Effect of limited airflow on under hood temps

This is a race car, so I can do what I want.

Lotus (and others) have been successful in mid and rear engine cars (in race trim) limiting the frontal area, and drawing air up through ducting through the hood and a horizontal radiator.

I am thinking this optimizes cooling, limits frontal area, and pulls air over the car (creating down force) which would normally go under the car and create turbulence.

I am considering this in a front engine car.

Yes, I have the space.

I like the idea of limiting the amount of air that flows into the "dirty" engine bay (aerodynamically dirty). Seems to me, this is a huge parachute on the front of the car that dumps an enormous amount of turbulence under the car.

But I am also concerned about the changes in under hood temps related to the convection air moving (semi) freely around the engine.

I can segregate (or at least limit) exhaust and turbo heat sources.

I am confidant the efficiency of the water cooling will be much better, which should improve the water cooling system's ability to cool the engine.

But I am curious how much cooling capacity is contributed to the average engine via the convective air moving through the engine bay.

If I am essentially asking the engine to operate in a semi-sealed area with limited air flow, am I creating a disaster?

Discuss...

So you're planning on having as much air flow as possible through the radiator, but sealing off the engine bay somewhat? Sounds like a Boxster. You'll want to keep heat-sensitive items such as coils and brake hydraulics cool if possible.

You were right in the downforce idea, though. Moving air from the high pressure nose to the low pressure area on top of the hood will give you more front downforce, even if there's aerodynamic dirt underneath said hood.

In reply to Keith Tanner:

Yes. That's my plan.

Good point on the coils and brake hydraulics. Not hard to accomplish.

You can get away with very little to no air moving over the engine itself, this has very little effect on cooling. It's all about the heat exchangers.

The only things you have to watch out for when reducing airflow to the engine are the heat buildup causing plastic bits to melt, and the exhaust itself getting too hot (happens in F1, unlikely to be a real problem in most cases).

Seems like a lot of racing vette's have done this.

You still need to vent the engine- unless you are using materials that are robust to the convected heat coming off of the engine. Even if you block off the radiative heat from the exhaust, there's still a lot of heat there.

A side story related to engine heat.....

So Lotus has this great bonded aluminum car call the elise. Great idea- no real investment in some specialized weling equipment, you can bond metal to other things- bla bla bla. Nobody here can argue that it does not work charms in the Elise, right? It's main compromise that it's has an absolute limit of 150C. Go over that, and it fails. 302F for those who don't like SI.

The way the car is designed, the big thig that generates the heat is behind all of that. All of the stuff that passes up through channels and goes to the radiator in the front of the car is less than 250F (I know of no cooling system that will actually operate over that temp). And there is no bonded structure behind the rear bulk head. Which is also protected with insulation (in all 3 forms of heat).

So it will always work great.

Since it was so cool, so advanced, Aston Martin and it's overlords at Ford thought it would be a good idea to contract Lotus to design the DB9 chassis. However, it was not until the contract was mostly being done did anyone figure out that the engine can easily produce heat that is well over 150C/300F, and direct that onto a lot of metal.

Oops.

If you go an look at that car, you'll see a TON of vents, some of them are chimneys that limit the heat that the exhaust can put into the chassis.

(that was the project that I learned that Lotus was not all that great. Burst another bubble since I had learned in the previous couple of years the same thing about Cosworth)

anyway, just a story. Good idea, make sure you do some venting for underhood.

For inspiration for this project, check out the designs of the recent Panoz prototype- they tried really hard making a front engine racer. Worked ok.

This is the car- the Panoz LMP1

alfadriver wrote: (that was the project that I learned that Lotus was not all that great. Burst another bubble since I had learned in the previous couple of years the same thing about Cosworth)

Hey nobody's perfect. You know what they say about glass houses, Mr. Ford Employee

GameboyRMH wrote:

alfadriver wrote: (that was the project that I learned that Lotus was not all that great. Burst another bubble since I had learned in the previous couple of years the same thing about Cosworth)

Hey nobody's perfect. You know what they say about glass houses, Mr. Ford Employee

If you saw their original exhaust package, you would wonder, too. I was stunned.

You can bash on what we do all day, I'm ok with that. But I know what those two firms bring/brought. I put more stock in Roush engineers than either of them.

I like this idea. I have ever since seeing a GT40 and a Elise and realizing what was happening.

As far as front engined, as Alfa points out, the Vettes do it and it obviously is working very well.

