Comparing Warm Air Intakes (WAI) & Cold Air Intakes (CAI)

[Daox]

Stock intake systems create virtually no restriction at ecodriving rpms. No gains to be had there, sorry.

Pumping losses occur when an engine is not able to take in as much air as it wants to thus creating a vacuum in the intake manifold. For example, lets use a 2 liter engine. Every two times the engine spins around (4 cycle engine) it wants to suck in 2 liters of air. Restrictions occur upstream of the combustion chamber that limit the amount of air that enters the engine. The first item is the intake valve(s). These are controlled by the camshaft and really can't be easily altered. The next item upstread is the throttle. The throttle is used to decrease engine output so you don't accelerate like crazy. However, as you close the throttle, the engine has to work harder as it is still trying to suck in 2 liters of air. This decreases engine efficiency and is why accelerating at high loads is more efficient.

[tasdrouille]

Originally Posted by tjts1

Because of this input to the ECU, any efficiency gained from the low density hot air going into the engine will be lost because of retarded spark timing.

That is assuming a lot. Do you have anything to back this up? AFAIK IAT is not a primary determinant of timing.

The work around this problem would be to manually control the input form the AIT sensor in order to trick the ECU to advance timing. This seems extremely dangerous to me.

That is hardly dangerous unless you floor it, and is a non issue with knock sensor equipped cars.

[Daox]

IAT isn't a primary determinant of timing. However, it will effect timing to some degree. Again, this is going to depend on engine design and tuning.

[tjts1]

Originally Posted by tasdrouille

AFAIK IAT is not a primary determinant of timing.

I never said IAT is "the primary determinant of timing". Please don't try to misinterpret what I said. The ECU sets spark timing based on your RPM, then in addition to that it looks at load and intake air temperature. As the intake air heats up, the ECU retards timing and vice versa.
From my bmw repair manual.


From autospeed.

The resistor trick is based on this idea: you add the resistor to the engine coolant temperature sensor circuit, or the intake air temperature circuit. This tells the ECU that the temperature is different to its actual value, and as a result, the ECU adds more fuel (or less fuel, depending on the direction of the modification), or more ignition timing or less ignition timing (again, depending on which way the mod takes the perceived temperature).

http://autospeed.com/cms/A_110350/article.html

Originally Posted by tasdrouille

That is hardly dangerous unless you floor it, and is a non issue with knock sensor equipped cars.

At low RPM rpm, you don't need to "floor it" in order to reach maximum load and zero vacuum. The throttle body is sized for max load at max RPM. At low rpm you can reach max load at as little as 1/4 throttle. Hook up a vac gauge to your intake manifold and you'll see what I mean.

At this point we have no evident to suggest that a warm air intake will improve fuel economy by reducing the density of the intake air.

[tasdrouille]

AFAIK, the primary determinants of timing are rpm as you said, the coolant temperature, and the fuel quantity to be injected, which is determined by the air mass, which depends on altitude, humidity, ambient temperature, etc.

I never said, nor really thought, that the impact of a WAI on FE was primarily or in minor part due to reduced throttling losses from lesser air density. What I've said in the past though, it that a hot engine is an efficient engine. It's mostly the temperature, not the density.

I'll repost again here the references I've posted a couple times already so people can make up their own minds. It's not me saying it, it's research papers and mechanical engineering textbooks.

- Pre-heated intake mixture at low rotational speed improves combustion. (Chiu and Horng, 1992)
- Specific fuel consumption varies inversely proportional to the square root of the suction air temperature (Nakajima et al. 1969).
- Higher ambient temperature is found to increase the flame speed, the combustion reaction rate, the uniformity of the fuel-air mixture and reduce the heat transfer rate though the cylinder walls (Pulkrabek, 1997).
- For lower temperatures, only a small part of the injected fuel is vaporized, causing nonhomogeneity. As a result, lower flame speeds, higher unburned mixture, higher hydrocarbons and carbon monoxide emissions, and loss of power are observed (Pulkrabek, 1997; Heywood, 1988).

