General Motors has revealed that the Vauxhall Ampera can be powered by as many as three engines simultaneously.

GM had previously concealed these details because it did not want competitors to see a system that was not yet protected by patents, which will now be granted.

But the revelation that there are circumstances when the Ampera's petrol engine is physically locked to the electric motor has prompted accusations that GM's definition of the Ampera as an E-REV is inaccurate, and that the car is in effect a hybrid.

The previously unrevealed engine is the Ampera's electric generator, which can be switched to act as an electric motor when the car is cruising at high speed on motorways to supplement both the main electric traction motor, and the 1.4 litre petrol engine that recharges the battery and provides an electrical boost to that main motor.

The reason for this unusual coupling is energy consumption - electric motors become less efficient at high revs, and this system allows the main traction motor's speed to drop from the 6,500rpm that would otherwise be needed to propel the Ampera when it's cruising flat-out at 100mph, to a more efficient 3,200rpm.

The generator-motor runs at 1,500rpm in this circumstance. A complicated set of planetary gears allows the outputs of all three motors to be blended seamlessly together - as with a hybrid like the Toyota Prius. This is how the argument that the Ampera is really a hybrid has come about, GM previously claiming that the Ampera is only ever propelled by its electric motor, the petrol engine acting only as a remote electricity generator.

It still maintains that this is the case, arguing that if the traction motor was removed, the Ampera would no longer drive, despite the planetary gears being fed with torque by the motor-generator and the petrol engine. This is because those gears are set-up to feed all the propulsion effort through the traction motor.

It's complicated stuff, and the arguments will doubtless continue, but the benefit is an Ampera that's 10-15 percent more economical because of this ingenious system, resulting in its electric-only range, with the petrol engine dormant, increasing by one or two miles.

GM is also claiming that the battery's range has also increased, from the previously claimed maximum of 40 miles to '25 to 50 miles' as a result of improved battery technology. Previously the battery could only be depleted to 50 percent of its reserve in order to preserve its life - GM provides an eight year, 100,000 mile warranty - but that figure now rises to 65 percent, without affecting the warranty.