Report: Toyota Will Also Recall Sai, Lexus HS250h
BASF and Embrapa’s Cultivance Soybeans Receive Approval for Commercial Cultivation in Brazil

Recent Hybrid Braking Complaints Highlight Regenerative Braking Design Issues

Toyota regen systems (2)
Comparison of regenerative/hydraulic (friction) braking ratios under normal braking. Left: 2001-2003 Toyota Prius (THS). Right: 2004-2009 Toyota Prius (THS-II). Source: Toyota Motor Company. Click to enlarge.

by Jack Rosebro

Recent brake feel and application issues in Ford and Toyota hybrids (earlier post) have drawn attention to the inherent complexity of hybrid and electric vehicle braking systems, which typically combine regenerative braking with conventional friction braking, antilock braking and vehicle stability control functions. Such issues are often addressed through software upgrades, either by reflashing a control module’s read-only memory or replacing the module altogether. Both Ford and Toyota have indicated that they intend to resolve their respective issues via changes in software.

Regenerative Braking. Regenerative braking is accomplished by allowing a vehicle’s electric motor-generator to spin freely, generating an alternating current which is rectified to direct current to charge the vehicle’s battery pack. The motor-generator must be connected in some manner to the vehicle’s final drive such that the vehicle’s coasting motion will cause it to rotate.

The rate of regenerative braking, or regen, is constrained by the size of the motor-generator as well as the amount of charge that the battery pack can accept at a given time. A typical hybrid passenger car with a relatively small (<20 kW) electric motor will, for example, have limited charging abilities during regen. Likewise, a vehicle with a battery pack that happens to be near the upper range of its charge window (~70% to 80% for many hybrids) will also be limited in its ability to convert braking force to stored energy during a given braking event.

Some early hybrid vehicles employed regen in tandem with a conventional hydraulic braking system. Hydraulic brake force was applied to slow the vehicle via friction braking when the driver applied the brake pedal, and was supplemented by regen braking during most braking events. The simplicity and redundancy of such a system was offset by reduced recuperation of braking energy; at no time would the vehicle be slowed by regen braking alone.

However, almost all of today’s hybrid vehicles combine regen and friction braking with a control strategy that allows the vehicle to be slowed by as much regen braking as possible. Engineers have sought to increase the rate of energy recuperation, and therefore overall powertrain efficiency, with succeeding generations of regenerative/ hydraulic braking systems.

Friction Braking. Most hybrid vehicles use an electrohydraulic system to activate the vehicle’s conventional friction brakes. In a non-hybrid vehicle with an electrohydraulic braking system, applied brake pedal force is read by pressure sensors which then transmit a signal to a brake control unit. The brake control unit typically uses a motor-driven hydraulic pump and control solenoids to apply individually calibrated brake force to each of the vehicle’s four wheels. A pedal feel emulator, sometimes referred to as a stroke simulator, smooths out brake pedal feel to give the driver the tactile feedback of a conventional braking system’s pedal response.

Input from the vehicle’s anti-lock brake system (ABS), which monitors individual wheel speed for signs of lock-up, may affect a given wheel’s brake pressure. A vehicle stability control (VSC) system, if the vehicle is so equipped, monitors steering angle, acceleration force, and body roll changes, and can also affect brake force. A typical electrohydraulic braking system is designed so that it can function as a purely hydraulic system if current to the system’s electrical circuits is interrupted.

Blended Braking. However, a hybrid vehicle which blends friction and regenerative braking must transmit applied brake force data to a powertrain control unit, which then calculates the desired friction-to-regen ratio. As the control parameters—applied brake force, vehicle speed, steering angle, rate of deceleration, and battery state-of-charge, among many others—are continually changing, and as a greater number of control units contribute data to braking force calculations as compared to a non-hybrid system, software design for involved modules can be particularly challenging.

