You will find many references to the Prius having a CVT, which is generally taken to mean a "Continuously Variable Transmission". Toyota calls the Prius transmission "ECVT" which is either "Electrically Controlled Variable Transmission" or "Electrically Controlled Continuously Variable Transmission", depending on where you read it. So is it "Continuously Variable" or not? We shall see that the answer is both yes and no.
CVTs have been around for a while and at first it doesn't seem that Toyota has broken new ground here. This, however, is entirely false, because the ECVT in the Prius works in a completely different manner from any other CVT put into a production car. It is so different, that calling it a CVT is misleading. However, using this semi-familiar term at least explains why the pitch of the engine sound doesn't rise as you accelerate.
A conventional, or "step", transmission, selects from a number of fixed gear ratios between the input side (the engine) and the output side (the drive train to the wheels). In "low" gear, the spin rate of the engine is reduced towards the wheels so that it can power a slow-moving car while spinning fast enough to develop the necessary torque. As the car picks up speed, the spin rate of the engine increases, making a sound we are all familiar with. At some point, depending on how hard he wants to accelerate, the driver, or the transmission itself, shifts to the next higher gear. The engine spin rate drops and then increases again as the car continues to accelerate. In this way, we work our way through the gears, keeping the engine spinning at a rate that delivers the power we need at whatever speed we're traveling and avoiding "over-revving" the engine.
As well as reducing spin rate from engine to wheels, a low gear multiplies up the engine torque in the same proportion. This is why acceleration is greater in lower gears and drops off as we (or the transmission) shift to higher gears. To achieve the best acceleration, race car drivers delay upshifting until the engine reaches the "red-line", the spin rate above which it will begin to suffer damage. A car's published 0 to 60 time is measured using a similar strategy and will be much worse if the driver doesn't want to push the engine towards this limit.
The more gear ratios we have, the more choices we have in setting the engine spin rate for any given speed. This allows us to call for power when we need it by revving high but to cruise economically by lowering the engine spin rate and reducing losses associated with the fast-moving internal engine parts. A conventional CVT has an infinite (or a very large number) of gear ratios. It selects a ratio that can deliver to the wheels the power being demanded by the driver, but keeps the engine spin rate low within this constraint to improve economy and engine life. You can imagine that as the car picks up speed, the transmission continually shifts into a slightly higher gear. The result is that the pitch of the engine tone stays fairly constant and the acceleration falls off smoothly, instead of in steps, as the higher gear ratios have a smaller multiplying effect on the engine torque.
The advantages of a CVT (selection of the best gear ratio at all times to balance performance and economy) are considerably offset by efficiency and reliability problems. Efficient and durable toothed gears, the mainstay of step transmissions, cannot be used. Instead, an arrangement of belts and pulleys is necessary with a mechanism to vary the effective diameter at which the belt passes around the pulley.
The Prius transmission produces one of the effects of a CVT but not the other. The spin rate of the engine can be selected to produce the required power but otherwise to spin no faster than is necessary to maintain fuel efficiency. The Prius engine tone therefore sounds as if the car has a CVT because it does not rise as the car picks up speed. Instead, it rises and falls with power demand, in other words, how hard you press on the accelerator pedal. The Prius transmission does not, however, multiply up engine torque at low vehicle speed. This is because it has only one gear ratio. Effectively, the engine is coupled to the wheels as if the car is always in top gear. This would be a crippling limitation, if not for the presence of a powerful electric motor in addition to the gasoline engine. With this motor adding its considerable torque, people have said that the car feels as if it's always in bottom gear!
The central component of the Prius transmission is an epicyclic gear that Toyota calls the "Power Split Device" (PSD). This type of gear is also known as "sun-and-planets" because it consists of a number of "planet" gears surrounding a central "sun" gear. The planet gears are on shafts fixed to a "planet carrier", which revolves around the same axis as the sun. Unlike real planets, they are all the same size and all the same distance from the common center of rotation. The planet gears are surrounded by and mesh with an inside-out gear called the "ring". This also revolves around the same axis as everything else. Click on the diagram at right to get a closer look.
The Prius internal combustion engine (ICE) is connected to the planet carrier. As it rotates, the planets mesh with and tend to push both the sun gear (in the middle) and the ring gear (around the outside) in the same direction as the planet carrier. By careful choice of the size (and hence number of teeth) of the sun and ring gears, Toyota has arranged 72% (actually 2.6 divided by 3.6) of the torque to go to the ring and 28% (actually one over 3.6) to go to the sun. This is hard to visualize, so instead think about a straight bar 3.6 feet long being used as a lever. Each end rests on a bathroom scale and you stand 2.6 feet from one end. 72% of the pressure of your weight registers on the closer scale and the remaining 28% registers on the other. The epicyclic gear achieves the same effect with rotational pressure, i.e. torque.
Now we understand how the torque of the ICE is split into two directions. The ring gear, which receives the bigger part, is connected via the usual reduction gears to the differential and hence to the wheels. This is how the ICE pushes the car. The sun gear, which receives the smaller part of the torque, is connected to a motor/generator called MG1. For the moment, let's forget than MG1 can act as a motor and imagine it acting as a generator. The ICE driving the planet carrier drives the sun and MG1 spins. A computer adjusts the electrical power drawn from MG1 so that the generation drag balances the torque passed through from the ICE. So, the ICE pushes the car with 72% of its torque and a generator with 28% of its torque. We'll find some use for the generated electricity later.
