Hydraulic Retarder Fundamentals

Question 1

Identify four (4) of the major hydraulic retarder components.
a)
b)
c)
d)

Answer 1

1. Any four (4) of the following are correct.
   a) retarder rotor (rotating assembly)
   b) retarder housing (stator)
   c) oil pump and oil supply circuit
   d) oil cooler (heat exchanger)
   e) retarder control valve and control circuit

Question 2

2. Explain the purpose of the hydraulic retarder rotor.

Answer 2

2. The retarder rotor is the rotating member of the hydraulic retarder. It is attached to a powertrain member output. As the rotor vanes strike the oil in the retarder housing, resistance occurs. That drag is transmitted to the driveline, thereby slowing the vehicle.

Question 3

3. Explain the purpose of the retarder/stator housing.

Answer 3

3. The retarder housing encloses the retarder rotor and contains the oil supplied during retarder operation. The close clearance between the rotor and the housing causes the rotor to drag as it passes through the oil. This drag slows the vehicle. A great deal of heat occurs due to this reaction.

Question 4

4. Explain the purpose of the oil cooler used in the hydraulic retarder system.

Answer 4

4. A great deal of heat is generated as the retarder operates. The amount of heat absorbed by the oil must not exceed 121°C (250°F). The oil cooler, whether an oil-to­ coolant or an oil-to-air design, helps maintain the oil temperature within safe limits.

Question 5

5. The only type of oil cooler used in the hydraulic retarder system is the oil-to-air design.
   a) true
   b) false

Answer 5

5. b) false; The hydraulic retarder system could also use an oil-to-coolant type of oil
   cooler.

Question 6

6. Identify the three (3) possible locations where hydraulic retarders could be
positioned.
a)
b)
c)

Answer 6

6. a) between the torque converter and transmission
   b) on the transmission output shaft
   c) on the driveline

Question 7

7. Explain the basic operation of a hydraulic retarder..

Answer 7

7. The operator moves a mode selector switch or lever in order to activate the hydraulic retarder. The vehicle operating conditions may be such that the hydraulic retarder is activated automatically. The retarder control valve allows oil to flow into the retarder housing. The rotor vanes strike the incoming oil and a resistance begins. When the retarder housing is full of oil, considerable drag is on the rotor and it slows down. Since the rotor is attached to an output powertrain member, the vehicle slows down. The action of the churning oil changes the kinetic energy created by the moving vehicle into heat energy.

Question 8

8. The hydraulic retarder changes the heat energy created by the vehicle moving over the road into kinetic energy.
   a) true
   b) false

Answer 8

8. b) false; The hydraulic retarder changes the kinetic energy created by the moving
   vehicle into heat energy. An oil cooler must dissipate the heat energy absorbed by the oil.

Question 9

9. The hydraulic retarder operates on the basis of a change in fluid friction.
   a) true
   b) false

Answer 9

9. a) true