• 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. Found on http://thinkexist.com/dictionary/meaning/trunk_piston/
1) Engine technology 
• 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.