Introduction to Diesel Train Physics

Aim - this section describes the general key physics of diesel Locomotives operation.


Index

Introduction

Power

Tractive Effort

Adhesion

Load Hauling Performance

Useful References


Introduction

The Diesel locomotive as the name implies is driven by a diesel engine. The diesel engine, or prime mover, produces the power to drive the locomotive forward and pull the train. The power produced by the prime mover is transmitted to the driving wheels, by a number of different transmission systems including mechanical transmission, electric traction motors, or hydraulic drive systems. The diesel typically is cheaper to operate then a steam locomotive, and some of the other advantages include the fact that they can be operated in multiple units with a single driver, they can be started or sindexped almost instantaneously compared to a steam locomotive which requires time to build up steam.

To understand the performance of a diesel locomotive the following three subjects will be considered:

  • Power
  • Tractive Effort
  • Adhesion

Once the above three elements are considered in conjunction with the train resistances, we can model the overall performance of the train.

Top


Power

The amount of force (or effort) available at the rail to drive the locomotive forward will be determined by the diesel engine (prime mover) and the transmission system which converts the mechanical enegry of the prime mover to an energy force at the rails. In the case of a diesel electric DC locomotive, the prime mover drives a DC electric generator, which in turn supplies electricity to the DC traction motors which then drive the locomotive wheels. Each of these power conversions suffer from some form of loss, which means that the full power of the prime mover, is not available to the rails for the diesel's tractive effort.

The following diagram from the BR Deltic Locomotive test report shows the power outputs at the end of each conversion stage. By studying the diagram we can note the following:

  • Output at Engine Shaft (Prime Mover) - This is the power that is provided by the diesel prime mover, and in the case of the Deltic locomotive it was 3,300hp. Typically, in some instances, this is the "rated power" of the locomotive. In other instances, power required by the auxiliary devices, such as the fan, batteries, etc is subtracted from the prime mover output power to give a "power available for traction" value, with this value being quoted as the "rated power" of the locomotive. In both cases this power is supplied to the generator, etc, and thus the power supplied to the rail will be less then this value.
  • Generator Input - This the power that is supplied to the generator in order to produce the energy required to drive the traction motors. It will be noted that there has already been a small reduction in power to support the operation of auxiliary equipment, such as auxiliary generator, radiator fan, etc.
  • Generator Output & (Traction) Motor Input - This is the power that is available to drive the traction motors, and again a small amount of power has been lost due to generator.
  • Rail HP - This is the power that is available to drive the locomotive forward and measured at the wheels of the locomotive. Again, power losses have occured due to work in the traction motors and gearing transmissions.
  • Drawbar HP - This is the power that is available to drive the locomotive forward, and power losses have occured due to losses in the traction motors and gearing transmissions. The main loss in power in this instance is due to the resistance to movement that the locomotive presents.
Diesel Efficiency

Often the amount of power produced at each stage is converted to a fraction of the main output power, and this demonstrates the efficiency of the power conversion process. Ideally this should be 1 (or 100%) in an "ideal" world, however this is never the case, and will always be a lower value then the input power value.

The diagram below shows the power produced by the traction motors of the locomotive, and the tractive force, superimposed onto the same diagram. From the diagram it can be seen that there are broadly three zones of operation for the motors as follows:

Because of this power characteristic, diesel locomotives are often called "Constant Power" machines.

Diesel Tractive Force and Power

Top


Tractive Effort

In specifying a diesel locomotive it is quite common to define a design tractive effort curve against the locomotive speed. The diagram below shows an example of one.

Diesel Tractive Effort

The following points are worth noting from the diagram:

Relationship between Power and Tractive Force

The Rail HP can be converted to tractive effort (force) by one of the following formulas, which can be rearranged to convert between Tractive Force and Power or vicer versa.

Imperial: TF (lbf) = (375 x Rail HP ) / speed (mph)
Metric: TF (N) = Rail Power (W) / speed (metres per sec)

Given that most diesel locomotives have specification information for the tractive forces, we can use the above formula to calculate the maximum rail hp at the design speed of maximum continuous force point, which is then assumed to be constant. Once determined we can then convert backwards again to find the tractive force at any speed.

In more recent years diesel locomotive using AC traction motors have become more popular, as they can produce higher tractive effort (force) then DC locomotives, because of the following possible modifications (see Republic Locomotives site for a more detailed explanation):

Top


Adhesion

The adhesion between the wheels and the rail will determine the amount of force that can be applied to moving the train, and hence the amount of load that the locomotive can haul. A more detail description of adhesion can be found on the adhesion page, and whilst this page principally deals with adhesion on a steam locomotive, a diesel locomotive has similar issues with adhesion. The main difference between steam and diesel traction is that the tractive force of a diesel is constant for the full wheel rotation, whereas due to the operating rods attached to a steam locomotives wheel the force is not even around the full roatation of the wheel.

As with the steam locomotive the diesel will suffer wheel slip if the rotational force exceeds the adhesive force.

Top


Load Hauling Performance

Once the parameters are determined for the ENG file, load hauling performance tests should be undertaken to confirm that the locomotive performance is within acceptable bounds for the locomotive being modelled.

Often in test reports the Traction Draw Bar Tractive Effort (Traction DBTE) is used to describe the pulling power of the locomotive. This is calculated by considering the impact of the locomotive resistance, trailing load resistance, and gross weight ratio, on the tractive effort calculated above. A detailed explantaion of how to calculate this value can be found on pg 48 of the test report in the Useful References section below. This is the amount of force that is available to overcome the resistance offered by the trailing load behind the locomotive. The graph below shows the Traction DBTE for a diesel locomotive, as well as the trailing load resistance for a 300 ton train on various different gradients.

So for example, with a load of 300 ton the Traction DBTE will vary with speed as shown by the green curve. If the train is climbing a 1 in 80 gradient the trailing load resistance is shown by the red curve. The point where the two curves cross designates the "balancing speed", ie the maximum speed that the locomotive can travel when climbing a 1 in 80 grade with a 300 ton load.

Diesel Performance

Top


Useful References

British Railways Test Report 2000 HP Diesel Electric Locomotive #10203

British Railways Class D16/2 Diesel Electric Locomotive #10203

Top