Ask AutoblogGreen: Do electric vehicles need front air intakes?
A reader posed us an interesting question the other day about air intakes and grilles. Specifically: do electric vehicles need them at all and why do many internal combustion vehicles have such large examples. For conventional vehicles, an intake of some kind is necessary in order to dissipate the heat in the coolant that keeps the engine from melting.
While many modern cars, such as Audis, have what appear to be very prominent grilles these are actually mostly for stylistic reasons. If you actually take a close look at the grille area, you will find that much of it is actually closed off, particularly the upper areas. For optimum aerodynamics, air typically only flows through the lower grille. A prime example of this is the production version of the Chevy Volt. One of the changes from concept to production was closing off the main part of the grille.
Reader Torrey also noted that most electric vehicles like the Tesla Roadster and Dodge Circuit have grilles as well (see above). Even without an engine, these cars still have cooling requirements. The electronics, the motor and the battery all need to be cooled. The Roadster and Volt both use liquid cooled batteries with radiators in the front. Aerodynamicists always try to minimize the open area to cut drag but some openings are still needed. Thanks to Torrey for the tip!
Reader Comments (Page 1 of 1)
NeilBlanchard 10:07PM (5/21/2009)
Hiya,
I think they are *mostly* vestiges of ICE cars, and what we think a car should look like. When the power plant wastes over 80% of the fuel as heat, then you need a fair bit of cooling. I very much doubt that the batteries in a well designed EV waste even a fraction of the inverse of that...
Look at the Aptera 2e -- it has just the two small "nostril" intakes just inside of the headlights; and it has two air exhaust ports on the back, one on either side of the license plate.
Sincerely, Neil
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Ignatius 10:47PM (5/21/2009)
The air intake is also to keep the reaction going, as gasoline requires a certain amount of air mixed with it to get the optimal explosion to propel the pistons upward. It does, however, cool the radiator as well.
Luke 1:19AM (5/22/2009)
Ignatius,
I recommend you trace the air intake in your car sometime. The air intake is often behind a headlight, often the right-hand one (on my Ranger and on the Prius, though it was on the left in my Jetta TDI). The grill is almost entirely for the radiator, though it probably does help to set up the airflow to the intake.
Just because a car is electric doesn't mean it doesn't need a radiator, though. I've heard that many non-homebrew electric car designs use water-cooled electric motors, end even water-cooled batteries. Also, don't forget that the air conditioner requires a radiator-like device, as well (it's often integrated into the main radiator, though I've heard of designs where there's a separate mini-radiator for the A/C). So, I personally would have to look at the details of the design before I could say anything about what the grill should look like on an electric car.
I guess it would be fair to say that the radiator for a small car would probably be smaller, and that there's probably no reason it HAS to be located near the front bumper....
NeilBlanchard 5:59AM (5/22/2009)
Hiya,
The air intake for a engine passes through a relatively small plenum and duct -- my xA has a "cold" air intake that is ~3" in diameter. And I'm pretty sure that the ignited fuel pushes the pistons downward...
Many grills are oversized even for cooling an ICE: I have about 3/4 of the grill on my xA covered, and the remaining 4" x15" opening is close to ideal; though now that the weather is warmer, I will have to fine tune the air flow a bit more, as the fan comes on more than I'd like. Take a look at this:
http://ecomodder.com/forum/showthread.php/scion-xa-aero-mods-2969.html
Having a small grill opening is a huge advantage for good aerodynamics, and aerodynamics are HUGE for getting good efficiency out of a vehicle. Again, I point to the Aptera 2e for an example of a well designed electric vehicle, that has air cooled batteries, and A/C and heat for the passengers, too.
Sincerely, Neil
Jeff 11:02PM (5/21/2009)
In my experience, the limiting factor for how much power you can get out of an electric motor is how much current you can send through it before it overheats due to I^2 *R heat generation in the motor. If you have a motor rated for 10kW continuous, you can probably get 30kW out of it if you just pump higher current through it, though it'll overheat after a little while. Cooling the motor down will increase the time you can run at these higher outputs.
