Diesel Locomotives | The Railway Technical Website | PRC Rail Consulting Ltd

The Diesel Locomotive

The
modern diesel locomotive is a self contained version of the electric
locomotive.  Like the electric locomotive, it has electric drive, in the
form of traction motors driving the axles and controlled with
electronic controls.  It also has many of the same auxiliary systems for
cooling, lighting, heating, braking and hotel power (if required) for
the train.  It can operate over the same routes (usually) and can be
operated by the same drivers.  It differs principally in that it carries
its own generating station around with it, instead of being connected
to a remote generating station through overhead wires or a third rail. 
The generating station consists of a large diesel engine coupled to an
alternator producing the necessary electricity.  A fuel tank is also
essential.  It is interesting to note that the modern diesel locomotive
produces about 35% of the power of a electric locomotive of similar
weight. 

Figure 1: A BNSF diesel electric locomotive a GE ES44C4 type, a typical US heavy haul locomotive. Photo: PorsHammer.

Diesel-Electric Types

Like
an automobile, a diesel locomotive cannot start itself directly from a
stand. It will not develop maximum power at idling speed, so it needs
some form of transmission system to multiply torque when starting. It
will also be necessary to vary the power applied according to the train
weight or the line gradient.  There are three methods of doing this: 
mechanical, hydraulic or electric.  Most diesel locomotives use electric
transmission and are called “diesel-electric” locomotives. Mechanical
and hydraulic transmissions are still used but are more common on
multiple unit trains or lighter locomotives. 

Diesel-electric locomotives come in three varieties, according to the period in which they were designed.  These three are:

DC – DC (DC generator supplying DC traction motors);
AC – DC (AC alternator output rectified to supply DC motors) and 
AC – DC – AC (AC alternator output rectified to DC and then inverted to 3-phase AC for the traction motors).  

The
DC – DC type has a generator supplying the DC traction motors through a
resistance control system, the AC – DC type has an alternator producing
AC current which is rectified to DC and then supplied to the DC
traction motors and, finally, the most modern has the AC alternator
output being rectified to DC and then converted to AC (3-phase) so that
it can power the 3-phase AC traction motors. Although this last system
might seem the most complex, the gains from using AC motors far outweigh
the apparent complexity of the system. In reality, most of the
equipment uses solid state power electronics with microprocessor-based
controls. For more details on AC and DC traction, see Electric Traction Power and Electric Locomotives on this site.

In
the US, traction alternators (AC) were introduced with the 3000 hp
single diesel engine locomotives, the first being the Alco C630. The
SD40, SD45 and GP40 also had traction alternators only. On the GP38,
SD38, GP39, and SD39s, traction generators (DC) were standard, and
traction alternators were optional, until the dash-2 era, when they
became standard. It was a similar story at General Electric.

There
is one traction alternator (or generator) per diesel engine in a
locomotive (standard North American practice anyway). The Alco C628 was
the last locomotive to lead the horsepower race with a DC traction
alternator.

The
diagram (Figure 2) shows the main parts of a US-built diesel-electric
locomotive and these are described in the following paragraphs. I have
used the US example because of the large number of countries which use
them. There are obviously many variations in layout and European
practice differs in many ways and we will note some of these in
passing. 

Figure 2: Schematic of diesel electric locomotive showing the main parts of a standard US design. Diagram: Author.

Diesel Engine

This
is the main power source for the locomotive. It comprises a large
cylinder block, with the cylinders arranged in a straight line or in a
V. The engine rotates the drive shaft at up to 1,000 rpm and this drives
the various items needed to power the locomotive.  As the transmission
is normally electric, the engine is used as the power source for the
alternator that produces the electrical energy to drive the locomotive.

