Ray,
Since I brought this topic up, maybe I should try and answer you. As Brock
Tella says, I should probably draw a picture, but let's see if I can make do
with a few choice words.
The effect of rod length has to do with the relative effect of the
side-to-side motion of the crank rod throw (or offset as you call it). If
the rod were infinitely long relative to the radius of rotation of the rod
journal on the crank, then the piston would exhibit simple sinusoidal
motion. With decreasing rod length, the side-to-side motion becomes more
important and tends to pull the piston away from top dead center faster and
faster while also keeping the piston near bottom dead center longer. For
typical rod length to stroke ratios around 1.7:1, the piston spends quite a
bit less time around top dead center than around bottom dead center.
The assumption in this discussion is that cylinder volume is kept constant
for different rod lengths by changing the pin position in the piston.
Hope this helps.
Bob
>
>Gentlemen,
>
>I am a layman and not an engineer, but I have been reading this thread with
>some interest. Now though, I feel compelled to ask a question (albeit
>naive it may be). So please educate a few of us.
>
>You all keep talking about rod length dictating compression, torque and
>piston speed, but if the crank offset (correct term?) does not change, then
>why would piston speed and travel change? All I can see is that longer
>connecting rods would reduce cylinder volume, but piston travel and speed
>would remain constant.
>
>Is that not correct? If not, why?
>
>Regards
>
>Ray
>
>
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