How they test electric cars to establish range and MPGe
Cooley On Cars
Hey folks Cooley again got another one of your emails about high tech cars and modern driving this one comes in from Peter a he's up in vancouver washington says I'm in the market for a plugin vehicle.
I've noticed there's a veritable alphabet soup of range estimates, what makes these also different.
and what is the process used to measure EV range?
This is kind of a complicated era, Peter.
When you look at a car sticker it used to be pretty simple.
City, highway, average, very understandable.
Today, we're getting into a different measurement on top of those in a plug-in hybrid.
And that is we're going to add in MPGe.
Now, MPGe Has to be defined first.
Miles per gallon equivalent.
The construct here is that there is 115,000 BTU of energy in a gallon of gas, and that is equivalent to 33.7 KWH of electricity stored in a battery.
Okay, now there's our equivalent in terms of energy to energy.
For example a 2019 Nissan Leaf Plus has a 68 kilowatt hour battery.
Divide that by our 33.7 kilowatt hours, and you get about 2 gallons of electricity.
That doesn't sound like it's gonna get you very far does it?
Except an electric car is much more efficient than a combustion car.
EV Motors can run at about 85% efficiency.
Or a gas engine, at best, converts about 25% of the energy in a gallon of gas into motion down the road.
So the EPA is tested, that 19 leaf for example, and found that it gets an 108 miles out of electricity equivalent to a gallon of gasoline's energy, hence 108 MPG equivalent.
Now, on the plugin hybrids like you're looking at, you're gonna see 2 numbers, MPGe and MPG, because the car runs in 2 modes in a varying mix depending how you drive it You're gonna see those two separate until further notice.
There is no easy way to put them together into a single efficiency number because it's so highly dependent on how you drive the car, what kinds of driving you do.
Some folks with plug-in hybrids dip into the gas engine quite a bit Others almost never do, and have to worry about their gasoline going stale.
They run so much of the time on pure electric only, and there's no way to put that together mathematically.
Because we have to predict how you're going to use the car, which isn't possible.
Now just when you're thinking it's a simple math problem to figure out the electric range.
Take the kilowatt hours of battery capacity Divide that by something and get a number of miles.
Of course it isn't that simple because every car is engineered differently.
Some used to kilowatt hours in their battery more efficiently than others.
They are just better designed cars or more modern.
So what do you do is you do an EPA test cycle, as you asked about, to find out what a car will get [UNKNOWN], never mind what the inputs are to that.
And that test goes a little bit like this The car's battery is fully charged and then its packed over night to settle.
The car is then put on a dynamometer its not run on the road.
The first run it through a so called city cycle of driving behavior until the battery is depleted, then the batter is recharged from an AC source not a DC first charger The kilowatt hours that that recharge put back in are divided by the number of miles the car drove on the cycles just before it went empty, and there is your MPGe number, but not quite.
To account for real world usage, terrain, climate controls, things like that, there's kind of a fudge factor.
They take that number and multiply it by 0.7.
to shave a significant amount off to keep the number conservative and again embracing lots of different factors that may reduce your real world range on electric power.
Now interestingly the AAA just released a new study where they took five popular battery electric cars and ran them in different weather conditions to find out the effect on range.
From your HVAC system when you're running the heater, when you running the air conditioning, I get a lot of emails about this.
What they found is rather strugging.
If you run a battery electric car in 20 degree winter weather with the heater turned up to hit 75 you may save as much as 40% of your range of MPGE.
Now that compared to a 75 degree day which is kind of their index On the other hand if you're gonna be in a warmer climate, 95 degrees, and you're gonna run the A/C to get back down to 75, that could save around 17% of your range or MPG.
Clearly the lesson here is, EV's or plug in hybrids in cold weather running that heater take a much bigger penalty, at least according to AAA, than cooling off your electric car living somewhere hot.
So in some beater there are a ton of test techniques here.
I'm really oversimplifying, and they have real world fudge factors built in to make sure that they are not strictly technical.
And that they're looking at real world parameters that are gonna bring the number down.
Hopefully, you get something pretty accurate when you buy your car and hit the road.
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