Horses and Toyota hybrid power split questions
 

I originally asked this question in diyelectriccar but got crickets back; I guess it was not appropriate there so I will try it here. These questions have baffled me since I first read about the Prius. Friend of mine has two of them but every time I asked he kept going over MG1, MG2, and planetary gears while dancing around the real topic without ever approaching it. I will even be nice and use ICE instead of just engine (I normally use motor vs engine to differentiate between electric motor and gasoline/diesel engine). I am specifically mentioning the transmission from a Lexus GS450h because of my interests, but in principle it should apply to the other Toyota hybrids.

1. So I I read a thread called lexus gs450h transmission to a mustang? where it mentioned the gs450h transmission can do "800 hp in a burst for a few seconds." How would that work? And that leads to...
2. How much of the ICE does the Toyota hybrid system exposes to the driveshaft? By that I mean (AFAIK) the MG1 needs to be in some balance with the ICE; I take the ratio between ICE and MG1 power is addressed by the gearing ratio between them and the max RPM the MG1 can do. Doesn't that cost energy (V6 292HP + transmission 192HP for a total of ~340HP (Lexus GS - Wikipedia) means we have a lot of missing horses)? The same "case of missing horses" is also (unintentionally) mentioned in an official Lexus blog. What if there was a way to lock the ICE so you could get all of its power + MG2? Probably that would not work, which leads to...
3. In The Lexus GS450h motor thread there is a mention of the "purely series hybrid design (BMW i3, Nissan e-Power)"; are those like the Honda Insight system where the contribution from the electric motor can go from 0% to 100% but the ICE must stay on at all times? That thread seem to indicate the BMW and Nissan has a much more sophisticated system than that.

Regarding #3...

Toyota does a "series-parallel" setup. It can run the motor, the engine, or both.

"Pure series" would mean the electric motor puts all the power to the ground. The engine generates electricity, which recharges the pack and from there goes to the electric motor. One after the other, in series. There is no mechanical link between the engine and the wheels.

The Honda Insight is a "parallel hybrid"...the engine and the motor work together, to varying degrees, to drive the wheels. Side by side, rather than one after the other.

Thus, back to the Toyota setup, which can choose to do either or both.

Regarding #1: It says "If it can hit 800hp".

How does that work? Electric motors power output/ratings are all about how much heat they can handle. If that motor can do 400hp on a regular basis, it can put up with 800hp for a few seconds. Then it would need time to cool off before doing it again. Assuming you don't burn up any (copper) windings in the process. You just cram more electricity in to it to get it to output more. Until it melts.

#2 sounds very much like your previous thread, reworded. You'll probably find someone more suited to answer this on the Prius forum. Someone who's taken the time to understand it fully. Or you can go do that reading yourself. Oh, there might be someone on here who knows it inside and out, so give it a chance, I suppose.

Most likely it limits the ICE to not go too beyond its peak torque RPM so often, as the electric motors provide an effect analogue to a CVT enabling the ICE to remain at higher efficiency levels.

To the best of my knowledge:

Wheel speed is based on the ratio of MG2 and ICE(+MG1) RPM, which are both connected to the wheels via a planetary gearset. During normal driving MG2 or the ICE can be entirely stationary, or MG2 can even spin backward, to get the desired wheel speed. As the RPM on one goes up, RPM on the other needs to decrease, to maintain the same road speed. You can't get max horsepower out of both simultaneously because of the power bands for each - if the ICE is at redline, MG2 will need to be spinning much more slowly or even backward, preventing it from producing peak horsepower at any reasonable road speed. Additionally, MG2 may also not be able to produce peak horsepower without additional power sapped from the ICE via MG1 - the battery alone in these types of hybrids often isn't adequately large to provide peak energy requirements of the electric motor.

I can't speak in any detail about the system because there's relatively little I've seen published on it, but I have a decent understanding of how series-parallel hybrids with planetary gears.

Makes me wonder to which extent the ICE power consumed by the MG2 while it spins backward (eventually working as a generator at that circumstance) would count to keep the power at the maximum combined rating. Having resorted to a hybrid Corolla taxi a couple of times, I was quite entertained looking at the display showing the energy flow between ICE, electric motors and battery.

From what I've read, the engine drives the planetary gears of the "power split device".

The ring gear around the planets is connected to both the Motor (MG2), which is on the output shaft, and the differential.

The Generator (MG1) is connected to the sun gear.

