Basic Steam Locomotive Action
The simplest way to understand a steam locomotive is to follow the path towards making it work.
A Diesel or gasoline powered engine is called an internal combustion engine. Fuel is burned inside a cylinder, resulting in a controlled explosion, pushing the piston backwards. The piston turns a weighted crank and rotary motion happens.
A steam engine is called an external combustion engine. There is a fire, not in the cylinders, but in a firebox. This fire transfers heat into a boiler filled with water, raising the water temperature to the boiling point, creating steam. Steam is water vapor in gaseous form. 1 CC of liquid water expands to almost 1600 CC's of steam vapor. Because the boiler is made of steel, this large amount of high temperature water vapor is kept at high pressure, and needs to escape confinement.
There are three ways for the high pressure steam to escape. One, is for the locomotive engineer to open the throttle, safely venting a measured amount of steam into the cylinder, pushing the pistons backward. The pistons are connected by piston rods, which turn large, weighted wheels, moving the locomotive.
Two, is for excess steam escape through a safety valve on top of the boiler. Enough pressure will push the valve open and excess steam vents harmlessly into the atmosphere.
The third method is for steam to push so hard on the boiler, with no way of escaping, that the steel tears, and the boiler explodes. This is a catastrophic boiler failure, and destroys the locomotive. 470 is designed to prevent this from happening, and the safety valves are checked before every trip.
Once steam has passed through the locomotive cylinders, it is vented up through the smoke stack, creating a draft, which draws cinders and fire gases out of the engine. This causes the familiar "chuff- chuffing" sound we associate with a steam locomotive. Most steam locomotive exhaust is made of solid ash and cinders, mixed with water vapor.
Steam is very powerful, and more is generated than the locomotive needs to move. For this reason, some can be tapped from the boiler and used to turn a steam dynamo, or generator, which produces electricity for the headlight and cab lights of the locomotive. Steam also powers pumps on the side of the locomotive, to fill the boiler with water and pump air into the air-brake storage tanks.
How a Steam Locomotive Boiler Works
When you boiler water in a kettle, on the stove, the heat from the stove passes up through the kettle bottom and heats the water, creating steam. A locomotive boiler is arranged differently in order to get the most energy from the fire to heat a very large container of water.
Think of a locomotive firebox as a steel box inside a tank of water. A fire which burns coal or some other fuel, is created and kept fed by the locomotive fireman. Like all fires, the combustion gases must be vented to keep the fire from going out. The locomotive firebox has large pipes coming out of it, called flues. Smaller pipes are below the flues, called water tubes. Inside the boiler, water runs all around the sides and lengths of these pipes. The hot fire gases roar through the pipes from the firebox to the smokebox, and out the smoke stack. This makes efficient use of the heat to boil water and make steam.
Steam, being water turned into a gas, is less dense than water. It rises to the top of the boiler and collects under the steam dome. From under the steam dome, the steam travels through what is called the dry pipe to the throttle valve, which the engineer uses to dispense a controlled amount of steam to the cylinders to move the pistons.
Boiler Chassis, Drive Rods and Wheels.
Heated water expands volume and changes from a liquid into a gas. The gas is 1600 times the volume of that same amount of water and is kept under high pressure inside the boiler. The locomotive engineer controls the escape of high pressure water vapor (steam) by use of a valve called the throttle.
How does that move a train? Perhaps the best way to understand how the energy of heated water, converted into steam, is converted into mechanical energy, is to trace the path on a diagram. The illustration on the following page shows the entire process, but for sake of brevity, look at Point 14 , the throttle valve. Using the diagram, a controlled amount of steam is passed through the throttle into Point 16, the dry pipe. In this illustration, as in locomotive 470, steam enters Point 20, the superheater tubes where it is used to help raise the temperature of water inside the boiler even more. After passing through the superheater tubes, the steam passes into the blast pipes, Point 22 (looks like a large question mark in the picture). Point 23 is the steam chest. This is a large steel saddle casting, which contains the locomotive valves and Point 24 cylinders. The steam, still under high pressure, pushes back on the piston Point 25 forcing it to slide backwards, pushing the crosshead Point 26 and main rod Point 27 as it moves.
