Vehicle speed effect in IAT

[kmh001]

Was gonna post this in nefarious' intake thread but it's too far off that topic.

I discovered something interesting while testing the prototype TMIC and monitoring the various temps available on the Dashhawk: Using an SRI the IAT naturally decreases as vehicle speed increases, but only up to a point. At around 80kmh the IAT began to increase pretty much every time.

Maybe there is an air-damming effect going on where at a certain speed the volume of air entering the engine bay exceeds the outflow capacity resulting in hot air from the radiator core/TMIC/engine pooling and eddying around in the engine bay.

It would be interesting to find out if an MPS3 running an SRI gets the same result.

Time to do some aerodynamic testing, I love engineering challenges like this.

[ted]

kmh001-spot on! Extracting hot air from engine bays is quite an art, and often forgotten. As speed gets up, the volume of air in becomes too much for it to extract. I actually had to lift the rear of my bonnet on the old MX5 up a bit to get some air out as over 170km/h it would start to lift the middle of the bonnet otherwise. And on that car, a 3" hole was cut into the back of the plenum and fed air to the intake from the base of the windscreen. So as a simple test, you could stick a couple of extra washers on the rear hinges on the bonnet, or pull out the rubber strip at the back.

And because of the high pressure and negative pressure areas there, its quite interesting to see the difference between a ram flow style setup and one that is letting the engine suck it in from a good pool of cold air.

[Nirvandan]

Thats not good...

[LYNCHY]

Wow crazy......Side vents on bonnet?

[kmh001]

I aim to find out Lynchy. As Ted alluded to, there is a high pressure area at the base of the windscreen which would normally cause air to flow into the engine bay, rather than let it out.

Fluids like air can do some counter-intuitive things like sudden flow-reversal at certain speeds. For example I tested the Mitsubishi V6 inlet manifold on my crude home-made flow bench and discovered that at a certain flow rate air began to be sucked out of no1 cylinder due to the inertia of the flow to the other 5 cylinders. So it wasn't the physical size of that manifold that limited how much power could be made, it was the internal flow characteristics that were self-limiting.

So something like a duct or whatever that may look like a solution at one speed, can become a problem at another speed if it isn't thoroughly tested across the full operating envelope.

Like I said, this will be a fun challenge.

[shakespeare]

I'm looking forward to this

With my CAI the temp decreases then stabilises as the vehicle speed increases, I will monitor this over the next few weeks.

[ted]

Actually Mal, it occured to me that you were talking at some point about a new TMIC. I take it this one is a fair bit larger etc. Does it utilise the same plastic top/shroud? I wonder if you get the same effect if it was using the plastic top as from memory that thing had some bits that may have been acting as a bit of a shield or diverter for the air that comes through the intercooler. Could be talking crap here, just going on memory of what that plastic thing looked like.

And unlike non TMIC cars, these things have a volume of now hot air which needs to escape, but the direction of flow is top downwards rather than front to back. And as speed increases, the pressure under the car may be a bit different with the volume of air that comes off the wheels etc.

Vents in the front guard used to be fantastic way of clearing hot air but that probably doesn't apply to cars of today where everything is so compact and covered.

[mpsgarage]

Actually Mal, it occured to me that you were talking at some point about a new TMIC. I take it this one is a fair bit larger etc. Does it utilise the same plastic top/shroud? I wonder if you get the same effect if it was using the plastic top as from memory that thing had some bits that may have been acting as a bit of a shield or diverter for the air that comes through the intercooler. Could be talking crap here, just going on memory of what that plastic thing looked like.

And unlike non TMIC cars, these things have a volume of now hot air which needs to escape, but the direction of flow is top downwards rather than front to back. And as speed increases, the pressure under the car may be a bit different with the volume of air that comes off the wheels etc.

Vents in the front guard used to be fantastic way of clearing hot air but that probably doesn't apply to cars of today where everything is so compact and covered.

Some one is closer then what they thing. As we had found somethings out when the test car was in a tunnel some months back. so it was back to the drawing board for some products.
Wish I could buy a wind tunnel, Gee I had some fun.

[kmh001]

Actually Mal, it occured to me that you were talking at some point about a new TMIC. I take it this one is a fair bit larger etc. Does it utilise the same plastic top/shroud? I wonder if you get the same effect if it was using the plastic top as from memory that thing had some bits that may have been acting as a bit of a shield or diverter for the air that comes through the intercooler. Could be talking crap here, just going on memory of what that plastic thing looked like.

And unlike non TMIC cars, these things have a volume of now hot air which needs to escape, but the direction of flow is top downwards rather than front to back. And as speed increases, the pressure under the car may be a bit different with the volume of air that comes off the wheels etc.

Vents in the front guard used to be fantastic way of clearing hot air but that probably doesn't apply to cars of today where everything is so compact and covered.

That's correct Ted, I wasn't referring to the TMIC specifically, only that I noticed the IAT issue while testing the prototype intercooler.

On that note, I recommended to Hyperflow that we go with a thinner core to increase the size of the pathway for air exiting the core, because if it can't get out it wont go in no matter how big the duct is.

Now I'm wondering if we could find a reliable low pressure area somewhere under the car and run a duct to draw air out of the TMIC?

[rd415]

Not sure if I am correct, but I believe that there is a low pressure area at the bottom of the windscreen. Back to my old days playing with A9X and L34 Torana's, the scoop was reversed to draw air from this low pressure area.

