Early internal combustion engine-powered locomotives used
gasoline as their fuel. Soon after Dr. Rudolf Diesel patented
his first compression ignition engine in 1892, its
application for railway propulsion was considered. Progress was
slow, however, because of the poor power-to-weight ratio of the
early engines, as well as the difficulty inherent in
mechanically applying power to multiple driving wheels on
swivelling trucks (bogies).
Steady improvements in the Diesel engine's design (many developed by Sulzer Ltd. of Switzerland, with whom Dr. Diesel was associated for a time) gradually reduced its physical size and improved its power-to-weight ratio to a point where one could be mounted in a locomotive. Once the concept of Diesel-electric drive was accepted the pace of development quickened. By the mid 20th century, the Diesel locomotive had become the dominant type of locomotive in much of the world, offering greater flexibility and performance than the steam locomotive, as well as substantially lower operating and maintenance costs. Currently, almost all Diesel locomotives are Diesel-electric.
Adaptation of the Diesel engine for rail use
The world's first oil-engined railway locomotive was built for the Royal Arsenal, Woolwich, England, in 1896, using an engine designed by Herbert Akroyd Stuart. It was not, strictly, a diesel because it used a hot bulb engine (also known as a semi-diesel) but it was the precursor of the diesel.
Following the expiration of Dr. Diesel’s patent in 1912, his engine design was successfully applied to marine propulsion and stationary applications. However, the massiveness and poor power-to-weight ratio of these early engines made them unsuitable for propelling land-based vehicles. Therefore, the engine's potential as a railroad prime mover was not initially recognized. This changed as development reduced the size and weight of the engine.
The world’s first Diesel-powered locomotive was operated in the summer of 1912 on the Winterthur-Romanshorn Railroad in Switzerland, but was not a commercial success. In 1906, Rudolf Diesel, Adolf Klose and the steam and Diesel engine manufacturer Gebrüder Sulzer founded Diesel-Sulzer-Klose GmbH for the manufacture of Diesel-powered locomotives. Sulzer had been manufacturing Diesel engines since 1898. Prussian State Railways ordered in 1909 a Diesel locomotive from the company, and after test runs between Winterthur and Romanshorn the Diesel-mechanical locomotive was delivered in Berlin in September 1912. During further test runs in 1913 several problems were found. After the first world war broke out in 1914, all further trials were stopped. The locomotive weight was 95 tonnes and the power was 883 kW with a maximum speed of 100 km/h.
Adolphus Busch purchased the American manufacturing rights for the Diesel engine in 1898 but never applied this new form of power to transportation. Only limited success was achieved in the early twentieth century with direct-driven gasoline and Diesel powered railcars. General Electric (GE) entered the railcar market in the early twentieth century, as Thomas Edison possessed an outstanding patent on the electric locomotive, his design actually being a type of electrically propelled railcar. GE built its first electric locomotive prototype in 1895. However, high electrification costs caused GE to turn its attention to Diesel power to provide electricity for electric railcars. Problems related to co-coordinating the Diesel engine and electric motor were immediately encountered, primarily due to limitations of the Ward Leonard electric elevator drive system that had been chosen.
The first significant breakthrough occurred in 1914, when Hermann Lemp, a GE electrical engineer, developed and patented a reliable direct current electrical control system (subsequent improvements were also patented by Lemp). Lemp's design used a single lever to control both engine and generator in a coordinated fashion, and was the prototype for all Diesel-electric locomotive control systems.
In 1917, GE produced an experimental Diesel-electric locomotive using Lemp's control design, the first known to be built in the United States. Following this development, the 1923 Kaufman Act banned steam locomotives from New York City because of severe pollution problems. The response to this law was to electrify high-traffic rail lines. However, electrification was uneconomical to apply to lower-traffic areas.
In response to the Kaufman Act, New York City railroads approached Ingersoll-Rand to build a prototype Diesel switching locomotive (shunter), the AGEIR boxcabs. The resulting unit was fitted with an electrical generator and traction motors supplied by GE, as well as a form of Lemp's control system, and was delivered in July 1925. These locomotives demonstrated that the Diesel-electric power unit could provide many of the benefits of an electric locomotive without the railroad having to bear the sizeable expense of electrification.
In the mid 1920s, Baldwin Locomotive Works produced a prototype Diesel-electric locomotive for "special uses" (such as for runs where water for steam locomotives was scarce) using electrical equipment from Westinghouse Electric Company. Industry sources were beginning to suggest “the outstanding advantages of this new form of motive power.” In 1929, the Canadian National Railway became the first North American railway to use diesels in mainline service with 2 units, 9000 and 9001, from Westinghouse.
