Developments in Road Vehicle Braking Systems

Transcription

1 Developments in Road Vehicle Braking Systems Professor Andrew Day. Road vehicle braking systems have developed substantially since the 1970 s. All that was required of a braking system back then was that it brought the car safely and controllably to rest. The circuits were split, either front / rear (longitudinal) or diagonal split (now the most popular on modern FWD cars). A typical car braking system will have the total braking force split e.g. about 70 / 30 front / rear. It is common experience to feel a tendency to dive under braking which is the result of braking weight transfer between the axles. The major loads are illustrated in the diagram below. Each tyre might make a contact area of only 0.01 m 2

2 The effect of water (a just wet surface) is to reduce adhesion and hence deceleration by almost half at 70 km/h. Braking efficiency is limited by the braking force developed at each wheel, which is ultimately limited by the tyre-road adhesion: ηb = max deceleration a vehicle can achieve (for like road surfaces) max theoretical efficiency The effect of a worn road surface is to reduce adhesion by almost half. In response to the above, highway maintenance authorities apply top dressing to recover the surface adhesion characteristics. An additional benefit of top dressing is to provide a water-proof surface layer. Cars have a typical tyre-road adhesion coefficient of k 0.85, commercial vehicles slightly less at The rate of braking is defined as z = rate of deceleration / g This means that a car s maximum deceleration will be 8.34 m/s 2. In context, this means that the physical braking distance travelled from 100 km/h will be 46m, add this to an associated thinking time of 19m means that the minimum stopping distance will be 65m.

3 Modern car braking systems. 1. Electronic brake force distribution (EBD) If the front / rear brake distribution is fixed, then the maximum braking efficiency will be available for only 1 rate of deceleration. However, modern cars overcome this problem by dynamically varying the ratio of front / rear brake force by monitoring slip s = (V ω r). Where V = road speed, ω = wheel angular velocity, and r is the dynamic or rolling radius of the wheel. 2. Anti-lock braking system (ABS) A number of hydraulic and mechanical systems were developed in the 1970 s, however it was not until (1990 s) that the Ford Escort (mk 3) introduced a mechanical ABS system on a mass produced car. Modern ABS systems work by detecting incipient wheel lock, they then modulate the actuation force on the locking wheel. 3. Traction Control System (TCS) and Electronic Stability Control (ESC)

4 The technology of ABS, used to prevent wheel lock on deceleration, may be programmed to inhibit wheel spin on acceleration. This is TCS. The TCS system acts as a torque transfer mechanism (similar to the mechanical limited-slip differential). 4. Electronic Stability Control (ESC) Approximately 10 years ago it became possible to correct understeer and oversteer by further extension of the ABS / ESC technology. Examples include the Ford torque vector control and McLaren s brake assisted steering. These corrections are applied without driver intervention. It is possible to couple the braking effort to the engine power, via the ECU, therefore reducing engine power, if required. Correction of understeer and oversteer. Understeer is corrected by applying brake effort to the inside rear wheel to generate a a correcting yaw moment, while oversteer requires brake effort to the off-side front wheel to apply the required yaw correction moment. Many accronyms exist for each of these systems, collectively they take the name: Electronic Stability Control (ESC). ESC has been mandated within the EU since 2010 for cars, commercial vehicles and buses.

5 5. A note of caution No amount of electronic control can overcome the laws of physics and these systems do reduce driver feedback, however the ability to drive a car on slippery surfaces safely is a major benefit. Without ESC With ESC The maximum adhesion force available from the tyre / road contact is a fixed quantity for any given set of driving conditions. This means that if a substantial amount of force is consumed resisting side-ways forces, little is left to resist longitudinal forces such as braking, therefore it is much easier to bring a wheel to the point of lock when applying the brakes while driving through a corner hard,. All friction force available for braking (no cornering force). Reduced friction force available for braking (active cornering force). Locus of total friction force 6. Advanced electronic controls These include: Hill Descent Control (HDC), invented by Jaguar-Landrover at the launch of the Land Rover Freelander. The company s objective was to provide a safe vehicle, capable of steep off-road descents that would be useable by a customer base unfamiliar with the demands of off-road driving. The latest Land Rover products include a version of this that permits the driver to adjust the required speed dynamically.

6 For commercial and 4WD vehicles, Rollover Stability Control (RSC) has been developed. Emergency Brake Assist (EBA) is a system that senses an emergency brake application by the driver from the speed of the brake pedal movement. It has been found that in emergency braking situations, the driver often does not apply enough brake force so the EBA then automatically augments the brake effort.

7 Many others systems exist and have been deployed, or are under active research and development, including the Electric Parking Brake (EPB). Others, potentially useful, include: Hill Start Assist (HSA), Adaptive Cruise Control (ACC), and Advanced Emergency Braking System (AEBS). AEBS will be required for new commercial vehicles within the EU from A thorough technical description of all of these modern braking systems, and others, for example regenerative braking is provided in Prof A Day s book. Braking of Road Vehicles, Andrew Day, Publisher: Butterworth-Heinemann, Date: 2014 ISBN-10: , ISBN-13: NG Pollard (19 November 2014)