Analysis of the Dripping Phenomenon of PT Injectors in Cummins Engines

In the early 1980s, Shandong Bulldozer General Factory introduced the D85A-18 crawler bulldozer from Komatsu Corporation of Japan, which was equipped with a Cummins engine. The features of the Cummins engine PT fuel supply system: ① The fuel supply volume depends on the fuel pressure provided by the PT pump to the fuel injector and the absolute time it takes to open the fuel injector metering hole. ② The PT pump is a low-pressure pump, with an output pressure generally ranging from 0.8 to 1.2MPa. The pressure of the fuel injected by the fuel injector is between 68 and 136MPa. In the PT fuel system (Figure 1), the position of the fuel tank is higher than that of the PT pump and PT injector.

The structure of the 1PT fuel injector

The PT fuel injector is a mechanical open fuel injector. There are 7 or 8 spray holes evenly distributed on the nozzle of the fuel injector, with a diameter of approximately 0.2mm. The axis of the spray holes forms a 17° Angle with the plane perpendicular to the 17th axis of the fuel injector plunger.

Analysis of the Dripping Phenomenon of 2PT Fuel Injectors

According to the relevant data of the PT fuel system, the fuel metering stage of the PT injector starts when the piston moves to the position of 44° crankshaft Angle after the top dead center of the intake stroke, and ends when the piston moves upward to the position of 28° crankshaft Angle before the top dead center of the compression stroke. During the fuel metering stage of the injector, the fuel intake line is connected to the pressure chamber of the injector. It is then connected to the cylinder through 7 or 8 spray holes on the nozzle of the pressure chamber. The fuel injector structurally makes it possible for the fuel dripping phenomenon.

From the working process of the fuel injectors, it can be seen that when the engine stops, except for the PT fuel injectors of 2 or 3 cylinders which are in the fuel metering stage, the other PT stage fuel injectors can return fuel, that is, the fuel inlet and outlet ports of the fuel injectors are connected. Due to the fact that the PT fuel system uses a high-positioned fuel tank (the tank position is higher than that of the PT pump and PT injector), if the check valve 2 of the PT fuel system does not close tightly, the fuel in the tank will pass through the check valve, the return fuel line, and then through all PT injectors except those in the metering stage to enter the fuel line. Finally, the fuel is drip-into the cylinder through the fuel injector in the fuel metering stage, thus forming the fuel dripping process of the PT fuel injector. The fuel dripping oil circuit: fuel tank - check valve - fuel injector return oil circuit - fuel injector in the fuel injection and return stage - fuel injector intake oil circuit - fuel injector in the metering stage - cylinder.

3 Preventive Measures

We once encountered a problem where the Cummins engine of the D85A-18 bulldozer was difficult to start. After inspection, it was found that the engine circuit was normal and there was oil in the fuel line. When starting, it could be seen from the cooling fan blades that the crankshaft could rotate at a very small Angle and the reverse torque was very large (the moment the start button was released). We concluded that there was fuel dripping from the PT injector in two or three cylinders of the engine. We turned the crankshaft in the opposite direction twice and started successfully, but a considerable amount of fuel was sprayed out from the exhaust pipe, which proved that our judgment was correct.

Install a fuel switch under the fuel tank. When the machine is shut down, turn off the fuel switch under the fuel tank to prevent the source of fuel leakage. Such faults have never occurred again since then.