Do rear brakes activate always, despite regenerative braking?

Regen is much as if you were in a conventional front-wheel-drive car and downshifted in order to slow the car. It acts through the drivetrain, and therefore only on the front wheels. The friction brakes, whether front or rear, are uninvolved, until more slowing is needed.

 

someone here @Mendel Leisk ? said a scanner should pad movement from the beginning of pressure on the pedal

 

When I got my brake discs replaced recently, my brakes were insanely squeaky for a while. This allowed me to experimented with it to figure out the answer to exactly this question.

Basically the rear friction brakes are on, to some extent, whenever you press the brake pedal, even slightly. At this level it's to keep the car balanced, just like in a traditional car, by splitting the braking between the front (regen) and rear (friction) brakes. The harder you press the brake pedal, the more regen and rear friction braking occurs, until......

The front friction brakes come on once you're applying enough brake pressure to exceed the maximum regen (you can see this on the hybrid system information screen's graph, when the light blue curved bar hits the lowest it can go while braking).

As to how much is regen and how much is friction braking depends on many things (e.g. battery level, battery temperature, driving mode, D or B, etc). At this harder braking level the braking is split between regen on the front, and friction on both the front and rear.

Below around 4 to 5mph it's always purely friction braking.

The simulated engine braking (not pressing any pedals), is entirely achieved with a small amount of regen.

If the battery gets full or hot, or you switch into B (engine Braking) mode on the gear selector, then the ICE will rev up when you take your foot off the accelerator, and more so as you press the brake. This is genuine combustion engine braking (converting kinetic energy from the wheels into heat in the engine) and replaces what braking the regen be would have normally been doing in these circumstances, and more so in B mode. If the battery isn't full while in B mode, some regen still happens, just not as much as when in D.

 

Friction brakes are also in use 100% if you are coasting in NEUTRAL, which is not generally recommended.

 

All I know: anytime the car's been washed (or driven in rain), then sits a few days, with the rust build-up on the rotors, you hear the brakes with every push of the pedal, and it seems like all four corners. For the first block or so.

 

It may be similar (if a bit more complicated) in an AWD HV. In my gen5 rav4, the Toyota rear motor brake regen PID always shows zero, but I think this is wrong as the car shows rear motor regen on the dash MID. Also Toyota only supplies one wheel cylinder (friction brake) sensor PID, so I can not distinguish the front-back effect. But you can still see (MG2 = front) regen torque versus friction braking (Wheel Cyl. sensor voltage) in the attached log.

 

I'm seeming to remember past discussions and braking-force-vs-time graphs indicting that initial foot pedal braking is mostly friction, because it is quicker and more certain. But then the ECUs quickly switch it over to almost all regenerative braking, as available (battery temperature limits and other factors permitting), though a small amount of friction braking always remains. Is this true of Prii too, or just some other designs?

Included graphs were something like this one (not Prius-specific):


I'm not remembering the front vs rear distribution of the friction element in this mode, though the regen element is certainly just at the front.

Of course, this friction portion doesn't apply to pure D- and B-mode engine braking without the foot pedal.


Separately, I'd also note that the Prius maximum regeneration is often less than many people think. It is easy for traditional ICE drivers to initiate braking too late for efficient regen energy recovery, necessitating more brake force than regen can provide, thus adding wasteful friction to reach the full requested braking level. Or use pulse braking with pulses far above the regen limits, then wasting regen potential during the pauses between pulses, as my spouse was trained a half century ago and simply will not change old habits.

 

I take such 'idealised' graphs with a grain of salt. Here is a closer view of the real log above. When the vehicle speed is decreasing quickly, we can be sure that the brake pedal is active, and often there is a lot of regen. before any wheel cyl. sensor voltage. Also (not shown here but in other logs), it does not take long downhill to "fill" the traction battery; then there is engine braking (seen as negative torque by ICE {with +ve rpm}, often with +ve torque by MG1), and I expect more friction braking if the brake pedal is pushed.

 

Still looks like a highly idealised 'example' to me. Must surely differ depending on how hard the brakes are pushed.

Often regen. alone in my (rav4 hybrid) logs.

Is there nobody on PriusChat with:
(a) a Prius, (b) an OBD reader capable of logging readouts from Toyota PIDs, and (c) interest to explore such matters?

 

As an example, here is a log that shows (a) friction braking at low speed, (b) regen. without friction braking at higher speed (c) engine braking when traction battery SOC rises and car goes downhill, (d) some heretical things ...

 

One possible complication is that the time period on which the OBD reader re-fetches the PIDs may be slow enough to miss interesting things.

Old-time member hobbit once put some LEDs on the dash wired directly to the fluid pressure sensors, to display the pressure changes in actual real time.

Of course, that was in a Gen 2, where the wiring from the actuator with the sensors to the ECU was accessible.

 

Modern OBD readers are fast (100 PIDs per second with OBDLink MX+). That should show everything implied in the idealised examples. But, in support of the input from ChapmanF, Toyota uses various refresh times for the Toyota car PIDs. I suspect that these must be very short for things that control braking.

I only logged once per second (the reader can go much faster, and you can choose only to log when a PID reading changes, but files may get big unless the number of logged PIDs is limited). You can see the 1 second effects in some of the stepped lines above.

Humans have trouble looking at multiple momentary gauges per second, but logs can reveal much more.

 

That article by Hobbit, linked by ChapmanF, is excellent to understand better how far from the idealised examples is the real braking algorithm in a Prius. Thanks for the link ChapmanF. My guess is that all the wheel cylinder sensors in current Toyotas feed into the CAN bus to be used in VSC ASB etc. But so far I can only find one (and it is not specified which one) in the PIDs licensed by Toyota.

 

Gen 2 Prius had separate pressure sensors measuring the actual pressures to all four wheel cylinders (plus two master cylinder sensors and one accumulator sensor).



Gen 3 has no wheel cylinder sensors. It does have a single "front wheel cylinder pressure sensor" for the pressure supplied in common above the FLH and FRH valves (plus one master cylinder and one accumulator sensor).



Gen 4 has only accumulator, stroke sensor, and servo pressure sensors.

 

So what would Hobbit use to sense friction braking in a gen 4?

 

Psrv would be the closest available proxy for what's going out to the wheel cylinders, but there's only one of it, and it's too far upstream to tell you what the ABS actuator might be doing to vary the individual wheel pressures.

 

I see PIDs for ON/OFF Status of each of the ABS Solenoids shown in the Brake Actuator Assembly (SFRH etc). Could that help?