A Tea Kettle and A Pressure Cooker
Heat is transferred from an outside source to water. When the water is energized, molecules lift off as vapor. They press against the kettle sides until they find an open vent to escape into the atmosphere.
A pressure cooker is locked shut so steam cannot escape. Energized vapor pushes on all sides of the container but can only escape through a safety opening. That opening has a special weight on it. Steam pressure lifts that weight, allowing pressure to be released safely, but keep much inside the container. Pressure cookers are dangerous if not used properly.
External Combustion VS. Internal Combustion
How does an Engineer control a steam locomotive?
This is the cab of a typical steam locomotive from the 20th Century. The Engineer sits on the right, the Fireman on the left.
470's cab when it was retired from service.
A steam locomotive is almost entirely controlled by steam pressure driving all its parts, even to make electricity for the lights. A Diesel locomotive is almost entirely controlled with electricity made by a Diesel engine turning a generator.
How is a Diesel locomotive controlled?
1. Air brake pressure (locomotive and train). 2. Electrical amperage for traction motors on wheels. 3. Diesel motor oil pressure and temperature gauges. 4. Electrical switches for locomotive. 5. Locomotive headlight switch. 6. Dynamic brakes. 7. Throttle 8. Reverser 9. Ground/fault switch 10. Attendant call. 11. Bell 12. Sander 13. #1 truck sander 14. Circuit breaker 15. Locomotive air horn.
How Does a Diesel Locomotive Work?
Steam locomotives required many people to maintain their parts, fuel and water them, remove the ashes and soot and lubricate them. Many steam locomotives became "individualized", needing special hand-forged parts. Steam locomotives could be very dirty while operating. Even though one steam locomotive could often perform the same work as three Diesel locomotives, they were expensive to keep running.
Diesel locomotives are mass produced somewhat like automobiles. They use standard parts which can be exchanged with replacements from a factory. Diesels burn easily stored oil as a fuel, and use it economically. The engineer's cab is enclosed and can be heated or cooled as desired. It takes far fewer people to keep a Diesel locomotive clean and operating.
The heart of a Diesel locomotive is an engine, sometimes called the "prime mover". It burns oil inside cylinders, much like a truck, but these engines are much larger. The prime mover turns an electric generator at different speeds. This generator produces high-voltage electricity which is sent on cables to electric traction motors. The traction motors are directly geared to the locomotive wheels. The cab of the locomotive has controls for the Diesel prime mover and the amount of electricity being drawn by the traction motors. This makes a Diesel easier to control with different loads at different speeds.
Diesel locomotives also have electric communication cables which can connect many locomotives together, with only one engineer and crew to control them. This is called "multiple unit" or MU control.
Steam locomotives are often remembered for the soft chuffing and whirring of parts while steam passed through it, making it seem alive. This led people to calling them "choo-choo's". Diesel's make rumbling noises from the prime mover and a "grrrrr" sound as the traction motors engage. This has led some people to call them "growlers".
A modern Diesel Electric locomotive, type SD-40, built by General Motors.
#3 Controls #12 Prime Mover #10 Generator #19 Fuel tank #18 Trucks (wheel assemblies)with electric motors.
This is a steam locomotive designed to pull fast passenger trains. The wide drive wheels covered more distance in a single rotation.
Maine Central 470, just after retirement in 1954.
Boston & Maine 494, a much older steam locomotive from the 1890's.
On line references to learn more.
Locomotive Valve gear and steam flow http://trumpetb.net/loco/rodsr.html
https://thumbs.gfycat.com/HorribleHelplessGilamonster-size_restricted.gif
Pacing Nickel Plate Berkshire #765 https://www.youtube.com/watch?v=JkzK_gLM3cY
Cab ride https://www.youtube.com/watch?v=xwtNdbN1CqA
Steam Locomotive Controls https://www.youtube.com/watch?v=UrV8x01LLNo
This Teaching Supplement was written by Richard Glueck for New England Steam Corporation Owners and Restorers of Maine Central 470. All content is used by permission of the owners and originators. This material may be distributed without charge.