The wheels on the train go round and round

[Deggesty]

Cylinder, steam pressure, speed and such are all a careful balance. What limits the speed of a locomotive is the movement of steam in and out of the cylinder. You cannot keep letting steam in until the piston reaches its travel end or there would still be steam flowing in when the piston starts to come back to exhause. Cutoff (reducing the time steam enters the cylinder) stops steam flowing well before the end of the stroke so the steam is no longer flowing when the piston reaches the end. Steam is mechanical and moves about the speed of sound. It cannot be started and stopped instantaneously.

Comments about larger drivers means the cylinder on a large drivered locomotive still goes back and forth the same as other engines but exerts much less force on the wheel surface. Torque is fixed as radius times force because that’s what the driving rod exerts. The large wheel radius means the force at the wheel radius is less. Steam engines were designed to go fast (passenger – with large fast driving wheels) or pull hard (freight – with smaller wheels and more force on the wheel radius).

This all is a balance of:
how much steam pressure pushes steam into the expanding cylider
how big the pipe is from the throttle to the cylinder valves to allow more steam flow
how fast and where in the cycle you can close the steam and open exhaust valves
how fast you can move the cylinder without excess forces on the rods
and so on

Locomotives were a progression of designs that emphasized different characteristics. In the 150 years of steam design many parameters could be designed in and tested. Some worked well, others not so well.

Early cutoff of the steam also makes better use of the steam–after admission is cut off, the steam continues to expand, forcing the piston away from the a

[RME]

So the general consensus is that there is no gearing in a standard steam engine, the relationship between rod and wheel size effectively sets the gear ratio?

In my opinion, the answer is yes in the sense you’re asking. The driver tires, as they wear, decrease the diameter appreciably, which increases the effective torque per stroke all else kept equal (schlimm, I pointedly avoided ceteris paribus here) while having a slight effect on diameter-speed-related concerns.

I would add a bit to what tree68 has said: notching up the gear is a bit like varying the spark advance AND fuel-injection modulation on an IC engine. Some of the more advanced poppet-valve gears also vary the timing in the ways systems like VANOS do, and it is possible (although usually done only with dumb relief valves) to alter the analogue of effective compression ratio somewhat. But the mechanical advantage is basically the same for a reciprocating locomotive as it is from piston to crankshaft in a motor, and it seldom if ever pays to play with that aspect of the design.

The history of early steam is replete with approaches that combined gearing with reciprocating engines; even the Fontaine locomotive approximates gears with friction wheels. The second Harrison locomotive used gearing to approximate the effect of large drivers without invoking problems of effective short stroke and very high wheel diameter, and had the state of materials and machine tooling been better at that time, some sort of change-speed gearbox might easily have been provided to allow good starting torque and good high-speed efficiency (of just the kind provided an IC engine by clutch/TC and multispeed transmission). Note the plethora of gear and chain ‘assistance’ in the locomotives in the original Seraing testing, and how (interestingly) some of the answers to near-zero rigid wheelbase in Europe involved not gears but fancy lateral and swivel arrangements with what