The following is a simple explanation of how an engine works. Most people hear "pistons, crankshaft, connecting rod, cylinder, blah blah blah blah", and think it's way to complicated to understand... The truth is that small engines are really simple, and the operating principle is really simple. The problem is that it seems very hard to find a really good, relevant and simple explanation of how an engine works. So I've decided to write my own article about it, giving you the absolute simplest explanations possible.
The easiest explanation is to look at a bike: Your legs provide the power, and all they do is move up and down. The pedals take the up/down motion and convert it into a circular motion. The chain is connected to the front sprocket (gear), and transfers the energy to the rear sprockets, and turns the back wheel. So in the end, the up and down motion from your legs is converted into forward motion on the bike.
An engine works exactly the same way as a bike. An engine takes an up/down motion (from the piston) and turns it into a circular motion (the piston connects to a crankshaft, which then turns under power from the piston). Then, we attach something onto the end of the crankshaft (like a chain for motorcycles, a blade for lawnmowers, a propeller for an airplane or a transmission for a car) and turn this circular motion into something productive.
A piston is like your leg... except it has no muscles. On it's own, it can't move up and down. So, the question is, "where/how/what gives the piston the energy to move up and down?" This is where gasoline comes in. The piston slides up and down in a sealed cylinder, and when the piston reaches the top of the cylinder, the fuel is ignited, which creates an explosion. Because the piston is in a sealed cylinder, the explosion forces the piston down.
Back to the bicycle analogy, if you think about it, unless you have foot straps on your pedals, the only force you're applying is a downward force. Again, it's the same with an engine: the only force applied to the piston is a downward force from the explosion; it's momentum that carries the piston back up to the top where it gets more energy from another explosion. Remember: the explosion from the gasoline is the energy. The explosion forces this piston up and down in the cylinder. The up and down motion (just like a bicycle) is converted into a circular motion, which is then used for productive means.
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