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When will we see a series hybrid racing?

While Formula One and endurance racing sanctioned by the ACO (which organises the Le Mans 24 Hours) have shown leadership in recent years by encouraging or mandating the use of hybrid systems in racing, these have all been parallel hybrids – that is, engines that drive the wheels and whose output can be augmented by electrical or mechanical means. However, we have yet to see what might be ultimate expression of the hybrid system – a series hybrid.

This type of hybrid still uses an internal combustion engine, but uses it only to drive a generator; the engine itself has no direct mechanical connection to the wheels. It has been used widely and successfully in passenger applications for decades, mainly in diesel-electric trains, and this type of powertrain has much to recommend it.

Most of us will be primarily involved with race engines, although a small minority of us (myself included) have had periods in our careers during which our main occupation has been hybrid systems. The implications of using a series hybrid approach have profound implications on the design of the engine that drives it. Logically, we would use the engine only at full throttle at a single speed; we can design the engine to be optimised at this single speed, at which it would run after a short warm-up period. This should mean that the engine becomes much more efficient as a whole – everything would be optimised for this one speed. When we don’t need the engine we would simply switch it off.

The closest approach racing has had to this was the Williams FW15C CVT car. This wasn’t a hybrid but used a constantly variable transmission. If you see footage of this car in a straight-line test, it sounds very strange – there are no gear changes and no change in engine tone, just an incessant fixed sound under acceleration. A series hybrid powertrain would have the same ethos, except there would presumably be periods where the engine could be turned off completely. The single-speed engine has many advantages: it makes estimating fatigue life for each component much easier, and therefore it should be easier to arrive at an optimum design.

The electric side of the powertrain, assuming that the first series hybrid is an electric system, replaces the electrical components of the parallel hybrid and the car’s usual transmission. There is a large generator attached to the engine, linked to a motor or motors that drive the wheels. In an optimal system, we might see an independently controlled motor driving each wheel. This is certainly what we would want in a passenger car if safety is our primary concern.

If we want motor racing to remain a visual spectacle though, we might choose to have only the rear wheels driven. The battery is still required as a reservoir of energy, with the size depending on how much captured energy from braking events we would like to store. The maximum power that the battery can discharge can be added to the maximum power that the engine can develop. A large battery also requires large propulsion motors to handle the battery’s extra power. There is a trade-off between the extra mass of the battery/motors and the fuel that the system allows us to save, and the extra mass they add to the vehicle and the energy involved in braking and accelerating this additional mass.

Such a series hybrid system is an intriguing engineering challenge – one day we might drive such a passenger car. Will we ever see it in racing though? Perhaps even the technologically adventurous ACO isn’t that brave – it might ruin the visual and aural spectacle of racing irrevocably.

Written by Wayne Ward

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