Main Alternator

The
diesel engine drives the main alternator which provides the power to
move the train.  The alternator generates AC electricity which is used
to provide power for the traction motors mounted on the trucks
(bogies).  In older locomotives, the alternator was a DC machine, called
a generator.  It produced direct current which was used to provide
power for DC traction motors.  Many of these machines are still in
regular use.  The next development was the replacement of the generator
by the alternator but still using DC traction motors.  The AC output is
rectified to give the DC required for the motors.  For more details on
AC and DC traction, see the Electronic Power Page on this site.

Auxiliary Alternator

Locomotives
used to operate passenger trains are equipped with an auxiliary
alternator.  This provides AC power for lighting, heating, air
conditioning, dining facilities etc. on the train.  The output is
transmitted along the train through an auxiliary power line.  In the US,
it is known as “head end power” or “hotel power”. In the UK, air
conditioned passenger coaches get what is called electric train supply
(ETS) from the auxiliary alternator.  

Motor Blower

The
diesel engine also drives a motor blower. As its name suggests, the
motor blower provides air which is blown over the traction motors to
keep them cool during periods of heavy work. The blower is mounted
inside the locomotive body but the motors are on the trucks, so the
blower output is connected to each of the motors through flexible
ducting.  The blower output also cools the alternators.  Some designs
have separate blowers for the group of motors on each truck and others
for the alternators.  Whatever the arrangement, a modern locomotive has a
complex air management system which monitors the temperature of the
various rotating machines in the locomotive and adjusts the flow of air
accordingly.

Air Intakes

The
air for cooling the locomotive’s motors is drawn in from outside the
locomotive.  It has to be filtered to remove dust and other impurities
and its flow regulated by temperature, both inside and outside the
locomotive.  The air management system has to take account of the wide
range of temperatures from the possible +40°C of summer to the possible
-40°C of winter.

Rectifiers/Inverters

The
output from the main alternator is AC but it can be used in a
locomotive with either DC or AC traction motors.  DC motors were the
traditional type used for many years but, in the last 10 years, AC
motors have become standard for new locomotives.  They are cheaper to
build and cost less to maintain and, with electronic management can be
very finely controlled.  To see more on the difference between DC and AC
traction technology try the Electronic Power Page on this site.

To
convert the AC output from the main alternator to DC, rectifiers are
required.  If the motors are DC, the output from the rectifiers is used
directly.  If the motors are AC, the DC output from the rectifiers is
converted to 3-phase AC for the traction motors. 

In
the US, there are some variations in how the inverters are configured.
GM EMD relies on one inverter per truck, while GE uses one inverter per
axle – both systems have their merits. EMD’s system links the axles
within each truck in parallel, ensuring wheel slip control is maximised
among the axles equally. Parallel control also means even wheel wear
even between axles. However, if one inverter (i.e. one truck) fails then
the unit is only able to produce 50 per cent of its tractive effort.
One inverter per axle is more complicated, but the GE view is that
individual axle control can provide the best tractive effort. If an
inverter fails, the tractive effort for that axle is lost, but full
tractive effort is still available through the other five inverters. By
controlling each axle individually, keeping wheel diameters closely
matched for optimum performance is no longer necessary. This paragraph
sourced from e-mail by unknown correspondent 3 November 1997.

Electronic Controls

Almost
every part of the modern locomotive’s equipment has some form of
electronic control. These are usually collected in a control cubicle
near the cab for easy access. The controls will usually include a
maintenance management system of some sort which can be used to download
data to a portable or hand-held computer.

Control Stand

This
is the principal man-machine interface, known as a control desk in the
UK or control stand in the US.  The common US type of stand is
positioned at an angle on the left side of the driving position and, it
is said, is much preferred by drivers to the modern desk type of control
layout usual in Europe and now being offered on some locomotives in the
US.

Cab

The
standard configuration of US-designed locomotives is to have a cab at
one end of the locomotive only. Since most the US structure gauge is
large enough to allow the locomotive to have a walkway on either side,
there is enough visibility for the locomotive to be worked in reverse.
However, it is normal for the locomotive to operate with the cab
forwards. In the UK and many European countries, locomotives are full
width to the structure gauge and cabs are therefore provided at both
ends.

