When Braking, how much is "Brake" how much is Regen?

Only near the limits of adhesion, which is extremely rare unless you are on the track. Aside from icy conditions, I never once had to get near the limits of adhesion in the 13 years I owned my 2004. A 25 second stop from 55 is at less than 1/10th of the traction limit, but that level of stop is common.

Regeneration is 100% front wheels and the back friction brakes (and the fronts) don't apply at all until you are too slow for regeneration to work. That was 7mph in my 2004. Might be lower in my Prime, not sure yet.

I had my brakes looked at on my 2004 before I sold it. In nearly 150,000 miles I only used 20% of the original brake pad thinkness. If they were getting applied to keep braking balanced, I would have worn out at least one or two sets of rear pads.

 

I used this on the way to work this morning. It was good info.

 

I don't know, about rear brakes just being along for the ride. I had the experience of driving a Civic with one rear (drum) brake disabled, after a botched brake fluid change.

Trying to do a hard stop at red light, I ended up over the cross walk, anti-lock brake mechanism going off like crazy. And due to it being one side only disable, there was a strange side-to-side rocking, just as you stopped.

Two front brakes only I suspect would be a lot worse.

 

As I keep saying, you only need front-rear brake balance when near the limit of adhesion. A hard stop is near the limit of adhesion and the Prime will use the friction brakes on any hard stop from any speed and so will be balanced (probably 70/30 front/rear or so) assuming the brakes are working properly. But on normal, gradual stops, it will be front-only using regenerative braking and there's no need for any rear braking at all.

 

The Civic in that condition always felt "off", pretty much every time I applied the brakes, I needed increased stopping distance, really had to adjust. To be fair: maybe if both rear brakes were disabled it would have at least evened things out. But having experienced that for the few days having the Civic in that condition, I would say all four brakes are contributing; it's an orchestra, and take two of them out: you'll feel it.

Sounds like a good myth busters episode.

 

I guarantee you that the brakes are not contributing on a normal stop on a Prius, until near zero speed. It's all regen, and thus all front-wheel braking.

 

Yes, I've heard of Prius brakes lasting 200k miles due to so much of it being regen.

 

This is from my log, first time I checked ours. Car was purchased in November 2010. Front and rear pads both start out around 10 mm thickness, front's slightly more:

Saturday, March 15, 2014
km: 43017
* inspected front and rear brakes, pads disassembled and lubed, calipers pins relubed
remaining thicknesses:
front left outer:7.9
front left inner:7.7
front right outer:7.9
front right inner:7.5
rear left outer:7.7
rear left inner:7.1
rear right outer:7.2
rear right inner:6.4

 

I had mine checked around 130,000 miles (210,000 km) and they were 80% of new thickness.

 

@stevepea, you might find this post relevant to your original question

 

I hadn't been following earlier, but this is one of those questions that comes up repeatedly on PriusChat, often with a lot of oversimplification and nobody looking it up.

These graphs are in the New Car Features manual for the 1st generation Prius. There's a similar one in the 2nd gen NCF, and I haven't grabbed one out of the 3rd or 4th gen book because my techinfo subscription isn't active this weekend, but there's not much change to the physics over the years.

The limit on the battery acceptance capacity is a maximum amount of power. The power is the regen braking force ✕ the rpm of the wheels, so at higher speeds it takes less regen force to max out what can be safely shoved into the battery. The graph on the left shows this. At the lowest vehicle speeds, the regen force drops off steeply just because the wheels aren't turning fast enough to make many volts. Just above the lowest speeds, there's a region where the regen force can be constant, because it's limited just by what MG2 and the inverter can do, not by the battery capacity. Then at speeds increasing from there, the regen force has to drop off, to keep the force ✕ speed within the battery's limits, the remainder of the braking demand being met by friction.

The graph on the right shows the picture in a more intuitive way, where the horizontal axis is time as the car is brought to a stop. The dashed line shows "driver's demand" (pressure on the pedal), which, at first, can be met by regen alone, but as the driver presses harder, at some point friction has to be brought in, and that point will depend on the speed. As the vehicle slows, though, more regen force becomes available, and takes over more of the braking, leaving less to be done by friction. That continues until the car has slowed to just 7 or 8 mph and can't produce much regen force any more, and friction is brought back in to finish stopping the car.

The exact power and speed points shown get changed over the different Prius generations, but the principles are the same.

The charge bargraph on the dash is a useful indicator of how much charge is going to the battery, but I don't think I've ever seen a Toyota document promising you can rely on it to know how the brake force is being apportioned exactly.

-Chap

 

The New Car Features books for the fourth-generation Prius and Prius Prime includes graphs similar to the one on the right, showing braking force over time. The graph on the left has been replaced by this series of graphs, which I think is not quite as clear:

The division between hydraulic and regenerative braking is determined in real time, through a function Toyota calls Regenerative Braking Cooperative Control. The New Car Features book explains:

During braking, the skid control ECU calculates the required regenerative braking force and transmits it to the hybrid vehicle control ECU. Upon receiving this signal, the hybrid vehicle control ECU transmits the actual regenerative braking control value to the skid control ECU. Based on this result, the skid control ECU calculates and executes the required hydraulic braking force.​

It’s not fully explained in the book, but this arrangement allows the HV control ECU to provide less than the requested amount of regenerative braking, or indeed none at all, when the the HV battery is already at a high state of charge or shouldn’t be charged for some other reason, such as ambient temperature.

Unlike a locomotive or subway car with dynamic braking, a Prius doesn’t have resistance grids to dissipate excess electrical energy, so when the HV battery can’t accept the energy, or if there is a malfunction in the hybrid system, the required braking force must come entirely from the hydraulic (friction) brakes.

 

I'm pretty sure I've heard one of my Prii (don't remember at the moment whether the g1 or g3) bring in the spin-the-engine-to-dump-power tactic under certain circumstances even when not in B mode. I know the g3 will do so on long descents with cruise control set. I think I dimly remember noticing it also when I used the brake, on a similarly long descent, but that memory is fuzzy and I don't have any long descents around here to run out and try it.

-Chap

 

That’s a good point; I’ve noticed this a few times on my fourth-generation Prius, too.