Engine Compartment Venting
 

Hi,

Currently, I am venting the engine compartment through gill slot openings in the belly pan. This is making lot of turbulence under the car, it's like having a window open. I am considering venting though the front wheel wells, where the air is already disturbed. Anybody have any thoughts or comments on engine compartment venting?

This is what has been done on the Aerocivic and on my own car with a full, front to back belly pan. The openings in the wheelwells are plenty large enough to let the air escape. Most of us on this forum already have some of the grill blocked so there is minimal cooling air going through the engine compartment. Watch your ScanGuage. I think you'll be fine.

According to Hucho, the very best arrangement is to vent into the area on top of the hood. Venting into the wheel wells and into the underneath are equal and are half as good Cd wise, according to his book.

I think that would vary quite a bit from car to car no? Wheel wells have different designs, underbodies have different designs (or lack thereof), and hoods have different angles and shapes.

Venting under the car could be marginally better if there's one of those higher diffusers.

Pull out the seal for the hood at the base of the windshield...

Venting on top would put a lot of dirty air over the car body. Im going to try venting into the wheel wells and close off the openings in the under body belly pan and shoot a tuft video again.

You may think what you like, as may I, as may any of us. I'm only reporting from a reliable handbook on the subject. The original paper is: BUCHHEIM, R., DEUTENBACH, L.-R., LUCKOFF, H.-J., "Necessity and Premises for Reducing the Aerodynamic Drag of future Passenger Cars," SAE Paper No. 810185, Society of Automotive Engineers, Warrendale, Pa., 1981.

Reference please:)

There are some ultra high performance cars that vent radiator air over the hood for better aerodynamics. One possible downside for production cars is the chance of coating your windshield with glycol while driving, if there is any sort of cooling system failure. Also, high performance cars are designed to be driven with big engines and pedal to the floor, generating much more waste heat than your average hypermiler. IMHO if you are already using some sort of grill blocking we are splitting hairs here. Try sealing the underbody and monitor your engine temperature.

]

it is sealed, read the first post!

Try reading this paper "Interference Effects of Cooling Air-Flows with External Aerodynamics" . It covers how engine compartment cooling effects overall drag.

http://tech.jsae.or.jp/ijae/download...x?pno=20114635

Edit: I think you want to keep your under tray gill slits :)

Interesting paper.

The red looks better than the blue. I don't know why the square back and notch back are so different, very strange. The blue defiantly had more air flowing, because of the bigger opening.

I don't think this paper applies completely, since my wheel wells are covered the air can not escape through the sides, but must exit under the body through the tire clearance holes in the belly pan.., my grill opening is minimal, I will be keeping most of the blue closed.

well you might consider you could be doing more harm than good. On the other hand the biggest difference it made was 0.004 in Cd over or under the base line.

I would like to keep the flow smoother under the car to try to get better pressure recovery on the boat tail bottom surface.

The question is does smother flow on the under side of the boat tail really going to get a net gain by disrupting air flow some where else?

What I think you are trying to say is that flow around the wheels is already disrupted and that a bigger disruption there won't matter. In this paper they show for both the notch back and the square back that isn't the case as drag increased when they blocked off the transmission tunnel. If this is the case for your car I can't say and I don't think any one else will be able to unless they have a wind tunnel that can measure Cd to the 3 digit after the decimal point.

My car doesn't have a transmission tunnel.

"None of our men are 'experts.' We have most unfortunately found it necessary to get rid of a man as soon as he thinks himself an expert because no one ever considers himself expert if he really knows his job. A man who knows a job sees so much more to be done than he has done, that he is always pressing forward and never gives up an instant of thought to how good and how efficient he is. Thinking always ahead, thinking always of trying to do more, brings a state of mind in which nothing is impossible. The moment one gets into the 'expert' state of mind a great number of things become impossible.” " ... Henry Ford

Over the hood. Will help reduce lift from air under the car.

Very interesting study. Slightly at odds with Buchheim, et. al., but interest anyway.

One take away was that the aero drag from cooling is linear with the amount of cooling air. Ecomodders have been proving that for a while;)

Maybe you could consider NACA vents on your belly pan, those should reduce the turbulences under the car siginificantly, comparing to grill like vents you've mentioned in a first post.

NACA vents are low drag intake vents, not meant for exhaust.

http://en.wikipedia.org/wiki/NACA_duct

I am considering streamlining the engine compartment, ducting the flow so it expands from the slit in the bumper to the radiator (diffuser) and then narrowing again to the exit and try to match the external flow velocity with minimum turbulence. I don't know if it can be done in practice, but I had something similar on an earlier saturn and that car would blow leaves out the back. I'll post progress on sc1 aeromods thread. I ordered more rivets, hopfully they will arrive today.

Check out google for "reverse NACA duct"

http://i108.photobucket.com/albums/n...E/DSC_3962.jpg

Bring back the NACA duct..... - PistonHeads

NACA can work both ways. I've seen NACA-like outlet working as an exhaust tip, but can't find a photo right now to link it. It was just build as reverse of NACA inlet. So it was wider at the very end of the outlet and it was tapering to the front and the actual exhaust pipe. I'm hoping you get the idea from my description.

