Full Grill Block - Where best to get air for cooling?
 

With the grill fully blocked and no airflow through the radiator and going up a steep hill the engine will overheat fairly quickly but on the flat with gentle driving it can take 15 minutes (maybe the full journey) before needing any air for cooling. It seems widely accepted that there is an aerodynamic advantage to keeping the grill fully blocked until cooling air is needed.

Question - When the radiator does need some air for cooling, where is the best place to get it?

I could open a flap in the grill but that increases the amount of air flowing under the car and that appears to be bad for fuel economy.

Alternatively I could use the radiator cooling fan to suck air from behind the front splitter through the radiator. That way the air that ends up under the car is air that was already under the car so maybe has no overall effect on fuel consumption, except for the 10-30W (depending on air temperature) of power needed for the fan. A few guesses suggests that keeping the grill blocked might save a KW of engine power so using a few 10s of W for the fan may give a good return...

Has anyone tried sourcing the cooling air from anywhere but the grill? Are there any figures available for the results? Any knowledge if it is a good or bad idea?

As a general rule, if you're going to block part of the grille, leave the center open. The reason for this is that if you have the sides open, you are spoiling clean airflow coming from the very front of the car. It is instead best to pull from the stagnation point and let anything outside of that flow around cleanly.

I'm almost positive there are threads that discuss this. Look up "bottom breathers" like the Corvette. It uses the high pressure zone in front of the air dam to shove air into the radiator.

http://images.corvettefever.com/tech...ment+front.jpg

Or just punch a 25 square cm hole in the lower middle of your grille block, since it won't affect that much.

Or rig up a flap in the lower middle of the block, with a hinge so it can open when you need it. Actuate by cable or solenoid.

The radiator fan can pull some decent power, and alternators aren't that efficient. So, the fan should be avoided. However, if it only comes on 1-2 times on your trip, I'd just leave it.

On my Prius, I run a full grill block all winter unless I'll be going on the highway. Then, I remove one of the pipe insulation strips from the grill. That allows for plenty of cooling.

I have a variable speed fan that keeps the coolant returning from the radiator 10 degrees C below the thermostat opening temperature thus allowing the thermostat to mix warm and cool coolant and keep the engine temperature constant.

In normal running the fan stays off for 10-15 minutes and then runs slowly, if I go up a steep hill it speeds up. The amount of power used is typically 15W while the grill block appears to be saving several KW. So even though alternators aren't 100% efficient it doesn't matter as long as I save more than I use...

Thanks for the search tip, helps to know what to search for.

They still seem to have the intake in the high pressure area though, and in a place my car doesn't have. The obvious places to put a hole and maybe flap on my car are in the lower grill block or in the undertray in front of the radiator which is a low pressure area so would need some extra effort from the fan possibly in return for gains in aerodynamics.

Well, the stagnation point is a good high pressure area. You've got one of those.

What car is this? Tip: add a garage entry. Then your vehicle shows up with all your posts, beneath your username. Potentially useful context.

I think you might be reading into the full grill block too hard. I think there are a lot of vehicles on here that might have a "full grill block", where the entire upper or lower grill is blocked, but one of the 2 is left partially open, or there is a gap in the body work, or at the edge of the hood that allows air flow. My grill is completely blocked, but it pulls air like the corvette pictured above. it's not that differnt or special from any other grill opening, it's just not surrounded by body work on all sides, it still uses some amount of frontal area to grab air to direct through the radiator.

Yeah, grille blocks don't generally make the engine compartment airtight.

Edit: looks like it's already been stated. I'll just leave it here anyway.

The stagnation point will be a high enough pressure area to get air through your radiator, especially if you duct it. You don't need a big cantilevering front end to make it work; almost all active members here have some form of grille block, so just check the project threads. You can have a reasonable grille opening and still not cause unreasonable drag. It's a balance.

What car is it?

Air in at the stagnation point.

Highly evolved cooling systems have a fixed intake and a flap in the exit. (P-51)

Best location for the exit is at the widest part of the body, where the air is moving the fastest. (Edison 2)

Not quite the right image but it shows the grill area:

http://www.carsbase.com/photo/MG-ZS_mp147_pic_35664.jpg

Yes, I realise that most people don't completely block the grill, and most people try to avoid using the fan, but I have a fan that I am happy to use and which will only use the amount of energy needed so I have a choice of where to draw the air from. The question is, which is the best place to draw the air from? Maybe it is from in front of the grill adding to the amount that ends up under the car, maybe not...

