By Pete Melville, Hybrid and Electric Vehicle Repair Alliance
Increasing the performance of a car is a subject that goes back almost as far as the invention of the car itself. For combustion engines, the basic formula is to get more air into the engine. Whether that’s through making the engine bigger, fitting forced induction to push more air in, increasing the rotation speed, improving the flow of the intake or exhaust, or increasing the number of valves per cylinder, they all have the same objective – more air (and of course fuel mixed with it) = more power.
Of course, when it comes to a vehicle powered by electricity, the obvious answer is that you need more current or more voltage. But the limiting factor isn’t always what you expect. In this article, we’ll look at three aspects of increasing performance: acceleration, top speed, and towing capacity.
Firstly, let’s consider acceleration, which, if you’ve driven one, you’ll appreciate is one thing electric vehicles do pretty well. Whilst there are high-performance models from companies like Porsche, Tesla, and BMW, even a run-of-the-mill 60bhp Kangoo van pulls away quite well when nipping about town.
There are a few factors at play here – one is probably that the lack of noise makes the acceleration feel effortless, therefore perhaps is only a perception rather than a real factor. Another is that a combustion engine has such a narrow speed range that it requires multiple adapters to make it useful for driving a car, and these need to be swapped over whilst on the move. These gear changes take time, which of course slows down our acceleration and is another win for the single-speed electric vehicle. But the biggest difference of course is that the electric motor can provide full torque as soon as it starts turning. The inverter can also control the torque (via current) and the speed (via frequency) separately. Some situations such as motorway cruising require low torque but high speed, whilst others such as driving up a kerb, need more torque but less speed.
So, we’re already off to a good starting point, but how do we increase the power of our electric car? I’ll introduce two cars here to make my point – one is the Nissan Leaf and the other is the Mercedes-AMG GT e-Performance, a plug-in hybrid. The Nissan Leaf comes in two versions: one with 148bhp and the other with 215bhp. Those tempted to put the 215bhp motor into the lesser car might be surprised to learn that the two motors are in fact the same, only details that differ from the older Leaf’s 110bhp motor. Nissan are fitting powerful motors into all their cars, but only letting people use part of it by setting current limits in the inverter software. What’s more, the more powerful version is only available if you buy the longer-range car with the larger battery, so what about people who want to go fast but not far?
It’s not all about marketing – the larger battery not only stores more energy, it also has a higher maximum discharge rate. It’s the same with Tesla, who offer various battery pack sizes. You don’t need much range to do a ¼ mile sprint, but drag racers prefer the large battery versions as it’s not always about the amount of energy you can store, it’s also about how quickly you can get it out. The same is of course true of 12V batteries, a Land Cruiser doesn’t have a bigger battery than a Micra so you can leave it parked up for longer periods, it’s because its larger engine takes more amps to crank over. When you compare a plug-in hybrid to an electric car, if they’re both driving at the same speed using electric power, the PHEV’s small battery must work harder than the bigger one in the EV, which also affects longevity. Talking of plug-in hybrids, let’s get back to that long-named AMG that I mentioned.
The current range of AMG hybrids all have a 200bhp electric motor on the rear axle in addition to their petrol engines, equivalent to getting an already fast car and adding the powertrain from the faster version of the Leaf we talked about. AMG appear to have broken the laws of physics, as this plug-in hybrid has a battery capacity of only one-tenth of the Leaf, but with the same discharge rate. If we scaled that up to the Leaf size, we could pull 2,000bhp out of it, which would be quite some Leaf. There are various improvements in the design of the cells to achieve this and a very significant difference in the cooling system. The Leaf, unusually by modern EV standards, doesn’t have a battery cooling system at all. This is quite normal for phones and laptops, but in a car, it does mean it isn’t always performing optimally, and it means we need some safeguards around maximum charge and discharge rates, especially if we want consistent power all the time. You can’t have a car with 300bhp which then reduces to 150bhp if it starts warming up, so they set the power limit all the time so it’s a bit more consistent.
