The amount of air in an otto engine isn't fixed - which, by definition, is throttling ;)

I'm speaking of a butterfly valve used to throttle the engine. if you have a diesel or one of the new (bimmer? mercedes?) variable intake valve throttled engines, I withdraw my statement.

I don't mean that there's always the same amount of air in the cylinder, just that at a given amount of fuel expended, there's a certain amount of oxygen needed. at a given speed you have to exert a given amount of energy to counter drag. you need a certain number of oxygen molecules (mass) to match the amount of fuel burned (at least most engines are programmed to do so). opening the butterfly valve throttle makes it easier to get the same mass of air (more volume cuz its warmer) into the engine to meet the demands of the fuel/power output. We're both right.

Bob,
Do you mean to say, intake air temperature of 175-180 deg?
Previously I'd seen 60-80 mentioned and also 100 deg.

I measured engine bay air temps 95-120 deg. F when outside air was about 55, just two days ago.

Thanks guys.

My question is about the ideal air temperature to support combustion. So far I've seen answers ranging from 60-80 up to 175-180. Yikes. Still looking for more information on that please.

Below are the separate but related concerns - which really don't bear on what I'm asking:

I do understand about warmer air being less dense. Generally with fuel injection the system will properly adjust fuel quantity to match the actual amount of oxygen available for combustion regardless of temperature.

And yes, sucking warmer air through a given diameter opening pulls in less oxygen than if pulling warmer air. So with warmer air you have a reduced max air intake potential and also an increased ability to keep intake quantity down at low throttle.

Conversely, colder air being more dense lets you suck in more of it which lets you burn more fuel for more power. When the reduced temperature compromises the efficient combustion of the mixture the power benefit reduces or disappears.

Again, any other information on ideal air temperature to support combustion?

Yup, I really meant 175 degrees. Over 200 and it will retard spark for sure, and fuel economy will drop.

The guys above are also correct that if the air was too hot and therefore the amount of fuel injected to match it was too small so that the engine didn't produce enough power to move the car through the air, you'd end up having to press harder on the accelerator, or downshifting, defeating the purpose of using the hotter air. I think whats really happening is that the cars are engineered to be able to produce enough power to go up a hill with a full load and pulling a small trailer or with roof racks. This extra power is normally wasted as I see it.

Ok, try this. Fill the tank to the brim, feed it as cold of air as you can (just like a cold air kit would provide), and drive it at a steady speed on the highway (using cruise to remove variability), then return and fill the tank again, and then feed it air at 175 degrees and drive the same route at the same speed, returning to fill the tank at the same pump under the same conditions. You'll need to make this at least 20+ miles each trip unless you have a scangage or trip computer that keeps accurate track of fuel used as the injectors are activated. I'd bet you the economy picks up by over 10% when the intake air temp was 175 degrees.

BTW, I have one of those Volvo airboxes, but it won't fit anywhere in my Saturn. Another tweak I came up with would be to let it have cold air instead of hot anytime you press hard on the accelerator. I also have mine rigged with a resistor that fakes the airtemp a bit higher still, in order to get it to run a bit leaner as well, like 15.5 to 1 instead of the 14.7 to 1 normally used. Nobody here but me believes it works. They think the Scangage gives false readings. Given that, I think its funny how the total fuel used about matches what the Scangage said I'd used on a 350 mile fill.

Thanks cheapybob.

For those of you who don't have a Volvo 240 airbox at home to look at, it has two inlets, one is cold air from just behind the radiator grill and the other is preheated air from the exhaust manifold. There's a (mechanical) thermostat controlled damper inside that selects one or both inlets as appropriate. It measures the outside cold air and closes off the heated air source over 60 deg. F. It closes off the cold air inlet when the outside air is below 40 deg. F, thus pulling straight preheated air. Between 40-60, both are open.

This afternoon I "restored" the airbox heat controls to oem status. Well, pretty much oem. I added a digital thermometer to the box with display on the dash so I can see what it's sending to the engine.

First test, outside air was about 45 deg. F. Airbox temps varied from high 40's to about 70 but were mostly in the mid-50's. At least I'm getting started. I think the intake air should likely be warmer than that for best FE.

My next step will be to insulate the longish tube/duct that carries the manifold-heated air. And to block off a bit of the oversize cold air intake duct. My earlier mods to the car took some of the restrictions off that cold air intake so there's relatively too much cold air going in, thus the temps colder than desired.

If that doesn't work to get my intake temps up higher, I can try ditching the manifold preheat system and pull in air that's gone through the radiator. I measured that the other day; typically it's 95-120 deg. F. I could mix in a little cold air from in front of the radiator and end up around 80. Just don't know if it will be consistent enough in different conditions and seasons.

Lastly, I can't go too hot. We Volvo buffs are aware that too hot (as in straight manifold-heated air in summer) will fry the air mass meter, and those are expensive.

Too bad about your air mass meter, and thanks for the info on the airbox. I would guess the meter can handle 120 degrees, or it wouldn't survive on normal hot days in hot areas, but then again, you might already know its limits.

Good discussion here. Don't forget that when you place fewer reactants into the combustion chamber to produce the same RPM's, you are wasting less fuel. I believe there must be a lot of wasted power generated in pounding the piston down only to have it reverse course and come back up again.

However, the real question is: Are we placing fewer reactants into the combustion chamber when using warm air? After thinking about this for several months and reading this discussion, my hypothesis is: NO.

Here's my reasoning:

- When the piston travels downwards, a vacuum is created in the resulting void volume, which allows a certain amount of air molecules (and fuel of course) to "diffuse" in through the intake valve to fill up that volume.
- This should be the same regardless of the density of air in the intake manifold.

However, one has to ask the question, given a decent background in fluid mechanics and piping design: If the pressure drop across the intake valve is HIGHER with cold air (less vacuum in IM, same vacuum in chamber), wouldn't that drive a higher flow rate of reactants into the chamber in the time the intake valve is open? I.e. with warm air the pressure drop across the valve should be LOWER (vacuum in IM is higher, and therefore closer to the vacuum in the chamber), so we have a lower flow rate of reactants into the chamber during the time the intake valve is open.

Or does the increased viscosity in the cold air offset the presumably increased flow rate due to a higher pressure drop? My educated guess is that pressure drop trumps air viscosity.

Now, in the span of 5 minutes, I have changed my hypothesis to YES, warm air DOES result in fewer mols of reactants in the combustion chamber per stroke/RPM.

My head hurts. Help!

I agree- warm air is less dense, which means more empty space between molecules, and that coupled with less gas being introduced, means less reactants.

If the throttle was opened more, this would mean:

1. less pumping losses from maintaining the vacuum AND 2. higher compression (less vacuum means the cylinder fills more completely and there is more fuel/air to compress). Higher compression means better efficiency.

Oh- but wait...

Jim- OK, I see the dilemma now- cold air= more dense but more vacuum means the cylinder fills less, but warm air is less dense, but the cylinder fills more completely because there is less vacuum to prevent filling. Hmmmm

I change my answer to "I'm not sure if there are less reactants with warmer air" but even if the amount of reactants were equal, warm air combustion should be more efficient because of higher compression

Compression is a fixed ratio relating the BDC volume to TDC volume of the cylinder. What your sentence describes is intaking additional air/fuel. Compression cannot be changed simply by altering the intake charge.