Researchers at the University of Warwick (UK) are developing a flywheel and infinitely variable transmission (FIVT) for application in a mechanical hybrid powertrain. The work on FIVT is the outcome of a larger mechanical hybrid project created by the university to assess the potential of mechanical hybrid systems—flywheel, pneumatic and hydraulic—to compete with electrical hybrid systems.

The Warwick FIVT unit consists of an IVT (infinitely variable transmission) and a flywheel assembly. Any IVT can be used in the FIVT system; the Warwick researchers have been using the Torotrak toroidal IVT unit in their studies up to now.

While most flywheel hybrid systems spin their flywheels at high speeds, the FIVT flywheel spins up to the maximum speed of the engine, although it can potentially reach higher speeds under braking if the IVT unit has an input speed range greater than the engine speed range.

By spinning the flywheel at lower speeds than other systems, the FIVT delivers increased efficiency with reduced stresses, according to the team. This, in turn, allows less expensive flywheel materials to be used—steel, for example, instead of carbon fibre. Denser materials also could reduce packaging dimensions. In their simulations for the US FTP cycle, the Warwick researchers used a 600 mm diameter, 9 mm thick steel flywheel which stored 400 kJ of energy.

In simulations for a 17-tonne bus and a 2.6-tonne SUV (Chevrolet Suburban), they found that the FIVT provided the greatest increase in fuel economy across different cycles, although the electric parallel, flywheel parallel and FIVT outcomes were similar. One significant downside to the FIVT is in an idling scenario; for long idles, the energy in the flywheel can be exhausted.

There are two other proposed systems that integrate a flywheel with an IVT, the first developed by Leyland in the 1970s, and the second a more recent system by Kestrel Powertrains. Kestrel also uses the Torotrak IVT. Both these systems use a gearset to connect the flywheel, the researchers say, which requires extra packaging space, increases the cost, and reduces the fuel economy gains due to the additional losses in the gearset.

The researchers have developed a more detailed simulation tool to support refinement of the flywheel design (material, size, clutch, auxiliary strategy, etc.) and to develop a more detailed control strategy.

A Kestrel system was built into a London bus for practical testing, the results are aiming towards 50% + fuel and emissions saving compared to the baseline double deck buses, for an investment of <?5,000 per bus. This leads to less than 1 year payback, according to Kestrel. Retrofit is practicable because it fits into the space of a conventional engine and gearbox. The fully mechanical system consists of a conventional engine, energy storage flywheel, an epicyclic gear set to select engine or flywheel drive, and a Torotrak transmission.

Source : english.chinabuses.com

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