Diesel Locomotive FAQ

1. What is a diesel locomotive?
Actually, it is more properly called a diesel-electric locomotive. The concept is relatively simple: An oil-burning engine turns an alternator or generator which in turn produces electricity that powers traction motors that connect to the axles of the locomotive. This process is much more efficient than the external-combustion steam locomotive.

2. I want more details.
The gasoline engine, like in an automobile, has a thermal efficiency (the conversion of fuel into work) of 8 or 9%. The diesel engine, however, has a thermal efficiency of about 30%. Unlike in a gasoline engine in which the fuel is ignited by spark plugs, the fuel in a diesel engine ignites because of the air pressure inside the cylinders. The air in the cylinders is raised to about 500-600 psi which raises the temperature inside to about 1000 degrees F. Oil injected into the hot air ignites and expands. The expanding gas forces the piston to move down and this turns the crankshaft that is connected to the generator(DC) or the alternator(AC) where electricity is produced. When the piston rises again from the momentum, the gas is expelled from the cylinder and the cycle begins again. The generator or alternator then provides power to the traction motors. Then you're on your way!

3. What was the first diesel locomotive ever built?
The first gas-powered railcar was built in 1910 by General Electric. The first commercial diesel locomotive was built, also by GE, in 1918 for the Jay Street Connecting Railroad in New York. Although not particularly successful, it sparked further interest that led to the construction of a diesel locomotive demonstrator in 1924. Later that year, GE, in cooperation with the American locomotive Company(Alco), began marketing diesel yard switchers to the railroads. Thus, quietly and with little fanfare, the diesel locomotive made its first appearance on the nation's railroads. Who could have guessed that those little switchers would start a revolution that would forever change the face of railroading?

4. What is a "cab" unit?
A cab unit has no external walkways and the carbody is the same width as the locomotive itself. The carbody is also critical to the structural integrity of the unit. Indeed, if the carbody is removed without extra support, the body of the locomotive may collapse. (This feature made the cab unit somewhat difficult to service for heavy repairs and led to the development of the hood unit)

5. How do you describe the wheel configuration of a diesel locomotive? What does A-1-A mean?
This is a little tricky. Powered axles are given letter designations while unpowered ones are assigned numbers.
For example, the Alco PA is an A-1-A,(first truck) A-1-A(second truck) diesel. This means the first axle is powered, but the second is not. Therefore, the second axle is given the number 1. The next axle is powered, but since the axle before it is given a number and is not powered, we start over again in the alphabet and assign it the letter A. Whew! Another example is the F unit. It is called a BO-BO because all axles are powered and the O designation means that each axle is powered directly by the traction motors and not by a link with other powered axles.

Alco PA A-1-A, A-1-A


6. What are dynamic brakes?
In a diesel locomotive, the traction motors can be turned into generators powered by the turning wheels. The electrical current that is produced is converted into heat that absorbs a large amount of the train's energy and causes it to slow down. The extra heat is dissipated by the dynamic brake grid. This is very useful on downgrades and saves a considerable amount of wear on brakes, thus lowering maintenance costs. This also allows longer trains to be pulled since there is more control and less of a risk of a loss of braking on a long train.

7. When were the E,F and PA, FA units introduced?
The EA was first introduced in 1937 by EMD. The F first ran in 1939. The PA and FA appeared in 1946.

8. What is the difference in an A unit and a B unit?
An A unit has a cab for the crew, the B unit does not. The B unit is
also called the booster unit. The booster is designed to be run in tandem with an A unit.

9. Who built diesel cab units?
The Electro Motive Division of General Motors built the popular E and F units. EMD dominated the diesel field. A distant second was the American Locomotive Company, or Alco. It built the beautiful PA and FA locomotives. Baldwin and Fairbanks-Morse trailed behind EMD and Alco by a huge margin. Baldwin built the Sharknoses, Babyfaces, and Centipedes. FM built the Erie units and C-Liners. Except for EMD and FM, the other companies had built steam locomotives in the past. Of the companies mentioned above, only EMD remains today.

10. How many cab units were built?
Counting all builders, about 9000 cab units were constructed from the thirties to the sixties.

