NW-Mailing-List Digest, Vol 31, Issue 8

[NW Mailing List]

> Both steam and diesel are constant horse power machines.  The max hp of the

> prime mover of a diesel can be maintained continuously even if the current

> would damage the traction motors. and A steam locomotives boiler can

> maintain its max boiler horsepower continuously even if it is escaping from

> the safety valves. 



That is fundamentally not possible.  The ability to maintain maximum boiler pressure does not equate to the ability to maintain maximum power, given that the pistons are not moving.  I recall from University Physics I that power is equal to work done over time (delta t), and work is force times distance (more accurately, the integral from initial force to final force).  In both the steam and diesel, that force is pressure applied to piston area, and higher piston speed equals more power (as distance over time has increased).  The difference lies in that that work done over time for a diesel is irrespective of train speed.  With a reciprocating steam locomotive, in order for it's cylinders to produce power, the locomotive has to be moving, as that is what produces the time element needed to produce power.  Higher locomotive speed allows the piston speed to increase, thus increasing distance over time and consequently, power output.

Robb Fisher
RFDI