A 100kW motor at 90% efficient still pumps out about 10kW of heat, so imagine 10 hair dryers going, that's a decent amount of heat.
When talking about the 80% waste heat coming out of an engine, keep in mind that a good deal of that energy is being vented off out the tail pipe and therefore the radiator doesn't have to take care of the full 80%.
All that to say I think EVs still need a decent amount of fresh air, though not quite as much as conventional vehicles (which also need air for combustion). For the Aptera, they really put a heavy emphasis on aerodynamics, so they might be running their motors at pretty conservative outputs.
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Mattias 4:53AM (5/22/2009)
You are right, BUT: When going 120mph+ with my small diesel its mechanical power is 100kW, lets assume a thermal efficieny of 1/3, that means it produces about 200kW heat. This is a little bit more than the 10kW or maybe 20kW that you correctly assume.
I guess that a Mini style side mounted radiator would completely do the job for the electric motor. But there might be another small radiator necessary to keep the battery pack cool.
Ron Fischer 11:38PM (5/21/2009)
Radiators and front openings are sized for the car's cooling needs when idling at a standstill in the hot desert with the radiator fans running. Nearly any other driving situation will provide much more cooling due to airflow.
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John Pelletier 7:22AM (5/22/2009)
i seem to remember a certain ecomoded white honda civic that had air vent flaps that the moder could open from inside the drivers seat. he needed to do this on hill climbing and such when the motor is working really hard. obviously though there is more need for air intake on an ICE car then there is on an electric. keeping the grill makes it seem like another car, which for MOST people is a major factor, hence the love it or hate it prius. it just does not look like a typical car
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Rahul 9:18AM (5/22/2009)
I agree that all cars need some air cooling. Take the fully electric Maini Revai cars from India. Grilles are on it. So i guess it is a necessity rather than a decorative piece for all cars. But big electric cars may not need that big grilles like the Audi or the Bentley cars.
Rahul
Indian Car Advisor (Carazoo.com)
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Jeff 10:35AM (5/22/2009)
The grill on the Reva is only for the AC. The motor and electronics are all in the back of the car, and there's a bit of a scoop on the underside to direct air flow towards the motor and a couple heatsinks sticking through the bottom side of the car. The batteries are also cooled by a fan that simply sucks air out of the battery compartment, nothing connected to the front grill. That car is tiny and only has about 10kW of power so there isn't all that much waste heat there.
RJ 9:40AM (5/22/2009)
I have read over and over that the grill openings are made oversized for worse case conditions such as towing a trailer uphill in 110 degree weather.
Though my car isn't electric, I have been playing with grill blocks on my Honda Civic. I was concerned about overheating, but having noticed any adverse effects to the engine temps.
Here is a blog post I wrote, along with a picture here:
http://www.greencarmpg.com/grill-block-and-fake-vent-covers.html
I will probably redo the work to make it a little nice. And maybe add an air dam.
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lne937s 9:59AM (5/22/2009)
Take a look at the grill opening needed for a 900hp internal combustion car... (it is actually under the bumber- everything else is just stickers)
http://blog.news-record.com/staff/spotter/archives/COT%20burton%20bristol%2006.jpg
http://jayski.com/schemes/2006/COT/46COTwyler.jpg
or a 400hp + sports car intended for every day use:
http://www.autobahnpower.com/main.asp?sitepages=CorvetteZO6
The grill opeings on most cars are mainly for styling purposes. Most of the "grill" is not needed on either an internal combustion car of an electric car. For a typical car, you only need a few inches of opening near the ground. With much greater thermal efficiency, there is very little need for a significant grill opening on an electric car...
If people were less concerned about what looks "cool", we would have much more efficient cars.
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Tim Templeton 11:18AM (5/22/2009)
Da, Maybe the manufacturer are doing it to simply give the design some personality or to make the electric car noticeable as part of their product line. (as in the case of the picture which is easily definable as a Chrysler product, Charger, Viper Ram, etc). If the vehicle had no "look" then it would most likely resemble a big dildo.
