Glossary: Understanding Pistons

If the engine is considered as the heart of an automobile, then the piston can rightly be considered as the heart of the engine. They are an integral part in reciprocating engines and have been an area of constant research. Engineers are thriving for better designs and materials to build pistons which can perform more efficiently and live a longer life.

What is a piston?

A piston is a solid block shaped like an inverted bucket, which slides freely inside the cylinder. It also supports the piston rings and the piston pin. One end of the piston is fixed to the bearing of the connecting rod, while the other end which faces towards the cylinder head, is exposed to the gases.

What are its functions?

1) Fuel is combusted during the power stroke in the presence of air. The combustion of the charge (fuel and air) exerts a PRESSURE on the surface of the piston. This gas PRESSURE is transmitted to the connecting rod by the piston.

2) They act as a guide for the connecting rod’s travel path within the cylinder.

3) They house the piston rings. Piston rings are circular rings, which are fitted along the circumference of the piston. They are fixed on the upper part of the piston below the piston head. They act as a seal between the upper part of the cylinder and the crankcase. They also aid in transferring some of the heat from the piston to the cylinder walls and act like a regulator for the oil entering into the combustion chamber from the crankcase. Usually around three rings are used (2 compression rings and 1 oil ring), but the number of rings used vary with bore size and other design factors. There is always a gap between the two ends of a piston ring in an uncompressed state. This is because the rings will expand due to the effects of pressure and temperature and should not break.

4) They absorb heat and transfer it to the cooling system. Not all the heat generated due to combustion can be converted into mechanical work. Considering the fact that a diesel engine is only about 40% efficient, a major amount of heat has to be dissipated which may otherwise damage the components. The piston plays a major role in absorbing and transferring the extra heat to the cylinder walls.

What are the design considerations?

1) It has to be kept as light as possible to reduce vibrations and inertia loading on the bearings and further to reduce frictional losses. Due to large difference in pressure inside the cylinder during the engine’s operation, pressure waves are created which when in propagation, resulting in vibration. By keeping the parts lighter in weight, the vibrations can be reduced which means smoother engine operation. Further more, increased weight of piston increases the load on the connecting rod and crankshaft which would demand thicker section designs to increase strength. Another issue with pistons are frictional losses, that are caused due to the contact of moving parts. The piston has to be designed with a suitable material which is lighter as to reduce frictional losses.

2) It has to operate without major changes in dimensions or properties in all temperatures, and various forces it encounters during its operation.
The piston has to slide up and down inside the cylinder and thus there is a small clearance (gap) between the piston and the cylinder to allow this motion. The clearance shouldn’t be large as the fresh gases as well as the combusted gases can pass through this gap and enter into the crankcase which leads to many problems (crankcase dilution, increased blowby losses). Similarly large amounts of oil from the crankcase can enter into the combustion chamber which may cause many problems (blue smoke, spark plug/injector fouling, high oil consumption).
During operation, the piston is exposed to large amounts of heat due the combustion of the gases and hence the piston will expand. This expansion should be within limits to maintain the clearance. Larger clearance results in “piston slap” where the piston collides with the cylinder walls at indefinite time intervals causing noise and damage to the piston rings. Thus a piston will have to be designed in such a way that the clearance is maintained constant at any given temperature. If the piston expands infinitely, then it leads to a condition termed “engine seizure“, where the piston is unable to move inside the cylinder.

3) The piston has to operate emitting as little noise as possible.(explained above)

4) It should provide sufficient bearing area to prevent undue wear. The lower part of the piston is called the piston skirt. The piston skirt acts like a bearing and prevents undue wear.

5) It must dissipate heat to the cylinder walls with as much efficiency as possible.

6) The material used for the piston should not react with the lubricating oil at high temperatures. Another big advantage of using aluminium over cast iron for making the piston is that aluminium runs at a lower temperature than cast iron (about 200 degrees centigrade) as aluminium dissipates heat better than cast iron. Because of lesser temperatures, the lubricating oil does not break down and form carbon deposits on the piston.

What are the materials used for making pistons?

Almost all the pistons found in modern engines are made up of aluminium alloy. Cast iron (grey cast iron) was previously used and now it finds its application only in two stroke engines. The one big advantage of using cast iron is its expansion due rise in temperature is lesser (lower coefficient of thermal expansion) than aluminium alloy. Thus a piston made of cast iron does not get affected by temperature and the clearance between the piston and the chamber remains constant even at high temperatures.

Aluminium alloy is a combination of aluminium and silicon. It is the amount of silicon though, which determines the pistons overall strength verses wear resistance properties. Silicon also controls the rate of expansion of the piston as the material becomes hotter. There are traces of many other metals including copper, nickel, manganese and magnesium, all of these adding to the overall behavior and strength of the piston.

Piston rings are made of fine grain alloy cast iron and usually coated with chromium to reduce corrosion wear and increases hardness. The piston pin which supports the connecting rod is made of case-hardened steel.