Can we determine the limit of efficiency for the AWIC?

ciper · Post by **ciper** » Sat Jan 24, 2004 3:36 am

I would assume a calculation could be done if we knew the amount of surface area inside the intercooler, the amount of air flowing through it and the temperature of that air.

We should assume an infinite supply of cool water running at 10gpm, since we only care about the AWIC and not the attached system.

Any "thermohydrolic" engineers on the board?

RSSTIBOY said that around 300HP the Subaru unit becomes unable to keep up, wouldnt the number depend on the temperature (turbo efficiency) of the outlet air?

Id like to know so that I dont end up using an undersized heat exchanger on my car.

vrg3 · Post by **vrg3** » Sat Jan 24, 2004 7:31 am

I don't have any answers, so I'll just say what comes to mind...

ciper wrote:I would assume a calculation could be done if we knew the amount of surface area inside the intercooler, the amount of air flowing through it and the temperature of that air.

I think it would be a lot more complicated than it initially seems. For one thing, we'd have to know the geometry of the heat exchanger so we could account for boundary layers and such. Seems to me that it would involve a finite element analysis. Let's also not forget to take the pressure drop into account when evaluating a heat exchanger.

We should assume an infinite supply of cool water running at 10gpm, since we only care about the AWIC and not the attached system.

Is there something special about 10 gallons per minute?

How would we define "cool?" Is it reasonable to use ambient temperature?

RSSTIBOY said that around 300HP the Subaru unit becomes unable to keep up, wouldnt the number depend on the temperature (turbo efficiency) of the outlet air?

I think it would have to. He must have been thinking of some particular compressor when he wrote that.

Id like to know so that I dont end up using an undersized heat exchanger on my car.

It seems to me that as long as the pressure drop is manageable you could always bump up the capacity of the pump and reservoir. I guess we want to know whether the intercooler or the rest of the system is the limiting factor. For some reason, my gut says it's the latter.

rsstiboy · Post by **rsstiboy** » Sat Jan 24, 2004 9:06 am

yeah, that horsepower is based on the factory VF12 turbo (which is a 'tad' smaller than the TD05)

scottzg · Post by **scottzg** » Sat Jan 24, 2004 9:58 am

with this whole awic thing, i think y'all are forgetting a major aspect; the air moving through the intercooler is moving at a far greater rate than the water. What i mean to say is, larger gains would be seen by doubling the size of the intercooler than by doubling the size of the radiator. I can't demonstrate this mathematically, im not good at that. Much of the turbo lag is caused by having to pressurize the piping, so increasing the size of the awic would not harm the lag so much.

To sum up what im saying: the awic setup may be good for 300hp, 320 with a larger radiator, but with a larger ic, it would be good for much more, and not harm the lag as much as a fmic would.

sorry i cant be more concise, im a generalist.

vrg3 · Post by **vrg3** » Sat Jan 24, 2004 4:31 pm

scottzg wrote:the air moving through the intercooler is moving at a far greater rate than the water.

Well, can't you pump the water through as fast as you want?

And fast-moving air is supposed to be one of the advantages of air/water intercoolers over air/air intercoolers, right?

scottzg · Post by **scottzg** » Sat Jan 24, 2004 6:58 pm

vrg3 wrote: Well, can't you pump the water through as fast as you want?

yea, but pumping the water through faster means its getting cycled through the radiator faster too. And it seems logical to me that the superheated air will cool more with more time in the cooler than with a cooler that is a fraction cooler. I dont know why though.

And fast-moving air is supposed to be one of the advantages of air/water intercoolers over air/air intercoolers, right?

Wouldn't it have to be moving at the same rate to produce the same power?

rsstiboy · Post by **rsstiboy** » Sat Jan 24, 2004 11:45 pm

you have to increase the size of both for it to work properley, then get bigger pipes to connect the two and get a bigger water pump, it wasn't financially viable to do it, I could get more HP from using a FMIC for the money it was going to cost me to upgrade the AWIC. hence the choice I made.

carfreak85 · Post by **carfreak85** » Sun Jan 25, 2004 2:45 am

Well if you want to make lots of HP real quick (like at the races)..... just use colder water..... Ice water is our friend!

rsstiboy · Post by **rsstiboy** » Sun Jan 25, 2004 11:56 am

unfortunately its not quite that easy, cold water soon gets hot just like normal temperature water, for drags yes, for track work or street no.

evolutionmovement · Post by **evolutionmovement** » Sun Jan 25, 2004 7:31 pm

If you could somehow route an A/C duct to a sleeve around one of the pipes... Or have a way of switching it so you don't need to run it in the interior at the same time. Ford has a patent on an A/C cooled AWIC, I just need to go dig it up and I'm lazy. Or you could just live up here right now - my car's cooling system won't even heat up past the 1/4 mark unless I'm on the highway for a while with the heat on low. And no, there's nothing wrong with the thermostat. The car's fine when it's over 10F.

