Engines · Marine · Demo
A car engine screams at 6 000 rpm; a ship engine is a three-storey building turning at 100 — and it’s the more efficient one.
86 mm is a 2.0-litre car engine; 960 mm is the biggest marine diesel ever built. Stroke, revs, cylinder count and combustion cycle all follow the bore — nothing else is being dialled in behind your back.
Everything derives from the bore: stroke ratio, BMEP and mean piston speed follow real practice from car to ship, then power = BMEP × displacement × firing rate. The two ends land on a real 2.0 L car engine (~110 kW at 5 900 rpm) and the RT-flex96C (~80 MW at 102 rpm) — the middle is honest interpolation.
In plain terms
Scale a piston engine up and everything changes character. A car engine displaces two litres and revs to 6 000; the biggest marine diesels displace 25 000 litres, stand taller than a house, and turn at about 100 rpm — bolted straight to the propeller with no gearbox at all. They burn the cheapest fuel afloat and still convert over half of it into work, which no car engine comes close to. None of that is exotic technology: it is the same four bars of physics stretched to a different size.
What wears an engine is not rpm but piston speed — the metres per second the piston actually travels. All piston engines live near the same limit of roughly 8–20 m/s. A car’s 86 mm stroke can hit that at 6 000 rpm; a ship engine’s 2.5-metre stroke hits it at barely 100. The revs fall out of the geometry: double the stroke and you must halve the rpm.
A propeller is most efficient when it is huge and slow — a 9-metre screw at 100 rpm moves far more water, far more gently, than a small fast one. A car engine needs a gearbox because its 6 000 rpm is useless to wheels; a ship engine needs none because its natural speed IS propeller speed. The crankshaft bolts straight to the shaft, and to reverse the ship, the whole engine stops and runs backwards.
Heat escapes through surfaces, and big cylinders have little surface for their volume — so less of the burn leaks into the cooling water. Add unhurried combustion at 100 rpm, very long expansion, and turbocharging, and the giant two-strokes exceed 50% thermal efficiency: the most efficient combustion engines ever built, against roughly 30–35% for a good car engine.