Turbo/Compression/Timing

[Shagbark]

My take on smoke in the rearview: This is excess fuel, as you state, but it continues to burn (to some degree) after the power stroke (ie. into the exhaust stroke in extreme cases). This can really cook exhaust valves etc. More air (via turbo) will allow the combustion to complete within the duration of the power stroke, thus reducing EGT, as there is less burning fuel exiting the cylinder. With a turbo, you should see higher peak combustion pressures (it's doing more work), but w/o adding fuel, the EGT should go down. Diesels aren't air/fuel ratio sensitive in the way gassers are.

I knew I was missing something. That makes what Al said about more complete combustion lowering EGTs make perfect sense. Building a fire in the exhaust manifold would definitely increase EGTs.

[asavage]

Is this why turbos are more efficient? meaning they get better power, economy and less noise?

Exhaust-driven turbosuperchargers recover "waste" heat energy (note the drop in exhaust temp after the turbo) and use it to pump more air into the combustion chamber, increasing volumetric efficiency (think of VE as the amount of energy the engine expends just pulling air into itself, as a simplification). So you literally get something for nothing -- well, more-or-less.

On cars, turbos are rarely added to increase fuel mileage, but they can be tuned that way. On diesel industrial and commercial equipment, they are there primarily to increase horsepower without add'l fuel expense.

The reduced noise is just a side benefit, and again most mfgrs don't take the added noise reduction and leave it alone, they de-content the muffler to make it lighter and cheaper and the overall noise level remains the same