Not having seen a generalized overall description of the Prius' operation, I've come up with the following, referring to my attached & updated drawing, PRIUSPWR. I'll appreciate comments.
How the 2004 Toyota Prius works
Ref. Dwg PRIUSPWR rev. Oct 6, 2004
A 4-cylinder internal-combustion engine (lower left) and two electric motor-generators (“MG1” and “MG2”, left & right, respectively) are connected to the wheels of the car by means of certain special gearing plus the conventional differential gearing depicted at the right. The engine and MG1 connect via the special gearing (the planetary gear-set), and MG2 connects directly, to what would be the drive shaft in a conventional car. There is no clutch; all mechanical elements are permanently interconnected.
The planetary gear-set can be thought of as being like a conventional differential gear-set in that mechanical power is applied to (or taken from) an assembly of “planet” gears acting together (analogous to one of the wheels in the differential system) and a “sun” gear (analogous to the other wheel) while a resultant power appears at a “ring” gear (analogous to the drive-shaft). Depending upon the occasion, engine power may flow from planet-gear assembly to ring gear and thence to the wheels, provided that MG1’s torque sufficiently impedes the turning of the sun gear. MG1 power may flow from sun gear to planet-gear assembly and thence to the engine (for starting the engine) when torque from the wheels or from MG2 sufficiently impedes the turning of the ring gear. MG1 power may also flow from sun gear to ring gear at the same time as the engine-power flow. And excess wheel power (as when going downhill, for instance) may flow from ring gear to sun gear and thence to MG1, given sufficient resistance-to-turning of the planet-gear assembly (as, for instance, when the engine is off). Excess wheel power may also flow into MG2 as will be shown.
Both MG1 and MG2 are electrically connected, ultimately, to a 200 volt nickel-metal-hydride storage battery (upper left) through respective electronic power-inverters, which act to change between ac (alternating-current) and dc (direct-current) power, and a converter which acts to change between the fixed 200 volts of the battery and a variable dc level of up to 500 volts. The variable level connects to the inverters.
MG1 and MG2 are what are called step-motors. Each consists of a rotating element (the rotor), incorporating a powerful permanent magnet, and a stationary element (the stator) incorporating three rotationally-spaced coils of wire. Under computer control, rapid trains of electric-current pulses are applied from the inverters to the coils so as to produce rotating magnetic fields that pass through the rotors. Those fields interact with the fields of the permanent magnets to produce rotational torques on the rotors. The torques produce the mechanical power outputs when the rotors turn.
But MG1 and MG2 can also act as electric generators. While the trains of pulses are applied to them, and should the magnetic fields of the rotors’ magnets be caused—by external forces from, say, excess wheel power—to “get ahead” of the stators’ rotating fields, that excess mechanical power will be turned into added electrical power within the motor-generators. That added power will then automatically flow back through the inverters and the converter into the battery. In that way, power taken from the battery to produce pulse trains can be replenished in a recharging process. And charge-replenishment can come about not only from excess wheel power but also from the power of the engine itself.
The motor-generators MG1 and MG2 can turn freely when no electric pulses are applied to their stator coils. This condition occurs when the accelerator pedal is not depressed (neglecting a small excitation that is applied to simulate the “creep” action of conventional transmissions).
The engine runs essentially “on” or “off”, with its throttle set to run close to “wide open” for best efficiency. Because of that, there must be some way to vary the power that it delivers to the wheels. That is the primary function of MG1. MG1 acts, in effect, as clutch-and-throttle for the engine; as the means by which the relatively-constant rotation rate of the engine’s output shaft is coupled to the varying rotation rate of the car’s wheels, through the planetary gear assembly and the conventional differential. As noted, with no electrical excitation at all to MG1 its rotor will spin freely. In that case, rotation of the planet-gear assembly, caused by the engine, will impart no motion to the ring gear but rather will merely spin the sun gear and thus freely spin MG1’s rotor. Only when MG1’s inverters deliver the proper pulse trains to its stator coils, from the battery and via the voltage-converter, will MG1’s rotation become impeded, allowing engine power to flow from planet gears to ring gear and thence to the wheels. MG2, on the other hand, acts primarily in delivering power derived from the battery directly to the wheels, to augment engine power when required.
While MG1 is doing its throttling function, it is actually receiving some mechanical power from the engine—because it is impeding the rotation of the sun gear. That mechanical power becomes converted, in the manner described, into electrical power that finds its way back into the battery. Thus not much power is wasted in implementing its power-control function.
The engine is shut off at every opportunity. Whenever it is to be run, it is “spun up”, almost unnoticeably, by power from MG1. During reverse operation it does not run; only the electric motors propel the car.
There are numerous computers that implement the various control tasks. In addition to the control functions described here, accelerator-pedal position and brake-pedal force are monitored so as to control the engine and the motor-generators, and also to modulate brake-pressure. The latter action is done as a function of the “dynamic braking” that occurs whenever possible. In that action, excess wheel power flows through MG1 and MG2 as recharging power into the battery, helping to slow the car and also to diminish brake wear.
An additional voltage-converter and a small 12 volt lead-acid battery provide for auxiliary power, and a separate inverter powers the air conditioning system from the 200 volt battery.
Ken Herrick Rev. October 9, 2004
C converter then through the inverter to energize MG2 and power the DIFF/Wheels? Why does the 200VDC need to be "converted" before being "inverted" (made AC).
levan · updated 
