Piston Speed

[dnewton3]

Rod length has no bearing on stroke distance; stroke is determined by the throw of the crank rod journal.
One full crank rev covers 2x the stroke (down and up); so the total piston distance traveled per rev is 2x the stroke; 7 inches.
The rest is just UoM conversions.

(3000 rev / min) x (7 in / rev) x (1 min / 60 sec) x (1 ft / 12 in)

avg piston speed is approximately 29.17 feet per second.

 

[tmorris1]

Four or two stroke?

Wouldn't matter

 

[burbguy82]

ok....Ill show my work for credit right or wrong.....

3000 rpm / 2 = 1500 rpm, since the stroke is the same up and down, for simplification/cancellation....so the piston is moving up 1500 times per minute and down 1500.....so we are really talking 1500 strokes per minute........

1500x3.5 inches per/min= 5250 in/min

5250/12 inches= 437.5 ft/min

437.5/60 seconds

7.291 ft per second?

thanks to @Molekule for getting our brains going on something other than thick vs thin!

guess not

 

[Mainia]

So choosing the key word "linear" would dictate a centered crank to bore center, vs an offset crank to bore center in modern day engines, that would give an ever so slight mathematical difference in the up vs down? At least acceleration would be slightly different?

 

[MolaKule]

Dnewton3 has explained it fully.

Rod length has no bearing on stroke distance; stroke is determined by the throw of the crank rod journal.
One full crank rev covers 2x the stroke (down and up); so the total piston distance traveled per rev is 2x the stroke; 7 inches.
The rest is just UoM conversions.

(3000 rev / min) x (7 in / rev) x (1 min / 60 sec) x (1 ft / 12 in)

avg piston speed is approximately 29.17 feet per second.

 

[Leo99]

Those being corny with the "0 ft/s" answer would be correct had @MolaKule asked for avg velocity rather than speed. Avg velocity = 0 ft/s since average linear displacement is 0.

Wasn't trying to be corny. Been a long time since I learned about vectors and speed vs velocity. Saying "linear speed" instead of "speed" made me think there was a possible vector in play. Guess linear speed vs rotational speed wasn't necessary as pistons only travel in a straight line and I wasn't sure why "linear" was a necessary descriptor. So, I thought vector might be reason and there is still reason to argue why the average linear speed/velocity is 0 ft/s.

 

[MolaKule]

Wasn't trying to be corny. Been a long time since I learned about vectors and speed vs velocity. Saying "linear speed" instead of "speed" made me think there was a possible vector in play. Guess linear speed vs rotational speed wasn't necessary as pistons only travel in a straight line and I wasn't sure why "linear" was a necessary descriptor. So, I thought vector might be reason and there is still reason to argue why the average linear speed/velocity is 0 ft/s.

No reason to argue. The piston speed is 0 at engine shutdown or at TDC OR BDC.

Google epi-eng.com for a tutorial on the mechanics of pistons and the terminology used:

http://epi-eng.com/piston_engine_technology/piston_motion_basics.htm

 

[PWMDMD]

That's one way to solve the problem.

Always use units to make sure certain units cancel when making any calculation.

The power of dimensional analysis! I just had this conversation with my freshman in college - it doesn't just give you the final units, it validates that you set the calculation up correctly.

 

[PWMDMD]

Wasn't trying to be corny. Been a long time since I learned about vectors and speed vs velocity. Saying "linear speed" instead of "speed" made me think there was a possible vector in play. Guess linear speed vs rotational speed wasn't necessary as pistons only travel in a straight line and I wasn't sure why "linear" was a necessary descriptor. So, I thought vector might be reason and there is still reason to argue why the average linear speed/velocity is 0 ft/s.

We could slightly change the question to “average velocity magnitude during one stroke” and then the average velocity vector would give the same answer as the average scaler speed - 29.17 ft/s.

 

[CDNozzle]

The power of dimensional analysis! I just had this conversation with my freshman in college - it doesn't just give you the final units, it validates that you set the calculation up correctly.

