Air Temperature

[Fleet 500]

From what I've read...
cool air for power.
warm air for fuel mileage.

That's why muscle cars had hood scoops- to bring in outside (cooler) air into the engine to make more power.

[KingNothing]

Eleventy billion *C

Thats what like 55*F?

Thats kinda what i told my chemistry teacher...she wasnt amused

[hightower99]

But the more heat that is stored in the mass the higher it’s temperature will become which will reduce overall effectiveness.

Yes and that means the higher mass will maintain a lower temperature (because it will store less energy per unit mass).

I have been assuming that for both cases the heat transfer into and out of the IC has been the same. Since the cold and hot side areas are the same, for both the low and high mass ICs, then the surface temperature must also be the same. This would imply that the IC will reach the same steady state temperature.

Ah Ok now I understand. In that case yes both ICs would achieve the same overall temperature. But it would take the high mass IC quite awhile to achieve the same temperature and up till then it would cool the IA more effectively (relying only on it's own thermal properties). But in that case it would be better for a drag car application because when it eventually gets hot enough to need the cooling air, the cooling air would be flowing rather quickly.

True, but since we are only talking hypothetically/theoretically it doesn’t seem unreasonable as it helps isolate the affects of the change. As apposed to changing mass and wall thickness which will result in coupled effects.

Ah I thought that would be the case. Well I personally agree that it would make the discussion more simple but I also think that it would take away much of the real world relevance (which is what I wanted to discuss).

Air doesn’t transfer heat well to air, because of its very poor thermal properties. Generally speaking air can be considered a thermal insulator. Nevertheless, you won’t get better heat transfer from hot air to cold air by putting aluminum in the middle. Regardless of how good it’s thermal properties, the aluminum will only serve to increase the thermal resistance between the hot and cold sources.

How will putting aluminium between the sources increase thermal resistance? if the thermal resistance between an air/aluminium contact is much less than between air/air then how does that not help? Also we did this experiment at school where we saw what happened with a thermal viewer. The aluminium plate seemed to help alot.

warm air for fuel mileage.

That's why muscle cars had hood scoops- to bring in outside (cooler) air into the engine to make more power.

How does warm air increase mileage?

Also muscle cars had hood scoops because they looked cool first and foremost. A slight ram air effect was sometimes created that helped at speed (but due to bad design this wasn't always the case). Also it was rare that the hood scoop was directly connected to the intake and therefore it was rare that it helped the engine ingest cooler air at all.

Just alittle FYI

[Alastor]

Yes and that means the higher mass will maintain a lower temperature (because it will store less energy per unit mass).

Initially, but if the heat transfer rate out of the IC is decreased then it will have to store more total energy. If there is less energy per unit mass depends on the boundary conditions.

Ah Ok now I understand. In that case yes both ICs would achieve the same overall temperature. But it would take the high mass IC quite awhile to achieve the same temperature and up till then it would cool the IA more effectively (relying only on it's own thermal properties). But in that case it would be better for a drag car application because when it eventually gets hot enough to need the cooling air, the cooling air would be flowing rather quickly.

I agree that the large mass IC seems best suited to drag racing. The additional mass could be beneficial when the engine is under heavy load but the vehicle isn’t moving. Since the races are relatively short peak operating temperatures are likely to be lower, mitigating the negative effects of slower thermal response.

Ah I thought that would be the case. Well I personally agree that it would make the discussion more simple but I also think that it would take away much of the real world relevance (which is what I wanted to discuss).

Discussing the real world is fine, it is just the non-linearity can make it hard to quantify changes speaking hypothetically. Nevertheless, for what we are talking about there is probably a good chance that the wall is isothermal anyway. So changing it’s thickness might not even matter.

How will putting aluminium between the sources increase thermal resistance? if the thermal resistance between an air/aluminium contact is much less than between air/air then how does that not help?

How is the thermal resistance between air -> aluminum -> air much less than air -> air.

[Fleet 500]

How does warm air increase mileage?

I really don't remember the technical aspects; it's just something I read years ago. If I happen to comes across an article about that in one of my magazines, I'll post it here.

Also muscle cars had hood scoops because they looked cool first and foremost. A slight ram air effect was sometimes created that helped at speed (but due to bad design this wasn't always the case). Also it was rare that the hood scoop was directly connected to the intake and therefore it was rare that it helped the engine ingest cooler air at all.

