汽车制动系统-毕业设计外文资料翻译
Automobile Brake System
The braking system is the most important system in cars. If the brakes fail, the
result can be disastrous. Brakes are actually energy conversion devices, which
convert the kinetic energy (momentum) of the vehicle into thermal energy
(heat).When stepping on the brakes, the driver commands a stopping force ten
times as powerful as the force that puts the car in motion. The braking system
can exert thousands of pounds of pressure on each of the four brakes.
Two complete independent braking systems are used on the car. They are the
service brake and the parking brake.
The service brake acts to slow, stop, or hold the vehicle during normal driving.
They are foot-operated by the driver depressing and releasing the brake pedal.
The primary purpose of the parking brake is to hold the vehicle stationary while
it is unattended. The parking brake is mechanically operated by when a separate
parking brake foot pedal or hand lever is set.
The brake system is composed of the following basic c omponents: the “master cylinder” which is located under the hood, and is directly connected to the brake pedal, converts driver foot’s mechanical pressure into hydraulic pressure. Steel “brake lines” and flexible “brake hoses” connect the master cylinder to the cylinders” located at each wheel. Brake fluid, specially designed to work in
extreme conditions, fills the system. “Shoes” and “pads” are pushed by cylinders to contact the “drums” and “rotors” thus causing drag, which (hopefully) slows the car.
The typical brake system consists of disk brakes in front and either disk or
drum brakes in the rear connected by a system of tubes and hoses that link the
brake at each wheel to the master cylinder (Figure).
Basically, all car brakes are friction brakes. When the driver applies the brake, the control device forces brake shoes, or pads, against the rotating brake drum or disks at wheel. Friction between the shoes or pads and the drums or disks then slows or stops the wheel so that the car is braked.
In most modern brake systems (see Figure 15.1), there is a fluid-filled cylinder, called master cylinder, which contains two separate sections, there is a piston in each section and both pistons are connected to a brake pedal in the driver compartment. When the brake is pushed down, brake fluid is sent from the master cylinder to the wheels.
At the wheels, the fluid pushes shoes, or pads, against revolving drums or disks. The friction between the stationary shoes, or pads, and the revolving drums or disks slows and stops them. This slows or stops the revolving wheels, which, in turn, slow or stop the car.
The brake fluid reservoir is on top of the master cylinder. Most cars today
have a transparent reservoir so that you can see the level without opening the cover. The brake fluid level will drop slightly as the brake pads wear. This is a normal condition and no cause for concern. If the level drops noticeably over ashort period of time or goes down to about two thirds full, have your brakes checked as soon as possible. Keep the reservoir covered except for the amount of time you need to fill it and never leave a cam of brake fluid uncovered. Brake
fluid must maintain a very high boiling point. Exposure to air will cause the fluid
to absorb moisture which will lower that boiling point.
The brake fluid travels from the master cylinder to the wheels through a series
of steel tubes and reinforced rubber hoses. Rubber hoses are only used in places that require flexibility, such as at the front wheels, which move up and down as well as steer. The rest of the system uses non-corrosive seamless steel tubing with special fittings at all attachment points. If a steel line requires a repair, the
best procedure is to replace the compete line. If this is not practical, a line can be repaired using special splice fittings that are made for brake system repair. You must never use copper tubing to repair a brake system. They are dangerous and illegal.
Drum brakes, it consists of the brake drum, an expander, pull back springs, a stationary back plate, two shoes with friction linings, and anchor pins. The stationary back plate is secured to the flange of the axle housing or to the
steering knuckle. The brake drum is mounted on the wheel hub. There is a clearance between the inner surface of the drum and the shoe lining. To apply brakes, the driver pushes pedal, the expander expands the shoes and presses them to the drum. Friction between the brake drum and the friction linings brakes the wheels and the vehicle stops. To release brakes, the driver release the pedal, the pull back spring retracts the shoes thus permitting free rotation of the wheels.
Disk brakes, it has a metal disk instead of a drum. A flat shoe, or disk-brake pad,
is located on each side of the disk. The shoes squeeze the rotatin g disk to stop
the car. Fluid from the master cylinder forces the pistons to move in, toward the disk. This action pushes the friction pads tightly against the disk. The friction between the shoes and disk slows and stops it. This provides the braking action. Pistons are made of either plastic or metal. There are three general types of disk brakes. They are the floating-caliper type, the fixed-caliper type, and the
sliding-caliper type. Floating-caliper and sliding-caliper disk brakes use a single piston. Fixed-caliper disk brakes have either two or four pistons.