Batteries

Just
like an automobile, the diesel engine needs a battery to start it and
to provide electrical power for lights and controls when the engine is
switched off and the alternator is not running.

Traction Motor

Since
the diesel-electric locomotive uses electric transmission, traction
motors are provided on the axles to give the final drive.  These motors
were traditionally DC but the development of modern power and control
electronics has led to the introduction of 3-phase AC motors.  For a
description of how this technology works, go to the Electronic Power Page on
this site.  There are between four and six motors on most
diesel-electric locomotives.  A modern AC motor with air blowing can
provide up to 1,000 hp.

Pinion/Gear

The traction motor drives the axle through a reduction gear of a range between 3 to 1 (freight) and 4 to 1 (passenger).

Fuel Tank

A
diesel locomotive has to carry its own fuel around with it and there
has to be enough for a reasonable length of trip.  The fuel tank is
normally under the loco frame and will have a capacity of say 1,000
imperial gallons (UK Class 59, 3,000 hp) or 5,000 US gallons in a
General Electric AC4400CW 4,400 hp locomotive.  The new AC6000s have
5,500 gallon tanks.  In addition to fuel, the locomotive will carry
around, typically about 300 US gallons of cooling water and 250 gallons
of lubricating oil for the diesel engine.

Air Reservoirs

Air
reservoirs containing compressed air at high pressure are required for
the train braking and some other systems on the locomotive.  These are
often mounted next to the fuel tank under the floor of the locomotive.

Air Compressor

The
air compressor is required to provide a constant supply of compressed
air for the locomotive and train brakes.  In the US, it is standard
practice to drive the compressor off the diesel engine drive shaft.  In
the UK, the compressor is usually electrically driven and can therefore
be mounted anywhere.  The Class 60 compressor is under the frame,
whereas the Class 37 has the compressors in the nose.

Drive Shaft

The
main output from the diesel engine is transmitted by the drive shaft to
the alternators at one end and the radiator fans and compressor at the
other end. 

Gear Box

The
radiator and its cooling fan is often located in the roof of the
locomotive.  Drive to the fan is therefore through a gearbox to change
the direction of the drive upwards.

Radiator and Radiator Fan

The
radiator works the same way as in an automobile.  Water is distributed
around the engine block to keep the temperature within the most
efficient range for the engine.  The water is cooled by passing it
through a radiator blown by a fan driven by the diesel engine.  See Cooling for more information.

Turbo Charging

The
amount of power obtained from a cylinder in a diesel engine depends on
how much fuel can be burnt in it.  The amount of fuel which can be burnt
depends on the amount of air available in the cylinder.  So, if you can
get more air into the cylinder, more fuel will be burnt and you will
get more power out of your ignition.  Turbo charging is used to increase
the amount of air pushed into each cylinder.  The turbocharger is
driven by exhaust gas from the engine.  This gas drives a fan which, in
turn, drives a small compressor which pushes the additional air into the
cylinder.  Turbocharging gives a 50% increase in engine power.

The
main advantage of the turbocharger is that it gives more power with no
increase in fuel costs because it uses exhaust gas as drive power.  It
does need additional maintenance, however, so there are some type of
lower power locomotives which are built without it.

Sand Box

Locomotives
always carry sand to assist adhesion in bad rail conditions.  Sand is
not often provided on multiple unit trains because the adhesion
requirements are lower and there are normally more driven axles.

Mechanical Transmission

Figure
3: A diesel-mechanical locomotive is the simplest type of diesel
locomotive.  It has a direct mechanical link between the diesel engine
and the wheels instead of electric transmission.  The diesel engine is
usually in the 350-500 hp range and the transmission is similar to that
of an automobile with a four speed gearbox.  Other parts are similar to
the diesel-electric locomotive but there are some variations and often
the wheels are coupled. Diagram: Author.

