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Module 5:Tractive Effort

Module 5:Tractive Effort. Module Objectives. Learn About Tractive Effort (the Amount of Force at the Wheels Available for Moving a Train) Understand the Effects of Combining Steep Grades and Horizontal Curvature. Energy Utilization.

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Module 5:Tractive Effort

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  1. Module 5:Tractive Effort

  2. Module Objectives • Learn About Tractive Effort (the Amount of Force at the Wheels Available for Moving a Train) • Understand the Effects of Combining Steep Grades and Horizontal Curvature

  3. Energy Utilization The two primary aspects of transportation energy efficiency: • Resistance – How much work is required to move something • Energy Efficiency – How well energy is converted into useful work

  4. Train Resistance • For our purposes, Resistance is the combination of forces that work against a train’s movement. • Early measurement of railcar resistance simply involved piling on weight, w, and determining how much was needed to make the car move. • If the weight, w, is great enough to overcome the static friction, the car will start to move. Resistance is typically measuredin “pounds per ton” (in U.S.) Friction

  5. Trains VS. Trucks • Trains require less energy to move than trucks because of smaller resistive forces: • Lower friction factor (0.18) • Smaller contact area • Steel wheels on steel rails • Rubber tires on pavement • Less wind resistance per pound of load • Size of contact area?

  6. Horsepower  Tractive Effort Power to auxiliaries (cooling fans,motor blowers, locomotive controls, air Brake compressor, etc) Loss in generator (or Alternator/ rectifier) Converting crankshaft Power to electricity Traction motor losses Gear friction × HP 308 = TE ( ) lb-f V Power to move, lift, & accelerate weight of locomotive itself (varies with speed, grade, Curvature, loco/ train Weight ratio) • Tractive Effort • The point of contact between the wheel and the rail is where the torque of the motor is converted to a force that is called tractive effort.

  7. Traction Forces W TE = m TEmax x W m =Adhesion W = Weight on Drivers

  8. Adhesion Adhesion • Percentage of a locomotive’s weight on its driving wheels, converted into tractive effort. • DC locomotives can achieve a maximum peak of 30 % adhesion on dry rail. • Through advanced control, AC locomotives can achieve up to 40 % adhesion in many weather conditions.

  9. Factors Affecting Adhesion Wheel/Rail Contact Factors Wheel-Rail materials Hertzian (Rolling Wheel) Stresses Rail Metal Creep (Longitudinal Movement) Track factors Rail surface condition Rail profile irregularities Curvature of track Vehicle factors Mechanical Loco weight and axle weight distribution Weight transfer Speed Wheel size variation Electrical Torque control method Traction motor characteristics Power circuit configuration Slip-slide control

  10. Train Resistance • Two Elements: • Rolling Resistance (On Level Tangent Track) • Friction related • Journal, wheel, track quality, track modulus • Aerodynamics of equipment • Grade & Curve Resistance • Track Profile & Alignment related • Change in potential energy head

  11. Rolling Resistance • Rolling Resistance • The resistance for train movement on straight and level track can be determined by W. J. Davis formula: R=A+BV+CV² Where R= Train rolling resistance A= Rolling resistance component independent of train speed B= Train resistance dependant on speed C= Drag coefficient based on the shape of the front of the train and other features affecting air turbulance V= Train speed • The Train resistance on level tangent track is the sum of locomotive and train resistances.

  12. Freight Train Speed V Resistance 8 7 Air Resistance (CV2) "Flange" Resistance (BV) 6 "Journal" Resistance (A) 5 CV2 4 3 2 BV 1 A 0 10 20 30 40 50 60 70 80 90 100 At low speeds, journal resistance dominates, as speed increases air resistance is increasingly most important Resistance (lbs./ton) Speed (mph)

  13. Davis Equation Modifications • The Davis Equation has been substantially updated to reflect modern developments, but its basic form is the same. • Ro= 1.5 + 18N/W + 0.03V + CaV2/10,000W (CN Equation 1990) where: Ro = resistance in lbs. per ton N = Number of Axles W = Total Weight in Tons of Locomotive or Cars V = Velocity of Train (MPH) a = Cross-sectional Area of Vehicle in ft2 C = Canadian National Streamlining Coefficient • See AREMA Manual for Railway Engineering Chapter 16 - 2.1.3 for latest version of formula

  14. Resistance Versus Speed 10,000 Ton Train 60,000 50,000 40,000 Resistance (lbs.) 30,000 20,000 10,000 0 0 10 20 30 40 50 60 70 80 Speed (mph)

  15. Grade Resistance 1% Grade = Approximately 20 lbf / ton

  16. Curve Resistance 10 = 0.8 lbf / ton

  17. Compensated Grade Gc = G + Dc * 0.04

  18. Typical Freight Consist Typical modern locomotive Typical modern freight consist 6000 horsepower,212 tons 100 cars x 143 tons each = 14,300 tons

  19. Speed/Tractive Effort Curve Modern Locomotive - Level Grade 180,000 At low speed, tractive effort is limited by adhesion, not power 160,000 140,000 120,000 100,000 Tractive Effort (lbs.) 80,000 60,000 40,000 20,000 0 0 10 20 30 40 50 60 70 80 Speed (mph)

  20. Maximum Possible Train Speed? 180,000 160,000 140,000 120,000 100,000 Tractive Effort (lbs.) 80,000 60,000 40,000 20,000 0 0 10 20 30 40 50 60 70 80 Speed (mph) About 58 mph. This is referred to as the “balancing speed, why? Modern Locomotive - Level Grade At low speed, tractive effort is limited by adhesion, not power Tractive Effort Tractive force = resistance @ 35,000 lbs. Train Resistance

  21. Resistance V Acceleration Q: How much force is available for acceleration at 15 mph? • Available acceleration declines as speed increases • At balanced speed it is zero. • So, therate of acceleration declines with speed. A: 135,000 - 15,000 = 120,000 lbs. Q: How much force is available for acceleration at 35 mph? A: 59,000 - 21,000 = 38,000 lbs.

  22. What If We Need More Power Capability of “multiple unit” control makes this possible Tractive Effort (lbs.)

  23. Freight Time/Speed Graph 70 60 Speed (mph) 50 40 30 20 10 0 0 500 1000 1500 2000 2500 3000 Time (seconds) This curve depicts a single locomotive. How will the curve change if a second locomotive is added? How long will it take for this train to reach 40 mph? About 900 seconds = 15 minutes

  24. Time/Speed Graph Now how long will it take to reach 40 mph? Speed (MPH) About 300 seconds≈ 5 minutes

  25. Freight Distance/Speed Graph How many miles until this train reaches 40 mph? Speed (MPH) About 75,000 feet≈ 14 miles Velocity Profile h (ft) = 0.03 V2

  26. 2 Unit Distance/Speed Graph Speed (MPH) With 2 locomotives, only about 25,000 feet ≈ 5 miles

  27. Horsepower:Trailing Tonnage Long-distance train with few stops along the way. Low horsepower:trailing ton ratio. 12,000 hp: 14,300 tons = 0.83 hp:ton Typical of many freight trains High-speed train with frequent stops and starts. High horsepower:trailing ton ratio may be as high as2 to 4 hp:ton Typical of intermodal trainsand may be even higher for passenger trains

  28. Stopping the Train 6,000’ • Safety is the most important consideration! • Adhesion Limits Braking Ability. • Stopping Train Vs. Motor Vehicle.

  29. Stopping Distance V Speed Stopping a train can often take a mile or more

  30. QUESTIONS?

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