Steve

ciper · Post by **ciper** » Sun Jan 25, 2004 7:55 pm

The AC design is true, it is on the new lightning. A light on the dash tells you when the intercooler is "charged" and then you have fun for a while until it needs to "charge" again.

I said 10gpm because its easy to get pumps near this range and you dont want the water to move too fast or the amount of heat it removes goes down.

I think ScottZG has it right. The reason I want to use the Liberty AWIC is that its cheap (relative), it fits perfectly and it looks stock.

Isnt the VF12 far outside its optimum range at 300 HP? I dont memorize the turbo specs but it seems to me that with a larger turbo 300 wouldnt be the upper limit.

"How would we define "cool?" Is it reasonable to use ambient temperature?" Id say cool is keeping the water at 100 degrees? I swear I remember reading that outlet temperatures on Air-Air systems are way above that.

rsstiboy · Post by **rsstiboy** » Mon Jan 26, 2004 12:18 am

its not that far out of its efficiency range, but does create enough heat to charge the liquid in the cooling system in a few hard boost sessions.

Post by **Legacy777** » Mon Jan 26, 2004 3:36 pm

I can't promise I have answers, but I'll take a look at what I can find in my thermo and heat transfer books if I get time tonight.

NemesisEJ22t · Post by **NemesisEJ22t** » Mon Feb 09, 2004 6:44 pm

The best way to find this out would be to take 2 temp sensors and measure air temps before and after the intercooler at different pressures. You could then compare those numbers to what you should get with the ideal gas laws for air (PV=RT). That would give you a rough estimate of how much cooler the air is than it would be if there was no intercooler. I think this is right, i have had only 4 weeks of thermo so far though, so don't believe me yet.

Speeder · Post by **Speeder** » Tue Mar 09, 2004 6:06 am

I've been hunting around for a used air water intercooler, they are very rare. If any of you fellows want to sell one please let me know.

There are formulas available but I'm sure the Subaru engineers already selected ideal core dimensions for the length of air and water passage. At some point an increase can be negative because of either pressure drop to the air or heat soaking of the water. However, adding length by increasing quantity of passages is a safe bet. They also probably made a careful selection on the turbulator (little fin) selection to eliminate laminar flow and increase heat transfer without undue drag.

There are also careful calculations they probably made to proper dwell time of water through the core (which is why we need thermostats on radiators). Faster flow isn't better.

So what's left (aside from ice water, larger radiator, etc.)? Try using a water mist on the radiator side to utilize evaporative cooling.

puckaveli · Post by **puckaveli** » Tue Mar 09, 2004 6:44 pm

Just so you guys know the stock Legacy W/A IC flows more air than any stock top mount Air/Air with the inlet pipe Subaru has ever made (except one). That is with the inlet pipe. Every top mount without the inlet pipe will flow over 250cfm put with an inlet pipe they all flow under 200cfm. Only the Ver2 STi IC flows over 200cfm. The W/A IC has a larger and shorter inlet pipe so it will give you better efficiency and higher flow than any air-air IC. The legacy w-a IC core size is a tad smaller than a Ver2 IC which is 2853cm2.

The water-air IC will always be able to give you more than 100% efficiency and air-air can only do 100% (rarely) since it can't lower intake temps below outside air temps. You can push as much as air as you want to through an IC but if it doesn't reduce the air temp it's pointless. Until you get into real high boost numbers flow will not be an issue, temp will be. I think find a way to keep the water cold will be the key to making the water-air IC work well. It already flows more than any stock air-air you just need to keep it cool.

LegacyT · Post by **LegacyT** » Wed Mar 10, 2004 1:40 am

The water-air IC will always be able to give you more than 100% efficiency and air-air can only do 100% (rarely) since it can't lower intake temps below outside air temps

No intercooler can give you 100 percent efficiency. An air water intercooler is basically a system with a radiator. The coolant in an AWIC can only, and thats at 100 percent efficiency be cooled to ambient, however this will never happen. The coolant, it can now only cool the intake air, at 100 percent efficiency, to the tempeture of the itself, which is already above ambient. again 100 % efficency will never happen in real life. So an AWIC just like any air-air IC will never give you 100% efficiency when compared to whats cooling it, ie ambient air.

