Week 4: Rail Propulsion, Coupling, and Braking

Text (44p.): Ch. 4 (55-80), Ch. 6 (105-124)
Supp. Text: Ch. 7 (90-118), Ch. 10 (156-178)

Locomotives

Electric Locomotives

• DC or AC, provided via 3rd rail or overhead wires
• less slip due to non-pulsating power (good for grades)
• high acceleration at low speed (good for commuter trains)
• VERY ROUGH ... 1000 HP ~ 10,000 pounds tractive effort
• 3000 HP at 40 mph, 30,000 pounds effort is nominal
• Multiple Unit Operation

Click here to read about the basic history of the Diesel Electric Locomotive (from UP)

• Most are DC 600 V, but newer technology is ... Click here for info on UP AC Locomotive
• 4 hp/ton = 90 mph on rail (8 hp in car)
• Capacities for a 3,000 hp unit:
  - engine cooling water = 300 gallons
  - engine lube oil = 250 gallons
  - fuel = 3,000 gallons
  - sand = 2 tons

  TE = 375HPe/V where:

  TE = tractive effort, limited by friction forces (about 0.30 max, with sanding can approach 0.40) (see Fig. 4-1 below)

  tractive effort = coefficient of adhesion * weight of drivers

  max. adhesion occurs with slight slip, though quickly reduces with significant slip (kinematic friction)

  HP is the rated HP (or .93 of gross HP)

  V is speed, mph

  e is efficiency, usu. 0.82 - 0.83

  therefore, for 3000 hp, TE = 20,000 pounds (at 45 mph)

  most trains are dispatched assuming 0.18 adhesion (track conditions play an important role in determining friction)

  ruling grade may not be the steepest grade along the route, if that grade is short (whole train may be longer than the grade)

  If the grade is short enough, it may be surmounted as a momentum grade (trading kinetic for potential energy)

  Locomotives are rated for tonnage, depending on their power and weight

source: Wright and Ashford's Transportation Engineering, third ed.)

Let's diiscuss some of the figures from Armstrong's Text:

• Click here to see figure 4-1
• Click here to see figure 4-2
• Click here to see figure 4-3

Trains and Coupling

• coupler strength = 350,000 lbs ... what happens if more tractive effort is needed to move the train?
• helper units (if manned, on back of train), if not, 2/3 way back)
• rotary shank couplers for unit trains
• long shanks for long cars
• connecting is automatic, decoupling requires manual effort
• draft gears absorb shock, friction

Brakes

• Fail safe principal, 70-110 psi required in system to start train, release or pressure activates brakes
• No more than 15 psi can be lost in system, no more than 1 psi/min lost if brakes applied
• Freight brakes must be fully released to be re-applied (pax trains use more expensive, empty load brakes)
• Braking is more important to capacity than power to accelerate.
• Brakes pressure equlization takes time - 700 fps vs speed of light for electro-pneumatic systems (not common in freight)
• Dynamic Braking is using the motors as generators, dissipating the heat through a bank of resistors - saves wear and tear on brakes and wheels
• Brake force = coeffiecient of friction between shoe and wheel time brake force. Must not slide wheels flat on empties, so maximum brake force is a fraction of the weight of an empty car, say one-half.
• When the car is full, the brake force must be at least 13% of the grossa car weight for steel shoes. If the cooficient of friction is only 0.20, the retarding force is only about 2.5% of gravity. How long would it take such a car to slow from 70 mph (100 fps) to a stop, if maximum pressure were able to be applied? 0.025g = 0.8 fps/s, so, about 125 sec. If constant decceleration applied (it doesn't), this would mean the train would travel well over a mile to stop.) Thus, the train may over-run signals if spaced too closely. Of course, this is worst case, and the locomotives provide much braking force. However, the impact on capacity is well appreciated.
  
• Air Brakes
• EP Brakes
• Vacuum Brakes

Some photos and information about selected locomotives