pre65 wrote:Does the Prius battery keep it's charge by recovering energy, or do you have to give it a regular external charge ?
The battery is kept charged by the engine and regenerative braking - so you don't plug it in, just fill it up with petrol.
The idea is that in stop-start traffic, the electric motor does most of the work, with the engine off - in that scenario a normal car is at its least efficient. If you're in a long traffic jam, you'll hear the engine kick in to top up the battery if the charge level gets low. At more steady speeds, the petrol engine can then work at its greatest efficiency. Also, the engine works on the Atkinson cycle rather than the Otto cycle, which increases its efficiency still further but also drastically affects torque - it would not be able to move the car off from a standing start by itself, it would stall. So the electric motors (it actually has two) also help it out in that regard.
The two electric motors combine to make possibly the most elegant CVT gearbox in the industry. It's one planetary gear set - that's it. The primary motor attaches to the ring gear, this is also connected to the differential and driveshafts. A secondary motor which can drive in either direction, is connected to the sun gear, and the petrol engine is connected to the planet carrier.
With that setup, the speed and rotational direction of the secondary motor relative to the primary determines the gearing between the petrol engine and front wheels - or even whether the engine runs at all. The motor drive system also replaces the function of starter motor and alternator - two less things to go wrong.
Really quite a cleverly engineered machine - I guess that's why I like it.