Fluid Coupling

In
a diesel-mechanical transmission, the main drive shaft is coupled to
the engine by a fluid coupling.  This is a hydraulic clutch, consisting
of a case filled with oil, a rotating disc with curved blades driven by
the engine and another connected to the road wheels.  As the engine
turns the fan, the oil is driven by one disc towards the other.  This
turns under the force of the oil and thus turns the drive shaft.  Of
course, the start up is gradual until the fan speed is almost matched by
the blades.  The whole system acts like an automatic clutch to allow a
graduated start for the locomotive.

Gearbox

This
does the same job as that on an automobile.  It varies the gear ratio
between the engine and the road wheels so that the appropriate level of
power can be applied to the wheels.  Gear change is manual.  There is no
need for a separate clutch because the functions of a clutch are
already provided in the fluid coupling. 

Final Drive

The
diesel-mechanical locomotive uses a final drive similar to that of a
steam engine.  The wheels are coupled to each other to provide more
adhesion.  The output from the 4-speed gearbox is coupled to a final
drive and reversing gearbox which is provided with a transverse drive
shaft and balance weights.  This is connected to the driving wheels by
connecting rods.

Hydraulic Transmission

Hydraulic
transmission works on the same principal as the fluid coupling but it
allows a wider range of “slip” between the engine and wheels.  It is
known as a “torque converter”.  When the train speed has increased
sufficiently to match the engine speed, the fluid is drained out of the
torque converter so that the engine is virtually coupled directly to the
locomotive wheels.  It is virtually direct because the coupling is
usually a fluid coupling, to give some “slip”.  Higher speed locomotives
use two or three torque converters in a sequence similar to gear
changing in a mechanical transmission and some have used a combination
of torque converters and gears. 

Some
designs of diesel-hydraulic locomotives had two diesel engines and two
transmission systems, one for each bogie.  The design was poplar in
Germany (the V200 series of locomotives, for example) in the 1950s and
was imported into parts of the UK in the 1960s.  However, it did not
work well in heavy or express locomotive designs and has largely been
replaced by diesel-electric transmission.

Wheel Slip

Wheels
slip is the bane of the driver trying to get a train away smoothly. 
The tenuous contact between steel wheel and steel rail is one of the
weakest parts of the railway system.  Traditionally, the only cure has
been a combination of the skill of the driver and the selective use of
sand to improve the adhesion.  Today, modern electronic control has
produced a very effective answer to this age old problem.  The system is
called creep control.

Extensive
research into wheel slip showed that, even after a wheelset starts to
slip, there is still a considerable amount of useable adhesion available
for traction.  The adhesion is available up to a peak, when it will
rapidly fall away to an uncontrolled spin.  Monitoring the early stages
of slip can be used to adjust the power being applied to the wheels so
that the adhesion is kept within the limits of the “creep” towards the
peak level before the uncontrolled spin sets in.

The
slip is measured by detecting the locomotive speed by Doppler radar
(instead of the usual method using the rotating wheels) and comparing it
to the motor current to see if the wheel rotation matches the ground
speed. If there is a disparity between the two, the motor current is
adjusted to keep the slip within the “creep” range and keep the tractive
effort at the maximum level possible under the creep conditions.

Diesel Multiple Units (DMUs)

The
diesel engines used in DMUs work on exactly the same principles as
those used in locomotives, except that the transmission is normally
mechanical with some form of gear change system.  DMU engines are
smaller and several are used on a train, depending on the
configuration.  The diesel engine is often mounted under the car floor
and on its side because of the restricted space available.  Vibration
being transmitted into the passenger saloon has always been a problem
but some of the newer designs are very good in this respect.

There
are some diesel-electric DMUs around and these normally have a separate
engine compartment containing the engine and the generator or
alternator.

Diesel Engine Background

The
diesel engine was first patented by Dr Rudolf Diesel (1858-1913) in
Germany in 1892 and he actually got a successful engine working by
1897.  By 1913, when he died, his engine was in use on locomotives and
he had set up a facility with Sulzer in Switzerland to manufacture
them.  His death was mysterious in that he simply disappeared from a
ship taking him to London. 

