The short answer is yes. But why is it that each time you people sit down for five minutes to write a question, I have to sit down for two hours to write an answer?
When the exhaust port opens, a pressure pulse starts moving through the exhaust pipe. It is reflected at the end cone and it should be back at the cylinder just before the exhaust port closes.
Next a part of this reflected pulse bounces off the partly-closed exhaust port and a residual pulse starts moving down the exhaust pipe. This residual pulse too is reflected by the end cone and starts moving back to the cylinder. Ideally it will arrive at the exhaust port just when the port opens again. Then the cylinder pressure and the pressure of the residual pulse combine their energy and the resulting pulse will be stronger than the pulse from the previous cycle. And the combined pulse from the next cycle will be stronger still, and so on; we have achieved true resonance.
Some may argue that we want a low pressure in the exhaust pipe when the port opens because then the spent gases will experience less resistance while leaving the cylinder. But that is not true. Gas flow depends on a pressure difference ratio. But once that ratio reaches 2, the flow velocity will reach Mach 1, the speed of sound. Raising the pressure difference any further will not raise the flow velocity any further.
The cylinder pressure at exhaust opening can be as high as 7 bar and the pressure of the reflected pulse will be about 2 bar. Thus the pressure ratio is well above 2, so lowering the pressure in the exhaust duct outside the cylinder will not do any good to the flow.
What has the exhaust timing got to do with the 'true resonance' I mentioned above?
The initial pulse starts moving at Exhaust Opening and it has to be back at Exhaust Closing, or a little earlier. This pulse travels with the speed of sound and its journey up and down the exhaust pipe will take t seconds.
The residual pulse starts moving at Exhaust Closing and it has to be back at the next Exhaust Opening. This pulse also travels with the speed of sound and its journey up and down the exhaust pipe will also take t seconds.
So from EO to EC takes t seconds and from EC to EO also takes t seconds. In English: the exhaust port should be open just as long as it should be closed.
Assuming that the crankshaft rotates with a uniform speed, this means that the crank angle during which the exhaust is open must be equal to the crank angle during which the port is closed. So both angles must be 180°.
I developed this line of thought some 40 years ago, but when I first published it in 1978 (in the motorcycle magazine Moto73 of which I was the technical editor) everybody called me crazy. Some people still do, but I got used to it
.
Above I made a couple of assumptions. The crankshaft does not rotate with a uniform speed, but at high revs the deviation is negligible. In case you really want to know, I did the math for the Aprilia RSA125. At a nominal rpm of 13,000 the minimum rotation speed is 12970 rpm @ 107° after TDC and the maximum value is 13031 rpm @ 356° aTDC. What's more significant: the deviation in crankshaft position from truly uniform rotation is always less than 1°. So that really is negligible.
Second assumption: both the initial pulse and the residual pulse move with the speed of sound. Not true: the pulse pressures in exhaust waves are so high that acoustics rules do not apply any more. We are dealing with gas dynamics here and the stronger a pulse, the faster it moves. Since the residual pulse is weaker than the initial pulse, they move at different speeds. But we will leave this aside for now.
Third assumption: the initial pulse starts moving as soon as the exhaust port starts opening. More or less true, but we are not interested in the first weak appearance of the pulse; we want to know when the pulse reaches its maximum amplitude. And that requires a certain amount of open exhaust port area. It turns out that for our desired theoretical exhaust timing of 180° we will need a geometrical exhaust timing of about 190°, depending on the shape of the port: does it open gradually or does it open over its full width all at once.
The obvious question will be: why has the Aprilia RSA125 a geometrical exhaust timing of about 200°? True, at 190° the maximum torque value would be higher, but the engine would not want to rev because the blowdown time.area would be too small.
The 200° are a compromise: a bit less torque and a bit more revs; as long as the torque decline is smaller than the rpm rise, we gain horsepower.
Originally Posted by Frits Overmars
Reply With Quote
)
Bookmarks