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Electric cars have a problem. No. It’s not about their limited range. Or slow charging times. Or limited charging infrastructure. It’s about their inability to efficiently cruise at high speeds.
This is not a new problem for the auto industry. The petrol and diesel engines have suffered from narrow torque bands since their birth. Engineers found a way around this problem by adding multi-speed transmission. As long as a car’s engine stays between a specific RPM range and has enough torque available there to overcome the weight, frictional and aerodynamic drag, it can cruise efficiently. The onus of changing gears can be upon you or an auto-box.
Unlike petrol and diesel cars, electric cars don’t have a multi-speed transmission. So, how do they attain high speeds? By revving some more. A Tesla Model S, for example, can rev up to 18000rpm. With a fixed-ratio drivetrain, the motor adds 1kmph of speed for every 90rpm. Compared to an internal combustion engine, 18000rpm is downright bonkers.
To see how it delivers that performance, let’s consider the middle-of-the-road Model S 85’s performance graph. That’s the green line shown below. The solid line is for the torque in Nm and the dotted line is for power in kW. As you can see, its peak torque of 445Nm comes in pretty much as soon as it starts rolling. It stays strong till about 78kmph (7000rpm) and then starts dropping. The power curve keeps ramping up till it reaches 323PS and then stops making any progress as the torque drops off. But when the torque really starts to catch the gravity at about 130kmph (11700rpm), the power starts dropping too.
If you’ve fallen for the horsepower trick that carmakers play all the time, then allow us to break it down for you. Power is a result of torque at a given RPM. There’s also a constant involved, but in simple terms, know that Power = Torque X RPM.
So, the strong acceleration you feel in any car is actually the result of strong torque at those RPMs. The graph here clearly shows the relation between the two. While the torque is strong, the power keeps rising with RPMs. But, depending on how rapidly the torque drops, the power starts fading too.
Based on the Tesla Model S power graph, it’s clear that the motor is not making the same kind of torque beyond 7000rpm.
Sky-high revving electric motors aren’t just limited to Teslas. The Renault Zoe also revs to an astounding 11300rpm. Its motor adds 1kmph to the speedometer reading for every 82rpm increase in the motor revs. It makes 110PS of power from 3395 to 10980rpm. That should give you an idea about where the torque starts its downward journey. The figures sing the same song. Its peak torque of 225Nm is available between 1500 and 3390rpm. That number is like deja vu. In typical electric vehicle fashion, there would be a healthy amount of torque below 1500rpm as well, but in this case, it peaks at 1500 and stays put till 3390rpm.
Judging by its peak torque figure, we can say that you’ll experience the best of its performance up to about 42kmph.
Now, let’s talk about the efficiency part of an electric motor and why you shouldn’t rev it hard.
The above graph shows the battery power consumed at a given speed (yellow), aerodynamic drag (red) and the power required to compensate for the increase in aero drag (blue). As you can see, the actual power consumption and power required to deal with the aerodynamic drag at 30kmph is same. However, once the speedometer starts climbing, actual power consumption increases drastically – far beyond what’s needed to compensate for the aero drag.
The reason for this exponential increase in battery power consumption is inefficiencies in the electric motor itself at higher RPMs. That’s not unlike petrol and diesel engines. Similar to a combustion engine, revving the guts out of an electric motor can not only cause excessive damage, overheating and premature failure but will also consume excessive battery. In simple terms, cruising at 50kmph and 100kmph is not the same in an electric car with a fixed ratio transmission. You will get better range per charge if you cruise at a slower speed in an electric vehicle. How slow can you cruise without hampering the range? Well, that depends on the peak torque rpm of the electric motor.
Here’s a video of Tata Nexon electric getting a boot-full of accelerator:
Things worth noting here are the estimated range, distance covered during the test displayed in the trip meter and power consumption. In the beginning, the estimated range is 101km, the trip meter reading is 4.8km and average power consumption for that trip was 132Wh/km. By the end of this short video, the range drops by 2km while the distance covered is only 0.8km. The power consumption at the end is 163Wh/km.
I did the number-crunching for you and found out that during that 800m run, it consumed about 280Wh/km. So if you drive it at its top speed, you can expect to go about 108km on a full charge of its 30.2kWh battery pack. That’s an efficiency of just 3.6kmpu. Tata expects the typical usage to stay under 100Wh/km or 10km per unit (kmpu) of electricity. That’s because the regen is also expected to add some range back into the batteries.
It means that despite its 129PS of power and 245Nm of torque output, the Nexon electric isn’t meant for the highway. It’s supposed to be used in the city. Tata Motors also confirms our suspicion by not offering cruise control in the Nexon electric. The petrol and diesel models of the Nexon facelift get the highway-friendly feature. If you intend to use your Nexon electric on the highway – at highway speeds – then expect far less than the advertised 312km of range.
