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      09-30-2013, 07:38 AM   #127
Boss330
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Our resident offline expert has corrected my post with the following info:

Quote:
In your last post with the wrench illustration, you mention that the calculation to establish by how much the force reduces based on the bore reduction to be very complex. Actually it isn't. It is very well reflected in the formulas I provided.

From your wrench example:

T=F x d

Where the force is defined by the cylinder pressure acting on the piston area (P x (B/2)^2 x pi) and the distance is the vectorial product of the half stroke relative to the crank position (S/2 x sin(Teta)). Hence:

T = P x (B/2)^2 x pi x S/2 x sin(Teta)

What is interesting is that due to the geometric characteristics, any increase in stroke for a given displacement will be negated by the same exact amount by the bore reduction. For a given cylinder pressure, torque is therefore displacement dependent only regardless of the bore and stroke relationship. To demonstrate this, take the definition of displacement and substitute it in the torque formula:

D = (B/2)^2 x pi x S

and Torque becomes:

T= P x D/2 x sin(Teta)

Tada , torque depends on cylinder pressure, displacement and crank angle only. No bore or stroke.
Which makes sense as I had done that calculation myself and realized that I came out with the same torque numbers for both the N55 and S55 config... Something which certainly didn't side with my view on this matter.

However, I suspect "cylinder pressure" might be a keyword here... And that's where the complex calculations are coming into play.

The issue with cylinder pressure, or "Power Density", is illustrated in swamp2's post above. This is where the dog is buried and the "secret" behind why one concept is preferable over the other in different areas (like a F1 engine vs a HD truck).
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Last edited by Boss330; 09-30-2013 at 07:57 AM..
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