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Old 08-27-2008, 12:43 PM   #16 (permalink)
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Originally Posted by MazdaMatt View Post
Does this meant hat a turbo adds a HUGE advantage in FE to a deisel? 14psi means double the o2 in the air (not quite, due to compression heating, i suppose, but close).
Yes and no. I oversimplified when I said that the limiting factor was how much O2 you could stuff into the cylinders. Of course once all the fuel is burned you can't get more power no matter how much O2 you have.

Turbos on diesels do improve fuel economy. Here's why:

We've all seen old diesels pouring black smoke from their exhausts. That smoke is unburned fuel because a non-turbocharged diesel simply can't get enough O2 when it is under load. Turbocharging the exact same engine will supply enough O2 to completely burn the fuel, improving FE. However, a diesel not under heavy load will not have its FE improved by a turbo.

Turbos on diesels work differently than on gas engines. Diesel turbos only supply compressed air to the cylinders, while gas turbos supply a compressed air/fuel mixture to the cylinders. That's why diesel turbos can increase FE while gas turbos almost always decrease FE. Gas-engine turbos always put more fuel into the cylinders under boost, while diesel turbos do not (but they allow more fuel to burn more efficiently under load).

I'm not convinced by the claims of automakers of new turbocharged gas engines that increase FE. I suspect the increased FE comes from use of a smaller engine rather than from the turbos per se.

AH! as I was writing this I see that lunarhighway posted something that supports this thinking... a 0.9L turbo engine replacing a 1.2L NA engine.
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