11. Exactly why did the diesel-electric locomotive replace the steam locomotive?
A. Thermal efficiency of a diesel is about 30%, compared to 6-7% for a steam locomotive. The diesel is therefore much more fuel efficient.
B. Diesels develop maximum horsepower and efficiency over a wide range of speeds. Steamers have a very narrow speed range in which they reach full efficiency.
C. Diesels can be operated in multiple units(MU) under only one set of controls. This means that 1 unit can control many other units. This allows once crew per train and greatly reduces labor costs.
D. Dynamic Braking allows good speed control on downgrades and reduces brake repairs. Longer trains are also allowed with better speed control.
E. Maintenance costs are very low in comparison to steamer. Diesel locomotives have an availability of 90% or better, compared to 30-40% for the average steamer. Standardized and modular design played a major role in the diesel's advantage over steam. A diesel could replace about 10 steam locomotives.
F. Fewer fuel and water stops.A diesel requires little water. Diesels allowed the retirement of $50,000,000 worth of equipment to supply water to very thirsty steamers.
G. A low center of gravity enables higher train speeds on curves.
H. Unlike steam locomotives, diesels do not stress the track with the pounding force of reciprocating components. Track maintenance is reduced as a result.
I. Since diesel locomotives were standardized, they made good collateral on bank loans. This meant that railroads could borrow money easier and upgrade to diesels even if the financial condition of the road was not good.
J. The average steamer after World War II was 20 years old and out of date. Although modern steam was able to get within striking distance of the diesel in terms of availability and efficiency, (but they were still more expensive to operate than diesels) it was logical to replace aging steamers with diesels.
K. The rising cost of coal and inability to find spare parts also hastened the demise of the steam locomotive.

12. Just how much money did the diesel save the railroads?
The Baltimore and Ohio Railroad provides a good example. From 1945-1957, a 12 year period, the transition from steam to diesel resulted in considerable savings in train operations. During this period, total fuel costs dropped from $23.6 million to $21.2 million. In 1945, fuel costs averaged 18% of all transportation costs. By 1957, this had dropped to 11% of total cost. This continued to drop and hit 8.5% in 1960. The cost of water dropped from $954,000 in 1945 to $147,000 in 1960. These figures are particularly impressive when you factor in inflation. When you add all these totals up, you can see that the diesel was a lifesaver for the railroads at a time when increased efficiency was vital in competing with trucks, automobiles, and airlines.

The Pennsylvania Railroad serves as another example. A 1947 study compared the economic performance of the TI and Q2 steam locomotives to 6000 horsepower sets of diesels. (4 1500 hp freight units and 3 2000 hp passenger units. No distinction was made between builders) On the passenger side, a T1 cost $1.67 per mile to operate and a 6000 hp diesel set cost $1.30. For freight trains, a Q2 cost $2.37 per mile and a 6000 hp diesel set cost $1.94. These figures factored in maintenance, fuel, and other related costs, but did not take into account reduced expenses for labor with the elimination of steam helpers, reduced train crews because of multiple unit operation, and fewer trains required by using diesels. A 1951 study, again not distinguishing between builders, put the cost of operating a 1500/1600 hp freight unit at $0.88 per mile, and a 2000 hp passenger unit at $0.73 per mile. As these facts indicate, a railroad could achieve substantial savings in short and long term operating costs by dieselizing as quickly as possible.

13. Why use an electric transmission? Why not a hydraulic or direct drive transmission?
To put it simply, electrical transmissions match engine capabilities better than other systems. The electric transmission allows the engine to operate at the most efficient RPM and horsepower for the conditions of a railroad.

14. Why are diesels left to idle so much instead of being shut down?
Most diesel locomotives use water as coolant. If the unit is left shutdown for an extended period of time in freezing temperatures, the water will freeze. This could damage the locomotive since water expands as it freezes. Most railroads have rules that locomotives cannot be shut down in temperatures below 40 degrees unless the water is drained. Since starting diesels is time consuming, they are frequently left running even in warmer weather. Fuel consumption drops to only a few gallons per hour in idle mode.

15. How are diesel locomotives cooled? Water is used as collant in most locomotives.

16. Why don't they use anti-freeze in diesel locomotives?
Most diesel locomotives do not use anti-freeze because the chemicals in anti-freeze can be harmful to certain types of metal used in the primer mover. There is also the problem of leaks allowing oil and anti-freeze to mix, causing a loss of much of the lubricating ability of the oil. In addition, anti-freeze can be very expensive and a large amount would be necessary for a locomotive. The new 6000 hp units from EMD and GE are capable of using anti-freeze, as well as the caterpillar engines used in some units.