Mark,

ciper · Post by **ciper** » Wed Mar 10, 2004 1:43 am

You can cool the AWIC fluid below ambient, and acheive greater than 100%

Just look at the system on the F150

LegacyT · Post by **LegacyT** » Wed Mar 10, 2004 2:06 am

Ahh but thats a whole different realm of intercooling using a refigerant, not ambient air as a cooling source. And even then the coolant temp cant reach the same temp as refrigerant thus its cannot be 100% efficent. Mankind has not yet devised any system which is 100 percent efficent it simply can't happen.

Mark,

ciper · Post by **ciper** » Wed Mar 10, 2004 2:10 am

It is more than 100% because the normal cooling method is still outside air:

the front heat exchanger for the fluid is cooled by outside air
the front heat exchanger for the AC is coolded by outside air

vrg3 · Post by **vrg3** » Wed Mar 10, 2004 2:15 am

Yup, it can temporarily exceed 100% efficiency by using active cooling.

I've seen some pretty cool pictures and descriptions of speedboats with turbocharged engines and intercoolers that sit in the water, using the ocean (or lake or whatever) as a heat sink. They can continuously cool the intake charge below ambient air temperatures.

LegacyT · Post by **LegacyT** » Wed Mar 10, 2004 3:08 am

ciper I thought you knew this, A/c is not cooled by the condenser, The condensor only liquifies the compressed refrigerant. After the expansion valve the refrigerant evaporates, and drops below ambient. your not cooling with outside air, your using the change of state of a chemical with such characteristics to lower the temperature. Even so, the f150 AWIC isn't full time. It can only do 20 seconds bursts or so by dumping stored "chilled" coolant into the system. If your using ouside air to cool an AAIC or AWIC it cannot reach ambient or 100% efficency. In the case of the F150 your not comparing the temperature of the intake air temp to outside, but intake air temp to cold refrigerant temp, since the refigerant is cooling it. And still then it will not be 100 % efficent

vrg3, yes using cold water as a heat sink will probobly lower intake temps below ambient, however you cant state that since intake temp is below ambient that the IC system is over 100% efficient, since ambient air is not the cooling factor, but the cold lake water or ocean water is. So when comparing intake temp to the cooling source ie the water, it will not be the same. the intake temp will always be higher then the cooling source temp, depending on the efficiency of the IC system, but will never be 100%

Do I sound like a broken record

Mark,

vrg3 · Post by **vrg3** » Wed Mar 10, 2004 3:14 am

It all depends on how you define efficiency. When talking about forced induction, typically intercooler efficiency is defined as the ratio (T_out-T_in) / (T_in-T_ambient).

An air conditioner does rely on ambient air to cool the condenser. The whole system relies on being able to discharge heat into passing air. The liquifying process you describe is endothermic. That's where the actual cooling comes from.

ciper · Post by **ciper** » Wed Mar 10, 2004 3:45 am

LegacyT: Dont try to sound smart and change the discussion around. When the gas is compressed the rise in temperature is drained off by the radiator in the front to outside air.

I know the AC idea isnt running all the time. Its still cooled by outside air. I see it as storing energy and using it when needed.

It all depends on how you define the efficiency. Sure we are fudging by saying the outside air temp is 80 and the cooling fins are at 60, but the argument is still valid. When you are on the specific topic of cooling air from a turbo entering an engine the calculation is performed using outside air temperature and the internal temperature.

Ill reduce my argument to this. Car audio enthusiasts add a capacitor so they can store "unused power" when its not needed and apply it to high draw peaks.
Or another idea would be how many large company install a flywheel. It can provide 60-180 seconds of power to the entire building while a generator starts up

Are you trying to convince me that this idea is somehow flawed, that instead I should add a larger battery and bigger alternator? Or in the second case that I should instead use a large UPS?

Kostamojen · Post by **Kostamojen** » Wed Mar 10, 2004 4:38 am

puckaveli wrote:Just so you guys know the stock Legacy W/A IC flows more air than any stock top mount Air/Air with the inlet pipe Subaru has ever made (except one).

You know, I think ive noticed that... Its got a very good air flow system set up thats going to be able to push a nice amount of air, probably pretty decent even fore higher rpms.

I still cant find a radiator for this damn thing though :p

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Can we determine the limit of efficiency for the AWIC?

Can we determine the limit of efficiency for the AWIC?

Re: Can we determine the limit of efficiency for the AWIC?

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