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?
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.
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.
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