Why 80 HP electric motor?

Because the car is capable of delivering power to that motor (MG2) from the battery and from the MG1 generator simultaneously.

 

... and in fact, not only is the car capable of doing that, it has to: if MG1 is not delivering some of the engine's power through the transmission electrically, the gears won't do it mechanically either.

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That's true of the engine torque, but I can't see saying that about power in all circumstances. At a road speed of about 61 mph (2769 RPM ring gear speed), if the engine speed is 2000 RPM, then MG1 speed will be zero, so it can't be delivering any of the engine's power through the transmission electrically. All engine power is then going through the ring gear (aside from losses).

It's certainly true that in circumstances when MG2 is producing anywhere near 80 mechanical hp, most of its input electrical power has to come from the engine via MG1.

 

Well hang on, if MG1 gets ~28% of the gas engine's output, that is already pretty close to the 20kW nameplate capacity on MG1. Figure 26hp gets through with efficiency losses.

So from a standing start, you floor the pedal and you've got the engine delivering about 71hp routed mechanically to the wheels, 26hp routed electrically through the transmission from MG1 to MG2, and maybe 36hp in contribution from the battery = 133hp total.

The 26hp contribution from MG1 is the lesser of the influences on MG2 power supply.

Also let's keep in mind that an 80hp (60kW) motor-generator is an awesome way to recharge a battery. You build it the same way whether you plan for peak power to be motored or generated.

 

Torque output, not necessarily power output. We also need to figure individual RPMs before computing the power split, which is not fixed.

 

MG1 receives about 28% (1/3.6, to be exact, ignoring losses) of the engine's torque output, but not that fraction of it's power output unless the engine, MG1, and the ring gear are all rotating at the same speed. The fraction of engine power MG1 receives ranges from 100% to 0%---or even negative in a sense.

From your 71+26 hp, I gather you seem to be assuming the engine's claimed peak power can go to the wheels from a standing start, but that's impossible for multiple reasons. For one, 97 hp at zero wheel speed would require infinite torque at the wheels, which ain't happening. For reason #2, peak engine power requires corresponding engine speed of 5200 RPM, which, with wheels at zero speed, would require MG1 to spin at 18,720 RPM, far above its speed limit.

if MG can contribute only 20 kw (according to you), and the battery can contribute only 36 hp (according to Abernabe), that adds up to only about 63 hp, well below 80. Hmm...

 

Fair point that peak power could only occur sometime after it started rolling.

I still think we don't see the battery and MG1 adding up to MG2's full potential because it only needs that full potential in generator mode when braking/charging the car.

 

Good catch, thanks, I said that less than carefully. MG1 still has to supply a torque at that point to make the mechanical power path work, but just a stator field holding the rotor stationary is enough ... so the only power involved there would be some loss in the stator windings.

Using a stator field to hold a rotor stationary doesn't seem like a very efficient way of using a motor (100% going to I²R loss), so I kind of wonder if the firmware prefers to avoid that zero-rpm region by operating the engine a bit above or below the corresponding engine RPM).

 

Considering that the I²R loss shouldn't really be meaningfully different at zero rpm than at other low rpm, I'm not sure there is any real gain from moving it slightly off of zero.

 

Yeah, I'm not sure either. But the zero RPM point is the one where the loss is exactly 100% of the power being handled there, which seems like a pretty hard minimum for efficiency, at least for the MG in isolation.

On the other hand, move away from that point, and now you have more engine power taking the electrical path, which may be inherently less efficient than the mechanical one overall.

 

I've wondered about that too. It can't totally "avoid that zero-rpm region" and still operate as smoothly as it does. I assume that condition, with all power taking the mechanical path, is most efficient overall, despite the stator-holding electrical loss.

 

Good point. Would near-zero MG1 speed somehow be significantly better than zero in regard to winding temperatures?

 

Right, it couldn't be a hard "avoid—never operate there", that would mean a discontinuity.

But they could put different weights on the firmware's preference for different operating regimes.

 

Maybe someone should drive their Gen3 around while logging motor speed data to see what it does.

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