17. What is the main limit in locomotive operation? Explain.
The amperage limit of the traction motors. The tractive effort of a locomotive is proportional to the torque of the traction motors. High torque, and hence high tractive effort, demands high amperage in the traction motors. However, the level of amperage the motors can withstand is quite limited. If the motors are overloaded for too long, they can burn out. As speed increases, amperage decreases and voltage increases. This is the reason locomotives have a minimum speed and a starting tractive effort. (STE) The STE can only be maintained for a short period of time. The result is that additional locomotives may have to be assigned to a train simply because of traction motor limitations when the extra power is not necessary. (particularly on a long and slow coal train on a steep grade.) This is one reason the new locomotives with AC traction motors are so popular with the railroads, there traction motors can withstand much higher thermal loads than DC motors, and can therefore pull heavier loads with fewer units. (In some assignments)

18. What is a flashover? What causes a flashover? A flashover occurs when the brushes in the commutators of DC traction motors touch each other because of a unusual movement or bump, such as when a locomotive goes over a grade crossing. If the brushes touch each other, an electrical short can occur almost like a lightning strike. Obviously, this can severely damage the traction motors. Locomotives with DC traction motors must have the throttle reduced slightly when going over a grade crossing to reduce the risk of a flashover. Newer locomotives with AC traction motors have no commutator brushes and therefore do not have to be throttled down at grade crossings.

19. What is shunting? As the speed of a locomotive increases, the traction motors generate large amounts of extra electricity that is simply not needed. This creates resistance in the motors (called counter-emf) and reduces the amount of amperage going into the motors, which limits speed. Shunting is a process in which the resistance is reduced by lowering the flow of electricity to the magnets that create the EM field in the motors without reducing the amperage. This lowers the resistance the traction motors face. (Think of shunting as like shifting gears in a car)

20. What is transition? A procedure that reduces the resistance the traction motors (see shunting) face by changing the proportion of amperage and voltage while not changing the output of the alternator. This reduces counter-emf. This allows the locomotive to develop more amperage and volts as needed. Transition may cause a brief interruption in tractive effort and can result in a broken coupler if slack develops and then the cars snap back as TE resumes. (Think of transition as like shifting gears in a car)

21. Do all locomotives have transition and shunting? No.

22. What is Turbocharging? This device uses exhaust gases from the diesel engine to drive a turbine that supplies highly compressed air to rapidly remove (scavenge) exhaust gases from the cylinders. This increases the compression in the cylinders, and helps to cool the cylinders and cylinder heads. Increased compression results in higher efficiency in burning the fuel, and hence, more horsepower. Turbocharging can increase the power output of a diesel engine by 30-50%, depending on various factors. For example, a GP38-2 and GP40-2 have the same prime mover. However, the GP40-2 can develop 3000 horsepower with its turbocharged engine compared to the 2000 horsepower of the non-turbocharged GP38-2. (In the cab unit days, Alco had turbochargers, but EMD did not.)

23. What are blowers? In 2 cycle engines without turbochargers, blowers are used to provide air to the cylinders.


 "How Diesel-Electric Locomotives Operate: The Last 25 Years,
Including ACs
" from PEAT. This book is the resource for the serious railfan. An excellent newsletter, "Railroad & Locomotive Review", is also available.

 "The American Diesel Locomotive" Brian Soloman. MBI Publishing. 2000.

 "Modern Diesel Locomotives" Hans Halberstadt. MBI Publishing. 1996

"History of the B&O Railroad" John F. Stover. Purdue Pub. 1995

 "The Encyclopedia of Trains and Locomotives" C.J. Riley. MetroBooks. 1995

 "The Railroad Encyclopedia" Edited by Anthony Lambert. Eaglemoss Productions. 1996
 "Black Gold-Black Diamonds: The Pennsylvania Railroad & Dieselization, Volume II" Eric Hirsimaki. Mileposts Publishing. 2000.

Diesel Era Magazine

 LocoNotes Mailing List

Railway Technical Web Pages site.