Diesel fuel’s incompressibility necessitates a positive displacement pump, specifically a reciprocating pump, to achieve the high injection pressures required by diesel engines. This pump delivers fuel to the injectors, which act as pressure relief valves with a pre-set pressure.
Each injector is fed by a steel line connected to a piston and cylinder within the pump. A mechanism within this system controls the piston stroke, determining when the pump transitions from fuel recirculation to positive displacement. Initially, the piston stroke recirculates fuel at low pressure back to the filter and tank. Once the designated point is reached, the pressure in the line increases rapidly, forcing the injector (pressure relief valve) open. The remaining piston stroke delivers a precise amount of fuel into the combustion chamber.
The driver controls this transition from recirculation to injection via the accelerator pedal. At idle, most of the piston stroke is dedicated to recirculation. When the accelerator is fully depressed, the majority of the stroke is in positive displacement mode, maximizing fuel delivery.
This system’s design makes it highly sensitive to air. Since air is compressible, its presence prevents the pump from generating the necessary pressure to open the injectors. If air enters the system, it must be purged slowly through the low-pressure recirculation lines. Older Mercedes models featured a manual hand pump for this purpose. Modern vehicles typically rely on extended cranking to purge air, utilizing the injection pump itself. While not ideal for the starter, battery, and glow plugs, occasional use shouldn’t cause significant damage. Consistent air intrusion and extended cranking, however, could lead to premature wear.
Therefore, a normally starting and idling engine likely indicates a healthy injection pump. Unlike centrifugal pumps, positive displacement pumps are less prone to cavitation (vaporization) due to their high output pressure exceeding the fuel’s vapor pressure. Cavitation can cause damage in centrifugal pumps, but is less of a concern in diesel injection systems.
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A Diesel Injector Cross Section Leak Off test is crucial for diagnosing potential issues within the fuel injection system. This test measures the amount of fuel that leaks back past the injector needles, indicating their sealing performance. Excessive leak off can lead to various problems, including hard starting, rough idling, reduced power, and increased fuel consumption. Understanding the mechanics of the injection pump helps contextualize the importance of regular maintenance and troubleshooting, including the diesel injector cross section leak off test, to ensure optimal engine performance.
