Facts about Intercooling

[Grant]

Since the role of ducting is to simply transport charge air, the actual route it takes is of little concern, other than designing it primarily for the lowest possible pressure drop and matching the air flow stream into the inlet manifold.

Question in regards to this. If the air travels further, doesn't it gain more opportunity to heat up? I would expect that a shorter path to/from the intercooler would mean that the air would stay at the lowest temperature you could expect. Happy to be corrected as my theory is based on logic rather than experience.

Grant

[turbotom]

Since the role of ducting is to simply transport charge air, the actual route it takes is of little concern, other than designing it primarily for the lowest possible pressure drop and matching the air flow stream into the inlet manifold.

Question in regards to this. If the air travels further, doesn't it gain more opportunity to heat up? I would expect that a shorter path to/from the intercooler would mean that the air would stay at the lowest temperature you could expect. Happy to be corrected as my theory is based on logic rather than experience.

Grant

Theoretically you are correct, but in practice you need to look at the time the charged air spends in the inlet tract to get an idea of how much actual temperature gain there will be due to the inlet tract going over the top of the motor.

Lets look at the volume of air consumed by the engine first.

A normally aspirated 4 litre engine, will consume 4 litres in a complete 4-stroke cycle (2 complete revolutions)

In simple terms, by using a turbo (or a supercharger) you increase the effective displacement of the motor by a factor of the boost pressure.

Given standard atmospheric pressure is 14.7 pounds per square inch, a turbo boost of 5 psi will result in an increase in effective displacement by 5/14.7 * 4 = 1.36 litres.

So at 5psi, our 4 litre XR6 T engine, is similar to a 5.36 displacement NA motor.

At a boost pressure of 11 psi, which is I believe typical in the APS phase 2 and Nizpro Cobra systems, the effective displacement is a whopping 6.99 litres.

note this is equivalent to a 429 cubic inch engine. No wonder they go so hard

getting back to the air in the inlet tract......

I don't know the actual dimensions and volume of the inlet tract, but will use a figure of 15 litres as an example. If someone knows the exact value, they can post it and substitute the value in the following calcs.

The volume I am using, is of the inlet tract after the exit from the intercooler, and into the inlet manifold.

At 5000 rpm we have

1) 5000 / 60 = 83.3 revolutions / second.

With 11 psi of boost, we have an effective displacement of 6.99 litres.

It takes two complete revolutions for an entire 4-stroke cycle, so the motor will consume half of that effective displacement every revolution....

2) 6.99 / 2 = 3.496 litres per revoluton.

3) so from results of 1) and 2) above, the engine will consume 83.3 * 3.496 = 291 litres every second.

Now, if the inlet tract volume is 15 litres, then the air travelling from the exit of the intercooler to the inlet manifold will take

4) 15/291 = 51.5 milliseconds to traverse the pipe.

To determine the heating effect of the engine bay on the air within the inlet tract then becomes a thermal dynamic exercise, but in simple terms, the shorter the time period that the air is exposed, the less the effect.

So theoretically Grant, you are absolutely correct.

But How much of an increase will occur when the air is exposed to an elevated temperature for approx. 50 milliseconds?

As a footnote.... If you can run 15 psi in an XR6T engine, its the equivalent to a 496 cubic inch engine

I love turbos!

before I get flamed by all the techos out there.... the figures I have used are approximations, and the idea is to give a quantified image of the time it takes for the air to get to the engine from the intercooler.

Actual values of ambient pressure due to altitude and environment and the volume of air due to temperature will also affect the actual flow rates, but as an overall indicator I think this might explain the reason that the location of the pipe is NOT as significant as it might appear.

cheers

tom

Edited 26/07/04 08:02 AM by turbotom

[aps]

Under bonnet heat, indeed high heat anywhere in a motor vehicle is an issue that bugs all of us. The issue however is sometimes elevated to legendary status primarily by those with a top mount intercooler such as that in a standard WRX. In those cases, it can be a problem because the intercooler itself is in a high temperature environment – and has a high thermal inertia – ie once hot it takes a while for it to cool.

In simple terms, an intercooler has a high surface to volume ratio. That is, much of the air passing through it has good contact with passage walls – after all it is designed to transfer heat.

Ducting on the other hand has a very small surface to volume ratio when compared to an intercooler core because a very small fraction of the air passing through a duct has contact with the duct surface. This makes a duct of the diameters we find in an XR6T a very poor temperature transfer device.

