PatATE: Asymmetric Exhaust and Transfer in the two-stroke engines
A common characteristic of the ported two-stroke engines is the presence of "piston controlled" exhaust and transfer ports on the cylinder liner.
The exhaust, in order to give time to the gas pressure into the cylinder to fall before the beginning of the transfer, has to open substantially earlier than the transfer.
Inevitably the exhaust has to close substantially after the transfer.
This, in turn, means that the entered, during the scavenging, air or air-fuel mixture has plenty of time to escape from the cylinder before the closing of the exhaust.
And because the transfer needs a long duration in order the engine to breathe freely at higher revs, the duration of the exhaust becomes, inevitably, too long (for instance, as long as the transfer duration plus another 50 crankshaft degrees).
Another significant issue of the conventional ported two-strokes is the extreme temperature on the exhaust port and on the side of the piston skirt that slides over the exhaust port, and the low temperature (in comparison) at the opposite side of the cylinder / piston; it is at the exhaust side of the cylinder liner where the scuffing starts in most cases.
Fig.1 shows a conventional two-stroke engine of the prior art, having the cylinder, the piston and the crankcase sliced.
Figs. 2 and 3 show a PatATE engine; it is the engine of Fig. 1 slightly modified:
the exhaust port becomes a "hybrid port" (4), hybrid in the meaning that for a portion of the cycle it operates as an exhaust port, while for another portion of the cycle it operates as a transfer port;
it is added a transfer passageway (6) for feeding compressed air or air fuel mixture (typically, but not necessarily, from the crankcase) at the external (relative to the combustion chamber) side of the hybrid port;
it is also added an exhaust passageway (5) for providing a path to the exhaust gas exiting from the hybrid port towards the exhaust,
they are added some auxiliary exhaust ports (8) at the sides of the hybrid port;
there is also added a valve (7), or valve assembly, to control the communication of the hybrid port with the transfer and exhaust passageways.
For the rest numbers shown in the drawings: (1) is the cylinder, (2) is the combustion chamber, (3) is the piston, (9) is a conventional transfer port.
In the Fig. 2 the valve closes the top end of the transfer passageway at left: any gas exiting from the cylinder through the hybrid port goes to the exhaust passageway.
In the Fig. 3 the valve closes the exhaust passageway: any gas exiting from the top end of the transfer passageway goes into the cylinder through the hybrid port.
In the above drawings they are shown the (original) exhaust port, the hybrid port, the transfer passageway, the auxiliary exhaust ports, the intake port (when the piston is at the TDC the intake port opens by the piston allowing fresh mixture to enter and fill the crankcase) and the (original) transfer port.
In the above drawings (40 degrees after the BDC) gas from the cylinder exits through the exhaust port (prior art two-stroke), or through the auxiliary exhaust ports (case of PatATE), while compressed gas from the crankcase enters through the transfer port (prior art two-stroke), or though the transfer port and though the transfer passageway and though the hybrid port (case of the PatATE) and scavenges the cylinder.
The exhaust continues strongly in the prior art two-stroke, while the exhaust is almost finished in the PatATE.
In the above drawings the difference is even bigger: in the prior art engine the exhaust continues (the transfer is closed), while in the PatATE the transfer continuous (the exhaust is closed).
The previous are also shown in the following timing diagrams:
The timing diagram at left is quite simple: the exhaust opening (Ex-O) and closing (Ex-C) are symmetrical, the transfer opening (Tr-O) and closing (Tr-C) are symmetrical, too.
The exhaust duration is necessarily substantially longer than the transfer duration.
And because the transfer needs a significant valve-time-area (otherwise the engine cannot have high specific power), which requires a significant height of the transfer ports and consequently a significant duration of the transfer, the exhaust has to open substantially earlier than the transfer (even in expense of substantial energy loss because the pressure in the cylinder is still high).
In comparison, a PatATE two-stroke needs not to open the exhaust too early (which increases the energy milked by the fuel during the "longer" expansion, which means increased fuel efficiency); and despite the late exhaust opening, the duration of the transfer can still be long (longer not only in comparison to the transfer duration of the conventional, but longer even than the exhaust).
And because the transfer ends later than the exhaust, the volumetric efficiency of the engine can be higher even without a tuned exhaust (a tuned exhaust provides a better volumetric efficiency in a narrower rev and load range), with the quantity of the fuel escaping unburned to the exhaust getting smaller.
The previous mean, among others, lower emissions and better fuel efficiency.
They also mean more time for the mixing of the fuel with the air in case of direct injection two-stroke: with the exhaust closing earlier, the injection starts earlier (without the risk of fuel lost in the exhaust) which gives more time for the preparation of the mixture before the combustion, which gives more complete combustion, cleaner exhaust and higher power output.
Another significant issue is the lubrication and the reliability of the two-stroke engine.
