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In an Internal Combustion Engine, why have eight cylinders in an engine? Why not have one big cylinder of same displacement of eight cylinders instead?

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  • $\begingroup$ I gave you some hints with an answer but i downvote you because though interesting it is not a physics equation and you did not search before asking $\endgroup$ – veronika May 5 '19 at 19:27
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Here are several more reasons for multi-cylinder engines.

If one increases the diameter of a single piston much beyond four inches, and the engine is intended to run efficiently at speeds up to 4000 or 5000 RPM, then there is not quite enough time for the fuel/air charge to burn to completion if ignited by one spark plug. This means that large pistons require dual ignition, meaning that room has to be found in the cylinder head for a second spark plug.

In addition, the vibrations generated by the up-and-down motion of large pistons and the crank shaft to which they are attached are significant and can only be partially balanced by rotating counterweights on the crankshaft.

For these two reasons- along with the other mentioned in the first answer- it is better design practice for car, aircraft, and truck engines of modern manufacture to have multiple small cylinders instead of one very large one. Low speed boat engines are the exception; it is still common for inboard engines (especially diesels) in low-speed boats to have single cylinders and develop between 50 and 100 HP, but those engines run at speeds between 250 and 500 RPM.

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Eight cylinders spread the load out during the engine's cycle. This means the whole motion will be much smoother than having one big cylinder that just jerks the engine to bits as it lobs it around after unleashing a big explosion then "recharges" till it's time to go again.

Note however that having "one big cylinder" is not unheard of - the first Harley-Davidson motorbikes had them.

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  • $\begingroup$ Spreads the power strokes, the load is probably constant. And 6 cylinder inline engines are smooth due to how the primary and secondary balance forces are arranged. $\endgroup$ – user207455 May 5 '19 at 15:28
  • $\begingroup$ @SolarMike, for an inline engine, the construction of the crankshaft determines the cylinder firing order and timing. Even though the cylinders are inline, their firing order is evenly spread out relative to the position of the number 1 cylinder, keeping the engine operation "smooth". $\endgroup$ – David White May 5 '19 at 15:53
  • $\begingroup$ @DavidWhite Have you seen the masses built into crankshafts to balance to eccentric forces due to the connecting rods..? and some engines have extra shafts just for balancing... $\endgroup$ – user207455 May 5 '19 at 15:55
  • $\begingroup$ @SolarMike, yes, I've seen them, and I now better understand your original statement, given the clarification. $\endgroup$ – David White May 5 '19 at 15:58
  • $\begingroup$ FWIW, motorcyclists call a bike with one big cylinder a thumper $\endgroup$ – Solomon Slow May 5 '19 at 16:20
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It is not possible to make a " very big" cylinder because the stress depends on the radius: for this reason all pressure vessels are thin and tall. The cylinder of an engine is a pressure vessel.

The force exerted to the piston is squared when you increase the diameter but the volume ( you can read weight ) is cubed That means a bad proportion between power and weight.

It can't be very tall either because the motion of the piston is not sinusoidal and a part of the force is exerted on the wall. We must therefore make the walls thick but after it is difficult to cool the engine

These are the most important reasons and you can find more if you read a good book about engines but all reasons have to do with geometry

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    $\begingroup$ So how big is your "very big"? This engine has a bore of 38" approx see gcaptain.com/emma-maersk-engine $\endgroup$ – user207455 May 5 '19 at 22:12
  • $\begingroup$ @Solar Mike Emma maersk is near the limit of this very big this ship is 13000 teu ships with 20000 teu either have two engines or they move slower than emma $\endgroup$ – veronika May 6 '19 at 4:17
  • $\begingroup$ It’s about multiple cylinders and why they are better - if you read my other comments it’s about balancing. $\endgroup$ – user207455 May 6 '19 at 5:51

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