Throttle Mode and Cutoff Mode
by Fred Severson
1. July 2009
Prototype steam locomotives have two ways to control power to the driving wheels. One method is to control the steam pressure supplied to the steam cylinders using the throttle. The second way is to control the amount of cutoff (Johnson Bar). The engineer, of course, has the option of controlling the locomotive with either or both cutoff and throttle to maximize efficiency. If the locomotive is solitary or working the yard with few cars, the engineer will usually set the cutoff at its minimum and use the throttle to control power and speed. At minimum cutoff (usually about15%), steam is injected for only this small percentage of the piston stroke. At the end of the piston’s travel, the steam is reduced in pressure and temperature, which produces a steam exhaust with a softer and wetter sound. If there is no need for the extra power supplied from cutoff, it is much more efficient to leave cutoff at its lowest setting and use the throttle for acceleration. This is called Throttle Mode.
If the locomotive is pulling a heavy load on the mainline, the engineer will usually set the steam pressure with the throttle to supply the amount of steam he will need at full speed at minimum cutoff on level track for the load he is pulling. Since a steam locomotive is most efficient at minimum cutoff, the throttle is usually set to provide just enough steam at maximum speed at the minimum cutoff position. During acceleration, the locomotive’s power is controlled using the Johnson Bar to set the cutoff. When cutoff is at maximum (usually about 85%), steam is being injected into the cylinders during most of the piston stroke and is exhausted at full pressure and temperature. This provides the characteristic “bark” of an accelerating steam locomotive. We call this Cutoff Mode.
If the locomotive encounters an increasing grade, the speed could be maintained by increasing cutoff but this is an inefficient use of steam. Instead, the engineer is likely to increase the throttle to provide the extra pressure needed to maintain speed at minimum or low cutoff. Conversely, if a descending grade is encountered, the throttle is usually reduced.
For the model train, we only have one power control knob such as the voltage setting to the track in analog or speed steps in DCC. In keeping with the prototype, we would want the cutoff to be minimum at any steady state speed regardless of load. The steady state sound volume should be a linear function of the model power control knob setting to model the increased steam pressure necessary to maintain the speed.
For the model we use inertia term, CV3 + CV23, to specify the trainload. We specify that the locomotive be in Throttle Mode, if the total inertia is below a value, CL, which we call “Chuff Cutoff Minimum Inertia”. CL can be set in CV51.10 and ranges from 0 to 255. For instance, if CL is set to 40, then the locomotive operates in Throttle Mode for inertia settings (CV3 + CV23) less than or equal to 40. If the inertia is set to any value above CL, the locomotive is in Cutoff Mode. The default value for CV51.10 is 50.
During acceleration we model the steam exhaust sound volume Sound of Power™ (SoP) to be function of both the throttle setting and CV3 + CV23.
During deceleration, the SoP sound volume is reduced to a minimum level until the locomotive reaches steady state or stops.
Throttle Mode (CV3 + CV23 <=CL)
Since there is no help from cutoff during acceleration in Throttle Mode, we would expect the engineer to set the throttle temporarily higher depending on load and then back off to steady state when final speed is reached. This is done automatically in the model. If the model train operator sets the model power control knob to a the level he thinks will be suitable for steady state operation, he will hear labored chuff at higher volume in proportion to load and then hear the volume decrease when steady state speed is achieved.
If the model power control knob is decreased, the sound volume will decrease during deceleration and then increase to the appropriate level for the selected lower throttle setting when the speed reaches steady state.
Cutoff Mode (CV3+ CV23 > CL)
In Cutoff Mode during acceleration, the amount of SoP cutoff and the volume are automatically set by the load and controlled by the throttle setting. During acceleration, the model power control knob directly controls the amount of cutoff but the range is different depending on load. For instance, if the load is high, the model power control knob will affect cutoff over the entire cutoff from 15% to 85%. If the load is lower, the model power control knob will affect cutoff from 15% to a smaller maximum. The “bark” of the cutoff and its volume will be higher if the load is high and the throttle is set high.
When the model control knob is increased in either Throttle Mode or Cutoff Mode, the locomotive will accelerate with increased Sound of Power™ (SoP) until it diminishes when the loco reaches steady state as described above. In other words, the full Sound of Power continues through the entire acceleration period and only diminishes when steady state is reached rather than a gradual reduction in SoP as the locomotive speeds up. This is in keeping with observations of how steam engineers operate their locomotives. Since the prototype accelerates slowly in most cases, the engineer usually does not pull back on the Johnson bar until final speed is nearly achieved. The rate at which the Sound of Power backs down after steady state represents the approximate time it takes an engineer to pull back on the bar (2-5 seconds).
When decelerating, the rate that the cutoff and volume drops represents the time it would take an engineer to physically pull the throttle and/or Johnson bar to reduce speed.
The Sound of Power setting in either Throttle or Cutoff Mode is set by the difference between the current internal speed step and the requested speed step on the Model Power Control Knob. In other words, if the Model Power Control Knob had been set at 15% and the locomotive is moving slowly in steady state and the Knob is turned up to 50% then the SoP would be based on the difference (50% - 15%) and produce moderate labored sounds. If instead the throttle had been moved to 85% than the SoP would be greater based on the difference (85% - 15%). If the locomotive had been moving at 50% steady state and the throttle was increased to 85% then the SoP would again be more moderate. This is in keeping with my observations of how steam engineers operate their locomotives. If the required speed increase is small, there is less need to apply full cutoff or throttle but if the required speed increase is large, there is more need to apply more power. It is a trade-off between keeping schedules and minimizing fuel and water consumption.
If the user wants more Sound of Power during acceleration, he can increase Model Power Control Knob more than necessary to achieve steady state and then back down to the desired steady state setting. The SoP will respond to the maximum setting during the acceleration period rather than the final steady state setting.