Solution #1 – Multi-Speed Transmission
Driving a manual transmission in stop-and-go traffic is a lot of pain in a petrol or diesel car. But in an electric car, it won’t be as miserable. Even when you’re fully stopped, an engine keeps revving. While the engine is idling, you keep your car from crawling past a stoplight either by braking or by pressing the clutch pedal and disengaging the drivetrain. An electric car doesn’t rev at idle. So, you don’t need to do any of those things. Even when rolling off in an EV with a stick shift, you won’t have to lift off the clutch carefully. In fact, to get moving, you don’t even need to use the clutch as there’s no mismatch of speed between the engine and the wheels – they’re both at 0rpm.
A transmission adds ~40kg of weight to a mass-market car but you can use it to get better mileage and speed. It can allow you to drive your electric car efficiently at high speeds. It doesn’t need to be a 5- or 6-speed gearbox like normal petrol or diesel cars. Even 2- or 3-speed transmissions are more than enough to efficiently drive an electric car on the highway – at triple-digit speeds. Those who don’t like the manual transmission can just slot it in their preferred gear and drive it like an EV with a single-speed transmission.
Porsche is already doing it. Could you imagine the embarrassment of buying a Porsche and not being able to drive it on the Autobahn? You can’t drive a Tesla the way you should on the German expressway with no speed limit. That’s why the Porsche Taycan has a 2-speed transmission. So, even after hitting 120 – 130kmph, it can continue to pull like crazy to its top speed of 260kmph.
A multi-speed manual transmission, though, could be especially exciting for enthusiasts as it allows us to gain some control over the vehicle. With a manual gearbox, a driver can better exploit the electric car’s bottom-end torque and enjoy an engaging drive slotting gears.
Since it’s not a completely new concept, it shouldn’t cost a lot in R&D. The tech already exists. It just needs to be adapted to work with electric cars. If Porsche can do it, so can others. Its benefits aren’t just limited to high-end cars. While the performance-focused electric cars can get better top speeds out of a multi-speed transmission, a budget car could leverage it for a better driving range.
Multiple electric motorcycle manufacturers – Indian and foreign – have decided to add multi-speed transmissions to their vehicles, which further validates this theory. eMotion’s Surge, Tacita T-Cruise and Kymco RevoNEX are in line to get a multi-speed transmission. It’ll allow their electric motors to efficiently utilize whatever little juice their batteries can store in them. It’s all the more important for a motorcycle to keep its weight in check as the rider has to manage it at parking, city, and cruising speeds. The fact that electric motorcycle makers are choosing multi-speed transmission instead of more batteries to get longer range suggests that adding more batteries to achieve similar range may have a worse weight penalty.
Here’s what the founder and CEO of eMotion, Pranav Singanapalli, had to say to justify adding a gearbox to the Surge electric motorcycle:
“We have 3 reasons – the first is we want to preserve the experience of riding a conventional motorcycle. We want to keep the experience familiar to a traditional bike owner. Secondly, it is the efficiency – the motor has an rpm when it delivers the best efficiency, and we can maintain that with the gearbox and thirdly we can offer great torque and acceleration without compromising the top speed.”
Solution #2 – Add More Battery
Adding more battery is one of the options to get a decent range from an electric vehicle on the highways.
The obvious problem with this is the cost. Although the cost of lithium batteries has dropped drastically in recent years, it’s still the single most expensive component in an electric vehicle. Making EVs more expensive isn’t going to help them spread the good word.
The not-so-obvious issue with adding more battery is the kerb weight. If you only occasionally need to drive on the highways, lugging that extra weight of the batteries around the city makes little sense. Not to mention, its adverse effects on the life span of the car’s tyres, brakes and suspension. It’s good insurance, but a heavy one – literally.
Solution #3 – Empty Slots For Renting Battery
Let’s say your car has slots for 50kWh worth of batteries but you only need 200-250km of range from the electric car for 99% of the time. Well, then just ask for a car with a 25kWh of battery and leave the rest of the slots empty but easily accessible. When you want to make a highway trip, rent 25kWh batteries from the dealership for a week or two, fill up those empty slots and enjoy ~500km of range at modest speeds. Or about 300km at highway speeds. This strategy will also keep the upfront costs of the vehicle down and leave the option of adding more battery if and when needed.
Having a serviceable hatch mechanism for storing batteries also opens up the possibility of significantly cutting the revival times and lowering the upfront costs of the car. You can buy the car without the battery. Just lease them instead. Rather than waiting for hours recharging your car along the highways, it could be possible to just replace the batteries at an outlet. It drastically cuts down the time required to top up and hit the road again. Since you don’t own the batteries, you don’t need to worry about the condition of the replacement either. Third-party battery makers can also chip in to offer competitive prices for a replacement battery pack for your car.
References: Electrek
(The article is written by Mahesh Yadav. Mahesh is car and motorcycle lover. But unlike most enthusiasts, who can’t have enough, he believes that great things come in small packages. As a fan and firm believer of the ‘Just enough. Just in time.’ theory, he loves the underappreciated vehicles that offer just enough to meet the consumer needs for as little cash as possible. The list of his favorite cars includes the Eicher Polaris Multix, Tata Nano, Honda Brio, Mazda Miata, MG E200, and the likes.)