(Functional) hood scoops actually did work. Some were non-functional, to look cool, as you said.
With a hot engine, the underhood temperature is well above 100 or 110 degrees. Outside air is much cooler which was the purpose of the hood scoops... to bring in cooler (denser) air to create more power.
Car & Driver (Jan., 1969) tested 6 muscle cars; here is what they said about the tested '69 Dodge Coronet Super Bee with 383 and optional cold-air induction system...
"The test car was powered by the standard 335-hp 383 cu in V-8 breathing through the optional cold-air induction system that feeds fresh air into the carburetor through two hood scoops. Cloudsburts and storms will never see the inside of your engine because a red knob under the instrument panel (labeled "carb air") has been provided so that the driver can manually close the scoops.
Of course, this little feature makes it very easy to see if Dodge is really giving you your money's worth in performance or if the scoops just add up to expensive decoration. Dodge does not speak with forked tongue. In a standing 1/4 mile, the open scoops are worth exactly one mile per hour and slightly more than one-tenth of a second in the elapsed time."
The car ran a 14.04 sec @ 99.55 mph 1/4, btw, with a 5.6 sec 0-60 mph time.

[hightower99]

How is the thermal resistance between air -> aluminum -> air much less than air -> air.

Because the aluminium is able to take the heat energy from the hot air very easily and it can also give that heat energy to the cold air very easily. Also Aluminium has a thermal conductivity almost 9500 times greater than air

With a hot engine, the underhood temperature is well above 100 or 110 degrees.

You mean degrees F right? because under hood temps shouldn't be much higher than engine temp (80-90C) in fact when the car is moving the temps should be roughly half that. Also a hood scoop will not help to the engine ingest cold air unless it is directly connected to the intake system. Just having a whole in the hood isn't going to do you any good. Also that the hood scoop was worth 1 MPH and about a tenth of a second in a 1/4 mile. that is such a small gain that it could be down to driver skill or even good track conditions...

[Fleet 500]

You mean degrees F right? because under hood temps shouldn't be much higher than engine temp (80-90C) in fact when the car is moving the temps should be roughly half that. Also a hood scoop will not help to the engine ingest cold air unless it is directly connected to the intake system. Just having a whole in the hood isn't going to do you any good. Also that the hood scoop was worth 1 MPH and about a tenth of a second in a 1/4 mile. that is such a small gain that it could be down to driver skill or even good track conditions...

Outside air is going to be cooler than underhood air, no question about it. Don't forget, there is a hot radiator and exhaust manifolds under the hood. And the engine itself is very hot (try touching a hot engine!).

Driver skill and/or track conditions isn't really a good answer. Because there have been other tests in which the scoops were closed and a car was recording trap speeds of 97-97.5 mph with a lot of runs (like 10 or so) and 98.5-99 with the scoops open.
There were usually quite a few 1/4 mile runs for a road test and the average was run was printed.
Same thing with 0-60 mph runs. For instance, the Car & Driver (April, 1967) test of a '67 383 Plymouth Barracuda. To quote: "In 4 runs, 0-60 mph times varied between 6.5 and 6.8 seconds."

[Alastor]

Because the aluminium is able to take the heat energy from the hot air very easily and it can also give that heat energy to the cold air very easily. Also Aluminium has a thermal conductivity almost 9500 times greater than air

It doesn’t matter how much better the thermal conductivity of the aluminum is, the fact that it is finite means that some thermal resistance is induced.

As well, aluminum has a thermal conductivity almost 400 times better than water. So it doesn’t seem consistent that you agree a direct air to water heat transfer would be ideal.

In the most simple case the heat is transferred via conduction. So for air -> air the thermal resistance between the and hot and cold fluids is zero. In the case of air -> aluminum -> air the thermal resistance is non-zero and directly proportional to the aluminum wall thickness.

[hightower99]

Outside air is going to be cooler than underhood air, no question about it. Don't forget, there is a hot radiator and exhaust manifolds under the hood. And the engine itself is very hot (try touching a hot engine!).

Yes I know my point was that the air under the bonnet isn't as hot as you would think especially when you are moving.

It doesn’t matter how much better the thermal conductivity of the aluminum is, the fact that it is finite means that some thermal resistance is induced.

As well, aluminum has a thermal conductivity almost 400 times better than water. So it doesn’t seem consistent that you agree a direct air to water heat transfer would be ideal.

