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...