Support>Tutorials>Railroad Operations
by Bill Hobbs
Primary Controls
There are two primary controls for a steam locomotive: the throttle or regulator and the reverser or cutoff lever. The throttle works simply enough by controlling a valve that controls the mass flow rate of steam to the steam chest of the cylinders. This also affects the pressure of the steam at the steam chest. If the use rate of steam is greater than the flow rate, the available pressure will drop. Less pressure means less energy available to do work in the cylinders.
The reverser is more complicated in its operation. As the name suggests, it can control the direction of movement of the locomotive by shifting the side of the cylinder to which steam is being admitted during a stroke. It also controls the timing of the cylinder events of admission, expansion, exhaust, and compression. For our immediate discussion, admission and expansion are the most important. The portion of the stroke during which steam is admitted to the cylinder determines the volume of steam used during each stroke. The longer admission occurs, the more the force developed depends on the volume of steam and the less on the expansion of that steam. It is more efficient to admit as little steam as possible and extract as much energy from it through expansion as possible.
Starting a train is generally done by using a long cutoff (say70%) and easing the throttle open until the train starts to move. Long cutoffs minimize the likelihood of slipping by smoothing out the power stroke. Once the loco reaches about 6 mph, resistance is at a minimum and it is time to open the throttle more and ease back on the cutoff until the train reaches the desired rate of travel.
It is most efficient to run a steam loco with as short a cutoff and as high a throttle setting as consistent with the work being done.
A steam locomotive is a constant force machine. The cylinders can deliver a constant force at any given speed until one of the following two factors reduces the force:
(1) The use of steam outruns the boilers ability to produce steam and the available pressure drops. This is the major cause of loss of force;
(2) The steam pressure inside the cylinder falls below the available pressure because the valve movement is so rapid that it does not allow sufficient steam into the cylinder to reach the available force. This is called "wiredrawing". It becomes more pronounced as speed increases.
The two factors combined can result in a significant loss of pressure at high speeds. The longer maximum boiler pressure can be maintained and transmitted to the cylinders, the longer the force curve will stay flat.
When running, hit the F5 key twice and keep an eye on the HUD displays for steam generation and usage. When usage exceeds generation, boiler pressure will drop. Try keeping the two as closely matched as possible. If too much steam is being produced (when doing light work, for instance), let the fire mass decrease some and/or add water to the boilers to keep from wasting steam through the pop off valves. I let the fires burn down quite a bit when working light because it saves fuel and water.
Blowers, Injectors, Firing, etc.
A steam loco contains a good example of a self-regulating system. Exhaust from the cylinders exits through the blast pipe in the smoke box where it creates a partial vacuum on its way out the stack. This vacuum pulls air in through the grate or burner and moves exhaust gases from the fire box through the flues and tubes to the smoke box where they mix with the exhaust steam. As the rate of exhaust increases, so does the flow of gases, increasing combustion and evaporation. The physics involved is the same as used in an airbrush.
When a loco is not moving, the rate of combustion on the grate is between 5 and 10 lbs of coal per sq ft of grate area per hour, or an equivalent amount of fuel oil. Under “forcing” by exhaust steam, this rate can exceed 200 lbs on modern locos! As the rate of combustion increases, it becomes less efficient (because of the rate of movement of gases through the system giving less time for heat exchange) until a point is reached at which additional combustion does not result in additional evaporation – the grate maximum. The curve is upward parabolic in shape.
The blower (N key in RailWorks) acts in the same way as exhaust steam, creating a partial vacuum in the smoke box. It generally consists of a ring of steam pipe sitting horizontally around the blast pipe in the smoke box having a series of holes that point upwards towards the stack. The blower is needed to raise the combustion rate on an idle loco, or to increase the draft for a moving loco that has been poorly fired, is using low-grade fuel, or has a blast pipe arrangement not adequate for the steam usage rate. The cost is the live steam used. Leave it off if it is not needed.
Firing is best done on as even a basis as possible. When the loco is idle or doing light work (shunting, drifting, or idle) the grate mass can be allowed to drop from the ideal mass accordingly. I have run with as little as two-thirds of the “ideal” mass under such conditions. When the rate of steam usage is going to be high, I like to get the fire mass close to the ideal and keep it there through smooth firing. Don’t just throw in a few scoops and quit. Fire at a rate to keep the fire mass at the level needed to support the steam usage rate.
