The fuel gallery is charged with low-pressure fuel (15 to 75 psi). This permits fuel to flow into and through the barrel ports when the plunger does not obstruct them. The plunger reciprocates within the barrel; it is loaded by spring pressure to ride its actuating cam profile. Therefore, the actual plunger stroke is constant.
The Bosch P7100 injection pumps use a port-helix fuel metering system. This term describes its scroll-shaped (helix) fuel-metering method. The plunger is milled with a vertical slot, or cross and center-drilled with helical recesses. The function of the vertical slot or cross and center drillings is to maintain a constant conduit between the pumping chamber above the plunger and the helical recesses. Thus, whatever pressures exist in the pumping chamber must also exist in the helical recesses.
The fuel delivery stroke begins when the plunger is forced upward by the cam profile and the plunger's leading edge traps off the spill port. As the pressure rises in the pump chamber, it acts first on a delivery valve and next on the fuel confined in the high-pressure injector line transmitting diesel to the injector nozzle, and finally delivers a fuel pulse to the cylinder. This happens at the precise moment that upward travel of the plunger exposes the helical recess to the spill port. High-pressure fuel is spilled back to the charging gallery, causing a rapid collapse of pressure in the pump chamber, line, and nozzle. The injection pulse ceases when there is no longer sufficient pressure to hold open the delivery and nozzle valves. Port opening always occurs while the plunger is moving upward, that is, not at plunger TDC or beyond. This is required because the pressure in a port-helix pump element is designed to rise through the delivery stroke, thereby producing smaller atomized droplets from the injector toward the end of the effective stroke.
Timing Is Everything
Fuel delivery timing is critical during all engine operating conditions and speeds. In a diesel engine, fuel is injected into the cylinder slightly before the piston completes its compression stroke. The objective of the injection pump is to match the operating conditions with the proper amount of fuel to create the desired air/fuel ratio.
The goal is to smoothly and evenly transmit force developed in the cylinders to the flywheel or torque converter. The job of the injection pump is to manage the cylinder pressure (by injecting fuel) and cause it to peak when the crankshaft throw angle offers little mechanical advantage, and to diminish as the crankshaft throw is driven to a 90-degree angle with the connecting rod when mechanical advantage is highest. Most diesel engines are engineered to attempt to produce peak cylinder pressure 10 to 20 crankshaft rotation degrees after TDC.
Fuel delivery timing is extremely critical in a diesel engine and impacts power, fuel economy, and emissions. If the injection timing is overadvanced, a loss of power will be experienced, along with possible internal engine failure such as piston damage. Excessive gray/black smoke can be a sign of an overly advanced pump setting. If the injection timing is retarded (too late), a loss of power will result along with excessive smoke, high exhaust gas temperature (EGT), and if extremely late, severe white smoke.
It turns out that many of the symptoms of incorrect pump timing are shared by timing the injection on either side of the optimum specification. For this reason, it may be very difficult to determine if the timing is advanced or retarded only by examining the exhaust color or the engine's power output.