0. Introduction

Auxiliary braking device^[1] is used to decelerate and control the speed of moving automobile(especially down long slope). The absorbed power of auxiliary brake in a short time is relatively small, but it is unchangeable (or basically unchanged)in a long time. As a braking system component, it can improve the reliability of automotive braking system, alleviate the main braking system load of vehicle and extend the service life of braking system. Some statistics show that because of the use of retarder, the average failure rate of driving brake can reduce by 48. 12%, the consumptions of braking disk and brake drum reduce by 42. 04% and 50. 78% separately. The brake disk life expectancy of passenger bus with retarder is about(15~ 75)× 10⁶ km. On the other hand, the brake disk life expectancy of passenger bus without retarder is only about(5~ 25)× 10⁶ km.

At present, main auxiliary braking devices with mature technology and suitable installation are engine retarder, engine exhaust brake, eddy current retarder(ECR), permanent magnet retarder(PMR) and hydraulic retarder(HR)^[1]. Engine retarder and engine exhaust brake achieve the deceleration by making the engine do negative power. They are generally applicable to diesel engine. But the braking torques of the others act on driving shaft directly to reduce automotive speed.There is no restriction on engine type for the others.

Auxiliary brake system is regarded as an essential for commercial automobile according to the traffic laws in many countries. The traffic laws in Germany require that a passenger bus with total load more than 5. 5 t and a truck with more than 9 t, must install auxiliary brake. Many buses, construction automobiles, heavy transport automobiles are equipped with hydraulic retarder in Nordic, the western United States, Japan and other hilly areas.

July 1, 2002, Ministry of Transportation of PRC implemented traffic industry standard JT/T325— 2002. It provides that high-Ⅱ level of medium-sized passenger cars, high-Ⅰ, high-Ⅱ, high-Ⅲ level of large passenger cars and the super-sized passenger cars should be equipped with necessary retarder. Ministry of construction of PRC promulgated and implemented industry standard CJ/T162— 2002 in 2002.It provides that ultra-Ⅱ, ultra-Ⅰ, advanced urban and suburb bus should be installed with retarder.

1. Structures and working principles of auxiliary braking devices

1.1 Engine brake

Engine brake device mainly consists of block, solenoid valve, control valve, adjusting screw, driven piston, active piston and other major components. The role of block is to integrate these components of engine brake device into an assembly, as shown in Fig. 1. Solenoid valve is mainly used to conduct engine oil. Control valve produces low-pressure oil area and high-pressure area during engine braking. Driven piston is the component of implement during engine braking. Active piston produces high pressure in high-pressure oil region. The gap between driven piston and engine exhaust with T-shaped platen is adjusted by adjustment screw.

Figure  1.  Engine brake device

1-Solenoid valve; 2-Low-pressure oil; 3-Adjusting screw; 4-High-pressure oil; 5-Active piston; 6-Rocker arm adjusting screw; 7-Strut; 8-Exhaust rocker arm; 9-Exhaust valve; 10-T-shaped platen of exhaust; 11-Driven piston; 12-Spherical check valve; 13-Engine oil port

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Engine brake has many advantages^[2].Firstly, the manipulation is convenient and timely, as long as driver releases accelerator pedal, engine brake will play a role. Secondly, the braking torque acting on wheel is even, especially, be suitable in muddy and snow-clad roads. It can reduce or avoid the deviation and sideslip caused by the differences of braking torques on both sides of wheels.

However, the brake torque of engine brake varies with shift change. It will be high when speed is low(at lower shift).Moreover, its torque cannot be adjusted.

1.2 Engine exhaust brake

The working principle of engine exhaust brake is the obstruction of butterfly throttle valve in the exhaust channel(Fig. 2) and stopping the oil, so that engine compresses air during the process of compression and exhaust^[3]. By increasing the exhaust resistance of piston and crankshaft, the engine becomes an air compressor that absorbs kinetic energy, increases power consumption and forces engine to reduce speed. It will slow down driving wheel rotation through power train, and achieve the purpose to reduce automotive speed.

Figure  2.  3D model of engine exhaust brake valve

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The most common exhaust brake form is the electromagnetic pressure control, which is shown in Fig. 3. Three pneumatic cylinders(2, 4 and 14) respectively control exhaust brake valve 3, inlet silencing valve 1 and flameout manipulation arm 13. Regular closed solenoid valve 15 is in series control circuit involving exhaust brake switch 7, clutch switch 10 and accelerate switch 11.It also controls compressed air pipeline from storage pot 5 to pneumatic cylinder. Any one of disconnect switches will cause solenoid valve turn off and relieve exhaust brake.

