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Author:  david rosenthal [ Sat Jan 22, 2011 12:38 am ]
Post subject:  Question
Now this is a subject that has been discussed before but the question is

"how do you calculate piston speed in a engine"

Now I am not being a smart arse but some will be surprised as to the correct calculation for it.
Now I don't know how to do a poll but some answers. Add what you like,but "your a dickhead " has already established as fact

1 don't know and don't give a sh*t as long as they go up and down
2 RPM X stroke.
3 it is a average of RPM x stroke
4 depends on how much the block stops it from going out to the fresh air.
5 measure it
6 how long is a piece of string
Author:  MiniKit [ Sat Jan 22, 2011 2:01 am ]
Post subject: 
I did a bit of research on this a while back and saved this page from a website.. It made my brain hurt, and then it started to make sense so I stopped looking at it. 8)

This should make you choke on your drink.. :wink:

Maths stuff to hurt your eyes
Author:  david rosenthal [ Sat Jan 22, 2011 11:59 am ]
Post subject: 
MiniKit wrote:
I did a bit of research on this a while back and saved this page from a website.. It made my brain hurt, and then it started to make sense so I stopped looking at it. 8)

This should make you choke on your drink.. :wink:

Maths stuff to hurt your eyes


yep, it's interesting and the maths :shock: :shock: So this opens up a complete new field. AIR in, exhaust out :)
Author:  Wombat [ Sat Jan 22, 2011 1:12 pm ]
Post subject: 
When we used to drive from Melbourne ( Torquay actually) to Byron or Sunshine Coast in one non stop run so the kids would sleep most of the way - I used to do mental arithmetic to keep me awake - like how many explosions had gone off in the cylinder in the last kilometre and stuff like that - now speed of the piston - that's one I hadn't thought of. :lol:
Author:  850 GT [ Sat Jan 22, 2011 11:00 pm ]
Post subject: 
Is that piston speed on up stroke or down stroke, plus you have stall time at top off stroke and then the piston velosity after the bang. :lol:
Rod length also changes piston speed :?
Author:  GT mowog [ Sun Jan 23, 2011 3:23 am ]
Post subject: 
Wombat wrote:
When we used to drive from Melbourne ( Torquay actually) to Byron or Sunshine Coast in one non stop run so the kids would sleep most of the way - I used to do mental arithmetic to keep me awake - like how many explosions had gone off in the cylinder in the last kilometre and stuff like that - now speed of the piston - that's one I hadn't thought of. :lol:


I used to do similar. I remember on one trip, working out the crank spun (near enough) 1.5 million times from Newcastle to Adelaide.
Author:  Hunter2 [ Sun Jan 23, 2011 6:01 am ]
Post subject: 
Ouch this hurts. I've just run out the numbers and we expect to drive over 22,000 kilometres this year on our One Lap "drive about". More numbers are not really needed.

However unlike the piston which only momentarily slows to a near stop at the top and bottom of each stroke, we hope to stop the Window Van, get out and see the sights on a regular basis.
Author:  Mick [ Sun Jan 23, 2011 9:14 am ]
Post subject: 
Its a nice piece of mathematics. Not so good for the long trip mind occupiers however, PI is an irrational number you know and I hate radians per second...
Author:  Morris 1100 [ Sun Jan 23, 2011 1:16 pm ]
Post subject:  Re: Question
david rosenthal wrote:
Now this is a subject that has been discussed before but the question is

"how do you calculate piston speed in a engine"

Now I am not being a smart arse but some will be surprised as to the correct calculation for it.
Now I don't know how to do a poll but some answers. Add what you like,but "your a dickhead " has already established as fact

1 don't know and don't give a sh*t as long as they go up and down
2 RPM X stroke.
3 it is a average of RPM x stroke
4 depends on how much the block stops it from going out to the fresh air.
5 measure it
6 how long is a piece of string


I read one book that said piston speed is not the problem, it is the speed of acceleration of the piston that causes the problems.
Author:  slinkey inc [ Sun Jan 23, 2011 7:06 pm ]
Post subject: 
Machine Dynamics, 2nd year Mechanical Engineering.

It is easier to use scale diagrams of velocity triangles called the 'graphical method' or similar depending on the text book. The graphical method can give relatively accurate results for a whole lot less effort.

Or write a program that takes the required inputs in Matlab and that should spit out quality and repeatable results fast and efficiently.
Author:  david rosenthal [ Mon Jan 24, 2011 1:20 am ]
Post subject: 
when I started doing them back in 1970 we used scale drawings and trigonometry . all the calcs were done using a log book and slide rule.
Morris 1100 is correct about the acceleration being harsher because the piston has to deal with a accelerating force that is deminishing rapidly.

Years ago when I was working on a ship with a big slow speed 2 stroke cross head engine, we heard a funny noise from one cylinder and after the chief had checked it out we opened the scavange port door and had a look while it was running. The whole piston and conrod was chattering as it went downwards. The accelerating force had bend the piston rod [a hollow round shaft ] and tore the stuffing box to pieces. Before we were able to shut it down the rod broke and went into the crankcase. the crank picked it up next time around and punched the whole lot out thru the crank case door.

The cause was over stress from too high peak pressure in the cylinder. A leaking air start valve. The whole rod was a mass of cracks and it finally broke on the palm bolt flange.
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