[ELI5] How does a piston engine work?


Edit – How does having multiple pistons affect the output of the engine?

In: Engineering

When gaseous fuel and air burn, they get super hot and expand. This expansion is contained within a cylinder, with a moving sealed piston head in it. The expansion forces the piston head outwards. The piston head is connected to a wheel so that the wheel gets spun. Once the piston reaches the farthest point (top dead center) the gas is released and the spinning wheel pushes the piston back down. This cleans the remaining exhaust gas out of the cylinder. Then, the spinning wheel pulls the piston back up, sucking fresh air in. Then the wheel pushes the piston back down, compressing the air. At bottom center the fuel is injected into the cylinder and ignites either due to heat from the gas being compressed or due to a spark generated by an electrical spark plug. The cycle then starts anew.

There are many types of piston engines. They use various methods to form a high pressure mixture of air and fuel at the top of the cylinder and then ignite it. When the air and fuel burn it will heat up causing it to expand. This pushes the piston down with more force then it used when compressing the air in the first place. The piston is connected to a crank which turns the up and down motion of the piston into rotating motion. The energy from the piston being pushed down is used to spin a flywheel connected to this crank which continues to rotate to make the engine replace the burnt gasses with new air and fuel and compress it so the cycle starts again.

Fuel ignites, causing small explosions which pushes the piston. The piston is attached to a wheel thing, so it moves from linear motion to rotational.

There’s some details (other comment or mentioned some), dealing with pressures, vaporization, ignition mechanism, compression, air mixing, cycling, etc, but that’s more detail than necessary to understand the basics.

to add to the previous explanations (which have mostly covered what happens in a single cylinder), having multiple cylinders has two main effects

the first is that it smoothes out the motion and torque produced by the engine.

Imagine pedaling a bicycle with only one leg, you’d get a moment of force propelling you along while you pushed down, then you’d freewheel until your leg and the pedal reached the top before you could push down again, so you’d get a sort of jerking forward motion. Having two legs (and pedals!) lets you deliver that force more evenly. For the rider it means a smoother ride, for combustion engines (operating at 1000’s of RPM) this can create a *lot* of vibration, which can literally start shaking things apart on your car/boat/plane etc

the second is space efficiency. A 2 litre engine with 1 cylinder means the whole 2L of cylinder needs to be accommodated, whereas the same 2L can be acheived by (say) four smaller cylinders, which can fit into a smaller overall footprint and incidentally, provide less vibration per cylinder the large one (edit: there’s some trade off, as adding more cylinders will result in a slightly longer engine, like the long V-12’s you see that take up 3/4 the car’s length!)