References:
Chiu, C.P., and Horng, R.F., 1992, “Effects of Intake Air Temperature and
Residual Gas Concentration on Cycle-to-Cycle Combustion Variation in a
Two-Stroke Cycle S.I. Engine Equipped with an Air – Assisted Fuel Injection
System”, JSME International Journal, Vol. 37, N.4, pp. 957-965.

Nakajima, K., Shinoda, K., and Onoda, K., 1969, “Experiments on Effects
of Atmospheric Conditions on the Performance of an Automotive Gasoline
Engine”, SAE Transactions, SAE 690166, pp. 745-766.

Pulkrabek, W.W., 1997, “Engineering Fundamentals of the Internal
Combustion Engine”, Prentice Hall, Inc.

Heywood, J.B., 1989, “Internal Combustion Engine Fundamentals”,
McGraw-Hill Book Co.

Finally, I'll post the most useful freely available paper I could find:

Changes of Low Load Engine Parameters by Temperature of Mixture

In this research, they were able to reduce BSFC at low RPM and low load by 7% at 195 F vs 100 F. Also, indicating that density reduced throttling losses have not much to do with it, they measured almost the same throtte position at those two temperatures.

[wizardwizard]

i think this refers to the air pressure in the intake tube while at 1500 rpm @ 25 mph and how this pressure would change immediatly when you hit the gas pedal it starts taking in more air wich means less air would be in the intake area and more air in exaust.
when you press the gas, the pressure changes, the gas is held up by water pressure from the tank as long as you have gas, the air pressure is not as stable as this water pressure so you could have a situation where you cant go any faster cuz you cant get enough air into the intake like if you had a front air dam.
the pressure of the air intake fluctuates in an unstable way as your engine uses more and less of it compared to the flow of the gas which is constant because water (liquid) cannot be compressed or decompressed physically (maybe with chemical reaction)
by using a ram air or cold air, you ensure that the intake air pressure stays up when it is being sucked at varying amounts and changing in a way that is unstable compared to the gas intake pressure which is constant.
it makes it easier for the engine and computer to keep the right fuel air timing ratio if it can control the amount of air and gas it is using with instruments that work 100% of the time.

[Markmysite]

This thread died back in May.... hmmm. It was getting pretty heated ... I would've loved to read more ... lol.

It seems that the debate on this will go on for a long time. Why? Because not all engines are the same. Yes, they function in similar ways, but some cars with MAP sensors will react differently to WAI than those without. The ECM measures the difference between upstream and downstream and calculates mixture based on that in some cars, but not on others. I'm not aware of a comprehensive list that tells you which do and don't either.


Still, I think that most modern cars will not benefit from just putting a WAI on today because the ECU will just readjust because of the sensors. After the O2 sensor is warmed by the exhaust gases it will provide the data for air/fuel ratios.

Ahhhh, but what if you were to use the old foil trick? Carefully wrap the O2 sensor in aluminum foil and use a zip tie to hold it in place (leaving a small path for some air flow)? This would trick the ECU into runner leaner yes?

Well darn, the new O2 sensors don't have that vent hole in back. Rather, they use a wire. Can you limit the flow in a wire? Hmmmm


If you can fix teh O2 sensor issue you've got it solved. You would have the MAP sensors and AIT sensor inline on your WAI they should be taken care of.

Yes, knocking may occur on some cars so you obviously would need to run higher octane gas ... or just forget the whole thing. I think it's worth a try.... especially in the winter.

Who's in?

[tasdrouille]

Well, to sum it up the theory is sound, but ymmv.

I would advise anyone to try it. If the timing is getting pulled back tune for cooler air or get rid of it. If not leave it there.

[tasdrouille]

Just bringing more water to the mill. This is from a 1989 research paper by Honda on their F1 1.5L turbo engine. The following images are at WOT



70 C is close to 160 F. They found that past that knock could become a problem and BSFC would go back up a bit.

I'll throw in fuel temp and boost too cause that can be interesting.

[rkcarguy]

My opinion, the WAI is a limiter that slows your acceleration and reduces your WOT power output by inducing a hotter and therefore less dense air/fuel charge to the engine. Measured side by side I'd bet the CAI equipped car would be near identical mpg as a WAI equipped one, but obviously will walk away under acceleration.
Just put a wedge under your gas pedal so it only goes to 75% WOT and enjoy the same gains instead...