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And in some other conditions too.

For example, my 1L km+ done Ikon 1.8D used to emit blue smoke with erratic idling in cold starts(25C ambient temperature); this was later narrowed down to the failure of cold advance solenoid. Once this was set right, then there was no smoke. It had it's injection timing advanced, so it produced high kmpl numbers and some high coolant temperatures as well(100+ C). When timing was retarded(from advance, i faced cold starting troubles). Now i have sold the car so can't experiment with it.

The bolero DI and Tavera DI(isuzu 4JA1) used to give black smoke when at starting- probably FIP injecting more diesel to aid the cold start.

The Premier Padmini 137D that i still have- it gives white smoke if accelerator pedal is depressed beyond the engine's torque producing capability(hope i meant it right)- but the friendly mechanic says it has to do with FIP injection timing- it is a bit retarded, hence the white smoke.

These are the facts i have; hope some Sherlock gives a theory based on all these :)

Hmm, persuasively presented but I'm afraid there are factual errors here.
Excess fuel will always result in black smoke only, never white.
White smoke is the result of an engine being a tad starved of fuel and having surplus air.
The reason why black smoke can be seen from perfectly healthy vehicles is that during acceleration, before the turbo has spooled up, there is a lack of air in the cylinder, which is the same as too much fuel.
Faulty fuel metering or timing will result in white smoke only if the amount of fuel injected is less than optimal for the amount of air present in the cylinder.
If the fuel injected errs to surplus injection the car will emit black smoke.

And sir, my learning is not from Google, as you are well aware. :)

Excellent. So then let's discuss some more smoke! :)
The leanest mixture injected in a diesel would be then at idle. Why is there no white smoke in that case?
And therefore normally aspirated diesels should not emit black smoke during acceleration - is that correct?

And finally, humour me. What component of the gases being emitted from the tailpipe lend the colour to the smoke?

All I know is:
Black = soot / carbon particles. (I hope I am right...)

So then,
white = ?
and
blue = ?

Because there is no excess air. The fuel being injected is matched to the naturally aspirated air at that rpm.

No, that is not correct.
Naturally aspirated engines will also emit black smoke during acceleration till the air stream being inducted attains the pressure (velocity and mass) to match the fuel being injected. There is always a small lag due to inertia of physical masses. Fuel can be increased much more rapidly than air induction. Air through the intake scoop, filter and manifold takes a little longer to get distributed to each cylinder. Once the air intake stream exactly fills the partial vacuum created by the downstroke of the pistons there will be no black smoke.


You'll have to seek a chemist to humour you. I am an engineer who knows how engines behave, not the chemistry and materials science behind them. :)

In a N-A engine, the same volume of air is sucked in at EVERY induction stroke at a given altitude, under the same conditions of induction manifold and air filter setup. So why black smoke ONLY while accelerating and not during steady state driving at high speeds, where much more fuel is being injected than at idle? Conversely, why is the engine not running ultra-lean (and, as you say, emitting white smoke because it's fuel-starved) at idle?
No Sir, the explanation lies in physics, not chemistry. Also, an engineer knowing about engine behaviour DEFINITELY WILL know about combustion byproducts, whether it's physics or chemistry or any other branch of science (engine behaviour would otherwise have been termed engine psychology). And as a self-professed non-Google engineer, if you deny humouring a mechanic like me, you are announcing to the forum that mere mechanics like me do not deserve to receive a share of your vast knowledge - and I find that hurtful. :eek:

Isn't white smoke an indicative of moisture in the combustion mixture? Like when a gasket blows causing coolant to be sucked in minute amounts? Blue smoke is when engine oil is being combusted?

Just saw this yesterday when a new Verna CRdi revved past and a thick black smoke ensued.

Do these cars not fail the PUC checks?

You have completely missed the fact that the engine rpm changes radically and therefore the volume of inducted air per unit of time changes drastically also!

I do believe this is now taking the form of a duel rather than a discussion.
I'm afraid it was not I that decided the curriculum of what my engineering course should cover and what was beyond the scope of the course, which, by the way, was a B.Tech. course and not a research scholar course. There are many things that I do not know.
And it is clear that you know the answers and that they lie in physics.

I frankly state that I am not aware of the chemistry and the materials science that are involved.

Let us consider the volume of inducted air per stroke/combustion cycle, and not per unit of time.
Why would you think that? :confused:
I didn't do a B.Tech. course, neither was I ever a research scholar in physics, chemistry or the sciences related to combustion of hydrocarbons. As I said earlier, I'm just a mechanic! :)

Appealing to everyone reading this: Somebody answer please...

This is a very odd plea. :eek:
Since the discussion is about an engine at idling and during acceleration I hope it is eminently clear that 'time' cannot be divorced from this discussion!
RPM is a measure that has 'time' inherent in it.

I don't want to butt into this discussion but just want to clarify that the quantity of air getting into the cylinder in an NA engine is not the same at every induction stroke across different RPM. Cylinder filling varies depending upon RPM and intake tracts which are designed to aid better cylinder filing at certain RPMs to increase efficiency. So volume of air in the cylinder per induction stroke will not be the same at different RPM.

Please do! Welcome, Sankar!
If **different air volumes for different RPM** is correct even in an N-A engine, why utilize a turbo?

Still want an answer to the questions, if possible...
In what way does a turbo change that equation?

And of course, the most important question to which I am looking for an answer...

Of course it is possible.
A turbocharger helps to increase the specific power output of an engine.
A far larger quantity of fuel can be burnt successfully because of the vastly increased quantity of air that a turbo supplies to the cylinders.

Black - burnt but not complete
white - not burnt at all, why white ? maybe because its atomised or misty
Blue - I think people above have said its oil

Why do diesel engines not measure air (or O2) and inject only required amount of diesel and prevent the soot?

What i stated above is correct regarding air volume at different engine speeds. Airbox, intake pipe,inlet manifold, intake port, valve timing etc influence the flow of air into the engine and the flow is the most efficient at a certain RPM range.

The turbo is used to force more air into the cylinder and further increase efficiency/power.

But i don't want to, I don't want to play mechanic and engineer, i'm neither nor :uncontrol

Carry on :thumbs up