In 1935, Krauss-Maffei, MAN and Voith built the first Diesel-hydraulic locomotive, the V140 in Germany. Because the German railways (DR) was very pleased with the performance of the locomotive, Diesel-hydraulics became the mainstream in Diesel locomotives in Germany.
The first regular service of Diesel-electric locomotives was in switching applications. General Electric produced several small switching locomotives in the 1930s (the famous "44-tonner" switcher was introduced in 1940) Westinghouse Electric and Baldwin collaborated to build switching locomotives starting in 1929. However, the Great Depression curtailed demand for Westinghouse’s electrical equipment, and they stopped building locomotives internally, opting to supply electrical parts instead.
Diesel-electric railroad locomotion entered the mainstream when the Burlington Railroad and Union Pacific used Diesel "streamliners" to haul passengers. Following the successful 1939 tour of General Motors' EMD's FT demonstrator freight locomotive set, the transition from steam to Diesel power began, the pace substantially quickening in the years following the close of World War II.
Diesel engines slowly eclipsed those powered by steam as the
manufacturing and operational efficiencies of the former made
them cheaper to own and operate. While initial costs of diesel
engines were high, steam locomotives were custom-made for
specific railway routes and lines and, as such, economies of
scale were difficult to achieve.[ Though more complex to
produce with exacting manufacturing tolerances (1/10,000th of
an inch (0.0025 mm) vs. 1/100th of an inch (0.25 mm) for
steam), diesel locomotive parts were more conducive to mass
production. While the steam engine manufacturer Baldwin offered
almost five hundred steam models in its heyday, EMD offered
fewer than ten diesel varieties.
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews) and the operating environment is much more attractive, being much quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel engines can be started and stopped almost instantly, meaning that a diesel locomotive has the potential to incur no costs when not being used. Steam locomotives require intensive maintenance, lubrication and cleaning before, during and after use. Preparing a steam locomotive for use can take many hours, especially if the locomotive is being fired from cold. However, it is still the practice of large North American railroads to use straight water as a coolant in diesel engines instead of coolants that incorporate anti-freezing properties. This results in diesel locomotives being left idling when parked in cold climates instead of being completely shut down. Still, a diesel engine can be left idling unattended for hours or even days, especially since practically every diesel engine used in locomotives has systems that automatically shut the engine down if a problem such as a loss of oil pressure or coolant loss occur. A steam locomotive, by comparison, may be kept in readiness between uses with a small fire to maintain a slight heat in the boiler, but requires regular and frequent attention to maintain the fire and the level of water in the boiler.
Moreover, maintenance and operational costs of steam locomotives were much higher than diesel counterparts even though it would take diesel locomotives almost 50 years to reach the same horsepower output that steam locomotives could achieve at their technological height. Annual maintenance costs for steam locomotives accounted for 25% of the initial purchase price. Spare parts were machined from wooden masters for specific locomotives. The sheer number of unique steam locomotives meant that there was no feasible way for spare-part inventories to be maintained. Steam engines also required large quantities of coal and water, which were expensive variable operating costs. Further, the thermal efficiency of steam was considerably less than that of Diesel engines. Diesel’s theoretical studies demonstrated potential thermal efficiencies for a compression ignition engine of 36% (compared with 6-10% for steam), and an 1897 one-cylinder prototype operated at a remarkable 26% efficiency. By the middle of the twentieth century, Diesel locomotives had effectively replaced steam engines.
A diesel-mechanical locomotive uses a mechanical transmission in a fashion similar to that employed in most road vehicles. This type of transmission is generally limited to low-powered, low speed shunting (switching) locomotives, lightweight multiple units and self-propelled railcars.
The mechanical transmissions used for railroad propulsion are generally more complex and much more robust than road versions. There is usually a fluid coupling interposed between the engine and gearbox, and the gearbox is often of the epicyclic (planetary) type to permit shifting while under load. Various systems have been devised to minimise the break in transmission during gear changing, e.g. the S.S.S. (synchro-self-shifting) gearbox used by Hudswell Clarke.
Diesel-mechanical propulsion is limited by the difficulty of building a reasonably sized transmission capable of coping with the power and torque required to move a heavy train. A number of attempts to use Diesel-mechanical propulsion in high power applications have been made (e.g. the 1,500 kW (2000 horsepower) British Rail 10100 locomotive), although none have proved successful in the end.