Powerwise

When the engine and all motor-generators are spinning the engine is feeding the planetary gears that splits 50% of it's power to the Generator (MG1) and the other 50% to the output shaft that the Motor (MG2) is connected to. How much of the Generator's (MG1's) output is directed to the battery or to the Motor (MG2) will affect total output.

For an example if the engine is supplying 100hp, 50hp go to the wheels an 50hp go to the generator. If all of that 50hp is directed to the motor, then you get 100hp (theoratically, if there werent' any losses) at the wheels.

But if 25hp is directed to the motor (MG2) and 25hp is directed to the HV battery, then 25hp + 50hp means you get 75hp at the wheels.

But if the motor (MG2) receives not only that 50hp power from the generator (MG1) but also another 50hp from the battery you now have 150hp at the wheels. (100hp from engine divided into 50hp MG1 and 50hp output shaft and the 50hp from MG1 and another 50hp from battery into MG2 that's on the output shaft = 150hp).

Speedwise

If the engine is spinning faster than the output shaft (MG2), then the Generator (MG1) is spinning that many times faster. For an example, Engine speed is 3,000RPM, output shaft (MG2) 2,000RPM, so Generator (MG1) is 4,000RPM.

If the engine is spinning the same speed as the output shaft (MG2), then the generator (MG1)is spinning the same speed, For an example: For an example, Engine speed is 4,000RPM, output shaft (MG2) 4,000RPM, so Generator (MG1) is 4,000RPM.

If the output shaft (MG2) is spinning faster than the engine then the Generator (MG1) is spinning that many times slower. For an example: Output shaft (MG2) is 3,000RPM, Engine is 4,000RPM so Generator (MG1) is 2,000RPM.

Any time the engine is spinning half the speed the output shaft (MG2) is the Generator (MG1) is stopped. For an example: Output shaft (Motor MG2) 2,000RPM, engine 1,000RPM so Generator (MG1) is 0RPM.

Any time the engine is less than half the speed the output shaft (MG2) is then the Generator (MG1) spins backwards. For an example: Output shaft (MG2) is 1,000RPM, Engine off 0RPM so Generator (MG2) is -1,000RPM.

As brought out before, the main challenge is probably thermal. Some have supercharged their Prii and found that they don't overpower the generator (MG1). It just tends to absorb a lot more power and they end up with a much more fully charged HV battery all the time. The Motor (MG2) should also be capable of delivering more power.

I don't remember what is the rev limit for the ICE while both MG1 and MG2 are operating in the Prius, but it's set below 5000RPM

ICE rev limit??

There's also a rev limit for MG1 which is why the engine starts after 36mph on a Gen 2 Prius IIRC. (it's different for different cars. The Avalon can keep the engine off up to 48mph).

The rev limit for MG2 is the same limit as max vehicle speed since it's connected directly to the output shaft. I have no idea what that is though as I've never driven my Prius more than 50mph.

Yes. I don't know if the reason is to protect the power-split device or to keep the overall efficiency.

Stubby79, you are right. In fact I did not realize I had another thread already, but I think this one is a bit more well thought.

Regarding the Toyota system (#3), I think this is the crux of question #2: I really do not know how it works. For instance, if I were to quote https://priuschat.com/threads/power-...device.167349/, without MG1 the engine power cannot get to the wheels. That would be consistent with what you said about not being a mechanical link between engine and wheels.
That also means we are then limited by the RPM iand power the MG1 can handle (Gen 3 added more stuff to allow that motor to spin slower but the principle is the same).https://en.wikipedia.org/wiki/Hybrid...ve#Power_flows seems to imply there can be a mode where the engine alone powers the wheels; from the argument above
that is impossible in the Toyota/Aisin system.

Regarding the Insight Hybrid (#3), I can see its motor helping the engine specially at low RPM where it is not efficient. And then at high RPM it would be collecting energy for the battery (Kinda like in the Top Gear where Jeremy Clarkson drove the BMW Hybrid, where he said if you wanted to charge the battery you should drive hard/fast).

Regarding #1, what you are saying is that the 800HP (ignoring the "for a few seconds" part since I consider that is a given because of the issues you enumerated) all electric?

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?

It does seem that the Toyota system is aimed to provide efficiency first and foremost, with any power tied down to what the motors, specifically MG2, can deliver. In other words, there is no point in building an engine that can deliver 600HP at 7500RPM since how much of that power will get to the rear wheels depends on MG1 being able to (a) transfer some of that power to MG2 and (b) create enough resistance (which requires energy) so a fraction of that power can go to the ring gear (MG2 + wheels).