Adding an electric motor and battery to a vehicle is just one of many ways to create a hybrid. We've seen hydraulic hybrids being tested in a variety of commercial vehicle applications. Torotrak has an altogether different mechanical system based on flywheels. The Torotrak system first came to our attention a couple of years ago when the FIA proposed allowing kinetic energy recovery systems on Formula One cars. Subsequently, at least two F1 teams licensed the Torotrak system for use, but none are believed to actually be using it right now. The flywheel system has a number of advantages over electric hybrid systems, including cost, weight and size.

It does have limitations as well. The system has to be mounted in physical proximity of the drive hardware because of the mechanical connections needed. Also clearly it cannot be adapted to plug-in capability. However, Torotrak Technology Director Chris Greenwood told ABG at the SAE World Congress that several UK companies are currently testing the flywheel hybrid system in road-going applications in government-funded research programs. Among those is Jaguar-Land Rover. Continue reading after the jump.

Recently, Jaguar was reported to be working on a KERS-based system for the new XJ and a Torotrak-equipped sedan could potentially be the first to hit the road. Greenwood also told us that larger scale versions of the system are being tested on transit buses, where they work very well. The system is claimed to give fuel efficiency improvements of about 40 percent. The flywheel system is very well suited to this application because of the frequent starts and stops and returns over 70 percent of the kinetic energy to the wheels during boost mode. The flywheel can absorb energy much more quickly than nickel metal hydride batteries. Greenwood gave us a rough estimate of the system cost at about £700 or about $1,300, significantly less than any of the full electric hybrid systems.

One of the concerns about flywheel systems is safety in the event of a catastrophic failure. Greenwood explained that the flywheel is constructed with a steel hub and a steel rim where most of the mass is concentrated. The rim is completely wrapped in carbon fiber. If the rim fractures, the pieces are contained within the carbon fiber and, as it rotates, the fiber eventually turns into something akin to a bundle of cotton fibers bringing the whole thing to a controlled stop. The system has been tested repeatedly and failed flywheels have never perforated the housing. If Jaguar adopts the system, it could be on the road within two years.

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