Quote:
Originally Posted by Varn
My VW Diesel has the intake air for the engine come from the fender well.
Would using preheated air improve the mileage? I don't have any instruments and have to rely on calculating efficiency based on gas mileage.
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No, a cold air intake (CAI) is better than a warm air intake (WAI) on a diesel. A WAI improves fuel economy on a gas engine for 3 reasons:
1) The warm air is less dense, so the throttle opens up more to get the same air, therefore throttling losses are reduced.
2) The warm air improves the flame speed
3) The warm air improves the vaporization of the fuel
A diesel doesn't have a throttle, so the first point obviosly doesn't apply.
A diesel is primarily mixing-controlled combustion, so there's isn't a flame front propogating through a premixed mixture like there is with a gas engine, so the second point doesn't apply either.
The third point may apply in theory to a diesel, however, the effect is going to be extremely small. This is because in a diesel the fuel is directly injected into the air after it's been highly compressed (not into the cool, low temp intake like in a gasoline engine). Plus diesels run much higher injection pressures than gasoline engines (30,000+ psi on modern common rail diesels), so most of the atomization and vaporization comes due to the pressure effects. Therefore intake temp with have a fairly small effect on vaporization in a diesel.
So, on a diesel the benefit of a cold air intake is for two reasons.
1) Cold air = more dense air = higher air-fuel ratio and/or reduced pumping loses
2) Colder air improves the thermodynamic efficiency of the combustion cycle
Regarding the first point, the cold air is more dense so your turbo will provide more air, increasing the air-fuel ratio, and thus the combustion efficiency. This is if the diesel, doesn't have a computer-controlled variable geometry turbo, that will adjust to keep the AFR the same. If it does, there still is a benefit. The turbine will not have to close down as much to give the engine the air flow that it wants so the pumping losses will reduce.
The second point is a little trickier to grasp. Theoretical diesel cycle efficiency is dependent on the ratio of specific heats (lower=better). Cooler air has a lower specifc heat ratio, therefore it improves the combustion efficiecny. See this calculator tool here:
The Diesel Engine
BTW, I'm working on a CAI/ram air system on my truck. I hope to be posting pics soon.