Published on August 31st, 2020 |
by Steve Hanley
August 31st, 2020 by Steve Hanley
The EPA testing regimen for rating the range of electric cars is based on the protocols it developed for testing gasoline and diesel powered vehicles. The city portion involves 18 stops over 7.5 miles at an average speed of 20 mph. Acceleration following a standing stop is done at a leisurely pace equivalent to a 0–60 time of 18 seconds. The highway cycle is 10.3 miles at an average speed of 48 mph.
The EPA recognizes that such low speeds may not give an accurate picture of how much range an electric car will have in real-world driving, so it plugs in a correction factor of 30%, an arbitrary number it came up with in 2011 when the numbers for the Toyota Prius were shown to be wildly optimistic. The entire EPA testing protocol is set out in the 32 page SAE J1634A standard.
The purpose of the EPA numbers, of course, is to allow people shopping for an electric car to make some sort of informed comparison between different models but they are little more than a guide. As we all know by now, how far your electric car can go before it needs charging will depend on a number of factors. Driving uphill uses more power than driving downhill even if the incline is slight. Running the heater and air conditioning drains power from the battery. Fast starts take more power than slow starts. Cold temperatures reduce range no matter what brand of electric car you are driving.
Real World Range
Car and Driver does its own range tests but uses a speed of 75 mph to get its highway numbers. Its tests are done on real roads, not a dynamometer, so its findings are actual, not theoretical. In its experience, Teslas driven at 75 mph typically miss their EPA highway range by 27 percent while all other EVs have missed the mark by 22 percent on average. This has led the magazine to call for the development of a new EPA testing protocol, one that more accurately reflects real world performance.
Car and Driver reports there are three principal factors at work that tend to give Tesla an advantage over other manufacturers when calculating the expected range of their cars. First, the EPA allows automakers to use any one of three testing protocols. One of them leads to the 30% correction factor most commonly used by the EPA but the other two are more favorable. For instance, the protocol for the Tesla Model 3 Standard Range Plus results in a correction factor of 29.5%, giving that car a slightly higher range estimate. But the protocol used by Tesla for the Model Y Performance results in a correction factor of only 24.4%, which means its range number is 18 miles more than it would be if the normal 30% correction factor were applied.
C&D is quick to point out that everything Tesla is doing is in total compliance with applicable rules and regulations. Car companies have struggled to obtain the highest possible fuel economy ratings possible ever since miles per gallon became a significant issue after the OPEC oil embargoes in the 1970’s. Whatever Tesla does to improve its stats, its all above board, the magazine says. Tesla has played the electric car game longer than other car companies and so has gotten better at finding legal ways to make the rules work to its advantage.
Second, Tesla has also gained an advantage by making its cars more efficient. Less weight means more range. Recently Tesla started using the world’s largest high pressure casting machine to make a rear subframe for the Model Y that is 30% lighter than the stamped subframe used previously. That will pay dividends in terms of the range for the cars built with the new technology, even if it is just a few miles. Tesla has also recently revealed what it calls the octovalve, an improvement to the heat pump for the heating and air conditioning system that led to a 10% increase in the range of the taller, heavier Model Y.
Two small motors running at higher loads are more efficient than one large motor running at a lower load. Permanent magnet synchronous motors are more efficient than induction motors. When Tesla switched two smaller motors for the Model S and Model X and put a permanent magnet synchronous motor in the front, range for both cars increased by 10%. The new Model S Long Range Plus was put on a diet that sliced 140 lbs off its weight, added low rolling resistance tires and substituted an electric oil pump for the rear motor that is more efficient than the mechanical pump previously used.
The subject of wheels and tires is an interesting one. C&D reports that based on information supplied to the EPA by Tesla, customers who opt for 21 inch wheels on a Model S Long Range Plus “will cut the range by nearly 80 miles. You won’t find that information on Tesla’s website.” So people who buy that car and go for the optional wheels and tires are more than offsetting all the gains Tesla achieved with its engineering prowess, which sort of defeats the purpose of buying a car branded as Long Range Plus in the first place.
Third, the size of a car’s battery pack and how much of that power is usable also have a significant impact on range. Larger batteries have more usable power available. In general, batteries are happiest if they stay between 20% and 80% of their capacity. Here again, Tesla seems to have a significant edge on the competition. For instance, Porsche says its new Taycan has a 93.4 kWh battery but only 83.7 kWh of that is usable.
Car and Driver says, “Based on the limited data we have, it seems that Tesla allows its cars to use more of a pack’s capacity than other manufacturers do. We suspect that’s partially because the company puts some of the responsibility on the driver to choose how high to charge the battery, noting that anything above a 90 percent charge should be reserved only for trips, not everyday use.”
The Take Away
What the magazine reveals is that Tesla rules the range rating not by cheating the way traditional manufacturers did for years but by being smarter and constantly improving their products. It is only a matter of time before those new casting techniques and climate control technologies trickle down to the Model 3 or the Model S if it receives the refresh many think is coming soon. But the takeaway is that EPA range is a useful concept that may help you decide between various electric cars but it is not an iron clad guarantee you will get the same range on the roads you normally drive.
We have to agree with Car and Driver that the EPA needs to come op with a new range test, one that does a better job of reflecting reality. But even if a better test comes along, there will always be big differences between one driver’s experience and another’s. If you focus exclusively on range, you risk missing all the other reasons why driving an electric car is such a delightful experience.
Have a tip for CleanTechnica? Send us an email: [email protected]
Latest Cleantech Talk Episode