With the piston approaching its TDC, combustion happens into the cylinder.
At about the same time the piston port aligns with the "fuel" port; due to the subpressure into the crankcase, air-fuel mixture (through, say, a carburetor) is sucked in the space within the piston, underside the piston crown.
At about the same time, and due to the crankcase subpressure, air enters through the intake ports and through the intake passageways, and fills the crankcase.
During the expansion and before the middle stroke, the piston covers the asymmetric intake ports and the intake ends.
Later, after the middle stroke, the asymmetric transfer port opens. But the control surface of the connecting rod seals the transfer control port of the piston, not allowing hot gas from the combustion chamber to enter into the space within the piston or into the crankcase.
Soon after, the exhaust port opens and the pressure inside the combustion chamber drops quickly.
Soon after (the piston is still before the BDC) the conventional transfer ports open and compressed air from the crankcase enters into the combustion chamber and scavenges the burned gas out of the cylinder; this continues until the closing of the conventional transfer ports after the BDC.
Until the closing of the exhaust port, a part of the air inside the combustion chamber escapes to the exhaust.
A little before the closing of the exhaust, the asymmetric transfer port opens (if desired, it can open even after the closing of the exhaust port).
Rich air-fuel mixture from the space within the piston (in front of the transfer control port) enters into the cylinder and mixes with the air therein.
As the space within the piston, in front of the transfer control port, runs out of air-fuel mixture, air from the crankcase enters in the piston from its right bottom open end; a little before the closing of the asymmetric transfer port, a gust of this air cleans the passageways through which the space within the piston communicates with the combustion chamber.
The asymmetric transfer port closes, and the compression starts. The squeeze near the TDC completes the mixing of the air-fuel mixture with the air, and accelerates the flame propagation during combustion.
And so on.
The inner piston structure is such that in order to go to the crankcase and mix with the air therein, the fuel-air mixture entered through the "fuel" port and the respective piston port is restricted to follow a "corridor" formed inside the piston:
It moves upwards near the piston crown, then it continues horizontally to the right (cooling the piston crown) to arrive above the transfer control ports.
Then it moves downwards to the crankcase.
But before this, the asymmetric transfer port opens and, due to the pressure difference between the crankcase and the combustion chamber, the air-fuel mixture enters into the combustion chamber with the exhaust port substantially closed (which means: the fuel cannot escape to the exhaust before participating in one, at least, combustion).
The following animation shows an indirectly injected arrangement.
The "fuel" port of the previous arrangement is gone.
Click here for more details (stereoscopic animation).
The injection resembles with the "oil jet cooling system" used in several four-stroke engines wherein: "a jet of pressurized engine oil is directed to the underside of the piston to help dissipate the extreme heat generated during sustained high rpm operation".
In the present case the fuel, landing onto the underside of the piston crown, is evaporated and mixed with air, cooling the piston and the wrist pin (it may also lubricate the wrist pin).
The oil scraper ring (at the middle of the piston) never passes over ports.
The oil scraper ring keeps the lubricant into the crankcase (as in the 4-stroke engines), allowing plain bearings to be used.
At the BDC the lower compression ring abuts on an area of the cylinder liner whereon, 180 crankshaft degrees earlier, was abutting the oil scraper ring.
I.e. when the piston passes from its BDC, the lower compression ring abuts onto an oiled area of the cylinder liner, making unnecessary the conventional "total loss" lubrication of the ported 2-strokes.
Here is a PatATeco version combining the four-stroke lubrication with the crankcase scavenging:
The upward motion of the piston causes subpressure which suctions air into the intake manifold and into the crankcase through the reed valve and through the throttle valve at right.
The throttle valve can, alternately, be located away from the reed valve, at the left side (relative to the cylinder) of the intake manifold, nearer to the crankcase; when it is closed it isolates the crankcase from the rest intake manifold and from the transfer ports.
In case of tuned exhaust / high revving, the engine can operate with the reed valve permanently open (7x24) and with the upper side of the intake manifold isolated, by the closed throttle valve, from the crankcase, reminding -in a way- the PatTwo engine.
After the TDC, the downward motion of the piston compresses the air trapped into the crankcase and into the intake manifold. With the ports open by the piston, compressed air from the intake manifold either scavenges the cylinder, or enters inside the piston waiting the asymmetric transfer ports to open.
With the proper form of the air ducts, the oil droplets in the air exiting from the crankcase are collected and then they are returned back into the oil pan for recirculation, so that the air in the intake manifold near the cylinder ports remains rid of lubricant.
It reminds the "stratified charge" (click here for an explanatory drawing of it) with the difference that here the "oiled air" in the crankcase remains away from the cylinder.
A tuned intake manifold in cooperation with a tuned exhaust . . .
The following animations show another arrangement wherein the control surface on the connecting rod that seals the transfer control port is spherical, minimizing the "idle" volume of the passageway from the space inside the piston to the combustion chamber.
In case it is used in a chainsaw, for instance, a carburetor feeds the "fuel" port and the space within the piston with rich fuel-air mixture.
The crankcase is fed with air through conventional piston-controlled intake ports.
During the scavenging, compressed air from the crankcase passes, through the conventional transfer ports (yellow) into the combustion chamber pushing out the burned gas.
A little before the closing of the exhaust port, the asymmetric transfer port opens and rich fuel-air mixture enters into the combustion chamber mixing with the air therein.
The squeeze completes the mixing and increases the flame propagation speed.
For the control of the engine a throttle valve can control the flow through the air intake ports and another throttle valve in the carburetor can control the flow through the "fuel" port.
In the simplest case the two throttle valves are driven by the same shaft.
The promising (a decade ago) Direct Fuel Injection (DFI) in the two-strokes proved in practice inadequate to make them comparable to the four-strokes (emissions / mileage).
One of DFI limitations comes from the small time interval between the injection and the ignition: the higher the revs, the smaller the required size of the fuel droplets, otherwise a part of the fuel is liquid during the combustion (carbonization of the fuel, unburned fuel in the exhaust, etc).
For smaller fuel droplets it is required a more sophisticated, expensive and power consuming fuel injection system.
In smaller cylinder capacities (in two-strokes for chainsaws, for instance) wherein the revs increase substantially, the DFI is out of the question not only for cost / complication / weight reasons, but also because the substantially smaller time interval between the direct injection (which starts after the closing of the exhaust port) and the ignition (which happens several degrees before the Top Dead Center (TDC)) makes the DFI a problem, not a solution.
In a PatATeco two-stroke the fuel cannot pass to the exhaust before the following combustion. In comparison, a significant part of the fuel provided to an indirectly injected (or carbureted) two-stroke exits from the combustion chamber without having participated in a combustion.
In the PatATeco two-stroke the scavenging is realized by compressed air entering into the combustion chamber though conventional symmetrical transfer ports.
The scavenging is not restricted any longer by the loss of fuel to the exhaust, which is a crucial limitation in the tuning of the state of the art indirectly injected two-strokes.
With the proper sensors (for instance, with a fast Oxygen sensor like those used in the MultiAir engines of FIAT), and feedback, the control over the two-stroke engine can be simple and effective.
In the previous PatATeco designs a buffer of rich air-fuel mixture is formed inside the piston, underside the piston crown, waiting the asymmetrical transfer port(s) to "open".
Plenty of time is offered to the fuel (some four times more than in a DFI) to evaporate and mix with the air before the ignition.