Types of Pistons in Combustion Engines

Pistons are components found in most machines such as reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders and some machines with other similar mechanisms.

The piston is a moving component contained by the cylinder and is generally made gas tight by the piston ring. In an engine, the purpose of the piston is to transfer force from the expanding gas in the cylinder to the trunk shaft via the piston rod and/or connecting rod.

In a pump, the function is reversed and force is transferred from the trunk shaft to the piston for the purpose of compressing or expelling the fluid in the cylinder. In some engines, the piston can also act as a valve by closing and opening ports in the cylinder.

Pistons are generally cast or forged from aluminum alloys. For better strength and fatigue life, some pistons are generally forged.

Generally, pistons are designed with a certain degree of ovality and taper profile, meaning they are not perfectly round, and are larger in diameter near the bottom of the skirt than at the crown.

There are many types of pistons if they are classified based on their nature, as in this article, the pistons are classified as coming from internal combustion engines:

1. The Trunk Piston

The Trunk Piston

Trunk Piston is a piston that is relatively long to its diameter. This piston is generally located in one unit with the crosshead cylinder.

The connecting rod on the piston is generally tilted for more movement, so there is a side force that acts between the piston side and the cylinder wall.

The piston rod is a piston design that was first proposed in a reciprocating internal combustion engine. This piston in its application is widely found in gasoline engines and diesel engines, but now it is rarely found.

One of the characteristics of rod pistons is that they have an additional groove for the oil ring under the gudgeon pin, apart from the ring located between the gudgeon pin and the crown.

This piston is generally designed separately from the crosshead cylinder and instead the gudgeon pin is placed directly in the piston.

2. The Deflector Piston

The Deflector Piston

A deflector piston is a piston used in two stroke engines and has crankcase compression. The compression of the crankcase causes the piston to have a gas flow in the cylinder which must be carefully directed to provide efficient cleaning.

The deflector piston has a cross scavenging system, where the transfer port (the hole that allows substances to enter the cylinder) and the exhaust port are on the side opposite the cylinder wall.

In the deflector piston, there is a simple rib mechanism that causes the crown to develop into a large asymmetrical protrusion, generally accompanied by a steep surface on the inlet side and a smooth groove on the exhaust side.

This mechanism allows the piston to prevent mixing that occurs due to substances entering from one port to another. This is intended to divert the mixture into the top of the piston (around the combustion chamber).

However, on the deflector piston, the cross scavenging system is not as effective as expected. Most current engines rely more on a ported schnoodle system that places a pair of transfer ports on the sides of the cylinder and pushes the gas flow around a vertical axis instead of a horizontal axis.

3. The Crosshead Pistons

The crosshead piston

Crosshead pistons are pistons generally used for slow speed diesel engines and require additional support for side force.

This piston has a large piston rod that extends downward, and has a second piston that is smaller in diameter.

The large piston is responsible for sealing the gas and carrying the piston rings, while the small piston serves as a trunk guide.

In the crosshead piston, the lubricating oil is not exposed to the heat of combustion, thus avoiding contamination by combustion particles, not easily damaged by heat, and lighter and less viscous oil can be used.

This is the main advantage of crosshead pistons when compared to other types of pistons.

Crosshead pistons are not used for high speed engines because the friction between the two pistons on the crosshead piston is half that of the trunk piston.

4. The Racing Piston

racing piston
The Racing Piston

Racing pistons are pistons that are generally used in racing car engines. The strength and rigidity of a racing piston is usually much higher than that of a passenger car engine, while weighing much less in order to achieve the high engine RPM required in a racing car.

5. The Slipper Piston

The Slipper Piston

Slipper pistons are pistons that are generally used for gasoline engines, where they are small in size and tend to be light.

These pistons can generally be reduced to a piston crown (support for the piston rings) and leave only the piston skirt remaining to stop the piston from rocking in the bore.

The purpose of the reduction is none other than to reduce reciprocating mass, and make it easier for the piston to balance the engine and allow this piston to move at high speeds.

In racing, the slipper piston can be configured to provide very light movement while maintaining the rigidity and strength of the full skirt.

In slipper pistons, the reduction of inertia is carried out in order to allow an increase in the mechanical efficiency of the engine or the force required to accelerate and decelerate the reciprocating part causing more friction of the piston with the cylinder wall than the fluid pressure on the piston head.