Compound steam engines

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Answers to some previous questions will be forthcoming within a general interest item now being prepared .

Any new questions – now is the time .

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Copy of pm sent to Eric Cox a little while ago :

Hi Eric ,

I don’t visit this site very often now but I’ve just had a look through recent postings and seen your question on compound engine cylinders .

As regards simple design engines in smaller sizes very exact calculations can be done it is true but the fact is the answers always come out the same for any given steam pressure and temperature !

Basically the higher the pressure and temperature of the ingoing steam the bigger the ratio between HP and LP cylinder areas has to be .

Some examples :

For 90 psi + mild superheat ratio would be between 1 to 2 and 1 to 2-1/2 on area . So for 1 inch dia HP about 1.5 inch dia LP would be ok .

For 250 psi + high superheat ratio would be between 1 to 3-1/2 and 1 to 4 on area . So for 1 inch dia HP about 2 inch diameter LP would be ok .

A lot depends in reallity on how the valve gears on HP and LP are set . Standard practice is to have both valve gears doing the same thing both being notched up at same rate .

The above is for two cylinder compounds . Three or more cylinder compounds follow the same general principle but need a bit more thinking about for a practical design .

Couple of notes :

Whatever the design is the engine will only work properly as a compound over a limited range of pressure temperature speed etc . Away from this ideal range engine will still work but probably not much more efficiantly than a simple engine . Clever valve gear adjustments can improve the working range a lot .

Not all compounds work as compounds anyway – if ingoing steam is too wet and area ratio of pistons too high the steam will condense all the way to water in the LP cylinder and both cause damage and be ineffective in producing power .

All compounds really need at least mild superheating of the ingoing steam and benefit a lot from higher superheats .

Its abroad subject – if you have any particular engine in mind or just want to ask questions then please get back to me .

Regards ,

MikeW

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