The first law of thermodynamics as applied to cars: burn fuel to transform chemical energy into movement (kinetic energy) and quite a lot of heat is emitted by the cooling system and exhaust. Then hit the brakes and convert the kinetic energy to heat that is radiated by the brake discs. All fuel ends up as heat. The point of any KERS is to capture and store the kinetic energy the vehicle needs to lose to slow down, before it’s turned into heat, then use it when you want to accelerate.
We’ve all become familiar with the various forms of hybrid vehicle using a petrol or diesel engine, along with an electric motor and a regenerative braking system that puts energy back in the battery as the vehicle slows down. The best known in the UK is probably the Toyota Prius range, which has been in production since 1997. The Prius Plug-In is an example of the ‘strong hybrid’ type of vehicle, one that can run for a considerable distance on recovered energy. But the big disadvantage of vehicles of this type is that batteries are heavy and expensive. Each battery pack contains 10 to 15kg of lanthanum, and each Prius motor contains 1kg of neodymium, both rare metals that are difficult to mine and refine. And just like the smaller batteries that power our gadgets, car batteries need to be replaced as their ability to hold a charge declines.
A mild hybrid stores much less energy and cannot be driven for long on that alone, it is there to boost the power available for acceleration. In racing, KERS is designed to give a car a brief advantage accelerating out of a bend, or attempting to overtake. On the road it’s more about reducing fuel consumption.
Car engines rarely need to give full power for more than a few seconds at a time, but we need to have power in reserve for acceleration. To produce that power, we have a bigger engine than we need most of the time, which means more weight and higher fuel consumption. KERS makes it possible to build a lighter car with an engine designed for economical cruising, with the KERS providing a boost when you really need it.
There are many ways of storing energy, some more practical than others in a vehicle. Batteries of course, compressed gas, and flywheels are all contenders.
Servé Ploumen, Technical Expert Vehicle Energy Analysis at Ford Research and Advanced Engineering Europe in Germany told us that Ford had analysed all the options, and built a demonstration vehicle using an electrically powered flywheel system before deciding to concentrate on a mild hybrid set-up using batteries. Strong hybrids like the Prius use high voltage battery packs which require serious safety precautions and specialist components, and in common with other German manufacturers, Ford is developing a system using 48 volts (The LV 148 standard) which is inherently safer and means that many of the components are not very different to standard automotive parts. Delphi Automotive, who have worked on developing equipment for the LV 148 standard, say that a modified belt-driven alternator running in motor/generator mode can reduce CO2 emissions by 15 per cent.
PSA Peugeot-Citroën also looked at the various options and came to a different conclusion. The ‘Hybrid-Air’ platform they announced in 2013 and showed at the Geneva Motor Show in March this year uses compressed Nitrogen to store energy. PSA Peugeot Citroën are looking to the world market for mid-range cars and believe batteries are too expensive, especially for developing markets. The system features a variable output hydraulic pump/motor combined with an automatic transmission and a storage cylinder running the length or the car. The cylinder is full of nitrogen and fitted with a piston. Under braking, oil is pumped into the other end of the cylinder and the piston moves along compressing the gas. Nitrogen is used rather than air, because compressed air and oil can be somewhat explosive. PSA Peugeot Citroën claims that ten seconds worth of braking is enough to fully charge the cylinder. In city driving, the cylinder is also recharged directly by the engine running at its optimum speed, rather than constantly cycling up and down as it would normally. The company claims it can run on air alone for 60 to 80 per cent of the time in city driving. On the open road, the car runs conventionally most of the time, but with extra power stored in the gas cylinder for overtaking and acceleration.
Flywheels have been used as energy stores for centuries and the electric Gyrobus developed in the 1940s and 50s had a three-tonne flywheel, allowing it to run off-grid. In recent times, two projects stand out. Williams developed an electrically driven flywheel system for F1 that was overtaken by the shift to pure electrical systems, and Flybrid Automotive developed a mechanically driven flywheel system which began life using a Torotrak continuously variable transmission (CVT) to connect the flywheel to the drivetrain. Williams sold their project to GKN who are developing it for use in road vehicles, whilst Flybrid was acquired by Torotrak and are now developing the technology for buses, excavators and passenger cars, incorporating new clutched flywheel transmission technology.
Using an electrically powered flywheel simplifies the layout and control system which resembles a conventional electric hybrid, but with a motor/generator and flywheel replacing the battery pack. The GKN system has been used to great effect in the diesel engined Audi R18 e-tron quattro which won the 24 hours endurance race at Le Mans this year. The flywheel provides a power output of 228bhp, with a maximum flywheel speed of 40,000rpm.
The GKN system is adaptable to different types of vehicles and is already achieving fuel efficiency savings of over 20 per cent on prototype buses being trialled in London.
Flybrid units are also being trialled on buses, in this case by Wrightbus in Northern Ireland, and the company is working with various car manufacturers to develop custom systems. The company is currently running a Volvo S60 demonstrator, which achieves an impressive 25 per cent fuel efficiency improvement over an equivalent powered vehicle. In this set-up, the front-wheel-drive Volvo has a Flybrid unit integrated in the rear differential. Flybrid use a steel and carbon fibre flywheel running at up to 60,000rpm. To reduce friction, it runs in a vacuum chamber fitted with patented sealing technology. The Volvo offers two modes: Hybrid and Sport.
In Hybrid mode, the engine cuts out as you brake, and kinetic energy from the rear axle is diverted to the flywheel. When you accelerate, up to 81bhp is available from the flywheel, but the engine output is limited, resulting in much the same performance as the standard set-up, but using less fuel. During sustained high speed motorway driving, the system uses ‘boost and cruise’ which charges the flywheel, then cuts the engine for a mile or so, improving the fuel consumption.
In Sport mode, the flywheel is charged in the same way, but the power from the flywheel is added to the 255bhp from the engine. The extra energy is diverted from the engine when the car is cruising to ensure there is always some energy available in the flywheel for extra acceleration, such as overtaking manoeuvres. Flybrid claim that diverting some of the power in this way doesn’t increase fuel consumption because it kicks in when the engine is running at less than maximum efficiency, so adding a little load actually makes it more efficient.
Flybrid KERS can also be closely integrated into the vehicle’s transmission system, but on cars like the Volvo, putting it on the back axle gives a more even weight and power distribution and improves handling. Flybrid has also built a demonstrator with Jaguar, in this front engine, rear-wheel-drive set-up, the KERS unit is integrated into the differential.