With the hybrid port used as the "main" exhaust port (through which it exits the biggest portion of the burnt gas, which is also the hottest gas) and then as the "main" transfer port, the peak temperatures on the cylinder and on the piston skirt drop significantly, which means more reliable engine, lower possibility for scuffing, lower specific lube consumption (which also means a cleaner engine and a lower-running-cost-engine: because the cost of the lubricant is comparable with the cost of the fuel in several state-of-the-art modern two-stroke engines (sleds, ATVs, aero-engines etc).
Another advantage of the late transfer closing is that the two-stroke engine gets more compatible with turbo-charging and super-charging, which means that the already top specific power of the two-strokes can be even higher.
In the timing diagram of the PatATE, at right:
Ex_H_O is when the hybrid port opens by the piston (High Exhaust Opens) and the exhaust starts,
Ex_L_O is when the auxiliary exhaust ports open (Low Exhaust Opens),
Tr_L_O is when the (conventional / symmetrical) transfer port opens (Low Transfer Opens),
Tr_H_O is when the hybrid port turns to transfer port (High Transfer Opens),
Ex_H_C is when the valve closes the exhaust passageway (High Exhaust Closes),
Tr_L_C is when the (conventional / symmetrical) transfer port closes (Low Transfer Closes),
Ex_L_C is when the auxiliary exhaust ports close (Low Exhaust Closes),
Tr_H_C is when the hybrid port closes (High Transfer Closes).
The Exhaust Duration can be equal or larger or smaller than the
The unique valve shown to control both, the transfer and the exhaust passageways, is not obligatory: a valve can control the exhaust passageway and another valve can control the transfer passageway.
The valve can be actuated in various ways, for instance mechanically (a camlobe on the crankshaft and a linkage is all that is required), hydraulically, electro-magnetically etc.
The following drawings show the case wherein an electromagnet is used to actuate the valve of the PatATE:
At left, the PatATE1 engine is at 10 degrees before the BDC and the valve is sealing the transfer passageway (the hybrid port is used for exhaust), while at right the PatATE engine is at 10 degrees after the BDC and the valve is sealing the exhaust passageway (the hybrid port is used for transfer).
Strong electromagnets are used in several modern two-strokes, for instance for the direct injection of fuel in the Rotax / Bombardier / Evinrude E-TEC two-stroke engines.
Here it is shown a PatATE wherein the "Valve" is a "Rotary Valve":
Here is a timing graph of the above rotary-valve PatATE:
Here the rotary-valve PatATE is shown from a different viewpoint:
Here they are shown the cylinder liner, the rotary valve and a slice from the top end of the casing. The "slide-to-slide" step is 30 crankshaft degrees.
The rotary valve spins in synchronization with the crankshaft.
In the specific case the angular speed of the rotary valve equals with the angular speed of the crankshaft.
Different speeds (for instance: the rotary valve spins at half crankshaft speed) give different characteristrics / options.
The width (along the periphery of the cylinder) of each hybrid port (there are two) is 90 degrees.
The duration of the hybrid ports is 180 crank degrees (the piston starts opening the hybrid ports at 90 degrees before the BDC and closes them at 90 degrees after the BDC).
There are two intake ports.
In the following graph it is shown the Exhaust Ports area and the Transfer Ports area versus the crank angle:
Full Speed Rotary Valve
In the following version of the PatATE there is only one hybrid port on the cylinder liner, and the rotary valve spins at crankshaft speed:
In the above animation the engine is shown complete. The hybrid port is closed by the piston while the intake port is partly open.
In the above animation the width of the hybrid port along the periphery of the cylinder liner is nearly 180 degrees.
The duration of the hybrid port is 180 crankshaft degrees.
The hybrid port is partly opened by the piston, while the intake port is completely closed by the piston.
In the following animation the exhaust is almost done while the transfer is near its maximum; the intake port is closed by both, the rotary valve and the piston.
The port at the lower side of the (red) rotary valve, in cooperation with the (blue) piston, controls the intake port of the engine.
In the following graph it is shown the Exhaust Port area and the Transfer Port area versus the crank angle:
The following animation shows the position / motion of the rotary valve of the PatATE just before the opening of the hybrid port by the piston.
During the compression and the expansion the rotary valve keeps rotating.
At the beginning of the exhaust the outer side of the hybrid port is fully "uncovered" by the rotary valve, so that the complete hybrid port is dedicated to the exhaust.
The rate of the exhaust opening is about double as compared to the rate of the exhaust opening in a similar conventional 2-stroke.
According the Yamaha RD350LC port-map (click here to download), the exhaust port extends on the periphery of the cylinder liner for some 80 degrees while the hybrid port of a similar PatATE extends along the periphery of the cylinder liner for nearly 180 degrees.
In case of divided load (Portable Flyers with two intermeshed counter-rotating propellers, Electric Power sets with two counter-rotating generators, Marine Outboard engines driving two counter-rotating screws etc) the synchronization gearing (not shown) runs unloaded and the basis of the engine is perfectly rid of vibrations.
The compact combustion chamber is shared between the two opposed pistons (the instant pressure on the two piston crowns is the same).
The scavenging is not of the through or uniflow type.