The meaning of the flat torque mentioned, is not to cut the peaks of a typical torque curve in order to make it flat, but to raise the entire curve at its maximum.
Any other (non flat) torque curve cannot achieve the same acceleration.
When Toyota tried to replicate / to copy the VTEC of Honda (with their Corolla VVT-i, I think) the hole in the torque curve (at the transition point) was so deep that, according the testers-journalists, the car was dangerous for the typical driver, because for the average driver it takes a lot of crucial time to get out of a deep torque hole.
The lack of torque at specific conditions (i.e. the deep holes in an, otherwise, strong torque curve) was a lethal characteristic of the Bimota V-Due 500 2-stroke, that, after killing several motorcycle drivers, caused the bankruptcy of the famous Italian company.
Here is the port map of the famous Aprilia RS250 of 1999 (two cylinders in V90, oversquare design with 56mm bore and 50.6mm stroke):
The porting is focused on the faster blowdown: the “peak” (?) exhaust duration is 209.3 degrees and the “typical” (?) exhaust duration is 193.4 degrees, while the duration for the side transfer ports is only 129.6 degrees and the duration for the boost port is only 127.3 degrees.
The transfer is not only 64 degrees narrower than the exhaust, but its maximum (at the BDC) is not bigger than the maximum of the exhaust.
The above way of presenting the porting of a 2-stroke does not fit with the asymmetrical porting of the PatATE.
Here is a different presentation of the porting of a, say, modified to PatATE Aprilia RS 250:
The ports of the Aprilia RS250 are shown by black line.
The red triangular is the exhaust of the PatATE RS250, the blue triangular is the transfer of the PatATE RS250.
Going from top to bottom:
The exhaust of the PatATE starts opening substantially later (at, say, 87 degrees before the BDC) than the exhaust of the original RS250 (which opens at 105 / 97 degrees before the BDC).
The (red) exhaust opens at a higher rate than in the RS250.
Soon the exhaust area of the PatATE gets larger than the exhaust area of the RS250 (and as mentioned before, the Aprilia RS250 is focused on a larger exhaust area for the sake of a faster blowdown).
Some 20 degrees later the transfer (blue) of the PatATE starts opening at a very slow rate, giving time to the blowdown to complete. At the angle wherein all the transfer ports of the original RS250 open, the area of the transfer of the PatATE is still quite small, with a substantially smaller rate of increase.
At the BDC the rate of exhaust closing and the rate of transfer opening of the “PatATE” are about the same.
At the BDC the area of the exhaust port is still larger than the area of the original RS250 exhaust port.
As the piston moves towards the next (lower in the diagram, see the beige arrows) TDC, the transfer of the PatATE strengthens and the exhaust weakens / reduces until it closes completely.
For another 20, or so, degrees the transfer of the PatATE remains open to complete the transfer, while the exhaust is closed.
According the plot, the transfer remains open several degrees later than the transfer of the original RS250; also the exhaust of the RS250 remains open several degrees after the closing of the transfer of the PatATE.
On the same diagram, going from left to right is like moving along the periphery of the cylinder, with the zero angle being at the center / middle of the exhaust ports.
The exhaust of the RS250 covers, at maximum, about 120 degrees on the periphery of the cylinder, the transfer of the RS250 covers the rest cylinder but has, necessarily, substantially shorter height.
The exhaust and the transfer of the PatATE extend, each, for some 180 degrees along the periphery of the cylinder, but they are substantially asymmetrical relative to the BDC: the exhaust is significant before the BDC (blowdown, upped half of the plot) while the transfer is significant after the BDC (filling of the cylinder by the compressed gas in the crankcase, lower half of the plot).
The height of the transfer and the height of the exhaust are equal.
It is a long post.
However this is a different and useful way to present the porting (and not only) of a 2-stroke engine.
Even for the conventional 2-strokes, in a diagram like this one can put the relative pressures (or temperatures, or gas velocities etc) without the need to explain whether they refer to the downwards motion of the piston (expansion stroke) or to the upwards motion of the piston (compression stroke).
Thanks
Manolis Pattakos
