With reference to the following slide taken from the above animation:
A: the one inlet port (there is another one for the left piston).
B: inlet hole on the piston skirt.
C: exhaust port.
D: transfer port.
E: opening formed between the tilting valve and the respective piston port (the spherical piston port formed around the wrist pin, (in the case of the left piston it is the K, L)).
F: scavenging pump space
G: passageway between the scavenging pump space F and the transfer passageway H.
H: transfer passageway (it ends at the transfer ports D, which are controlled by the piston skirt)
I, J: lips of the tilting valve of the left piston.
K, L: port formed on the left piston (it is sealed by the tilting valve lips I and J during a part of the cycle).
M: space inside the right piston (permanently in communication with the inlet port A through the hole B on the piston skirt)
N: space inside the left piston.
In the above slide, the pistons are at the beginning of the compression; the exhaust port C is just closed (the transfer port D is already closed).
The space F increases, Air / mixture from the space M inside the piston (wherein the crankshaft is arranged) passes to the space F through the opening E (anti-diametrically there is formed another opening).
The vacuum in the space M inside the piston fills with air / mixture entering through the inlet port A and the hole B on the piston skirt.
After the TDC, the tilting valve seals the respective port on the piston isolating the space M inside the piston from the space F.
During the expansion the volume F decreases, however there is no way the trapped gas to escape.
When the transfer port D is finally opened by the piston, the compressed charge into the F and H spaces enters and scavenges the cylinder.
As the piston moves towards the BDC it pushes the air / mixture from the space F to pass to the combustion cylinder through G, H and D.
Before the closing of the transfer port D, the tilting valve "opens" (E) and air / charge from the space M inside the piston accelerates going to the spaces F and H and, through the transfer port D, into the cylinder until the transfer port D to get closed by the piston.
The vacuum in the space M inside the piston makes air / mixture to enter through the inlet port A into the space M, so, at the end of the transfer there is a significant flow of charge through the inlet port A towards the space M inside the piston and towards the increasing space F at the back side of the piston (like the overlap in the 4-strokes).
During the compression stroke the space F fills with fresh charge, and so on.
TUNING
The "tuning" depends more on the intake and transfer, than on the exhaust.
With the exhaust ports already opened, the transfer ports open and the scavenging begins.
The transfer ports open earlier than in a conventional 2-stroke. The pressure into the "scavenge pump" (sealed by the "back-side" of the piston whereon the tilting valve is still closed) allows such timing.
Later the tilting valve opens and the scavenging continues based on the inertia of the gas column formed along:
the open tilting-valve-port on the piston,
the space into the scavenge pump,
the transfer passageways,
the transfer ports,
the combustion chamber,
the exhaust ports,
and the exhaust.
Fresh charge from the "crankcase" (i.e. from the space inside the piston) feeds this "inertia" column, while the crankcase is fed from the intake, through the intake port, with fresh charge (air-fuel-oil mixture).
When the piston finally closes the transfer ports, the suction of air-fuel-oil-mixture from the intake continues due partly to the subpressure created into the scavenge pump space as the piston (with the tilting valve open) moves towards the TDC, and partly to the inertia of the gas column formed along:
the intake,
the intake port,
the crankcase,
the scavenge pump space,
and the scavenge passageways.
After the TDC the tilting valve closes and the air-fuel-oil-mixture trapped inside the "scavenge pump" is compressed waiting the transfer ports to open again by the piston.
And so on.