Figure  3.  Exhaust brake device with electromagnetic air pressure control

1-Inlet silencing valve; 2-Pneumatic cylinder; 3-Exhaust brake; 4-Pneumatic cylinder; 5-Storage pot; 6-Battery; 7-Exhaust brake switch; 8-Signal lights; 9-Clutch pedal; 10-Clutch switch; 11-Accelerate switch; 12-Fuel control arm; 13-Flameout manipulation arm; 14-Pneumatic cylinder; 15-Solenoid valve

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While accelerator pedal is relaxed, exhaust brake switch 7 is closed, which lightens signal lights 8. A loop circuit, involving clutch switch 10, solenoid valve coil 15 and accelerate switch 11, is formed at last. Solenoid valve 15 has a suction force which can turn off exhaust pipe and open intake airway. Then three pneumatic cylinders are filled with compress air. The implementation of exhaust brake is done by stopping diesel engine and shutting off the intake and exhaust pipe of engine. Engine speed lower rapidly so that automotive speed reduces quickly.

Engine exhaust brake structure is simple, light weight, low cost, easy operation, so it is widely installed on medium and heavy diesel automobiles. But they are different for petrol automobiles, because of small engine compression ratio, relatively low braking efficiency, complex structure, as well as valves being prone to die.

Engine exhaust brake application is earlier in China, and many heavy-duty trucks have equipped engine exhaust brake.

1.3 Eddy current retarder

The principle of eddy current retarder (ECR)^[4-6] is transforming automobile kinetic energy into thermal on the basic of electromagnetic theory to achieve automobile braking. Eddy current is generated on rotor which rotates in a magnetic field. The heat is disseminated through the convection and radiation into surrounding environment. Eddy current retarders are divided into disc-type retarders and drum-type retarders in according with the structural arrangements of stator and rotor.

The mechanical structure of disc-type eddy current retarder is shown in Fig. 4, which comprises stator and rotor components. Stator with eight electromagnetic coils is fixed on automotive chassis(frame, rear axle gearbox casing or shell)through bracket. The magnetic polarities of two adjacent wire coils are opposite. Rotor connected by transition flange is distributed at each side of stator. There is air duct on rotor to facilitate heat dissipation.

Figure  4.  Disc-type ECR structure

1-Rotor disk; 2-Core; 3-Excitation coil; 4-Rotor axis; 5-Bearings; 6-Fixation; 7-Airgap; 8-Terminal

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The part mechanical structure of drum-type ECR is shown in Fig. 5, 6, which also comprises stator and rotor components. Stator generally has 8, 10 or 12 high-permeability cores with radial distribution. Excitation windings setting up on the core form magnetic poles. Two adjacent excitation windings with opposite poles are in series or in parallel. The formation of magnetic pole is independent each other. Rotor is cylindrical structure, it is connected with shaft rotating freely through connecting flange. In order to distribute the heat generated by eddy current promptly, rotor usually has many cast heat dissipation skewed slots.

Figure  5.  Physical map of drum-type ECR

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Figure  6.  Model of drum-type ECR

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There are many advantages of ECR: simple structure, quick response, no significant time lag, low production cost, adjustable braking torque and being easy to achieve intelligent control. In addition, there is no contact friction, no wear, low failure rate, easy maintenance and high reliability.

However, its deceleration capacity and service time are restricted by rotor temperature. The retarder with larger volume and higher quality is impacted by its surrounding airflow conditions and ambient temperature, but also consumes a certain amount of electrical energy.

At present, ECR is installed on many large and medium-sized luxury passenger buses, such as French Telma, Spaish Frenelsa, Japanese Hino, Shanghai Shenwo, Zhengzhou Yutong, Dongfeng Nissan, Xiamen Jinlong, Suzhou Jinlong and so on. The manufacturers of ECR in China are Shenzhen Terca Traffic Technology Co., Ltd., Huaian Huiming Auto Parts Manufacturing Co., Ltd., Yangzhou Hongquan Industrial Co., Ltd., Zhejiang Wanan Group Co., Ltd., Zhejiang Shengzhou Boat Lake Auto Parts Factory, Wuxi Sansheng Technology Co., Ltd., Jiangsu Chaoli Electric Co., Ltd., Yangzhou Hualu Electrical Ltd., Jinan Engine Plant of China National Heavy Duty Truck Group Co., Ltd., Zhejiang Ruili Group, Jiaxing City Newman Machinery Co., Ltd. and so on.

