Chuff Rate Question

[blwfish]

Conventional steam locomotives exhaust twice per cylinder per rotation of the driving axle. So a two-cylinder exhausts four chuffs per driving axle rotation, and a three-cylinder such as a UP 9000 or SP 4-10-2 yields six chuffs.

Mallets - ie compounds - sound like larger versions of a two-cylinder, even though there are four cylinders. The high pressure cylinders exhaust into the low-pressure ones, where the steam expands again before going up the stack. Since it's the exhaust beats that we hear, a Mallet like the C&O 2-6-6-2's sound like big 2-cylinders. A simple articulated such as an N&W A or UP Big Boy/Challenger is effectively two two-cylinder locomotives, so eight exhaust beats.

There are some exceptions to all of these rules, such as the N&W class Y 2-8-8-2's, which were fundamentally compound locomotives, but which were often started as simple locomotives. Once the train was started the Y would be switched back to compound operation for the run.

There were also various oddball locomotives, such as the Baldwin 60000 4-10-2, which was a 3-cylinder compound, meaning high pressure went to the center cylinder, which exhausted into the two outside cylinders - hence it sounded like a conventional 2-cylinder locomotive. (The UP 9000's were three-cylinder simple locomotives.)  D&H and B&O each produced a number of odd duck compounds in the 20s and 30s, but they're obviously not Bachmann models.

[MarkInLA]

Thanks to Tom (above)..Very clear advice...Mark

[MarkInLA]

To those who are still wrestling with the 4 chuffs per 1 driver revolution, what  those knowlegable are trying to get across which many folks, especially newbees, young kids and non train buffs will usually and understandably assume is that all the mechanical activity concerning side rods and pistons, ALL of it is at the opposite position on the other side from the one they're looking at. I did too at 8 yrs old. This is a natural human assumption. But, it's not correct !  If the side rods on one side of loco are at say, 12:00 ( looking at wheel like a clock), the other side will be sitting at either 3:00 or 9:00..If you're seeing them at say, 4:30, other side is at either 7:30 or 1:30, a quarter of the distance apart, not half the distance apart..Thus the drivers are said to be 'in quarter' or 'quartered'
Why ? Because if they were exactly opposite there would be a mechanical position occurring where they'd get stuck, not able to decide which is pushing piston forward OR back, and would lock up...Also it would cause the loco to rock like crazy even if you could have it this way...They are not half way apart on the clock. They are a quarter away from one another ALL the complete 360 degs. of motion, all the time...This, too, is what is responsible for the 2 chuffs per side equalling 4 per one revolution..

[blwfish]

Good point, Mark.  Again, the three-cylinder locomotives are a little different, since they can be set 120 degrees apart and never be stuck in the same way.

[Desertdweller]

Wow!  What complex questions!

2 cylinder engines: 4 chuffs per axle revolution.  Chuffs are not made when steam is admitted to the cylinder, they are made when the steam on the other side of the piston exhausts. (In other words, steam going in is silent, but steam going in results in other steam on the backside of the piston going out.  Steam engines are two-cycle engines, there is no compression or separate intake and exhaust strokes.  The piston just moves back and forth, alternately admitting and discharging steam as it moves).
The exhausting steam causes a venturi effect inside the smoke box, drawing draft through the fire box.

The drivers are "quartered" (crank pins 90 degrees offset on opposite ends of the axle) so the engine cannot be stopped with both cylinders at top dead center at the same time.  If this happened, the engine wouldn't be able to start.  There would be no space for incoming steam.

Three cylinder steam engines use a crank-shaped axle that the center cylinder pushes on.  Since there is 360 degrees in a circle, the crank and the main rod pins are located 60 degrees apart.  A 3-cylinder engine will have 6 chuffs per axle revolution.

A 4-cylinder steam engine:  this depends on how it is set up.  If you have a duplex drive 4 cylinder locomotive, with side rods linking each set of drivers on each side, you would get 4 chuffs per axle revolution just like a 2 cylinder engine.
But if the engine sets are not linked to each other by side rods (like in a Pennsy T-1, or in an articulated locomotive), both engine sets can and often do run out of synch with each other.  One engine can easily lose traction and slip while the other keeps adhesion.  You would then get a sound like two 2 cylinder engines working, not necessarily together.

[Desertdweller]

I forgot to include the difference between the sound of a compound articulated and a simple articulated.

A compound articulated, as has been mentioned, feeds its low-pressure (front) cylinders with steam exhausted from the high pressure (rear) cylinders.  So the front engine would stay in synch with the rear.  If the chuffs would stay in synch with each end would depend somewhat on driver quartering and valve timing, but would have to be pretty close for it to work at all.

Simple articulated are really just two independent steam engines working off one boiler, all 4 cylinders are high pressure.  These could easily get out of synch with each other.

