Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Piston ring pressure?

  1. Jul 26, 2009 #1
    I am very mystified by how piston and piston rings work (obviously they do). My questions are related but are in 3 parts:

    1. What is highest pressure of the gas/liquid in a particular piston? (race car engine, construction machines...) and what is the approximate pressure in Kpa?

    2. With the amount of liquid pressure that has to be contained by the piston ring against the wall of the piston how does it not prevent the piston from moving up and down due to the friction?

    3. Can the piston seal be square or linear versus circular or is that the most efficient shape for pistons?

    thanks - sorry for the generality of the quesitons.
  2. jcsd
  3. Jul 26, 2009 #2


    User Avatar
    Science Advisor

    1. Depends greatly on the specific engine, but typical cylinder peak pressures are on the order of 5,000 kPa.

    2. There is not a lot of outward radial force on the rings, only enough to hold them firmly against the cylinder wall--they are relatively easy to push through by hand. And the cylinders are continually sprayed with oil at the bottom end of the piston.

    3. Round is the easiest shape to machine the cylinders to the tolerances required. Thus, the pistons and rings must also be round.
  4. Jul 26, 2009 #3

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    see post on piston ring sealing dated May25-09, 04:53 PM in this forumn
    also the cylinder bore is round but all pistons are oval...the piston pin boss has more material and the piston skirt is egg shaped to expand once the engine comes up to operating temperature. the cylinder pressure will dramaitcally vary die to compression ration and camshaft used..overlap will significantly reduce effective cylinder pressure but VE will go up due to more fuel / air packed into the cylinder..it gets complicated fast in the IC

    also see forumn Feb14-09, 04:36 AM #1

    Mgt3 is Offline:
    Posts: 25 Horsepower - Please help - Confused
    Last edited: Jul 26, 2009
  5. Jul 26, 2009 #4
    thanks for the quick responses guys.

    so all piston or hydraulic machines have oil as the liquid/gas in the chamber to prevent friction?

    Do you think it is possible to create a ring seal for sealing water from entering the chamber (or is is usually oil b/c of the lubrication factor). the ring would be moving up and down the shaft rather than just rotating around it's axis.
    Last edited: Jul 26, 2009
  6. Jul 27, 2009 #5
    The worst case pressure is when the intake manifold is zero, or even worse, turbo boosted, and the compressed gas is heated due to adiabatic compression (similar to diesel, and then the gas further expands due to ignition. So 50 atm (5 MPa) is reasonable. By the way, the minimum piston ring length for a given piston cross sectional area is a circle.
  7. Jul 28, 2009 #6


    User Avatar
    Science Advisor
    Gold Member

    Sorry Bob, this isn't quite right. The highest pressure is seen under peak load; while inlet manifold pressure obviously controls this, there are circumstances where IMP is zero (or 'turbo boosted') and peak cylinder pressure isn't being achieved (under transients for instance). 50 bar is on the low side for peak cylinder pressure; for a medium duty engine 150 bar is no big deal for hundreds of hours at a time.

    Gloo, I'm not sure what you're asking. Are you wondering why we use oil rather than water as a lubricant?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook