In both gasoline and diesel engines, the fuel and air must be properly mixed to obtain efficient combustion. In gasoline engines, mixing of the fuel and air takes place outside the cylinder. Depending upon the design of the system, mixing will occur in one of two places: (1) within the carburetor in the carburator-type system or (2) at the intake ports in the fuel injection-type systems. In both designs the proper mixture is forced into the cylinder to be compressed. In the diesel engine, however, fuel in the form of small particles is sprayed into the cylinder after the air has been compressed, thus mixing takes place within the cylinder. If each particle of fuel is to be surrounded by sufficient air to burn it completely (that is, if proper air-fuel mixture is to be obtained), the air in the combustion space must be in motion. This air motion is called TURBULENCE.

Various means are used to create turbulence. Design of engine equipment and parts and, in some engines, a process called precombustion enter into the creation of proper turbulence within the cylinder of an engine.

METHODS OF CREATING TURBULENCE

Fuel is distributed in the cylinders of a diesel engine by injection nozzles, which atomize the fuel and direct it to the desired portions of the combustion space. FUEL INJECTION creates some turbulence, but not enough for efficient combustion.

In 2-stroke cycle engines, scavenging-air PORTS are designed and located so that the in-take air enters the cylinder with a whirling or circular movement. The movement of the air continues through the compression event and aids in mixing the air and fuel when injection occurs. While fuel injection and the ports in 2-stroke cycle engines aid in creating air movement, additional turbulence is created in most engines by special shapes in the COMBUSTION SPACE. These shapes may include the piston crown and that portion of the cylinder head that forms part of the main combustion space. In some engines, auxiliary combustion chambers are provided as part of the combustion space to aid in mixing the fuel and air.

Even though there are many types of combustion chambers, all are designed to produce one effect-to bring sufficient air in contact with the injected fuel particles to provide complete combustion at a constant rate. Combustion chambers may be broadly classified under four types: open, precombustion, turbulence, and divided chamber. The last three terms are more commonly used to identify auxiliary combustion chambers; all are associated with the process called PRECOMBUSTION.

Of the three types of chambers, the open combustion chamber is the simplest in design. The fuel is injected directly into the top of the combustion space. The piston crown and (in some designs) the cylinder head are shaped to cause a swirling motion of the air as the piston moves toward TDC during the compression event. There are no special chambers to aid in creating tur-bulence. Open combustion chambers require higher injection pressures and a greater degree of atomization than other types to obtain the same degree of turbulence and mixing.