Regenerative brake

A regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into electrical energy. This electrical energy is then stored for future use or fed back into a power system for use by other vehicles.

Regenerative brakes in electric railway vehicles feed the generated electricity back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use.

Regenerative braking should not be confused with dynamic braking, which dissipates the electrical energy as heat.

Traditional friction-based braking is still used with regenerative braking for the following reasons:

* The regenerative braking effect rapidly reduces at lower speeds.
* The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. No regenerative braking effect can occur if another electric vehicle on the same supply system is not currently drawing power or if the battery or capacitors are already charged. For this reason, it is normal to also incorporate Dynamic Braking to absorb the excess energy.

Regenerative brake: The motor as a brake

Regenerative braking utilizes the fact that an electric motor can also act as a generator. The vehicle’s electric traction motor is reconnected as a generator during braking and its output is connected to an electrical load. It is this load on the motor that provides the braking effect.

An early example of this system was the Energy Regeneration Brake, developed in 1967 for the Amitron. This was a completely battery powered urban concept car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile.

Regenerative brake: Electric railway vehicle operation

During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive wheels turn the motor armatures, and the motors act as generators. Either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking) provides the braking load.

For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction.
Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings.

Regenerative brake: Comparison of Dynamic and Regenerative Brakes

Dynamic brakes (“rheostatic brakes” in the UK), unlike Regenerative Brakes, dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include forklifts, Diesel-electric locomotives and streetcars. If designed appropriately, this heat can be used to warm the vehicle interior. If dissipated externally, large radiator-like cowls are employed to house the resistor banks.

The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics. With DC supplies, this requires that the voltage be closely controlled. Only with the development of power electronics has this been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).

A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.

Regenerative brake: Use in motor sport

The governing body of international motor sport, the FIA, has allowed the use of 60 kW “Kinetic Energy Recovery Systems” (KERS), in the regulations for the 2009 Formula One season.

Electronic brakeforce distribution

Electronic brakeforce distribution (EBD) is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle’s brakes, based on road conditions, speed, loading, etc. Often coupled with anti-lock braking systems, EBD can apply more or less braking pressure to each wheel in order to maximize stopping power.

In a hydraulic brake system not equipped with EBD, there is a fixed front-to-rear brake force bias which is determined by the hydraulic components (for example, caliper piston diameter). This bias may be shifted under heavy braking, by means of a mechanical proportioning valve, to prevent rear-wheel lockup. EBD instead applies brake force precisely through electronic control. It recognizes that driving conditions, braking situations and vehicle weight distributions are unique and constantly changing. Working together with Anti-lock Braking System (ABS), EBD uses sensors to determine which wheels would provide maximum braking for the conditions – whether that’s the front or rear wheels, the left or right. The final result is more precise and effective braking under all conditions, and also makes the car much more stable under heavy braking, reducing front end dive.

Engine braking

Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression braking is a common legal term for the same mechanism. Large trucks use a device called an engine brake to increase the effectiveness of engine braking.

Engine braking: Design

Compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the forward momentum of the vehicle as well as the angular momentum of the flywheel. When a driver downshifts to spin the engine at high angular velocity (or RPM) without pressing on the gas pedal, the engine converts energy from the vehicle’s speed, which is kinetic energy, into a temperature increase in the fuel-air mixture. These hot gases are exhausted from the vehicle and heat is transferred from engine components to the air.

This energy conversion occurs because most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to air charge during compression to directly ignite the mixture when the fuel is injected.

Engine braking: Advantages

The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gases are ejected from the vehicle very quickly and the gases also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.

This engine heat is taken away by the engine’s integrated cooling system: usually a liquid circulation system and a radiator. Disc or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they use their mass to retain heat without producing temperatures that would deform and damage the brakes.

Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have less leverage on the engine. This is what allows cars to slow down using their relatively flimsy engine parts. The engine maintains a high rotational speed to dissipate a lot of power without forcing too much strain on the engine.

The engine brake is used in large diesel vehicles because the rate of conversion of mechanical energy into waste thermal energy is low compared to the mechanical returns to kinetic energy from the air-spring effect in the engine.

Engine braking: Applications

Engine braking is always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged. This is often called engine drag.

Active use of engine braking (shifting into a lower gear) is only advantageous when it is necessary to control speed while driving down very steep and long slopes. It should be applied before regular disk or drum brakes have been used, leaving the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract the acceleration due to gravity.

Improper engine braking technique can cause the wheels to skid, especially on slippery surfaces such as ice or snow, as a result of too much deceleration. As in a skid caused by over-braking, the car will not regain traction until the wheels are allowed to turn more quickly; the driver must reduce engine braking (shifting back up) to regain traction.