For locomotives powered by both external electricity and diesel fuel, see electro-diesel below. For locomotives powered by a combination of diesel or fuel cells and batteries or ultracapacitors, see hybrid locomotive.
In a Diesel-electric locomotive, the Diesel engine drives an electrical generator whose output provides power to the traction motors. There is no mechanical connection between the engine and the wheels. The important components of Diesel-electric propulsion are the Diesel engine (also known as the prime mover), the main generator, traction motors and a control system consisting of the engine governor as well as electrical or electronic components used to control or modify the electrical supply to the traction motors, including switchgear, rectifiers and other components. In the most elementary case, the generator may be directly connected to the motors with only very simple switchgear.
Originally, the traction motors and generator were DC machines. Following the development of high-capacity silicon rectifiers in the 1960s, the DC generator was replaced by an alternator using a diode bridge to convert its output to DC. This advance greatly improved locomotive reliability and decreased generator maintenance costs by elimination of the commutator and brushes in the generator. Elimination of the brushes and commutator, in turn, disposed of the possibility of a particularly destructive type of event referred to as flashover, which could result in immediate generator failure and, in some cases, start an engine room fire.
More recently, the development of high-power variable-frequency/variable-voltage (VVVF) drives, or "traction inverters," has allowed the use of polyphase AC traction motors, thus also eliminating the motor commutator and brushes. The result is a more efficient and reliable drive that requires relatively little maintenance and is better able to cope with overload conditions that often destroyed the older types of motors.
A Diesel-electric locomotive's power output is independent of road speed, as long as the unit’s generator current and voltage limits are not exceeded. Therefore, the unit's ability to develop tractive effort (also referred to as drawbar pull or tractive force, which is what actually propels the train) will tend to inversely vary with speed within these limits. (See power curve below). Maintaining acceptable operating parameters was one of the principal design considerations that had to be solved in early Diesel-electric locomotive development and, ultimately, led to the complex control systems in place on modern units.
The prime mover's power output is primarily determined by its rotational speed (RPM) and fuel rate, which are regulated by a governor or similar mechanism. The governor is designed to react to both the throttle setting, as determined by the engine driver and the speed at which the prime mover is running.
Locomotive power output, and thus speed, is typically controlled by the engine driver using a stepped or "notched" throttle that produces binary-like electrical signals corresponding to throttle position. This basic design lends itself well to multiple unit (MU) operation by producing discrete conditions that assure that all units in a consist respond in the same way to throttle position. Binary encoding also helps to minimize the number of trainlines (electrical connections) that are required to pass signals from unit to unit. For example, only four trainlines are required to encode all throttle positions.
North American locomotives, such as those built by EMD or General Electric, have nine throttle positions, one idle and eight power (as well as an emergency stop position that shuts down the prime mover). Many UK-built locomotives have a ten-position throttle. The power positions are often referred to by locomotive crews as "run 3" or "notch 7", depending upon the throttle setting.
In older locomotives, the throttle mechanism was ratcheted so that it was not possible to advance more than one power position at a time. The engine driver could not, for example, pull the throttle from notch 2 to notch 4 without stopping at notch 3. This feature was intended to prevent rough train handling due to abrupt power increases caused by rapid throttle motion ("throttle stripping," an operating rules violation on many railroads). Modern locomotives no longer have this restriction, as their control systems are able to smoothly modulate power and avoid sudden changes in train loading regardless of how the engine driver operates the controls.
When the throttle is in the idle position, the prime mover will be receiving minimal fuel, causing it to idle at low RPM. In addition, the traction motors will not be connected to the main generator and the generator's field windings will not be excited (energized) — the generator will not produce electricity with no excitation. Therefore, the locomotive will be in "neutral". Conceptually, this is the same as placing an automobile's transmission into neutral while the engine is running.
To set the locomotive in motion, the reverser control handle is placed into the correct position (forward or reverse), the brake is released and the throttle is moved to the run 1 position (the first power notch). An experienced engine driver can accomplish these steps in a coordinated fashion that will result in a nearly imperceptible start. The positioning of the reverser and movement of the throttle together is conceptually like shifting an automobile's automatic transmission into gear while the engine is idling
Placing the throttle into the first power position will cause the traction motors to be connected to the main generator and the latter's field coils to be excited. With excitation applied, the main generator will deliver electricity to the traction motors, resulting in motion. If the locomotive is running "light" (that is, not coupled to a train) and is not on an ascending grade, it will easily accelerate. On the other hand, if a long train is being started, the locomotive may stall as soon as some of the slack has been taken up, as the drag imposed by the train will exceed the tractive force being developed. An experienced engine driver will be able to recognize an incipient stall and will gradually advance the throttle as required to maintain the pace of acceleration.
