trunk piston
In a single-acting engine, an elongated hollow piston, open at the end, in which the end of the connecting rod is pivoted. The piston rod, crosshead and stuffing box are thus dispensed with.
trunk piston: hollow piston that saves a rod and box
A trunk piston is an elongated, open-ended hollow piston used in single-acting engines, most commonly large slow-speed marine diesel engines. Instead of a solid piston connected to a separate rod via a stuffing box, the connecting rod pivots directly inside the hollow piston itself. This eliminates the external piston rod, crosshead, and the stuffing box seal that would normally be needed to prevent leakage at the cylinder head.
The piston is machined as a thin-walled cylinder with an open bottom end. The connecting rod's small end bearing sits inside this hollow chamber, held by a pin that allows articulation as the rod rocks during operation. Cooling oil or seawater can be circulated through the hollow piston to manage heat, which is critical in large engines where piston face temperatures can exceed 300 degrees Celsius. The open end faces the crankshaft side of the cylinder, away from combustion.
Why it matters in marine engines
Trunk pistons are dominant in large two-stroke marine engines operating at 100 to 300 RPM. They are lighter and more compact than an equivalent solid piston with external rod assembly, which reduces the height of the engine block and improves structural efficiency. The absence of a stuffing box also means fewer leak paths and simpler maintenance on vessels operating far from repair facilities. In contrast, small and medium-speed four-stroke engines typically use solid pistons with external rods and crossheads, which provide better load distribution and easier rod replacement.
The hollow design creates a trade-off: trunk pistons experience higher bending stress on the piston walls because the combustion load is carried across the hollow section like a thin-walled beam. Designers compensate by using high-strength aluminum alloys or ductile iron, and by careful control of clearance between piston and bore. Scuffing between the outer piston wall and cylinder liner is a known failure mode if cooling is inadequate or if fuel quality leads to deposits on the piston skirt.
The term "trunk" refers to the piston body itself, which resembles a storage trunk or hollow case. The piston is often called a "trunk piston" even when not contrasted with any other type, because the name simply describes its hollow, enclosing geometry. Some older texts refer to this arrangement as an "open piston" or "hollow piston," but trunk piston is the standard naval and diesel engineering term.