Dear Car Talk: A question about electric vehicles! How is an EV cooled and heated?
I assume an electric motor runs the compressor for cooling. And I assume heating is done by either a heat pump or with resistance heating.
My question is, since electricity is used for both of these things, how does this affect the range? — Jim
Dear Jim: Good question, Jim.
The answer is complicated. There are different systems, and lots of variables. Unless you count “water bottle” and “electric socks,” you’re right that there are two types of heaters used in EVs. There are resistance heaters (basically, a toaster), and heat pumps, which pull heat out of the air — even cold air. A heat pump is far more efficient.
To get a sense of how they affect range, we called our pal Tom Voelk, who tests lots of EVs for his “Driven” car review series on YouTube.
Tom says that in his experience, the effect on range varies, depending on the actual outside temperature, the size of the cabin, and how toasty you want your cheeks to be. But for old style, toaster/resistance heaters, Tom says range can drop by a third or even more in sub-freezing weather. For EVs with heat pumps, he’s seen range drop by around 15% under similar conditions. Quite a bit less.
Of course, all that loss in range is not just due to using the heater. The loss is greater when using heat because it’s, what? Cold out! And batteries are already less efficient in cold weather.
When you’re using air conditioning, it’s usually hot out. Unless you’re a weirdo. So, the batteries are already working much more efficiently. Tom says that with constant air conditioning, he typically sees about a 10% drop in range. You can reduce those losses if you use what’s called pre-conditioning.
A number of EVs now allow you to pre-condition the cabin before you drive away. So, you can tell the car you’re leaving for a recreational root canal at 8 a.m., and the car will either heat or cool the cabin while it’s still plugged into your home charger — before you drive off and use up any range.
You can also reduce your heating demand by using a seat heater and heated steering wheel, which keep your most sensitive parts warm without having to turn the whole cabin into a blast furnace.
I hope that gives you a ballpark idea, Jim. And I sincerely hope I haven’t done Tom Voelk’s career any lasting damage today by associating him with Car Talk.
Dear Car Talk: I have a 2020 Toyota Avalon. I recently put new Michelin tires on the car and immediately noticed a 10% decline in mileage, coinciding with the arrival of fall temperatures.
My mileage fell even more when temperatures this winter dropped into the single digits. Is this for real? — Joe
Dear Joe: I don’t think your tires should take all the heat here, Joe. New tires will have some negative effect on mileage. New tread provides better grip than worn out, flatter tread. But that better grip increases friction and rolling resistance.
By how much? Well, it depends on the tires you buy. If you replace your tires with a similar type of tire (like an all-season passenger car tire), the decrease in fuel economy should be far less than 10%.
On the other hand, if you go from a low-rolling-resistance, high-fuel-economy tire to a performance tire with a soft, sticky rubber compound designed to maximize cornering grip, you will see a larger drop in mileage.
Somehow, I don’t see you signing up for a track day and running through chicanes at 90 mph in your Toyota Avalon, Joe. So, assuming the tires you got are similar to what you had before, and they’re properly inflated, I think it’s more likely your drop in mileage is caused by seasonal factors.
When it gets cold out, cars typically use more gasoline. There are behavioral reasons for that: you spend more time warming up the car, you take more short trips and drive places where you’d normally walk, and you may use heated seats, defrosters, and other accessories that use energy.
Then there are mechanical reasons why your mileage is lower in the winter. Your tire pressure drops one PSI for every 10 degrees the temperature drops, and lower tire pressure means lower mileage. Your oil and fluids are more viscous and take more energy to circulate. And winter blends of gasoline have slightly less energy per gallon.
So, I’d wait and see if your previous mileage, or something closer to it, returns with the warmer weather, Joe. I’m guessing it’ll come pretty close.
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