As the throttle is moved to higher power notches, the fuel rate to the prime mover will increase, resulting in a corresponding increase in RPM and horsepower output. At the same time, main generator field excitation will be proportionally increased to absorb the higher power. This will translate into increased electrical output to the traction motors, with a corresponding increase in tractive force. Eventually, depending on the requirements of the train's schedule, the engine driver will have moved the throttle to the position of maximum power and will maintain it there until the train has accelerated to the desired speed.
As will be seen in the following discussion, the propulsion system is designed to produce maximum traction motor torque at start-up, which explains why modern locomotives are capable of starting trains weighing in excess of 15,000 tons, even on ascending grades. Current technology allows a locomotive to develop as much as 30 percent of its loaded driver weight in tractive force, amounting to some 120,000 pounds of drawbar pull for a large, six-axle freight (goods) unit. In fact, a consist of such units can produce more than enough drawbar pull at start-up to damage or derail cars (if on a curve) or break couplers (the latter being referred to in North American railroad slang as "jerking a lung"). Therefore, it is incumbent upon the engine driver to carefully monitor the amount of power being applied at start-up to avoid damage. In particular, "jerking a lung" could be a calamitous matter if it were to occur on an ascending grade.
PROPULSION SYSTEM OPERATION
As previously explained, the locomotive's control system is designed so that the main generator electrical power output is matched to any given engine speed. Given the innate characteristics of traction motors, as well as the way in which the motors are connected to the main generator, the generator will produce high current and low voltage at low locomotive speeds, gradually changing to low current and high voltage as the locomotive accelerates. Therefore, the net power produced by the locomotive will remain constant for any given throttle setting.
In older designs, the prime mover's governor and a companion device, the load regulator, play a central role in the control system. The governor has two external inputs: requested engine speed, determined by the engine driver's throttle setting, and actual engine speed (feedback). The governor has two external control outputs: fuel injector setting, which determines the engine fuel rate, and load regulator position, which affects main generator excitation. The governor also incorporates a separate overspeed protective mechanism that will immediately cut off the fuel supply to the injectors and sound an alarm in the cab in the event the prime mover exceeds a defined RPM. Not all of these inputs and outputs are necessarily electrical.
The load regulator is essentially a large potentiometer that controls the main generator power output by varying its field excitation and hence the degree of loading applied to the engine. The load regulator's job is relatively complex, because although the prime mover's power output is proportional to RPM and fuel rate, the main generator's output is not (which characteristic was not correctly handled by the Ward Leonard elevator drive system that was initially tried in early locomotives).
As the load on the engine changes, its rotational speed will also change. This is detected by the governor through a change in the engine speed feedback signal. The net effect is to adjust both the fuel rate and the load regulator position so that engine RPM and torque (and thus power output) will remain constant for any given throttle setting, regardless of actual road speed.
In newer designs controlled by a “traction computer,” each engine speed step is allotted an appropriate power output, or “kW reference”, in software. The computer compares this value with actual main generator power output, or “kW feedback”, calculated from traction motor current and main generator voltage feedback values. The computer adjusts the feedback value to match the reference value by controlling the excitation of the main generator, as described above. The governor still has control of engine speed, but the load regulator no longer plays a central role in this type of control system. However, the load regulator is retained as a “back-up” in case of engine overload. Modern locomotives fitted with electronic fuel injection (EFI) may have no mechanical governor, however a “virtual” load regulator and governor are retained with computer modules.
Traction motor performance is controlled either by varying the DC voltage output of the main generator, for DC motors, or by varying the frequency and voltage output of the VVVF for AC motors. With DC motors, various connection combinations are utilized to adapt the drive to varying operating conditions.
At standstill, main generator output is initially low voltage/high current, often in excess of 1000 amperes per motor at full power. When the locomotive is at or near standstill, current flow will be limited only by the DC resistance of the motor windings and interconnecting circuitry, as well as the capacity of the main generator itself. Torque in a series-wound motor is approximately proportional to the square of the current. Hence, the traction motors will produce their highest torque, causing the locomotive to develop maximum tractive effort, enabling it to overcome the inertia of the train. This effect is analogous to what happens in an automobile automatic transmission at start-up, where it is in first gear and thus producing maximum torque multiplication.