1.4 Permanent magnet retarder

The working principle of permanent magnet retarder(PMR)is similar to ECR's. It is just that rotor rotates in magnetic field, cuts the magnetic lines, and the eddy current magnetic force generated by magnetic field forms reversal torque to reduce shaft speed. The magnetic field of eddy current retarder is generated from excitation coil by controlling the on-off of windings to control the retarder. However, the magnetic field of PMR is generated by its inherent powerful magnets. The movement of permanent magnet can form moving magnetic field to control the work of retarder.At a work state, magnetic lines generated by adjacent magnets on a separate stent form closed loops traversing stator's magnetic pole pieces and rotor. Eddy current generated on rotor forms braking torque. At a disconnect state, the magnetic force lines must be shielded so that rotor is free from magnetic field by rotating magnet stent position. An activity permanent magnet changes its location through pneumatic device to form two working states: opening and disconnecting. Fig. 7 shows that the opening and disconnecting states of the magnetic circuit of permanent magnet retarder.

Figure  7.  Opening and disconnecting states of magnetic circuit of PMR

1-Ferromagnetic material; 2-Non-magnetic material; 3-Rotary drum; 4-Leakage magnetic line; 5-Permanent magnet; 6-Magnetic line

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Permanent magnet retarder includes two parts: rotor and stator. According to the shape of rotor, the structures of PMR^[7-8] are divided into drum-type and disc-type. The braking torque of disc-type permanent magnet retarder is only 700~ 820 N·m, and its residual hysteresis drag torque generally reaches 10 N·m. Because of its low braking torque and a certain hysteresis drag torque, its application is not extensive. Now, drum-type retarder is widely used in foreign countries.

The drum-type PMRs are divided into three types: magnet axial sliding type, magnet cage circumferential rotating type and magnet circumferential rotating type. Besides of the structures, the main differences between magnet axial sliding type and magnet cage circumferential rotating type are that the later's braking torque is about higher 12% than the former's. When disconnected, the former has no magnet axial sliding residual drag-free torque, but the later remains to 10~ 15 N·m. The former's temperature rises greatly, while the later's temperature is not obvious. Currently, large and medium-sized luxury buses in Japan are usually equipped magnet cage circumferential rotating type.

PMR does not consume energy because of the use of permanent magnets as excitation source, can maintain a constant power and durability in continuous work, and does not overheat. Its maintenance is easy. It only need to regular inspect its airgap. However, its magnetic field is limited, so the brake power is not large. The field is constant, so its braking torque cannot be altered.

At present, the manufacturers of permanent magnet retarder are Japanese Isuzu/Sumitomo Corporation and so on. In China, only Gala's bus produced by Guangzhou Isuzu Bus Co., Ltd. installs the PMR of Isuzu/Sumitomo Corporation. Jiangsu University together with Yangzhou Hongquan Industrial Co., Ltd. has developed a permanent magnet retarder already.

1.5 Hydraulic retarder

Hydraulic retarder is a device that converts automotive kinetic energy into liquid heat with the interaction between liquid and leaves to achieve braking. The main parts are fixed impeller and rotating impeller, which generally are installed on transmission. When it works, rotating impeller rotates driven by drive axle and transmission, and fixed impeller generates resistance torque to rotating impeller through liquid flow, so that automotive speed is reduced^[9].

The working principle(Fig. 8) of hydraulic retarder(Fig. 8) is that its rotor rotates with transmission output shaft. When there is no oil, rotor idles, the retarder is not at work state. When the retarder is filled with oil, a moment of momentum brought by rotor acts on oil, and drives oil to rotate. Oil rotating within the circle along leaves is thrown round wizard. At the same time, a fixed guide pulley leaves also produce an angular momentum effecting on oil. Oil outflows guide pulley and then flows into rotor, which forms the resistance moment acting on rotor to clog rotor's rotation, so that automotive deceleration is realized. The rotational energy of rotor is transformed into heat with the damping effect of oil, and is emitted into air through radiator. The nature of reverse torque is the continuous and smooth damping of oil. In addition, the decelerating torque acting on transmission's output shaft will not cause wheel's locking. Therefore, the braking impulsion of automobile is reduced greatly while braking.