Finally, we have to understand that the ring and the sun, both pushed by the ICE with a fixed fraction of its torque, are free to rotate at different rates. Although there is a fixed mathematical relationship between their spin rates and the ICE spin rate, one can speed up and the other slow down without changing the input spin rate from the ICE. Let's go back to our 3.6 foot lever. Take it off the bathroom scales and have two friends hold an end each. Push the lever 2.6 feet from one end. One friend feels 72% of your push and the other 28%. Now, if you push so hard that they can't stand still, either or both of them can move back and allow you to move forward. They can chose the speed at which they retreat independently of each other. For example, one could stand still and the other do all the retreating. With a short lever, this visualization doesn't take you far from your starting point, but our epicyclic gear works in the same way without limit. If the ring gear stays still, the sun gear can absorb all the rotation of the planet carrier by rotating faster.
This is our clue to how the epicyclic gear allows the ICE spin rate to be adjusted like a CVT. For any given road speed, the computer figures out how fast the ring gear is spinning. From the power demand, it decides how fast it would like the ICE to spin. Then it just solves a simple equation to figure out how fast MG1 must spin. Then, it adjusts the power drawn off by MG1 to speed up or slow down the ICE until the desired condition is achieved. None of this affects the fact that 72% of the ICE torque is sent towards the wheels. The wheels can even be stationary and this torque is still applied. By allowing the ICE to drive the car from stationary in this way, we do away with the need for a clutch or torque converter, actually eliminating a wear-prone and bulky component of the transmission.
The last step is to find a use for the generated electricity from MG1. A second motor/generator called MG2 is connected to the PSD ring gear and adds its torque to that coming out of the PSD from the ICE (which drives the planet carrier). So, the power that seemed to be going to waste is actually routed around the mechanical PSD by an electrical path and still ends up driving the wheels. In effect, ICE power is split (at the Power Split Device, of course), some following a mechanical path through the ring gear and some following an electrical path to the sun gear, MG1, the control electronics and to MG2. The ring gear and MG2 drive the wheels together through the reduction gears and differential.
Although we can gloss over this for now, to fully understand the Prius transmission we need to note that the division of power between the mechanical and electrical paths is not fixed, as the torque split is. Power is the product of torque and spin rate, so the power passing in each direction depends on the relative rates of spin of MG1 and the ring gear. You may see statements to the effect that 72% of the ICE power goes directly to the wheels and 28% is turned into electricity. This is not correct. Torque is split in this ratio, but the power split is variable, a fact that is used to advantage by the engine control computer.
We have discovered above how the Prius transmission uses an epicyclic gear, a generator and control electronics to adjust the spin rate of the engine without the use of either a step transmission or a conventional CVT. We also discovered that a fixed proportion (about 72%) of the ICE torque is sent mechanically to the wheels. Without the ability to change the gear ratio between the ICE and the wheels, we cannot multiply up the ICE torque to get high acceleration at low speed. Although we've solved the problem of letting the ICE spin at a suitable rate when the car is moving slowly, the torque coupling is equivalent to always being in top gear!
This problem is partly solved already by passing generated electricity from MG1 to MG2 which adds its torque to that of the ICE. Electric motors do not share with an ICE the problem of not generating torque at low speed. In fact, this is where they generate the most torque. If we run the ICE at, say, 2000 r.p.m., with the car barely moving, a lot of power passes from the ICE to MG1. In fact, until the car starts moving, all the power goes to MG1. Even though MG1 gets only 28% of the torque, it gets all of the movement! Since MG2, connected to the wheels, is not rotating very fast, it doesn't take much power to generate its maximum torque. The Prius can launch from a standstill at respectable acceleration up to about 10 m.p.h. using ICE power passing primarily through MG1 and MG2. Only about a fifth of the torque comes directly through the mechanical path during this initial acceleration.
At about 10 m.p.h., the Prius can no longer maintain the expected acceleration using only power from its small ICE, which is designed for efficiency and low emissions and not for power. As speed increases, MG2 needs more and more power to maintain its torque and will soon reach its power handling limit, whereupon torque will fall off. Toyota has sized MG2 so that it has enough power to keep accelerating the car at an acceptable level, but where is the extra electrical power to come from? The answer, of course, is the battery. MG1 does not pass electrical power directly to MG2 as we may have supposed above. The control electronics includes a device called an "inverter" that turns the generated power into direct current at several hundred volts. This is connected to the battery terminals. A second inverter for MG2 takes as much power as is needed from the battery terminals to drive MG2 and accelerate the car. If MG1 is not producing the power MG2 needs, the difference flows out of the battery. At other times, when the car is up to speed, excess power from MG1 flows into the battery to replenish the charge. This "battery boost" is what allows a 2800 pound car, which would otherwise need an engine of over 100 hp and a step transmission, to give good acceleration performance with a 70 hp engine and no step gears.