Recent Hybrid Braking Complaints Highlight Regenerative Braking Design Issues

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.