PatAT: Asymmetric Timing in the two-stroke engines
Intellectual Property: patent US 9,695,739
patent GB 2,528,748
In the conventional two-stroke engine the exhaust port and the transfer port are controlled by the piston making inevitable the symmetric timing (for instance as shown in the following plot at left, wherein: the exhaust port opens 65 degrees before the BDC and closes 65 degrees after the BDC, while the transfer port opens 20 degrees after the exhaust port and closes 20 degrees before the exhaust port).
With symmetric timing, the transfer port opens after the exhaust port and closes before the exhaust port.
Time is required to pass from the opening of the exhaust port till the opening of the transfer port (translated into crankshaft degrees) in order the pressure inside the combustion chamber to fall substantially.
The same time passes from the closing of the transfer port till the closing of the exhaust port (case of zero offset), giving the chance to a good part of the fresh air or mixture to escape from the open exhaust port under the action of the upwardly moving piston, reducing the torque and increasing the emissions.
With the PatAT the two-stroke operates on asymmetric timing (as shown, for instance, in the above plot at right, wherein: the exhaust port opens 55 degrees before the BDC and closes 55 degrees after the BDC, while the transfer starts 35 degrees before the BDC and ends 65 degrees after the BDC).
In the PatAT the source of pressurized air or mixture (the crankcase in most cases) communicates with the combustion chamber through transfer ports disposed in series with respective piston ports.
The transfer ports are controlled by the piston.
The piston ports are controlled by the connecting rod.
The source of pressurized air or mixture can, additionally, communicate with the combustion chamber through conventional transfer ports (that open by the piston after the exhaust port).
As the piston moves "downwards", it initially opens the transfer ports; but with the respective piston ports closed by the connecting rod, the combustion chamber cannot communicate with the crankcase (or, in general, with the scavenge pump).
The piston continues its downwards motion and opens the exhaust port; the pressure inside the combustion chamber drops quickly; the crankcase continues to remain sealed from the combustion chamber.
Later the connecting rod opens the piston ports (and the piston opens the conventional transfer ports, if any). The transfer takes place.
As the piston moves upwards, it initially closes the conventional transfer ports (if any).
Later the piston closes the exhaust port.
The crankcase is still communicating with the combustion chamber: after the closing of the exhaust port, air or mixture continues to enter (through the "connecting rod controllable" piston ports and their respective transfer ports) into the combustion chamber until the transfer ports to close by the piston.
While the engine remains as simple, as cheap and as compact as the conventional two-stroke, its operation is drastically improved.
In the PatAT arrangements shown in the animations below, the source of the pressurized air or mixture is external (it can be a piston compressor, a turbocharger, a roots compressor etc).
The separating plate at the middle of the piston seals the space underneath the piston crown from the crankcase.
Through openings in the cylinder and in the piston, the pressurized air or mixture enters into the combustion chamber either directly or indirectly (it initially goes into the space underneath the piston crown and then, through a piston port controlled by the connecting rod and then through a transfer port controlled by the piston, it gets into the combustion chamber).
The crankcase has four-stroke lubrication.
The scraper ring at the middle of the piston, above the wrist pin, scraps the surplus of oil from the cylinder liner back to the oil pan, from where it is cleaned and recycled.
The cylinder liner area the scraper ring sweeps, is rid of ports.
As in the four-stroke engines, the tiny quantity of lubricant that passes "above" the oil scraper ring lubricates the compression ring(s).
There are two "asymmetric" transfer ports disposed in series with respective piston ports (which are controlled by the connecting rod); there are also three conventional transfer ports (they open after the exhaust port).
The arrangement with the oil scraper ring fits, among others, with compression ignition (Diesel or gasoline fuel) engines: efficient lubrication of the crankcase and of the piston skirt (wherein the thrust loads are taken), asymmetric transfer, turbo charging, scuffing resistance etc.
The following even-firing Cross-Radial:
is as vibration free as the best V8, it has firing intervals equal to those of a V8 four-stroke, it has four-stroke lubrication (plain bearings, forced / splashed lubrication in the crankcase, oil scraper rings), it can utilize a central scavenging pump (a turbocharger, for instance), etc.
Click
here
or
here
for more animations of the Cross Radial PatAT:
In the above arrangement and animation they are shown the moving parts of a Cross-Radial PatAT from various viewpoints.
