Trunk pistons are long relative to their diameter. They act both as a piston and cylindrical crosshead. Because the rod is angled for much of its rotation, there’s also a side force that reacts along the side of the piston against the cylinder wall. An extended piston helps to support this.
Trunk pistons are a typical design of piston since the first days of the reciprocating combustion engine. They were used for both petrol and diesel engines, although high-speed engines have now adopted the lighter weight slipper piston.
A characteristic of most trunk pistons, particularly for diesel engines, is that they need a groove for an oil ring below the pin, additionally to the rings between the pin and crown.
The name ‘trunk piston’ derives from the ‘trunk engine’, an early design of the marine external-combustion engine. To create these more compact, they avoided the steam engine’s usual connecting rod with a separate crosshead and were instead the primary engine design to put the wrist pin directly within the piston. Otherwise, these trunk engine pistons bore little resemblance to the trunk piston; they were extremely large in diameter and double-acting. Their ‘trunk’ was a narrow cylinder mounted within the center of the piston.