Future Developments of Automotive Braking Systems

The rapid development of technical materials has afforded engineers the freedom to experiment with new technologies and processes which have, in many cases, led to remarkable improvements in braking performance. One such development has been the carbon-ceramic disc brake rotor. Disc brakes to surpass drum brake market in the next few years The carbon-ceramic brake disc had its commercial premiere at the IAA in Frankfurt in 1999. The use of this high-tech material has revolutionized brake rotor technology: In comparison to the conventional grey cast iron brake disc the carbon-ceramic rotor weighs around 50 per cent less, reducing vehicle unsprung mass by almost 20 kilograms. Further significant advantages include: Improved braking response Reduced brake fade Thermal stability at elevated temperatures No hot judder Superior pedal feel Improved steering behavior High abrasion resistance resulting in longer life (Up to 4 times the life) Minimal brake dust A special feature of carbon-ceramic rotors is the ceramic composite material they are made of. Both the carbon-ceramic brake disc body and the friction layers applied to each side consist of carbon fiber-reinforced silicon carbide. The principle matrix components are silicon carbide (SiC) and elemental silicon (Si), with carbon fibers providing reinforcement of the material. Silicon carbide, the main matrix component, gives the composite material exceptional hardness. The carbon fibers make for high mechanical strength and provide the fracture toughness needed in technical applications. Carbon fiber-reinforced silicon carbide materials combine the unique properties of carbon fiber-reinforced carbon (C/C) and polycrystalline silicon carbide ceramics. The resulting quasiductile properties of the ceramic composite material ensure its resistance to high thermal and mechanical loads. The carbon-ceramic brake disc is manufactured using carbon fibers which are given a special protective coating and then cut into short fiber sections of specific thickness and length. The production process including preparation of the fiber mixture, production of the body and the bell mounting as well as the final machining of the assembled brake rotor takes approximately 20 days. Image credit: www.carbonceramicbrakes.com During the production process a preform is pressed together with binding resin to a so called "green body." The final ceramic component is produced after carbonizing this assembly at 900 °C and thereafter siliconizing it at 1700 °C, while exposed to high vacuum. Using "lost core" technology, a plastics matrix, which defines the design of the cooling vane geometry, burns out without leaving residues during the carbonizing process. Property Unit Material C/SiC material, general C/SiC for carbon- ceramic brake disk Gray cast iron (GG-20) Density g cm-3 1,8 ... 2,9 2,45 7,25 Tensile strength MPa (=N mm-2) 10 ... 240 20 ... 40 200 ... 250 Modulus of elasticity GPa 20 ... 240 30 90 ... 110 Flexural strength MPa (=N mm-2) 20 ... 210 50 ... 80 150 ... 250 Elongation at break % 0.05 ... 0.8 0.3 0.3 ... 0.8 Thermal shock resistance (second thermal coefficient K') W m-1 26.500 ... 46.000 > 27.000 < 5.400 Thermal stability °C 1350 1350 approx. 700 Maximum operating temperature (brake disk) °C 1400 non-oxidizing 900 700 Linear coefficient of thermal expansion K-1 1.0 ... 3.5 2.6 ... 3.0 9 ... 12 Thermal conductivity W m-1K-1 20 ... 150 40 54 Specific heat capacity (cp) kJkg-1K-1 0.6 ... 1.7 0.8 0.5 An important feature of carbon ceramic brakes is that they can withstand temperatures of up to 1600 degrees Celsius. However, higher operating temperatures come at a cost: With intense heat being generated from the friction on the surface of the rotor, the resultant high temperatures can damage other unprotected parts. Another challenge, when fitted to road cars, is that carbon ceramic brakes require at least ten to fifteen stops to effectively warm up the rotors to achieve optimum braking. This is obviously exacerbated in cold weather conditions and can pose a real problem in emergency stops. In order to apply the high clamping loads required by modern brake systems servo-assist braking comes as standard fitment. However, with hybrid and electric vehicles gaining market share across all segments, vacuumless braking will be the next step towards bridging the gap between the braking technology used on non-EVs and fully electric vehicles. [eventPDF] Vacuum independent braking technology Safety component manufacturer, TRW’s latest innovation in braking is its Integrated Brake Control (IBC) system; a vacuum independent technology which simplifies the brake system while enhancing performance. The IBC replaces the electronic stability control (ESC) system along with the vacuum booster and the associated cables, sensors, switches, electronic controllers, and vacuum pump. Integrated brake control relies on a fast acting brushless motor to drive a precision ball-screw actuator to ensure superior braking performance and ESC capability. The system is easily integrated with driver assist systems such as radar and camera to enable features such as Automatic Emergency Braking (AEB). IBC also offers considerable packaging and weight