Further to this, an intercooler passage (at least that on a good intercooler) has a good deal of internal finning (since a picture is so much better than words, there’s a detail shot of internal finning about half way down http://www.aps-350z.com/350z/intercooler/intercooler.htm). The idea is to vastly improve the contact surface area with air passing inside the internal passages to further improve heat transfer. A duct has no such finning, again making it an even less capable heat transfer device.

Now, looking at the ducting on an XR6T, the diameter is large (small SA/VOL ratio) and the gas speed is fairly high relative to the small capacity Japanese turbo engines that most have experience with. This means that even at idle, the air through an intercooler duct spends very little time in the duct before being consumed by the engine. This further reduces the total amount of heat transferred to the air inside the duct because it just slips through. At idle, any temperature that is transferred to the air inside the whole length of intercooler ducting would be measured in small fractions of a degree (indeed you would be hard pressed to measure it with high quality temperature probes). At any engine speed above idle, you wouldn’t have a hope of measuring the difference.

So whilst common sense dictates that heat transfer through a duct should be an issue, in an XR6T it is so insignificant (are there degrees of insignificance? that one wonders why we spent so much brain power thinking about it.

The real issues then are on a macroscopic scale that involves the whole system. Put simply:

- How cold is the air entering the manifold?

- What pressure drop is there from the turbocharger to the inlet manifold - total?

But this discussion is probably left for another time because I can hear my wife inviting me to sit down with her for a platter of warm mezzethes and a glass of lusciously chilled ouzo. Oh, and while we’re at it in the later discussion, debunk Brian’s furphy regarding the relationship of core thickness to engine coolant temp.

George

APS Web Team … and other stuff

[aps]

Actual values of ambient pressure due to altitude and environment and the volume of air due to temperature will also affect the actual flow rates, but as an overall indicator I think this might explain the reason that the location of the pipe is NOT as significant as it might appear.

cheers

tom

Hi Tom, I saw your message after hitting the "add reply" button.

Firstly, spot on and secondly, even at low engine speeds and power levels (power levels are the result of air consumption rate and encompass engine volumetric efficiency), the time spent in a duct is very short indeed.

Further to this, the thermal inertia of a duct is also very small. This means that the duct has very little energy to transfer to the charge air thereby further reducing the tiny effect it may have.

George

APS Web Team ... and other stuff

[Geeseman]

thanks George and Tom

[ducatijb]

Bloody good post Tom...put it into perspective for me anyway...

[macka'sxr6t]

To my way of thinking, I have to agree with George.

The lenght of the ducting really has no bearing on air charge temp increasing, unless the air temp outside of the ducting is significantly higher than the air temp inside the piping. In which case the piping would then absorb the heat, becoming a heat conductor and increase the air charge temp.

If the air charge temp on the inside of the piping is greater than that outside it would then become like the intercooler and become a heat reducer, with the heat radiating out from the pipe duct, but because there is no significant air flow in that part of the engine bay the cooling effect would be mimimal but it would still be there.

Has anyone measured the temp of the engine between the heat shield and the pipe ducting and compared that to the air charge inside the pipe ducts to varify which category the piping falls into.

The actual amount of boost that you are running will effect the intake air charge and intercooler efficiency more than the lenght of the ducting. By increasing the boost (air compression) you are increasing the friction between the air molecules which generates the heat in the first place that's why it's more important to find an intercooler that can handle your boost.

[bcl]

Oh, and while we’re at it in the later discussion, debunk Brian’s furphy regarding the relationship of core thickness to engine coolant temp.

George

Furphy, is it.

So you’re telling me that no matter how wide (thick) the Intercooler core, and therefore how much you block air flow to the radiator, there will be no impact on engine temperature whatsoever.

I also note that on your APS web site you recommend in fine print that after fitting the APS stage 3 with bigger intercooler “For motorsport applications other than drag racing: fitment of a large capacity engine coolant radiator and engine oil cooler system”. I read this as meaning that if I do track work with this intercooler that a bigger radiator is recommended.

Just for your information your web site claims “APS shoehorned the largest intercooler core possible into the XR6 Turbo”. Your core is 89mm. I have seen a 100mm core fitted, without any bar cutting required, as I was looking at that as a future option.

Brian

[Guest jteale]

So you’re telling me that no matter how wide (thick) the Intercooler core, and therefore how much you block air flow to the radiator, there will be no impact on engine temperature whatsoever.

I have been wondering the same thing, the APS IC (or any vendors larger IC) does seem to block more of the cars radiator, would this not affect engine cooling?

Just like more info, Thanks...

[geea]

How does a larger intercooler affect throttle response. Does there come a point when the sizeof the intercooler would affect throttle response enough to make the car slower in certain circumstances?

Geea.