The
diesel engine is a compression-ignition engine, as opposed to the
petrol (or gasoline) engine, which is a spark-ignition engine.  The
spark ignition engine uses an electrical spark from a “spark plug” to
ignite the fuel in the engine’s cylinders, whereas the fuel in the
diesel engine’s cylinders is ignited by the heat caused by air being
suddenly compressed in the cylinder.  At this stage, the air gets
compressed into an area 1/25th of its original volume.  This would be
expressed as a compression ratio of 25 to 1.  A compression ratio of 16
to 1 will give an air pressure of 500 lbs/in² (35.5 bar) and will
increase the air temperature to over 800°F (427°C).

The
advantage of the diesel engine over the petrol engine is that it has a
higher thermal capacity (it gets more work out of the fuel), the fuel is
cheaper because it is less refined than petrol and it can do heavy work
under extended periods of overload.  It can however, in a high speed
form, be sensitive to maintenance and noisy, which is why it is still
not popular for passenger automobiles.

Diesel Engine Types

There
are two types of diesel engine, the two-stroke engine and the
four-stroke engine.  As the names suggest, they differ in the number of
movements of the piston required to complete each cycle of operation. 
The simplest is the two-stroke engine.  It has no valves.  The exhaust
from the combustion and the air for the new stroke is drawn in through
openings in the cylinder wall as the piston reaches the bottom of the
downstroke.  Compression and combustion occurs on the upstroke.  As one
might guess, there are twice as many revolutions for the two-stroke
engine as for equivalent power in a four-stroke engine.

The
four-stroke engine works as follows:  Downstroke 1 – air intake,
upstroke 1 – compression, downstroke 2 – power, upstroke 2 – exhaust. 
Valves are required for air intake and exhaust, usually two for each. 
In this respect it is more similar to the modern petrol engine than the
2-stroke design.

In
the UK, both types of diesel engine were used but the 4-stroke became
the standard.  The UK Class 55 “Deltic” (not now in regular main line
service) unusually had a two-stroke engine.  In the US, the General
Electric (GE) built locomotives have 4-stroke engines whereas General
Motors (GM) always used 2-stroke engines until the introduction of their
SD90MAC 6000 hp “H series” engine, which is a 4-stroke design. 

The
reason for using one type or the other is really a question of
preference.  However, it can be said that the 2-stroke design is simpler
than the 4-stroke but the 4-stroke engine is more fuel efficient.

Size Does Count

Basically,
the more power you need, the bigger the engine has to be.  Early diesel
engines were less than 100 horse power (hp) but today the US is
building 6000 hp locomotives.  For a UK locomotive of 3,300 hp (Class
58), each cylinder will produce about 200 hp, and a modern engine can
double this if the engine is turbocharged. 

The
maximum rotational speed of the engine when producing full power will
be about 1000 rpm (revolutions per minute) and the engine will idle at
about 400 rpm.  These relatively low speeds mean that the engine design
is heavy, as opposed to a high speed, lightweight engine.  However, the
UK HST (High Speed Train, developed in the 1970s) engine has a speed of
1,500 rpm and this is regarded as high speed in the railway diesel
engine category.  The slow, heavy engine used in railway locomotives
will give low maintenance requirements and an extended life.

There
is a limit to the size of the engine which can be accommodated within
the railway loading gauge, so the power of a single locomotive is
limited.  Where additional power is required, it has become usual to add
locomotives.  In the US, where freight trains run into tens of
thousands of tons weight, four locomotives at the head of a train are
common and several additional ones in the middle or at the end are not
unusual.

