Diesels loading up?
- Thread starter The motor guy
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Black smoke is un burnt fuel.
Diesels can wet stack from idling too long or being run without a load but it doesn't cause black smoke. Wet stacking will cause exhaust slobber and white smoke. Wet stacking happens because diesel engines don't have a throttle to limit the amount of air entering the engine and without load or being idled to long causes the cylinder temperature not to be high enough to burn all the fuel.
Diesels can wet stack from idling too long or being run without a load but it doesn't cause black smoke. Wet stacking will cause exhaust slobber and white smoke. Wet stacking happens because diesel engines don't have a throttle to limit the amount of air entering the engine and without load or being idled to long causes the cylinder temperature not to be high enough to burn all the fuel.
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Lugging.
diesel engines don't run at any specific air fuel ratio so the lean, rich doesn't apply like it does with gasoline engine technology
basically, black smoke is more fuel than air at a given time than can be burned cleanly .. which is different than incomplete combustion which generally is indicated by white smoke from a diesel.
Incomplete combustion might occur on a cold start where not all the cylinders are working up to speed, and you get this unburned fuel and white smoke out the stacks. Black smoke can just be from a plugged air cleaner or over fueling.. over fueling being the more common. someone mentioned lugging, which is basically overloaded operation and over fueling at low engine speeds.
if the smoking problem is on a modern diesel with emission controls, the black smoke problem is caused by TAMPERING and fools with a heavy foot.
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Yes, modern diesels don't take long for the turbo to catch up and clean things up. So a puff of smoke isn't going to last long. Kind of a bummer they have all these emission controls now. Diesels quit blowing smoke 30 years ago.
nowadays they have that smoke catcher on them otherwise known as a Diesel Particulate Filter ( DPF),,, but what with electronic controls and HPCR injection they really dont smoke until something is either wrong with them or somebody messed with them.
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Pretty much that's what I was saying
after about 1980 even when they were mechanically injected in stock form they had smoke controls.... 2 stroke Detroit's had the throttle delay built into the rack, and Cummins used the AFC ( Air Fuel Control) to prevent overfueling until they had good manifold pressure... others did the same thing but I'm not familiar enough with those to say what exactly... kinda like most of the old stuff had a smoke limiting device built in, just because there is a requirement not to bellow black smoke..
OVERKILL
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This is a great explanation (and love the pics!)
OP: Remember, a diesel doesn't typically have a throttle plate, so their air intake is unrestricted. The throttle controls the amount of fuel the engine receives, which in turn dictates RPM and power output.
When you "floor" a gasoline car with a carburetor, the accelerator pump dumps a shot of extra fuel in to prevent a lean condition (characterized by a bog or pause, which a properly setup carburetor avoids with a properly cammed and sized accelerator pump) before sufficient fuel flow through the jets is established by virtue of the venturi effect.
When you floor a gasoline car with fuel injection, the injector pulse width doesn't just go to wide-open, there's a fuel map, relative to load (which is based on either mass air flow or MAP reading) and engine RPM, as well as an accelerator pump function to make the process seamless.
When you floor a mechanical diesel, you instantly increase the fuelling level to maximum at the speed the pump is spinning. This massively increases the amount of fuel entering the cylinders, which in turn results in a period of over-fuelling until engine RPM (and boost, in the case of forced induction) catches-up.
With a computer controlled diesel, this is a bit more elegant, but the same phenomenon is in effect, fuelling (because you are trying to increase RPM and power output) necessarily increases (because that's what commanding more from the throttle does) and there is a brief period of over-fuelling where you may see some dark haze or black exiting the exhaust due to this.
The other thing that will cause black to be observed from a modern emissions diesel is a regen, where it is trying to clean-out the DPF and intentionally over-fuelling. This will happen in steady-state operation, just cruising down the highway for example.
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This can be seen even on V16 generator sets - bring on another large motor and it’s fed fuel instantly to keep cycles where the ECM is set … Seconds later - no smoke …
Used to love the 18 wheelers starting up in cold weather while driving country roads.
Between the top of the piston and that space "quench area" down to the first piston ring, on a diesel it is very wide. If a diesel is too cold or receive too light of load, liquid fuel can begin to puddle in that area. Also the entire exhaust is too cool and the carbon will adhere to all of the surfaces.
The other thing is if the entire cylinder temperature is too cold then complete combustion will not be occuring during the throttle up. So you will get lots of smoke until the temps rise.
yes... old diesels without aftertreatment or computers are prone to these problems, far more than the current stuff with exhaust aftertreatment... modern engine with aftertreatment will actually run the intake grid heater during warmup and also run the engine at varying idle speeds to keep the aftertreatment system warm enough to function..
the modern stuff also has quite a bit of fine tuning which is meant to eliminate engine smoke at all times.. multiple injection events during each injection cycle as well as precisely tailored injection have really cleaned up the beast..
but the old stuff, where people would leave it slow idling for hours at a time, fuel and oil literally puddle in the cylinders and when someone finally hits the throttle all that crap would go up in smoke and go out the tailpipe... sometimes you would see flaming chunks of carbon going out the stacks.. but that was the good old days, when you walked thru a parking lot full of running diesels and your eyes were watering from the fumes... not so much anymore.
I have a 68 JD3020 tractor, mechanical injection and no turbo. If you would like to observe the phenomenon you describe in its natural state you should visit a farm with some old tractors.
Take a step back and look at your statement from a logic and/or engineering perspective. How does the black smoke build up inside the exhaust? Where does it hide? The exhaust is a fixed size and not like a balloon that can expand.
Have you put your hand near the tail pipe of even a heavy duty pickup? The exhaust comes out with significant force and when the throttle is pressed the exhaust air pressure increases immediately. The exhaust is also extremely hot. My EGT gauge runs 800-1200F when towing my 36' TT. It cools down by the tail pipe but it is still very hot.
As others have stated this phenomena is from unburnt fuel. Diesels don't have a throttle plate like gasoline engines, they free flow air all the time. How engine speed is increased is through increased fuel and when this happens the exhaust on older models turns black. Once the turbo is spooled up it increases the air pressure and volume of air and the exhaust returns to the normal color.
FWIW, none of this happens on the new emissions controlled vehicles if they are still compliant.
Just my $0.02
Diesels are ALWAYS lean...when they get closer to stoichiomnetric they can burn things.
JHZR2
Staff member
This has happened on properly tuned, efficiently running diesels long before there was anything to regen.
I agree about the nooks and crannies.
Fuel control especially on turbocharged diesels, is a delicate thing, but if the conditions are right you can see it getting worked out of any diesel that doesn’t have a dpf.
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