Decoding electric racing cars

Electric cars have been around for longer than their petrol and diesel brethren, yet discussing their features can sound like another language entirely. Current E is on hand to demystify the Formula E Spark-Renault racing car with a little help from friendly electric vehicle design and engineering experts Cliff Rassweiler and Chris Vagg.

Formula E’s introduction of electric racing cars to a widespread audience requires getting to grips with a new set of baffling technical terms. Not a rocket scientist, atomic physics lecturer or electrical engineer? Never fear – handily, most of the principles can be easily compared to internal combustion engine cars, whose terminology is pretty well understood.

The system

Let’s dispel some of the terrifying technical jargon immediately. At heart, all cars need a method of propulsion – usually an engine or a motor – and an energy supply to power that motion. This system is called the powertrain.

In F1, the big howling V8 engine (V6 in 2014) is simply turning the chemical energy of the fuel into kinetic energy to move the wheels. In the Formula E racing car, the same thing happens, but the energy is drawn from batteries and it is an electric motor rather than a V8 doing the transforming work.

Motors and power

So far, so straightforward. (Although the motor may be called a motor generator unit, or MGU for short.)

When we play top trumps with internal combustion engine cars, we talk about torque and horsepower (hp) – or more commonly, brake horsepower (bhp). Generally, more torque provides more acceleration, and increased horsepower means increased top speed.

Electric motors are rated by torque and horsepower, too. But we also talk about how much current the motor can handle (in amps) and the voltage (in Volts) that it runs at. More current means more acceleration. Higher voltage means higher top speeds.

The White Zombie, a drag racing electric vehicle, jams 1,800amps into its twin motors to give it lightning-fast acceleration over a very short distance. It reaches 60mph from rest in 1.8 seconds. Its system voltage is just 355V however. In contrast, the Buckeye Bullet, which is competing for the record of world’s fastest electric vehicle and so needs the best possible top speed, has a system voltage of 825V.

Multiplying the current by the voltage gives the power of the system, measured in Watts (W). A single horsepower is equal to 746W. That’s how we translate the peak power of the Formula E racing car, reputedly 200kW, as 268hp (rounded to 270hp in most places).

Battery capacity

The other critical factor in electric racing is the power source – the batteries. These are a bit like the fuel tank in a conventional racing car: too big a tank will weigh the car down and reduce speed; too small a tank and the car must throttle back or risk running out of fuel well before the finish line.

Rather than a fuel tank, electric cars have a battery pack that includes battery modules (collections of cells, which are the smallest and simplest battery unit), and control and thermal management systems. The whole thing may be referred to as a rechargeable energy storage system (RESS).

Instead of measuring capacity in gallons or litres, batteries are rated in kiloWatt hours (kWh). The entire system in the Spark-Renault car will have a 30kWh capacity. That means that the pack can supply 30,000W (around 40hp) for one hour. Put another way, if the car was running continuously at the top end of the motor’s capacity – 200kW – it would drain the battery in nine minutes.

Different battery types come with different benefits and drawbacks, just as petrol and diesel deliver different operating and performance characteristics. The Spark-Renault will be using lithium-ion batteries, which cram in a lot of power yet are lightweight. They are relatively expensive to produce compared to other batter types.

Economy

When looking at the fuel economy of a car with an internal combustion engine, we use the familiar terms miles per gallon (mpg) or kilometres per litre (km/l). For electric cars, we use Watt hours per mile or kilometre (Wh/mile or Wh/km).

An electric car might typically be expected to run at around 250Wh/mile in everyday road use, but would consume something nearer 1,250Wh/mile on the race track.

This is probably the least sexy consideration for the average racing fan or commentator, but is hugely important in the construction and operation of the car. The design engineers at Williams (which is providing the battery packs), McLaren (the motors and ECU ‘brains’) and Spark (overall build of the car) must figure out how to build a system which has the capacity to make it to the finish line, but be lightweight enough to make for good racing, and fit inside the sleek shell designed by Dallara.

The teams must then analyse how their drivers put down all that power in the race, reigning them in if the batteries are being drained too fast or spurring them on if there’s power left in reserve. Not too dissimilar from conventional motor racing, then.

Road car relevance

Some top-line racing series feature technological innovations that never leave the track, making it difficult for even the most ardent fans to keep up with and understand the gadgets and gizmos.  

In stark contrast, Formula E aims to be directly relevant to road cars, which means that understanding the phraseology used during races will help the everyday motorist better understand their own electric vehicle, and vice versa.  

Current E graphic inside the Formula E racing car

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