Regen Braking Threshold before Friction brakes kick in

Down to about 7 mph in a regular 3rd-generation. I don't know about the 5th, but would be surprised if the limit is as high as 18 mph.

 

No, there is no such threshold on Gen 4 Prius Prime. Regenerative braking can fully replace friction braking at all speeds down to zero. Gen 5 Prius Prime should have even better regenerative braking.

 

The topmost graph in #3 does not show regenerative braking fully replacing friction at all speeds down to zero. It shows the regenerative portion (gray shaded) tapering off as it closely approaches the rightmost ("stop") point, leaving only hydraulic braking at that point. That is similar to the same graph from earlier generations.

The other thing that is worth noticing in that graph is how friction braking can be a larger proportion of the total early in your braking, while the speed is still high, with the regen braking force rising along that curve, to be most of the braking force once your speed is lower.

The next pair of graphs show that effect a different way to clarify it. "At high speeds" shows that only a small regen braking force is possible, while "at mid or low speeds" shows more regen force is possible then.

The reason is that the power being regenerated is the product of regen force ✕ speed, and the limit on regen is how much power can be accepted, so at higher speeds, the power limit is hit at lower braking force.

You don't normally have to think much about any of that; the car does the math for you (and even, as the first graph shows, keeps re-doing the math as you slow, and increasing the proportion of regen as appropriate).

One thing that means, of course, is that when your speed is high, you have very little regen available before the friction brakes will be brought in.

 

Another factor is the charge level of the battery. Regen isn't going to happen if it is full.

 

Power will be diverted from the battery, but the regen braking will still slow you saving your friction brakes correct?

The way I've always understood it is, if you keep the braking "bar" above the horizontal dotted line, you are ONLY using regen braking to slow the vehicle.

This might also be understood as, if the bar is below the dotted line, there will be no additional regen braking, only friction. Although this does not speak to whether or not friction braking is occurring above the dotted line.

Regardless, there have been accounts of Prii coming into service w/ 100k miles having like-new brake pads. I think that bodes well for the idea that friction braking can be near-wholly avoided if you keep the bar above the dotted line.

Thoughts?

 

There will still be some braking happening via the powertrain, and wasting the energy by twirling the engine, rather than putting it back in the battery.

Some people call that engine braking, and use regen for when energy is being reclaimed into the battery. Others may use the term "regen braking" for both.

 

Yeah, but that's negligible—like 1%—at low brake-pedal depression. The brake energy is the area under the curve (integral of brake force times distance), and that white area is no more than about 1% of the total area at low brake force. At gentle braking, friction brakes don't seem to be applied until the car is almost at standstill (about 1 mph) as far as I can tell on my Gen 4, and I see the regenerative power on the display up until then.

 

So do the aerodynamic drag, tire rolling resistance, and even gravity when you are going uphill, the last of which is actually true regenerative braking, as gravitational energy is conservative like electromagnetic energy, but the first two and drivetrain drag are not regenerative as they are dissipative forces.

 

Regen slows the car by making electricity. That electricity has to go somewhere. The Prime has a big enough battery that charging up the battery isn't an issue on most drives. For some living at the top of a hill, coasting down can put enough charge into the full battery that the car spins the engine to slow the car. This takes the car out of EV mode. It will go through a warm up cycle, and the engine won't shut off until it is done.

I mention the state of charge because the battery's current condition is another factor the system takes into account. Others are things like how fast the brake pedal is applied. Which makes it difficult to state a specific speed at which the car switches from regen to friction.

My gen2 didn't have a braking graph. That older model could easily go over 100k before needing new brake pads. Don't drive like you're in a movie chase scene, and the regen system will help with efficiency and brake wear.

The graph you presented is an illustration of how the system can work, not what is actually happening; an Example of Cooperative Control. The bottom graph for when the car is in neutral also applies to low speeds, and it has no regen braking at all.

 

For seeing what is actually happening, I drive an actual Gen 4 Prius Prime. Friction brakes don't kick in down to about 1 mph in gentle braking.

How can there be regenerative braking when the motor–generator is disengaged from the wheels (when the car is in neutral gear)?

 

Moreover, I obtained 115 true mpg in long-distance round-trip freeway stop-and-go HEV driving in warm weather in multiple occasions. If regenerative braking didn't work at low speeds, this would be impossible. Perhaps it didn't in previous generations, but on my Gen 4, it certainly works down to about 1 mph, as evidenced by actual driving, multifunction display, and new-car features manual.

Now, find another vehicle that gets 115 mpg on a long-distance round trip!

 

There can't, but the chart title isn't "When neutral is selected", it's "When neutral is selected and low speeds."

 

As I said, that "low speed" is about 1 mph for gentle braking on Gen 4 Prius Prime, which agrees with the topmost figure that shows that friction braking is negligible at low speeds for gentle braking.

You need the friction brakes to bring the car to a complete standstill, but you don't need them at speeds above about 1 mph for gentle braking on a modern EV like Gen 4 Prius Prime. Again, older Prius generations might not have had this capability—I don't have any experience on them.

 

Last but not least, I don't even use the brake pedal on my Gen 4 Prius Prime most of the time—it is hypermiling with one-pedal driving. So, how can the friction brakes be applied without using the brake pedal? LOL

I usually only use the brake pedal to bring and keep the car at complete standstill, when the speed is down to about 1 mph.

 

Another point: if you have to brake, regen braking is better than non-regen braking that wastes all the energy. Regen only wastes some (there are conversion losses both on putting energy into the battery and on getting it back out).

To the extent you can arrange your driving to minimize any kind of braking at all, that's better than regen anything.

 

Which vaguely answers the original question that was asked before discussion digressed into other, more varied, situations.

 

No, it does not. It says nothing at all about a speed threshold, which the OP asked.

It is about 1 mph in gentle braking for Gen 4 from my experience. It could be 0 mph if you don't use the brake pedal at all and do one-pedal driving.

 

The top one of the graphs you posted shows friction braking taking over an increasing percentage of the braking force as speed nears zero, apparently near 100% at very low speed. That's logical. For gentler braking than the graph assumes, the speed at which regenerative force begins to decline would probably be lower, but would still exist.