A friend of mine just asked me the question of "On a steam engine such as 765, there are 4 beats/chuffs per revolution, but what causes the initial "accented" chuff?" I was not really sure of the correct answer, so am posing this here. Thanks for your help.
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If one exhaust beat is louder than the others, it's because the valves are not set perfectly "square." All the exhaust beats should be the identical.
Setting the valves on a steam loco is more art than science. Differences of 1/64" can make HUGE changes in the way the engine sounds and performs.
but what causes the initial "accented" chuff?"
A starting, the initial louder exhaust is produced by the first rotation of the drivers, since the Engineer introduces more steam inti the valves/cylinders, in order to get the locomotive to move and start thew train. Upon immediate forward movement, the Engineer then reduces the throttle quite a bit, so as not to slip the drivers. Thus the first exhaust sound may be much louder than the successive exhausts, until the Engineer increases the throttle again in order to maintain/increase forward momentum.
All this "throttle action" generally takes place during the first 1 to 3 revolutions of the drivers.
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