Figure  8.  Working principle of hydraulic retarder

1-Stator; 2-Rotor; 3-Shell; 4-Radiator; 5-Control valves system; 6-Pump; 7-Tank

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Hydraulic retarder is usually connected to the power input shaft of automotive cardan shaft. The driver can control the fluid-filled volume of hydraulic retarder by manipulating button and pneumatic valves, which are next to steering wheel. It can impose different braking torques to limit or slow down automotive speed according to a need, so driver can ensure automotive safety in mountains and a smooth deceleration while braking. The heat transformed by hydraulic retarder together with engine heat, is cooled by radiator and fan.

The braking torque of Hydraulic retarder is proportional to the square of rotor speed, and to the fifth power of retarder's cavity effective diameter. Therefore, hydraulic retarder can provide a greater braking torque compared with other retarders when automobile running at high speed. The feature of the type is as follows: no mechanical components wear, low noise, braking stability, long life, but a long response time.

At present, German ZF and Voith have produced hydraulic retarder, and their branches have been built in Shanghai and Suzhou separately. In China, Jilin University, Beijing Institute of Technology, Jiangsu University and other universities are engaged in researching hydraulic retarder.Shenzhen Terca has produced a prototype of hydraulic retarder. Many bus companies have already equipped hydraulic retarder on automobile, such as Zhengzhou Yutong, Suzhou Jinlong, Anhui Ankai and so on.

2. Auxiliary braking key technologies

2.1 Improvement technology of auxiliary braking torque^[10-11]

While engine auxiliary brake is at work state, engine becomes an air compressor that generates a reverse torque and consumes automotive energy to achieve a braking goal. To realize engine auxiliary brake by improving engine components and adding other parts primarily, many higher reliability requirements have been put forward on them. The invalidation of the implementation should be prevented to ensure the normal operation of engine.

Other retarders make effect through converting automotive kinetic energy into heat and being distributed, thus it must make sure the energy conversion efficiency. The magnetic induction intensity of rotor should be properly improved. Because of the demagnetization and magnetic saturation effects of eddy current, when the magnetic induction intensity reaches a certain value, the braking torque will decline by the study of electromagnetic distribution. It is well known that the density distribution of eddy current is not even, but as the law of exponential decay. Because of the skin effect of eddy current, eddy current is mainly distributed in the vicinity of the magnetic pole surface of rotor. Its penetration depth is very small, and reduces with the improving speed of rotor. Not only the materials of stator iron core, permanent magnet and rotor, but also the airgap between rotor and stator has a direct impact on the intensity of eddy current.

Stator core and rotor are kinds of soft magnets, made of soft magnetic material, which is one type of ferromagnetic materials. In order to guarantee the reliable performance of the retarder, it is better to select the low working point of soft magnet for the rich margin of braking power. Low-carbon steel or alloy steel and other materials are often chosen as rotor materials. There are many characteristics of alloy steel, such as high magnetic permeability, low coercivity, anti-fatigue, thermal stability, corrosion resistance and so on. In order to increase rotor life, the unfavorable factors, such as the stress and magnetic effect, can be eliminated through a specific heat treatment process.

The airgap between stator and rotor is one of important structural parameters, because the relative air permeability rate is 1, even a very small airgap will also generate a great reluctance. Therefore, it is better that the airgap is smaller. Smaller airgap can make critical speed down, and make the biggest braking torque transfer to low speed direction possibly, but also improve the maximum braking torque. An appropriate airgap is generally between 0. 76 to 1. 70 mm.

The 0. 15 mm copper layer is plated on the surface of low-carbon steel as rotor, so that the electric conductivity of rotor increases, and eddy current and braking torque are correspondingly increases. In fact, peak torque may increases by 40% within normal operating speed range of automobile. For particular design, peak torque can reach a maximum value for a copper layer thickness of 0. 5 mm. Copper layer concentrates current because of the higher conductivity of copper relative to steel. Copper layer can make braking torque increase, because the concentrated current increases linearly with the layer thickness, while the heat goes as the square of the current. When the layer thickness increases, machine airgap effectively increases(copper has a permeability in essential vacuum), which decreases the coupling capability of stator and rotor, and decreases induced current. Hence, there exists a real layer thickness that leads to a maximum peak torque.