Performance was enhanced when sealing the engine bay completely to the bonnet and only allowing air from in front of the screen to be drawn into the carby.

I also believe that the aeroplane wing shape over the top of the bonnet causes the low pressure area, so it didn't work on vehicles with flat bonnets hence the shape of the A9X bonnet scoop, to force the air to speed up over the scoop.

[mpsgarage]

when did a Torana work in the first place!

[rd415]

when did a Torana work in the first place!

OH! That's not nice.

Good car back in their day - dam easy to work on

[mpsgarage]

so was a mini, But you still can't drive a mini in the rain & remember it came from the land of rain. Every one stuff's it up But Holden are just good at make below average cars.

[rd415]

I'm showing my age, love mini's too - mate of mine has a mini that until recently held the lap record at wakefield - mind you it has a 4age motor grafted onto a mini gearbox - engineering marvel - think about it - welding the end of a mini crank onto the Toyota crank, morse chain instead of gear drive to box - WOW.

Just to think I thought you were good guys and was just about to order an exhaust for my MPS6

[mpsgarage]

I started my racing in a mini,
Now I look back that was a crazy car; carb inside the dash. wheels were 10X10 & the steering wheel was like a pencil.
But it was faster around the track then most of the so call muscle cars when my dad was behind the wheel.

[kmh001]

Not sure if I am correct, but I believe that there is a low pressure area at the bottom of the windscreen. Back to my old days playing with A9X and L34 Torana's, the scoop was reversed to draw air from this low pressure area.

Performance was enhanced when sealing the engine bay completely to the bonnet and only allowing air from in front of the screen to be drawn into the carby.

I also believe that the aeroplane wing shape over the top of the bonnet causes the low pressure area, so it didn't work on vehicles with flat bonnets hence the shape of the A9X bonnet scoop, to force the air to speed up over the scoop.

That's correct Russel, except as I mentioned above, it's a high pressure area at the base of the windscreen. The reason for the high pressure area is due to stagnant flow, which results in higher static pressure than fast flowing air. The A9X bonnet scoop, similarly to the VL Group A bonnet scoop, was aimed at the high pressure area and the pressure gradient between there and the inlet manifold results in lots more air being rammed into the engine intake than would be the case with a engine that has to waste energy sucking it in.

This is also why the cabin ventilation air inlet is placed at the base of the windscreen on almost all cars. With a bit of speed, air will flow into the cabin due to the high pressure even without the fan being turned on. Think if it this way, if you had low pressure at the base of the windscreen, the flow would be reversed and the air inside the cabin would be sucked into the vents and would flow up over the outside of the windscreen.

[rd415]

Thanks Mal - still a bit confused though.

You cannot blow air down the throat of a carby without the exact same pressure being at the fuel level in the float. Blow down a carby and the engine will stop, too much air not enough fuel too far away from stoichiometric. Carburettored cars rely solely on the venturi effect and atmospheric pressure in the float chamber to operate.

Turbo carby cars mainly draw through for that reason, Blow through turbo have the whole carby in a sealed box (very hard to achieve).

So you can see where I am confused. I still believe that it is a low pressure area - regarding the air vent you mention, it is at the lower pressure so cool air can be drawn in at the low pressure air using the vaccuum effect from the rear vents.

[2XS]

I've used a basic wind tunnel setup.

I've stuck bits of wool (5cm long, stuck with masking tape) to various parts of the car and driven at speed down the road. You can then see what direction the wool is blown. Certain areas change direction at different speeds.

As commented, the base of the windscreen is certainly a sight to see. For other parts of the car I used a small digital video camera. It's good for the underside of the car too.

[kmh001]

That's not as basic as you think Troy. Major aircraft manufacturers still use tuft testing because there's nothing like real world conditions for collecting real world data.

Russell I'll try to answer your question by breaking it down, but if you look up Bernoulli it may also help. His principle published in Hydrodynamica (1738) explains a lot, including how a spray gun, venturi and wing work.

1. Static pressure decreases with increasing airflow speed. Here's an extract from one of my pilot text books that depicts it rather well.



2. The area at the base of the windscreen is a high pressure area due to flow stagnation at the junction of the bonnet and windscreen, which slows down the airflow so the static pressure goes up. This diagram shows the air pressure around the body of a car. Blue is low pressure, red is high.



3. Air will always flow from a high pressure area to a low pressure area, due to the pressure gradient. A big enough pressure gradient can lift a 400 tonne aircraft into the air, it can also cause a flow-reversal, even at high speed. Air can even flow backwards over the surface of a wing travelling at 1000 kmh due to a pressure gradient.

4. The consumption of air by the engine will create a low pressure area around the intake. Therefore air will flow toward the intake from areas of higher pressure.

5. Ducting the intake to the high pressure area, as the A9X does, will provide significantly more airflow to the intake and the air will easily travel forward down the duct (no matter how fast the car is going) because it has already slowed down in the stagnation area.

[rd415]

Okay - I understand most of what you say, particularly Beroulli's law as it was a subject I taught to students to get them to understand about pressure differences. Used an aeroplane wing to explain how a carby venturi works and rigged up a venturi using a biro case/ plastic tube and air to show atomisation - also sealed the container of liquid to demostrate how without air pressure on the surface of the liquid there is no venturi effect.

What I can't quite grasp is why it is a high pressure area at the bottom of the screen because the faster the air passes the screen with more likely glancing blows lowering the static pressure therefore the lower the pressure so it's not a high pressure area.