Compounds, Yes or No?


| November/December 1975



Advance tandem compound

26 HP Advance tandem compound. Courtesy of O. R. Aslakson, 317 N. 9th Street, New Rockford, North Dakota 58356

O. R. Aslakson

317 N. 9th Street, New Rockford, North Dakota 58356.

Noticing Walt Johnson's inquiry in Anna Mae's 'Soot in the Flues', IMA July-Aug. '75, asking information about compounds, I will try to come up with something that might help. I believe articles in regard to our hobby might be appreciated by beginners and some of the 'Old Timers' too. Welcome to the club, Walt.

I have threshed with three different compounds, one a cross-compound and two tandem-compounds, all three Advance engines. I have a good opinion of compounds, especially for steady work like threshing, I am probably in the minority in regard to this.

Possibly a little information on a 'simple' (not compound) cylinder might be of help. First, almost all traction engines are 'double-acting', the steam acting on both sides of the piston to produce the reciprocating motion. Let us assume that the piston is just starting its power stroke, the valve in the steam chest has just opened to admit steam to push the piston to the other end of the cylinder. At this same position of the piston, the valve is allowing the other side of the piston to start its exhaust stroke. When the piston has traveled a little more than half way thru its stroke, the valve cuts off the flow of live steam and the expansive power of the steam acts on the piston till it nears the end of the stroke. When the piston reaches and passes the other end of the cylinder ('dead center') the same sequence of 'admission', 'cut-off' and 'expansion' occurs on the other side of the piston. This is a very elementary explanation, the point of cut off is variable on most engines, and there are other things to consider, such as 'lead', 'cushion' etc. Condensation and back-pressure are 'enemies' of efficiency on both simple and compound engines.

A tandem - compound is a single crank engine with separate, tandem (in line) cylinders. Each cylinder has its pistons and valves travel The pistons and valves travel together, being fastened on the same piston rod and valve stem. The steam from the boiler is admitted to the small 'high pressure' cylinder, goes thru the sequence of admission, cut-off and expansion the same as a simple, but instead of exhausting up the stack, it is admitted to the steam chest of the large 'low pressure' cylinder and goes thru the same sequence here before being allowed to escape to the atmosphere. In other words, more full use of the expansive power of the steam is utilized. (There are differences of opinions of how efficiently it does this).

Another form of tandem-compound is the Woolf, it uses a single valve and steam chest to distribute high and low pressure steam to the respective cylinders.