One thing I still haven't seen explained, and I have searched, is what happens if blowdown is adjusted to extremes... ie, 10 degrees of blowdown vs 40?
btw, old cat, that's a nice link...very interesting
Degrees alone does not tell the entire story either. One must also consider the port opening size (square mm is common) and shape. Time area and angle area are two "measurements" that take these into account.
For those unfamiliar with angle area:
"i understand
time area . but ive never found a good answer for what angle area meens. is there a simple explanation for the definition of angle area ? i thought it had something to do with crank angle but im not sure."
"Frits Overmars
OK, here goes.
In the beginning there is a cylinder. With just one rectangular port, to keep things simple. That port has three linear dimensions: the distance from cylinder top plane to the top edge of the port, the distance from top plane to bottom edge of port, and the port width. That's all.
Now we put that cylinder on a crankcase containing a crankshaft with a certain stroke, a con rod with a certain length, and a piston.
Rotate the crank and the piston will descend until it starts to open the port. Keep rotating the crank, and after a certain number of degrees the piston will close the port again. Now we can say that in addition to its linear dimensions the port has also a timing.
Next: angle.area.
Let's assume that the port starts to open at 90° before Bottom Dead Center. We rotate the crankshaft 1° further and then the port will be open a little bit, say x mm. The open area is now x mm high times the port width. And this piece of open area will stay open all the way from 89° before BDC right until 89° after BDC on the next upstroke. That little piece of open port area has an angle.area of 178° * x * width.
We rotate the crankshaft 1° further still, and the port will open a little bit more, say another y mm. This additional open area will stay open from 88° before BTC till 88° after BDC. So this second little piece of open port area has an angle.area of 176° * y * width. And so on.
When we add all these pieces of open port area times the number of degrees that they are open, we find the total angle.area of the port.
Finally: time.area.
Now we look at a
running engine. The angle.area of a port is a fixed value, but the time.area is not. How much time it takes to rotate the crankshaft 1 degree, depends on the rpm, as Graham explained above. Double the rpm, and that time will be halved, and so will be the time.area.
Emptying and charging the cylinder requires a certain time.area. If the rpm rises too high, the engine will run out of breath because the available time for emptying and charging the cylinder will become too short."