Hybrid Traction System
  with a flywheel energy storage system

Improved regional passenger transport systems are under consideration for urban and suburban areas world-wide. Present solutions comprise electric trams and diesel buses operating at a particular high frequency of start and stop cycles. Diesel powered vehicles are unattractive because of pollution and ride quality; electrification is associated with high costs and visual intrusion.

The presented hybrid traction system with a flywheel energy storage enables brake energy recovery and load leveling. This allows for energy efficient vehicles of high quality, able to meet strict emission limits and to provide a standard of performance expected of modern electrified systems without involving electric supply infrastructure.

Project overview

Characteristics and benefits of the hybrid traction
  system with a flywheel energy storage system

  • Ride quality and drivability of the vehicle equal to those of fully electrical systems
  • Smaller PMU compared to conventional traction systems
  • PMU operates in the best point of efficiency at constant power
  • Dual mode capability of vehicle
  • Wide choice of Prime Mover Units, including fuel cells
  • Emissions from less than EURO 5 standard to zero
  • Energy saving and CO2 reduction up to 40%
  • No electric supply infrastructure
  • Reduced operational costs
  • System structure performs best for urban passenger transport with high frequency of starts and stops

This technology was demonstrated successfully in two prototype vehicles: A flywheel bus built by CCM (Phileas in Eindhoven) and a flywheel tram (from the previous EU-project ULEV-TAP).

Project description

ULEV-TAP 2 aims to extend hardware development from the previous EC sponsored project based on series electric hybrid concept for the light rail application. The resource allocation has been targeted for vehicle ready hardware. There will be no attempt to integrate the hardware in the present project due to budget cut-downs. However a detailed market study will be done to show the potential for light rail vehicles using advanced propulsion. 

The project now aims to concentrate research and development effort on the central hardware required for a series electric hybrid drive based on the flywheel technology energy storage system, diesel engine driven prime mover unit and a supervisory control unit for safe, reliable and optimised drive.

The project is financed to 50% by the European Union, the other half is invested by the principal contractors. Imperial College London,as an assistant contractor receives 100% funding.

Expected results

The project ULEV-TAP 2 will result in highly optimized hardware components for electric hybrid drive including a prime mover unit (PMU), a premium power unit (PPU) based on flywheel technology and the integrating supervisory control unit (SCU) for optimised energy use in a series hybrid system configuration.


The overall achievements targeted in the project are the following:

  • Improvement in overall system efficiency by 40% as compared to diesel
  • ULEV and EURO 3 emissions standards by 50% reduction by hybrid configuration
  • Reduction in maintenance cost by 50% as compared to diesel electric
  • Reduction in the investment costs by approx. 33% as compared to electrified systems
  • Minimisation of noise to EU standards (20 dB lower than diesel)