Hot or cold intake air? Best diesel mileage.

[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.

[Diesel_Dave]

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.

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.

[Varn]

Thanks Dave, Somehow I figured it might be different than a gasser.

Heeding your advice, I went out before I started it up today and plugged my intake back into the fenderwell where it was hanging since last summer.

Would like to see your CAI once you get it finished.

[Varn]

As a follow-up question, if cooler is better, does a heat exchanger between the turbo and the intake improve efficiency as well

[Diesel_Dave]

Originally Posted by Varn

Thanks Dave, Somehow I figured it might be different than a gasser.

Heeding your advice, I went out before I started it up today and plugged my intake back into the fenderwell where it was hanging since last summer.

Would like to see your CAI once you get it finished.

Thanks, so far I have part of it done. What I've done so far is what's shown here:
Home Deopt CAI on a 3rd Gen - Dodge Cummins Diesel Forum
(Not me, just copying somone else.)

So I have the pipe going down under the fender right now. What I'm still working on is connecting that pipe to the fog lamp hole to get ram air. Other folks have done it with dryer vent hose, but I think I've got a better way of doing it by using a roof pipe vent boot behind the existing fog lamp hole grille. I have that part in place already and it looks nice. I just have to connect the boot to the pipe.

[NachtRitter]

Originally Posted by Diesel_Dave

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 more 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

I know what you meant by #1, but it might be worth a quick edit since the rest is well written and good advice

[Diesel_Dave]

Originally Posted by Varn

As a follow-up question, if cooler is better, does a heat exchanger between the turbo and the intake improve efficiency as well

Yes, it's called an intercooler/charge air cooler. Most modern turbodiesels have them. Typically it's right in front of the radiator.

[blownoiler]

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.

Is your injection system run by a computer? The older mechanical injection systems can benefit from an underbonnet air intake when ambient temps are very low, the computerised systems generally give better results with cooler air, but you can heat the fuel for better mileage if you are running alternative fuel, but I wouldn't suggest that you heat low sulphur diesel as pump/injector wear may be accelerated.

[Hersbird]

What about direct injected gas motors? Would cold air intake be better for the same reasons?

[Varn]

My computer is what I use to contact others on the Internet. Not needed in my car.
Even with indirect injection and a boxy body it runs at 50 mpg plus. I have felt that colder air containing more O2 would be more economical than less O2 although cold weather has a myriad of other issues with less air pressure in tires, More electricity needed, thicker lubricants and denser air. Have also read that an intercooler can increase mpg by cooling the intake charge. I have had several turbo cars both gas and diesel but never ran any back to back testing.

I run fuel that is labeled 20% biodiesel and regulated by the federal government. I am not about to put anything bogus that will reduce the life of my engine or car. It has made it nearly 3o years at close to 10,000 miles a year so the VW engineers and designers obviously know something about making an efficient running vehicle.

Originally Posted by blownoiler

Is your injection system run by a computer? The older mechanical injection systems can benefit from an underbonnet air intake when ambient temps are very low, the computerized systems generally give better results with cooler air, but you can heat the fuel for better mileage if you are running alternative fuel, but I wouldn't suggest that you heat low sulfur diesel as pump/injector wear may be accelerated.