Building the 4 Rotor- some details...
Removed from the front of the rear
engine are the front cover, oil pump, thrust bearing, counterweight, water pump, etc. Pretty much
wiped clean with the exception of the stationary gear and modified rear ecc. shaft. The 3/8" plate
bolts to the rear engine first, as internal bolts are needed to seal passages exposed when the front cover is eliminated from the rear engine.
A round donut shaped pilot ring was machined, which lightly pressed into the recess in the exposed front
stationary gear on the rear engine, and into the seal groove of the rear stationary gear of the front
engine. This pilot ring served to accurately align the front and rear engine's eccentric shaft bores, while allowing the e-shaft coupling to pass thru it's center. O-ring grooves in the pilot ring seal the front engines' rear stationary gear to the adapter plate, preventing oil leaks from inside the front engine's rear housing.
The 2 lower exposed (outside bottom) tension bolt holes are bored out and the stock bolts
replaced with very long bolts that go thru both engines, to pre-tension the lower part of the
engine, as I used only front and rear motor-plates, and no mid-plate support.
At the heart of the design is the coupling, which has to take a lot of abuse, yet remain small enough to fit thru a main
bearing to allow assembly. The coupling design
requires welding up the eccentric shafts (or building new shafts from scratch), which lengthens the full diameter portion of the shaft adjacent to the main bearing journals, so that the coupling can be machined into them. It is a tapered/pinned design that requires a draw bolt passing thru the bored out center of the front ecc.
shaft, threaded into the male half of the coupling (which is machined into the rear e-shaft), to draw the two ecc. shaft tapers together. The tapered portion
was about 1-1/2" long, with a series of (12) 3/16" dia. x 2" long aluminum pins arranged in a circle
about the taper parting line. As the pin holes are bored parallel to the e-shaft centerline, the parting line passes thru the pin at an angle, with 1/4" of each end of the pins firmly rooted in each of the shafts. The drawbolt thru the bored out center of the front e-shaft pulls the coupling together, and allows the front engines' thrust bearing to work for
both shafts.
Because the design removes what would have been the 2 middle counterweights, the
two engines were indexed the same (#1 rotors for each fired at the same time). This puts the two
middle eccentrics 180 degrees (in rotation) from one another, thus canceling the need for center
counterweights to maintain balance.
For the oil pump, I use the big oil pump on the front of the front engine, which feeds both engines, and modify the
drive ratio. By machining off and welding the toothed part of another oil pump sprocket onto the
eccentric shaft sprocket, it is possible to drive the oil pump at twice speed (1:1 instead of 1/2 speed), which roughly doubles the volume. The special drive chain required
is made from parts of 2 chains put together, as I could not find a source for the odd pitch chain.
A separator/de-airiation plate was made that fit across the entire bottom of the assembled
4 rotor, which helps seal the area between the engines for the full length, baffled and gated
custom built oil pan. A special larger pickup tube is made and the oil circuits "ported" to ease
the demands on the pump.
The water pump housing is modified so that the output can not
flow into the front engine from the front housing, but instead is re-directed into an external distribution manifold. From here, the coolant is split and directed into the top of the center housing of each engine. The modified rotor housing coolant flow
is into each center housing from the top, clockwise (from the front) around to the top on the right
side, cooling the combustion side of the engine first, then to exit ports that return the coolant to the radiator. No thermostat is used, only center housing outlet restrictors
to regulate coolant flow rates and engine temperature.
For the ignition system, my 1st design fired only 1
plug per rotor, and allowed using the normal point type distributor. Points were adjusted to fire
leading/trailing at the same time, with leading plug wires going to the front engine, and trailing wires
to the rear. A more modern ignition would be a direct fire using double posted "waste spark" coils,
one post for the front engine, one post for the rear. 4 coils would be used.
Starting the beast was
harder using 1 plug per rotor, so a 24v
starting system was used. The added voltage increased cranking speed, but the starter would overheat at anything
over 20 secs.(melted the solder in the brushes). A system that uses 18 volts cranks almost as fast,
as is not nearly as hard on the starter. If you use both plugs per rotor, the 18v system will work for
you.
During the duration of the project, which included quite a few nites racing spread out over a year and a half, the couplings experienced no damage at all. The draw bolt, which I felt would be the first external indicator of failure, maintained it's assembly torque with no sign of any loosening. When the engine was finally torn down (after an oil line was torn off during a race), the coupling and all it's components looked like new.
As proof of this, when the engine was assembled, the coupling pins and taper was coated with loctite, which I felt would fill any minor discrepencies, and improve the strength of the coupling. Upon diassembly, the shafts were quite hard to seperate, as the loctite bond was still intact.
For the type of racing that the engine was built for, eliminating as much rotating mass as possible is a much
sought after goal. The effects on accelleration can be very surprising. By joining the two engines as I did (using only 1/2 the normally required flywheel
and counterweight mass), the engine rev'ed much faster than a single engine by its self. Although
the exhaust note might be more pleasant if the engines were phased 90 degrees from one another
for an even firing order, the reduced mass and synchronized peak torque pulses make for less
stress on the coupling, and the reduced rotational inertia over that of two separate engines far
outweigh any performance advantages of the smoother sounding 90 degree firing order.
By using the tapered/pinned design, the loads on the coupling are effectively spread out over a much larger cone shaped area. Less stress concentration, than what would have been present with any other type of useable coupling we could dream up, was an added bonus. All this in such a compact and relatively inexpensive design.