closing the inlet before BDC (say 4 o clock instead of 6 o clock) :Farnborough wrote: ↑20 Dec 2025, 09:58The current (up to this year) has been closing inlet valve early, substantially before bdc to my understanding, to enhance a dynamic compression ratio. That over and above the stated geometrical calculation.
A std and accepted measurement normally would involve the volume swept by piston from the bottom of its stroke up until it reached tdc, with calculation of remaining volume now in combustion chamber against that swept volume. That’s conventional static/geometry calculation to give a design specification.
By closing the inlet valve early, and before the piston has reached the bottom of it's stroke, while pressurising the cylinder from turbo input to above atmospheric during that phase, this can include that cylinder volume before the piston reaches the bottom of its stroke on intake to give a dynamic compression ratio above the static specification.
That's my understanding of what they have done up until now. Its no secret, in other words.
Pat Symonds, I believe, gave a good lecture on this topic with lucid discussion points about what they are doing.
- isn't this expanding the charge before BDC, so how can the numerator in the CR calculation 'dynamically' have the 4 o clock to 6 o clock volume 'added' to the standard stroke (6 o clock to 12 o clock) ? Shouldnt it be subtracted instead, because not only is the gas expanding from 4 to 6, it also means less gas is getting ingested by closing inlets early. I will read up more on this, but at the outset, it doesn't sound intuitive.
Nevertheless, going back to static/standard definition of CR,
CR(cold) = ((1600/6) + x ))/x = 16 ; => x=17.778 cc
CR(hot) = ((1600/6) + y ))/y = 18 ; => y=15.686 cc
Irrespective of the shape of the 'head' of the compression chamber, this signifies a thin slice (vol x-y = 2.092cc) getting removed from somewhere near one of the ends of a cucumber. With a bore dia of 80mm (defined in the spec) means this cucumber slice is 0.416mm thick.
Can't they design a piston head with exotic materials that expands by 0.416mm when operating at optimal engine temp v/s when it's cold ? My guess is that such a thing already exists (for all its worth, the CR for the 2014 to 2025 era might be exceeding 18)