Reports of irregular braking behavior from some 2010 Toyota Prius hybrids as well as some 2010 Ford Fusion and Mercury Milan hybrids appear to be focused on the activation of ABS in conjunction with both regenerative and friction braking. Some Prius drivers have reported inconsistent brake pedal feel and momentary reduction in braking force when transitioning from high-traction surfaces to low-traction surfaces. Ford describes their issue as a “different brake feel”, possibly accompanied by “visual indicators and a chime”, which also indicates the possible engagement of ABS during transitions in the ratio of regen to hydraulic braking.

Toyota expects to announce a fix for the 2010 Prius this week, while Ford has stated that it will be notifying affected vehicle owners by mail. Toyota reports approximately 200 complaints and four accidents in both Japan and the United States, but does not indicate whether or not injuries have occurred; Ford does not cite the number of complaints or accidents, but notes “no injuries” related to the issue.



These are just software issues. Thankfully nobody has been hurt. But this is almost trivial and will be rectified quickly.


ABS + energy recuperation braking would most likely always give a different fealing than regular power assisted brakes.

Electronic could certainly reduce the perceived difference.



Toyotas built in Japan (and in Asia) use brake pedals made by Denso Corp while Toyotas made in North America use brake pedals made by CTS Corp.

The Denso Corp pedals have had no brake jaming problems.

That is probably why our 2 Toyotas (imported models) have had not brake pedal problems.

Should our future Toyotas also be imported models?


Having driven a my 2001 Prius for more than nine years and having rented a 2007 and 2010 model, I can attest to the refinements of the second and third generation braking experience over that of the first generation models.

All, however, can demonstrate some disturbing characteristics, in varying degrees, to an inexperienced driver:

Encountering a pot hole, while braking hard, can induce a heart-stopping feeling of brake loss . . . like sliding on ice for 1/2 second . . . before the brakes regain their composure.

When braking hard, as one of the front wheels enters the pot hole, it becomes airborne, unloaded and hence the wheel stops rotating -- it locks up. The anti-lock braking system detects the locked up wheel and disengages regenerative braking. Since more than 70% of the braking force is provided by regenerative braking and its used by both front wheels, the system takes away more than half of the braking effort until the wheel encounters pavement and starts turning again. Once the ABS is satisfied the wheel is turning reliably, it re-engages regenerative braking -- restoring all the braking force that was in play before the pot hole was encountered.

Experienced Prius drivers, well aware of this potential, are on the lookout for pot holes while braking. We either coast past them before applying the brakes or we brace for the sickly brake-loss experience, knowing its a temporary condition and brake normally.


HarveyD: Denso pedals are being recalled, as well. It's not just CTS.

Roger Pham

Thanks, storky, for sharing your experience.
I have no such experience in my 2007 Prius, however, I hardly ever brake hard, as that defeats the purpose of regenerative braking. I always brake very gently but well in advance in order to avoid friction braking altogether.

I am a believer in simplicity, and think that the owners should have an option to disable the ABS and the VSC (Vehicle Stability Control) altogether. I learned to drive in the days when cars are bone simple, with carburetor and breaker-point ignition...and did have a few scary moments while driving in the ice, but was able to quickly regain control. All modern drivers now should do some driving simulation in computer simulators in all types of road conditions in simulated cars without ABS and VSC in order to learn the true art of driving. Those who are certified to be able to handle icy roads and other road conditions in the simulator may feel the confidence to disable the ABS and the VSC in order to simplify work loads for the car's computer and avoid disaster.
There will be no electronic substitution for a well-trained driver.



I have opposite information from Japan and it seems to confirm the lack of this problem on our Toyotas.



As the graphic accompanying this article demonstrates, only panic stops on the 2nd Generation Prius are supplemented in any significant way by hydraulic brakes. National Renewable Energy Laboratory testing in 2002 established that, short of a panic stop, energy recovery is not affected by the rate of deceleration. Whether heavy of light on the brake pedal, you recover the same amount of energy over the same change in velocity.


HarveyD: CTS and Denso accelerator pedals are indeed both part of the recall.

However, you mentioned brake pedal issues. I had assumed that it was a typo, and that you were referring to accelerator pedals.

