Calculating port positions

[paul]

This is probably waaaay too basic for anyone here but it might be useful to complete tyros (like me). I added this article to my web-site, if anyone would care to look it over and let me know if I've made any boo-boos...

www.artefact-rescue.co.uk/articles.php?lng=en&pg=140

Ta

[paul]

Update: added a free program to do the calculation for you (link at bottom of that page)! No error checking as yet so only enter numbers!

[havoc]

I don't know what is correct, but I calculated it more or less the same way as you did, but took T to be perpendicular to the line crankpin-pivot (instead of being perpendicular to crankshaft-pivot). But I guess the difference will be so small that in practise it won't make a difference.

[paul]

Updated again: Error checking is included in the program

[baggo]

Hi Paul,

Strictly speaking Havoc's way of calculating the angle is the more accurate. The maximum angle does not occur when the crankpin is at 90 degrees and 270 but slightly before and slightly after (unless the con rod is infinitely long!). All you've got to do is to use sin to calculate the angle rather than tan. The difference is very small i.e the actual angle in your case is 10.06 degrees rather than 9.9 making the port distance 2.88mm rather than 2.83. Hardly enough to make much of a difference ! (but you did ask )

John

[paul]

Ah yes John I see that now - the maximum would occur fractionally below the position of the crank-pin in my diagram (when the piston rod was tangential to the circle described by the pin). I'll modify the diagram/text/program tomorrow when the speciality beer has worn off!!

Cheers Havoc too!

[paul]

Modified version of the article and software is now in place