2.2 Improvement technology of cooling capacity of auxiliary brake

Devisers must also improve the heat dissemination rate while enhancing the energy conversion efficiency of ECR. ECR and PMR generate a torque to hinder transmission shaft's rotation through eddy current's induction. Rotor structural design^[12] will affect the cooling capacity of the retarder. With the rise of rotor temperature, the braking torque will decline. The reasons are as follows: when the electrical conductivity of rotor decreases, relative permeability becomes smaller, the magnetic flux leakage of rotor disk increases, and magnetic induction intensity gradually becomes smaller. At the same time, there are magnetic effect, magnetic saturation and other factors. At present, the designers adopt natural air-cooled(air duct, cooling leaves)form on eddy current's retarder. A perfected design for cooling duct and leaves will increase heat exchange rate between air and rotor.

2.3 Reducing technology of residual hysteresis drag torque for permanent magnet retarder

In order to solve the hysteresis drag torque problem of permanent magnet retarder because of the rise of magnetic flux leakage, Kawasaki and Sakamoto design a magnet circumferential rotating type retarder, which is currently used by most manufacturers. He Ren from Jiangsu University in China has developed a magnet circumferential rotating type retarder with hierarchical control. Its stator structure is improved. The type retarder will be expected to increase braking torque by 12% than before. Recently, Kanagawa has invented a new type of PMR. Not only braking torque is enhanced, but also there is not residual hysteresis drag torque while releasing brake.

2.4 Matching technology between auxiliary brake performance and automobile

It is noted that clutch should reasonably match with the performance of whole automobile with auxiliary brake. The reserve coefficient of clutch should be increased appropriately. The inappropriate selection of the reserve coefficient or the unreasonable match between clutch and the retarder, as well as the improper use of the retarder will greatly increase the slipping probability of clutch, shorten the life of clutch, and even burn clutch plate frequently.

To automobile with auxiliary brake, there are special requirements for the design of main reducer gear and its mounting shaft in order to improve the life of rear axle. The driving and driven gears of main reducer will mesh in positive and negative states alternatively when retarder works and does not work. In the alternative process, the stress states of gears will change, and then impact the loads of gears. On the other hand, the noise of reducer will be high when the gears oppositely mesh. At the same time, the changes of gear force states will impact the mounting shafts of the gears and other certain parts.

Driver should operate eddy current retarder with caution when automobile runs on bad road conditions. Driver should not let the retarder be on high shift directly or only use the retarder to reduce automotive speed, especially at higher speed^[13]. When main braking system and the retarder work simultaneously, ABS should stop working automatically. The reason is that when automobile is running on the road with lower adhesion coefficient, and the retarder works suddenly, there is likely to make shaft be locked. ABS would make main brake completely release, thus main brake torque will disappears.

3. Technical features and effectiveness indexes of auxiliary brake

3.1 Technical features

In general case, the auxiliary brake can meet the requirements of smaller braking intensity.It will be beneficial to reduce the temperature of master brake for commercial automobile or urban public bus, and to improve their driving safety. In emergency case, it can improve automotive braking intensity while auxiliary brake and main brake working at the same time. Literature[5] analyzes the distribution curve of ideal braking force for automobile with auxiliary brake when emergency braking. When auxiliary brake works, the total braking torque of rear wheels increases, which increases the locked possibility of rear wheels so that automotive heading angle α becomes larger. But generally, automotive braking stability will not be impacted too much, as long as whole automobile and optional auxiliary braking devices match properly.

3.2 Effectiveness indexes^[14]

The basic performance requirements of auxiliary brake are as follows: good braking performance at low speed, quick responds system, smooth brakeing torque, good thermal decay performance and so on. According to the requirements, the evaluation indexes of auxiliary braking performance are proposed to evaluate the merits of its performance and to provide a theoretical basis for the study of its test methods.

(1) Rated braking torque M_max.The larger the largest braking torque generated by retarder is, the bigger the deceleration generated by instant braking force is, the better the performance of eddy current retarder is.

(2) Average braking torque M. During deceleration, greater average brake torque means that the braking power is greater, the energy consumption per unit time is larger, as well as the performance of retarder is better.

(3) Thermal decay coefficient K. Thermal decay coefficient reflects the braking torque of retarder decreases with temperature rise level. The smaller thermal decay coefficient means the smaller drop of braking torque at high temperature, and the better thermal decay performance of retarder.