The brake system assemblies are actuated by mechanical, hydraulic or pneumatic devices. The mechanical leverage is used in the parking brakes fitted
in all automobile. When the brake pedal is depressed, the rod pushes the piston
of brake master cylinder which presses the fluid. The fluid flows through the pipelines to the power brake unit and then to the wheel cylinder. The fluid
pressure expands the cylinder pistons thus pressing the shoes to the drum or disk.
If the pedal is released, the piston returns to the initialposition, the pull back
springs retract the shoes, the fluid is forced back to the master cylinder and
braking ceases.
The primary purpose of the parking brake is to hold the vehicle stationary while
it is unattended. The parking brake is mechanically operated by the driver when a separate parking braking hand lever is set. The hand brake is normally used
when the car has already stopped. A lever is pulled and the rear brakes are approached and locked in the “on” position. The car may now be left without fear of its rolling away. When the driver wants to move the car again, he must
press a button before the lever can be released. The hand brake must also be able
to stop the car in the event of the foot brake failing. For this reason, it is separate
from the foot brake uses cable or rods instead of the hydraulic system.
Anti-lock Brake System
Anti-lock brake systems make braking safer and more convenient, Anti-lock
brake systems modulate brake system hydraulic pressure to prevent the brakes
from locking and the tires from skidding on slippery pavement or during a panic
stop.
Anti-lock brake systems have been used on aircraft for years, and some domestic
car were offered with an early form of anti-lock braking in late 1990’s. Recently, several automakers have introduced more sophisticated anti-lock system. Investigations in Europe, where anti-lock brakin g systems have been available
for a decade, have led one manufacture to state that the number of traffic
accidents could be reduced by seven and a half percent if all cars had anti-lock brakes. So some sources predict that all cars will offer anti-lock brakes to
improve the safety of the car.
Anti-lock systems modulate brake application force several times per second to
hold the tires at a controlled amount of slip; all systems accomplish this in
basically the same way. One or more speed sensors generate alternating current
signal whose frequency increases with the wheel rotational speed. An electronic
control unit continuously monitors these signals and if the frequency of a signal
drops too rapidly indicating that a wheel is about to lock, the control unit
instructs a modulating device to reduce hydraulic pressure to the brake at the
affected wheel. When sensor signals indicate the wheel is again rotating
normally, the control unit allows increased hydraulic pressure to the brake. This
release-a pply cycle occurs several time per second to “pump” the brakes like driver might but at a much faster rate.
In addition to their basic operation, anti-lock systems have two other things in
common. First, they do not operate until the brakes are applied with enough
force to lock or nearly lock a wheel. At all other times, the system stands ready
to function but does not interfere with normal braking. Second, if the anti-lock
system fail in any way, the brakes continue to operate without anti-lock
capability. A warning light on the instrument panel alerts the driver when a
problem exists in the anti-lock system.
The current Bosch component Anti-lock Braking System (ABSⅡ), is a second generation design wildly used by European automakers such as BWM,
Mercedes-Benz and Porsche. ABSⅡ system consists of : four wheel speed
sensor, electronic control unit and modulator assembly.
A speed sensor is fitted at each wheel sends signals about wheel rotation to
control unit. Each speed sensor consists of a sensor unit and a gear wheel. The
front sensor mounts to the steering knuckle and its gear wheel is pressed onto the
stub axle that rotates with the wheel. The rear sensor mounts the rear suspension member and its gear wheel is pressed onto the axle. The sensor itself is a
winding with a magnetic core. The core creates a magnetic field around the winding, and as the teeth of the gear wheel move through this field, an alternating current is induced inthe winding. The control unit monitors the rate o change in this frequency to determine impending brake lockup.
divided into three parts: signal processing,
The control unit’s function can be
logic and safety circuitry. The signal processing section is the converter that receives the alternating current signals form the speed sensors and converts them into digital form for the logic section. The logic section then analyzes the
digitized signals to calculate any brake pressure changes needed. If impending lockup is sensed, the logic section sends commands to the modulator assembly. Modulator assembly
The hydraulic modulator assembly regulates pressure to the wheel brakes when it receives commands from the control utuit. The modulator assembly can maintain or reduce pressure over the level it receives from the master cylinder, it also can never apply the brakes by itself. The modulator assembly consists of three
high-speed electric solenoid valves, two fluid reservoirs and a turn delivery
pump equipped with inlet and outlet check valves. The modulator electrical connector and controlling relays are concealed under a plastic cover of the assembly.
Each front wheel is served by electric solenoid valve modulated independently
by the control unit. The rear brakes are served by a single solenoid valve and modulated together using the select-low principle. During anti-braking system operation, the control unit cycles the solenoid valves to either hold or release pressure the brake lines. When pressure is released from the brake lines during anti-braking operation, it is routed to a fluid reservoir. There is one reservoir for the front brake circuit. The reservoirs are low-pressure accumulators that store
fluid under slight spring pressure until the return delivery pump can return the fluid through the brake lines to the master cylinder.
