Steam & Excursion > Mind bender solution.

Fans,

Let's have some fun.

See who can figure-out how train propulsion actually works --- with a direct-drive, reciprocating Steam Locomotive.

Here's the puzzle:
Presume that there is No driver slippage.
What actually happens in the relationship of the piston to the cylinder, as the engine begins to move and pull a load of cars?
What changes occur in the relationship of the driver crank and frame, once the cylinder dead-centers are passed?  
What role does the front cylinder head play in propelling the train?
With respect to the 'ground',  how does the piston's velocity vary?  Why?

( Hint: Imagine that there is an open crack in the frame, just above the axle of the Main Driver.
What happens in that crack as the driver is powered [under strain] through 360-deg of rotation?).

To be polite: If you're familiar with the solution, wait for others to wrestle with the puzzle and have
the time to respond, before pouncing.

Puzzlers: You will astounded by what you discover about our recip-engines...
You are exploring mental territory explored by Newton, Watt, George & Robert Stephenson,
George Westinghouse, all the Loco builders, and Master Mechanics.....

Each had their "Ah-Ha! Moments", as they reasoned-through the forces & what actually was happening
in propulsion  --- as the Iron Horses labored against their trains..

Have a Fun.

Wes Camp

 



Edited 3 time(s). Last edit at 01/13/20 19:24 by wcamp1472.

wcamp1472 Wrote:
-------------------------------------------------------
> Fans,
>
> Let's have some fun.
>
> See who can figure-out how train propulsion
> actually works --- with a direct-drive,
> reciprocating Steam Locomotive.
>
> Here's the puzzle:
> Presume that there is No driver slippage.
> What actually happens in the relationship of the
> piston to the cylinder, as the engine begins to
> move and pull a load of cars?

You can think of the steam pressure pushing the locomotive forward against the piston which is in effect stationary in relation to the earth. The forward head then is moving away from the piston. Everyone seems to glorify the piston as the strongman but the heads are actually the bit connected to the drawbar. On the forward stroke, power is sent via the axles to the frame. I think lol.

. > What changes occur in the relationship of the
> driver crank and frame, once the cylinder
> dead-centers are passed?  

Maximum torque is achieved when the crank pin ia 90 (0 and 180 deg position) deg to the frame and diminishes to 0 at the 90 and 270 position. This. Means power pulses are transmitted to the frame via the cylinders and saddle. I imagine that little crack would grow and shrink with each revolution. Especially since the the drivers are quartered. The backstroke of the cycle is slightly more effective because of the radial pattern the main rod traces from the wrist pin of the crosshead.
> What role does the front cylinder head play in
> propelling the train?


With respect to the 'ground',  how does the
> piston's velocity vary?  Why?

The piston must travel the effective distance of the bore with each revolution of the driver BUT at each end, the piston comes to a complete stop. This means the speed of the piston relative to the cylinder is an exponential function then reversed with the highest speed achieved when the crank pin is 90 degrees to Earth up or down, or the piston at halfway between fwd and aft travel. In relation to the ground the piston travels forward faster than the train and at one point is stationary to the ground on the aft stroke.
>
> ( Hint: Imagine that there is an open crack in the
> frame, just above the axle of the Main Driver.
> What happens in that crack as the driver is
> powered through 360-deg of rotation?).
>
> To be polite: If you're familiar with the
> solution, wait for others to wrestle with the
> puzzle and have
> the time to respond, before pouncing.
>
> Puzzlers: You will astounded by what you discover
> about our recip-engines...
> You are exploring mental territory explored by
> Newton, Watt, George & Robert Stephenson,
> George Westinghouse, all the Loco builders, and
> Master Mechanics.....
>
> Each had their "Ah-Ha! Moments", as they
> reasoned-through the forces & what actually was
> happening
> in propulsion  --- as the Iron Horses labored
> against their trains..
>
> Have a Fun.
>
> Wes Camp
>
>  

Ok, how close am I?



Edited 2 time(s). Last edit at 01/14/20 08:56 by Check_A1E_Perf.

> With respect to the 'ground',  how does the piston's velocity vary?  Why?

The piston movement with respect to the ground  varies as a "Cycloid" curve, modified by the changing angle of the rod as the crank moves moves above and below the axle.

