View Single Post
Old 03-14-2013, 04:45 PM   #20
Rick Vessell
Tech Elite
 
Rick Vessell's Avatar
 
Join Date: Jan 2002
Location: Mars
Posts: 2,302
Trader Rating: 9 (100%+)
Default

Quote:
Originally Posted by Roelof View Post
I have seen a video about Cosworth engines where the engineers did calculate the zero headclearance at max rpm and indeed you could see a complete clear squish band on the piston. Flex is there and it is needed to stay alive. I just calculated it, at 44.000rpm the (lineair) acceleration of the piston in an half stroke a 3 gram piston can create about 30kg of force. With the conrod making 5 to 6 grams in total it can go up to 50kg, that is huge! It is for sure pushing and pulling hard on the crankshaft,



You need to upload them....
Quote:
Originally Posted by MantisWorx View Post
I understand what you are saying but that movement is not soley from the crank but more from the other moving parts. The crank is only one part of the equation and since the crank is the hardest and most unlikley part to actually "flex" the movement you see is from the soft piston deflecting,rod deflecting, dual bushings and wrist pin all of which will move/flex more than a crank. Why would the actual crank flex before any of the other moving soft parts? I want to bring up the fact that movement and flex are two completely different things. A resonance can and will cause flex where as movement is just simply from tolerances on moving parts. I am a little confused on which one you are attempting to explain!!

Flex is present and necessary. Every crank I've inspected that broke the crank pin was due to the hardening being done incorrectly. The crank pin was too stiff and did not flex enough when loaded by the piston assembly. If the piston assembly was a 'soft' as you're implying in your arguement, the bushings and piston would beat themselves to pieces in short order once the engine accelerated above 10000 rpm.

The rod bushings are made of the material they are for long service life and lack of friction on the steel crank and wrist pin. Yes they are 'softer' than the parts they ride on but hardly 'soft'. They are intended to be the wear items in the assembly to prevent failure of the more expensive and harder to produce crankshaft and wrist pin.

Per my calculations, an engine running at 44000 rpm (Roelof's example) with a 18mm stroke (typical average for a .21), gives 36mm of piston vertical travel per revolution. That works to 1,584,000mm per minute. This is equal to 62,362 in. per minute approx. That gives you a piston speed of 5196.850 ft/sec or 3543.307 mph or 4.654 mach. That's just an approximation covering the entire travel. Don't forget the piston comes to a stop twice per revolution and then accelerates the opposite direction to peak speed about mid stroke before starting to decelerate at the opposite end. Add to that cylinder pressure on the compression side of the stroke and the spike at combustion. You can be fairly certain that EVERY part in the reciprocating assembly is flexing/bending/moving, whatever you want to call it most if not all of the time. That sound you hear when the car goes by is the exhaust note, sure. It's also the sound of the piston breaking the sound barrier many times a second. Find a video from Lostallo, Switzerland when they run 1/8 open. The amount of time spent at WOT per lap is around 13-14 secs. The front straight alone account for over half that time. Listen to the cars as they pass the camera. Amazing does not describe it.

Yes, rods do stretch, bushings deflect, wristpin bend, bearings have runout, but the crank also flexes both in whole and at the crankpin when running. I've been studying these engines for a long time and I'm still amazed they actually run and don't destroy themselves instantly, especially they way they are treated by the majority of the users that have them.
Rick Vessell is offline   Reply With Quote