汽车制动Word文档格式.docx

1、 Car Braking Systems IntroductionA conventional passenger car braking system consists of a pedal operated master cylinder that supplies fluid under pressure to friction brakes at each wheel. To assist the driver, it is customary to include a vacuum servo that provides a helping force in proportion t

2、o the input. For safety reasons, the master cylinder is of tandem design incorporating twin pressure chambers. Each chamber connects to at least two of the vehicle brakes to prevent total loss of brakes should one circuit fail.The friction brakes fall into two main categories that are determined by

3、the type of rotor to which the braking force is to be applied. The first type is a rotating disc that has friction pads clamped at both sides and the second takes the form of a rotating drum. In this case, friction linings are forced against the inner diameter of the drum.Most modern cars have disc

4、brakes fitted at the front wheels but the rear wheels have either disc or drum. The rear brakes almost universally include manual parking.Figure 1: Layout of typical braking systemThe figure 1 shows the layout of a basic car braking system. The major functional parts are the pedal, booster （vacuum s

5、ervo）, master cylinder, brake hoses and the disc and drum brakes at the wheels. The parking brake is manually operated using a cable system.Tandem braking systemsTandem braking systems are tailored to suit the weight distribution on the two axles. For Rear Wheel Drive （RWD） cars the system is split

6、front to rear and for Front Wheel Drive （FWD） cars the system diagonally connects a front and a rear brake. Both systems facilitate the fitment of Anti-lock Braking Systems （ABS）The front/rear splitFigure 2a: The front/rear splitThis system, （see Figure 2a）, is the simplest of all the tandem braking

7、 systems with each axle connected to a separate circuit.It is in everyday use for larger RWD cars where the rear axle carries sufficient weight to provide a satisfactory vehicle deceleration should the front circuit fail. If the rears fail, the fronts can be applied.The diagonal split. Figure 2b: Th

8、e diagonal splitThe system shown in Figure 2b connects each front brake to its diagonally opposite rear brake. It is almost universally used for FWD ears where the heavier weight on the front wheels must be utilised in each hydraulic circuit. This is to ensure adequate retardation for the part circu

9、it failed case. For this case, however, the braking force is applied to only one of the front wheels and this generates an adverse force at the steering wheel unless the steering offset is reduced to a minimumTandem master cylinderA master cylinder converts the drivers applied force into a hydraulic

10、 pressure to apply the vehicles brakes. In addition, it automatically adjusts for the increasing volume at the brakes to cater for pad wear. In this way the drivers pedal travel remains substantially the same irrespective of how much the brake pads have worn.The conventional tandem master cylinder c

11、onsists of a pair of pistons spaced apart in a single bore to form two chambers. Each chamber is vented to a fluid reservoir at atmospheric pressure and connects to one part of a dual circuit braking system. The primary piston operates from the brake pedal input and the secondary piston responds to

12、pressure generated in the primary chamber by movement of the primary piston. The initial movement of the pistons isolates the pressure chambers from the reservoir. The material for the reservoir is semi-transparent to ascertain the fluid level and it houses a float operated switch to give a fluid le

13、vel warning.Most master designs are similar but the method of isolating the reservoir from the pressure chambers and the way in which fluid is admitted, whilst “bleeding” the brakes, varies.American Standard （AS）Figure 3: AS/AS Tandem master cylinderFigure 3 illustrates a tandem master cylinder havi

14、ng pistons equipped with AS seals. The seals not only provide sealing and recuperation but also act to isolate the reservoir from the pressure chamber.Actuation of the brake pedal moves the primary piston so that its recuperation seal passes over a small ventilation hole. Once isolated from the rese

15、rvoir, pressure builds up in the primary chamber and the secondary piston moves to close its own ventilation hole. Thereafter, the two pistons move further down the bore displacing fluid under pressure to the brakes. The secondary piston travel equates to the volume of the braking circuit to which i

16、t connects and the primary piston travel equates to the volume of the whole braking system. Typical piston travels for a diagonal split braking system are 16mm for the primary and 8mm for the secondary.To remove aerated fluid from a brake, open the relevant brake bleed screw and actuate the master c

17、ylinder. Then close the bleed screw and release the applied force to allow the master cylinder plungers to return. This allows fresh fluid from the reservoir to pass over the AS seals. Repeat until the air is removed from the fluid.Centre Valve （CV）Figure 4 shows a typical CV master cylinder design.