To V or not to V

Diesel
engines can be designed with the cylinders “in-line”, “double banked”
or in a “V”.  The double banked engine has two rows of cylinders in
line.  Most diesel locomotives now have V form engines.  This means that
the cylinders are split into two sets, with half forming one side of
the V.  A V8 engine has 4 cylinders set at an angle forming one side of
the V with the other set of four forming the other side.  The
crankshaft, providing the drive, is at the base of the V.  The V12 was a
popular design used in the UK.  In the US, V16 is usual for freight
locomotives and there are some designs with V20 engines. 

Engines
used for DMU (diesel multiple unit) trains in the UK are often mounted
under the floor of the passenger cars.  This restricts the design to
in-line engines, which have to be mounted on their side to fit in the
restricted space.

An
unusual engine design was the UK 3,300 hp Class 55 locomotive, which
had the cylinders arranged in three sets of opposed Vs in a triangle,
in the form of an upturned delta, hence the name “Deltic”.

Tractive Effort, Pull and Power

Before
going too much further, we need to understand the definitions of
tractive effort, drawbar pull and power.  The definition of tractive
effort (TE) is simply the force exerted at the wheel rim of the
locomotive and is usually expressed in pounds (lbs) or kilo Newtons
(kN).  By the time the tractive effort is transmitted to the coupling
between the locomotive and the train, the drawbar pull, as it is called
will have reduced because of the friction of the mechanical parts of the
drive and some wind resistance. 

Power
is expressed as horsepower (hp) or kilo Watts (kW) and is actually a
rate of doing work.  A unit of horsepower is defined as the work
involved by a horse lifting 33,000 lbs one foot in one minute.  In the
metric system it is calculated as the power (Watts) needed when one
Newton of force is moved one metre in one second.  The formula is P =
(F*d)/t where P is power, F is force, d is distance and t is time.  One
horsepower equals 746 Watts.

The
relationship between power and drawbar pull is that a low speed and a
high drawbar pull can produce the same power as high speed and low
drawbar pull.  If you need to increase higher tractive effort and high
speed, you need to increase the power.  To get the variations needed by a
locomotive to operate on the railway, you need to have a suitable means
of transmission between the diesel engine and the wheels.

One
thing worth remembering is that the power produced by the diesel engine
is not all available for traction.  In a 2,580 hp diesel electric
locomotive, some 450 hp is lost to on-board equipment like blowers,
radiator fans, air compressors and “hotel power” for the train.

Starting

A
diesel engine is started (like an automobile) by turning over the
crankshaft until the cylinders “fire” or begin combustion.  The starting
can be done electrically or pneumatically.  Pneumatic starting was used
for some engines.  Compressed air was pumped into the cylinders of the
engine until it gained sufficient speed to allow ignition, then fuel was
applied to fire the engine.  The compressed air was supplied by a small
auxiliary engine or by high pressure air cylinders carried by the
locomotive.

Electric
starting is now standard.  It works the same way as for an automobile,
with batteries providing the power to turn a starter motor which turns
over the main engine.  In older locomotives fitted with DC generators
instead of AC alternators, the generator was used as a starter motor by
applying battery power to it.

Governor

Once
a diesel engine is running, the engine speed is monitored and
controlled through a governor.  The governor ensures that the engine
speed stays high enough to idle at the right speed and that the engine
speed will not rise too high when full power is demanded.  The governor
is a simple mechanical device which first appeared on steam engines.  It
operates on a diesel engine as shown in Figure 4. Modern diesel engines
use an electronic governor system that replicates the requirements of
the mechanical system.

Figure
4: Diagram of a traditional diesel engine governor. It consists of a
rotating shaft, driven by the diesel engine. A pair of flyweights are
linked to the shaft and they rotate as it rotates. The centrifugal force
caused by the rotation causes the weights to be thrown outwards as the
speed of the shaft rises. If the speed falls the weights move inwards.
The flyweights are linked to a collar fitted around the shaft by a pair
of arms. As the weights move out, so the collar rises on the shaft. If
the weights move inwards, the collar moves down the shaft. The movement
of the collar is used to operate the fuel rack lever controlling the
amount of fuel supplied to the engine by the injectors. Modern engines
use electronic governors. Diagram: Author.

