PowerTrain Components

The diagram below is a schematic of the Prius powertrain.  "Schematic" means that it shows the essential features, but takes liberties with detail.  In particular, I have grossly simplified the way in which the internal combustion engine (ICE) drives the planet gears in the power split device (PSD) and the way that the ring gear is connected to the silent chain sprocket and motor/generator 2 (MG2).  It is, however, accurate that the shaft from the ICE (blue) passes through the shaft of MG1 and the sun gear (yellow) in order to reach the planet carrier on the other side of the PSD.

The following sections contain descriptions of the various powertrain components.  You can read them by just scrolling down, or click on parts of the diagram to see their descriptions in any order you wish.  There's also a paragraph about what's missing from the Prius that would be present in a conventional car.

Internal Combustion Engine

The Prius has an internal combustion engine (ICE) which is unusually small for a car of this size (1300 kg).  This is made possible by the presence of the electric motors and battery, which supplement the ICE when power demand is high.  A conventional car, with an engine sized for hard acceleration and climbing steep hills, almost always operates that engine with low efficiency.  Maximum efficiency generally occurs at around half of the engine's peak power output.  A small engine can operate closer to this maximum efficiency because power demands encountered in normal driving are a larger fraction of its peak power.  The possibility of using a small engine in a hybrid vehicle is called "engine downsizing".

In addition to being downsized, the Prius engine uses many techniques to improve efficiency and broaden the range of conditions under which high efficiency is achieved.  The engine uses the Atkinson cycle, rather than the usual Otto cycle, which improves efficiency particularly at lower power by reducing "pumping loss".  Limiting the maximum spin rate to 4500 r.p.m. allows lightweight parts to be used, reducing inertia and friction losses.  The crankshaft is offset from the cylinder axes so that during the combustion stroke the force from the piston is transmitted to the crankshaft through a straight rather than tilted connecting rod.  The valves have narrow stems and low force springs to reduce energy lost in operating the valves.

I have devoted an entire topic to the Prius' Internal Combustion Engine.


The Prius has two electric motor/generators.  They are very similar in construction, but different in size.  Both are three-phase synchronous AC permanent magnet motors.  This sounds more complicated than it really is.  The rotor (the part that spins the shaft) is a just a big, powerful magnet and has no electrical connections.  The stator (the part that stays still and is fixed to the rest of the car) contains three sets of windings.  When current is passed in one direction through one set of windings the rotor is attracted to a particular position.  By passing current sequentially through each set of windings first on one direction and then in the other the rotor can be made to move from one position to the next and therefore rotate.  This is a simplistic explanation, but captures the essence of this type of motor.  If the rotor is spun by an outside force, an electric current flows in each set of windings in turn and can be used the charge the battery or power the other motor.  Thus, the same device can be a motor or a generator depending on whether current is pushed into the windings to attract the rotor magnet or drawn out and something else spins the rotor around.  This is even more simplistic, but will serve for the depth of these explanations.

Motor/generator 1 (MG1) is connected to the sun gear of the power split device.  It is the smaller of the two and is rated at a maximum power of about 18 kW.  Traditionally, its role is described as starting the ICE and controlling the ICE spin rate by generating a variable amount of electrical power.  Motor/generator 2 (MG2) is connected to the ring gear of the power split device and therefore to the reduction gears and hence the wheels.  Thus it is capable of directly powering the car.   It is the larger of the two and is rated at a maximum power of 33 kW.  It is sometimes described as the "traction motor" and its traditional role is to power the car as a motor or recover braking energy as a generator.  Both motor/generators are water-cooled.  My use of the word "traditional" here prefigures discussion elsewhere on this Web site in which I suggest that the roles of the two MGs are not so easily classified.  This can be ignored for our immediate purpose.


Since the motor/generators operate from three-phase alternating current and the battery, like all batteries, produces direct current, some electronics is needed to convert between the two.  Each MG has an "inverter" which performs this function.  The inverter knows the position of its MG's rotor from a sensor on the shaft and switches current through the windings as required to keep it spinning at the desired speed and with the desired torque.  The current in each winding is changed as each magnetic pole of the rotor moves past the winding and on to the next.  In addition, the inverter switches the battery voltage onto the winding and then off again very fast to vary the average size of the current and hence the torque.  Using the "self-inductance" of the motor windings (an electrical property which resists current change), the inverter can actually pass a larger current through the winding than is drawn from the battery.  This only works when the winding voltage is less than the battery voltage, hence energy is conserved.  However, since the size of the winding current determines the motor torque, this current multiplication allows a very high torque to be achieved when the motor is turning slowly.  Up to about 7 m.p.h., MG2 is able to apply a torque of 350 newton metres to the reduction gears.  This is what makes it possible to "launch" the car at an acceptable acceleration without the use of step gears to multiply up the ICE torque.