If you have the room and the rules to do it, I think it's brilliant. I'd be VERY curious to see if there is a pure aerodynamic benefit.

exhaust manifolds are the only thing I can think of that would had massive heat underhood if the radiator is sealed off. (NA application.) Make sure you have enough flow for that and have at it. (electrics/hydraulics concerns have already been mentioned)

On the CHAMP cars we used what we called a header box to contain/control radiant and convective heat from the primary pipes. Basically it was a heat shield that attached to the heads at the exhaust flanges and covered all of the pipes all the way to the collectors. There was a small dedicated cool air duct to blow thru them, which helped keep the temps acceptable. Adding some small dedicated ducting to the items that need to stay cool are fairly simple and really don't need to be very big for a dedicated racecar (as long as the speeds are kept up).
Ducting the radiator flow up and over the hood does provide a pure aerodynamic benefit, but the downside is if the rad springs a leak the coolant winds up on the windshield. One benefit is potentially smaller core size and or smaller inlet size. Both reduce overall drag. The flow to the underbody is less about dumping turbulent air from the engine bay as it is using the clean flow to it full extent. The pressure under the front of the car is lower than nearly anywhere else on the car (except maybe the top/front of the hood). That gives you a tool to use in ducting air across the items you want cooled.

In reply to Flight Service:

Even if you got rid of the exhaust, you still have a lump of metal that has water in it which is about 200F. While that does not seem like a lot- that can build up a lot of heat. And being behind that large lump of heat, it will end up in the driver's compartment. 200F will burn you, eventually. Which is to say that driver comfort does also need to play in on the the whole heat rejection equation, especially for a non-sprint/autocross kind of car.

For the most basic analysis of where the energy from the fuel goes in an internal combustion engine, you can use 1/3 to the wheels, 1/3 out the tailpipe, and the last 1/3 as waste heat from the engine itself of the related cooling systems.
As AlfaDriver notes, the heat in the engine bay will try to get to the surrounding stuff (cockpit etc). Heat shields and dedicated cooling air are the next steps.

Don't 4th gen f bodies do something similar? My '94 Z28 had ducts under the nose that pulled air up and through the radiator, which then either exited under the car or up past the cowl, since there was no tray under the front.

Is this on an SVX? (based on your name). Those already have somewhat limited cooling and really high underhood temps, in part due to the lack of grille, the width of the engine, and the low hoodline. I'd hesitate to reduce airflow any more than it already is.

In reply to gearheadE30:

I think that cutting some clever vents in the hood would -increase- the flow.

If you go to YouTube and watch a bunch of Roadkill Videos, you will note that the hood seems to come off as a universal overheating fix. So, there must be something to it!

Almost forgot, my parents are Corvette folk. If you pop the reverse-opening hood just to the latch, the water temperature drops a very large amount. It does work. In that vein, the C2 design was originally supposed to have hood vents on eitehr side near the windshield. As the story goes, the designers moved them to the leading edge and made them non-functional.

What got sold:

What got raced:

What was designed:

Now, I've never had my parents '67 up fast enough to know, but reportedly the front end gets very light as a result of this screw-up.

Peter Brock can probably tell you more for certain, I have a hunch he may know something about it.

The Superformance coupes added the 4 holes to the top while the originals either had no vents or a small cutout of the back side of the center bulge. I don't have any vents on my FFR hood and while it does get hot under the hood nothing has been a problem so far.

alfadriver wrote: All of the stuff that passes up through channels and goes to the radiator in the front of the car is less than 250F (I know of no cooling system that will actually operate over that temp).

I'm sure you know this, but high pressure (2 bar or higher) cooling systems will operate at temperatures slightly exceeding 120C (250F). These are most common in race cars but some people don't know that BMW used 2-bar cooling system pressures in pretty much everything (including the E46) up until recently. On the non-M E46, an electronically controlled thermostat makes the engine run at a little under 120C when under light load (highway cruise) to improve fuel economy, and down to 80*C for heavy load conditions. Interesting stuff.

Don't know how hot these things run, but there is very little ambient circulation around that engine. The hood scallop seals tightly against that ducted fan shroud.

Maybe not the best example, but landspeeder engines are completely sealed off from the outside world. In place of radiators, they use big coolant reservoirs or a heat exchanger into an ice bucket.

GameboyRMH wrote: Maybe not the best example, but landspeeder engines are completely sealed off from the outside world. In place of radiators, they use big coolant reservoirs or a heat exchanger into an ice bucket.

For most of those guys, a tank of water is a big enough heat sink for the 5 miles that they are driven. They all use some kind of heat exchanger, too- replacing the radiator. That way, the engine can run hot, and the heat sink tank barely needs to warm up.

But the packaging. Holy cow those guys are creative. Love looking at those cars. Huge engine(s) in tiny spot(s).

In reply to gearheadE30:

No, it's not an SVX. I know how to control temps in those!

The SVreX was my first Challenge car. I sold it 10 years ago, but the name stuck.