savings compared with current ESC systems – weighing just 3.8 kg as opposed to 7 kg for a conventional system, and a further 3Kg for hybrid vehicles, which may require an electric vacuum pump. Integrating IBC into a Hybrid Enabling Brake Control allows TRW to cover regenerative braking market requirements for all powertrains and brake circuit split variations, as well as reducing CO2 emissions and improved fuel economy by enabling maximum energy recovery. The high modularity of the EBC460 Slip family of Control Systems provide cost-efficient solutions, while the optimized packaging and low weight allow installation covering small cars through to luxury vehicle segments. SCB is compatible with vehicle powertrain architectures that have reduced to no vacuum availability, such as direct-injection engines, diesel engines, and fuel-cell powertrains. SCB is a hydraulic-boost brake system with integrated electronic stability control functionality in one package. According to Dan Milot, Chief Engineer of Advanced Control Systems for North America Brake Engineering at TRW Automotive, "Slip control boost (SCB) saves fuel by being an integral part of the regenerative braking system." The SCB system replaces traditional boosters, master cylinders, and vacuum pumps with an electrohydraulic control unit and master-cylinder reservoir. Opposition systems supplier Continental recently unveiled another EBS innovation: The electric drum brake which is set for series production in 2017. The system consists of two actuators which are integrated into the carrier plate of the rear axle drum brakes plus the associated control software. The electric park brake is controlled by the Electronic Stability Control (ESC) ECU. According to Continental the design of the electric park brake which is equipped with a double direction hydraulic cylinder and a mechanic locking device is well suited to application in compact cars. The combination of a cost-effective drum brake with integrated actuators and activation through ESC yields a low-weight system with high efficiency and low manufacturing costs. A logical progression of the EBS would be a brake-by-wire system. However in order to achieve this, the few remaining hydraulic components in the brake system would need to be replaced in order to achieve the level of sensitivity required. The electronic wedge brake simplifies brake by wire systems The electronic wedge brake (EWB) developed by Siemens VDO Automotive (SV) is an interesting development that could be well suited to EBS. It could also be used in conventionally powered vehicles to provide a range of electronic "brake-by-wire" features. These might include a "soft-stop" function, whereby the system brings the vehicle to a smooth halt by automatically reducing the braking force just before it stops, or a "hill-hold" controlled by software, which would prevent the vehicle from rolling back when pulling away on a gradient. Although the EWB requires a constant electrical current, it is not affected by problems associated with the onboard power supply. "If the current to one of the wheels were disrupted, the system would automatically compensate by distributing the braking power to the others," says Karsten Hofmann, head of Chassis Product Marketing at SV. And if the vehicle’s entire power supply system fails, an emergency battery ensures that all vital functions are maintained until the fault is remedied. Image credit: Siemens VDO The brake calipers (1) span the brake disk (2) from two sides. The brake disc is braked by a pad (3) which is moved by an electric motor (4) by several rollers (5) along wedge-shaped inclined faces. For each wheel, the electronic wedge brake has a control unit consisting of a brake pad, a mechanical transfer system, two electric motors for precise positioning, four sensors measuring wheel rotation and therefore the speed of the vehicle, the forces on the brake and the position of the wedge. Whenever the driver applies brakes, the system transmits the force electromechanically to the wheels, which are electronically networked with one another. Depending on the sensor readings and the braking force applied by the driver, the two electric motors move the brake pad over a series of rollers along a gradient, or wedge. The position of the rollers on the inclined surface determines the pressure point of the brake pad. When the pad presses against the disk, the latter is immediately retarded. As soon as a high braking moment is generated by increasing the frictional forces, the electric motors either hold the brake pad in position or move it back over the roller bearing thereby returning to the best position for optimum braking. Because the system only moves micrometers, response times of the order of milliseconds are achieved. Image credit: Siemens VDO Using a sophisticated algorithm the integrated software in the EWB system also replaces the ABS anti-lock braking system and electronic stability program (ESP). Whereas it takes 140 to 170 ms for a conventional ABS to generate full braking power, the EWB only needs around 100ms, an advantage which, according to Siemens engineers, should make a difference on uneven and icy roads. "Our system can control each individual wheel faster than any hydraulic