It may seem counterintuitive that locomotives with smaller driving wheels should achieve higher efficiency than those with larger driving wheels, but it is a fact that high engine rotation rates offer a number of technalogical benefits. It may also seem counterintuitive that long strokes (such as used on GWR 2-cylinder locos) also offer advantages over short stroke engines.
High Rotation Rates: In many of his technical papers, Livio Dante Porta stressed the importance of long strokes and high rotation rates, and in the latter case recommend the adoption of a maximum rotation rate defined in AAR standards of 504 rpm (= 8.4 Hz, or revs per second), when determining the size of a locomotive's driving wheels.
The advantages of high rotation rates can be summarized under three headings:
- Leakage: Where not properly controlled, steam leakage is one of the principle causes of efficiency loss in steam locomotives. Leakage past piston and valve rings is hard to detect or to measure, but it tends to be a constant amount regardless of engine speed. Hence its importance increases in slow moving machines such as those used in shunting, and its importance decreases as engine speed increases.
- Wall Effects: At high engine speeds, steam has less time to lose heat through contact with the cylinder walls, hence a reduced efficiency loss. In his Compounding paper, Porta claims that the temperature drop (where there is no condensation) is roughly inversely proportional to the rotational velocity to the power of -0.3 (i.e. ω-0.3).
- Lighter Hardware: Small wheels imply lighter hardware, hence higher power-to-weight ratio
The 5AT's 1880 mm (6' 2") driving wheels were ideal in terms of meeting the AAR requirement when operating at its maximum continuous operating speed of 180 km/h (113 mph).
Note: It is much more difficult to design high wheel rotation rates into a heavy freight locomotive given the fact that even if fitted wth very small driving wheels of 1.22 (4' 0") diameter, the locomotive would need to operate at 115 km/h (72 mph) to achieve a rotation rate of 504 rpm. Big end clearance restrictions make it impossible to design wheels that are small enough to achieve this rotation rate at "normal" freight train speed.
Long Piston Stroke: Advantages of long stroke, small diameter cylinders include lower clearance volume and lower heat transfer losses (i.e. lower wall effects) - see Questions about the 5AT Technical Specification. In his Compounding paper, Porta implies that long stroke, small diameter cylinders help to reduce steam leakage, however he does not specifically state this, and (given the nature of leakage which in Porta's designs is limited to the ring gaps, it is hard to see that cylinder diameter would have any significant effect on leakage.