Intake temps and IC piping questions 1

[MNRaptor]

I have heard in the past that a reduction in intake temperature of 10 degrees roughly translates to an increase in power of around 1%. My question is if this is at all accurate or if there is a better formula for determining power gains from intake temp reduction.

In a discussion on another board the topic of specific materials used in IC piping came up. I believe that Stainless will be the best material due to it's more insulating properties over alluminum. Many do not believe the difference would be worthwhile to even be concerned with siting that the air is moving too quickly in a turbo application to be effected by the pipe's temperature.

Does anyone have input on this in either direction?

 

[patprimmer]

Use aluminium them wrap it with an insulating blanket if the intake charge is cooler than the surrounding atmosphere. If the intake charge is hotter than the surrounding atmosphere, the conductivity actually helps cool the charge and the duct becomes a very low efficiency inter cooler

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MNRaptor]

What about the thoughts that the charge is moving too quickly to be effected by the piping in any significant manner. Is there likely to be a change of more than a few degrees in temp between different materials?

I should say the question assumes the intercooler is somewhat efficient at cooling the charge and in that regard is the suggested alluminum with an insulating wrap going to further the reduction in temp or just maintain it more effectively than Stainless would?

Thanks for the time and info in advance, we do a lot of intercooler systems and are always looking to improve them. Currently we do all our systems in SS piping, but if there is indeed a performance gain to be had by switching to wrapped alluminum, I am definately interested.

 

[CCycle]

The Chemical Rubber handbook lists the density of 20 deg C air as 1.2047 gm/l.
It lists 10 deg C air as 1.2472 gm/l which is a 3.53 % increase.

Converting to Fahrenheit indicates 1.9% density change for 10 deg dt.

I would say their figures are quite conservative at room temp.
But at higher temps ten deg would not be as large a difference % wise.

 

[patprimmer]

I cannot say what temperature profiles will exist on your engine. You need to measure some temperatures under various conditions then decide if and where to insulate. I have seen measurable if not significant gains where the air ducts were insulated as appropriate, even on a normally aspirated engine. Although small, it can be enough to get a winning advantage on a race car.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Retracnic]

The old "1% HP gain for every 10 degree drop in inlet temperature", is a decent rule of thumb. Like everything else automotive, your personal results may vary. Unfortunately there isn't a more reliable formula that can be calculated readily. There are simply too many other variables involved.

As for which materiel to use, that is most assuredly aluminum. Aluminum is a far better conductor of heat than any grade of stainless steel. In the case of the typical air-to-air intercooler setup, we want as much heat as possible to escape, from the intake charge, out into the atmosphere. To accomplish this, the heat must pass through whichever metal we chose to construct our system from. Obviously, we want to give this heat as little impedance as possible.

As patprimmer mentioned, as long as our outside ambient temperature is lower than our intake temperature, then all of our piping will contribute to the overall intercooling effect. In the cases where inlet temps may be lower (such as plumbing that runs too close to the exhaust manifold, downpipe, or turbine), we would like to use an insulating wrap to protect our plumbing from becoming heat soaked.

As far as your theory about the speed of the charge being too great for the material to make much difference...

There is no doubt that in a high performance turbo system, a great volume of air passes through the system in a very short period of time. Just like a hair dryer. The air that passes through a hair dryer is only in contact with the heating elements for a brief instant, yet the temperature is raised dramatically. Much like the hot air that passes through your intercooler...

I promise, ditching the SS piping and switching to wrapped (where needed) aluminum will produce measurable results on an engine that is at normal operating temperatures. No doubt about it.

Regards,

Bryan Carter

 

[crysta1c1ear]

The Chemical Rubber handbook lists the density of 20 deg C air as 1.2047 gm/l. It lists 10 deg C air as 1.2472 gm/l which is a 3.53 % increase.

10°C is 283.15°K
1/283.15 = 0.00353170

So a 1° temperature change will give a 0.353170° change in air density, and a 10° change will give ten times that, hence 3.53%.

The old "1% HP gain for every 10 degree drop in inlet temperature", is a decent rule of thumb.

Maybe, but who is to say my engine is the same size as your engine. I might be running a microturbine 'engine on a chip' with a generator for a refuelable alternative to a battery. (See for example

http://tinyurl.com/6s345)

1HP wouldn't even be close then! Best to stick to percentages.

A quick sample calculation of air density ....
D=P/RT
P=1 atm, 1013 millibars, 101325
R (gas constant) = 287.05
T 10°C = 273.15+10= 283.15
D=1.2466441 on my calculator.

That's the 1.247 g/l figure above, since g/l and kg/m³ are the same thing.

 

[CCycle]

Crystal1, Interesting observation and right on with the calculations. Evidently the CRC tables are theoretically derived.

It seems like I remember working with air years ago. Air is not an ideal gas quite and deviates from the gas law by some one or two percent around room temperature. But I may not be remembering it right. I believe I have some ASHRAE info on this if I can find it. My reason for using the tables was that they would be corrected. But obviously as you point out, they are straight gas law.

 

[MNRaptor]

I agree with several of the points brought up on the piping however, this piping exsists typically in an engine bay where heatsoaking the pipes becomes a problem on a normal basis. I agree with wanting alluminum as the material for the IC itself as that is it's job to transfer the heat from air to air in this case. The piping itself however is only a means of transport to get the charge to the TB, it is running somewhat next to the engine and underhood temps on these cars are pretty high.

Would it not be smarter to run a pipe made from a more insulating material so the heat will not be transfered into the charge, rather be insulated from it? Will wrapping the alluminum pipe still be an advantage in this situation as the outside air will never be as cool as the intake charge exept for the first few minutes of run time. So for example wouldn't it be better to insulate the Stainless pipe even in that case to prevent heat soak as opposed to using the alluminum? I apologize, I should have explained the application in better detail in the first post.

