The piston, with its connecting rod and bearing, transfers the force of the combustion and expansion of the power stroke to the crankshaft.
The piston itself, its rings, and the piston or gudgeon pin are together called the piston assembly.
The cutaway shape on this piston allows it to clear the counterweights on this rotating crankshaft.
The shape of the piston crown depends on the shape of its combustion chamber, and its compression ratio.
In diesel engines, the combustion chamber may be formed totally or in part in the piston crown, depending on the method of injection, so they use pistons with different shapes
The piston crown may be flat, concave, dome, or recessed.
The piston must stand up to great heat and pressure. It also must change direction from about 10 times a second to up to hundreds of times a second.
In most engines, the weight of the pistons is important for engine balance.
This is why pistons should only be replaced in matched sets.
Some pistons are forged, while others are cast aluminium alloys.
All pistons expand as they heat up. As there is more metal near the gudgeon pin, this area tends to expand the most.
To allow for this, many pistons are machined into a slightly oval shape. This is called cam grinding.
Then, as the piston heats up and expands, it becomes round.
Other methods to control expansion include steel struts or ribs, expansion slots in the skirt, or slots called heat dams that restrict movement of the heat.
Piston rings
Piston rings keep a tight seal within the cylinder to stop the heat and pressure in there from escaping.
They also stop oil passing up into the combustion chamber.
New rings and cylinders have minor irregularities and when these wear off, the rings will make a better fit.
To help this along, the rings can be given a porous coating. It’s softer and wears more quickly than the ring material which is usually cast iron.
To prevent wear, the face of the piston ring can be coated with a harder material like chromium that operates well against cast iron without scuffing.
They are split so they can be fitted into grooves in the piston, and to expand against the cylinder walls.
When they’re removed, their diameter’s larger than the piston’s. So when they’re installed they’re compressed and the gap is almost closed. Tension in the rings keeps them against the walls.
There are 2 main types of piston rings - compression rings and oil rings.
Compression rings must seal against compression loss during the compression stroke, or the air-fuel mixture won’t be fully compressed.
They must also seal properly during the power stroke, or combustion gases are forced past the piston into the crankcase. This is called blowby.
A plain compression ring has a rectangular section. It is held against the walls by combustion pressure behind the rings.
A tapered ring seals against pressure too but its slightly tapered face helps scrape oil off the walls as well.
Faced rings are designed to better resist heat and wear.
A ring that is chamfered or grooved exerts increased pressure against the walls. It is also called a torsional ring.
Its shape creates internal forces in the ring so that when it is installed, it twists slightly upwards.
During intake, the ring scrapes surplus oil off the walls.
During compression, they tend to slide over the oil and not carry it into the combustion chamber. In the power stroke, combustion pressure forces down on the top of the ring and also against its back. This straightens it so that it has full-face contact with the cylinder walls for effective sealing.
Oil-control rings prevent excessive oil working up into the combustion chambers.
It can be a one-piece ring that depends on its own tension to hold it against the cylinder walls. Slots in the ring and holes in the piston behind the ring let oil return to the sump.
Many oil-rings are segmental types with 3 or 4 segments. It has 2 side rails and an expander, which also acts as a spacer for the rails. They depend on the expander to hold them against the walls. The expander is made of thin steel with a series of crimps to give it an outward spring force.
Connecting rod
The connecting rod connects the piston to the crankshaft. It is fastened to the piston at its little end, by a
piston pin, also known as a
gudgeon pin.
In some engines the pin is a press fit in the small end of the connecting rod.
In others, it is clamped to the connecting rod with a clamping bolt.
Another method lets the pin float in both the piston and connecting rod, and it is held with circlips. There is a bearing in the small end of the connecting rod.
The big end of the connecting rod has a detachable cap, and carries 2 halves of the big end bearing. The big end is attached to the crankshaft at the crankpin journal.
Connecting rods must be very strong and rigid, and as light as possible. They are subject to stretching, compressing and bending, so they are highly stressed.
They are cast or forged to form an H near the small end and an I near the big end.
This shape provides greater strength to resist the stresses than a solid rod of the same mass.
To maintain engine balance, all the connecting rods in an engine are a matched set.
Compression ratio
An engine’s compression ratio can be a guide to the power it can generate.
It’s not always obvious whether one engine is bigger than another. The size of the engine block can be misleading. Two blocks can be the same size but one has cylinders bored out to larger volumes.
The standard measure of size is called displacement. Displacement is the volume a piston displaces in the cylinder as it moves from its lowest point, or bottom dead center, to its highest point, top dead center. This is also called swept volume. Notice that displacement does NOT include the volume above top dead center.
Engine size is then the sum of the displacements of all of the cylinders of the engine. It is called total engine displacement. For this engine it is 2 litres or approximately 120 cubic inches.
Another guide to engine power is Compression ratio. It compares two volumes in the cylinder. One is swept volume plus clearance volume. That’s the volume above top dead center. The other is the clearance volume only. Putting these volumes into a ratio gives us the compression ratio - 6 to 1.
The larger the first volume, and the smaller the second, the higher the engine’s compression ratio and the more powerful the engine.
