Horses and Toyota hybrid power split questions

[Ecky]

Originally Posted by raubvogel

While I understand that the engine and MG2 have different power bands -- I expect electric motors to reach their peak HP at lower RPMs than an engine -- the part of having MG2 spinning backwards confuse me: is it doing that to collect engine power that is not necessary to keep the desired road speed (like the MG1) or actually "fighting against" the engine? Also, how would that work if I have pedal to the floor and want to drive at its top speed?

Also, shouldn't a reduction gear on MG2 be able to shift where its peak power is compared to the engine?

In some cases it's actually necessary for MG1 or 2 to fight against the engine, to sap some of its energy and send it to the battery or the other motor.

Here are two cases:

1) Imagine flooring the accelerator pedal from a dead stop. The engine kicks to life. If MG2 were stationary, it would be like dropping the clutch on a manual, you'd stall the engine. MG2 actually needs to spin backwards to provide a gear reduction between the gasoline engine and the wheels. The higher the gas engine needs to rev to generate power, relative to wheel speed, the faster MG2 needs to spin backwards.

2) Traveling at normal highway speeds, the engine is spinning at a fixed RPM. In order to get the proper gearing to match that RPM to road speed, MG2 also needs to be spinning forward. However, this energy needs to come from somewhere, and the battery would only last a short time. So, MG1 is constantly working against the gasoline engine to generate power to send to MG2.

[Ecky]

Case #3) Low road speeds - where MG2 needs to be *approximately* stationary for the gasoline engine to keep constant road speeds and normal RPM. MG2 might in some cases work against the gas engine, if the battery needs to be filled, or might actually send some power back to MG1 to assist the gas engine, while it's working against it.

Because the electric motors are also the gearing for the transmission, it's sometimes necessary to make their power split work in seemingly illogical ways in order to solve gearing issues, or keep the gasoline engine in its island of efficiency.

[Isaac Zachary]

The "power split device" on Toyota hybrids is nothing more than a planetary gear set, which is basically a differential.

You can imagine it like this:

• Take a car and lift one of the drive wheels up off the ground and disconnect the brakes to it. Don't mind that it's now a three wheeled car. We'll just call it "the wheel on the ground" from now on for this mental exercise (although in an actual Toyota hybrid this "wheel" is still inside the transmission and is chained to another differential that goes to two drive wheels.)
• Now connect the engine directly to the differential. For an example, put the manual transmission in high gear and leave it there. You will not be shifting gears nor using the clutch (because in a Toyota hybrid there are no other gears nor a clutch).
• Now put two electric motor/generators on the car, one at each drive wheel. In other words, one on the drive wheel that's still on the ground (this is MG2) and one on the drive wheel that's now lifted up off the ground (this is MG1).

How does this work?

• The engine is off when you first start the vehicle, so both wheels are stopped.
• If you apply momentary power to the MG1 on the wheel that's up off ground it now "pop" starts the engine. The other wheel doesn't turn because you have the brakes on or that wheel is in park.
• But now the running engine that's connected directly to the differential in high gear will be turning the wheel that's up off the ground at a high speed, even at idle. By making the MG1 at that wheel generate some electricity the differential now splits the torque to the two wheels. So the wheel on the ground gets half the torque and the wheel that's off the ground and generating electricity with MG1 is also taking on half the torque.
• You can now gun the engine, but to keep the free wheel from just spinning out of control it now has to ramp up it's generator and take on more torque. The torque will always be split 50:50 between the wheel on the ground and MG1 on the free wheel.
• Now you can use the electricity that's being generated from the MG1 on the spinning wheel up off the ground to do a couple of things.
  • You could use it to charge the HV battery.
  • Or use it to provide MG2 that's on the wheel on the ground some power. (Or both charge the HV battery and power MG2.)
  • Another option would be to power MG2 on the wheel on the ground not only with all the electric power being generated from MG1 but also with whatever electric power the HV battery can provide.
• Let's say you continue to accelerate and maintain a steady engine speed (RPM).
  • Of course The wheel on the ground with MG2 on it will continue to speed up.
  • But the wheel off the ground with MG1 will slow down. (As long as engine RPMs stay the same).
  • This change of speed will continue until you reach highway speeds, at which point both wheels will be spinning forwards at about the same speed.
• But now let's say you want to regen brake.
  • The engine is now left to idle.
  • MG1 on the wheel that's off the ground stops generating electricity, so now that wheel spins freely essentially putting the engine in neutral.
  • And now MG2 on the wheel on the ground starts generating electricity.
• But let's say the engine power is cut and you go into "EV" mode.
  • Besides the engine turning off, MG1 continues to neither power nor generate, therefore leaving the engine in "neutral" so to speak.
  • But since the engine is no longer turning, the freely spinning wheel with MG1 now has to spin backwards. (Have you ever lifted both drive wheels off the ground and turned one of them and watched the other turn backwards with the engine off?)
  • To get out of EV mode while the car is still moving MG1 on the wheel off the ground simply has to start generating electricity again (unless you're going too slow). Since it's spinning backwards this will put torque on the engine to spin it forwards and start it again. Once the engine is up to speed, MG1 on the wheel that's off ground will now be spinning forwards again.

[orange4boy]

Somewhere there is a video or gif of this concept which makes it easier to conceptualize.

I have a gen 1 power split device that I tore down a while ago so I got to see in intimate detail how it's put together.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

Low road speeds - where MG2 needs to be *approximately* stationary for the gasoline engine to keep constant road speeds and normal RPM. MG2 might in some cases work against the gas engine, if the battery needs to be filled, or might actually send some power back to MG1 to assist the gas engine, while it's working against it.

Because the electric motors are also the gearing for the transmission, it's sometimes necessary to make their power split work in seemingly illogical ways in order to solve gearing issues, or keep the gasoline engine in its island of efficiency.

Reminds me the 2 times I took a hybrid Corolla taxi, and was quite entertained watching the power flow displayed at the multimedia screen. Sometimes it was not so easy to understand why the power flow went the ways it did.

[Isaac Zachary]

My own opinion is that "power split" is an incorrect term. It's more like a "torque split" device since it splits torque between the wheels and the generator. How much power is "split" depends entirely on the speed of each part, whereas torque is always 50:50. At a stop 0% of the power is going to the wheels and 100% (minus losses) is going to the generator.

Again, my own opinion.

[raubvogel]

So, the only way to transfer all the power from engine to the wheels is to have the planet gears locked with respect to the outer gear, so engine and outer gear are rotating at the same RPM. For that to happen MG1 must also be rotating its sun gear at the same RPM. Even if we assume infinite supply of electricity to both MG1 and MG2, MG1 still is the limiting factor.

That does put a damper on my original plan (using the GS450h transmission), but that is OK: this thread is not about a project of mine but to understand the Toyota hybrid system. I think I now understand it and its limitations.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

My own opinion is that "power split" is an incorrect term. It's more like a "torque split" device since it splits torque between the wheels and the generator.

Considering the torque converter fitted to conventional automatic transmissions, makes sense.