How well do you know about piston design and features? Here’s a helpful guide on everything you need to know.
An engine’s economy and efficiency depend primarily on the smooth functioning of the piston. Pistons are fundamental parts of every internal combustion engine. They have reciprocating motion and rapidly convert heat energy into mechanical power and vice versa.
How a Piston Works
A piston works by traveling up and down within a cylinder when an engine produces power. A piston’s primary purpose is to withstand the expansion of gases and transmit it to the crankshaft.
In other words, a piston transfers the force of the explosion to the crankshaft, which, in turn, rotates it. Pistons typically come with rings that serve as seals between them and the cylinder wall.
The piston is expected to function within the cylinder with minimum friction. It must be capable of withstanding high explosive forces within the cylinder. It is also expected to withstand incredibly high temperatures above 2,000 degrees Celsius. It must be lightweight but incredibly strong.
Piston design varies from one engine to another. This is because it depends considerably on the design of the cylinder head. The top of a design is referred to as the crown or head.
Pistons are designed with specific features that perform some functions when an engine is operating.
In general, low-performance and low-cost engines have and utilize flat-head pistons. But in a few cases, especially when the piston comes incredibly close to the valves, engineers create a valve relief in the crown. The piston crown generally receives the bulk of the initial force and pressure that the combustion process causes.
The piston pin area is subjected to a considerable amount of force as a result of rapid directional changes. The area is also exposed to thermal expansion that the transfer of heat from the head to the body of the piston causes. This occurs due to the mass within the piston pin area and the thermal expansion properties of cast aluminum alloy.
The pistons that some high-performance engines use come with a raised dome that significantly controls combustion while increasing the compression ratio.
Some engines come with dished pistons in order to create the desired shape of the combustion chamber and the cylinder head. If the crown has some part of the combustion chamber, you can easily – and more accurately – control the compression ratio.
But this particular design has a flaw: an incredibly large amount of heat ends up passing through the piston and its rings.
The Band Between the Grooves
Pistons come with grooves cut right into their circumference in order to fit the piston rings. The bands between these grooves are called ‘Lands.’ The primary role of lands is to considerably support the rings against pressure from the gas.
The lands also efficiently guide the rings, making them rotate smoothly and freely in a circular route. Supporting webs readily transmit the force of the explosion from the crown to the piston-pin bosses. This alleviates the large loads from the ring grooves.
The Different Piston Shapes
Some pistons are cast and then machined at factories into an elliptical shape. This is an oval shape in which half is the mirror image of the other half. The shapes of pistons are crucial as they offer a significant advantage in conforming to the ever-dynamic dimensions of the cylindrical bore.
When the piston is cold, it showcases an elliptical shape. But as the engine reaches its usual operating temperature, the piston pin bore area starts expanding much more than other thinner sections of the piston.
At the typical operating temperature of the engine, the shape of the piston becomes circular, matching the cylinder bore for combustion efficiency and improved sealing.
Some pistons have a taper-like design. The smallest diameter of the taper is the piston crown. This tapered shape efficiently compensates for thermal growth and thermal expansion. Thermal growth refers to the considerable increase in the size of a material when exposed to prolonged heat, with little or zero changes back to its initial or original dimensions.
The tapered design of the piston allows it to move freely within the cylinder bore irrespective of the heat applied to the piston head.
It is known that some Briggs & Stratton engines make use of a barrel-shaped piston skirt. The piston is designed that way in order to provide smoother transitions during the directional changes of the piston. The piston rolls right into the cylinder wall as it changes direction at the end of a stroke.
This significantly minimizes noise while spreading the force of that directional change across a much greater surface. It also helps in reducing side loading on the piston skirt.
Some piston designs come with piston pins offset from the center of the piston. The appropriate orientation of the piston pin offset is marked by an arrow or a notch on the piston head.
The earliest material used in making pistons was cast iron. However, the benefits of using lighter materials for engine balancing of modern engines are now the order of the day. A well-designed piston should be capable of resisting the high combustion temperatures of an engine. This is why alloys such as hiduminium and Y alloys are used to obtain the desired properties.
Pistons are constructed from aluminum alloys via a casting process. The pistons used in many racing vehicles require better fatigue life and strength, so they were forged.
Billet pistons are sometimes used in racing engines since they don’t rely heavily on the architecture and size of available forgings.
Major parts of the piston and their respective functions
- Piston rings
Piston rings are those pieces of split rings mounted on recess areas of the pistons. An engine usually has 3 piston rings, though the ring can sometimes be one, depending primarily on the engine type.
- Piston bearings
Bearings are semicircular metal piston parts that enhance the effectiveness of the piston’s movement. It is located at the exact points pivotal rotation occurs and fits within the bores of these points.
- Piston skirt
A piston skirt is a cylindrical material attached to the round portion of a piston. This skirt is constructed from cast iron to resist its self-lubricating properties and wears.
Grooves on the skirt allow piston rings to sit perfectly. The primary function of piston skirts is to move up and down the cylinder.
Pistons are one of the greatest parts that assist the function of the combustion cycle. They are enclosed within cylinder blocks that use piston rings to provide no space for gas to escape.
Pistons assist in transforming heat energy into mechanical power and vice versa. They move upwards and downwards within the cylinder to contract and expand the air-fuel mixture. This makes pistons highly inevitable in internal combustion engines.