aniretaKrace

Ok so me and my co-worker got in a debate this morning on engine theory....

apparently this guy sitting next to my friend was saying that in theory, no matter how built a motor is, the piston speed should never exceed 25 m/s.

I took the parameters from the GSXR motor we used at ODU caz there the ones i know off the top of my head and did some math

with the car tuned for 8k rpm we were at 22.6 m/s So, if that theory is true I guess we must have done something right.

8000 rpm = 133.33 rev/s
133.33 rev/s divided by .0425 m = 5.6667
5.6667 * 4 cylinders = 22.67 m

BUT with larger engines this does not equate the same and to multiply by 8 cylinders for an F1 car or something it comes out way higher than 25 about 45m/s

I’m wondering if this is just per cylinder or if this even exists at all. anybody else heard of this?
Just curious

RandomTask

First, seeing how your on a crankshaft, the speed of the piston is not linear. It correlates sinusoidal to the stroke. So while yes, the ‘average’ speed at 8k RPM of that motor would hit 22.6 rpm (I didn’t check your math - I just did and its wrong), that’s not the “max” speed. This website gives the math behind it. It would in actuality be higher than that. I have a feeling your friend confused ‘max’ with the ‘mean’ piston speed. Regardless, a motorcycle engine an FSAE care utilizes definitely revs well passed the 8k mark which would blow the 25m/s max speed out of the park but I’ll continue to prove a point.

I was slightly curious to why he would say 25m/s is the limit. I’m assuming he’s talking about it referencing a reliability standpoint. Problem with that is materials and processes change. You couldn’t take a mid ‘80’s gm 2.5L “Iron Duke” above 6k w/o a rod finding its way out yet now they have Honda’s that love hanging around the 9k mark. This is due to A.) Better design, B.) Better material, and C.) better control over processes. A.) Better design from better understanding of materials and FEA. B.) Better materials – Higher strength materials become more readily available and cheaper, as such company’s are more willing to use them. C.) better control – tighter tolerances can be better achieved.

Another major factor is piston weight. Remember, this thing is accelerating and decelerating from 0-100-0 in fractions of a second, that’s a LOT of force exerted.

So when your friend says 25m/s, ask them why that’s the limit then call them a fucktard.


Edit: Yeah, why did you multiply by 4? re-doing your math; a '04 GSXR motor has a stroke of 42.5mm = .0425m.
@ 8000 rpm/60 = 133.33 revolutions per second
* 2 (remember a revolution is a complete up and down movement)
=11.333 m/s (22.66 if you pegged it on the rev limiter of 16k rpm)

A modern F1 engine as a stroke of 41.5mm and usually spin 19k.
=19000/60 = 316.66
*2= 633.33
633.33*.0415=26.28m/s mean speed.

Lastly, the lS7 has a 101.6mm stroke and revs to 7k.
.1016*(7000/60)*2 = 23.7m/s And thats for a large displacement production motor.

Again, max speed is NOT the same as mean.

kashimiru

what does the # of cylinders have to do with anything?

edit: a 2009 gsxr1000 w/ a stroke of 57.3 mm and a rev limiter set at 13,500 has a piston speed of 25.7850 m/s. just google search piston speed and plenty of calculators and pages about it come up

shawn22

Why is a GSXR motor only spinning at 8000rpm?

Piston speed, as far as I've ever heard of it, is measured in a single individual cylinder. The speed is going to be the same for all cylinders, you cannot combine the 4 piston speeds as they will not make the pistons be moving any faster.

Silverbullet86

# of cylinders have nothing to with piston speed.

+1 on material.

the piston speed limit would be dependent on how much shear the end of the rod, piston wrist, and piston wrist holes can take without any deformation. Infact it should be designed way under the material's yield point.

wtf is this. the units dont make any sense, neither does whats going on. 22.6 m is not 22.6 m/s.

U = 2Ns <-- Taj O. Mohieldin
pistons speed = 2 * RPM * stroke length

Fabrik8

NASCAR piston speeds are north of 29 m/s (mean, not max).

I'm not sure where that 25 m/s figure came from, but I'm guessing it's based on something like engine reliability for a non-race engine or something like that..

If you have strong enough con rods and pistons, can keep the cylinders filling properly, and can control friction well enough to make the piston speed worthwhile, very high piston speeds are possible. You don't really want broken parts, don't want the airflow into the cylinder to go supersonic, and obviously don't want friction to subtract more power than you gain with such high piston speeds.
Acceleration is most of the problem though. F1 pistons have an acceleration of something like 10,000Gs, which puts a bit of a strain on..well, everything.
The whole F = m*a thing tends to drive a lot of this..