汽车制动系统
制动系统是汽车上最重要的系统。如果制动系统坏了，结果会很严重。
制动器实际上是能量转化装置，制动器能将车辆的动能（动量）转化为热能（热）。当司机踩制动踏板时，司机施加的使汽车停下的力相当于推动汽车
的力的十倍。制动系统能施加在每个制动器上的力有几千磅。
汽车上装有两套完全独立的制动系统，即行车制动系统和驻车制动系统
行车制动用来减速、停车或者控制正在正常行驶的车辆，它是通过司机踩下和释放刹车踏板来控制的。驻车制动的主要目的是使车辆在无人看管时
保持静止状态。驻车制动是机械式操作，司机可以通过一个独立的驻车制动
刹车脚踏板或者刹车手柄来操控。
制动系统由下列基本部分组成：“制动主缸”，安装在发动机罩的下面，直接与制动踏板相连，将司机脚部的机械压力转化为液压压力。钢制的“制动管路”和柔韧的“制动软管”将制动主缸与安装在各个车轮上的“制动轮缸”连接起来。制动液，特别设计工作在极端环境下，充满制动系统。“制动蹄”和“制动衬块”在制动轮缸的推动下分别与“制动鼓”和“制动盘”接触而产生阻力，从而（有希望）使汽车减速。
典型的制动系统由前部的盘式制动器和后部的盘式或鼓式制动器组成，
制动器用管道与制动主缸相连。
基本上，所有的汽车制动器都是摩擦式制动器。当司机开始制动时，控制装置迫使制动蹄或者制动衬块挤压车轮上的旋转的制动鼓或者制动盘。制动蹄与制动鼓或者制动衬块与制动盘之间的摩擦使车轮减速或者停下，从而使汽车被刹住。
大多数现代制动系统里都有一个充满液体的缸体，叫做制动主缸，它包含两个独立的部分，每个部分都有一个活塞，每个活塞都和司机驾驶室的刹
车踏板相连。当踩下制动踏板时，制动液就被从制动主缸压到各个车轮。
在各个车轮处，制动液推动制动蹄或制动衬块与旋转的制动鼓或制动盘
接触。静止的制动蹄或制动衬块与转动的制动鼓或制动盘之间的摩擦，使制
动鼓或制动盘减速或停止，这将使车轮减速或者停止，进而使汽车减速或者停止。
制动液储液罐在制动主缸的顶部。今天大部分的汽车的制动液储液罐都是透明的，以便你不必打开盖子就能看到液面的高度。制动液的液面会随着制动衬块的摩擦而下降，这是正常状况不必担心。如果液面高度在短时间内明显下降或者下降了三分之二，请尽快检查你的制动系统。保持制动液储液罐的盖子是关闭状态，除非你需要加油，绝不要让凸轮制动液暴漏着。制动液必须保持非常高的沸点，暴漏在空气中将造成制动液吸收水分而沸点降低。
制动液通过一系列钢制管路和强化的橡胶管从制动主缸到达车轮处。橡胶管路仅仅用在对柔韧性有要求的地方，如前轮处，管路会随着形式上下移动。剩下的系统管路用不锈无缝钢管通过特殊配件相连。如果钢制管路需要修理，最好的方式是更换相同的管路，如果这不实际，也可以用为制动系统维修专门生产的专用接头配件来修理。决不能用铜制管路来维修制动系统，
那样很危险而且违反规定。
鼓式制动器，包括制动鼓、一个制动轮缸、回位弹簧、一个制动底板、
两个带摩擦片的制动蹄和支承销。制动底板固定在驱动桥壳的法兰盘上或者转向节上。制动鼓安装在轮毂上。制动鼓的内表面与制动蹄摩擦片之间有一段间距。制动时，司机踩下踏板，制动轮缸是制动蹄张开压在制动鼓上。制
动鼓与摩擦片之间的摩擦使车轮制动，使车辆停止。解除制动时，司机放开踏板，回位弹簧拉回制动蹄从而使车轮能自由转动。
盘式制动器，有一个金属盘代替了制动鼓。制动盘两侧各装有一个平的制动蹄或者叫盘式制动摩擦衬块。制动衬块挤压旋转的制动盘使汽车停止。
来自制动主缸的制动液迫使轮缸的活塞向制动盘移动，从而推动摩擦衬块紧紧地压在制动盘上。摩擦衬块与制动盘之间的摩擦使制动盘减速和停止，这就是制动的过程。活塞是塑料或者金属制的。盘式制动器有三种基本类型，
浮钳盘式制动器、定钳盘式制动器和滑动钳盘式制动器。浮钳盘式制动器和滑动钳盘式制动器用一个活塞，定钳盘式制动器有两个或者四个活塞。
