Ideal Intake Air Temp??

[brucepick]

I recall a thread discussing controlling the intake air temp with temp sensors and servomotors. Where?

Most importantly, what's the ideal intake air temp?? Somehow I recall reading that it's 60-80 deg. F but I have no idea if that's correct.

Reason I'm interested - my Volvo 240 came oem with an intake air temp regulator in the air box. Mine and most of these by now have the regulator disabled for various reasons. I can restore it to oem or set it up any way I want. I have a digital thermometer that I'll set up to read IAT so I'll know the temp of air going into the engine.

Thanks, guys!

[Erik]

The 87 Honda civic engineers decided it should be 100 degrees F and set up the system to regulate itself to achieve this temp. But this car has a carb, it might be different for EFI.

[brucepick]

Originally Posted by Erik

The 87 Honda civic engineers decided it should be 100 degrees F and set up the system to regulate itself to achieve this temp. But this car has a carb, it might be different for EFI.

Thanks.

My guess is, fuel + air ignites optimally at some particular air temperature. Maybe slightly different depending on compression. I think what does change with fuel injection is, an AMM sensor it is not fooled by air temp changes. So it can be designed to accept incoming air of a wider range of temperatures.

Anybody else have input on ideal intake air temperature?

[SL8Brick]

bruce, I'm by no means knowledgeable on this topic...but I was under the impression that colder air is denser air and dense air is more condusive to maximizing ignition efficiency.

Maybe someone can set me strait on this?

[froggy81500]

Originally Posted by SL8Brick

bruce, I'm by no means knowledgeable on this topic...but I was under the impression that colder air is denser air and dense air is more condusive to maximizing ignition efficiency.

Maybe someone can set me strait on this?

colder, denser air is typically what people want for more power, not efficiency. More oxygen molecules in colder air requires more fuel and produces more power.

Warmer air, conversely is not as dense and needs less fuel to maintain stoich.

[trebuchet03]

Originally Posted by brucepick

Thanks.

My guess is, fuel + air ignites optimally at some particular air temperature.

Given the range of potential air temps possible for a car... Nope - given air and fuel (in proper mixture), it will ignite It's the sensors in the car that change that, which seems to vary from mfr. to mfr. Go as hot as possible - just before engine management doesn't like it

[cheapybob]

175 to 180 degrees will give optimal economy that I've seen. Over that I seem to lose too much power, possibly due to spark retard from ping. If you can get 175 degrees, you can calculate the difference in fuel consumed by taking the percentage of weight reduction of using the hotter air. Of course, this assumes your car injects fuel to match oxygen at 14.7 to 1 by weight like mine does.

[kamesama980]

before you guys get all hot air=efficient gung ho and stick a blow torch in your engine bay, the reason it's more efficient to bring hot air in (to a point) is that yes, the air is less dense and less fuel to match the oxygen BUT you still need to produce the same amount of power to travel down the road at a constant speed. the reason for the increase in efficiency is that to counter the power LOSS from warm air, you open the throttle wider. what is the throttle? the biggest restriction to your car breathing freely. warm air does NOT=efficiency, warmer air results in less throttle restriction which results in efficiency

[trebuchet03]

Originally Posted by kamesama980

before you guys get all hot air=efficient gung ho and stick a blow torch in your engine bay, the reason it's more efficient to bring hot air in (to a point) is that yes, the air is less dense and less fuel to match the oxygen BUT you still need to produce the same amount of power to travel down the road at a constant speed. the reason for the increase in efficiency is that to counter the power LOSS from warm air, you open the throttle wider. what is the throttle? the biggest restriction to your car breathing freely. warm air does NOT=efficiency, warmer air results in less throttle restriction which results in efficiency

However you have to slice it... Hot Air --> less fuel

It's not so much the lack of restriction - it is restricting, but less air is physically in the cylinder. It's less pumping losses -- higher load

[kamesama980]

at a given speed, you have a given drag, which you need to expend a given amount of energy to counteract. the energy in gas is fixed and to maintain a stoich mixture, so is the air. therefore you have the same air in the cylinder but yes, less pumping loss due to sucking it through a bigger hole (more open throttle)

[trebuchet03]

Originally Posted by kamesama980

the energy in gas is fixed and to maintain a stoich mixture, so is the air. therefore you have the same air in the cylinder...

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

[kamesama980]

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.

[brucepick]

Originally Posted by cheapybob

175 to 180 degrees will give optimal economy that I've seen. Over that I seem to lose too much power, possibly due to spark retard from ping. If you can get 175 degrees, you can calculate the difference in fuel consumed by taking the percentage of weight reduction of using the hotter air. Of course, this assumes your car injects fuel to match oxygen at 14.7 to 1 by weight like mine does.

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.

