How does regen braking actually stop the car?

The Prius control system imitates an automatic transmission. If you let up on the brake, the Prius will try to creep forward. This was done to make operation consistent with what most drivers expect. If you press firmly on the brake, the control system will stop sending power to the motor. As you let up, a small amount of electricity will be routed to MG2 to make the car creep. Make sure you keep enough pressure on the pedal to avoid wasting power at a stop.

Tom

 

One more way to look at it: The Prius stops in regen mode exactly the same way it goes in EV mode. This is an over simplification, but if the battery were connected directly to the motor, you would have a simple off/on acceleration. To smooth out the acceleration, a controller is inserted between the battery and motor that switches the power on and off thousands of times per second. The higher the ratio of on time to off time, the more power is delivered to the motor. The controller is bidirectional, so it works exactly the same way in reverse for regenerative breaking.

If this were a simple DC motor, reversing would be as simple as reversing the polarity of the field winding. The the motor would essentially be fighting against the direction of forward momentum and electricity is produced. In this case, we can't exactly reverse the polarity of the permanent magnets, so things get more complicated. To be honest I don't even really understand it fully, and I had to take fields & waves in college. Basically I think in an AC/permenant magnet motor it all comes down to the phase of the drive signal relative to the position of the armature/rotor. If you drive it "in phase" you generate forward motion. If you drive it "out of phase" it fights against the forward momentum of the vehicle and electricity is generated. This is why the motors have fairly precise position sensors, and not just simple tach/rpm sensors like a regular car.

One way to think of it is like the timing between piston position and the firing of a spark plug in an ICE. There is an optimal timing at any given rpm for when the "push" of each cylinder detonating will add to the forward speed of the motor. Firing too early or too late would actually create resistance to forward motion (among other things like blowing up your engine ) Each cylinder in the ICE is kind of like one of the 3 winding phases in the motor. The controller monitors the position of the motors rotor, and determines both how hard to "fire" each winding based on pedal position, and when to "fire" based on whether you're trying to speed up or slow down.

Sorry if that was redundant with what was said before. As someone else mentioned I figure its just worth saying it in as many ways as possible as we all visualize things differently!

Rob

 

One of my favorite websites is HowStuffWorks - Learn how Everything Works! . Here is a link that has a general overview of the things being discussed: Howstuffworks "Improving Fuel Economy"



Some of the information is a little dated but the site usually covers most subjects in a holistic way.

 

The reversal of current flow doesn't happen at 8mph but much slower, at least in my car I haven't seen it below about 5km/h.
One good reason for this powering the car as and after stopping once the brake pedal is released is it reduces shock loading on the transmission.

Your Prius can't read minds, not yet but I think the new model might. So when you remove your foot from the brake pedal a SMALL electrical current is directed MG2 to take up any backlash in the final drive, this prevents shock loading on the transmission. When you drive a manual car you generally would take the shock out of the system with your clutch technique, you don't notice it because you have practised it for years. You don't bunny hop like when you were learning do you?
Automatic transmissions do the same as the Prius as a side affect of their design.
So by putting a little power to the wheels as you take your foot off the brake takes up any lash in the transmission before you firmly plant your size 10 1/2 on the accelerator.
When you are dealing with massive amounts of torque like MG2 can produce you need to take precautions.

The amount of energy used to provide this feature is minuscule so I wouldn't get too worked up over it, a manual car would waste far more energy slipping the clutch just to get moving. Remember there is no magic in a clutch, unlike a torque converter it doesn't multiply available torque it just allows the ICE to maintain a crank speed where it has sufficient torque to move the vehicle. That's why you need to rev the engine a bit higher in a manual car when doing a hill start, there is a little more torque at mid revs than low and you can used kinetic energy stored in the flywheel.

 

Also, let me know if this is correct...

Light regen with no brake pedal pressed recovers energy more efficiently than quickly recovering energy with the brake pedal pressed. So if there's nobody riding your nice person, it's better not to use the pedal if you can help it. I've heard this light regen is around 50% efficient at capturing energy

Better yet, if you can find the sweet spot on the accelerator where you're "coasting in neutral" between regeneration and using engine/motor power, use the road and your wind resistance to slow down. The energy monitor will show no arrows at <= 40 mph and it'll show electric only at higher speeds, generally (once warmed up) The prius rolls so well and is so aerodynamic that this only really works for planned deceleration.

 

Attila's study shows the brake pedal value 17 is the most energy regenerated.
The new 2004 Toyota Prius : How to brake

Ken@Japan

 

Wow, thanks for the info. I always thought the regen efficiency was limited by the charge rate the battery pack could accept and above that, it was wasted. If these #'s are valid, it looks like operating the MG at partial loading decreases braking efficiency and then the battery charge-rate limit kicks in at higher deceleration rates. Also, the higher rates might be prompting the friction brakes to kick in.

 

Hi P&G,

Attilla's numbers indicate that at that optimum braking rate the regeneration efficiency is quite good, ie better than 80 % (i calculated it out once from his results).

The efficiency of reuse is only around 40 % however. That is the efficiency of regneration times the efficiency of use. This is probably due to the Prius being a Hybrid car. In a Hybrid car, the battery cables, battery size, motor size and inverter are all sized to provide just the right power to keep the engine in efficient operation. Which is considerably downsized compared to a pure EV. So at the high loads of an electric only accelleration, there is allot of resistive loss. One might get better electic efficiency if one accellerated much more slowly, but that is not going to be practical traffic, unless one is on a downhill.

Still, 40 % reuse of braking energy is a whole bunch better than 0 % in standard cars.

 

Thanks, good explanation. I wont be so aggrivated about it now...