LEC Drivetrain & Transmission 3 — Questions and Answers
Question 1: In a diesel-hydraulic locomotive drivetrain, what component replaces the traction motors used in diesel-electric designs?
- A hydraulic torque converter and mechanical gearbox (Correct answer)
- A steam-powered hydraulic pump
- A fluid coupling connected directly to the driving axles
- A set of high-ratio gear reducers driven by the diesel engine
Correct answer: A hydraulic torque converter and mechanical gearbox
Diesel-hydraulic locomotives use a hydraulic torque converter (and sometimes a fluid coupling) combined with a mechanical gearbox to transmit power to the axles.
Question 2: What is 'dynamic augment' in the context of locomotive drivetrain forces?
- The additional tractive effort produced when sanding is active
- The cyclic vertical force variation caused by rotating and reciprocating drivetrain components (Correct answer)
- The increased power output when transitioning from series to parallel motor connection
- The torque boost provided by turbocharging at higher engine speeds
Correct answer: The cyclic vertical force variation caused by rotating and reciprocating drivetrain components
Dynamic augment is the periodic variation in vertical wheel force caused by rotating wheel/axle imbalance and reciprocating parts, affecting track loading.
Question 3: What does the term 'gear ratio' refer to in the context of a locomotive axle drive?
- The ratio of engine RPM to maximum speed in miles per hour
- The ratio of traction motor pinion teeth to bull gear teeth on the axle (Correct answer)
- The relationship between main generator voltage and traction motor current
- The ratio of driving axles to total axles in the locomotive
Correct answer: The ratio of traction motor pinion teeth to bull gear teeth on the axle
The axle gear ratio is the number of teeth on the axle-mounted bull gear divided by the number on the motor pinion, determining the speed/torque trade-off.
Question 4: A locomotive with a higher axle gear ratio compared to a standard unit will MOST likely have:
- Higher top speed but less starting tractive effort
- Lower top speed but greater starting tractive effort (Correct answer)
- Identical performance but improved fuel economy
- Better performance on curves due to reduced centrifugal force
Correct answer: Lower top speed but greater starting tractive effort
A higher gear ratio multiplies torque more at the wheels but limits rotational speed, resulting in more pulling power at the cost of maximum speed.
Question 5: What is the function of the traction motor suspension system (nose suspension) in a locomotive?
- It isolates the traction motor from the carbody to reduce noise
- It allows the motor to be rigidly mounted to the truck frame while the axle flexes vertically
- It permits the traction motor to move with the axle over track irregularities while transmitting torque (Correct answer)
- It provides a hydraulic cushion between the pinion and bull gear during starting
Correct answer: It permits the traction motor to move with the axle over track irregularities while transmitting torque
Nose-suspended traction motors are rigidly attached to the axle bearing housing on one end and pivot from the truck frame on the other, allowing axle movement while maintaining gear mesh.
Question 6: During a power-off coast, what should a locomotive engineer expect regarding drivetrain behavior?
- The traction motors will generate braking force through back-EMF
- The drivetrain will freewheel with no engine braking effect in diesel-electric units (Correct answer)
- The main generator will automatically switch to motor mode
- The axle gear boxes will lock to prevent wheel spin
Correct answer: The drivetrain will freewheel with no engine braking effect in diesel-electric units
In diesel-electric locomotives with throttle at idle/off, the traction motors are disconnected from the circuit and the drivetrain freewheels with minimal engine braking.
Question 7: What is the significance of 'continuous tractive effort' rating for a locomotive?
- The maximum force the locomotive can exert for 10 seconds during starting
- The sustained pulling force the locomotive can maintain indefinitely without overheating (Correct answer)
- The average tractive effort measured over a standard test run
- The tractive effort available only when all traction motors are operating
Correct answer: The sustained pulling force the locomotive can maintain indefinitely without overheating
Continuous tractive effort is the maximum sustained pulling force achievable without causing traction motor overheating, measured at a specific continuous speed.
LEC Drivetrain & Transmission 3
In a diesel-hydraulic locomotive drivetrain, what component replaces the traction motors used in diesel-electric designs?