system and thus keep them on course better," says Hofmann. During any retardation a significant amount of kinetic energy is converted into heat and noise, prompting engineers to investigate innovative methods to store this for useful work at a later stage. Regenerative braking systems Kinetic energy stored in a moving vehicle is proportional to the mass and speed of the vehicle through the equation E = ½mv². The energy conversion efficiencies from chemical to electrical (battery), DC current to AC current (inverter), electrical to mechanical (motor), and torque to force (transmission and wheels) are all quite high and work just as efficiently when returning energy into the battery. Battery-to-wheel conversion efficiency is also quite good (up to 80%). With input and output efficiencies of 80%, respectively, a significant regenerative braking efficiency of about 64% is possible. Regenerative braking systems are common in Hybrid, Plugin-hybrid and full EV vehicle applications where the kinetic energy, converted under braking, is stored in high-energy battery packs. When required, this energy is used to drive an electric motor which is in turn used to augment the ICE, when used in a hybrid, or propel the EV. Recently, companies such as the Schaeffler Group and Protean Electric have been studying in-wheel drive systems that take the concept one step further: Using an electric motor in each driven wheel, the components required for drive, deceleration and driver assistance are all installed in the integrated wheel hub drive. Protean, who have several vehicles under test, make the following claims: • Highest torque density of any of today's leading electric drive systems. • 75 kW (100 hp) peak power and 1,000 Nm (735 lb.-ft.) peak torque in each motor. • Mass of only 31 kg (68 lbs.) per motor. • Fuel economy improvements up to 30 percent in hybrid configurations, as compared to the existing vehicle, depending on battery size. • Superior regenerative braking capabilities, which allow up to 85 percent of the available kinetic energy to be recovered during braking. Crucial to these regenerative systems are expensive high energy storage batteries, such as Li-ion or NiMh. There have been many studies into alternative storage systems, including recent developments in ultracapacitors. Regenerative braking using ultracapacitors "Ultracapacitor" technology can store up to 20 times more energy than conventional electrolytic capacitors. Despite this impressive advance in energy storage, ultracapacitors are still far from being as efficient as electrochemical batteries. Even a Lead-acid battery can store at least ten times more energy than an ultracapacitor. Nevertheless, they have better specific power, and can be charged and discharged thousands of times without performance deterioration. Image credit: www.mazda.com These characteristics when used with normal electrochemical batteries can improve the transient performance of electric vehicles whilst increasing the useful life of the batteries. Using ultracapacitors fast battery discharge during acceleration, or sudden charge during regenerative braking can be overcome. Added to this, ultracapacitors allow regenerative braking even when the batteries are fully charged. An ultracapacitor-bank control system is used to enable increased vehicle acceleration and deceleration with minimal loss of energy, and degradation of the main battery pack. The system uses an insulated-gate bipolar transistor (IGBT) Buck-Boost converter, which is connected to the ultracapacitor bank at the Boost side, and to the main battery at the Buck side. The battery voltage is used as a measure of the car’s instantaneous state: When the vehicle is accelerating, the battery voltage goes down, which is a signal for the control to take energy from the ultracapacitor. During regenerative braking, the battery voltage goes up, and the control activates the Buck converter to store the kinetic energy of the vehicle by charging the ultracapacitor. The measurement of the current on both sides maintains the current levels within specified ratings. The battery state-of-charge is used to change the voltage level of the ultracapacitor at particular values. If the battery is fully charged, the voltage level of the capacitors is kept at lower levels than when the battery is partially discharged. To optimise the charge and discharge of the ultracapacitor the control system measures the battery voltage, battery state-of-charge, car speed, instantaneous currents in both terminals (load and ultracapacitor), and the actual voltage of the ultracapacitor. This last indication monitors the level of energy stored in the ultracapacitor. A microprocessor measures all the variables and generates the Pulse-width modulation (PWM) switching pattern of the IGBTs. Although these systems are very effective in regenerating and storing kinetic energy for electrical power generation, the vast majority of vehicles produced (In the foreseeable future) are not hybrid or EV’s. Because of this, several companies are studying Mechanical Kinetic Energy Recovery Systems for near and medium term fitment. Author: Peter Els

Future Developments of Automotive Braking Systems

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