I don't really have a choice of exit, the engine is very close behind the radiator so there is no way to do anything other than let it go over and around the engine and then out the bottom of the engine bay. I would also have a lot of cooling issues if it didn't, for example the transmission is air cooled. The undertray extends a little passed the radiator then stops.

best place
 

*If you're going to leave the radiator as it is,the ideal configuration is to create an airtight inlet duct which is as deep as the height of the radiator,as wide as the radiator,and 1/6th the height of the radiator,with the inlet right above the splitter,at the forward stagnation point.This might require a nose extension.
*If you were to lean the top of the radiator forward,again,with an airtight inlet duct,you could pull all the air from under the nose and at the forward stagnation point of an airdam,as older Corvettes and Pontiac Trans Am Firebirds utilized.This WOULD probably necessitate a nose extension in order to accommodate the exotic inlet.
*If you don't want to compromise the appearance of the car,then you'll have to settle for something less than ideal.You just may not have enough distance between the radiator face and the grille to allow proper air diffusion over the entire core face.
*Whatever your choice,you'll want to pull the air from the stagnation area.Otherwise you'll have single-fault failure mode for cooking all the mechanicals.

i figured it was all just working off the higher pressure in front of the air dam, and lower pressure behind
all my air goes under my bumper, using stock vertical radiator. older vettes / 4th gen camaros/ firebirds leaned back. the radiator leans forward on the latest vettes, but they duct the air off the top 1/3 or so of the radiator out the hood

working off
 

yes,the frontal stagnation point is nothing but barometric pressure plus the ram component of the dynamic pressure.
By the position behind the radiator core,any pathway that the exiting air can communicate with is at a lower static pressure by virtue of Daniel Bernouli's principle of the pressure/velocity tradeoff.
Your Saturns nose has the grille cut right into this stagnation area and is harvesting cooling air with high efficiency.It would embarrass many 'new' cars.
The '82 Trans Am picked up a Cd 0.015 drag reduction ( 4.75% ) with the bottom-breather setup combined with a 25-mm lower (1-inch) airdam.
'Bummer about these configurations is that so many of these airdams were torn from the cars while negotiating 'normal' driveway inclines.:o

Here's a picture to go with aerohead's analysis:

http://ecomodder.com/forum/member-fr...1215138521.jpg

Turn the undertray into a partial bellypan, and have the flap slide instead of hinge.

That's a nice idea but I don't want to prevent air circulating around the engine bay, a belly pan seems risky. It was designed and tested with a large airflow keeping everything cool, having put in the grill block, at times in hot weather I am opening the grill in order to cool the engine bay more than the coolant in the radiator.

I like the image of the Corvette in post #2 but I don't have an air dam and the bottom under the radiator is flat. I'm not sure what effect adding an air dam would have, there is a bit of a tunnel running under the grill which may be intended to give highest flow under the centre of the car and shape the flow further back, or maybe it is just styling! I'm sure adding an air dam would have a significant effect though, seems like another project.

The original question hasn't really been answered but I think I'll take the easy option and just put a vent in the centre of the grill block with an iris to control the flow and control the iris based on the coolant temperature at the exit of the radiator and override it when engine compartment temperatures get too high. Then I can try to test whether opening the iris or turning up the fan power uses least fuel.

Thanks for the replies :)

TL;DR: Right where the factory put it. :)

"The original question hasn't really been answered"? I think you are disrespecting aerohead's responses. You will need to particularize the general responses for yourself in any case. What kind of car is that in the picture from WaWaWoum, anyway.

As for myself, the point I was trying to make, apparently not particularly well, is that you could leave the grille completely unblocked, put sliding louvers in a belly pan and gain dynamic control of the airflow from there. You should be able to seal the engine compartment completely, pipe in coolant and intake air and have it work just fine. If it doesn't, perhaps an oil cooler is called for. How are your transmission temps?

This is your real question. Be wary of 'widely accepted' until you verify it for yourself.

It's widely accepted because it's widely true. The main question is whether it's true for any particular car, and how much of an advantage there is. The answer may be a blanket yes (and very effective), or it may depend on your design.

