Productivity Tip: Everything You Need to Know About Pistons

Many people say they know what a piston does, but how many of them understand the many different types of pistons, their functionality, and how to keep them operating at peak productivity?

Fret not, this week’s tip answers all these questions and more to help engine and compressor operators maximize the performance of their machines.

What is a piston?

A piston in a compressor, pump, or internal combustion engine is the moving component contained within a cylinder. It is designed to transfer the force provided by expanding gas transferring linear motion to circular motion. To accomplish this, it is connected to the crankshaft by a wrist pin and piston rod.

Pistons were originally cast from iron, but today they are mostly cast from aluminum alloys containing about 85% aluminum.

Types of pistons

Most of us are familiar with the pistons in an automotive engine. They are connected to the crankshaft by a one-piece connecting rod. However, there are other designs that are necessary to accommodate larger engine arrangements.

One such design is a “crosshead” piston, which contains a two-piece connecting rod, where crosshead bearings and guides facilitate motion. Though larger in size, the crosshead piston’s function is identical to that of pistons in small automotive engines. You may find these in large stationary engines and compressors, often used to either generate electricity or transmit natural gas up a pipeline. There are other types of piston designs, such as trunk, slipper and deflector pistons – none of which are covered here as they are less common.

 

Functionality

As the piston moves up and down, sealing is provided by the piston rings, which fit into a circumferential groove on the piston.   In an engine, the piston:

  • Provides the vacuum needed to facilitate the introduction of the air/fuel mix into the cylinder
  • Compresses the gases
  • Transfers power to the crankshaft and
  • Pushes spent gases out of cylinder

 

Typically, there are 2 to 3 rings on a piston. The top two rings provide compression, while the bottom ring is used for oil control. Most rings are of tapered design cast from iron, and they are coated with chrome and/or nitrided to reduce wear.

Lubrication fundamentals

Lubricant is needed for the interface of the cylinder, piston rings, and piston grooves. Lubrication is provided by the oil in the crankcase, typically delivered by crankshaft splash or by pressurized assist (in larger engines). Pressurized assist is particularly prevalent in two-piece rod designs, where the crosshead and slide need to be lubricated.

Piston interface contact is largely sliding, with the lubricant providing an oil wedge under a hydrodynamic regime. This wedge is stressed by the oscillation of the rings and the higher loads encountered at the piston turn around areas, which may disrupt the lubricant film.

To provide proper lubrication, an oil is needed that furnishes the proper viscosity under the given loads (VI Improved), is able to withstand heat (anti-oxidation additives), and fortified to reduce the wear effects (anti-wear additives) caused by sliding friction.

I hope this week’s tip helped provide some more detail around piston performance and maintenance, but if you have any further questions or comments, don’t hesitate to reach out!

Anonymous
  • 1.  Seizure of piston - loss of lubricant or coolant, overspeeding the engine, excessive load.

    2.  Broken piston - bad metallurgy, pre-ignition or detonation.

  • Porque sucede este tipo de daño ¿Gripado en el pistón o pistones rotos? ¿Cuál ha sido la causa de la avería?,como duda Rick Russo.

  • 1. What type of lubrication is best for pistons.  

    Answer:  To provide proper lubrication, an oil is needed that furnishes the proper viscosity under the given loads (VI Improved), is able to withstand heat (anti-oxidation additives), and fortified to reduce the wear effects (anti-wear additives) caused by sliding friction.

    2. How come the oil become black after a very little usage especially in diesel engines.  

    Answer:  There are a couple of reasons for this.  

    A.  When oil is new it contains basestock and additives in a new unaltered state.  As the oil is worked, it is exposed to heat which catalyzes reactions between components in the oil, and this may lead to a color change, typically darker than original.

    B.  The are contaminants from the engine operation which may cause the oil to darken.  Dirt, wear metals, soot and fuel are but a few.

    3. For pistons, does a higher viscosity oil is better?

    Answer:  This is a great question and the answer to this has changed over the years.  In the 1960's through the 1980's, the recommendation was a 10W-40.  Then as fuel conservation became relevant, the recommendation became a 5W-30.  Now most new vehicles recommend a 5W-20, depending on the ambient temperature.  What the oil manufacturers and OEM's have found is that with todays high revving engines, a lighter oil, does not yield any wear metals.  

    ExxonMobil has done a lot of testing in commercial engines as well to include large stationary natural gas engines.  20 WT oils in large NG engines has not yielded any more wear metals than a straight 40 WT, so overtime, I expect the recommendation for large commercial engines to change as well.  

    So to answer this question -  higher viscosity in engine oils is not necessarily better.

  • Hi,

    1. What type of lubrication is best for pistons.

    2. How come the oil become black after a very little usage especially in diesel engines.

    3. For pistons, does a higher viscose oil is better?

  • Теперь я знаю немного больше о двигателях.Благодарю Рик.

  • Piston ring wear has many causes, but that said it does not occur that often.  Some causes:

    1.  Loss of lubricant film.

    2.  Contaminant getting behind the rings preventing them from rotatings or flexing.

    3.  Improper installation keeping the rings from rotating, expanding, etc.

    4.  Deposits on the rings preventing free movement.

    I am sure there are more, but that is a good start.

  • what is cause of wear in rings of piston ? unsuitable lubricant ?