Opposed Piston Engine- Design issues

NW Mailing List nw-mailing-list at nwhs.org
Wed Jan 23 11:22:45 EST 2013

In the 1990's there was a good article in trains written by the FM Sales
Manager that discussed the problems with the opposed piston engine in
railroad service. This is worth tracking down.

As to one crankshaft delivering more horsepower that the other, this is
essentially not feasible. The opposed piston engine works by having two
pistons facing each other across one combustion chamber. thus, when
combusting fuel, the chamber has to all intents and purposes the same
pressure throughout that volume. Thus, each piston face sees the same
pressure or total force. The engine would have the same crank throw on
the upper and lower crank, thus, the same torque would be produced at each
piston and crankshaft. Now in a realistic case there are pressure
variations through out the combustion chamber, but they are relatively
small and dynamic, changing somewhat throughout the cycle. All in all, it
is still the same pressure or dynamic pressure throughout the cycle. the
two cranks are geared so that the output shaft is driven by the two gears
off of each crankshaft. Engineers call it the mean effective pressure,
mean being a mathematical term for average across a cycle.

Could each crank deliver significantly different torques? Yes, by
designiing different cylinder bores and different crank throws. ( If one
could get the gearing to survive!!) Would this be a rational and easy
design and development problem? HECK NO!!! And the engine would cost
much, much more with a much higher level of mechanical and maintenance
costs. So it doesn't make any sense to do this.

ALL two cycle engines are potential smokers due to the needs of the
scavanging cycle necessary to make the two-cycle 'go'. When accelerating
an engine one MUST provide more fuel to get the energy required to speed up
the engine. Pre- digital fuel injection with computers, this process was
mechanical and required over supply to 'fudge' the dynamically operating
system. This meant that any and all accelerations were necessarily rich
mixtures- over supply of fuel- to accomplish this. For these big diesels
turbocharger or supercharger lag enters into the picture. One has to
accelerate the engine to get the supercharger up to speed and for the free
running turbo charger, it can lag more.

Modern Diesels are virtually all four cycle to have better control of the
combustion process to control emissions, including the particulates
(smoke). And all are digitally controlled by computers to provide the
proper fuel supply variability to minimize any excess fuel supply. This
just can't be done in a two-cycle engine.

The design philosophy of the opposed piston engine is to utilize more of
the pressure in the combustion chamber with the two pistons rather than
'waste' the pressure pushing against a head so that the pounds of engine
per horsepower can be lower. For a opposed piston two cycle the weight of
the engine was lighter for a given horsepower than a two-cycle EMD-tpe
engine. But the penalty was greater complexity.

For maintenance, any work inside the engine required taking off one half of
the crank case, the upper crank shaft and the gear box. For an EMD
engine, just the local cylinder head and cylinder sleeve had to be removed
to get down inside. This component could be repaired on the bench and a
replacement pulled off-the-shelf.

Gary Rolih
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