn't have the control or brain to correct this and for what I've seen of Common Rail diesels the electronic system is nearly as poor. Now if you could pump in cool, damp air to complement the amount of diesel injected in all conditions you'd have quite a setup.
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Post by bxbodger »

Now if you could pump in cool, damp air to complement the amount of diesel injected in all conditions you'd have quite a setup.
Coming within the next 3 or 4 years, I should imagine: water injection systems are starting to appear on big diesel engines in the states to keep emissions down, and i don't suppose it'll be long before the technology is applied to the tiny engines you find in cars.
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