I bought my 2008 HCH a month age and have noticed that during braking the brakes often seem to "let up" so that I have to apply additional pressure to stop. I noticed this on the two that I drove before buying and the salesman [of course] said it was normal and that it has to do with the regenerative braking. It happens about 1/2 of the time. Have you experienced this? Is there a cure?

Side issue= first tank of gas = 37 mpg - not to impressive even tho I've checked tires and drive smoothly.

 

It is normal. It's a transition point between where regenerative braking stops and traditional takes over (sort of). Drive the car for about 1-2 months and you'll get used to it. 37mpg isn't terrible if you live somewhere that the weather is cold and your commute is short. I'm getting 47 mpg lifetime (average for this subset of HCHII drivers.) In the summer I get in the low 51's. Winter is in the low 40's (30-32 around town before the engine gets warmed up.)

Eric

 

I have found that if you pump your brakes and coast, you can continue to use the regenerative braking until you slow to a stop. I usually start pumping them way before I know I have to stop. That way your allowing the car to coast to a stop, using the regeneration, which will allow your FE to increase and your brakes will last longer. Plus, you may not have to come to a complete stop (if approaching a traffic light) because your slowing the act of stopping enough that the light may turn green before you even get there.

When ever I do feel the brakes "let up", I remove my foot and reapply it. The regeneration will kick in every time.

Jess

 

"It's a transition point between where regenerative braking stops and traditional takes over (sort of)"

Can someone please explain this?

I think it is that during low rates of acceleration the motor slows the car down enough by regen, and then when the motor can't slow down the car enoughthe brakes need to be applied for more stopping power. Put, the clutch needs to be disconnected while the car brakes are applied, and since the motor is attatched to the engine, when the clutch is disengaged the regen stops, while allowing the brakes to be applied. Is this correct?

 

Yes. I think it works something like that.

 

When you remove your foot from the gas at speeds over something like 18 mph, the gas engine essentially shuts off (the valves all close and no fuel is burned). At the same time, the computer calculates how much drag it will be okay to let the electric motor -- now generator -- put on the drivetrain for a safe deceleration rate. At very high speeds -- say, 70 mph -- that can be a lot of drag. Even without touching the brake pedal, you'll see 6 or 7 green "charge" bars light up -- almost max regen. At lower speeds, not so much, because that much drag would cause the car to slow down too quickly to be practical. At low speeds -- 25-30 mph -- watch your tach when you let off the gas. You'll actually see the rpms *rise* because the CVT changes the ratio to force the generator to spin faster and create more drag. Very cool computing.

When you touch the brake pedal, the computer sees this as a go-ahead to allow more regen, creating even more drag on the drivetrain that slows you down. At highway speeds, just touching the brakes is enough to call full regen (all bars). At lower speeds, you need to get farther into the brakes to get all bars regen.

Because at typical street speeds you're farther into the brakes, it can be hard to distinguish how much drag is from regen and how much drag is from the service brakes. The one time you *can* tell is when the car's speed becomes low enough (14-18 mph, in mine) that the computer shuts off regen, releasing that portion of drag. Turns out it's pretty significant -- it actually feels like the car *surges* in that moment. What's really happening is that you're using so little brake that once there's no drag, the car's rate of deceleration decreases noticeably.

I don't know about y'all, but I'm so used to it now that I unconsciously anticipate, compensate, and even plan for this effect in my coast-outs. And when I get in a different vehicle and there's no regen cut-out, I have to relearn how to stop. . . .

Regarding clutches, I'm pretty sure the auto-clutch in the HCH II only disengages the drivetrain under about 9 mph. (I'm basing that on the feel of the car when you hit the gas coasting at speeds under 10 mph, versus speeds higher than that.) Certainly the way the computer "turns off" regen at 14-18 mph could be by using the auto-clutch to disengage the drivetrain, although that speed seems high to me given how the car feels when you get back on the gas at that speed -- it doesn't feel like the clutch has to "re-engage." I had always just assumed that regen was shut off electronically. . . . Now I guess I don't know.

cheers --
doug

 

The only bad thing that I have noticed about this transition period is at high speed it can cause the rear to kick out in a panic situation. I believe the reason is that the fronts seem to be able to decelerate faster than the rears. This forces the rear to try to come around (like a spin). I had this happen once going around 75mph and I had to slam on the brakes, the car twisted to the left slightly (sitting in driver's seat point of view), I had to let off the brakes and place some opposite lock to correct it, and then hit the brakes again.