Some compound articulateds (like N&W Y-series) used high pressure steam in all cylinders for starting.  The big low pressure cylinders would quickly run the boiler out of steam if left in this mode.  These engines could get out of synch when starting, but would fall into synch when switched back to compound operation.

Les

[edpb]

For some time now I have been watching my Mogul and wondering if a real loco can start to move before a chuff is heard.  I hope this isn't too far off this topic.  --  Ed

[Tom M.]

I believe the answer is yes.  The chuff you hear is on the exhaust portion of the piston stroke.  As such, you would have to infuse steam first. Depending on the relative position of the cylinders, you could get at least a quarter turn of the driver before you get a chuff.

Regards,

Tom

[Doneldon]

I believe the answer is yes.  The chuff you hear is on the exhaust portion of the piston stroke.  As such, you would have to infuse steam first. Depending on the relative position of the cylinders, you could get at least a quarter turn of the driver before you get a chuff.

Regards,

Tom

edpb-

Tom is exactly correct.

                          -- D

[edpb]

Well, Tom and Doneldon, here it is Nov 30 and please believe me that I am very glad to have your replies.  I have my answer, and now I know where the chuff comes from. 

Ed

[electrical whiz kid]

Simply put, the sounds are 90 degrees apart from each other.  It is going to be pretty hard to distinguish left from right, unless you have a larger scale engine; considering that the [average] view of an HO scale locomotive would be that from the top of a three story building-not down at trackside; so to me, the fuss about sync-ing right and left is pretty pointless and would tend to be a waste of time; which I would rather engage in more worthy pursuits.

[wjstix]

Well the "chuff" is the sound of the steam from the cylinder exhausting up the stack, so you don't really hear left-right-left-right, just chuff-chuff-chuff from the stack.

On a compound Mallet, the steam exhausting from the high-pressure rear cylinders to the low-pressure front cylinders doesn't create a chuff. The chuff only occurs when the steam exhausts out the stack. So a Mallet like the Spectrum 2-6-6-2 would just have the same four chuffs per revolution of any other engine.

On a simplified engine, where all four cylinders get steam directly from the boiler, the steam from all four cylinders exhaust directly up the stack, so the exhausts can be in or out of phase (chuff-chuff...chuff-chuff...)

[ebtnut]

Not quite mentioned (although it can be inferred) - Simple articulateds, as noted, "chuff" independently for each engine.  If you see or hear recordings of simple artciulateds running, you will often note that effect.  What can also happen, if one of the engines slips a bit, or if the drivers on the two engines aren't quite the same diameter (wheels can be turned periodically to true up the tread and flange profile several times before the tire becomes too thin), the two exhaust sounds will slowly go in and out of sync as the loco runs down the track. 

[Selector]

Correction to what was stated a few posts above about compound steam locomotives staying in synch necessarly  T'aint necessarily so.  Both types of locomotives, simple and compound, get out of synch.   There are many videos on youtube of the UP Challenger out of synch, but also Y6b Mallets on the Blue Ridge with their drivers clearly out of synch.

There are four chuffs per revolution of any of the driver axles on a steam locomotive because there are necessarily, by virtue of their mechanical valve events, four emissions of exhaust steam during each revolution of any one driver.  The valves slide back and forth in their races, whether slot or cylinder, atop their respective pistons which are doing the same thing.  Between them are two ports.  The valves slide over the ports and away from them, closing and opening them respecitvely, depending on where the linkages force the valves to be in their reciprocating travel.  They are always either a little behind the movement of the piston in relation to passage past the ports or timed to open as the piston reaches its extreme Nand stops.  Unlike an internal combustion engine where the spark happens very close to top dead center, but not before it or you get knocking, the steam locomotive's pistons must begin their return trip down the cylinder before the steam is admitted to accelerate them, but it is also cut off by the valve passing over the port and closing it again, prior to the piston reaching its furthest extent at the other end of its travel.  By letting the amount of steam admitted to expand and to do the work it can of forcing the piston forward ahead of it, there is more economy than there is by letting the port stay open and continuing to admit steam until the piston ceases its motion and begins its movement back the other way once more.   That is what is meant by the term 'cut-off', which is when the admission of steam is cut off and what has been admitted is allowed to expand and spend itself.  This happens mostly at greater speeds, but not at start-up when lifting a train from a dead stop.

So, if two cylinders each have two exhaust events, and the two cylinders are cycled 90 degrees apart in the 360 degree cycle, there must be four separate exhaust chuffs that the ear can detect, which is always the case.

[wjstix]

Indeed, the drivers on compound or simple articulateds would tend to go in and out of phase with each other. But note that in a compound Mallet, only the exhaust from the front cylinders go up the stack, creating a "chuff" sound. So even if the drivers weren't in synch, you'd still hear one set of chuffs.