The piston velocity varies above and below the engine velocity by the ratio of the piston stroke (same as the crank circle diameter) to the wheel diameter.

For a reasonable stroke of about half of the wheel diameter, the piston velocity relative to the ground would vary from about half of the engine velocity with "rod down" to about 1.5 times the engine velocity with "rod up".

If you could make the crank diameter equal to the wheel diameter (without hitting track structure!) the piston would actually stop at "rod down", and would be twice the engine velocity at "rod up".    If you could make the crank diameter greater than wheel diameter, the piston  would actually move backwards relative to the ground at "rod down".   I think with a strange geared locomotive, you could actually make the crank circle diameter greater than the wheel diameter.

Bill   

Check_A1E_Perf Wrote:

> The piston must travel the effective distance of
> the bore with each revolution of the driver BUT at
> each end, the piston comes to a complete stop.
> This means the speed of the piston relative to the
> cylinder is an exponential function then reversed.......

Actually, a sine function - slightly distorted due to "angularity" of the main rod..

> ...  with the highest speed achieved when the crank pin
> is 90 degrees to Earth up or down, or the piston
> at halfway between fwd and aft travel. In relation
> to the ground the piston travels forward faster
> than the train and at one point is stationary to
> the ground on the aft stroke.

The only time the piston will not be moving relative to the ground is when the engine is stopped.  See sketch.  Point A (the center of the axle) is moving at the same speed as the locomotive.  Point B on the wheel tire (in contact with the rail) is instantanously not moving relative to the ground.  As the engine moves to the right the instantanous speed of point C relative to the ground is the same as the axle when the main pin is at r and at f, is faster at t, and slowest at b though not stopped.  So, Point C, and thus the piston, is always moving in the direction of travel, unless the engine is stopped.

-LD

Edit: Just noticed label for point r seems to have fallen off my graphic.  Its the black dot to the left of dot A.
Edit: Redrawn graphic per above and added sine wave as locus of point C's forward motion.
 



Edited 4 time(s). Last edit at 01/17/20 11:18 by LarryDoyle.

---

wcamp1472 Wrote:
-------------------------------------------------------
> Puzzlers: You will astounded by what you discover
> about our recip-engines...
> You are exploring mental territory explored by
> Newton, Watt, George & Robert Stephenson,
> George Westinghouse, all the Loco builders, and
> Master Mechanics.....
>
> Each had their "Ah-Ha! Moments", as they
> reasoned-through the forces & what actually was
> happening
> in propulsion  --- as the Iron Horses labored
> against their trains..
>

Crikey, don't forget Richard Trevithick!



Edited 1 time(s). Last edit at 01/14/20 09:59 by exhaustED.

LarryDoyle Wrote:
-------------------------------------------------------
> Check_A1E_Perf Wrote:
>
> > The piston must travel the effective distance
> of
> > the bore with each revolution of the driver BUT
> at
> > each end, the piston comes to a complete stop.
> > This means the speed of the piston relative to
> the
> > cylinder is an exponential function then
> reversed.......
>
> Actually, a sine function - slightly distorted due
> to "angularity".

I had originally put sine wave but doubted myself...
>
> > ...  with the highest speed achieved when the
> crank pin
> > is 90 degrees to Earth up or down, or the
> piston
> > at halfway between fwd and aft travel. In
> relation
> > to the ground the piston travels forward faster
> > than the train and at one point is stationary
> to
> > the ground on the aft stroke.
>
> The only time the piston will not be moving
> relative to the ground is when the engine is
> stopped.  See sketch.  Point A (the center of
> the axle) is moving at the same speed as the
> locomotive.  Point B on the wheel tire (in
> contact with the rail) is instantanously not
> moving relative to the ground.  As the engine
> moves to the right the instantanous speed of point
> C relative to the ground is the same as the axle
> when the main pin is at r and at f, is faster at
> t, and slowest at b though not stopped.  So,
> Point C, and thus the piston, is always moving in
> the direction of travel, unless the engine is
> stopped.
>
Got it. The relative speed gets faster the closer the crank pin is to the constantly moving forward axle and vice versa. That makes sense.
> -LD
>