Anatomy · Marine
Anatomy of a giant ship two-stroke
The biggest combustion engines on Earth stand five storeys tall, turn at walking-pace rpm, and are bolted straight to the propeller. This is a section through one cylinder — of up to fourteen.
The drawing catches the engine at its most two-stroke moment: the piston has just reached the bottom, the ring of scavenge ports is uncovered, the exhaust valve overhead is open, and pressurised air is spiralling up the cylinder, shoving the burnt gas out. One power stroke every revolution — that, plus sheer size, is where the power comes from.
Everything odd about the layout — the straight piston rod, the sliding crosshead, the single huge exhaust valve — exists to allow a stroke of 2.5 metres. And the long stroke exists because slow, fat cylinders waste the least heat: these are the most efficient combustion engines ever built, better than 50%.