How Toyota maximize re-gen

Both MG1 and MG2 can regenerate. If it's regenerating a lot of power then I think it's mostly MG2.

The amount of regeneration is not just determined by the speed. The amplitude and phase of the "excitation" waveform that the inverter supplies will determine the amount of motoring or regeneration that each provides. (Yeah kind of like you notion of "activating extra coils" but more sophisticated.)

 

Yep, you can think of it as the inverter providing the correct voltage waveforms to energize the coils in such a way as to produce a rotating magnetic field. The very powerful permanent magnet on the rotor then follow this rotating magnetic field.

To make it act as a motor the rotating magnetic field is phased so that it's poles lead those of the PM rotor, which produces an accelerating torque. To make it act as a generator the inverter phases this rotating field so that it lags behind the rotor magnetic axis. This causes a retarding torque and also causes the induced current to be out of phase with the inverter voltage, which causes the direction of current into the battery to reverse.

The amount of motoring or regeneration can be controlled by the inverter controlling both the strength of this rotating magnetic field and it's phasing relative to the permanent magnets of the rotor.

 

No, it spins forward to minimize the rotational rate of MG1 at higher speeds.

Note that Toyota does not maximize regeneration as there is a loss of energy as the HV battery charges and discharges. Excess energy is sent to the battery when you are slowing down, when the ICE has to run for heat, or when the ICE runs at a higher output power than needed to run at a more efficient operating point.

JeffD

 

BRAKE, not break...

 

Yes it's a fairly common little spelling slip up, but it's not as annoying as the mistake where someone replies to the wrong person.

 

All of the "gears" in a Prius are electrical/electronic. The Prius only has one set of gears, and they are fixed ratio. Everything else - low gear, top gear, reverse, and all in between - is nothing more than cleverly manipulated field currents in the two motor generators. Computers control the fields in MG1 and MG2, causing them to act as motors or generators, and controlling speed and braking forces. It is a very elegant system.

Tom

 

Point taken.

 

Yes, there is a regen braking limit, and I believe it is significantly less than many readers here expect. This limit is caused by the traction battery's power limit, which itself varies with battery temperature.

For a given battery power, the braking force will be inversely proportional to speed. I cannot compute actual braking forces due to not knowing system inefficiencies, but expect that the limit corresponds to moderate braking at street speeds, and light braking at highway speeds. For an approximate guide, watch the HSI CHG bar. The maximum regen should happen just as that bar reaches full length to the left.

For a guide, try some calculations, using the simple formulas P = F * v, F = m * a, and a battery limit of 27kw. From memory, I seem to remember that at 60 mph in a reasonably loaded Prius and neglecting conversion losses, maximum regen braking occurs at a braking deceleration of about 0.06 g. Hard friction braking in ideal conditions can be around 0.9 to 1.0 g.

Below that speed, MG2 is turning too slow to produce much power.

 

There are three things that put an upper limit on regeneration:

1) Traction of the drive wheels - Regenerative braking is done only through the front wheels. As soon as one front tire loses traction the game is over, and the braking system switches over to friction. This is the source of the infamous braking "surge" so often discussed on these pages, where the Prius "shoots forward" when braking over a bump.

2) Generator capacity - The Motor-Generators (MG1 and MG2) have a finite capacity. Even if more kinetic energy is available, the MGs cannot generate beyond their limits.

3) Battery capacity - The HV or traction battery has a finite capacity to absorb charge. It is limited by both the rate at which it can absorb charge and the total amount of charge that can be absorbed. These limits are influenced by the battery's State of Charge (SoC) and ambient conditions such as battery temperature.

In normal operation, #3 above is almost always the limiting factor. The MGs are sized to use power from both the battery and the internal combustion engine (ICE), so they have a capacity that exceeds that of the battery. Some of the extra regenerated power can be burned off as heat by spinning the ICE as an air pump, which is what happens with engine braking (B mode).

In addition to the above limits, the amount of braking force that can be produced through regeneration varies with speed. Work, or energy, is force times distance. In the case of regeneration, the force is the braking force, and the distance is the distance traveled while braking. Power is work per unit time. At higher speed, a car travels farther in the same amount of time, so for a given amount of braking force more energy is regenerated, or in other words, more power. Since regenerative power is limited, the faster the car speed, the lower the braking force before the regenerative limit is reached.

In a nutshell, because of this speed/power connection, at low speed you can brake relatively hard without friction brakes. At highway speeds friction will be added for anything but light braking.

Tom

 

Unfortunately, the OP and myself don't have 3rd gens so we have no HSI CHG bar. I can watch the amont of draw/regen from HV battery via the Bta XGauge on ScanGauge. I don't think I've seen regen ever get much above ~mid 90 amps or so, indicated by Bta of -90 or so.

 

It's because of differential which is between wheels and MG2. If you accelerate with one wheel on a slippery surface you will not move (without diff lock). Imagine that you are braking on this same surface, not an optimal way to brake with re-gen, VSC or EBD or ABS sense this situation and switch to friction brakes.