As the locomotive accelerates, the now-rotating motor armatures will start to generate a counter-electromotive force (back EMF, meaning the motors are also trying to act as generators), which will oppose the output of the main generator and cause traction motor current to decrease. Main generator voltage will correspondingly increase in an attempt to maintain motor power, but will eventually reach a plateau. At this point, the locomotive will essentially cease to accelerate, unless on a downgrade. Since this plateau will usually be reached at a speed substantially less than the maximum that may be desired, something must be done to change the drive characteristics to allow continued acceleration. This change is referred to as "transition," a process that is analogous to shifting gears in an automobile.
Transition methods include:
- Series / Parallel or "motor transition".
- Initially, pairs of motors are connected in series across the main generator. At higher speed, motors are reconnected in parallel across the main generator.
- "Field shunting", "field diverting", or "weak fielding".
- Resistance is connected in parallel with the motor field. This has the effect of increasing the armature current, producing a corresponding increase in motor torque and speed.
Both methods may also be combined, to increase the operating speed range.
- Generator transition
- Reconnecting the two separate internal main generator stator windings from parallel to series to increase the output voltage.
In older locomotives, it was necessary for the engine driver to manually execute transition by use of a separate control. As an aid to performing transition at the right time, the load meter (an indicator that informs the engine driver on how much current is being drawn by the traction motors) was calibrated to indicate at which points forward or backward transition should take place. Automatic transition was subsequently developed to produce better operating efficiency, and to protect the main generator and traction motors from overloading from improper transition.
These special locomotives can operate as an electric locomotive or as a Diesel locomotive. The Long Island Rail Road and Metro-North Railroad operate dual-mode diesel-electric/third-rail locomotives between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason, Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified, as the third rail power system is extremely hazardous in a yard area.
Diesel-hydraulic locomotives use a torque converter or hydraulic drive system to convey the power from the diesel engine to the wheels.
Hydrokinetic transmission (also called hydrodynamic transmission) uses a torque converter. A torque converter consists of three main parts, two of which rotate, and one that has a lock preventing backwards rotation and adding output torque by redirecting the oil flow at low output RPM. All three main parts are sealed in an oil-filled housing. To match engine speed to load speed over the entire speed range of a locomotive some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power.
Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range. A fluid coupling is similar to a torque converter but it lacks the stator. The output torque is equal to the input torque regardless of the ratio of input to output speed; loading the output shaft results not in torque multiplication and constant power throughput but in reduction of the input speed with consequent lower power throughput. (In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears.) The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple unit compared to a locomotive makes this an acceptable trade-off for reduced mechanical complexity.
Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics, but initial versions were found in many countries to be mechanically more complicated and more likely to break down. Hydraulic transmission for locomotives was developed in Germany. The bad reputation of diesel-hydraulic principle was caused by the poor durability and reliability of the Maybach Mekydro hydraulic transmission. The Mekydro consisted of a hydraulic torque converter followed by a four speed automatic mechanical gearbox. Voith developed a different solution using several torque converters, and it has proven to be extremely durable and very well-suited for the purpose.
In Germany and Finland, diesel-hydraulic systems have achieved extremely high reliability in operation. Persistent argument continues over the relative reliability of hydraulic systems, with continuing questions over whether data were manipulated politically to favour local suppliers over German ones. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in some European countries.
The high reliability of the German locomotives was paralleled by higher reliability of non-German locomotives built with German-made parts compared to that of the same designs built using parts made locally to German patterns under licence. Much of the unreliability experienced outside Germany was due to poor quality control in the local manufacture of engines and transmissions. Another contributing factor was poor maintenance due to staff accustomed to steam locomotives now working on unfamiliar and much more complex designs in unsuitable conditions, and failing to follow the unit-replacement maintenance methods that were part of the German success. It is notable that diesel-hydraulic multiple units, with the advantages of modern manufacturing techniques and improved maintenance procedures, are now extremely successful in widespread use, achieving excellent reliability.
The diesel-hydraulic locomotive has two distinct advantages over the diesel-electric. First, it is lighter for the same power output. This is particularly important for usage on branch lines allowing only smaller axle loads, which had been the case in Germany for a long time. Main lines, built for higher axle loads, had already been electrified there, which, e.g., was not the case in the US where diesel locomotives were used on main lines as well. Secondly, the factor of adhesion is better, meaning higher starting tractive effort relative to the locomotive weight. This is because in a diesel-electric all driven axles are driven by individual electric motors and can lose grip individually whereas in a diesel-hydraulic all axles are interconnected by shafts and universal joints. Thus, all axles must rotate at the same speed, which makes individual slipping of axles impossible. However this second advantage is eliminated if the electric traction motors have anti slip control, a feature often included in modern designs.