The Prius high-voltage battery consists of 228 cells of 1.2 volts each for a total nominal voltage of 273.6 volts.  The cells are arranged in 38 modules of 6 cells each and the whole lot is assembled into a unit that is fixed behind the rear seat.  You can see where it is from the bump at the bottom rear of the trunk.  The maximum current of the battery is 80 amps discharge and 50 amps charge.  This is remarkable, since each cell is similar in size to an ordinary D-cell such as you would use in a large flashlight.  In fact, the Japanese version used an earlier generation of battery cells that really were D-cells.  The rated capacity of the battery is 6.5 ampere hours, however, the car's electronics only allow 40% of this capacity to be used so as to prolong battery life.  The state of charge is allowed to vary only between 40% and 80% of the rated full charge.  Multiplying up the battery voltage and current capacity, its rated energy storage capacity is 6.4 MJ (megajoules) and its usable capacity is 2.56 MJ.  This is enough energy to accelerate the car, driver and a passenger up to 65 m.p.h. (without help from the ICE) four times.  Alternatively, it is enough to raise the car through 600 vertical feet.  To produce this amount of energy, the ICE would consume about 230 millilitres of gasoline (a little less than half a pint).  (These figures are provided only to give you a feel for the battery energy storage capacity.  The car should not be run without fuel and, even if it is, starting out with 80% of the rated full charge requires a long downhill run.  Most of the time, you only have about 1 MJ of usable energy in the battery.)

The Prius also has an auxiliary battery, which is not shown.  This is a 12 volt, 28 ampere hour lead-acid battery that lives on the left side of the car in the trunk.  Its purpose is to supply power to the electronics and accessories when the hybrid system is turned off and the high-voltage battery main relay is off.  When the hybrid system is running, the 12 volt supply is from a DC-to-DC converter running from the high-voltage system.  This will also recharge the auxiliary battery if necessary.

Power Split Device

The torque and power of the ICE and both motor/generators is combined and distributed by a planetary gear set called by Toyota the "power split device" (PSD).  Although not terribly complicated in construction, this device is tricky to understand and even trickier to see in the full context of the powertrain operation.  Therefore, I devote several other topics to discussion of the Power Split Device.  In brief, it allows the Prius to operate in both series- and parallel-hybrid modes of operation at the same time and gain some of the advantages of each mode.  The ICE can drive the wheels directly (mechanically) through the PSD.  At the same time, a variable amount of power can be drawn off from the ICE and turned into electricity.  This can charge the battery or be passed to one of the motor/generators to help drive the wheels.  The flexibility of this mechanical/electrical power split allows the Prius to improve fuel economy and control emissions in a way that would not be possible with a hard mechanical linkage between ICE and wheels, as in a parallel-hybrid, but without the electrical energy loss of a series-hybrid.

The Prius is often said to have a CVT or Continuously Variable Transmission and it is the PSD that is responsible for this.  However, a conventional CVT is just like a normal gearbox except that the gear ratio can be varied continuously rather than in a small number of steps (first gear, second gear, etc.).  I explain in another topic why the Prius PSD is most unlike a conventional CVT.

Silent Chain and Reduction Gears

The use of a chain drive is rather unusual, but all conventional cars have reduction gears between the engine and the axles.  Their purpose is to allow the engine to spin faster than the wheels and also to multiply up the torque produced by the engine to a larger torque at the wheels.  The ratios by which spin rate is reduced and torque is increased are necessarily the same (neglecting friction) due to the law of conservation of energy.  The ratio is called the "final drive ratio".  The final drive ratio of the 2001 model year US Prius is 3.905.  This is achieved in the following way:

If we perform the simple arithmetic 36/39 * 44/30 * 75/26, we get (to four significant digits) 3.905.

My guess as to why a chain drive is used is that it avoids the axial thrust (force along the shaft) which would be produced by the usual helical-cut gears used in automotive transmissions.  This could also be avoided by using spur (straight-cut) gears, but these are noisy.  Axial thrust is not a problem on the intermediate shafts and can be accommodated by conical roller bearings.  However, it might not be so easy to deal with on the output shaft of the PSD.  Remember, this is just my guess and may be completely wrong.

Differential, Axles and Wheels

There is nothing very unusual about the differential, axles and wheels of the Prius.  As in a conventional car, the differential allows the inside and outside wheels to turn at slightly different rates when the car is going round a corner.  The axles pass the torque from the differential to the wheel hubs and include a joint to allow the wheels to move up and down with the suspension.  The wheels are a light-weight aluminum alloy and are fitted with low rolling resistance high-pressure tires.  The tires have a rolling radius of about 11.1 inches, which means that for each revolution of the wheel, the car travels 69 ¾ inches.

What's Missing

The Prius powertrain may look complicated, but we must take into account several things that a conventional car needs that the Prius design has dispensed with.  These are:

So, the complexity of the Prius is not actually much greater than a conventional car.  In addition, new and unfamiliar parts such as the motor/generators and the PSD are actually likely to have higher reliability and longer service life than some of the parts that have been eliminated.

Last edited June 26, 2002.  All material Copyright © 2001, 2002 Graham Davies.  No liability accepted.