Love dimensional analysis. Despite how simple it is (or it can be), it blew my mind in high school when I first saw it in chemistry.

It's helped me solve many problems in undergrad before I even start writing down variables.

 

[cci]

No reason to argue. The piston speed is 0 at engine shutdown or at TDC OR BDC.

Google epi-eng.com for a tutorial on the mechanics of pistons and the terminology used:

http://epi-eng.com/piston_engine_technology/piston_motion_basics.htm

Great site -- thank you for posting that link.

Anecdotally, the older Harleys (Panhead through Evo) seem to last longer if kept under 4,000 rpm. The pre-Evo models were 3-31/32" stroke, Evos 4-1/8". Close enough to the 4" stroke modeled in the article to be instructive.

I always wondered what the dynamics were -- building strokers out of these engines, anything up to about 4-1/2" stroke seemed to hold together OK, anything past that the longevity of the engine deteriorated quickly with additional stroke. Going from 4-1/2" to 4-3/4" was a much more significant change than going from 3-31/32" to 4-1/2".

​

Quote from the article:​

​

The Mean Piston Speed at 4000 RPM for the example 4.000 inch stroke engine is:​

​

MPS (ft per minute) = 4000 x 4 / 6 = 2667 feet per minute.​

​

For purposes of rules of thumb, it is generally agreed that for an engine in aircraft service, 3000 fpm is a comfortable maximum MPS and experience has shown that engines having an MPS substantially exceeding that value have experienced reliability issues. Note that R / S has no influence on MPS, although it does affect PEAK piston speed (4390 fpm for the example engine {R / S = 1.525} at 4000 RPM).​

A graph of MPS as a function of RPM for those various strokes would be interesting.

 

[cci]

Looking at the numbers, it seems like something else must explain the difference in longevity.

The mean piston speed and peak piston speed are both only in the neighborhood of 20% greater from stock to 4-3/4", and if I'm understanding the math correctly, it's a linear function, right?

 

[MolaKule]

...and if I'm understanding the math correctly, it's a linear function, right?

See Dnewton's solution in post #21.

Yes, mathematically, mean piston speed is a linear function.

Dnewton3 has explained it fully.

Rod length has no bearing on stroke distance; stroke is determined by the throw of the crank rod journal.
One full crank rev covers 2x the stroke (down and up); so the total piston distance traveled per rev is 2x the stroke; 7 inches.
The rest is just UoM conversions.

(3000 rev / min) x (7 in / rev) x (1 min / 60 sec) x (1 ft / 12 in)

avg piston speed is approximately 29.17 feet per second.

 

[SubieRubyRoo]

Great site -- thank you for posting that link.

Anecdotally, the older Harleys (Panhead through Evo) seem to last longer if kept under 4,000 rpm. The pre-Evo models were 3-31/32" stroke, Evos 4-1/8". Close enough to the 4" stroke modeled in the article to be instructive.

I always wondered what the dynamics were -- building strokers out of these engines, anything up to about 4-1/2" stroke seemed to hold together OK, anything past that the longevity of the engine deteriorated quickly with additional stroke. Going from 4-1/2" to 4-3/4" was a much more significant change than going from 3-31/32" to 4-1/2".

​

Quote from the article:​

​

The Mean Piston Speed at 4000 RPM for the example 4.000 inch stroke engine is:​

​

MPS (ft per minute) = 4000 x 4 / 6 = 2667 feet per minute.​

​

For purposes of rules of thumb, it is generally agreed that for an engine in aircraft service, 3000 fpm is a comfortable maximum MPS and experience has shown that engines having an MPS substantially exceeding that value have experienced reliability issues. Note that R / S has no influence on MPS, although it does affect PEAK piston speed (4390 fpm for the example engine {R / S = 1.525} at 4000 RPM).​

A graph of MPS as a function of RPM for those various strokes would be interesting.

