82 Diesel Engine Technology Copyright Goodheart-Willcox Co., Inc. Their operating concept resembles an ignition distributor used on gasoline engines. A rotating part, which is called a rotor, is located inside the pump and distributes fuel at a high pressure to the individual injectors in the proper firing order. Electronic controls have been integrated in modern distributor pump systems to improve performance, fuel effi- ciency and better control engine emissions. Pressure-Time Injection Systems The pressure-time injection system (PT system) derives its name from two of the primary factors that affect the amount of fuel injected per combustion cycle. “P” refers to the pressure of the fuel at the inlet of the injectors. This pressure is controlled by the system’s fuel pump. “T” refers to the time available for the fuel to flow through the injector’s inlet orifice and into the injector cup. The amount of time that the orifice remains open is controlled by the engine’s speed. This system uses a camshaft-actuated plunger, which changes the rotary motion of the camshaft into the recipro- cating motion of the injector. The plunger movement opens and closes the metering orifice in the injector barrel. When the orifice is open, fuel flows into the injector cup. Metering time is inversely proportional to engine speed. The faster the engine is turning, the less time there is for fuel to enter the injector cup. A third factor affecting fuel delivery amounts in the PT system is the size of the orifice opening. The size of the orifice opening is set through careful calibration of the entire set of injection nozzles. The pressure-time injection system is no longer in use on modern engines. However there are some electronic fuel injection systems that work off some of the same principles as the older mechanical PT systems. Unit Injector Systems In a unit injector fuel system, timing, atomization, metering, and fuel pressure generation all take place inside the injector body that serves a particular cylinder. This compact system delivers high fuel pressures and was the first system to incorporate electronic computer controls for fuel delivery. Fuel is drawn from the tank by a transfer pump and is filtered. The pressurized fuel (50–70 psi or 344.75–482.65 kPa) then enters a fuel inlet manifold cast within the engine’s cylinder head. This inlet manifold feeds all of the unit injec- tors through a fuel inlet or jumper line. The fuel enters the individual unit injectors, where it is pressurized, metered, and timed for proper injection into the combustion chamber. Unit injector systems use a push rod-actuated rocker arm assembly or a straight camshaft-operated rocker arm assembly to operate the injector plungers and develop the high pressures needed for injection. Mechanical unit injectors used in older engines incor- porated a control rack and linkage to control the metering and timing of the fuel injection. More modern engines use electronic unit injectors that rely on a solenoid to control the timing and duration of the fuel injection. Hydraulic Electronic Unit Injector Systems Hydraulic Electronic Unit Injector (HEUI) systems are a modern fuel injection system built upon the same principles as earlier unit injector systems. These systems have many common features with other electronic unit injector systems, including the use of a lower pressure fuel transfer pump (30–80 psi) and an electronic solenoid to control fuel delivery. The primary difference of the HEUI system is that it does not use a camshaft-operated rocker arm or any type of mechan- ical linkage to develop the high injection pressures needed. Instead, this system uses high-pressure engine oil supplied to the injector to control and amplify the fuel injection, achieving pressures in excess of 28,000 psi. (193,053 kPa). Common Rail Injection Systems Common rail injection is the newest high-pressure direct injection fuel delivery system. An advanced design fuel pump supplies fuel to a common rail that acts as a pressure accumulator. The common rail delivers fuel to the individual injectors via short high-pressure fuel lines. Based on a variety of engine operating conditions, the sys- tem’s electronic control unit precisely controls both the rail pressure and the timing and duration of fuel injection. Injector nozzles are operated by rapid-fire solenoid valves or piezoelectric-triggered actuators. High pressures, fast switching times, and a variable rate of fuel discharge make common rail injection the logical choice for current and future on- and off-highway diesel engines. Engine Performance Terms and Formulas The following sections define many of the key terms and formulas used to describe diesel engine operation and calculate engine performance characteristics. Bore and Stroke Bore is the diameter of the cylinder. Stroke is the dis- tance the piston travels between its top dead center and bottom dead center positions. See Figure 4-43. The stroke length is twice the center-to-center distance between the crankshaft’s main journals and the rod journals. Displacement Cylinder displacement is the total volume that is dis- placed in a cylinder as the piston moves from the bottom of its stroke to the top. Cylinder displacement is measured in cubic inches (cu. in.) or liters (l) and is calculated as follows: Cylinder displacement = π × r² × S, where π = 3.14 r = bore radius (1/2 bore diameter) S = length of stroke