Originally Posted by
joffy
right, i understand chips comment, that its the breathing efficiency, but if you take one cylinder at the bottom of the inlet stroke it will have a volume of .25L, if you then wanted to increase the amount of air (say double to make it easy) then the size of the cylinder hasnt changed, the amount of mass of air has doubled then surely pressure has to be doubled??
im not being awkward i just dont understand lol
If the inlet stroke involved it pausing at the bottom of the stroke for a while yes.
In an engine when its moving quickly it doesnt get a chance to fully fill with air, also it doesnt empty fully on the exhaust side either, and increased pressure in the exhaust manifold increases that effect.
If its not fully empty, it breathes in less.
Then there is the fact that breathing out against a higher exhaust back pressure means the piston has to do more work to push the gas out, so therefore there is less power left at the crank.
You just arent thinking of an engine as a whole enough, you are focusing on one little thing and missing the bigger picture.