Average piston speed does play a part, but potentially the more important factor when increasing stroke is that the compression height of the piston shrinks, eventually ending with the wrist pin bore in the oil control ring groove (or higher).

That also forces the skirt to be shrunk in length, which further reduces stability of the piston in the bore. Once the piston is rocking (even slightly), the wear pattern on both the cylinder wall and the rings are different- in a bad way. Rings sealing poorly and oil getting past the control ring are the usual hallmarks of going too far on reducing compression height.

Combine the copious oil usage along with accelerated wear patterns, and you understand why there really is such a thing as too many cubic inches (for a given cylinder block)!

 

[cci]

Average piston speed does play a part, but potentially the more important factor when increasing stroke is that the compression height of the piston shrinks, eventually ending with the wrist pin bore in the oil control ring groove (or higher).

That also forces the skirt to be shrunk in length, which further reduces stability of the piston in the bore. Once the piston is rocking (even slightly), the wear pattern on both the cylinder wall and the rings are different- in a bad way. Rings sealing poorly and oil getting past the control ring are the usual hallmarks of going too far on reducing compression height.

Combine the copious oil usage along with accelerated wear patterns, and you understand why there really is such a thing as too many cubic inches (for a given cylinder block)!

​

"That also forces the skirt to be shrunk in length, which further reduces stability of the piston in the bore. Once the piston is rocking (even slightly), the wear pattern on both the cylinder wall and the rings are different- in a bad way."​

Thank you for that -- it corresponds entirely to my observations disassembling worn engines, just didn't understand why.

". . . the compression height of the piston shrinks,"​

Can you say more about compression height of the piston?

 

[MolaKule]

Average piston speed does play a part, but potentially the more important factor when increasing stroke is that the compression height of the piston shrinks, eventually ending with the wrist pin bore in the oil control ring groove (or higher).

That also forces the skirt to be shrunk in length, which further reduces stability of the piston in the bore. ...

The yield strength of an aluminum piston is on the order of 29,000 psi. Where would one find this high a pressure in an ICE to shrink (squash) a piston back to the wrist pin bore?

My Nissan piston has an area of about 10.2 sq. in. A force of 290,000 lbs would have to be applied to exceed the yield strength of the piston. From where would this force emanate?

At combustion temperatures, the piston should actually elongate (expand) about 0.002."

 

[bdc101]

Love dimensional analysis. Despite how simple it is (or it can be), it blew my mind in high school when I first saw it in chemistry.

It's helped me solve many problems in undergrad before I even start writing down variables.

You learned it in high school?

I suppose that tells you what the kids are learning in high school these days (even when I was there 20 years ago)

I didn't learn dimensional analysis until mechanical engineering school -- and it's one of the few things from college that I continue to use on a regular basis!

 

[Japanese]

What is the piston's average linear speed in ft./second?

1. Multiply stroke by RPM:
2. Divide by 6: so 1750 ft per minute?

14.58 ft/s?

Does the piston weight matter too?

 

[MolaKule]

14.58 ft/s?

Yes, mathematically, mean piston speed is a linear function.

Dnewton3 has explained it fully in post #21.

Does the piston weight matter too?
No.

(3000 rev / min) x (7 in / rev) x (1 min / 60 sec) x (1 ft / 12 in)

avg piston speed is approximately 29.17 feet per second.

 

[SubieRubyRoo]

The yield strength of an aluminum piston is on the order of 29,000 psi. Where would one find this high a pressure in an ICE to shrink (squash) a piston back to the wrist pin bore?

My Nissan piston has an area of about 10.2 sq. in. A force of 290,000 lbs would have to be applied to exceed the yield strength of the piston. From where would this force emanate?

At combustion temperatures, the piston should actually elongate (expand) about 0.002."

Poor phraseology on my part; when the connecting rod or stroke is lengthened, and the compression height correspondingly decreased due to the fixed deck height of the engine, the piston skirt is also generally shortened by the piston designer to capture weight savings and shed unnecessary friction.

Better?