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      05-07-2013, 06:32 AM   #51
CanAutM3
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Quote:
Originally Posted by mkoesel View Post
This is why I said "sum of all forces" in an earlier post. But the pressure is still greatest at TDC and combustion typically occurs then (or near it at least). The force diminishes over time.
Agreed on the fact that cylinder pressure is greatest around TDC. However because of the crank/rod/piston geometry, no torque can be produced at TDC. The cylinder pressure conversion to torque is maximized at the piston mid course where the "torque arm" is greatest. Peak instantaneous torque for a given cycle therefore happens somewhere past TDC.

T = P * A * S/2 * sin Teta

T = Torque
P = Cylinder pressure
A = Piston Area
S = Stroke
Teta = Crank position angle where TDC=0


Quote:
Originally Posted by mkoesel View Post
The sledgehammer was an exaggeration to highlight the nature of the comparison, I revised it right after to tone it down. The point is that the force is applied all at once vs. over some interval of time.

It is a summation of the torque made in each cylinder. If all cylinders fired at the same time, two engines with the same stroke and differing cylinder counts (and every thing else normalized) would clearly yield the same torque curve. This would also be the case if the force acting on the piston decreased at the same rate in both cases.

We are deep into all of the complexities that led to me revising my original response. To be more precise at this point would mean applying the math to properly model the system. Il'l have to take a look at the graphs that Boss330 posted to see if they cover what I am saying.
I think we agree on the majority of concepts. However I think we are not aligned on the definition of "peak torque".

My understanding of the torque output of an engine is the mean output of all the cylinder pulses for a given RPM, as would be measured on an engine dyno. Peak torque is the highest mean output over the entire rev band. IMO this definition is consistent with manufacturer published data and all published dyno charts. With this definition, cylinder count for given displacement, volumetric efficiency and stroke values should not have an impact (except for friction losses).

However, looking at instantaneous torque, I agree, a bigger piston will yield a greater momentary peak torque. As I mentioned previously, I don't think this number properly reflects the capabilities of an engine and it is not a number that is comonly used to descibe an engine characteristics.

Looking forward to read your interpretation of Boss330's graphs .

Last edited by CanAutM3; 05-07-2013 at 10:46 AM.