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Hybrid puzzle of the day - can MG2 generate when not spinning?


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I thought I'd finally got to grips with understanding the general principles of how the hybrid system works on Lexus and Toyota cars. But today I found a new puzzle.

Can anyone help with this?

When the car is stationary in Park and the engine is spinning at around 1,000 rpm, motor/generator 1 (MG1) spins at around 3,600 rpm and generates electricity which is stored in the hybrid Battery. So far, so good.

But some of the engine's torque is also transmitted through the planetary gearset to MG2, which is connected to the road wheels. So without some sort of adjustment, the engine's power would tend to make the car move forwards, which doesn't happen when the car is in Park.

I had realised that for the car to be stationary with the engine running and the car in Park, MG2 needed to exert some force to prevent the car moving forward. I had assumed this would be by applying some of the electricity generated by MG1 to push MG2 in reverse to counter the force from the engine. That seemed fairly straightforward.

However, I was looking at the Hybrid Assistant app today and I noticed that with the engine running and the car in Park, the app was registering torque of around minus 9 for MG1. (I don't know what the units are - it doesn't say. Probably Newton metres). At the same time MG2 was recording torque of around minus 7.

A quick test showed that MG2 showed minus figures when Reverse is selected and plus figures when Drive is selected. So it seems obvious that this just shows the counter-force that MG2 exerts when in Park to stop the car edging forwards.

But it occurred to me that if MG1 is showing minus figures when it's acting as a generator, maybe MG2 does too. So the question is whether MG2 can act as a generator when it isn't spinning, just as it does when applying braking force to regenerate electricity. It can certainly exert torque when it isn't spinning. But is this because it's using electricity to stop it spinning or because it's creating resistance by generating electricity, again to stop it spinning?

It would be more efficient to have both MG1 and MG2 generate electricity when the engine is spinning and the car is in Park, compared with generating electricity in MG1 and then using it in MG2 to keep the car stationary.

So does anyone know the answer? Is MG2 acting as a motor or as a generator when the car is stationary in Park?

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48 minutes ago, NemesisUK said:

Anything in these Lexus EU tech docs shed light?

Many thanks for these. I haven't found anything in them that directly answers the question that I was wondering about but there's a bit that gets close to the question.

In the first document (Hybrid Control System Description), paragraph 1.b.(iv) talks about what happens when the engine needs to start. It says, "While the engine is cranking, to prevent the reactive force of the sun gear of generator (MG1) from rotating the ring gear and driving the drive wheels, current is also applied to motor (MG2) in order to prevent motor (MG2) from rotating. This function is called 'reactive control'."

This is in line with what I described above. But I think that by using the word "cranking" they are just talking about the moment when the engine is started and MG1 is generating the torque rather than the engine.

It may be that the "reactive control" is the same whether the engine is starting or the engine is running and it probably is. But it would still be interesting to know if a stationary MG2 can produce a "reactive control" when it is stationary but acting as a generator rather than a motor.

Thanks for producing these documents. They certainly help to clarify what is going on in the power-split device.

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1 hour ago, NemesisUK said:

More on MG1 and MG2

Thanks for that. This document (Hybrid Transmission System Details MG1 and MG2) seems to confirm that MG2 can apply torque while remaining stationary. It says, "When three-phase alternating current is applied to the three-phase winding of the stator coil, rotation of the magnetic field is created in the motor." So the magnetic field is rotating rather than the motor.

But it also seems to suggest that it can't create torque if it's stationary when acting as a generator. It says, "When generating power, rotation of the rotor creates a magnetic field, inducing current flow in the stator coil." So it would have to be spinning, as when braking, for MG2 to generate current.

If I've misunderstood this, maybe someone who knows about electric motor/generators can correct my interpretation!

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7 hours ago, Thackeray said:

So the question is whether MG2 can act as a generator when it isn't spinning

No. A generator produces electricity as a conductor cuts through a magnetic field (magnetic induction), therefore you must have movement.

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10 hours ago, Thackeray said:

I thought I'd finally got to grips with understanding the general principles of how the hybrid system works on Lexus and Toyota cars. But today I found a new puzzle.

Can anyone help with this?

When the car is stationary in Park and the engine is spinning at around 1,000 rpm, motor/generator 1 (MG1) spins at around 3,600 rpm and generates electricity which is stored in the hybrid battery. So far, so good.

But some of the engine's torque is also transmitted through the planetary gearset to MG2, which is connected to the road wheels. So without some sort of adjustment, the engine's power would tend to make the car move forwards, which doesn't happen when the car is in Park.

I had realised that for the car to be stationary with the engine running and the car in Park, MG2 needed to exert some force to prevent the car moving forward. I had assumed this would be by applying some of the electricity generated by MG1 to push MG2 in reverse to counter the force from the engine. That seemed fairly straightforward.

However, I was looking at the Hybrid Assistant app today and I noticed that with the engine running and the car in Park, the app was registering torque of around minus 9 for MG1. (I don't know what the units are - it doesn't say. Probably Newton metres). At the same time MG2 was recording torque of around minus 7.