The AMG features the most sophisticated battery cooling system we’ve seen (and yes, we do spend quite a bit of time looking at this sort of thing). The cells actually sit in a bath of coolant, which is the non-conductive type for obvious reasons. A pump circulates the coolant around the cells and into a heat exchanger, where the heat is removed by a more conventional water-based coolant. It can then go off to one of the car’s radiators (in addition to the oil cooler and air con condenser, there are seven of them). Sometimes the radiator cooling isn’t enough, in which case, the coolant can instead be refrigerated using a branch of the car’s air conditioning system. Having such a system for keeping the battery cool means you can run it at a higher temperature, a bit like if you have good brakes, you can drive faster. Keeping the battery at just the right temperature means you can optimise the discharge rate and get more power.
So now that we can accelerate faster, what can we do about our top speed? Although capable of exceeding the legal limit, some EVs have lower top speeds than an equivalent car with a combustion engine, so why is this and what can be done if we want to improve it?
This all comes down to that single-speed transmission we talked about. Because an electric motor can provide enough torque to pull away from rest, but keep going all the way to 80mph, virtually all manufacturers have just opted for a single gear. Small motors in big cars need to be lower geared to provide enough torque to pull away, which means they are spinning like a jet engine by the time you get to motorway speeds which isn’t ideal. That’s why some Volvo and Peugeot hybrids declutch the electric motor at high road speeds, to protect the motor from exceeding its maximum speed. Most hybrids from German manufacturers fit the electric motor on the input side of the gearbox, which of course means that whether on electric power or engine power, the electric motor can drive the wheels through several different gear ratios. That’s the key to top speed, and although our AMG’s electric motor drives the rear axle rather than the gearbox input, it does use a two-speed gearbox, a system also used by the BMW i8 and the Porsche Taycan. As well as increasing top speed by keeping the motor within its useable speed range, this can also improve acceleration, because the first gear can be a lower ratio. Although electric motors are not as sensitive as combustion engines, they are still more efficient at some speeds than others, so there can also be some efficiency benefits.
Another reason people look to increase performance is because they use their vehicle for towing, and this is another area where EVs can differ from an equivalent combustion engine vehicle. Although towing a trailer will reduce efficiency in any vehicle, EV charging can be quite awkward with a trailer, as the design of many charging points will force drivers to unhitch before charging. However, for shorter journeys, the high torque and weight of an electric vehicle would make it appear quite well suited to towing. And yet many of the first EVs weren’t rated to tow anything. Even the 400bhp, four-wheel drive Jaguar I-Pace can only tow 750kg, less than a Renault Clio. Hybrids are similar- the first three generations of Toyota Prius couldn’t tow anything at all. So why is this, and what can be done?
There are two factors at play here – one (for hybrids) is whether the electric powertrain is doing the work, and the other is the electric powertrain’s cooling system. As we know, burning petrol or diesel produces plenty of waste heat. In fact, for every £100 you spend on fuel, over £50 of it gets spent keeping the radiator and exhaust warm, with some of what’s left used for driving the car. For this reason, they have very effective cooling systems, capable of operating under a wide range of different conditions. Sticking a trailer on the back is one of those conditions.
The electric powertrain by contrast makes relatively little heat because it doesn’t burn anything. There’s still heat created by the internal cell resistance, and current flowing through power electronics and motor windings, and this needs to be managed in the right way. Putting a trailer on the back of the car means the car must work harder but doesn’t get rewarded by extra air through the radiator. It’s like driving up a hill that goes on for hours, and many electric vehicles simply weren’t designed with this in mind.
Some hybrids have much better towing capacities – if you look at the current Mercedes E-Class, the 2100kg towing capacity of the hybrid is the same as the non-hybrid. But when you look at how the car works, the motor is simply assisting the existing powertrain. Compare that to the front-wheel drive Nissan X-Trail ePower hybrid. On this vehicle the wheels are always driven by electric power, giving a smoother and swifter driving experience. But with the motor doing all of the work all the time, the towing capacity is just 670kg.
The good news is that if people want cars that can tow, manufacturers can easily make it so, with a few tweaks to the cooling system and another hoop to jump through in the type of approval process. There’s no technical reason why they can’t- they’ve simply not added the ability because they haven’t expected demand for it.
These days there are plenty of new models with towing weights over a tonne. The relatively small Mercedes EQA can tow 1,800kg, and the BMW iX can drag along 2,500kg. If the consumer demands more, there’s no reason why future models won’t have even higher towing capacities.
Disclaimer: please refer to the manufacturer’s data for exact specifications for towing capacities.