Sorry, that is not what the Toyota paper I posted above says. Please provide some justification for your opinion.

 

The instrumentation may depend on the year of your TCH. In the 2012 there is an ECO display on the left side. It shows charge current. In cruise, the controls are able to virtually pin the charge to full current. I do not believe cruise has the ability to use hydraulic brakes, only regen. My conclusion based on this is that hydraulic brakes are not used (at least in the 2012), until after the ECO charge needle reaches full scale.

 

If you pay attention while slowing down you can SOMETIMES feel the transition between them. Toyota engineers have that algorithm down pat for a seamless transfer between the two. The hydraulic brakes are also electronically controlled by the computer to help with that.

 

Ron,

The Toyota paper is describing the hybrid system only on the drive wheels. For the drive wheels, yes, they are either hydraulic or regenerative. The rear brakes however MUST be used or the butt end of the car will pass you on the way to doing a 360. The brakes are biased to the front but the rear brakes are there to provide stability during braking. Call your local Toyota dealer and talk to the tech that does their hybrid work. Don't talk to the idiot sales people or a service writer, talk to the guy that works on them and ask him. Failing that, write Toyota engineering and get the answer straight from the guys that design the system. I had several long talks with them during this years capstone design competition at Georgia Tech about the cars and how they work.

 

Please provide a reference.

 

There are two reasons I remain skeptical. One is that any use of the disk brakes is a waste of energy. Toyota is not leaving any stone unturned in the search for better mileage. Always using the rear disks would reduce energy regenerated through a permanent loss due to heat in the brakes.

The second reason is that the hybrid very aggressively uses regenerative braking to control speed going downhill in cruise control mode. I would be really surprised if they were applying the rear brakes in cruise control. Assuming that is true, then the butt end of the car would be passing you every time cruise applied regenerative braking.

 

Oddly enough, this is exactly wrong. Locking up just the rear brakes (on a flat surface) will cause the car to do a 180. Locking up the front brakes will cause the car to skid straight ahead (regardless of the steering wheel position). If you don't believe me, take a toy car (a matchbox car will do fine), put it on an inclined ramp, and put tape on the top of the rear wheels to keep them from spinning. Let it go down the ramp, and regardless of how you start the roll, the car will end up sliding down locked up wheels first.

The main purpose of the rear brakes is to offload as much of the braking loads from the front wheels as possible to minimize the likelihood of either locking up.

-- Alan

 

alan,

I agree with you when talking about locking up the rear brakes but that is not what we are talking about. When you press the brake pedal, the rear brakes will be used on the car (at a lower force than the fronts) to add enough drag to keep the car tracking straight.

Ron, we are talking about two different items. You are talking about he car using regenerative and engine braking on long grades to keep the speed from increasing. That is NOT braking with the pedal. We have been talking about pressing the brake pedal and using the brakes, not using the cruise control.

 

I decided to go check the ultimate source to resolve this disagreement, my car. Today I had an errand to run a few km away from home. On the way there I took every opportunity to use regenerative braking to slow the car down. However I was careful to not pin the regen current meter or come to a full stop. When I arrived, I quickly got out and touched the front brake disks and the rear brake disks to check temperature.

I do this as a matter of routine when I do a disk brake pad change. It is a quick way to tell if the brakes are dragging. However, when I check a conventional car, I only go a block or so, and come back. A dragging brake can get firecracker hot in no time. Also just using the brakes normally in a conventional car can make them too hot to touch, without getting burned.

So what was the temperature of the disks in this hybrid today? Front was just slightly warm, but no risk at all in touching the disk - perhaps 90 deg. F. I would estimate they had seen very minimal use. Rear brakes? They were definitely not as warm, and very hard to tell if the temperature was even above ambient -- perhaps about 80 deg. F. Conclusion? The disk brakes after all this regenerative braking testing, showed virtual non use. The bias in a bit more braking on the front was probably caused by the normal front to back distribution where the fronts do much more work than the rear. The regenerative braking cannot fully stop the vehicle as the generator force goes to zero as the speed goes to zero. This means the last bit of the stop always has to be the disk brakes. Of course I had to come to one full stop to get out and check the temperature.

I repeated the exercise on the trip home, with even more regenerative braking slow downs (but not stops to avoid that necessary disk brake to get a full stop). Brake disk temperatures were the same as the first test.

So, my car is telling me there is no automatic use of the rear disks when using regenerative braking. In fact what the temperatures are telling me is that these pads are likely to last a very very long time if you maximize your use of regenerative braking, and the fronts still may wear more than the rear.

The cruise use of regenerative braking is relevant to the discussion. If regen braking without rear brake operation causes front to back handling problems (rear end passing you), then cruise could not use regen only either.

I'm sure Toyota has gone through many iterations of braking control system, and it currently is probably one of the most complex systems in the car. However, everything I am seeing says the rear brakes in this 2012 TCH are NOT always being used during regen braking when using the brake pedal.

 

Awesome test. I love empirical data to answer questions. THANK YOU!

Being an avid radio controlled airplane hobbiest (and engineer), I learned a lot about the effects of drag and center of gravity. These lessons apply very well to cars, where the CG is nicely between the wheels always, and the CD mostly comes from the nose and windshield when not braking, and almost entirely from the wheels when braking. What we are discussing here is yaw instability due to front/rear brake. There are two interesting cases: when a wheel is locked up, and when no wheel is locked up. In the first case, always hope any locked up wheel is the front, because any rear without front lock up will lead to the yaw spin/instability. The matchbox toy car test I mentioned clearly proves that. Unlike an airplane, an unlocked wheel had enormous (relative to everything we are considering) lateral grip that prevents any yaw spin (and is why airplanes have big rudders at the rear, and why a front rudder would be unstable). That means that when there is no lockup, there is hardly any yaw stability issues to consider.

Bottom line, a well tracking (not locked up or close to slipping) set of rear wheels virtually eliminates any steering/tracking/yaw stability concerns of a braking car. Rear brakes have virtually no impact to steering ability, except when the braking is done through a differential (like regen is on the front, but is not done at all on the rear of a TCH). The only usefulness of rear brakes is to offload some of the braking forces from the front brakes.

-- Alan