(4) Braking efficiency. Greater braking efficiency means that the braking power of retarder is bigger while inputted the same electric power, and the performance of retarder is better.

(5) Braking performance of unit mass. It is just the ratio of retarder's rated braking torque to retarder's quality. Bigger braking efficiency means the better performance of retarder.

4. Development trends of auxiliary brake technology

4.1 Combined control technology of auxiliary braking device and main braking system^[15-17]

(1) With the development of electronic technology, the electronic control on the chassis of business automobile becomes more common. Auxiliary brake is generally combined with anti-lock braking system(ABS), or electronic brake force distribution(EBD).

(2) Auxiliary braking system is made up of two or more auxiliary brakes, such as the united control of engine brake and retarder brake.

4.2 Self-excited retarder(self-generating type)technology

The working electricities of above-mentioned two kinds of eddy current retarders are supplied by automotive original power systems^[17], which affects the usages of other normal electrical equipments. Self-excited retarder consists of a set of basic power generation devices and an original eddy current retarder, which is shown in Fig. 9. There are a set of armature windings and permanent magnet poles of generator in original drum eddy current retarder. When automobile normally runs, the armature windings of generator and the exciting windings of retarder are disconnected. There is only terminal voltage in power generation devices, but no exciting current is in the exciting windings of retarder. So the iron core could not form a magnetic field, and there is no braking torque. When driver downs brake pedal, the current is input the exciting windings, and retarder starts work. Because of the electromagnetic torque from generator and the braking torque from self-excited retarder, the total braking torque of this type retarder will increase by about 10% compared with the same model of electric eddy current retarder.

Figure  9.  Self-excited retarder

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The mechanical components of self-excited retarder are stator, rotor and other components, which are shown in Fig. 10. Rotor consists of eddy current induced rotary drum and flange with permanent magnet. Stator consists of the armature of generating device with coil and uniformly distributed core windings. Therefore, the cooling requirements for the type retarder should be higher than the others, and the power-generating capacity at low speed will lead the decline of braking effect.

Figure  10.  Mechanical structure of self-excited retarder

1-Iron core; 2-Generator armature; 3-Permanent magnet; 4-Stator; 5-Rotor; 6-Excitation winding; 7-Armature winding

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Self-excited retarder has light weight and small sizes. Particularly, there is no need to increase or intensify generators and batteries because of its own power generation capabilities. It will have broader market prospects for its one unique advantage that the inertia of drive shaft can be converted into braking torque.

Now self-excited retarder has been produced by Japan Sawafuji Motor. In China, Jiangsu Chaoli Electrical Ltd. has applied to the patent of its structure, and Jiangsu University is currently engaged in its research. But there is no Chinese passenger car to assemble the retarder.

4.3 Integrated design of retarder and transmission

An integrated retarder is installed between engine and gearbox. There is an integrated permanent magnet generator and motor, which can replace on-board generator and motor. Its structure is complex but has following advantages.

(1) Because the retarder system is self-excited, it requires no energy from automotive battery or alternator. Therefore, the battery is not drained during retardation cycle. The electrical power required to excite the retarder is generated by a PMG(permanent magnet generator), which converts the mechanical energy from the rotating drive shaft into usable electricity.

(2) The retarder system can be adapted so that PMG is utilized to charge automotive battery and furnish electricity to the retarder's operation, which eliminates the need of an alternator. The battery can supply power to PMG at starting mode via a start inverter, which eliminates the need of a starter.

(3) The retarder system can also be taken as an active/passive damper. The inertia developed by rotor acts as a passive damper which can replace flywheel. The retarder system can also employ active damping. The advantages of the damping are that mechanical stresses can be reduced, the lifes of transmission, differential and other driving components are extended.

5. Conclusion

With the greater attention to the safety of automobile from users, the scale expansion of retarder manufactories, updating technologies and decline costs, the retarder application market will be further expanded.Government should introduce mandatory regulations for retarder installation on all heavy vehicles, which may promote the further development of retarder market. At the same time, Government should expand industrial scale, adjust industrial structure, strengthen intellectual property protection and standardize industrial competitive practices, so that the research and exploiture of new products and the upgrades of existed products are promoted effectively.

The future trends of automotive retarder in technology development are energy saving, environmental protection and intelligentization, which are mainly described as follows: retarder's miniaturization and integration, self-excited retarder's increasing usage, humanity on retarder control and combined control of auxiliary braking device and main braking system.