Chapter 4 Principles of Engine Operation 79 Copyright Goodheart-Willcox Co., Inc. mix and distribute the injected fuel inside the chamber. There are two types of combustion chamber designs: direct injection and indirect injection. Direct Injection Chamber Modern high-speed diesel engines found in commer- cial trucks and passenger cars use the direct injection combustion chamber design. In this design, the fuel is injected directly into an open combustion chamber formed by a recess in the crown or top of the piston and the under- side of the cylinder head fire deck, Figure 4-38A. The fuel injection nozzle is located in the cylinder head and extends directly into the cylinder. The main advantages of the direct injection design are its high fuel efficiency and simplicity. In a direct combustion chamber, the fuel must atomize, heat, vaporize, and mix with the combustion air in a very short period of time. Fuel and air delivery to the chamber must be precisely controlled. During the intake and com- pression strokes, the shape of the cylinder head intake port creates an air vortex inside the cylinder. The shape of the combustion chamber also affects the airflow pattern at the end of the compression stroke when fuel injection occurs. Direct fuel injection systems operate at very high injection pressures (up to 30,000 psi). The position of the multiple-hole nozzle in the chamber is very important, particularly in direct injection systems that do not generate airflow patterns that assist in mixing the fuel/air charge. Indirect Injection Chamber For many years, indirect combustion chambers were used in most passenger cars and light truck applications. When compared to older direct injection systems, indirect injection engines operated more quietly and produced lower exhaust emissions. Today, the high injection pres- sures and precise electronic control available in direct injection systems have changed this. Indirect injection systems can be found on many existing engines but are no longer being considered for most new engine designs. Indirect injection systems use divided combustion cham- bers. Precombustion, swirl, and energy cell are the three major types. Precombustion Chamber In the precombustion chamber design, a separate precombustion chamber is located in the cylinder head or wall, Figure 4-38B. The volume of this chamber is usually 25%–40% of the combustion chamber’s top dead center volume. The precombustion chamber is connected to the main chamber above the piston by one or more passages. As air is compressed during the compression stroke, it pro- duces a large amount of turbulence while passing through the narrow passages into the precombustion chamber. When fuel is injected into the precombustion chamber, it partially burns, building up pressure. This pressure forces the mixture back through the passageway and into the cyl- inder (main combustion chamber), where complete com- bustion is ensured. Combustion is very smooth and quiet in these systems. Figure 4-38. Diesel engine combustion chamber designs. A—Direct injection is seeing more widespread use. B—The precombustion chamber design had been used for many years and was a popular design. C—Swirl chambers look like the precombustion chamber design, but contain more TDC volume than the precombustion chamber. Fuel injector Valve Quench area Combustion area Turbulence Piston A B C Valve Fuel injection nozzle Fuel spray Precombustion chamber ignites air and fuel in the main combustion chamber Main combustion chamber Combustion chamber Turbulence chamber Piston Cylinder block Intake valve Fuel injection nozzle Piston Cylinder block