Introduction:
Vehicles with electrified propulsion (hybrid, full EV, fuel cell) have the ability to brake electrically. When releasing the accelerator pedal, or with light braking, the electric motor acts as a generator. The vehicle’s kinetic energy is converted into electrical energy for the HV battery. The driving range increases when you brake gently often and the brake system gets the chance to do a lot of regenerative braking. More about that can be read on the page: inverter.
As of 2026, electric braking is still always used in combination with the conventional hydraulic brake circuit. In the event of an electrical malfunction, or on older vehicles during an emergency stop, the hydraulic brake circuit is (partly) activated. This serves as a backup. The following paragraphs show how manufacturers combine electric and hydraulic braking to ensure good comfort and to guarantee safety in the event of a failure of the electrical system.
Drive by wire:
The “drive by wire” brake system is intended to brake hydraulically with electrical assistance. There is no direct hydraulic connection between the brake pedal and the brake pistons in the brake calipers. With the brake pedal, brake pressure is applied to a so-called brake force simulator. The brake pressure is measured. An electric motor builds up the desired pressure in the hydraulic brake circuit. The drive by wire brake system offers the following advantages compared to the conventional brake system:
- No vacuum brake booster is used anymore, since the electric motor provides the required fluid pressure;
- Fluid leakage can be detected per brake and can be isolated. For that reason, a master cylinder is no longer required for two separate brake circuits;
- The driver does not notice any transition between electric and hydraulic braking at the moment when switching from regenerative braking on the electric motors to braking by pressing the brake pads against the disc;
- ABS system vibrations are no longer felt in the brake pedal;
- The (simulated) counterpressure in the brake pedal can be adjusted to the settings (comfort / sport).
The hydraulic diagram below shows the system used by BMW (DSCi). Operation is as follows:
When the driver applies the brake pedal, force is applied to the master cylinder (7). This master cylinder has two outlets: to the brake pedal force simulator (8) and to a decoupling valve. Through the blue line, the simulation pressure is passed on to the brake pedal force simulator. In this component, a counterpressure is created, which the driver recognizes as counterpressure in the brake cylinders. There is no physical connection from the master cylinder to the wheel brake cylinders. The simulation pressure is measured by a pressure sensor (5). Depending on the simulation pressure, the ECU controls the electric motor (10). This applies a working pressure in the brake pressure cylinder (9). A pressure sensor on the working-pressure side feeds the built-up pressure back to the ECU. The red connections in the diagram show how the working pressure reaches the wheel brake cylinders (1) via the valves. The pressure-holding valves (3) are open at rest, so brake pressure can be built up directly from the brake pressure cylinder. The pressure-reducing valves (2) are closed at rest.
Legend:
- Brakes
- Pressure-reducing valves
- Pressure-holding valves
- Decoupling valves
- Pressure gauges for brake pressure working circuit and simulator circuit
- Brake fluid reservoir
- Master cylinder
- Brake pedal force simulator
- Brake pressure cylinder
- Electric motor
- Diagnostic valve
- Yellow connections: supply and return to/from the brake fluid reservoir;
- Blue connections: simulation pressure;
- Red connections: working pressure (brake pressure).
If there is a leak near the brake pressure cylinder, or if there is an electrical malfunction causing the electric motor to be unable to build sufficient working pressure, then, to ensure safety, the decoupling valves (4) are energized. The connection between the master cylinder and the wheel brake cylinders is opened and the connection to the brake pressure cylinder is closed. Because the brake booster is absent, you must press the brake pedal harder in order to brake.
Combination of electric and hydraulic braking:
Fully electric and hybrid vehicles always have a combination of an electric and hydraulic braking system. The “brake by wire” brake system from the previous paragraph is not used often yet. In that system there is no direct connection between the brake pedal and the wheel brake cylinders. A powerful electric motor provides all braking force: even during an emergency stop. In that case, a brake booster is not needed.
On most electric and hybrid vehicles, a combination of electric and hydraulic braking is achieved as follows: during gentle (modulated) braking, regenerative (electric) braking takes place because the electric motors function as generators. During hard braking and/or in the event of malfunctions, the hydraulic system assists immediately. A brake booster is used here to increase brake pressure. So during deceleration there is interaction between the electric motor and the mechanical brakes. This system is sometimes also called “drive by wire,” although this term better fits the system from the previous paragraph.
The diagram below is based on the Toyota Prius 3. With the brake pedal (1), brake pressure is built up in the master cylinder (3). During gentle braking, braking is done only on the electric motors. The brake pressure simulator (4) provides counterpressure while pressing the brake pedal. The valve for the brake pressure simulator is opened under normal operating conditions. During hard braking, the lock valves (5) are opened and the valve for the simulator is closed. The brake calipers of the front wheels are supplied with brake pressure. Opening and closing the hydraulic valves (6) makes it possible for brake pressure to also reach the rear wheels. The brake pressure sensors (from left to right: p lv through p rv) measure the pressure and pass it to the ECU. Using a PWM signal, the hydraulic valves (5, 6 and 7) are regulated based on the desired brake pressure.
The system is designed so that in the event of a power failure, the brake pressure at the rear wheels is fully released and the pressure at the front wheels is controlled by the driver with the brake pedal.
Legend:
- Brake pedal
- Brake fluid reservoir
- Tandem master cylinder
- Brake pressure simulator
- Lock valves
- Hydraulic valves (closed from left to right)
- Hydraulic valves, front side closed, rear side open
- Pressure accumulator
- Hydraulic pump driven by electric motor
- Pressure limiting valve
- Yellow connections: supply and return to/from the brake fluid reservoir;
- Blue connections: brake pressure coming from the hydraulic pump;
- Red connections: brake pressure coming from the master cylinder (with valves open).
Hydraulic braking of the Toyota Prius 3 is done via the front wheels. The rear wheels are not connected to the master cylinder. On modern vehicles, including the Kia Niro, that is the case: all four brake cylinders are actuated by the master cylinder by means of two circuits.
When decelerating vehicles with a similar braking system, under certain conditions a switch takes place from electric to hydraulic braking. To keep the brake deceleration and the feel in the brake pedal smooth, “brake blending” is used on this brake system. This is described in the next paragraph.
Brake blending:
When releasing the accelerator pedal or during modulated braking, many electric vehicles decelerate exclusively using the electric motors. The kinetic energy is converted into electrical energy, which increases the driving range of the vehicle. The hydraulic brake system is hardly used. When a high brake deceleration is needed, the electric brake and the hydraulic service brake work together. The cooperation of the two braking systems is called “brake blending.” In earlier generations of hybrid and fully electric vehicles this was not smooth and the vehicle’s rate of deceleration changed when the hydraulic brake assisted. With current technology, the driver no longer notices the transition between the two braking systems. Note: this is not the technology used with drive by wire.
The graph shows the transition of the two braking systems while brake deceleration remains constant. The driver’s pedal force (a) remains the same for 10 seconds. When braking begins, the hydraulic service brake and regenerative braking on the electric motors work together. In the first six seconds we see that the deceleration due to regenerative braking increases. The electric motor functions as a generator and supplies the HV battery with the generated energy. The braking force of the hydraulic service brake decreases further and further until it no longer contributes. After approx. 7.5 seconds we approach a standstill of the vehicle and the electric braking force drops off. The hydraulic braking force increases again. After 8.5 seconds the vehicle is at a standstill. The driver keeps the brake pedal depressed briefly.
a: driver pedal force
b: deceleration due to regenerative braking (using the electric motor)
c: deceleration due to hydraulic service brake
d: deceleration requested by the driver
e: decrease in speed
d = c + b