Calling all smart types!

[paul_s13]

Hi all,

I've got a beauty of a question here, does anyone know if there is a calculation to find out the force on a conrod by the piston at a set rpm, basically I'm wanting to figure out rod stresses and the differences between the standard piston and a light weight piston.

Cheers!

[RMiller]

Good question. Do you know much about physics? I forgot most of the formulas, but the difference between 2 pistons will be due to the momentum P. Check through some formulas, and don't forget to incorporate Fg (force of gravity) although it won't change much. Calculate piston speed(at whatever rpm), then momentum (since you know the mass of each piston) and use the formula w/ force and momentum in it. Ha, I feel like I'm not helping. Good luck.

Now that I think about it, I think it'd be a pain to do this, but you still could. You would have to find the angles that the conrod is at to find the force at that particular angle. Ask a physics professor to help you.

[Notchbackca]

short answer: the force changes direction and magnitude constantly

[RMiller]

Heh, yeah.

[paul_s13]

hmm i thought as much, ok another Q how much does the standard piston weigh?

Paul

[Def]

Here's a link to one of the first lectures of the Internal Combustion Engines class I'm in:

http://www.me.gatech.edu/energ...s.pdf

The greatest stress on a conrod is at TDC after the exhaust stroke. Fastest piston speed is usually about 80-85* ATDC(not 90* as most people believe).

BTW - crankshaft offset = stroke/2

Most aftermarket pistons I've seen weighed are on the order of about +/- 50 grams from stock. So you really don't lose much there and might even gain some, although the rods can sometimes be a fair bit lighter(~50-100 grams). More material = more strength, so aftermarket pistons usually put a priority on beefing them up rather than making them light. Although I imagine some N/A applications could be as much as 100-150 grams lighter than stock, but would probably be just as strong/weak.

[fraz]

Rod length also has a big effect on piston.The longer the rod, the less acceleration on the piston at the top and bottom of the stroke.

[RMiller]

Hmmm. I see why the greatest stress in the vertical direction would be at TDC after the exhaust stroke, but the tensional strength of a conrod is really high. What I'm trying to say is the rod won't get pulled apart at TDC. However, during the combustion stroke the force is down but the conrod is at an angle. I believe this represents a greater stress on the conrod.

[Def]

Do an analysis and you'll see that's where the greatest stress is - TDC after the exhaust stroke with a vacuum on the piston face. Besides, most metals have a higher compressive strength than tension strength.

Rod length does slightly affect piston acceleration, but it is a REALLY MINOR effect compared to stroke length. It's to the point where you'd surprass the acceleration at all points with a 1mm increase in stroke to something like a 5-10mm increase in rod length.

[Notchbackca]

rod length effects the acceleration curve just as much as stroke, tho it is stroke that determines the maximum acceleration at a given rpm

[fraz]

Thats why im building a ca18det with 21mm longer rods

[paul_s13]

Its all good stuff!

I may have found pistons that weigh 250g and I've just taken out my engine and I want it to be a quck revving monster, also aiming to be the loudest (but nice sounding!) i've owned.

I've got till the next drift season to get it all done by!

[RMiller]

Def, I never would have thought the compressive stregth is greater than tensional strength of most metals...that seems so weird to me. Anyway, thanks for the info. I got a little mixed up about the angles, which didn't make any sense when I said it. Good stuff.