The combustion engine, while old, is still an impressive technological feat, as seen in its ability to remain the best way to power most vehilcles to this day. Internal combustion engines All work in the same general way, where some fuel is burned in a chamber, and the resulting energy from the explosions is used to move pistons, which in turn move a crank that can then be transferred into whatever energy is necessary to power whatever the engine is moving. The mechanical energy transferred to the crankshaft is much more useful in the terms of moving things than the chemical energy of the combustion itself is. The most common fuels for combustion engines are gasoline and diesel, as they provide a lot of energy when burned, and because of this are able to move an engine faster and with more power than other fuel types. Depending on the number of cylinders an engine has can determine whether of not the engine is balanced. What does this mean? Well, and engine with an odd number of cylinders, such as a 3 cylinder engine is inherently going to be unblanced, as there will always be one more piston in one position than the other. Because of this, most engines are designed with an even number of cylinders such as the inline 4, inline 6, flat 6, V6, V8, etc. engines. This allows an even number of pistons to fire in the same direction every time, balancing the engine and allowing for a more practical use, especially when the engine is in a car and you don't want a shaky ride, an engine with an even number of cylinders will combat the inherent imbalance of  an engine with an odd number of cylinders. This awesome video shows the internal workings of an actual engine in slow mostion, so you can see what actually goes on inside of an engine.