Injectors add water to the boiler. Again, think of the way a two-step airbrush works. One control turns on the steam (I or O keys in RailWorks); the second (K or L keys in RailWorks) controls the flow of water to the mixing valve. The water and steam mix jets at a high velocity against a boiler check value to enter the boiler. The steam water mix entering the boiler will cool the boiler and so there is a cost to adding the water. This fact can be used to keep from wasting steam through the pop-offs (Expert mode only, I believe). Once again, when pushing the loco hard, a steady replenishment of water is preferable to on-off operation. The fireman’s goal is to keep the steam pressure in the boiler as steady as possible for the engineer.
In RailWorks, be sure you turn off both the steam and the water flow when not using the injectors. If you don’t turn off the water flow (K or L keys) you will waste water.
Cylinder Cocks, Water Glasses, and Superheaters
There is logic to this combination of topics. I’ll start with a quick review of the basics of steam loco boilers. At the rear of the boiler (the back head) is the firebox. At bottom is the grate through which air must pass to support combustion. The top of the firebox is known as the “crown sheet”. It receives the greatest heat of any part of the firebox and the metal would readily melt if not covered on top by water. At the front of the firebox are a series of horizontal tubes that run the length of the boiler and exit in the smoke-box. These carry the exhaust gases from the firebox to the smoke-box and stack. About 80% of the heat exchange in the boiler takes place in the firebox, with the remainder occurring through the tubes. On a saturated steam loco, these tubes average about 2 to 2½ inches in diameter. The water level in the boiler must always be sufficient to cover the crown sheet, but should not fill the boiler entirely. The space above the water is used to gather steam as the water evaporates.
To get steam to the cylinders, there is a collection pipe which starts in the turret or steam dome and then runs horizontally through the boiler, turning downwards in the smoke-box where it divides into 2 or three sections, depending on the number of high pressure cylinders. The flow of steam is controlled by the regulator or throttle. Depending on the age and whether the loco is super-heater equipped, the regulator valve may be at the turret itself (old style), at the cylinder end of the dry-pipe, or at the cylinder end of the super-heater.
The water glass is one of the minimum of two methods required by US regulations for insuring that there is always water above the crown sheet. A second method is the so called tri-cocks. There are other methods but only these two satisfy the regulatory requirements. The loco may also be equipped with fusible plugs or float operated warning devices as secondary security. Should the water level drop over the crown sheet, it would quickly weaken and rupture allowing the boiler to reach atmospheric pressure. Since water evaporates at a higher temp under pressure, all the water in the boiler would flash to steam, turning the boiler into a giant unguided missile, quickly killing the crew on the footplate. Most boiler failures in the 20th century were attributed to crown sheet failure.
The water glass is set so the bottom is a little above the crown sheet level and its height is set to the maximum recommended water level for the boiler. When running a loco, it is good practice to keep the water at least at the 50% level. Given that running the injectors cools the boiler, there may be times that one will have to choose between letting the water level drop and loosing boiler pressure. So it is a good idea to always have enough water to allow this choice to be made.
Boilers can directly produce only saturated steam. The disadvantage of saturated steam in a locomotive is that when it is introduced to the cylinders, a portion instantly condenses. This represents a loss of energy, estimated at around 25% of the energy available in the steam. This water must be gotten out of the cylinders because it cannot be compressed by the piston. Saturated steam locos used some form of relief valves to deal with this problem. Large amounts that accumulated after the loco was sitting for a while called for the cylinder cocks to be opened. More about these in a moment.
To deal with this loss of energy, designers in the first decade of the 20th century worked on a practical way to add additional energy to the steam. The result was the superheater. To create a superheater, many of the small boiler tubes are replaced with larger ones. Inside the larger ones are smaller tubes that contain steam from the dry pipe, subjecting that steam to the exhaust gases and adding extra energy to the steam. This technique does not require additional fuel and can add sufficient energy to the steam to increase cylinder efficiency by 30-35%.
Superheating also reduces the problem of condensation in the cylinders. With the steam entering well above the saturated temp, a small drop of temp still leaves the steam at well above the evaporation temp. Further, less steam is required to fill the cylinder, reducing the volume required to run the loco.
This does not eliminate the need for cylinder cocks. There are still two problems to be dealt with by the cylinder cocks. First is the water that condenses when the loco is sitting idle. Second is the problem of boiler water “foaming” and entering the dry pipe as water. Even superheating may not convert all this water to steam, so it must be expelled from the cylinders.
One final thought about cylinders and regulators. When steam locos are “coasting”, it is still necessary to keep a little steam flowing to the cylinders in order to supply lubricants to the pistons. The best way for a loco to drift is to keep the reverser set for a long motion and keep the regulator “cracked” or 1-3% open. This won’t supply much power but will simulate prototype drifting. Some US roads even supplied their superheated locos with a separate valve known as a “drifting throttle” to supply saturated steam and lubricant to the cylinders.