Engine Pistons And Connecting Rods - How A Car Works

The power in an engine all comes from the force pushing on the top of the piston. That force is determined as the area of the piston multiplied by the gas pressure. Bigger pistons, and higher gas pressure will provide more power. On the whole, piston size is constrained by the design of the engine but the piston does have a vital role in maintaining high gas pressure by creating a gas-tight seal with the cylinder wall.

The top surface of the piston is called the crown (also head or dome ). There are various shapes of crown in production engines, but typically the crown will be flat, domed or dished.

[Various crown shapes]

Almost all modern pistons include valve reliefs which provide a clearance around the valves at the top of the piston stroke.

The crown, being the area directly in contact with the hot combustion gases, gets extremely hot. It is this area that expands the most and so there will be a slight taper inwards from the bottom of the piston to allow a greater clearance around this top land between the crown and top piston ring.

While we want a gas-tight seal, we also need the piston to run smoothly along the cylinder with minimal friction, so the piston needs some clearance . A typical piston will have a clearance of 0.1mm (0.004”) between itself and the cylinder wall - that’s around the width of a human hair. To maintain this clearance, the piston must be precisely machined, and the alloy that it is made from will be exactly specified to account for thermal expansion.

The small gap between the piston and cylinder wall is bridged by the piston rings , which fit into grooves on the piston in an area known as the piston belt . The spaces between these grooves are called ring lands .

The piston is attached to the connecting rod by a short hollow tube called a wrist pin , or gudgeon pin . This wrist pin carries the full force of combustion.

The piston is not only subject to vertical forces during combustion, but also side forces caused by the continuously changing angle of the connecting rod. Because of these side forces, the piston needs smooth surfaces to run against the cylinder wall and keep the piston guided vertically upright. The side surfaces of a piston are known as the piston skirt .

[Full skirt vs slipper skirt]

There are two types of skirt. The most basic is a full skirt or solid skirt, which is the classic tubular shaped piston. This design is still used on truck and large commercial engines, but has long been replaced on cars and motorcycles by a lighter design known as a slipper piston .

The slipper piston has part of the skirt cut away, leaving only the surfaces that bear on the front and back of the cylinder wall. This removal minimizes weight and reduces the area of contact between the piston and cylinder wall, thus reducing friction.

Modern production engines further reduce the friction between the piston and cylinder wall by using low-friction piston coatings , similar to teflon in a non-stick frying pan. These coatings are typically screen-printed on in a patch to the piston skirts - such as the illustrated graphite-based coating on a Ford Fiesta Ecoboost engine.

[Ford piston]

As the piston is pushed down on the combustion stroke, it will exert a sideways force in the opposite direction to the angled connecting rod. The direction of the cylinder on which this force acts is known as the thrust side, and both the piston and cylinder wall will suffer greater wear in this area.

[Diagram of thrust]

The piston gets incredibly hot, and needs to dissipate this heat efficiently. The heat from a piston goes to three places: As radiant heat into the combustion chamber, into the cylinder walls via the piston rings and down the connecting rod. Additionally, many engines cool the piston through the use of oil sprayed onto the underside.