18、 In this case, the pressure seal is conventional and the central valve provides the means to isolate the pressure chamber from the reservoir. In addition the central valve performs the function of allowing fluid recuperation whilst bleeding the brakes.Figure 4: CV Tandem master cylinderActuation of

19、the brake pedal moves the primary piston into its bore and its initial travel closes the central valve. Further movement pressurises the primary chamber and the secondary piston moves to pressurise the secondary chamber.To bleed the brakes, the master cylinder is actuated as before with the bleed sc

20、rew at the brake open. On release of the pedal force, the pistons return and fluid is sucked into the two chambers through the central valves.Although more complex than the AS master cylinder, the CV design is more amenable for use with ABS. In some cases, fluid from the ABS pump is forced back into

21、 the master cylinder （with brakes applied） to fully return the pistons. In these circumstances, the recuperating seals may pass under the ventilation holes which results in damage to the seals. The same conditions do not affect the seals of the CV piston and the central valves are designed to open a

22、t high pressure should conditions require it.4. Vacuum servoThe objective of a vacuum servo is to provide assistance to the drivers pedal force in proportion to the applied effort. The medium of the vacuum is chosen since it is readily available from an IC engines inlet manifold, particularly with t

23、he throttle closed. This happens when the driver transfers from accelerator to brake pedal. Another advantage of using vacuum is that the lack of air means that the medium is dry and the operating valve mechanism does not freeze.Figure 5: Vacuum servoA disadvantage of using vacuum is that the workin

24、g pressure is low （typically 0.7 bar） and the assisting piston must be sized accordingly. Vacuum Servo sizes are historically quoted in inches, i.e. 7”, 8” and 9” or in their equivalent area, i.e. Type 38, Type 50 and Type 64.A typical vacuum servo consists of a large piston housed within two thin m

25、etal shells to form two chambers. The large chamber connects to the vacuum supply and the small chamber, normally at vacuum pressure, is pressurised with atmosphere by operating the central valve mechanism. The valve assembly consists of an inlet and an outlet valve together with a reaction mechanis

26、m. The reaction device, usually a rubber disc, abuts both the input and output rods. In operation the two rods load the disc and produce a reactive force in proportion to the input rod area. A typical ratio between A2 and A1 is 4:1.When the driver pushes on the brake pedal to load the input rod, see

27、 Figure 5, the outlet valve closes and the inlet valve opens to admit air into the servo chamber. As a result, a pressure differential is created across the piston and it moves to push the output rod against the master cylinder. The output force acting over the area A2 pressurises the rubber disc. T

28、his pressure acts upon the input area A1 to provide a reaction to the drivers input. If A2 is four times the area of A1 the boost ratio is 4:When the input reaction is in the correct relationship to the applied force, the inlet valve closes. A decrease in input force opens the outlet valve and the p

29、ressure differential across the piston reduces. Should the vacuum fail, the brakes are applied by directly conveying the drivers force through the input and output rods to the master cylinder.Modern developments of this basic design include tie-bolts through the servo to connect the master cylinder

30、directly to the bulkhead. This feature reduces weight and deflection. Another improvement includes a device that determines the stop for the input rod so that a slight pressure differential develops across the piston to balance the force of the return spring. As a result, the servo piston takes up a

31、 position with both valves closed and keeps pedal travel to a minimum.Brake Pedal Travel RatioFigure 6: Pedal Brake TravelThe amplification of the force applied by the driver due to the brake pedal can be obtained from Figure 6 as shown below:Pedal Ratio = Rp = L2 / L1where: L2 = length from pivot t

32、o center of pedal pad L1 = length from pivot to pushrod attachmentPedal Travel = Lp = Lb Rp Lb = booster input travelBooster Input Force = Fi = Fp Rp Fp = pedal force （applied by driver）Basic Equation for brakingFigure 7: Forces acting on vehicle in brakingThe basic equation for braking is similar to that for acceleration except that here, all the external forces oppose motion as shown in Figure 7.