Quince

The Hybrid Synergy Drive and Ford Hybrid System both use the Power Split Device. It does not need an external starter and has no alternator. The Power Split Device meshes two AC electrical motor/generators (MG1 and MG2) and the gasoline engine through a single planetary gearset. MG2 is the big electric motor (90hp in the Fusion Hybrid, 60hp in the Prius) that turns the outer ring gear in the planetary gearset, and the outer ring gear directly drives the car's front wheels. (that is why the PSD system does not need a clutch or a torque converter, since the electric motor can exert maximum torque starting at 0 RPM, which a gas engine cannot). MG1 is the smaller 20hp electric motor that turns the sun gear in the middle of the planetary gearset. It acts as the starter motor (draws power from the hybrid battery) to crank the gasoline engine into motion whenever the gas engine is needed. The gasoline engine spins the middle planetary gears (through the planetary gear carrier) in the PSD planetary gearset. No separate alternator is needed because both MG1 and MG2 can act as electrical generators. MG2 can generate AC electric power when you step on the brake pedal, which converts the car's forward momentum into storable electricity. MG1, providing countertorque when the gasoline engine is turning, acts as a generator as well. The hybrid system includes a solid state inverter which can convert the AC power generated by MG1 and MG2 to DC for storage in the hybrid battery, or vice versa from DC to AC when MG1 and MG2 are running as motors. 12V needed by other systems in the car comes from the inverter too. The entire electrical system is managed by the computer. A huge side benefit of the PSD system is that the big MG2 motor can do the majority of the work in braking a car, saving brake pad wear tremendously. That is why cars like the Fusion Hybrid and Prius don't need to have their brake pads changed until well after 100,000 miles. (Not a typo.) Try that in a normal auto-tranny car and see what happens. :P That's the beauty of the Hybrid Synergy Drive / Ford Hybrid System. It is so incredibly simple, mechanically. The complicated part is the software and electronics that controls it all. How the Power Split Device system works: http://www.eahart.com/prius/psd The Honda IMA system is mechanically more complex with a conventional engine and transmission. It merely replaces the Transmission flywheel with a small disc-shaped motor/generator of the same size, and adds a battery pack in the back of the car to work with that small disc MG. It's simpler to integrate into a car because it builds on existing transmission and engine technology, and requires a much smaller battery pack because the MG is so small. Interesting note: The new Mercedes S400 BlueHybrid uses this IMA implementation, but I'm not sure if they have to pay Honda any royalties. To summarize, the IMA system is much closer to a normal car, while the PSD system is much closer to an electric car.