Tech Feature: Ford Hybrid Braking – UnderhoodService

Tech Feature: Ford Hybrid Braking

A hybrid vehicle is a different animal than many of us are used to seeing. After all, if the engine of a typical vehicle were to stall out at a traffic light, the driver of that vehicle would know something was wrong with it. If that same vehicle still drove with the engine off, then that would mean they were probably going down hill. But in the hybrid world, that is all normal. Several systems around the vehicle are impacted by two key elements.

A hybrid vehicle is a different animal than many of us are used to seeing. After all, if the engine of a typical vehicle were to stall out at a traffic light, the driver of that vehicle would know something was wrong with it. If that same vehicle still drove with the engine off, then that would mean they were probably going down hill. But in the hybrid world, that is all normal. Several systems around the vehicle are impacted by two key elements.

1. The engine does not run at all times.

2. The high voltage batteries need to be recharged on the drive.

The control side of a hybrid brake system contains differences, compared to its conventional cousins, to accommodate these conditions. All technical references here will be directed at 2005 to 2010 Ford Escape and Fusion Hybrid vehicles.

Regenerative Braking
A hybrid vehicle recharges the high voltage battery through use of regenerative braking. “Energy can not be created or destroyed.  It can only be transformed from one form to another.” We all heard that one in science class I’m sure. In the case of conventional friction brakes, energy of motion (kinetic energy) is being transformed into heat energy, via friction, and then dissipated into the air. With regenerative braking, instead of just wasting that energy by releasing it into the air, we are going to take it and place it in a box in the back of the car and save it as stored energy. Later, that stored energy will be converted back into kinetic energy that we will pour back into the body of the car. 

On a hybrid vehicle, the brake pedal acts as more of human interface to the ABS module for a request for braking.  Regenerative braking is the hybrid’s first choice for braking. The ABS module “tells” the PCM or TCM (model year dependent), that a request for brakes have been made and how much braking is desired. The PCM or TCM, then applies regenerative braking, via the traction motor inside the transmission, as needed. More regenerative braking increases the amount of electrical charge generated for the HV battery at the same time it produces more drag to slow the vehicle down. During many light braking events, the rear friction brakes will be lightly applied and the front may not be applied at all until the vehicle is at a stop or near stop. For this reason, it is normal for a hybrid to wear the rear brake pads two and three times faster than the front pads.

For 2005 to 2009 models, the HV system is rated at 330v.  2010 models are now equipped with a more efficient traction motor that lowers the HV voltage requirements to 275v.  For either system, you will notice bright orange wires that look like battery cables running throughout the vehicle.  Those are the HV battery cables.  Do not mess with those wires until you have been properly trained on that model vehicle.  Simply put, those orange cables can kill you.

2005 to 2008 Escape/Mariner EHB system

The 2005 to 2008 model Escape Hybrids use an electro-hydraulic brake (EHB) system. The EHB master cylinder is also referred to as an actuation control unit (ACU). The bore of the ACU is called a pedal feel emulator and is primarily used to simulate normal brake pedal feel for the driver.  The EHB system does not have a vacuum brake booster.

The pressure generated for the calipers is not, under normal conditions, the direct result of the driver’s foot pedal efforts. The pedal generated pressure  is normal only to stimulate a pressure sensor in the ACU that makes up the brake “request.”  Normally, the fluid pressure for the wheel hydraulics is generated by the pump and accumulator in the ABS HCU and is regulated by the teamed efforts of the ABS and PCM’s logic.

Under normal conditions, the driver’s pedal efforts are displaying to the ABS module as a “request” for braking. The ABS and PCM then divide the braking effects between the traction motor (for regenerative braking) and the friction brakes as needed. “Normal” braking for this system is typically that regenerative braking is providing most of the braking and the rear friction pads provide some to very little braking. The front may provide very little to no braking. Of course, at a stop or near stop the friction is applied to hold the vehicle from rolling.  

The higher the request for braking, the ABS module and PCM electronically calculate and decide how to apply friction and regenerative braking to meet the demand. From feather-light braking to panic stop, all brake applications are electronically calculated and delivered from a normally operating EHB system.

