Why is nearly every car powered by gasoline? Because gasoline has an extremely high energy density, because it is cheap and because it is easy and relatively safe to move around. In other words about 1,000 pounds of lead-acid batteries to store the same amount of energy that one gallon of gas contains. It will take you several hours to recharge the batteries but it takes about 15 seconds to pump one gallon of gas. This is why you don't see very many electric cars on the road.
Therefore, the purpose of a gasoline engine is to convert gas into motion so that the car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside the engine. Therefore, a car engine is generally referred to as an internal combustion engine. There is a such thing as an external combustion engine, which I will explain later in the project. All cars used internal combustion engines because of their advantages.
The closet model of a internal combustion engine, without actually being the engine is a potato cannon. This is where you take a big piece of plastic sewer pipe about 3 inches in diameter, and about 3 feet long, but a cap with a drilled hole on one end, put a spark plug(or something that creates a spark), spray some WD-40 into the piece, and put a potato in it, and you have created a potato cannon. This cannon can launch a potato about 700 feet. The potato cannon uses the basic principle behind any internal combustion engine: If you put a tiny amount of high-energy fuel (gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. If this cycle is repeating constantly it sets off tiny explosions hundreds of times per minute, and then you can harness that energy in a useful way, what you have is the core of a car engine.
Almost all cars currently use the four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also called the Otto cycle, in honor of Nikolaus Otto who invented it in 1897. The four strokes are known as:
I do not have an illustration to show this, so you will just have to imagine it. A device called a piston replaces the potato in the potato cannon. The piston is connected to the crank shaft by a connecting rod. As the crankshaft revolves, it has the effect of resetting the cannon. So the piston starts at the top, the intake valve opens and the piston moves down to let the engine take in a cylinder full of air and gasoline during the intake stroke. Only the smallest drop of gasoline needs to be mixed into the air for this to work. Then the piston moves back up to compress this fuel-air mixture. Compression makes the explosion more powerful.. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. Once the piston hits the bottom of its stroke the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe. Now the engine is ready for the next cycle, so it intakes another charge of air and gas, and repeats itself.
Note that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear. In an engine the linear motion is converted into rotational motion by the crank shaft.
Now go to the parts page to see how all the parts of a car work together to create motion.