well like the title said, i need to know how much force,, or psi, or whatever its measured in,to push a piston in normal running conditions on a 1.5. i didn't really know what to search for and figured this would yield the quickest results. i don't need a quick answer just someone to point me in the right direction. i know engines and this is for research. thanks in advance.

 

huh??

 

wouldn't your compression ratio have something to do with that...

 

That is going to totaly depend on your ring tension. LOL

 

are you talking about compression test numbers?

 

i guess that's what it would be eh? i didn't think that's what compression numbers were. but i could be wrong.

 

Eh?
Are you actually trying to see what it TAKES to push it?
If you tear down a motor, it doesn't take a whole lot to push it down by hand compared to what it really sees when it has fuel blowing up in the combustion chamber.
While running, it sees more force at WOT since you are introducing more fuel and air for a bigger explosion.
I'm sure somebody has to have figured it out before. Good Luck!

 

Back of the envelope:
The mean effective pressure (= average pressure = Pmep) can be estimated from
Pmep = (P * n) / (Vd * N)

where
P = power = assume 20 hp, cruising = 20*550 ft-lbs/sec * 12 in/ft = 132000 in-lbs/sec
n = number of revolutions per power cycle = 2 revs/cycle for a four stroke engine
Vd = displacement volume = 1493 cm^3/cycle for a D15B7 = 91.1 in^3/cycle
N = RPM converted to revs/second = 2500/60 when cruising = 41.7 revs/sec

so

Pmep = 132000 * 2 / (91.1 * 41.7) =~ 70 psi

Force = Pmep * Area of cylinder

Cyl diameter = bore = 2.95 in for a D15B7, so

Average Force = 70 * 2.95^2*Pi/4 =~ 478 lbs

Wiki has perfectly good introductory material on this. See http://en.wikipedia.org/wiki/Mean_effective_pressure

 

At what psi would be considered a dead cylinder?

 

0

 

awesome thanks honda lioness. that's what i was looking for.

 

nice math, though i jump on HT during school to escape school. and now it's like i'm back in engineering class..

 

AFAIC the best mechanical engineers--the ones who are the best designers and so have imagination and are the real brains--are the ones who laid hands on their cars prior to engineering college. They understand forces and torques intuitively and grasp what is discussed in many mech engineering classrooms much more quickly. Though of course it is not intuitive at all but a result of much trial and error and experience working wrenches etc. as a youngster. They have a much better foundation than those who have never worked extensively with anything mechanical and so can advance through their studies to real engineering and career success much more quickly.

The calculation of Pmep above is very crude (yet I think instructive on some basic points) and to me really begs questions like (1) what a graph of actual pressure vs. point in the cycle is for any one cylinder; (2) roughly what are the friction forces involved, because the Pmep above is only a crude estimate derived from a mish-mash of "indicated power" (= Pressure * Vd/sec) and "brake power" (what a dynamometer measures); (3) so a compression test measures some sort of peak pressure but does a cylinder see this pressure at, say, cruising speeds around 30-70 mph? Or only during acceleration? Or are the peak pressures a compresson test measures not all that useful as a measure of what the cylinders actually see during operation but just denote a gage of, say, general piston ring health?

No doubt some of the tuning threads around here at Honda-Tech discuss these matters and have people who have actually taken such measurements or read articles on same, if not at home then in an automotive course.

 

I think this is one of those questions that is phrased so badly it really has no answer... pressure required to push the piston down when the piston is at TDC is pretty frickin' high because you'd be trying to push the crank out of the block, but pressure required to push the piston down when the piston is at the centerpoint between TDC and BDC is pretty frickin' low because you have maximum leverage... to say nothing of how much pressure is "required" to keep an engine spinning at a constant rate when it already has inertia... what are you trying to determine, anyway?

 

Funny how right you are! The question can be interpretted different ways.
I thought they maybe wanted to know what force of the explosion inside the combustion chamber dependant on the fuel mixture.
Lioness seems to have explained the force to required to push the piston down from TDC.
You think it asks what force is required to move the piston/keep the piston moving at any given point of operation.

 

I trust you and others realize all I computed was a ballpark number. For one thing, it is an average of the force as the piston goes through its power stroke. For another, the use of 20 hp is a crude guess at the indicated power. I figure indicated power is somewhere between 15 hp and 40 hp for the popular older 1.5L Civic engine I referenced, moving steadily at a speed between 30 and 60 mph say.

I think what manufacturers are supposed to quote in their literature is the hp at the crankshaft, which should be some number less than indicated power and some number bigger than the hp transmitted to the halfshafts (since the tranny causes friction losses).

Engineering designers yada study these forces so they can optimize features like piston rod dimensions, cylinder wall thickness, oil requirements etc. The answer to the OP's question is not going to be that black-and-white and has to be qualified. Engineering is not an exact science yada. Blah blah; this is a poor medium for in-depth discussion of topics like this. I would be tempted to go look up automotive industry papers where I would presume pressure is actually measured during the cycle.