The Flybrid kinetic energy recovery system (KERS) incorporates Continuously Variable Transmission (CVT) technology sourced from the partnership of Torotrak Plc and Xtrac. Xtrac is using Torotrak’s full-toroidal traction drive technology for use in kinetic energy recovery systems within the motorsport industry. (Earlier post.) The FIA have defined the amount of energy recovery for the 2009 season as 400kJ per lap giving the driver an extra 80hp over a period of 6.67 seconds.
The mechanical KERS system uses flywheel technology developed by Flybrid Systems to recover and store a moving vehicle’s kinetic energy which is otherwise wasted when the vehicle is decelerated. The combination of gearbox-variator and flywheel form part of the driveline assembly. Energy is received from the driveline through the Torotrak CVT as the vehicle decelerates, and is subsequently released back into the driveline, again through the CVT, as the vehicle accelerates.
Compared to the alternative of electrical-battery systems, the mechanical KERS system provides a more compact, efficient, lighter and environmentally-friendly solution.
Although flywheel systems are not new, installations tended to be heavy and the gyroscopic forces of the flywheel were significant. To overcome these issues,Flybrid uses a smaller and lighter flywheel that rotates at more than 60,000 rpm. This advance in speed has been made possible by several key inventions for which the company is seeking patent protection.
Power transmission between the flywheel and the wheels is only limited by the capability of the CVT, according to Flybrid, and it is for this reason that they sought the Torotrak solution.
We believe the Torotrak solution offers the smallest and lightest package for the power output required and that the inherent torque controlled nature of the device ideally suits our application.—Jon Hilton, Flybrid’s Managing Partner
For F1 applications, the variator and flywheel each weigh less than 5kg in a system with a total mass not exceeding 25kg. The high level of mechanical efficiency combined with the variator’s ability to change ratio very rapidly helps to optimize flywheel performance. The transmission system selects the appropriate ratio depending on the torque demand and can change its 6-to-1 ratio within one revolution.
Flybrid has also filed various technical patents to tackle the key engineering issues of safety and noise. The flywheel is made from high-strength steel and composite material and has been designed with a high factor of safety in which the maximum stresses are significantly less than in the con-rod of a conventional internal combustion engine, according to the company.
The crash test procedure consisted of spinning the flywheel up to its full 64,500 RPM top speed before disconnecting the drive portion of the spin test rig and crashing the complete test chamber. Inside the test chamber was a F1 car representative light alloy housing within which was mounted the F1 car representative 400 kJ flywheel.
The test was performed at the Cranfield Impact Centre, a recognized F1 crash test facility, and involved a peak deceleration level of “more than 20g.” The exact level cannot be disclosed as the profile of deceleration was matched to the actual crash test data from a clients F1 car. After the test the flywheel was still spinning at high speed and was completely undamaged.
Flybrid has already secured one unnamed F1 team as a customer. The company is also well on its way to bench testing a flywheel KERS system adapted for road car applications using a Chevrolet V8 engine.
Flybrid, Torotrak and Xtrac all see the potential for wider application beyond motorsport—initially on high-performance road cars—both as an aid to performance and as a means of developing vehicles with reduced fuel consumption and CO2 levels. Applied to road cars the system supports the current motor industry trend for smaller powertrains; a lightweight kinetic energy recovery system providing a means of boosting acceleration and overall performance and economy independently of the vehicle’s internal combustion engine.
An ancillary flywheel is particularly suited to stop-start driving situations when real-world fuel economy is often at its worst. In these conditions, the variator can assist the launch of a vehicle which has slowed down or come to a standstill. In heavily congested traffic, where a car is frequently stopped and restarted, the system can help alleviate the heavy fuel consumption and emissions of greenhouse gasses normally associated with these conditions. However, unlike hybrid electric vehicles, a mechanical KERS system continues to provide the benefits of kinetic energy recovery throughout the speed range, and its benefits are maintained on the open road.
On a directly comparable basis, a flywheel system offers up to twice the efficiency of a kinetic energy recovery system that stores its energy in a battery. The overall in-out efficiency of a mechanical drivetrain feeding energy into a flywheel and back out to the vehicle again via an ancillary transmission system is approximately 65-70 per cent compared with 35-45 per cent for a hybrid battery-electric system. Fundamentally, this is because a purely mechanical system doesn’t have to convert the kinetic energy into electrical and chemical energy as with a battery system.
What this means is that with a flywheel each time the brakes are applied at least 65 per cent of the energy is available to re-accelerate the vehicle, whereas the best that can be achieved with existing battery technology is 45 per cent.—Jon Hilton
Flybrid and Xtrac will discuss the technical details of the flywheel KERS at the upcoming Global Motorsports Congress being held in Cologne on 5-6 November 2007.