The connecting rods and the pistons are properly machined to provide wide bearing surface wherein the heavy loads are taken, keeping at the same time the piston bore low and the crankpin short. A single plane bearing (the yello part around the crankpin) serves all the connecting rods "unconventionally" (it rotates inside their big ends, being secured to the crankpin).
Click
here
or
here
for animations of a Cross Radial PatAT having immovable combustion bowls:
With the combustion bowl immovable on the cylinder head, all the fuel (regardless of revs, load and injection timing) is injected into the bowl (and not in the squeeze area), enabling a faster and greener combustion of the gasoline fuel (click here for an explanatory youtube video), keeping the high fuel efficiency of the Diesels with lower nitrogen oxides (NOx) and particulate matter (PM) emissions.
In a variation of the above design, some special connecting rods and crankshaft are used: an "inner ring" is formed on each crankshaft web, while the big end of each connecting rod instead of being a complete ring it is formed as a "foot" covering a small portion of the periphery of the crankpin; the "foot" abuts onto the crankpin and cannot "lift" from there due to the "inner rings" action, like:
(master rod / articulated slave rods, click here or here or here to download
some controllable windows exe and GIF programs / animations)
the Cross-Radial with the four cylinders and with the forked connecting rods is a true "vibration free" engine (better balanced than the "master-slave-rods" Radial regardless of the number of cylinders of the later), it is also a true "symmetrical" engine: all the four cylinders run under the same conditions: same piston stroke, same piston motion profile, same connecting rod leaning (thrust loads), etc.
As the Radial.exe demonstrates, a typical Radial (master rod / slave rods) cannot help running with substantially different piston strokes in different cylinders.
Depending on the use, a second compression ring may be necessary.
For more power, more rows of cylinders can be added:
As a turbocharged compression ignition (running on Diesel or, preferably, on gasoline fuel for reduced emissions) it has the qualifications for extreme power to weight ratio and, at the same time, for top fuel efficiency (airplanes, helicopters, paragliding etc).
For smaller capacities, an Opposed-Piston Turbocharged PatAT compression-ignition driving two counter-rotating intermeshing propellers (not shown) has some advantages:
As the piston moves "downwards", it initially opens the transfer ports; but with the connecting rod blocking the piston ports, the combustion chamber cannot communicate with the crankcase.
The piston continues its downwards motion and opens the exhaust port; the pressure inside the combustion chamber drops quickly; the crankcase continues to remain sealed from the combustion chamber.
The piston passes from the BDC.
Later the upwardly moving piston closes the exhaust port.
The transfer process starts with the exhaust port already closed, and continues until the transfer ports to be closed by the piston.
The fresh charge (air-fuel mixture) has not the chance to escape from the exhaust before the combustion because there is no overlap between the exhaust and the transfer processes.
The combustion is expected stable even at partial loads.
The exhaust emissions are expected close to those of the four-strokes.
The "radical ignition" (HCCI) may get into play again.
The above arrangement demonstrates some options / choices / oportunities the PatAT asymmetric timing brings to the two-stroke.
In the following drawing the combination of the above arrangement with additional controllable transfer ports provides different modes of operation: green/economy mode at left, power mode at right.
With the yellow throttle blocking the "conventional" transfer port, the engine runs without overlap between the transfer and the exhaust.
With the yellow throttle down, the engine runs with all transfer ports active.
PatATi : Asymmetric Transfer and Intake
Rid of reed valves and of rotary valves, the engine can have asymmetric intake, too:
The piston, in cooperation with the connecting rod and the cylinder, controls the intake timing.
The intake port communicates with the crankcase through a piston port controlled by the connecting rod:
In the two-stroke, below, the bore equals to the stroke.
"H-beam" connecting rod.
The "connecting rod to stroke" ratio is 2.
Here is a "full balanced" Boxer PatATi:
Here is a "perfectly balanced" Opposed Piston PatATi (for Portable Flyers, etc):
Here is a single-cylinder / single-piston PatATi shown from various viewpoints (spot on the "intake piping"):
Here is a PatATi model engine (or RC engine) wherein the crankshaft is made of two pieces to enable assembly, with the casing (comprising the cylinder head, the cylinder, the cooling fins, the crankcase and the complete main crankshaft bearing) being one piece (only a simple cover - the green sliced part at left - needs to be added to seal the crankcase) :
