Piston parts include every section of the assembly from the piston head to the rod that links the component to the crankshaft. Every part plays a significant role in the working of the piston and, therefore, the engine. This section explains all the main parts of an engine piston, their purpose, and variations in terms of design.
Piston Parts Name and Function
The engine piston is deceptively simple-looking. However, behind the simple appearance is a complex design that consists of intricate geometry and sections. The conventional piston used in internal combustion engines or piston engines comprises these parts.
- Piston head
- Piston skirt
- Piston rings
- Piston pin
- Connecting rod
- Connecting rod bolt
- Piston bearing
The piston assembly parts are described more deeply below.
This part is also called piston crown, and represents the top surface of the piston assembly. The head suffers all the temperature and pressure extremes when the engine is running, with heat levels reaching up to 2500° and higher.
As you can expect, piston heads must possess higher abilities to resist heat and force. They are, therefore, usually made from special alloys. Their design should also facilitates an efficient combustion, which brings us to the design and function piston heads.
Piston head functions include bearing the pressures of combustion and enabling uniform combustion by swirling the air fuel mixture. The head also protects the parts below it from damage by heat. In order to perform these functions, the head’s surface is specially shaped.
Common designs include dome, dish, and flat top. The dome type features a raised middle, while the dish piston has a depression with raised edges. The flat top piston uses a flat profile. Dome and dish profiles help to regulate compression levels and prevent engine damage. The head can also be a wedge shape to provide clearance for the valves.
The piston skirt is the portion that surrounds the cylindrical section of the piston assembly, just below the crown. It’s usually grooved and normally made from cast iron or steel. To prevent the piston from binding in high temperature conditions, the skirt is often made in an elliptical shape.
The piston skirt function is to align the piston assembly in the cylinder. The piston movement is usually irregular. Without the skirt, that would mean the piston hitting the sides of the cylinder. The skirt, therefore, acts as the piston guide.
There are basically two designs for this part; full skirt and slipper skirt. A full skirt is a cylindrical shape and mostly used on the pistons of heavy duty vehicles. The slipper skirt is typically sized down to reduce weight.
These are circular pieces that go into the piston grooves located on the skirt. In most applications, there will be 3 piston rings. The rings are also usually a split design. This construction serves two purposes. First, it makes mounting the rings easy. Second, it helps with retaining the ring position.
The piston rings function includes sealing the combustion chamber by maintaining the correct piston, cylinder clearance. Another major purpose is to move engine oil up and down the cylinder walls. In that regard, there are often 3 types of piston rings; compression ring, wiper ring, and oil control ring.
The compression ring located on the upper section of the piston assembly. It’s also known as the pressure ring, and serves to prevent combustion gases from leaking down the cylinder bore. The wiper or scraper ring is the second ring. As its name implies, it scrapes oil from the cylinder walls. It also provides a sealing function.
The last ring, known as the piston oil control ring, wipes oil from the cylinder walls. In some pistons, this ring also has small holes machined into it. These help to pass excess oil so it returns to the crankcase. Piston rings are normally cast iron or steel. When more than one ring is used, the spaces between are called piston rings lands.
The piston pin refers to the shaft that goes through the skirt inside a bore called the piston pin bore. It can be a hollow piece or solid depending on the specific application. To make them hard wearing, these parts are commonly made from alloy steel.
The piston pin function is to act as the pivot for the connecting rod. This helps facilitate the piston movement when the engine is running. Due to the friction produced by the pivotal motion, the pin is usually lubricated by oil that flows to it through a hole in the connecting rod.
Just like with other piston parts, there are different configurations for the pin. It can be a stationary, semi-floating, or fully floating pin. The stationary pin uses a screw to connect to the piston bosses, and is not free to move at either end. On the other hand, the semi-floating and fully floating pins have their ends moving depending on the design.
The piston connecting rod is the part of the piston that connects to the crankshaft on one end and the piston pin on the other. Some people call it the conrod, which is the short form for connecting rod. Depending on the engine application, the rod can be steel, aluminum or iron. It’s also typically divided into 3 main sections; the small end, big end, and beam.
The small end of the piston rod refers to the part that joins with the pin, while the big end is the part that connects to the crankshaft. The beam is the piece that lies between the two ends. In some pistons, the beam is bored a hole to convey lubricating oil.
The main function of the conrod is to transfer the energy of expanding gases to the crankshaft. It rotates the crankshaft as the piston moves up and down or back and forth, converting linear motion into rotation. The rod also acts as conduit for the cylinder and the piston pin lubricating oil.
A piston connecting rod is usually either steel or aluminum and sometimes iron. Steel offers excellent strength and suits heavy duty engines, while aluminum offers weight reduction and fits lighter applications. Iron rods are more common in smaller engines. Most piston rods are also the forged types for sturdiness. Some are cast, especially those used on lighter engines.
Connecting Rod Bolts
The conrods bolts hold the connecting rod to the crankshaft and count as one of the piston parts. In order to bear the mechanical stress of the rod movements, the bolts are designed to flex a little. They are also made from different materials such as steel, aluminum, and even brass or nickel alloys.
The piston bolt function is to clamp the connecting rod to the crankshaft. This ensures the piston’s proper working, and smooth engine operation. The bolts must be correctly installed to allow that, though, and the right type of the bolt material used depending on the type of engine.
Design options for the piston bolts include hexagonal shape, flat shape, round, and embossed. They can also be the threaded type or unthreaded. Other variations include the cap type and through bolts. The bolts are among the parts of a piston assembly that you can separately replace.
Piston bearings are metal pieces or shells found at either end of the connecting rod. They are also called piston bushings and made from a variety of materials. Common ones include silicone aluminum, silicone copper, lead copper, and more. They’re also usually coated with materials to increase hardness.
The function of piston bearings is to allow the rod transfer motion to the crankshaft. They are of two types: the big end bearings and the small end bearings. The big end bearings are located at the point where the conrod connects to the crankshaft. Small head bearing are those found on the end where the rod joins with the piston pin. Piston bearings are replaceable parts.
A piston for car engine applications is made up of several parts. As we have seen, each part serves an important function. The parts of a piston are also made from different materials depending on its location and the amount of thermal and mechanical stress to be borne. Car pistons are also available in different types, as you will learn in the next section. Read on to find out how each type fits specific engines and applications.