A quick test showed that MG2 showed minus figures when Reverse is selected and plus figures when Drive is selected. So it seems obvious that this just shows the counter-force that MG2 exerts when in Park to stop the car edging forwards.

But it occurred to me that if MG1 is showing minus figures when it's acting as a generator, maybe MG2 does too. So the question is whether MG2 can act as a generator when it isn't spinning, just as it does when applying braking force to regenerate electricity. It can certainly exert torque when it isn't spinning. But is this because it's using electricity to stop it spinning or because it's creating resistance by generating electricity, again to stop it spinning?

It would be more efficient to have both MG1 and MG2 generate electricity when the engine is spinning and the car is in Park, compared with generating electricity in MG1 and then using it in MG2 to keep the car stationary.

So does anyone know the answer? Is MG2 acting as a motor or as a generator when the car is stationary in Park?

Torque is rotational force so presumably if something isn't spinning then there can't be any torque?

And as Colin states above, electricity is generated when a conductor cuts through a magnetic field so again, no movement means no electricity.

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The way I read it, it is the magnetic field which rotates by electronic switching off three phase current, much the same as the trains I drive. AC three phase motors/generators work differently to DC in that they need thyristor control to switch current on/off during the wave form which, due to the oval stator, is simulated. If you read it, nowhere does is state that the rotor is moving. Torque can be measured without movement, simply apply a socket or spanner to a very tight bolt and try to undo it, even when it won't move, you're still applying torque, just not enough. 

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9 hours ago, Herbie said:

Torque is rotational force so presumably if something isn't spinning then there can't be any torque?

 

7 hours ago, Paul Brooksbank said:

Torque can be measured without movement, simply apply a socket or spanner to a very tight bolt and try to undo it, even when it won't move, you're still applying torque, just not enough.

I think the problem in my mind was that I wasn't thinking about the difference between torque and rotation. Torque is applied to MG2 when the car is stationary and the engine is running but MG2 doesn't move. If there was no torque keeping MG2 stationary then the engine would tend to push the car forwards.

But what I'd forgotten to pay attention to was that in a motor electricity is converted into rotation, whereas in a generator rotation is converted into electricity. So in a motor if you don't have enough electricity to cause rotation you still nonetheless have torque - just not enough to cause rotation. Whereas in a generator if there's no rotation, there's nothing to be converted into electricity or to create any torque.

Incidentally, it's impressive how subtle the control of MG2 is. There's no sense in the car that it's doing anything but resting stationary while the engine runs. And yet MG2 is applying just enough force to prevent any movement of the car and doing it exactly the right amount to balance any slight variations of output from the engine.

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Not forgetting there is a transmission locking pawl that engages when Park is selected? I rarely used the foot operated parking brake in winter and can feel the car roll forward or backward until the pawl engages. This is on completion of a journey and when the car is warm and charged, so the engine is off. When starting off cold the engine doesn't immediately fire up and unless one is de-icing the screen the car is already moving.

The occasions one might actually have the engine fire up when stopped without the brakes applied (or the transmission pawl engaged) must be fairly rare?

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31 minutes ago, NemesisUK said:

Not forgetting there is a transmission locking pawl that engages when Park is selected? I rarely used the foot operated parking brake in winter and can feel the car roll forward or backward until the pawl engages. This is on completion of a journey and when the car is warm and charged, so the engine is off. When starting off cold the engine doesn't immediately fire up and unless one is de-icing the screen the car is already moving.

The occasions one might actually have the engine fire up when stopped without the brakes applied (or the transmission pawl engaged) must be fairly rare?

Yes, I would guess that torque is only applied to MG2 when the engine is running, and just enough to counter-balance the torque from the engine. So the parking pawl still holds the car stationary if the parking brake isn't on, whether the engine is running or not.

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Wow, what a great, informative thread 👍

It operation still battles me, but the sheer engineering complexity that has gone into the eCVT system never ceases to amaze.

For day to day driving it is such a smooth system, so I wonder why more hybrid cars don't use it?

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Great to see that people are interested to this extent but, even as an engineer by trade myself, this is too much for me I’m afraid.

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5 hours ago, Thackeray said:

Yes, I would guess that torque is only applied to MG2 when the engine is running, and just enough to counter-balance the torque from the engine. So the parking pawl still holds the car stationary if the parking brake isn't on, whether the engine is running or not.

Actually, I find that I can stop the car on an incline, allowing it to stop naturally, it holds position even with the engine switched off, drive still engaged and without my foot on the brake pedal. 

The batteries feed MG2 with enough current to hold the stator in position and therefore hold the car in a stationary position. If the batteries then deplete, the engine starts and feeds from MG1 into MG2. 

It is an ingenious system and any passengers in the vehicle who are accustomed to standard automatic vehicles (the cheap seats as I call them) are mind blown by the concept. 

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Having read the documents posted, I have to take my hat off to the individuals or team that put this system together. Talk about thinking outside the box , and it's proved to be very reliable.

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