Improvements in fuel economy of up to 22.4% have been demonstrated by a research vehicle fitted with a flywheel hybrid system including stop-start.

Developed by a consortium of British companies as part of the Government-supported Flywheel Hybrid System for Premium Vehicles (FHSPV) programme, the mechanically-driven flywheel system delivers up to 80bhp (82PS, 60kW) of recovered energy from a self-contained hybrid module.

In the industry-standard NEDC cycle, the flywheel hybrid including stop-start achieved an 11.9 percent improvement.

The consortium believes that mechanical hybrids solve many of the challenges associated with electric hybrids. There is no inefficient conversion of energy from kinetic to electrical to chemical and back. And the cost, weight, packaging and recycling issues associated with batteries are also eliminated.

“The research shows the potential of mechanical hybrids as an affordable alternative to battery hybrids,” confirms Prodrive’s Head of Vehicle engineering, David Hemming. “Both the fuel economy results and the driveability are impressive, even with early-stage calibrations and no other design optimisation.”

The FHSPV engineering development vehicle recovers energy via the rear differential through a continuously variable transmission (CVT) into a high-speed flywheel. When the driver reapplies the accelerator, the CVT smoothly transfers the energy back to the wheels. The flywheel and its drive system are installed adjacent to the rear axle, in the space normally occupied by the spare wheel and the whole system weighs 80 kg. Minimal body and packaging changes were required to integrate the system. There is no change to the driveline configuration.

Designed by Flybrid Systems, the flywheel is constructed from carbon composite and operates in a partial vacuum, allowing it to spin at up to 60,000rpm. The CVT, which manages the flywheel’s speed and the flow of energy in each direction, has been built by precision-engineering firm Xtrac using proven traction drive technology from Torotrak.