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’.
