26 HP Advance tandem compound. Courtesy of O. R. Aslakson, 317 N. 9th Street, New Rockford, North Dakota 58356
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.
The 'cross-compound' has side by side cylinders and cranks at 90 degrees same as a 'double simple'. The steam enters the small cylinder and exhausts into the large one, same as the tandem. The fact that the pistons do not travel together because of the positions of the cranks brings up some interesting points to consider. For example, when the small piston is starting its stroke the large one is approximately at half stroke. It would seem that possibly some loss of push would result because the exhaust from the small cylinder might be late in the large one. (I believe the capacity of the steam chest takes care of this.) On the other hand, if back-pressure on the small cylinder is considered, the position of the large piston in relation to the small one would seem to allow easier passage for the exhaust from the small one. Almost all cross-compounds have an 'intercepting valve', (sometimes called a 'simpling valve') to allow starting at any position of the cranks, no dead-center.
In regard to back pressure on the small cylinder of a tandem-compound, the fact that it is exhausting into a larger area, which is getting larger as the pistons travel thru their strokes, would have a scavenging effect that could more than cancel the loss to back pressure. Condensation is probably more of an evil than back-pressure; it affects both simple and compounds to a degree that would be hard to determine. Some cross-compounds used 'Super heaters' between the high and low pressure cylinders to correct some of the loss to condensation.
The 'stack talk' of a single engine is music to the ears of engineers and others. I also like the more mild and apparently effortless sound of a good compound at work.
I enclose two pictures of Advance compounds that are at our show. The 40 HP cross-compound is owned by Norman Pross, Luverne, N. Dakota. The tandem-compound is mine, I threshed three seasons with this one in the 1930's. We burn straw at the show.
No article designed for beginners would be complete without stressing the importance of carrying the proper water level. BE SURE YOU KNOW! I would refer to the article by Fred M. Freshette, Red Deer, Alberta in IMA Jan.-Feb. 1975. He has a very complete section on how to be sure that you are not fooled by the 'water glass'.