Only in the event that the ABS HCU can’t function to build pressure will the driver’s pedal pressure directly provide the hydraulic pressure for the calipers.  There are valves inside the HCU that allow direct passage from the pedal feel emulator chamber of the ACU, through the HCU and to the wheels. This is called “manual mode.” In manual mode, there will be no boost so the driver will have a very hard/stiff brake pedal.  Braking efforts will be very high for the driver and will likely prompt them to tow the vehicle for fear of having no ability to stop at all. The complaints of “no brakes” or “barely stops” may be the common terms used by customers whose brake system has entered manual mode (fail safe mode).  

The replacement of the friction material is the same as with a conventional system, however, the ABS module does test the brake hydraulic system’s integrity by pressurizing it during key-off events, such as opening the door or whenever the dome light is activated, and again when the key is switched on.

After four minutes have passed since the key was switched off, the accumulator discharges its stored fluid pressure back into the master cylinder reservoir. Before attempting friction material replacement, you must disconnect the battery to prevent the likelihood of an accumulator discharge event, or a system pressurization, during service. In addition to battery disconnect, Ford says to remove fuses 24 (50A) and 31 (50A) in the battery junction box, though conventional wisdom says just fuse removal or battery disconnect will prevent the system from pressurizing.  

If system bleeding is required, a capable scan tool must be used for the ABS service bleed. Plus, a pressure bleeder set to deliver a continuous 35 psi is needed during the bleeding process. Do not use a vacuum bleeder on this system.  This brake hydraulic system is diagonally split. The specified fluid is DOT 3.

2009 and 2010 Escape/Mariner and Fusion/Milan SBA system
For the 2009-up vehicles, the braking system has been modified. These vehicles now have a vacuum brake booster and use a Simulator Brake Actuator (SBA) as a human interface.  Although the older system could be said to be “brake by wire,” this system takes yet another step into the brake by wire concept, while at the same time providing a very conventional and proven fail safe mode as well as being a little more technician friendly.
This system is equipped with a brake pedal simulator also referred to as the SBA. The simulator appears to the driver as a typical brake pedal that they are accustomed to, and is also a hydraulic cylinder loaded to provide that familiar brake pedal feel. The simulator houses a brake pedal position/angle sensor that sends a signal to the ABS module.  This is an analogue voltage value that the ABS module uses to determine how much braking is being requested.

“Normal” braking requests commonly only signal the regenerative braking. Unlike the older model where the rear friction brakes are lightly applied for “normal” braking, the friction brakes are not applied at all for “normal” braking with this system except for when at a near stop or a stop to hold the vehicle from rolling. To achieve regenerative braking, the ABS module sends a request to the transmission control module (TCM).

If more than “normal” braking is required, then the ABS module will then signal the vacuum booster, via a solenoid, to apply the friction brakes by pushing a rod inside the master cylinder just like in a conventional brake system.  The brake pedal is mechanically connected to the brake pedal by a “Z” bar and an elongated eyelet at the end of the booster rod. Under heavier than normal, but not panic, braking, the eyelet provides enough free play that the mechanical brake pedal actions do not induce force into the brake booster.  This allows the ABS module to maintain control over the amount of regenerative braking versus friction braking for all braking demands from feather light to heavy, but not as strong as a panic stop.

In the event of a panic stop, the driver will be able to cram the brake pedal down fast and far enough to bottom out in the elongated eyelet and that mechanic action will override the electronic controls. This will apply the friction brakes in the conventional manner.

The elongated eyelet and clevis pin attachment to the brake pedal, also becomes the fail safe method for brakes in the event of a critical failure. Should a critical failure occur in the electronic brake control system, there will be no regenerative braking commanded and all friction brake control will be surrendered to the driver’s foot pedal action. This will provide the feel of “head room” or free play at the top of the pedal to the driver before braking occurs while in fail safe (or manual) mode.  Aside from the head room at the top of the pedal, the braking will feel almost normal after the pedal is pushed past the stack in the eyelet.  

Most vehicle owners may feel safe driving the vehicle into the repair shop in failsafe mode. A soft or low pedal maybe a common complaint from a customer who’s brakes are in failsafe mode. A critical failure in this system would be something that the ABS module must have to perform its duty, such as a failed pedal position sensor, vacuum or booster travel sensor, vacuum solenoid circuit or internal ABS logic or power/ground failure to ABS module. A non-critical failure, such as a single ABS wheel speed sensor failure is not likely to place the system into failsafe.

