The internal combustion engines can be classified according to ignition modes, type of fuel, type of cooling, state of intake, number of strokes, number of cylinders and arrangements of cylinders.

Generally speaking, the engine is the source of power that makes the wheels go around and the car move. We can easily conclude the principle of the engine as follows: The burning of fuel inside the engine produces high pressure in the engine combustion chamber. This high pressure forces piston to move, the movement is carried by connecting rods to the engine crankshaft. The crankshaft is thus made to rotate; the rotary motion is carried through the power transmission to the car wheels so that they rotate and the car move. (See fig 9-1)

In fact, there are two mechanisms and five systems inside the common gasoline engines. (See fig 9-2)

The two mechanisms are: crank and connecting-rod mechanism and valve mechanism. The five systems are: fuel system, cooling system, lubricating system, starting system and ignition system. The followings are the functions of the five systems.

The function of the fuel system (see Fig9-3) is to supply the engine with a mixture of air and fuel. For an example, in the gasoline engine, the fuel system does this by pumping liquid gasoline from a tank into the carburetor, a mixing device that mixes the gasoline with air. The mixture is delivered to the engine where it is burned.

The purpose of the cooling system (see Fig 9-4) is to keep the engine at its most efficient temperature. The combustion of the air-fuel mixture in the engine creates a very high temperature(as high as 2000 to 2700℃). The cooling system takes heat away from the engine. It does this by two methods: water cooling type and air cooling type.

Why does the engine need a lubricating system? The engine needs the lubricating system (see fig9-5) to supply all moving parts inside the engine with lubricating oil. In fact, there is no metal to metal contact, the moving parts float on films of oil, thus eliminating excessive wearing.

The engine requires a fourth system, the starting system (see fig9-6). The engine can not self-start. So it needs outside means to start for itself. Its purpose is to change the electrical current into the mechanical energy to push the crankshaft around.

The fifth is ignition system (see fig 9-7) .It is a special system for gasoline engine. (There is no ignition system in diesel engine) Its function is to produce high voltage pulse and deliver them to the combustion chambers.