My thoughts are that if the intercooler is efficient at doing it's job, the piping at that point should do it's best to maintain that lower temperature and not allow any heat transfer into the charge so the insulating material would seem a logical choice. I just am having a bit of trouble understanding why an alluminum pipe that is a better conductor wrapped to insulate it would be better than a pipe that is a better insulator to begin with.

I am just looking for understanding for myself so I am not a blind follower as the majority of the guys I race are.

I personally believe in taking every advantage in a race situation as Pat mentioned, I am not the one who believes the air is moving too fast to be effected. That is an argument that I want to make sure I am on the right side of.

The input on temp reduction vs power is helpful as well, thanks again.

 

[btrueblood]

Well, I think the rule is pretty close. A 3% change in intake density would mean 3% more thermal energy available to the engine.

But, if the thermal efficiency of the engine is around 30%, then the engine output (shaft horsepower) only changes 1%.

 

[spedtoe169]

I think its pretty much a toss between stainless and aluminum in terms of heat transfer. You should go with aluminum because of the weight savings. It's more than worth having to insulate some key tubes to prevent heat soak.

 

[GregLocock]

btrueblood

" A 3% change in intake density would mean 3% more thermal energy available to the engine.

But, if the thermal efficiency of the engine is around 30%, then the engine output (shaft horsepower) only changes 1%."

- you might want to have a rethink on that! Although you only get 1% of the expected horsepower, you were only getting 30% of the expected horsepower from the original engine, so the 1% is still 3%

Not my best explanation ever!

retracnic - centigrade or fahrenheit?


Cheers

Greg Locock

 

[patprimmer]

Greg

Sounds good to me.

MNRaptor

Although the conductivity of aluminium and SS are quite different, the conductivity of well insulated aluminium vs well insulated SS will be pretty much the same, so long as they are isolated at each end by a rubber joiner so as to insulate them from heat transfer from non insulated sections of pipe.

Measure some temperatures inside and outside the ducts under race conditions, then decide where to wrap the pipe.

Wrap it then measure again. Do A:F corrections as necessary.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mburgess]

From what I understood while the object of intercooling to reduce intake temperatures resulted in an increase power not only from the density change which was minimal but from the prevention of detonation & the ability to run more advance in the ignition curve. This however may not answer what is the best material to use. In motorcycle racing I make the airbox & ram ducts out of fibreglass/carbon fibre & wrap them in heat insulating tape.

 

[MNRaptor]

Thanks Pat, That is helpful, I just want to make sure we are doing things correctly in our search for power and also for our customer's sake. We do a lot of IC piping.

 

[crysta1c1ear]

mburgess said:
From what I understood while the object of intercooling to reduce intake temperatures resulted in an increase power not only from the density change which was minimal but from ....

We've said that cooler ambient air can give more air in a cylinder. Now we are saying cooler forced induction air doesn't give more air in the cylinder. I guess that's clear to most people here, but let me put in down on paper for the removal of doubt, as I've seen the compressor intercooling argument argued from both sides, and I think there are cases where each side is right.

I'm going to argue 'positive displacement' compressors fix the air density, and that the mburgess remark applies basically to these and not to turbochargers.

If you cool 1 kg of air in a sealed metal box the density doesn't change (same mass and volume), but the pressure drops. If you cool the same 1 kg of air in the same box with the lid off, the air contracts to a smaller volume, more air is sucked in, and the pressure will still be the same, (atmospheric); more air in the same volume box, the density has increased.

Now [figures for illustrative purposes only] let's say we have a 1.5:1 positive displacement supercharger and a 1 litre car engine. 1.5 litres of ambient air at 1.2 grams per litre (figures for 20°C from CCycle posting) is 1.8g and so the density is 1.8 g/l after compression.
Intercool the air its still going to be 1.8 g/l. This is the air in a sealed box scenario. No density change, and the mburgess remark is spot on. If the temperature of the air had gone up 100 degress before intercooling due to being compressed then its pressure would have been about 4/3 times as high: 1.5 * 4/3 = 2 atmospheres, before being intercooled back down to near 1.5 atmospheres.

Now if we take a forced induction system which isn't positive displacement - for the sake of argument an electric powered compressor completely independent of the engine designed to produce a constant 2 atmospheres output pressure (similar figure to the above example) - when we intercool the air, the pressure tries to drop and suck more air in. The compressor will then work harder to maintain the 2 atmospheres pressure. The end result would be cool air at around 2 atmospheres pressure. This would be at a density of around 2.4 g/l.

==

So we can take two compressors which without intercooling give 2 atmospheres pressure.

With intercooling added, the positive displacement pump gives the same amount of air into the engine - same density - just colder and less likely to pre-ignite.
With intercooling added, the pressure regulated compressor pumps more air. Like the normally aspired engine, colder air means higher air density and potentially more power.

===

And the point ...

I think the turbocharger in the initial posting is not a postive displacement device, so intercooling should provide a noticable air density increase in this case.

I agree with pats remarks about thermal conductivity not mattering if things are well insulated afterwards so making things out of the most thermally conductive material available in areas of doubt and then insulating or not according to ambient conditions and testing seems to be the intelligent solution.

 

[MNRaptor]

Thanks for all the responses. I will take the advice and do some testing when the tracs open again this spring.

 

[1Bluegrass]

crystalclear, very well put.
The cooling of the air after the charger has the main benifit of being able to alow more boost and ignition timing before detonation begins and is the primary reason for doing so.
Many relate the cooling effect applied to the density and does not do what they think it does.

Kenne Bell on his website tech section will say the same thing.
To improve power, cool the air before the blower and cool after for use of more boost and timing.
Happy holidays.