The vacuum brake booster has two sources for vacuum.  If the engine is running, then the engine will be that source. If the engine is not running, then an electric vacuum pump will become that source. Mounted on the booster is a vacuum sensor to monitor the amount of vacuum inside the booster. Mounted on the booster as well is a booster travel sensor to provide feedback to the ABS module about the booster’s position. Both are serviceable without replacing the entire booster. The booster and master cylinder in this system have a very conventional appearance aside from the added sensors mounted on the booster that are not very obvious at a glance.

There are no serviceable parts on the brake pedal simulator at this time. Should parts on the pedal simulator, such as the position sensor, fail, the entire assembly must be replaced.

Brake bleeding with this system also requires the use of a capable scan tool and pressure bleeder if air is in the master cylinder or the ABS HCU.  Simple caliper replacement should not require those things if no air entered the master or HCU.  This hydraulic system is also diagonally split.  Pedal-pump bleeding should be performed with the key off so that the free play in the clevis pin and eyelet will be minimized and to prevent the ABS module from interfering.  It is critical that the person operating the brake pedal pushes the pedal far enough to overcome the free play in the eyelet before judging the amount of pedal travel.  DOT 3 fluid is the factory specified fluid to use.
No specified warning are given for pad replacement. The self test that the older version performs is not shown to be performed by this system.  However,  to elevate the chance of confusion resulting in injury, it would be considered a good practice to disconnect the battery before attempting removal of the brake calipers on any Ford Hybrid vehicle.   

I hope you’ve enjoyed the information and get the chance to profit from it soon.  Especially when it comes to hybrids, play it safe and seek vehicle specific training.

At the heart of the Ford Hybrids is its advanced hybrid powertrain, an integrated system that uses a half-dozen key components to deliver seamless, efficient power.

“Ford’s history of leadership in the electric vehicles market aided the development of the Hybrid, and the technologies we’ re putting on the road this year are necessary steps toward the development of fuel-cell vehicles,” said Phil Martens, group vice president, Product Creation.

The Hybrid Principle
“Hybrids are all about increasing average thermal efficiency,” said Tom Watson, powertrain supervisor, referring to the amount of energy the vehicle can extract from its gasoline fuel. The Escape Hybrid’s increased efficiency comes from:

A “Right-sized” four-cylinder engine with electric drive boost. The four-cylinder gasoline engine easily meets the cruising needs of the vehicle with reduced fuel consumption. When needed, the electric drive system teams with the gasoline engine for the performance feel of a larger V-6.

An electronically controlled continuously variable transmission (eCVT). A planetary gear set connects the drive wheels to the gasoline engine and electric traction motor, so that the vehicle can move on any combination of electric and gasoline power, depending on what is most efficient at that instant.

Engine stop-start. When it isn’t needed to supply power, such as when coasting or while stopped in traffic, the gasoline engine automatically shuts off instead of idling inefficiently.

When circumstances warrant, a powerful starter motor can restart the engine within 400 milliseconds for seamless performance.

Regenerative braking. In traditional vehicles, the energy used to decelerate the car is lost as heat when the driver applies the brakes. Hybrids, on the other hand, can recover a substantial portion of what would otherwise be “lost energy” and store it in the battery for later use.

Electric drive. The Escape Hybrid can be driven up to 25 mph using the electric motor alone, thus using no gasoline and causing no emissions.

Electric-assist power steering. The electric assist system enables power-assisted steering when the gasoline engine is shut off. While driving at low speeds or coasting, it offers improved fuel economy, even when the engine is running, because it consumes less power than belt-driven power steering.

Full Hybrid
The Escape Hybrid is a “full” hybrid, meaning it has a high-voltage storage battery and the capability of driving on electric power alone. Full-hybrids can achieve 50 percent or more improvement in fuel economy during stop-and-go driving where the electric motor is most efficient.

Mild hybrids, in contrast, are distinguished by relatively small battery capacity and lack an electric-only drive mode, limiting their fuel-saving potential. Mild hybrids are unable to achieve as much improvement in fuel economy as full hybrids. When the driver calls for maximum acceleration, the gasoline engine and electric motor work in parallel, providing launch performance feel similar to that of a powerful V-6 engine.

In less demanding situations, the Escape Hybrid can run on its electric motor alone, its gasoline engine alone or the most efficient combination of the two.

