Author: Jules Carter, chartered engineer, director of advanced engineering, GKN Land Systems
Fans of motorsport no doubt have heard the question, ‘What has motorsport ever done for the real world?’ Those fans will be pleased to hear that there is now a very positive answer: it has developed a technology that can save fuel on buses, trucks and trains.
The principle behind the energy storage flywheel, which sits at the heart of GKN Hybrid Power’s Gyrodrive system, is very simple. Instead of any braking being done by the friction of a brake pad on a disk or a shoe in a drum where the energy is lost as heat, an electric generator in the driveline turns kinetic energy into electricity. This power is used to drive an electric motor to accelerate a flywheel; the spinning flywheel holds the energy until the vehicle needs to accelerate, at which point the cycle reverses. Electricity is generated as the flywheel slows down and it is used to drive the electric motor to help acceleration.
Flywheels are not new; they have been used as energy storage on buses before without any success. So the question is, what has Formula 1 done from those early days to make them ideal for the city buses of today and satisfy the need for low emission transportation? The answer to this lies in the material used in the flywheel.
Historically, flywheels used large steel rotors spinning at a few hundred revolutions per minute. In a flywheel, the energy stored is calculated by multiplying half the total inertia by the rotational speed squared. Therefore, the key factor is rotational speed. However, high rotational speed leads to high hoop stresses on the outside of the wheel – these are the stresses pulling the flywheel apart as it spin.
The engineers at the Williams Grand Prix team needed to store the energy in as light a device as possible. Therefore, if a flywheel was to work, they needed very high rotational speeds so the low inertia of a light flywheel was not a problem. In using carbon fibre wound round the flywheel, they developed a lightweight energy store that was capable of spinning at speeds of up to 40,000rpm. Regulation changes meant the flywheel could not be used in F1, but the technology instantly found a home in endurance motorsport where the long life and reliability made it ideal for the Le Mans 24 Hour race – firstly with class wins in a Porsche and then, for the last three seasons, helping Audi to win outright.
The development team behind the flywheel technology could see that this technology had huge potential beyond motor racing, realising the benefit of fuel saving from capturing braking energy in stop-start drive cycles on city buses and trucks. At GKN, we saw this potential and could provide the necessary investment for mass production. This led to us buying the Hybrid Power Company from the Williams Grand Prix team in April 2014.
Batteries and hybrid buses
The first such successful application has been in buses. It is a great coincidence that the total kinetic energy of a racing car is very similar to that of a double decker bus: half the mass times the velocity squared. A bus has a fraction of the velocity, but a lot more mass.
Hybrid buses have been on city streets for many years now, mostly funded by UK Government grants. They are expensive systems because they have predominantly relied on lithium ion batteries to store the braking energy. These battery packs are expensive, unreliable and do not have a long life. Anyone with an elderly laptop or phone will know that these batteries soon lose performance. For the hybrid bus, application engineers are looking to store the energy saved from the slowing bus for a few minutes until the passengers are on or the traffic lights change and the energy is again required. For this, the lithium ion battery is fundamentally the wrong choice for energy storage.
A battery charging or discharging is a chemical reaction and, as such, can only happen at a certain speed. Therefore, a battery that needs to absorb twice the power has to be twice the size. In sizing a battery pack to absorb all the braking energy of a bus, an engineer ends up with a very large pack that actually stores much more energy than is actually needed. This has huge weight and cost implications and explains why every hybrid bus needed a grant to cover the investment.
Even with these grants, operators are left with large bills for battery replacements every few years. Battery hybrid systems can add 2 tonnes to the weight of a bus; this results in the operator having to reduce the passenger carrying capacity of the bus, which results in the bus costing more and carrying fewer people.
The lightweight composite flywheel in the Gyrodrive system carries just enough energy to stop and start the bus from about 50kph. This is 1.5 MJ, or about the same energy as a double whiskey. The analogy to whiskey is not accidental, as it has almost the same calorific value as diesel, so every acceleration done with the Gyrodrive uses that much less diesel. Over the course of a day, these stops add up to many litres of fuel saving and an overall reduction in fuel use of about 25%.
Put another way, an average London city bus covers about 35,000 miles per year at an average 6MPG. This equates to 26,500 litres per year. The Gyrodrive system will save around 20%, depending on route i.e. a Gyrodrive-equipped bus will burn 5,300 litres less fuel every year, which is 14 tonnes of C02 not released.
Stop-start drive cycles are not restricted to buses, so it is easy to see that the potential for this technology lends itself to delivery trucks, refuse trucks and into other modes of transport such as commuter trains and city trams. In these examples, the flywheel’s electrical drive is the key; the flywheel can be viewed as a mechanical battery. For its power it is cheaper, lighter and will last the life of any vehicle.
Electric vehicles
Acting as a mechanical battery, the flywheel will also have a place in pure electric vehicles (EVs). In these vehicles, the lithium ion batteries have high energy storage capability but do not do well providing high transient power loads. They are further compromised if required to absorb high power levels under regenerative braking. Achieving these goals leads to battery designers using expensive chemistries and larger cooling systems.
Using a flywheel in the system allows the engineers to maximise the energy storage and life of the battery, whilst minimising cooling requirements and pack size. All the power peaks in acceleration and braking are taken by the flywheel with no reduction in life. Research is ongoing to demonstrate the benefits in battery pack life, the goal being that battery EV buses and trucks will not need expensive battery replacements after a few years – which is ideal for making our cities cleaner and quieter.
GKN’s investment in affordable hybrid technology paves the way for the development of fuel-saving vehicles in many new markets around the world; this is a testament for the engineers who could envisage how a motorsport technology could have real advantages in the commercial world of buses and trucks.
Jules Carter is the director of advanced engineering for GKN Land Systems, the UK-based manufacturer of driveline components for the agricultural and off -highway industries. In this role, he is responsible for the design and development of new products and systems focussing on hybrid and electric-drive systems. He is a chartered engineer and fellow of the Institution of Mechanical Engineers and a member of the IMechE Professional Review Committee. He has a BTech Hons in Automotive Engineering from Loughborough University.