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(speaking just to size, not RPM:) The square-cube law causes the burning gasses in a larger cylinder to bleed less heat away into the water jacket compared to a
by cellularmitosis 12y ago
(speaking just to size, not RPM:) The square-cube law causes the burning gasses in a larger cylinder to bleed less heat away into the water jacket compared to a smaller cylinder. This is the primary reason why the largest diesel engines in the world are also the most efficient.
Theoretically, the 14-cylinder Wärtsilä-Sulzer RTA96-C could be even more efficient if it were instead a gigantic single cylinder. There must be some other engineering or economic limitation which prevents us from manufacturing 25,000 liter cylinders.
edit: example: http://en.wikipedia.org/wiki/Square-cube_law#Engineering_examples http://en.wikipedia.org/wiki/Square-cube_law#Engineering_exa... "Watt recognized the problem as being related to the square-cube law, in that the surface area of the model's cylinder surface to volume ratio was greater than the much larger commercial engines, leading to excessive heat loss"
edit2: There is also a square-cube law with aerodynamic losses related to piston speed (how fast the air gets sucked into the engine: "pumping losses"). The slower you spin the engine, the lower your pumping losses are. However, I believe this plays a smaller role in affecting efficiency than the heat losses. (also, this engine is turbocharged, which changes things a bit).
- TylerE 12y agoA single cylinder that big would have SERIOUS vibration issues. Imagine dropping, oh, a 20,000lb weight from a height of 10ft. Every 6 seconds or so.
- cellularmitosis 12y agoThat can be overcome with balance shafts. http://what-when-how.com/automobile/engine-balance-automobile/ http://what-when-how.com/automobile/engine-balance-automobil...
- TylerE 12y agoTo a point. But the balance shaft for something that big and single cylinder to boot would be beyond massive.
- Evolved 12y agoLike what Tyler said, balance shafts to counter a single cylinder would be huge but also they'd add unnecessary complexity that could otherwise be overcome by adding additional cylinders instead. Furthermore, in order to obtain the same level of torque in a single-cylinder that a multi-cylinder engine creates you'd have to account for the power generated per cylinder. In the Wartsila RTA96C you have HP up to 109,000 (rounded) and torque up to 5,600,000ft-lbs [1]. These #s are for all cylinders. Assuming it is the 14-cylinder variant generating this power and you want to reduce this down to a single-cylinder package then you can easily deduce that the displacement would be around 25,480 liters (mentioned elsewhere in this thread). Assuming a stroke to liter ratio of 1.374mm/liter (2500mm stroke for each 1,820 liter cylinder) you'd end up with a 35,009mm stroke for a single-cylinder engine vs. a 14-cylinder engine. Since there are 25.4mm in an inch that's a 98-inch stroke for 14-cylinders vs. a 1,378-inch stroke for a single-cylinder or an 8-foot stroke vs. a 114-ft stroke. Finally, since you'll need a piston many times larger than the current setup for a single-cylinder vs. a 14-cylinder engine then you can imagine how much mass you have moving which can do some serious damage if it becomes unbalanced. Also you'd have an engine that instead of being long and relatively low is now very short and extremely tall (not something you want in a ship where capsizing is a very real concern). [1] http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96...
- jschwartzi 12y agoWhat's the minimum number of cylinders required for harmonic balance in a two-stroke inline engine? I'm just curious what the biggest engine you could build would be by cylinder volume.
- TylerE 12y agoSix cylinders, same a 4 stroke. The strokes doesn't actually matter.