diesel smoke

When diesels start moving from a stop they usually let out a puff of smoke and if the person floors it it's even worse. is this because the engine runs rich because the turbo hasn't spooled up yet? if this is so wouldn't a supercharger with a turbo feeding nto it be very well suited for a perfromance diesel truck because you'd have the instant response without the lag of having a masive turbo fitted?

I would say the turbo spools fairly quickly. Generally automotive manufactures put small turbos on cars, so I'm going to guess they are somewhat "undersized" on turbo diesels. However, if it were pretty big, turbo diesels have large displacement engines so its going to spool a large turbo faster anyways. I dont think the engine runs anymore rich before the turbo spools than a N/A car. Fuel is added where needed according to the CFM that the maf is reading. Once more CFM is drawn through the MAF because of boost, then more fuel is added. Thats one of the great benefits of having a MAF over a carburator. Theres no guessing the fuel map. Each value of CFM has a specific output of fuel. The reason your seeing black smoke is because the truck is already into boost and its adding fuel to compensate.

So to answer your question, I think lag is probably almost non existant and a supercharger (i.e. roots style) is not needed.

then what is the black smoke that comes out of the exhaust on large displacement diesels, i don't believe there is an maf on a diesel because you control the fuel going into the engine not the air.

then what is the black smoke that comes out of the exhaust on large displacement diesels, i don't believe there is an maf on a diesel because you control the fuel going into the engine not the air.

thats the burnt fuel.

Thats what i said. The MAF controls the fuel by reading the air thats being drawn into the engine.

No MAF on a diesel, no boost controls at all usually. Diesels don't have a throttle and are exposed to ambient air at all times. The only thing your right foot controls is how much fuel is injected. No foot, minimum fuel to maintain idle. Foot to the floor, max fuel that its designed to deliver.

Since diesels don't have a throttle, they don't have a "rich" condition. They always run lean; anywhere from 60:1 at idle up to about 16:1 at a well-tuned full load. To an extent, its very difficult to achieve a rich condition. Why? Because the more fuel you add, the more the turbine spins which spins the compressor, which puts more air in.

The black smoke you see is soot. Its a byproduct of combustion. Without getting too technical...

In a diesel the compressed air is so hot that when the fuel is injected (directly into the combustion chamber near TDC) it ignites. It continues to inject and burn fuel for a prescribed pulsewidth and pressure depending on where your right foot is. If you start to early its much like pre-ignition in a gas engine. The flame front starts pushing on the piston before TDC. If you inject it too late, the fuel is still burning when it leaves the exhaust valve. The resulting cool burn from this escaping combustion makes soot. Its the same stuff that comes off candles with wicks that are too big. The big flame is cool at the tip and doesn't properly burn the parafin in the candle. Hence, soot.

Older diesels made tons of soot. Extreme performance diesels make soot because they inject so much fuel that it can't help but keep burning after the power stroke is done. A modern-day common-rail injected diesel shouldn't make visible soot unless its used and worn. Worn rings, compression leaks, turbo bearing age, and several other factors make the engine drop its compression a little bit but the computer is still injecting the same fuel map as stock. That could contribute to the soot you see.

Typically, turbos make better diesel companions since they do respond to the load of the engine. A supercharger is just a belt driven thing and only works best in that trim at that elevation, with that quality of fuel, etc. Properly designed turbos on diesels don't need any boost control device other than the maximum speed that can be generated by the turbine. Newer designs all incorporate a wastegate for safety, but some early diesel turbos just blew what they could and it was fine. My 95 Powerstroke didn't have a wastegate at all. If you increased fuel, the pressure increased. End of story. They blew about 17 psi stock, but a chip would raise that to 23-27 pretty easily. A blow-off valve on a diesel is useless. There is no throttle, so the blowoff valve would just waste energy you just worked to create.

Since diesels don't need all of the more complex boost controls, ignition requirements, and heat stability of a typical gas engine, the turbo is well matched.

So I guess to answer the question properly, the black smoke is like half-burnt oil. It is a carcinogen if inhaled, but its so heavy that it just falls to the ground and easily biodegrades. Don't confuse it with diesel exhaust. It is NOT exhaust. True diesel exhaust is clear and colorless, just like proper gas exhaust.

Thats what i said. The MAF controls the fuel by reading the air thats being drawn into the engine.

Not quite. There is no MAF. The only thing that controls how much fuel is injected is the position of your right foot.

Yeah a little run down for deisal turbos is...

No throttle body (no BOV needed
No wastegate (actually, some of the new Izuzu engines equiped with Mistu turbos have em for emissions reason)
Now boost control really, with a compression ratio as high as a deisal you get an instant 20psi of boost even on the larger 1.50+ A/R.

(some of this stuff might sound wierd, but its just what i know from working on the ones at work, which are excavators and not deisal trucks. But i wouldnt imagine much of a difference between them)

yea, often trucks share the same engines at their heavy machinery brothers, www.scania.com (http://www.scania.com) is the website to a truck company, they use the same or similar engine for industrial and marine engines, volvo make trucks an excavators/plant vehicles too, they share the same engine variations, i belevie almost all, if not all are turbocharged. i beleive some volvo and scania (possibly other companies too) utilize BOVs and wastegates, im sure volvo truck have BOVs, and most likely internal wastegates. but this is all 9 litre up engine sizes.

When diesels start moving from a stop they usually let out a puff of smoke and if the person floors it it's even worse. is this because the engine runs rich because the turbo hasn't spooled up yet? if this is so wouldn't a supercharger with a turbo feeding nto it be very well suited for a perfromance diesel truck because you'd have the instant response without the lag of having a masive turbo fitted?
The diesels you'll see that on are the old mechanical injection versions.
Newer electronic control motors run so clean you'd never notice it's a diesel.

On the older mechanical injected engines the problem is exactly as you've described. The fuel pump has been adjusted for max power and torque when on boost, before the turbo spools up unburnt diesel gets spewed out the exhaust.

My mechanical injected turbo diesel has a boost device fitted to the back of the fuel pump. It has a diaphragm connected to the intake pipes. It won't give the engine full fuel until enough boost has developed.
The downside of such a device is it makes the turbo even slower to spool. The upside is no embarassing black smoke.

A variable vane turbo is the best solution to the lag problem.