[brucepick]

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?

[cheapybob]

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.

[brucepick]

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.

[cheapybob]

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.

[Jim Dunlop]

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!

[Erik]

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

[kamesama980]

Originally Posted by Erik

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.

[Erik]

Originally Posted by kamesama980

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.

I can see that the ratio of combustion chamber size to total cylinder size doesn't change, but doesn't total compression change depending on how much vacuum is present?

Ex. If I had a compression gauge and cranked an engine over with the throttle closed and then again with the throttle valve open, wouldn't I get a higher psi reading with the throttle open while cranking?

I should have said higher compression psi in my original post.

[brucepick]

Yikes.

I thought it was already established that a warm air intake was better for FE than a CAI.

[kamesama980]

Originally Posted by Erik

I can see that the ratio of combustion chamber size to total cylinder size doesn't change, but doesn't total compression change depending on how much vacuum is present?

Ex. If I had a compression gauge and cranked an engine over with the throttle closed and then again with the throttle valve open, wouldn't I get a higher psi reading with the throttle open while cranking?

I should have said higher compression psi in my original post.

you're right, but thats not usually talked about in those terms. you just described what we've been talking about with pumping losses and throttle plate restsrictions but backwards.

[jBubb]

I am testing different temps on my 99 Metro 3 cyl 1.0L as we speak.

I just received my ScanGauge a few days ago and I am waiting to burn through a tank to properly set it up for measuring FE.

I will play with different temps and let you all know what I find.

[bzipitidoo]

I would think the ideal is hot engine + cold air. Or, rather, dense air. Cold is one way to get denser air, lower elevation is another, and turbochargers are another. You don't see the benefits when they are masked by a long warm up time because it's colder outside. It also obscures things if the vehicle has lousy aerodynamics.

Turbo and super chargers make the engine more efficient by ramming more air in. True, that allows more fuel to be burnt, and consequently more power, but it does improve efficiency. Think about it: moving those pistons back and forth takes energy. The less movement there is per unit of power produced, the more efficient the engine. Now most people use this to burn more rubber instead of less gas. One would need to make the gearing taller to really benefit.

In the absence of a 'charger, colder denser air gives a little of the same benefits. The only reasons for hot air is fuel vaporization, and ever so slightly less air resistance. Better aerodynamics makes air resistance less important. For cold air, the advantages are more power per power stroke, and possibly a slight push from the air expanding even before combustion just from being rapidly heated by the engine when it enters the chamber during the intake stroke. As for vaporization, air is easy to warm up. Once warmed up, a properly designed engine should have no problem with that even in extreme cold such as -40 degrees.

Let's ask a similar question. Is FE better at sea level, or at 10,000 ft elevation? Anyone have any numbers on that? I expect sea level is better.

[Biffmeistro]

Originally Posted by brucepick

Yikes.

I thought it was already established that a warm air intake was better for FE than a CAI.

Yeah, but it's fun to argue! I think it has been established that it does, but now they're arguing why?

[jBubb]

Originally Posted by bzipitidoo

Let's ask a similar question. Is FE better at sea level, or at 10,000 ft elevation? Anyone have any numbers on that? I expect sea level is better.

Well, I live in CO around 5200 feet. I regularly go driving up into the mountains and usually see a noticeable drop in FE. Mainly because I have to slam the pedal down a lot more to achieve the same speeds at 5k feet.

It may not be as noticeable going from 5k to sea level tho. I would assume the o2 sensor would, to a degree, regulate the air/fuel mixture.

[brucepick]

Originally Posted by jBubb

... I would assume the o2 sensor would, to a degree, regulate the air/fuel mixture.

Yes. Taking readings to regulate the air/fuel mixture is it's job.

The air sensor located between the air filter and throttle body does its best to tell the computer how much air is entering the engine. Fuel is injected to match the amount of air. Oxygen sensor reads O2 present in the exhaust to detect whether the mixture is too lean or too rich. Mixture is adjusted accordingly, so it's a feedback-loop system.

Ya get more air and thus more fuel into the cylinder if it's colder air. But it burns somewhat more efficiently if it's warmer. Dunno the ideal temperature but I'm pretty sure it's well above the 20-40 degree temps we typically have in winter in my area!

[brucepick]

By now I've got the system pretty well restored.

Hot air hose is in place, wrapped with pipe insulation which is spiral-wrapped with duct tape. Cold air supply is also in place. Added thermometer sensor is in place inside the air box. New tube directs driving airflow at the small air inlet at front of airbox which feeds cold outside air to the thermostat inside. (I discovered the airbox thermostat is designed to read the outside air temp, not the resulting mix).

Intake air temperatures are in the mid-50's F with outside temperatures around 30 deg. I think warmer would be better but I haven't been able to achieve that.