Piston Ring Sets

What are Piston Ring Sets

In a reciprocating engine a piston ring is a split ring that slides into a groove on the outer diameter of the piston. In reciprocating engines piston rings have three main functions: to seal the combustion chamber to reduce gas leakage into the crankcase; to improve heat transfer from the piston to the cylinder wall; and to limit the amount of engine oil used by returning scraped oil to the sump.

The purpose of the piston rings is to close the gap between the piston and the cylinder wall. If this gap is too narrow the piston can seize in the cylinder due to thermal expansion causing serious damage to the engine. On the other hand if the gap was too wide the piston rings would not seal properly against the cylinder walls resulting in excessive blow-by (combustion gases entering the crankcase) and reduced cylinder pressure which would reduce engine performance.

The engine suffers friction losses due to the sliding motion of the piston ring inside the cylinder wall. Approximately 24% of the engine's total mechanical friction losses are due to friction induced by the piston rings. The design of the piston rings is therefore a compromise between reducing friction and providing good sealing and a respectable service life.

Types of Piston Ring Sets

Ring Types

The main types of piston rings used in internal combustion engines are compression rings and oil control rings. Suppliers in North America and other countries offer both types of piston rings. There are also compression rings and oil control rings that are CE marked or that meet SAE standards.

Different piston ring designs are required for different engines. Piston rings are fitted into specific grooves cut around the circumference of the piston. The oil control ring groove is usually located below the compression rings and the compression rings are usually mounted in the upper part of the piston.

Benefits of Using Piston Ring Sets

They seal off the gases produced during internal combustion help transfer heat to the cylinder wall and then lubricate and remove oil from the wall. It's important to use the right amount of oil. If your engine has too much oil it may produce blue smoke during combustion but if it has too little the engine could eventually seize up.

To ensure you get the most power from your engine the main function of the top compression ring is to seal off most of the combustion gases. Your engine will operate less effectively than it should if there is any failure or weakening of the piston ring in this area.

The intermediate ring helps with both tasks playing a finishing role in the combustion seal as well as scraping oil downwards. The bottom ring is primarily responsible for oil control ensuring that the correct amount of oil is used to lubricate the working surfaces of the cylinder.

The combined action of these three rings ensures adequate lubrication within the cylinder bore preventing undue wear at all times.

How to Choose the Right Piston Ring Set

Piston ring dimensions and shape

There are two ways of mounting piston rings. They can be mounted on rods or shafts or used as guides in cylinders. To provide the required clamping force and flexibility guide rings are slotted.

Depending on the shape of the slot piston rings can be used either as a seal or to allow lubrication or hydraulic fluid to pass through. As well as straight and angled grooves there are also form-fit variations with a Z or serrated design. Plastic piston rings have the advantage of being less complicated in shape as they are easy and inexpensive to handle and machine.

Different piston ring materials

There is a wide variety of materials used in the plastics industry. However not all of them are suitable for use in piston rings. In addition additives are required to improve the properties of the material for use in piston rings. The main differences between materials are their resistance to temperature thermal expansion and chemicals as well as improved properties such as elasticity coefficient of friction and mechanical stability.

Main functions of piston ring

In engines piston rings have the following main functions

Seal the combustion chamber to reduce gas leakage into the crankcase.

Improve heat transfer from the piston to the cylinder wall.

Ensure that there is an adequate amount of oil between the piston and the cylinder wall.

Limit the amount of engine oil used by returning scraped oil to the sump.

What are piston ring sets used for?

In engines piston rings have the following main functions: to seal the combustion chamber to reduce gas leakage into the crankcase; to improve heat transfer from the piston to the cylinder wall; to ensure that there is the correct amount of oil between the piston and the cylinder wall.

What materials are piston ring sets made of?

Pistons are made from low carbon steel or aluminium alloys. The piston is subjected to high temperatures inertia vibration and friction. The effects of differential thermal expansion between the piston and cylinder walls are reduced by carbon steels.

How many piston ring sets are required for one engine?

Cast iron and steel are typically used in the manufacture of piston rings. A key requirement for the materials used in their manufacture is wear resistance.

What is the lifespan of a piston ring set?

The total life of a piston ring for a large two-stroke 900mm diameter engine can be up to 24000 hours while for smaller 500mm bore engines it can be up to 16000 hours.

How does the quality of a piston ring set affect engine performance?

It appears that the engine's performance is dropping. Combustion gas leaks are caused by wear between the piston rings and the cylinder which has an immediate effect on performance.

The top and second rings are responsible for pressing tightly against the cylinder wall and sealing the combustion chamber preventing oil from entering and combustion gases from escaping.

As the oil ring moves down the cylinder it scrapes oil off the cylinder wall and deposits it in the oil sump. It is typical for some oil to burn during combustion as an incredibly thin layer of oil lubricates the ring/cylinder wall interface. However every engine has a different definition of 'normal' oil consumption.