In the most simple case the heat is transferred via conduction. So for air -> air the thermal resistance between the and hot and cold fluids is zero. In the case of air -> aluminum -> air the thermal resistance is non-zero and directly proportional to the aluminum wall thickness.

well in the air/air case the coolant and source have the same thermal properties so they need some material between them which is able to take the heat from the hot air and give it to the cold air. I have seen the effects in a school experiment. In the water/air IC you are relying more on the specific heat capacity to cool the air to a lower temperature (water has a specific heat capacity that is roughly 4.5 times that of aluminium). Also interesting is the fact that air has a higher specific heat capacity then aluminium. Also heat transfer directly from air/air does not occur at zero resistance. Cold air tends to stay cold and hot air tends to stay hot even when they are in contact with each other.

[Fleet 500]

Yes I know my point was that the air under the bonnet isn't as hot as you would think especially when you are moving.

The air around the engine is still hotter than outside air. That's why cold air induction systems are effective. Not a whole lot, but some.

[hightower99]

The air around the engine is still hotter than outside air. That's why cold air induction systems are effective. Not a whole lot, but some.

Yes outside air is cooler and properly designed cold air intakes do increase power (and mileage)...

But hood scoops have almost never been effective cold intakes... Ram air maybe... cold air NO...

[Alastor]

well in the air/air case the coolant and source have the same thermal properties so they need some material between them which is able to take the heat from the hot air and give it to the cold air. I have seen the effects in a school experiment.

It doesn’t matter if you put a material between the air. Air is a poor conductor of heat, that cannot be changed.

What was the experiment?

Also heat transfer directly from air/air does not occur at zero resistance.

If you believe that then you must also believe the same resistance will exist at the air/aluminum boundary as well.

Cold air tends to stay cold and hot air tends to stay hot even when they are in contact with each other.

This isn’t changed by putting aluminum in the middle, poor thermal characteristics are intensive to air.

[Fleet 500]

Yes outside air is cooler and properly designed cold air intakes do increase power (and mileage)...

But hood scoops have almost never been effective cold intakes... Ram air maybe... cold air NO...

Yep, ram air is effective (and not "maybe," but for sure).
Hood scoops help a little because underhood temperature is decreased slightly.
Here is some info I found from a muscle car magazine...
Because airflow increases as the square root of the numbers representing air density, an engine's output is radically improved when it's fed outside, rather than underhood air. A drop in engine bay temperature from 150 degrees F to a plumbed-in 70 degrees F, for example, can make roughly 7% more power in a 300-hp engine.
That's why that 383 Super Bee cut it's 1/4 mile time and increase its trap speed... the outside (colder) air brought in from the air induction unit.

[hightower99]

Yep, ram air is effective (and not "maybe," but for sure).
Hood scoops help a little because underhood temperature is decreased slightly.
Here is some info I found from a muscle car magazine...
Because airflow increases as the square root of the numbers representing air density, an engine's output is radically improved when it's fed outside, rather than underhood air. A drop in engine bay temperature from 150 degrees F to a plumbed-in 70 degrees F, for example, can make roughly 7% more power in a 300-hp engine.
That's why that 383 Super Bee cut it's 1/4 mile time and increase its trap speed... the outside (colder) air brought in from the air induction unit.

Ram air is only effective (at all) if it is plumbed directly to the intake and you are traveling very fast. The point is to try to push more air into the intake then the engine is able to suck in on it's own and that requires very high speeds.

Also hood scoops are only going to lower underhood temps because it is an open hole in the hood and it lets hot air out (therefore allowing no cold air in through the scoop). If it is connected directly to the intake then when you are moving it well help the engine breath cooler air. When you are stopped though the air will be heated.

Because airflow increases as the square root of the numbers representing air density,

WTF does that mean??? if density doubles the mass airflow doubles... simple.

[Fleet 500]

Ram air is only effective (at all) if it is plumbed directly to the intake and you are traveling very fast. The point is to try to push more air into the intake then the engine is able to suck in on it's own and that requires very high speeds.

Also hood scoops are only going to lower underhood temps because it is an open hole in the hood and it lets hot air out (therefore allowing no cold air in through the scoop). If it is connected directly to the intake then when you are moving it well help the engine breath cooler air. When you are stopped though the air will be heated.

What do you mean "if at all" effective? A good ram air system is definitely effective.
Also, an old Super Stock trick (late '50s/early '60s) was to adjust the rear of the hood to a position where it was a few inches higher in back. This was to allow the hot air from under the hood to be drawn out.

WTF does that mean??? if density doubles the mass airflow doubles... simple.

I was posting some of the text in the article.