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GKN’s Gyrodrive flywheel hybrid system is ideal for “big fuel consumers and lots of start and stop.” Therefore, buses offer the best application, as do other commercial vehicles such as refuse and recycling trucks and distribution trucks.

GKN develops electric flywheel hybrid system for buses

The core of GKN’s Gyrodrive flywheel hybrid system was developed by Williams Hybrid Power for a kinetic energy recovery system (KERS) designed for the Williams F1 team. GKN acquired the technology in April 2014 and has developed it for both endurance racing and heavy commercial vehicle use. The company recently demonstrated Gyrodrive as designed for use on city buses.

The system differs from purely mechanical flywheel hybrid systems, like that developed by Ricardo, in how it transfers kinetic energy between the driven wheels and the energy storage system. The essential components include a traction motor, driven from the vehicle’s drive axle, an electric flywheel, an inverter for the motor/flywheel unit, and the electronic control system.

When the driver starts to brake, the flywheel system is triggered, engaging the traction motor drive system, which will spin the motor (an EVOmotor designed by Williams) at up to 8000 rpm to generate electricity. The power generated is transferred by cable to the integrated motor/flywheel. The EVOmotor is of the transaxial flux type, offering higher torque and power densities than conventional radial flux motors. It is also very slim, aiding the packaging process.

Reinhard Reinartz, Director of Engineering and Technology at GKN Land Systems, is responsible for the Gyrodrive system. He describes the integrated motor and flywheel: “The rotor spins around the basic stator in a vacuum. The rotor is made up of two layers of composite material. The inner layer contains the magnetic material, coming out of the uranium enrichment industry. Then there’s a containment ring made of normal composite material just to keep it in place.”

The motor/flywheel can spin at speeds up to 45,000 rpm, although in the bus application speeds are generally in the 16,000- to 36,000-rpm range. The motor/flywheel is designed to be fitted under a passenger seat in the bus, so passenger capacity is not affected. The electronic control system is mounted under a second passenger seat. Altogether the system weighs 300 kg (660 lb), according to Reinartz.

When the recovered energy is needed to help power the vehicle, the process is reversed with the flywheel-driven motor generating electricity that is fed back to the traction motor and into the drivetrain.

Reinartz says that GKN’s expertise with packaging drivetrain components was one of the reasons that Williams approached the company.

“They have the system in Audis and other racing cars but leave it up to Audi to connect it to their traction motor,” he explained. “Here they asked us, ‘Do you have a gearbox?’ Yes, we have a GKN gearbox here and the EVOmotor. So we packaged this, which is the bread-and-butter mechanical portion, then we have the KERS system that is installed inside. It will basically fit in all types of buses. The only thing that we need is measurements and then obviously some data of the ECU of the gearbox and the engine.”

As Reinartz points out, if the system develops a problem, the bus can still be operated independently without it.

Both traction motor and motor/flywheel use a closed-circuit oil cooling system, intercooled via the engine coolant system. “We just plug that system into the bus’s system; that’s it,” said Reinartz. “Running temperature is 80°C.”

“In the morning when the bus operator starts, it only needs a couple of seconds to let that system spin up, so we have a mode to get it up and running. The working rev range is between 16,000 rpm and 36,000 rpm. Everything below, then it switches off and just idles. Then to work on it, there’s a switch-off mode and a discharge mode, so all the safety issues have been taken care of. EMC compliance has also been taken care of.

“The operating voltage is 500 V and energy storage is in the 1.2- to 1.8-MJ range, with a power rating of 120 kW. The weight of the motor/flywheel unit is around 60 kg. It’s not just the weight of the aluminum housing and the carbon composite, but there’s obviously copper in there, too.

“The beauty here, in comparison with a steel flywheel, is the efficiency. On its own it is 98% [efficient], but in the system completely, it’s 85%. That is the reason why we are claiming 20% to 25% fuel saving, depending on cycle,” said Reinartz.

“It’s still too big and too expensive for normal road cars. To get the best out of it, you need a big fuel consumer and lots of start and stop. The bus is the best application, then obviously others like refuse and recycling trucks and distribution trucks.”

The current Gyrodrive system is a parallel hybrid system, but GKN could develop a series hybrid variant. GKN is also considering the possibility of including batteries in the system or interfacing the system with hydraulics.

“In the end, there will be lots of portions of technology helping a big system in order to reduce fuel consumption and emissions,” he said. 

According to Reinartz, GKN is already looking at researching battery-electric buses with a Gyrodrive system incorporated. This could help to reduce the number of batteries needed and to smooth the charging cycle.

“Batteries don’t like deep cycling—draining completely, charging completely, so we could smooth out the cycling. The batteries could take over the moment the bus starts coasting. While it’s decelerating, you get a bit of energy back. Then while it’s accelerating you have the flywheel energy taking care of that mode,” he explained.

GKN can provide the system either as a retrofit, by replacing a drive shaft with a replacement modified to accommodate the traction motor, or supply it for OE fitment. The company has recently signed a deal to supply U.K. bus builder Alexander Dennis with an OE system. GKN believes the system can deliver more than one million cycles, so it should last for the life of the vehicle.

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