In any transport medium, the traction force is the prime one without which nothing can be moved. For Railways, the locomotives play the lead role. The locomotives are expected to have many qualities depending upon their usage either in high-speed coaching service or heavy load freight service.
The locomotive assigned to either goods service or coaching service should have a good load-speed relationship. Its riding quality should make a minimum disturbance to the track. In the run development of defects should be low.The other characteristics required are that the locomotives should be maintainable in short period and brought back to the service. They should have both side operation which avoids the turning work and time. They should be environmental friendly that is pollution free.
The main kinds of locomotives used nowadays are Diesel and AC locomotives worldwide. Both of them have their unique characteristics to compare and contrast.The diesel engine cannot produce high power at the start or at very low engine speeds. It is, therefore, necessary that the engine is decoupled from wheels while starting and till it attains a minimum working speed.
This is achieved by clutch and gear combination which is termed as transmission. As the power of the locomotive is very high, the electric transmission is preferred in which the engine is permanently coupled to a DC generator. The output of the generator is fed to traction motors through a control circuit which varies the torque-speed relationship. The traction motors are directly mounted on the axles and drive the axles through gears.
- Engine to produce power for transmission
- Governor to regulate the fuel input to Generator demand and to maintain constant engine RPM at each notch.
- Throttle to vary engine RPM and power output.
- Traction Generator to convert mechanical energy into electrical energy.
Expresser to produce compressed air for the braking and to operate various contractors in the control circuit.
- Radiator fan to cool the engine cooling water.
- Traction motor to convert the electric energy into mechanical energy for the driving wheels.
- Turbosupercharger to compress air before input into the engine for combustion.
- Over speed trip mechanism which trips the power if the engine RPM exceeds the predetermined limit.
- Wheel slip relay which reduces the power to traction motor if wheel slip takes place.
- Hot engine alarm gives an alarm if cooling water temperature exceeds the limit and cuts off the power if the engine remains hot for a specific time.
- Sand gear which sprinkles sand on the rails to improve adhesion.
Classification of locos:
- The locos are classified on wheel arrangements by 2-4 codes.
- B Bi-axle bogie with mechanically coupled axles.
- Bo Bi-axle bogie with independently driven axles.
- C Tri-axle bogie with mechanically coupled axles.
- Co Tri-axle bogie with independently driven axles
The resistance offered by a train to move from a stop is called Starting Train Resistance. The resistance offered by it to keep it moving at a specified speed is known as Rolling Train Resistance. In other words, force needed to start a train from the stationary position is starting resistance and force needed to keep a train moving at certain speed is rolling resistance.
The drawbar pulls exerted by locomotive has to be more than the train resistance to keep a reserve force needed for acceleration. Mathematically starting resistance can be expressed by the formula.
Rs = RVS + RG +RC + RA
Rs = Total train resistance at start
RVs = Vehicle starting resistance — Depends on bearing design.
RG = Grade resistance if any
RC = Curve resistance if any (On curves, the friction between rail & wheel increases and hence extra force is necessary).
RA. = Acceleration reserve to be divided depending upon acceleration need of the train.
The resistance during movement is called rolling resistance. It depends on speed and car body design but does not depend on bearing design. Mathematically it can be expressed as
RR = RVR + RG + RC + RA
Where RR = Total rolling resistance RG, RC, R A are same as starting a resistance.
RVR = Train rolling resistance on level straight track.
RVR = A + BV + CV2
Where A, B & C are constants depending on vehicle design, car body designs. ‘A’ depends on mechanical friction in bearing. ‘B’ Factors include flange friction, swaying and oscillation characteristics of the vehicle. ‘C’ Air resistance depends on body design. Aerodynamic design vehicles have less friction than box type vehicles. The values of A, B, & C are different for different vehicles.
The force at rail-wheel contact exerted by a locomotive is called Tractive Effort. The tractive efforts at the start get limited by the load on driving wheels and limiting frictional coefficient between rail and wheel (Adhesion). The locomotive is capable of producing much tractive effort at low speed because of its high horsepower. On high speed, it depends on loco horsepower and wheel diameter.
It depends on rail wheel friction. The adhesive percentage is defined as ratio expressed in percentage of tractive effort at wheel slip and vertical load on driving wheels.
Factors affecting adhesion:
It depends on the condition of rail-wheel contact surface e.g. wet, dry. Oily, slippery, etc. – Type of track and sleeper density – Rate of increase in torque applied to the wheel.
If torque applied to wheel exceeds the adhesive torque, wheel slip takes place. This reduces the friction between rail and wheel and further wheel slip takes place. Wheel slips reduce the tractive effort and hence haulage capacity and damage rail and wheel. Wheel slip should, therefore, be avoided.
Method to improve adhesion
- Clean rails regularly – Use sand while starting – Immediately cut off power if wheels slip.
- Equilibrium Speed/Balancing Speed
The train will go on accelerating till a tractive effort of power is more than rolling resistance. Once TE & TR (train resistance) become equal, the speed cannot be further increased. This speed is called equilibrium or balancing speed. The loco cannot continuously work on full power and hence potential TE should be substantially higher than TR for better train operation.
This also helps in attaining maximum speed quickly. The balancing speed of a WDM2 loco with a load of 4700 ton on the level gradient is 59 km/h. So it is desirable to use 2 WDM2 locos in MU for operating such a train.
Determination of Train Load
- The trailing load that can be attached to a locomotive depends upon.
- The ability of the locomotive to start the load on the steepest gradient on the section.
- Attain maximum permissible speed on most of the route
- Maintain minimum desired speed on the steepest gradient.
- Acceleration level desired.
Among 50 types of locomotives, only 15 types are manufactured more in India and they are all utilized all over the country widespread. While nominating a locomotive, its tracking effort, trailing load, the speed at which the train to run, and the gradient of the section on which the train is running are considered. Generally, the ratio of the traction power and the trailing load should be more than 1 and in India, it is around 1:1.1 to negotiate the midsection gradients.
The effect of grades on train operation is significant. For each percent of ascending grade, there is an additional resistance to constant-speed movement of 20 pounds per ton of train. This compares with a resistance on level, straight track of about 5 pounds per ton of train. A given locomotive, then, can haul only half the tonnage up a 0.25 percent grade that it can on the level at the constant speed.
It is always better to account the trailing load, the ruling gradient of the section and finally the traction power of the locomotive on the plain section and on the ascending gradients. The traction power of the locomotive varies with respect to dry and wet rail condition, fog and snow conditions, oily and greasy conditions which are the usual cause of wheel slips.