Isn't this fun?

Yes, fun :D

I wasn't expecting an answer specific to my car or even a general one for the model, just wondering if anyone had looked into the alternatives and what is worth trying. Looks like everyone takes the cooling air in through the grill though and have never tried anything else!

I think the bottom breather setup, simmar to the corvette, is about as different as you're going to find with a radiator in the typical placement. Many race cars will use that location and duct air out the hood. Off-road race vehicles will mount their radiators high in the rear / in the bed of the truck. Some mid / rear engined vehicles will have ducts on the sides of the car after the doors where the radiators are. Boxters have 2radiators in the front corners
All of those hinge on radiator placement. I'm sure with powerful enough fans, lots of ducting, and 7 alternators, it would be possible to draw air in from anywhere around the vehicle, and exhaust it where ever you see fit. You won't come out ahead running your radiator fan vs. a properly sized grill opening. After the losses from the fan converting elcrical power into air flow for the radiator, the alternator turning mechanical energy into electrical power for the fan, and the engine turning fuel into mechanical energy for the alternator. You're better off taking the more direct hit, using the engines power to create the air flow for the radiator

The race version of the car has an unusual bumper with a significantly smaller grill than the road version:

http://www.touringcars.net/photos/rc2006.jpg

http://gallery.xpowerforums.com/data...roes_Front.JPG

Of course it probably wasn't designed for fuel efficiency, but low drag would have been important.

alternatives
 

For the traditional radiator placement,the lowest entropy approach is to utilize the ram pressure and thereby maximize the pressure differential available.Cooling systems which contribute no more than 2% to overall drag can be realized in this manner.
Harvesting air from any other location implicates the addition of artificial pressure and starts to violate the second law of thermodynamics, as far as efficiency goes.
It's the tyranny of singularity in action.
If your fan ever fails,you've conceivably just trashed a $3,000 long block.
It's the primary driver for 'traditional' systems.

I think 2% drag is still consuming more energy than the fan is consuming even if you take into account the inefficiencies of generating the electricity, however I'm making a lot of guesses to get to that and I'm not sure how much drag use of the fan could save, hence the question...

Thankfully the designers of the car decided to remove that singularity and installed two fans which operate at different temperatures and off different sensors, plus I have a coolant over temperature alarm, plus if the temperature continues to rise the coolant pressure cap will release a cloud of steam and soon after that the low coolant alarm will illuminate. Only if you continue to drive after all that will you cause any damage.

A variable speed fan which controls the outflow coolant temperature.
WAAAAAY NEAT!!!!!! and first I have read about that being a factory design.

It means you have avery stable temperature of the coolant entering the engine, which without that system can vary by as much as 90 degrees sucking heat energy out of the engine when it's cold.

I would consider blocking the upper grille, which looks like it is integral with the hood and opens with same.

We think of cooling fans as either on or off, yours is a generation advanced from the old all or nothing fans.

Whatever you do block, start gradually, so as to avoid compromising the integrity of the super neat system you have already. Basically when temps drop below a certain point you want to restrict airflow due to the much cooler air flowing.

regards
mech

Well, no. 2000mc covered a lot of the examples I can think of. I will add the Edison2 that uses a gill-slit at the widest part of the body where the air is moving the fastest and is still in laminar flow. And of course my car is completely sealed in the front and take in air under the backlight. Here's someone who was looking at front engine/rear radiator:

http://ecomodder.com/forum/showthrea...ery-26356.html

Two-pager from 2008

http://ecomodder.com/forum/showthrea...anks-5796.html

...and

http://ecomodder.com/forum/showthrea...ear-21405.html

2%
 

The 2% is the passive penalty when driving at 40-mph or above,when no fan is necessary.It is just for the friction and turbulence of the heat exchanger and ducting.It represents the 'ideal'.The minimum of which cannot be improved upon.
The electric fan(s) remove the fans parasitic from the engine,and put it on the electrical system.
The 2% + loss is still there,plus any inefficiencies of the alternator drive,the alternator itself,powering the electric motor of a given efficiency,driving a fan of a given efficiency,through a restriction of some static pressure differential which would be dependent upon design.If you don't take advantage of the ram air,it is scientifically impossible to achieve lower than a 2% loss.