Fuel Injection

Ignition
is a diesel engine is achieved by compressing air inside a cylinder
until it gets very hot (say 400°C, almost 800°F) and then injecting a
fine spray of fuel oil to cause a miniature explosion.  The explosion
forces down the piston in the cylinder and this turns the crankshaft. 
To get the fine spray needed for successful ignition the fuel has to be
pumped into the cylinder at high pressure.  The fuel pump is operated by
a cam driven off the engine.  The fuel is pumped into an injector,
which gives the fine spray of fuel required in the cylinder for
combustion. 

Fuel Control

In
an petrol engine, the power is controlled by the amount of fuel/air
mixture applied to the cylinder.  The mixture is mixed outside the
cylinder and then applied by a throttle valve. In a diesel engine the
amount of air applied to the cylinder is constant so power is regulated
by varying the fuel input. The fine spray of fuel injected into each
cylinder has to be regulated to achieve the amount of power required.
Regulation is achieved by varying the fuel sent by the fuel pumps to the
injectors.  

The amount of fuel being applied to the cylinders
is varied by altering the effective delivery rate of the piston in the
injector pumps. Each injector has its own pump, operated by an
engine-driven cam, and the pumps are aligned in a row so that they can
all be adjusted together. The adjustment is done by a toothed rack
(called the “fuel rack”) acting on a toothed section of the pump
mechanism.  As the fuel rack moves, so the toothed section of the pump
rotates and provides a drive to move the pump piston round inside the
pump. Moving the piston round, alters the size of the channel available
inside the pump for fuel to pass through to the injector delivery pipe.

The
fuel rack can be moved either by the driver operating the power
controller in the cab or by the governor.  If the driver asks for more
power, the control rod moves the fuel rack to set the pump pistons to
allow more fuel to the injectors.  The engine will increase power and
the governor will monitor engine speed to ensure it does not go above
the predetermined limit.  The limits are fixed by springs (not shown)
limiting the weight movement.

Engine Control Development

So
far we have seen a simple example of diesel engine control but the
systems used by most locomotives in service today are more
sophisticated. To begin with, the drivers control was combined with the
governor and hydraulic control was introduced. One type of governor uses
oil to control the fuel racks hydraulically and another uses the fuel
oil pumped by a gear pump driven by the engine. Some governors are also
linked to the turbo charging system to ensure that fuel does not
increase before enough turbocharged air is available. In modern systems,
the governor is electronic and is part of a complete engine management
system.

Power Control

The
diesel engine in a diesel-electric locomotive provides the drive for
the main alternator which, in turn, provides the power required for the
traction motors.  We can see from this therefore, that the power
required from the diesel engine is related to the power required by the
motors.  So, if we want more power from the motors, we must get more
current from the alternator so the engine needs to run faster to
generate it.  Therefore, to get the optimum performance from the
locomotive, we must link the control of the diesel engine to the power
demands being made on the alternator.

In
the days of generators, a complex electro-mechanical system was
developed to achieve the feedback required to regulate engine speed
according to generator demand. The core of the system was a load
regulator, basically a variable resistor which was used to very the
excitation of the generator so that its output matched engine speed. 
The control sequence (simplified) was as follows:

1.  Driver moves the power controller to the full power position
2. 
An air operated piston actuated by the controller moves a lever, which
closes a switch to supply a low voltage to the load regulator motor.
3.  The load regulator motor moves the variable resistor to increase the main generator field strength and therefore its output.
4.  The load on the engine increases so its speed falls and the governor detects the reduced speed.
5.  The governor weights drop and cause the fuel rack servo system to actuate.
6.  The fuel rack moves to increase the fuel supplied to the injectors and therefore the power from the engine.
7.  The lever (mentioned in 2 above) is used to reduce the pressure of the governor spring.
8.  When the engine has responded to the new control and governor settings, it and the generator will be producing more power.