If you take a look at the colored pressure planes behind the cars, I think there may be a clue. The dark blue area, the lowest pressure area, on the notchback is much smaller than on the squareback. It looks to me like the notchback configuration is just barely good enough to see reattachment of the rear flow at the slightly extended trunk surface. That would make the dead air area smaller for the notchback. Scroll down to the point that the color planes for the notchback and the squareback are both visible, one above the other, and you will see the difference. There's lots of information in this paper and it takes a while for it to sink in. ;) They rarely do because the researchers must draw the constrains narrow enough to get it done in the allocated wind tunnel time. Even here, they ran out of time. They just can't cover the general cases. Even Hucho, when he practically owned the VW wind tunnel in the 70' and 80's, had to deal with budget constraints, and production schedules, which limited his use. Takes lots of money to turn on those big fans, even when you own them:(

Sorry, but show me one on an airplane that's mounted backwards..

Its hard to believe that a notchback will suck that much more air out of the front wheel well. Maybe the notchback has lower overall drag and thus the open wheel well adds a proportionally larger amount.

Cd of notchback with out moving ground simulation = 0.286
Cd of Notchback with moving ground simulation = 0.277

Cd of SquareBack/Estate with out moving ground simulation = 0.318
Cd of SquareBack/Estate with moving ground simulation = 0.319

http://delphi.com/pdf/techpapers/2012-01-0168.pdf

Cobra daytona coupe or ford GT both with hood venting and pretty darn fast daytona coupe was reported going over 190 on the british M1 highway i think they called it and reportedly was the reason the road got a speed limit! so if you can pick up almost 30 mph with the same engine combo , the aero cant be that bad,

But wasn't this primarily due to the different configurations at the rear of the two vehicles, and the amount of wake they created. The differences due to the ventioning of cooling air were minor by comparison. Did I miss something?

According to Hucho's text reference to the research of Buchhein, et. al., the over hood venting is far and away better than any other venting. It is widely used on race cars, but isn't very practical for street cars, since it dumps lots of hot air right into the climate control intake at the windshield base. But the delta Cd is .010 better than underneath or wheelwells.

The no compromise Lotus Elise uses top venting. Not sure where they pick up their cabin air intake.

I've toyed with trying the arrangement on my "test" Insight. I think I see a way to get clean cabin air through the fenders with NACA ducts. Big project and not sure when/if it gets done.

Book
 

If you can locate a copy of Paul Van Valkenburgh's book,Race Car Engineering. He has a section on measuring pressures on the outside of a cars body which would be invaluable,short of a wind tunnel,for siting cooling system exits.
For a 'clean-sheet' race car design you could slect one of the off-the-shelf designs depicted by Hucho or Korff.For a production vehicle you're going to be challenged because of the existing underhood architecture.
Professor Alberto Morelli probably spent millions of dollars perfecting the outlets on the CNR 'banana' car of 1978.
You could not have chosen a more difficult project to take on.
This is truly a case-specific-basis only type proposition.With the variety of vehicles out there,there won't be any one-size-fits-all solution.:o

I can't find a good illustration of where the Saturn ventilator opening are; but maybe this could help. A generic fiberglass hood scoop that feeds into a spoiler in front of the wipers. If the opening was mid-hood, tall and narrow with a low, wide exit and your exhaust vents were in the back corners of the hood... Just crazy enough that it might work?

My RS front bumper on my Focus uses these functional vents..

http://i102.photobucket.com/albums/m...0505-00466.jpg

There is baffles behind the rad to help duct out the air towards these side vents...

2012 Fiesta wheel well vents
 

Maybe, but I've always thought that that area, right at the base of the windshield, was a high pressure area. That is the theory behind using those kinds of hoods, that the high pressure "forces" air into air cleaners and carbs. I may be wrong?

I think that the outlet on the hood top has to be farther forward. It would probably be tricky to select the fore/aft position for such a top exit. One would need to look at the local pressures as aerohead suggested. And, Hucho suggests that the under hood vent work must be smooth to get any advantage. That requirement alone is a big stumpling stone on most front engine cars as there is a lot of stuff in the way:(

I wouldn't do something just at random. It is a tricky problem even for those who want to live with the disadvantages. I like to do aeromods, but I've been slow to act on this one.

In the cold light of day, it wasn't the best idea I ever had. I was thinking of an external duct on top of the hood, like a snorkel, to separate the vented air from the cabin intake. It would probably interfere with the wipers.

Depending on where the optimally-positioned vent and the intakes are, maybe vertical fences on the back of the hood would suffice.

I think I an going to put the gill slits back on.

67 Mustang had vents right behind the front edge of the hood for cooling , they help a lot over earlier no venting models, might be a problem with water entering if further back also helped to kill some of the lift those cars were famous for.

I believe the idea is to vent it to a low pessure area to better fill the void. Is there any way you can get it vented behind the car? Maybe by running some sort of vent inside your already made belly pan to the back of the car? Maybe 2 vents running along the exhaust. Hm.. Would that put too much pressure on your belly pan? Can someone comment on my ideas please.

That information I gave was for arcosine as he wondered about the Cd difference between the two configurations.

Testing showed no center turbulence improvement with venting through tire clearance holes.

Why not put the radiator in the rear, or the whole engine.

There is one (a good one too)—here's another: skin friction. A duct the length of the car would have to have a cross section some multiple of the vent area to move sufficient air.

I believe the famous shneider cup racing airplanes of the thirties , at least some of them used the skin of the plane and evaporative cooling or something along those lines, I think a vent right behind the radiator with maybe a sheild for rain would probably be the best, like and old fashioned cowl vent as used on fifties cars only right behind the rear edge of the radiator or close to it, the air here is going up and pulling a vacumn on the front of the hood on most cars I would think and this allows a bit of air to fill that and kill some lift as well.