DieselPrimer1

Diesel-Electric Locomotive Primer: the basics

Most people know the basics concerning how a diesel-electric locomotive works. But there are many who don't -- especially those new to the hobby! Let's take a quick look through an ALCO-GE switch engine to get a handle on things.

Here's our subject. This is a typical ALCO-GE switch engine, built for use on very many roads. This unit is a 1000 HP switcher, and it's an early one with no exhaust stack extension that you can see, and with Blunt trucks. The later change of trucks makes no difference to our discussion here. Now, let's look inside.

This is a basic diagram of the major equipment of the unit seen above. The basic parts are fairly obvious; a large diesel engine, mounted near the center of the locomotive (to equalize weight on the two four-wheel trucks) which has connected to it an electric generator. This generator supplies electric current to four electric motors, mounted in the trucks (one to each axle) which are called traction motors. These drive the wheels, which moves the locomotive and the cars coupled to it. On the right, at the front, we can see the radiator fan, which pulls air through the radiators to cool the engine. In the cab, we see the control stand, with the throttle and the reverser. The reverser's function is obvious -- it determines which direction the locomotive moves when the throttle is opened. (On this kind of unit, it also does more than this, but that's a subject for another article.) We'll refer to this drawing often.

This is the diesel engine/generator set removed from the locomotive. This is an ALCO 539T engine; it has six cylinders, and is turbocharged. The cylinders have a 12.5 inch bore, and the pistons have a 13 inch stroke. The engine runs at 740 RPM at full speed, and at that speed develops a maximum of 1000 gross horsepower. The actual input to the generator for this locomotive is 960 horsepower, and the generator is roughly 95% efficient at converting rotating power to electrical power. Thus, the output from the generator is roughly 910 horsepower.

Here is a later style of truck which was used under this kind of locomotive -- and under a great number of others built by EMD, Baldwin, Fairbanks-Morse and Lima-Hamilton as well. It is a General Steel Castings Corporation standard cast switcher truck. You can see the four cables connected to the nearer of the two traction motors; these hook up under the locomotive frame, and are connected to the generator by way of the control cabinet.

Why use electricity? A good question. The problem with diesel engines (and gasoline ones too) is that they have to be idling when not in use. That sounds obvious, but it means that you cannot just directly connect the engine to the wheels with shafts and gears. You must at least have a clutch. If you use this, you will have a very limited range of use; we all know that, in cars, you have different gears to allow the relatively small speed range of the engine to be used over a wide range of vehicle speeds. There are locomotive transmissions which actually work this way -- look for the article on Hydraulic Locomotive Transmissions on this site. However, these have limitations, especially when used in very heavy, low speed service. In the US, the electric transmission proved itself capable of handling heavy use (and punishment) early on, and was adopted as the normal design from then on. The motors can develop high torque while turning slowly while the diesel is at full speed and load.

If you're like me, you look at the outside of a diesel locomotive, and wonder two things. First, what does the diesel engine look like? Second, what do the controls look like? Well, here are the controls of the locomotive we've been discussing. The lever on top is the throttle; you pull it toward you to increase power. When it's all the way forward, an electrical contact removes electric power from the traction motors. This is called the "idle" position, or the "off" position in some cases. Pulling it toward you increases diesel engine speed and power, and thus generator speed and power; you get the idea.

Looking at this in a more technical way, what really has to happen in any diesel locomotive is that the throttle lever has to control the diesel engine speed. The device which is connected to the engine and which controls the amount of fuel the engine is given is called the governor, and on the outside of that governor is a "speed control shaft." Somehow, then, we need to get the throttle lever connected to this speed control shaft in order to be able to make the engine run as fast or as slow as we want to.

Look carefully at the diagram at the top. You will see that the throttle lever actually is mounted on a shaft, which connects to a linkage that runs underneath the generator and diesel engine, and then runs up the side of the diesel engine, to the governor.

Here is a very simple view of the governor. It is the object in the middle; only the various linkages are labeled here. The connection from the throttle is coming up through the floor, at the bottom, and connects to the turnbuckle jaw. It then operates a lever, which is connected to another lever on the left. This lever connects to the speed adjustment lever, which is actually mounted right on the speed control shaft. In other words, you have a direct mechanical link all the way from the cab, at the rear, to the governor itself. Various adjustments are provided for in this set of levers and links to ensure that the proper speed is obtained for a given position of the throttle, and to ensure that it doesn't get out of alignment.

There is one obvious problem with this setup -- you cannot run locomotives in multiple! In multiple unit setups, all of the locomotives are controlled from signals given by the control equipment in the leading unit. You obviously cannot connect a mechanical link through every unit; there must be a better way. There is; you can either use electrical signals, or perhaps air pressure in a hose, to get the throttle in the lead cab to finally operate the speed control shaft on the governor of every trailing unit. On the ALCO-GE switchers, they used electrical signals.

It's pretty obvious then that if we want to have multiple unit controls, we need new controls! And that's how it was with these units. On the left is the control stand from a unit with multiple unit controls. You can see that it looks very different from the other. That's because it isn't nearly as simple as just containing a shaft running through the floor -- it has to be able to produce distinct signals for each throttle position which can be sent to trailing units.

Very early, it was made fairly standard that units with electric throttles would have eight notches or throttle positions which provided eight power levels. That seemed to be enough; only much later would more be used on electric throttle units, and even that went away.

At right, we can see the actual controller portion of the control stand removed and disassembled. The throttle lever connects to a shaft which, at its base, has a number of cams on it at various angles. These are used to close or to open the various electric contacts you can see pulled out to the side. This means that for each throttle position, you have a sort of "coded combination" of signals -- one set for every engine speed. Now, all we have to do is find a way to operate the speed control shaft of the governor in a way that corresponds to the speed we want for each throttle notch.

Here is how they did it on ALCO-GE units with the 539 engine. This device is called a throttle operator. It has magnet valves (which open when supplied with electric power) and it also has air cylinders. The magnet valves let pressurized air into, or out of, the corresponding cylinder. The shafts of the cylinders are connected to a slightly complicated mechanical linkage system, which is also connected to the speed control shaft of the governor. By energizing magnet valves in various combinations, air cylinders are filled or emptied in various combinations, which moves the linkage a given amount for each combination. (There are eight combinations, if you didn't already guess!) This is how we get our eight speeds for the engine.

All we have to do with this setup is provide cables that connect the locomotives together, and then the leading unit's controller can send the signals to not only the throttle operator on the lead unit, but on the trailing units as well. These are called Multiple Unit cables, or MU cables.

This "throttle operator" setup is fairly complicated, and is not typical of most diesel-electric locomotives. ALCO-GE only used this on locomotives with 539 engines, and that's why you normally don't see them running in multiple with anything but their own kind. Most kinds, from most locomotive builders, that used electric MU actually used governors that contained their own electric devices and linkages -- the electric devices are called "solenoids," and simply move a shaft which runs through the solenoid. You then connect the four solenoids in this type of governor to a different mechanical linkage, and you then have just as good a way (if not better) to operate the speed control shaft of the governor. And, of course, there were very many locomotives built with Westinghouse equipment that used air pressure, controlled by the throttle lever, to operate an air cylinder connected to the speed control shaft. More air pressure, more piston movement in the cylinder, more engine speed. The basic principle, though, of finding a way to get a signal from one control stand to all of the trailing locomotives operating in multiple, was the same.

There we have it! The first "primer" on diesel-electric locomotives for our site. More subjects will be added as time permits and as need is indicated.