Quantum Chuff with Progressive Cutoff
by Fred Severson
Concurrent with the 1st release of the Revolution-U will be the release of the updated steam sound files that feature nothing less than amazing load/no load "Sound of Power"™ effects. You have to hear it to believe it - wave files are coming soon ... Here is a document that describes what's happening.
Introduction
There are two ways to control the power from a prototype U.S. steam locomotive, either the Throttle or the Johnson bar. The Throttle controls the amount of steam available and the Johnson1 bar controls the percentage of the piston stoke that steam is admitted into the steam cylinder. This percentage is also called Cutoff. The principle behind Cutoff is illustrated below in a simplified cut-away diagram of a steam cylinder. The valve at the top does double duty in most steam locomotives since it governs both the injection of steam from the boiler and the venting of steam from the cylinder. The Quantum Revolution-A sound decoder has 6 light ports or outputs. These are numbered 1-6 and must be configured for the following features using the Quantum Programmer. Default diesel settings (after 7-28-08) are shown below.
Fig. 1
The diagram below shows the progression of the piston as it moves from the left to the right. Steam is injected at “A” which cause the piston to move continually to the right until the value is moved over the opening at “B”, cutting off or blocking additional steam from entering the cylinder. The cylinder continues to move as shown in “C” but the steam pressure is reduced from the boiler pressure simply because the cylinder volume has increased.
Fig. 2
In other words, Cutoff controls the point during the travel of the piston where additional full pressure steam is prevented from entering the cylinder.
If the engineer uses the Johnson bar to set late cutoff, then full steam pressure is applied during the entire stroke of the piston. If he sets Cutoff early, then full steam pressure is only available at the start of the piston stoke. By changing the Cutoff, the engineer controls the force delivered to the locomotives wheels.
Cutoff is usually expressed as a percentage of the piston stoke. A 75% Cutoff means that full pressure steam is injected for 75% of the piston stoke. A 30% Cutoff means that steam has only been injected for 30% of the piston stoke. Because the control valves are opening and closing during the travel of the piston and the piston does not completely fill the cylinder area at the extreme limits of its motion, early Cutoff usually ranges from 15%-25% and late Cutoff from 75%- to 85%.
The steam exhaust cycle is shown in Figure 3.
Fig. 3
During the return cycle of the steam piston, the value opens exposing the steam in the cylinder to the outside air. The steam vents mainly from its own pressure but also because the piston moves to the left pushing the excess steam out the exhaust port. If a high percentage Cutoff was used, the steam is at maximum pressure at the end of the piston stoke and produces the familiar “Bark” heard when a steam locomotive starts out with a heavy load. If a low percentage Cutoff was used, the steam pressure is low at the end of the piston stoke which produces a much softer, wetter sound when the exhaust value is open. Engineers often use the Johnson1 bar as the primary power control for a steam locomotive.
Much of the details of how valve systems work on a steam locomotive are left out in this discussion since we are primarily interested in Cutoff and how it effects the operation and sound of a steam locomotive. If you want to learn more about how the valves allow steam to enter and exhaust from the cylinder, try the following link, which has a very good animated drawing of steam piston and value gear in action. Unfortunately it does not show cutoff action.
The Quantum Sound-Of-Power™ Chuff with Progressive Cutoff effect is based on the physical equations that describe prototype steam locomotive operation. In our case, we treat the DCC throttle as a Johnson bar. Low DCC throttle settings produce minimum Bark (low percentage Cutoff) and high DCC throttle settings produce maximum Bark (high percentage Cutoff).
We also have two modes for Sound-of-Power (SOP) operation in our Quantum system. Classic Throttle SOP and Semi-automatic SOP are set in bit 6 of CV 51.2. The default is 1, Semi-Automatic SOP.
In Classic Throttle SOP, the DCC throttle generates SOP from its lowest Bark to its highest Bark linearly as the speed steps range from its lowest value (1) to its highest value (126). In Semi-automatic SOP, the user sets the DCC throttle at the value he wants but the “Bark” or Cutoff is automatically set higher during acceleration and levels off to the value set by the operator as the train reaches its steady-state speed. This represents the engineer setting the Johnson bar to higher Cutoff, which is required to accelerate the train and then backing off to lower Cutoff as the train reaches the desired speed.
During deceleration, when the DCC throttle is turned down, the Cutoff is set lower until the final speed is approached whereupon the Cutoff or Bark increases to the correct value for that throttle setting. Again, this represents the engineer setting the Johnson bar for very low Cutoff until the final desired speed is obtained where he then increases Cutoff to maintain speed. If the throttle is brought done to very low speed steps, the volume remains at or above a minimum setting that is appropriate for the 15%-25% minimum Cutoff value. If the DCC throttle is set to zero, then the chuff sounds shut off completely. This would represent the engineer setting the Johnson bar to the Neutral position1 where no steam is injected into the steam cylinders.