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Randy C. 12:09PM (5/22/2009)
Gas cars break down like this:
1/3 of the energy goes out the radiator as waste heat
1/3 of the energy goes out the exhaust pipe as waste heat
1/3 of the energy actually turns the crank shaft
Now subtract from that final third the power to turn the water pump, oil pump, alternator, power steering pump and air conditioning. Then loose some more energy through friction losses in the drive train. If the car has an Automatic Transmission (A/T) add a little bit more loss to turn the fluid pump and the energy loss in the torque converter while the car is stopped. (That constant push you feel while the A/T is in gear is lost energy exiting in the form of heat in the transmission fluid. That's why A/T have oil coolers.)
So only 20% to 25% of the total energy that you paid for in that gallon of gas actually goes to pushing the car forward. I've seen it as low as 15%. See the waste?
While in an electric car you have very little waste heat to get rid of. There is only 1 moving part on the motor. Also electric motors are much more efficient at converting input energy to torque. And torque is what pushes you down the road, not horsepower. (Horsepower is a measure of how fast the torque is delivered.) An EV is over 80% efficient wall to wheels. That means 80% of the energy you pay for pushes you down the road and electric energy is actually cheaper to begin with.
You practically don't need big grill or vent openings with is kind of efficiency.
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Jason 4:19PM (5/25/2009)
Electric motors can be as high as 80% efficient, but you're not going to find a system that, when you include charging losses (notice that batteries get hot when you charge them: a loss of energy), discharging losses (again, batteries being discharged produce heat) and loses in the electric motor due to heat generated in the coils, is anywhere NEAR 80%. 50-60% is much more reasonable. The radiators on EVs need to be designed around the worst-case scenario: rapid discharge of the batteries (i.e. full throttle) at low forward velocity. Under those conditions, there is very little airflow, high heat rate in the batteries, and high heat rate in the motors due to both the starting current and the fact that at lower than optimal RPM (different on every motor, but typically set at cruising speed) the motors produce quite a lot of heat.
That, and you might want to re-examine how you define "what pushes you down the road" Horsepower and torque are linked by the velocity of the motor. What I think you mean to say is the torque available at your current velocity is what pushes you along. That, in fact, is precisely the horsepower available.
NeilBlanchard 2:05PM (8/29/2009)
Hi Randy,
Your ICE numbers are actually still too generous: about 87% is lost before it can get to the wheels: as heat from the engine, idling and not moving, some is lost in the drivetrain, some is lost to power the accessories; not the 60% you mention. Of the remaining 13%, half of that heats the air that the vehicle pushes aside, or heats the tires and the road surface from friction within the tires -- and some (about %6) accelerates the vehicle and then it heats the brakes when you stop -- leaving ~>1% to actually move the driver.
http://www.ted.com/talks/lang/eng/amory_lovins_on_winning_the_oil_endgame.html
Look at ~8:45 into the video for the numbers -- but watch the whole thing!
Rick 2:04PM (5/22/2009)
"While many modern cars, such as Audis, have what appear to be very prominent grilles these are actually mostly for stylistic reasons."
As an Audi owner, I can confirm, the air ducts next to the grills are completely functional. Also, the grill is completely in front of the radiator as well. Stylistic, yes, because the middle of the grill covers the bumper, but the top and bottom allow air through to the radiator.
That being said, I would MUCH rather prefer an EV that has a grill, air ducts, etc for stylistic reasons.
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AD 3:57PM (5/22/2009)
Aero v. cooling...
As a moss-back, I can look back to the E-Type Jag and see the tradeoffs between these two requirements. The original Series-1 E's (1961-1967) looked first to aero, and were terrible over-heaters in most American urban traffic. It wasn't until the Series-2's in 1968 that they were finally able to move enough air through the radiators at less-than-highway speeds, but the aero took it in the shorts.
And, let's not forget cooling requirements for any a/t's that these electrics and/or hybrids might be carrying around.
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comatus 9:03PM (5/22/2009)
1963 Studebaker Avanti.
Re-compute prejudices, please.
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