The goal for this idea is to have an engine which has
high efficiency, high power density, and low vibration.
The design is as follows: two or more cylinders are
arranged radially around a central crankshaft.
In each cylinder, on the side toward the crankshaft, is a
piston; all of these pistons
are connected to a single
crank on the shaft (just like in a regular rotary engine).
In each cylinder, on the side away from the crankshaft,
is a second piston, whose face is aimed inwards,
towards the first piston. The rear of each of these
outer pistons is connected, via a short connecting rod,
to a pair of long connecting rods (one along each side of
the cylinder), which in turn connect to the crankshaft.
The two cranks to which the long connecting rods
attach to are in phase with each other, and about 160
degrees away from the crank which drives the inner
pistons.
The inner pistons, the ones closer to the crankshaft,
cover and uncover the intake ports. The outer pistons,
the ones further from the crankshaft, cover and uncover
the exhaust ports. In the part of the cylinders between
the outer and inner pistons, there would be fuel
injectors (gasoline or diesel), and either glow plugs or
spark plugs.
Forced induction would be provided by a hybrid
turbocharger, with a variable geometry turbine.
Advantages:
Because there are two sets of moving parts (inner
pistons and outer pistons), their motions largely cancel
out, minimizing load on the crankshaft bearings, and
minimizing engine vibration. This results in lower
weight (smaller bearings, smaller engine mounts) and a
longer service interval.
Because each cylinder has two pistons, the pistons need
to move half the distance a single piston would in a
regular radial. Peak piston speed is cut in half, which
means that the maximum engine RPM is higher, which
increases the engine's maximum power, which increases
the engine's power density.
Due to the uniflow scavenging, the amount of fuel that
can be burnt per cycle is higher than if loop scavenging
were used; however, unlike most uniflow scavenged
engines, the exhaust is through a port which a piston
covers and uncovers, instead of through a valve. This
reduced parasitic loss, and increases available engine
power.
If the engine is to be built to use diesel fuel, then direct
injection is natural; if the engine is to use gasoline, then
the advantages of using direct injection should be
obvious.
As with all two stroke engines, forced induction is
necessary; by using a hybrid turbocharger with a
variable geometry turbine, the engine can operate
efficiently over a wide range of speeds, and turbo lag is
largely avoided.
The reason why the crank for the inner pistons would be
between 160 degrees out of phase from the cranks for
the outer pistons is as follows:
If the cranks were 180 degrees out of phase, then
dynamic equilibrium can be trivially achieved, the load
on the crankshaft bearings could be zero, and vibration
minimal... but the time the pistons spend at TDC would
be short.
If the cranks are 160 degrees out of phase, the relative
speed of the two pistons slows down close to TDC,
allowing more time for fuel to burn, which results in
more complete combustion, thus more power and less
pollution.
Also, I think it allows the port timing to be such that
expansion stroke is longer than the compression stroke,
producing something like the Miller cycle.
The advantage of this design over OPOC is of course
that it's not limited to just two cylinders. And I'm pretty
certain that the OPOC design uses a crank angle of 180
degrees (but I'm not sure since it's not mentioned in the
text of their website, nor is it clear in the pictures &
animations).