制动系统装配组件驱动方式有机械式、液压式或者气压式。机械式杠杆机构适用于所有汽车的驻车制动。当踩下制动踏板时，推杆推动制动主缸的活塞移动，制动液从油管流到动力制动单元再流到制动轮缸，制动液的压力
推开轮缸的活塞，从而使摩擦片挤压制动鼓或制动盘。如果放开制动踏板，
活塞回到初始位置，回位弹簧使制动蹄回位，制动液被压回主缸，制动停止。
驻车制动的主要作用是使车辆在无人看管时保持静止，驻车制动是机械式操作，司机通过一个独立的制动手柄来操纵。手刹通常在汽车已经停下时使用。当拉动手刹时，后制动系统被刹住并被锁在“打开”的位置，现在可以把汽车扔下而不用担心它自己移走了。当司机想要再次移动汽车时，他必须按一个按钮才能使手刹拉杆松开。手刹还必须能在脚刹失灵的情况下使汽车停止，因此，手刹系统采用拉索或杠杆，不同于脚刹采用液压系统。
防抱死制动系统使制动更加安全和方便，防抱死制动系统调节制动系统液压，防止制动器锁死，防止轮胎在光滑路面上或紧急制动时发生打滑。
防抱死制动系统已经用在航空飞行器上多年，90年代末期，一些国产汽车装备了早期形式的防抱死制动系统。现在，一些汽车制造商已经引进了更先进的防抱死制动系统。在防抱死制动系统已经应用十年的欧洲进行的一
项调查说明，如果所有的车都有防抱死制动系统，交通事故的数量将可以减少7.5%，所以一些人预计将来所有的车都会装上防抱死制动系统，以提高
汽车的安全性。
防抱死系统每秒调节制动作用力几次，以把轮胎的滑移量控制在一定范围内；基本上所有的系统都是以这种方式来完成的。一个或多个速度传感器产生交流信号，交流信号的频率随着车轮转速的提高而增大一个电控单元不
断监测着这些信号，如果其中一个信号的频率下降的太迅速，表明一个车轮将要抱死，电控单元命令调节装置减小受影响车轮的制动液压。当传感器信号表明车轮仍在正常转动时，控制单元允许制动器增大液压，这种收放循环每秒发生几次，对制动器的控制类似于司机反复释放和踩下踏板，但是速度更快。
除了他们的基本作用，关于防抱死系统还有另外两件事。第一，防抱死系统直到制动器抱死车轮才起作用，在其他时间，防抱死系统做好运行的准备，但是不干涉正常制动。第二，如果防抱死系统失灵了，制动器没有了防
抱死能力仍然能继续工作，当防抱死系统存在问题时，仪表盘上的一个警示灯会提示司机。
欧洲汽车制造商，如宝马、奔驰和保时捷，广泛使用的是博世的第二代
防抱死制动系统ABSⅡ。ABSⅡ系统包括：四个车轮转速传感器、电子控
制单元和调节装置。
装在每个车轮上的轮速传感器将车轮转动信号送到控制单元，每个速度传感器包括一个磁感应传感头和一个齿圈。前轮传感器安装在转向节上，它的齿圈安在转向轴上，随车轮一起旋转。后传感器安装在后悬架部件上，它的齿圈安装在车轴上。传感器是一个有磁芯的线圈，磁芯产生一个环绕线圈的磁场，当齿圈的齿穿过磁场时，线圈中会产生交流电。控制单元监测着频率变化的速度，以判断什么时候即将抱死。
控制单元的作用可以分为三部分：信号处理，逻辑计算和安全电路。信号处理部分是将从轮速传感器接收到的交流信号转换成数字形式，为逻辑计算做准备。然后逻辑计算部分分析这些数字化的信号，计算出制动压力需要的改变。如果觉察到即将抱死，逻辑计算部分将向液压调节器发出命令。
当液压调节器接受到控制单元的命令时，将会调节车轮制动器中的液压。液压调节器能保持或者减少来自制动主缸的压力，但是它不能自己启动制动器。液压调节器包括三个高速的电磁阀、两个储液器和一个装有进出口止回阀的输出泵。调节器的电子连接器和控制继电器隐蔽在塑料盖子下。
每个前轮都有一个电磁阀，受控制单元独立调节。两个后轮则只有一个电磁阀，采用低选择原则共同调节。在防抱死制动系统起作用时，控制单元循环控制电磁阀保持或者减小制动系统的液压，当防抱死期间制动系统液压被降低时，制动液被压入储液器。前制动线路有一个储液罐。储液罐是低压蓄能器，在低压下储存油液，直到回流泵将油液从管路送回制动主缸。