On
locomotives with an alternator, the load regulation is done
electronically. Engine speed is measured like modern speedometers, by
counting the frequency of the gear teeth driven by the engine, in this
case, the starter motor gearwheel.  Electrical control of the fuel
injection is another improvement now adopted for modern engines.
Overheating can be controlled by electronic monitoring of coolant
temperature and regulating the engine power accordingly. Oil pressure
can be monitored and used to regulate the engine power in a similar way.

Cooling

Like
an automobile engine, the diesel engine needs to work at an optimum
temperature for best efficiency.  When it starts, it is too cold and,
when working, it must not be allowed to get too hot.  To keep the
temperature stable, a cooling system is provided.   This consists of a
water-based coolant circulating around the engine block, the coolant
being kept cool by passing it through a radiator. 

The
coolant is pumped round the cylinder block and the radiator by an
electrically or belt driven pump. The temperature is monitored by a
thermostat and this regulates the speed of the (electric or hydraulic)
radiator fan motor to adjust the cooling rate.  When starting the
coolant isn’t circulated at all.  After all, you want the temperature to
rise as fast as possible when starting on a cold morning and this will
not happen if you a blowing cold air into your radiator. Some radiators
are provided with shutters to help regulate the temperature in cold
conditions.

If
the fan is driven by a belt or mechanical link, it is driven through a
fluid coupling to ensure that no damage is caused by sudden changes in
engine speed.  The fan works the same way as in an automobile, the air
blown by the fan being used to cool the water in the radiator.  Some
engines have fans with an electrically or hydrostatically driven motor. 
An hydraulic motor uses oil under pressure which has to be contained in
a special reservoir and pumped to the motor.  It has the advantage of
providing an in-built fluid coupling.

A
problem with engine cooling is cold weather.  Water freezes at 0°C or
32°F and frozen cooling water will quickly split a pipe or engine block
due to the expansion of the water as it freezes.  Some systems are “self
draining” when the engine is stopped and most in Europe are designed to
use a mixture of anti-freeze, with Gycol and some form of rust
inhibitor.  In the US, engines do not normally contain anti-freeze,
although the new GM EMD “H” engines are designed to use it.  Problems
with leaks and seals and the expense of putting a 100 gallons (378.5
litres) of coolant into a 3,000 hp engine, means that engines in the US
have traditionally operated without it.  In cold weather, the engine is
left running or the locomotive is kept warm by putting it into a heated
building or by plugging in a shore supply.  Another reason for keeping
diesel engines running is that the constant heating and cooling caused
by shutdowns and restarts, causes stresses in the block and pipes and
tends to produce leaks.

Lubrication

Like
an automobile engine, a diesel engine needs lubrication.  In an
arrangement similar to the engine cooling system, lubricating oil is
distributed around the engine to the cylinders, crankshaft and other
moving parts.  There is a reservoir of oil, usually carried in the sump,
which has to be kept topped up, and a pump to keep the oil circulating
evenly around the engine.  The oil gets heated by its passage around the
engine and has to be kept cool, so it is passed through a radiator
during its journey.  The radiator is sometimes designed as a heat
exchanger, where the oil passes through pipes encased in a water tank
which is connected to the engine cooling system. 

The
oil has to be filtered to remove impurities and it has to be monitored
for low pressure. If oil pressure falls to a level which could cause the
engine to seize up, a “low oil pressure switch” will shut down the
engine. There is also a high pressure relief valve, to drain off excess
oil back to the sump.

Sources:

The
Railroad, What it is, What it Does by John H. Armstrong, 1993, Simmons
Boardman Books Inc.;  BR Diesel Traction Manual for Enginemen, British
Transport Commission, 1962;   BR Equipment, David Gibbons, Ian Allan,
1986 and 1990;   Modern Railways;  International Railway Journal; 
Railway Gazette International;  Mass Transit;  Trains Magazine.