Cars are automobiles that can transport people. It is the main means of travelling for hundreds of millions of people all over the world. Cars have changed the way we live probably more than any other invention in history. At first only a few people had cars but after a while more and more people bought them because they improved the way people lived. Farmers with cars were able to bring their products to places that were farther away. The appearance of cities and towns also changed. More and more workers drove to their jobs and people started to move to suburbs outside the town centers. Automobiles give people many jobs. Millions of people around the world work in factories where cars are produced. Millions more work at gas stations, restaurants or motels that travelers stop at. However, cars also cause problems. Millions of people die in car accidents every year. Automobiles pollute the air that we breathe and parking space in cities is scarce because everyone wants to use their cars to get to city centers. Cars are very complicated machines and all systems in them work together. They power a car, control and steer it and make it comfortable for people to drive in. The heart of every car is its engine. It produces the power that turns the wheels and electricity for lights and other systems. Most automobiles are powered by an internal combustion engine. Fuel, usually gasoline or petrol, is burned with air to create gases that expand. A spark plug creates a spark that ignites the gas and makes it burn. This energy moves through cylinders in which pistons slide up and down. They are attached to rods that move a crankshaft. Normal car engines have four to six cylinders but there are also models with eight and sixteen cylinders. The turning movement is passed through the drivetrain to the drive wheels. The fuel system pumps petrol from the tank to the engine. Older cars used to have carburetors that mix fuel with air and send the gas to the engine. Some cars have a special fuel injection system that sprays petrol into the engine. Modern cars have turbo chargers that suck in extra air and therefore create more power. The engine and all parts that carry power to the wheels are called the drivetrain. It includes the transmission, drive shaft, differential, the axles and the drive wheels that move the car. While most cars have drive wheels in the front, some have them in the back. Cars that need to drive over all kinds of ground have a four-wheel drive. The transmission controls the speed and torque. When a car travels at a normal speed on a flat road it does not need so much torque to keep it moving, but when you want to start a car from a hill the engine must produce more power. Gears control speed and power of the engine in different driving conditions. In cars with manual transmission you have to change gears by pressing down the clutch with your foot and moving a lever. Cars with automatic transmission change gears without control by the driver. Lower gears give the car more torque and speed. When the car moves faster the transmission shifts to higher gears. The driveshaft carries the power to the axle which is connected to the wheels. It has several joints which make the axle and wheels moveable as the car drives on uneven and bumpy roads. The differential is connected to the rear end of the driveshaft. It lets the wheels turn at different speeds because in curves the outer wheels must travel a greater distance than the inner ones. Steering system of an automobileSteering system The steering system controls the front wheels. Turning the steering wheel makes them point to the left or right. Most cars have power steering; a hydraulic system makes it easier for the driver to turn the wheels.