 

I just love it when I see good thinking like the one in this thread.

Perhaps at this point I can add a bit or two, kind of "fill-in-the-gaps" thing. But first allow me to identify a few critical components that play a role in this Regeneration to hydraulic transition:
- The PCM. This module plays a critical role in actually controlling how the CVT works. It controls the activation of the hydraulic valves and solenoids that redirect the hydraulic flow responsible for normal CVT operation. The beauty about this module is that the governance routines are totally programmable- so visit your Honda dealer once in a while for your odd software update.
- The MCM. This is the motor control module. It measures not only the amount of assist and regen, but it also watches the position on the motor/rotor via any of its three phases. Proper monitoring of this position is critical for not only a smooth power train experience but also to provide a predictable power management routine with less stress to the active components.
- The IMA Motor/Rotor. It includes the phase and motor position sensors. These provide feedback to the MCM and PCM.
- The CVT. This is where the magic happens and in the absence of a shorter description it is where all the inputs produce the desired result.
- The BCM. This is the battery condition/control module and it is here where the battery management, power provisioning and energy recovery take place.

Now that we got some definitions out of the way, we can describe how regen happens (under normal operating conditions).
When decelerating, the CVT literally controls the rotor's speed and therefore the amount of recoverable energy.
Unless inhibited by another IMA component (MBM, BCM, etc) or sensor (emissions, PGM-FI, Power steering, etc), the PCM will cause the front wheels to drive the IMA motor at a speed matching the programmed recovery profile.

With this, the CVT responds by altering the ratios and therefore causing the motor (and camshaft) to spin faster or slower. When this happens the RPMS also change accordingly. Also depending on the transmission setting (D or S) the cam lobes may also be activated in the process.

Because of the design improvements on the HCH-II versus the HCH-1, the HCH-II can now recover more energy when decelerating since recovery ceases later in the game at 12km/h (7 miles per hour) and also because the motor/rotor now sports a higher density design (a true work of engineering art in my view).
Anyhow, the transition between the hydraulic brakes can be more or less abrupt depending on a many factors: Temperature is once again one such factor.
The condition of the CVT Fluid is also another since it has a impact on how smooth the forward clutch operates.
Also, the binary operation of the solenoids and valves play a role in this abruptness, that is why temperature and state of fluid play such a role in the transition.
The driver and the inputs he/she provides while braking is yet another.

What does this all mean? Well, a little lurching is normal. Too much is not. Keeping and eye on the condition of CVT fluid is a good thing but so is the regular visits to your dealer for the occasional PCM update.
And most important of all: modulate that brake pedal with the expectation of easing the abruptness of the transitions.

As Doug and others said, much of this is also controllable by the driver, especially when anticipatory braking is factored in.

By the way, all economy hybrids (Camry included) exhibit varying amounts of lurching and transition between the regenerative systems and the hydraulic braking system.


Fun stuff, heh? If anyone can add more, please jump right in.


Concerning livvie's remark:

Absolutely true.
However I believe the EBD feature on the HCH-II pretty much "neutralizes" that behavior, thus making it a little more predictable under similar circumstances.



Cheers;

MSantos

 

"The one time you *can* tell is when the car's speed becomes low enough (14-18 miles per hour, in mine) that the computer shuts off regen, releasing that portion of drag"

There's a graph shown in the 3-Stage VTEV IMA video (on world.honda.com) that shows how much energy is given to regen and how much is given to the brakes that shows what you're saying.

However, why would they let the rest of the energy be wasted. Wouldn't Honda have regen on the whole way so that they can capture as much energy as possible?

 

There are regulatory/design guidelines that govern the behavior of a vehicle's braking ability when approaching a potential stop. Basically, the primary braking system must be active and in effect - primarily for safety reasons.
All hybrids currently in the market are subject to this compliance.

Cheers;

MSantos