Tractive Effort of a Locomotive
The tractive effort of a locomotive is the force that the locomotive can generate for hauling the load. The tractive effort of a locomotive should be enough for it to haul a train at the maximum permissible speed. There are various tractive effort curves available for different locomotives for different speeds, which enable the computation of the value of tractive effort. Tractive effort is generally equal to or a little greater than the hauling capacity of the locomotive. If the tractive effort is much greater than what is required to haul the train, the wheels of the locomotive may slip.
A rough assessment of the tractive effort of different types of locomotives is provided in the following sections.
1 Steam Locomotive
The tractive effort of a steam locomotive can be calculated by equating the total power generated by the steam engine to the work done by the driving wheels.
Assume P to be the difference in steam pressure between the two sides of the cylinder, A is the area of the piston of the engine, d is the diameter of the piston of the engine, L is the length of the stroke of the engine, D is the diameter of the wheel of the locomotive, and Te is the mean tractive effort of the locomotive. Work done by a two-cylinder steam engine
= 2 × difference in steam pressure × area of the piston × 2 × length of the stroke
= 2P × A × 2 L = p Pd2L
It is clear from Eqn (25.14) that tractive effort increases with an increase in steam pressure difference and the diameter and length of the piston, but decreases with an increase in the diameter of the driving wheel of the locomotive.
2 Diesel Locomotive
Tractive effort of a diesel-elective locomotive can be assessed by the following
Te = 308 x RHP / V (kg) (25.15)
where Te is the tractive effort of a diesel-electric locomotive, RHP is the rated horsepower of the engine, and V is the velocity in km/h.
3 Electric Locomotive
The tractive effort of an electric locomotive varies inversely with the power of speed. The empirical formulae for calculating the approximate value of tractive effort are as follows.
For an dc electric locomotive: Te = a/V3 (25.16)
For an ac electric locomotive: Te = a/V5 (25.17)
where a is a constant depending upon the various characteristics of the locomotive.
The important characteristics of three types of tractions are compared in Table 25.2.
Table 25.2 Comparison of steam, diesel and electric traction