CV Transmission

I learned it from Graham's site because he explained it so well. Although it was for the Classic Prius, the fundamental is the same.

http://www.ecrostech.com/prius/original/Understanding/Contents.htm

Better 3d PSD in motion: http://eahart.com/prius/psd/

 

I just looked at his explanation and I mostly like it, except that he starts leading himself into the fog when he makes such a point of the Prius transmission being only sort-of like a CVT and offering a variable ratio of RPMs without inversely varying the torque the way every other transmission does ... leaving himself on a narrow misty path around the fact that the computer, inverter, MG1 and MG2 somehow manage to get you the torque anyway, without quite seeing that he's simply drawn his box too small. If you draw your "transmission" box so that the planetary gear set, MG1, MG2, inverter, and computer are all inside it, the fog evaporates leaving nothing more nor less than an honest-to-goodness CVT that gives you a range of RPM ratios varying inversely with torque ratios. Like any transmission, the power you get out (at one RPM times one torque) is roughly the power you put in (at another RPM times another torque) except for some losses (which in this case are partly frictional and partly electrical).

The idea should be familiar to a sailor since there are really all kinds of things you may have called a "transmission", such as the combination of a giant generator, some cables, a motor controller, motor, and reduction gears on shipboard - the "box" enclosing all the stuff that carries power from the prime mover to the screw.

I didn't throw the battery into the box; you would still have a CVT without it. The battery adds a way to handle peak loads while using an engine sized only for average loads; whenever the driving load is below engine capacity, the engine can be run a little extra hard to catch the battery up again. The battery also offers a place to put energy that otherwise couldn't be captured while driving downhill, braking or otherwise decelerating. That regeneration isn't as essential a part of the hybrid advantage as popular opinion seems to have it, but it does contribute.

All of the control of the CVT boils down to switching big transistors (found in the inverter) to accept varying-sized pulses of electric current from MG1 and send other varying-sized pulses to MG2; the pulses are timed very quickly and very precisely by the hybrid-vehicle computer, keeping careful synchronization to the rotation of MG1 and MG2.

There are also times when the computer is accepting pulses from MG2, sending pulses to MG1, or both: to collect energy while decelerating, to use MG1 to start the gasoline engine, or to achieve an overdrive ratio that has been dubbed "heretical mode" because it's one of those clever ideas so unexpected that your brain does a double-take and a couple back flips before saying "oh ... yeah ... that just gives a speed boost and torque reduction after all, like overdrive in any other transmission."

-Chap

 

The following key point of the eCVT(PSD) is also described at Graham's site.
"the total ICE torque is 72% to the ring(MG2:wheel) and 28% to the sun(MG1)"
http://www.ecrostech.com/prius/original/Understanding/PowerSplitDevice.htm

Ken@Japan

 

Dave, the main electric motor (MG2) on the Prius is connected directly to the wheels through fixed gearing. Your confusion centers around the Internal Combustion Engine (ICE) and MG1, which is a smaller motor/generator used as a starter motor and generator.

Since you are familiar with ships, let's look first at serial hybrid systems in which the ICE drives a generator which in turn drives a motor. The Prius does this with the ICE driving MG1 to make electricity which in turn drives MG2. It's a bit more complicated with the Prius, since part of the torque from the ICE is used to drive MG1 and part is sent directly to the wheels through the same fixed gearing used by MG2. Stated another way, part of the power goes through a serial hybrid drive system, and part of the power goes through fixed mechanical gears. The ratio of this power split changes depending on the required effective gearing. When a lower gear is needed, more power goes through the serial hybrid path.

That should get you started. Take a good look at the Power Splitting Device (PSD) and all will make sense.

Tom

 

This is the best one I have read...

Toyota Prius Hybrid Car E CVT Transmission Info

 

I thought this was a great site to see how the planetary HSD works....Also shows why the ICE kicks on >43 mph and that the ICE is limited to 4500 rpm....So the computer will never let you redline.