High Speed Chuff
Another improvement with Quantum Revolution steam sounds is our high-speed chuff. High-speed chuff on the prototype is characterized by a smooth blending and overlapping of the chuffs with variation in volume and timbre. Our models capture this quality as the chuffs change from highly distinct individual chuffs at low speeds to a continuous chuff roar that increases in rate over the entire high-speed range. Gone are the limited chuff range and irritating machinegun-like pounding at high speeds that characterize many model train sound systems. And changes in Cutoff or Bark with the throttle for the Quantum System are still apparent even at these higher speeds.
When you experience the new Quantum Chuff with progressive Cutoff, we suggest you leave the sound system in its default Semi-automatic setting and set your locomotive to a reasonable value for CV3 and CV4 (100 or so) to allow more prototype-like response to the throttle. When you start out with the throttle set high to accelerate your locomotive, you will hear the distinctive labored Bark as it starts out and accelerates. The Bark will decrease as it reaches steady-state speed. Since our SOP is based on the actual physical equations that govern the operation of steam locomotives, the Bark or Cutoff is continuously progressive with throttle and speed just like the prototype. Hence, you will not hear the Quantum Chuff abruptly change from one chuff sound recording to another as Cutoff changes.
Rod and Valve Clank
Quantum Revolution (QR) steam sound sets also model the sounds of Side Rod Clank. These sounds are apparent on prototype locomotives when the throttle or Cutoff is low and there is less force on the connecting rods. When the throttle is decreased on Quantum Revolution, valve gear and side Rod Clank can be heard as the locomotive decelerates. Rod Clank will start to occur as the speed decreases below a certain level and is quite apparent as the locomotive slows to a stop. If the throttle is not decreased all the way, then the Rod Clank will be heard during deceleration until the model reaches its steady state speed. This is consistent with prototype operation where the increased force to maintain steady-state speed applies more force to the connecting rods, which reduces its bearing slack and hence reduces the clank sounds. It is easier to hear the Rod Clank on the model in Semiautomatic SOP since the chuff volume is usually much lower during deceleration. If you are operating in Classic Throttle SOP, the Rod Clank will be more audible compared to the chuff sounds if you turn the throttle down to very low settings. Rod Clank rate on the Quantum System decreases realistically with locomotive speed and is synchronous with any chuff sounds that maybe present.
Power Loading
Quantum Sound-of-Power is not directly affected by changing values of inertia (CV3 and CV23). Intuitively it would seem that the more simulated load we apply to our locomotive, the harder it should work. We have all seen prototype videos or have experienced the raw power of a real slow-moving steam locomotive working hard to accelerate its heavily loaded train so why not do the same in the model. However, the reason the prototype sounds like it is working hard is because the throttle is wide open and at maximum cutoff. It is moving slow because even at full throttle it is having a difficult time moving its load.
The same should also be true of the model train if it was loaded with many cars. The difference is that model locomotives are usually way over-powered compared to the prototype and the rolling stock is often prototypically too light and roll too easily. If the model locomotive can pull its full allotment of cars without slipping, it can usually get up to near full speed on level track at full throttle.
To address this issue, we have added a new feature called “Power Loading™” which can be enabled in CV 56.4 under Quantum Configuration. When you select Regulated Throttle Control (RTC) with Power Loading, the more inertia you set in CV3 and CV23, the slower your locomotive will operate at the same throttle settings; and it will take more throttle to get it moving. If you have very little inertia set for CV3 + CV23, there is little effective “Power Loading”. But set CV3 + Cv23 at a higher value (say 100 to 380), and your locomotive will know there is a huge load behind it. Turn the throttle up and it will start moving at a much higher setting and at full throttle, it will move slower and the Sound-of-Power and Bark will be at its extreme limit.
Notes:
1) The Johnson bar is defined as a hand lever with multiple selection positions with a spring-loaded squeeze handle on the lever so that only one hand is needed to release the clutch, move the lever, then reengage the clutch into one of the selection positions. The Johnson bar on U.S. steam locomotives also controls the revering the locomotive. Foreign steam locomotives may not use a Johnson bar to control reversing or Cutoff.
Michael M. sent us the following note : This page has programs that animate many different value gear types allowing the user to adjust cutoff and well as many other things. Might go along with the QSU cutoff tip that had a link to an animation that did not show cutoff.