Atkinson-Cycle Engine Operates Efficiently
Escape Hybrid’s four-cylinder gasoline engine is an Atkinson-cycle variant of the conventional Escape’s Duratec 23 2.3-liter engine. The Atkinson cycle is similar to the familiar four-stroke cycle — intake, compression, power, exhaust — except the intake valve closes well after the piston begins moving upward to compress the air-fuel mixture. There are two key benefits of the Atkinson cycle.

First, it reduces the “pumping losses” associated with all gasoline engines. Additionally, because a fraction of the air-fuel mixture is released from the cylinder back into the induction system without being burned, the effective displacement of the engine is reduced. The power stroke, or the distance that burning fuel pushes on the piston, is longer than the effective intake stroke. This helps extract more energy from each drop of fuel.

High-Power Traction Motor Does Double Duty
In addition to its gasoline engine, the Escape Hybrid has a 70-kilowatt (the equivalent of 94 horsepower) permanent-magnet traction motor. This motor can propel the vehicle alone or team with the gasoline engine for a boost of power similar to — a V-6 engine. The electric drive system is most efficient at low speeds and at low loads — exactly the conditions where the gasoline engine is least efficient.

Electronically-Controlled Continuously Variable Transmission
In place of a conventional transmission is an electronically controlled planetary gear set that includes the traction motor and power-management electronics in one compact assembly.

The planetary gear set can vary the distribution of power among the gasoline engine, electric motor and the vehicle’s wheels. Because of this capability, the vehicle can run on the gasoline engine, the electric drive system or both — depending on the driving situation.

In addition, the planetary gear set acts as an electronically controlled continuously variable transmission (eCVT), meaning the gasoline engine can always run at its most efficient or powerful engine speed for a given vehicle speed. This eCVT contributes to efficiency and performance, as well as to refinement, because there are no defined shifting “steps” during acceleration.

Engine Stop-Start Saves Gasoline
Significant fuel savings can be realized by switching off the gasoline engine when it isn’t needed.
The traction motor provides a smooth method of stopping the engine when appropriate, making this process almost invisible to the driver. But a traditional reduction starter motor — a relatively lightweight device that cranks the engine at only a few hundred revolutions per minute — isn’t up to the task of restarting it instantaneously. Instead, the Escape Hybrid uses a much more powerful starter motor to restart the engine when needed, such as during strong acceleration from a stop. It can do so in less than 400 milliseconds and without the increased engine emissions associated with typical starts.

In the U.S. Environmental Protection Agency’s city driving cycle, the Escape Hybrid makes extensive use of engine stop-start and electric-only operation. During that kind of stop-start driving, the gasoline engine is off as much as 40 percent of the time.

Hybrid Storage Battery Offers 330 Volts
A 330-volt nickel-metal-hydride battery pack located beneath the rear load floor stores energy recovered during braking and powers the electric drive system. The Escape Hybrid, like a conventional vehicle, still has a 12-volt battery under the hood to power the vehicle’s lights and electrical accessories.

The hybrid storage battery consists of 250 D-sized cells in a sealed enclosure. Nickel-metal-hydride batteries have been used with excellent success in notebook computers and cell phones for years.

Hybrid-specific components, including the battery pack, are covered by a warranty of at least eight years or 100,000 miles. The entire vehicle comes with a bumper-to-bumper warranty for three years or 36,000 miles and complimentary roadside assistance.

Thermal management for optimal performance
“Batteries are like people,” Watson said. “They don’t like to work when they’re cold, and they get stressed when they’re hot.”

To keep the battery pack at its optimum temperature, it has a thermal management system to deal with temperatures from minus 40 degrees Centigrade (-40 Fahrenheit) to 50 degrees Centigrade (122 Fahrenheit). An electric heater and forced-air cooling system help keep the battery comfortable. While it’s never necessary — or even possible — to plug the Escape Hybrid into a charger, in the unlikely event that insufficient energy is available to start the gasoline engine, jumpstarts are possible. Sophisticated electronics within the Powertrain Control Module manage energy flow.

Electric Power Makes Steering Easier
Because the Escape Hybrid’s gasoline engine automatically stops to save fuel when possible, an electric power-assisted steering system replaces the traditional belt-driven hydraulic system.

An electronic control module detects the driver’s input torque at the steering wheel, instantaneously computes the proper amount of assist and commands a brushless electric motor to help control the steering mechanism.

The electric power-assisted steering brings several other benefits, including quieter operation. Because there is no power steering fluid, the chance of leaks is eliminated, as is fluid circulating noise. 

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