To clarify, none of the present issues are related to brake pedals assemblies.


I have experienced MAJOR loss of braking in trucks & SUVs due to descending on unpaved, very rough mountain roads ( more than once) and, on one occasion a faulty rear axel speed sensor.
On other occasions (towing, rough roads etc) braking seemed to be mostly gone but no sudden loss was evident.

I expect the regeneration braking adds some complexity to the process (and some unsettling driver feel) but ABS still brings benefits that are (judged) well worthwhile.

Of course we cannot possibly allow any slight irregularities during everyday braking.

At present, perfection is lacking and I am surprised lawsuits are (thankfully) not inundating auto makers.

But this may be because ABS faults, on autos with little or no regeneration, might not be as easily detected.

Our ability to judge stopping distances does not really kick in until deceleration is established; then, and only then, we know for sure whether or not we will rear end that car ahead.

They will work this out - maybe accept more wheel lockup or (sob) less regeneration.


BTW, I think the graphic is incorrect. THS (Toyota Hybrid System) was used in the Japanese release of the First Generation Prius (Model Year 1998-2000). THS II was used in the later release of the First Generation Prius (MY 2001-2003). The Second Generation Prius (MY 2004-current) uses the Hybrid Synergy Drive (HSD) system, not THS II.

Jack Rosebro

Storky: The graphic is correct, and is from a 2004 Toyota technical report on THS II.

1998-2000 and 2001-2003 iterations of the Prius used different versions of a powertrain that Toyota labeled Toyota Hybrid System (THS). The 2004-2009 Prius used a redesigned powertrain that Toyota simultaneously referred to as THS II and Hybrid Synergy Drive (HSD).

However, the THS II name fell into disuse, and Hybrid Synergy Drive remains the marketing term of choice for the company.

R River

Paradigm shift:

Before: Toyota - Good, Domestics - Bad...
Now: Toyota - untrustworthy, GM skeptical but improving, Ford - Good

Remember the demographic for Toyota is that of Buick 20 years ago - 55 to dead. These people abhor change and avoid risk. This will effect them long term. When a respected in the know guy like Steve Wozniak can diagnose his Prius problems, this trumps the biased media.

Personally, I can't stand the UAW and their BS, but this is exposing the nasty little habit of Japanese secrecy in engineering and the only if they think of it first. This issue and the acceleration problems are a Japan engineering issue at TMC. Not floor mats of N. American supplier issues. Akio should have known closing NUMMI was going to be an issue, especially since Japan imports will replace a good amount of the production.

Can't wait to see the venue that Akio chooses for his ritual suicide!!

Roger Pham

To put Toyota's issues in perspective:

--The braking problem in 2010 Prius is very minor and can be easily fixed with software update. Drivers should be reminded to brake gently and well in advance in any HEV, and not rely too much on anything man-made mechanical or electrical or otherwise. Drivers should also be reminded to use the parking brake and engine braking function upon failure of the main brake instead of panic, because brake failure could happen in any makes of cars.

--The acceleration problems in other Toyotas are very rare, like one in a million. Likely some faulty chips beside the "sticky" gas pedal issue. All the drivers need to do is simply shift the vehicle in neutral should the car starts to accelerate uncontrollably, and catastrophy can be averted. With our increasing dependence on micro chips, these one in a million failures will be quite inevitable. These type of malfunction could happen in other makes of cars as well, so the drivers should be taught to be mentally ready to shift to neutral and not to panic, and never be complacent that these thing cannot happen to other makes of cars.

--Future HEV's should have a mechanical shift lever to disconnect the engine from the drive train in case of computer malfunction.

--With respect to the 40,000+ traffic fatalities yearly in the USA, those handful number of people who died from allege malfunction from Toyotas represent an extremely minute percentage, the rest died from drivers' human errors. Should there be a complete recall of all human drivers? Short of that, should we have a stronger campaign to train better drivers? (instead of dependent on ABS, VSC, etc.)

The comments to this entry are closed.