DIESEL INJECTION SYSTEMS

There are basically three general systems of mechanical fuel injection: the constant pressure or common rail system, the spring pressure or accumulator type, and the jerk pump.
In the common rail system fuel at a constant pressure is maintained in a manifold connected to either cam actuated nozzles or with a timing and distributor valve and pressure operated nozzles. This pressure usually from 4000 to 8000 psi, is obtained by making the fuel manifold large and utilizing the compressibility of the fuel oil, using a pump of excess capacity and delivering fuel between each injection,....
and by passing fuel from the accumulator through a manually or governor controlled pressure regulating valve. The amount of fuel delivered per injection is controlled by injection pressure, total nozzle orifice area, and time that the nozzle valve is lifted.
In order to keep the fuel quantity injected independent of pump speed a accumulator or spring injection was developed. The basic system used upper and lower plungers in a common bore, the lower plunger was driven by an eccentric cam and the upper plunger was spring loaded. As the bottom plunger is forced up the fuel between the plungers is pressurized based on the spring force applied to the top spring. Fuel continues to pressurize until a delivery groove in the lower plunger indexes with the outlet passage. This pressurized fuel is then injected and continues until the upper spring forces the plunger downward and closes the outlet passage.
The injection pump in the jerk pump system is used to time, meter and pressurize the fuel. This is the most common and utilized system. The plungers are driven by a camshaft that is designed to control the injection characteristics of the engine. The spray duration in crank degrees still increases with speed and fuel quantity but not to the extent of the common rail system therefore the jerk system can be used on low, medium, and high speed engines.
The jerk pump system lead to the further development of distributor style pumps, unit injectors, the "PT" fuel system, and dual fuel pumps. New systems continue to be developed. Utilization of electronics in the fuel delivery system is getting more common. Some fuel injection manufactures are developing ways for their injection pumps to charge and discharge electronically in order to keep up with current standards for the diesel engine. New systems such as the HEUI (Hydraulically actuated, Electronically controlled, Unit Injector) are currently being used on several applications in all areas especially automotive. The HEUI System develops injection pressures as high as 18-24,000 psi by applying high pressure oil to the top of an intensifier piston. Since the area of the head of this piston is 7 times the area of it’s plunger a 7:1 pressure increase on the fuel beneath the plunger is achieved. By varying the oil pressure, injection rate can be controlled independently of the crank or cam. Thus injection timing, rates, and pressures are no longer dependent on camshaft position, speed or cam ramp velocity. This is all controlled by a solenoid actuated valve that determines when high pressure oil is applied to the piston.