 

Hi All,

One can think of the Prius transmission as an electric torque converting transmission. The overspeed of the engine, that the momentum of the car at low speed wont allow into the wheels, feeds MG1, which generates electricty that feeds MG2. MG2 then generates large torques with that power.

 

To sum it up,

Non-hybrid step gears:
High torque @ low speed = Low Gear (1st)
Low torque @ high speed = High Gear (6th or Overdrive)
Reverse = Reverse Gear

eCVT:
High torque @ low speed = Electric motor
Low torque @ high speed = Gas engine
Reverse = Electric motor (backward)

HSD takes advantage of having two engines and does the balancing act with transistors. The need to shift gear is gone. The shift takes place at the electron level.

 

HSD lets electric do the torquing.

 

Right. My only quibble with Graham's site is that he could have been clearer that this is a true statement about the PSD (planetary gear set, one component of the eCVT) but not a true statement about the eCVT as a whole. The system as a whole does indeed behave as a transmission that changes RPM in a continuously variable ratio and torque in the exact inverse ratio except for unavoidable frictional and electrical loss. Addition of the battery gives the further ability to smooth out peaky demand and downsize the engine.

-Chap

 

While very succinct, it obscures what is truly amazing about Toyota's Hybrid Synergy Drive (HSD). Obscures that which makes it so much better than Honda's hybrid technology. It was the HSD more than anything else that made me first want one of these vehicles. Seeing how they took what, at its core, was a simple differential (used on every car made) and turn it into a transmission was just a stroke of genesis. Toyota may not be able to sell the name HSD, but they will be licensing the technology for years.

 

After understanding the fundamental of HSD, I wanted to drive, OWN, and maintain the Prius. I wanted to feel how it was like to drive in EV mode. I wanted to experience the engine shutdown at stop lights. I wanted to see how smooth it was on acceleration. I wanted to feed the battery while re-gen braking.

I saw Prius as not just "another" car but a car with engine(singular for unity) of change. The advantages of HSD over step gear transmission became very clear and I was instantly sold. It was not until the federal government gave $3,150 tax credit AND the HOV access on my commute, triggered me to trade in my Celica (was a hatchback but 2 doors).

 

Not really a differential, but a planetary gearset like is used on almost all automatic transmissions. The difference is that there are no friction surfaces like brake bands to wear and heat. Their function is done my the motor/generator. This should make these units last a VERY long time, since heat and lubrication problems are what cause transmissions to fail, and this system faces a very benign environment for both.

 

http://www.ecrostech.com/prius/original/SideBars/PsdLikeDiff.htm

 

Well, his point is that it is like a differential that you massively stretch and bend until it is shaped like a planetary gearset. So it's like a differential that is shaped totally different than a differential.

And that is supposed to clarify?

Why not just call it what it is, a planetary gearset, familar to anyone who knows an automatic transmission (or even the Model T's manual planetary set)?

 

... and splits the torque at the gozinta to two gozoutas unequally, in exactly the way differentials don't.

-Chap

 

Sorry, I gotta correct your nomenclature. It's not a "gozouta," it's a "comzouta."

 

I would not because that implies that like an automatic transmission it has multiple gear ratios that change, which it does not. Both differentials and the PSD have a single fixed ratio set. Differentials equalize force at all three points by varying their speed and direction of rotation, as does the PSD. The only real difference other than a superficial allignment is that the rotational force is not equalized, but proportionally distributed in a fixed ratio in the PSD.

I think our difference in views is mainly based on what one see as the important advancement of the HSD. The differential analogy I (and others) use focuses on how it is nothing like a traditional transmission that requires multiple ratio gear switching nor is it like a true CVT where gear ratios continuously change, yet it accomplishes the the same function.

Just a difference in perspective

 

GM's 2-Mode hybrid transmission would be closer to the automatic transmission because it has 3 different gear ratios and shifts with the clutches.