What type of CVT is under the hood?

You listed these options:

• pulley-based - No
• toroidal - No
• hydrostatic - No

So it is none of the above.

It is a power-split transmission (PST), electrical type:

Bob Wilson

 

It fits in the "It's so simple that it's hard to understand" category.:rockon:

 

Its mechanically simple with complex software.

Let me try to explain in a simple way. Just think about the engine and mg1 at first. mg1 is just like your standard alternator, but it is more powerfull and connnected on the sun of a universal gear. The engine is on the planet carrier and the wheels attached through a differential to the ring gear. By turning mg1 you change how fast the engine turns the wheels. Mg1 is electrically controlled to allow any engine speed, to turn the wheels at any wheel speed. So you have an electrically controlled variable ratio of engine speed to wheel speed, so far so good. There is a problem, mg1 doesn't multiply the torque at the wheels it simply generates or consumes electricity. This is where mg2 comes in. It is also connected to the wheels through the differential and a different universal gear set to multiply its torque. The power that mg1 generated or consumed can be used in mg2. There we go, the torque is multiplied if the generated by mg1 is used by mg2. There is overdrive where mg2 feeds power to mg1 and it speeds up the engine.

The final wrinkle is the battery. The electronics can add power to the system or remove it. When you step on the accelerator the electronics determine how much power comes from the engine and how much power flows into or out of the battery. It then sets the correct amount of fuel into the engine and the speed of mg1 and mg2. Reverse is only electric on mg2.

 

Allow me to try to get closer to the basics. It's a planetary gear set used as a torque splitting device with power recirculation to control the effective gear reduction ratio (ice to wheels) from a negative infinity to some small less than unity reduction ratio to a positive infinity. The power recirculation is done my two electric motor/generators with a sophisticated control system.

The electric motor generators are also capable of charging the traction batteries or supplying additional propulsive effort.

The basic concept is many decades old and only became practical for cars in the past decade because of a combination of advances in electric motors and control systems and batteries.

 

We want to simplify our world so badly that we are just itching to drop it into a predetermined category. But this mechanism is different. I think the best description is "Hybrid Synergy Drive". It's different enough from the others, that if you made something like it you'd have to pay Toyota royalties to use it.

 

This still blows my mind. I can read about regular CVTs and other mechanical systems and get an "ah, so that's how it's done", but I only have that in part with the PSD for this car despite the great link you gave. In my head it's still a mess of gears that do magical things.

 

The "magical things" explanation isn't a bad one.

You can play with the sliders on the model and see what happens to MG 1 and 2 and the internal combustion engine and vehicle speed. You don't really need to have an engineering or mathematical understanding of the transmission to get a feel for how it works.

Even if you do get the math, the model adds a lot to visualizing what happens.

BTW, the 1908 Ford Model T used a single planetary gear set to get 2 forward gears, reverse and neutral. It was operated by two foot pedals and a handbrake lever. Not exactly intuitive.

 

You might also find the nomograph interesting, if you like graphical presentation of algebraic relationships.

http://priuschat.com/forums/gen-ii-prius-technical-discussion/43777-nomograph-my-2008-prius.html

 

There's a sort of half step that may help the newbies in understanding all this. It worked for me, and it might help some of you. Go read about planetary gear systems. Watch the demos and get to where you understand how they work. Notice that in each portrayal they are holding something still. As an example..."Hold the planet carrier still, turn the sun gear, and the ring gear turns at (some ratio)". This "holding something still" is done with bands or brakes. In effect it locks the appropriate part to the case of the transmission. So much for the planetary gearset.

Now, we're ready for the good part. What if, instead of holding the part still, you could push it around, or let the other gears turn it, or, exactly in between, hold it motionless when you want to? That is the trick of the HSD. And they use an electric motor to do just that. Push electricity into it and it will push the gears around. Let the gears push it around, and you get back some generated electricity. This added motor is Motor/Generator 1, or MG1.

Think about that for a few days. Then you'll be ready to read the technical stuff. I had just this bit explained to me in 2004 when the HSD appeared, and had no idea about the rest until I actually bought a Prius. Then I came here and I sure got an eyeful of technology! The animations are great!

 

That's a good way to start.:rockon:

 

I heard, "It's Full of Stars".

 

Fairy dust and butterfly kisses.

Tom

 

Hi RMulligan,

The easiest way to think of the Prius transmission for me, was to consider it an electric torque converter.

The engine is only loaded at the torque level which is optimum for efficiency, and the RPM is set by how much power is demanded. The power cannot make it all through the single high gear ratio to the wheels, so some goes to MG1 by way of the Power Split Device. MG1 turns it into electric power. That electricity then goes to MG2, which is a high torque motor, or low torque motor with a gearset (Gen III), and finally onto the wheels.

 

Understanding the Prius transmission is like differential equations:

• "What the f*ck? I know he used English but it made no sense!" - first day impression
• "That was an easy A." - after the final exam

Bob Wilson

 

I was siting the once relatively famous quote from "2001: A Space Odyssey" but I guess when it's actually 2010? Hey? We've got The Hybrids, The Cell Phones, The CVT's but where are my promised vacations in space!

 

Naw, it just needs a good 1 hour lecture and an hour of homework. Part of the problem is bad explanations and allusions to fairy dust.

who really wants a space vacation. I want my flying car, and a big private plane with a pool in it, and a monkey. I was promised a monkey wasn't I. And now I'm disallusioned, as everest base camp is really a dump, and no one is giving me a monkey or a flying car. How about a minature cow? Comon, can you just zone my neighborhood for minature cows. I'm not asking much.

 

I didn't think they had anything miniature in Texas

 

Amazing isn't it.

Next time you sit in a "conventional" car you will notice the jerk of the transmission even more.