aero-mods,installment#9(internal drag)

[MechEngVT]

vtec-e:

Engines typically reject equal amounts of energy through the drivetrain, exhaust, and cooling system. This is pretty typical of the "30%" efficient internal combustion engine. That means that if you are utilizing 80 kW to maintain speed or to accelerate your vehicle, at that time you must be rejecting 80 kW of energy through cooling or your engine will heat up until you require less power.

I don't think this will work, because you will be rejecting heat to air that will then circulate through your engine. This will return a substantial portion of the heat to the engine block, plus hotter air is less dense and will greatly diminish power, not to mention the decreased volumetric efficiency created by pulling air through a heat exchanger.

More gains are to be had by utilizing advanced lightweight radiator designs, appropriate ducting, electric water pumps and fans, and you'll be less likely to blow up your engine.

[vtec-e]

Thanks MechEngVT. I kind of figured it might be a bit much but didn't have the figures to test the theory. Ah well, back to the drawing board...

ollie

[jamesqf]

Originally Posted by vtec-e

Hypermiling doesn't work an engine hard does it? So remove the radiator and install a custom built (tubular?) heat exchanger...

Problem is, you have to design the system to handle the worst case, which might be for instance climbing over a Sierra pass on a hot summer day - something I do quite often, as it happens.

I drive an Insight, and have a ScanGauge which is usually monitoring coolant temp. (I run a partial radiator block most of the year, taking it out for climbs in spring & fall.) On level ground it will maintain its 195F thermostat temperature, unless it's pretty cold, so you could get away with a size reduction akin to my block. On a long climb, though, temps can go to 208-210F. With a smaller radiator, they'd go even higher.

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Something I've wondered about is whether the heated air could be exhausted at strategic locations in order to reduce drag. It seems you'd have a mild ramjet effect, since the air going through the radiator is heated, and so expands. Some of the diagrams seem to be taking advantage of this, but is there anything more explicit?

[MechEngVT]

Originally Posted by jamesqf

Something I've wondered about is whether the heated air could be exhausted at strategic locations in order to reduce drag. It seems you'd have a mild ramjet effect, since the air going through the radiator is heated, and so expands. Some of the diagrams seem to be taking advantage of this, but is there anything more explicit?

Yes, and although it doesn't look like it in the diagram that's exactly what setups B and C are attempting to do. In diagram B, it's showing the ducted expansion air exhausting around the front tires which can disrupt the turbulence that exists without a wheel skirt.

In setup C the idea is to take high pressure air from in front of the air dam, duct it through the radiator, and duct the warmed air out through hood-top vents into the low pressure area that typically develops above a car's hood. In a lot of racing applications the nose is blunt with a flat hood (stock cars, touring cars, side pods on open wheel) rather like a lot of production vehicles. This generates a low-pressure zone on top of the hood and causes frontal lift. Extending front air dams and installing chin splitters helps counter act this by reducing air flow underneath the car, but as shown in diagram C ducting the stall pressure air from the nose through the cooling system and injecting it into the low pressure zone will equalize the pressure. I'm convinced this is the most effective method. I've seen it work in a small-scale open wheel bike-engined car where designs A and D had caused repeated overheating issues, a switch to design C in a side pod with a smaller radiator produced reliable driving for hours on end. Granted this was in a vehicle that never saw above 50mph for more than 2-3 seconds so aero was not much of a factor the cooling performance worked very well.

[holypaulie]

Originally Posted by digitaldissent

maybe this will help illustrate things a bit

(I will try and get option B setup on my car)

I'll go with option C and I'm thinking to make hood vent like this guy http://www.240sx.org/links/installs/hood/hood.htm

What do you guys think about this mod?

[jamesqf]

I would guess that you'd also need an underbelly pan to close off the downward flow?

I sure wish I had a good CFD program, and some accurate numerical models of car shapes...

[holypaulie]

Originally Posted by jamesqf

I would guess that you'd also need an underbelly pan to close off the downward flow?

I sure wish I had a good CFD program, and some accurate numerical models of car shapes...

I have installed my belly pan few weeks ago. Now it's time for my hood vent mod. I don't know yet how wide should I cut opening and how much to open. I guess radiator width and an open an inch. Need some advise, please.

[holypaulie]

Anybody with advice please.

[i_am_socket]

Here's a good shot of "option c" on the Ford GT:


The intake opening on the front vs the hood exhaust opening seems to be roughly equivalent. Also seems to have a much gentler slope where it returns to the over-hood flow than that Nissan.

Once I get the time/money/space/balls to play under my hood, I'd probably go for "option B".

[Otto]

Per Hucho et al, that vent would work best if moved forward about a foot. Air incoming at the highest pressure stagnation point on the nose, then drawn out at the lowest pressure point right about at the leading edge of the hood.

This will also draw off that pesky under-hood hot air, keeping the intake manifold cooler, for better performance.

My Porsche 944 Turbo is similarly shaped, and its turbo intercooler is under a header panel between the pop-up headlights, so I plan such a vent to extract air in similar fashion.