Basic working principle and composition of diesel generator PT fuel system
The PT fuel system is a patented product of Cummins Engine Company of the United State. Compared with the fuel system of a typical diesel engine, the PT fuel system has its own unique features in composition, structure and working principle.
The basic working principle of PT system
The PT fuel system changes the fuel injection by changing the fuel pump’s delivery pressure and the inejctor’s fuel injection time. Therefore , it is named PT fuel system or pressure-time system.
according to the hydraulic principle, the flow rate of the liquid flowing through the orifice is proportional to the pressure of the liquid, the time of circulation, and the cross-sectional area of the passage. The PT fuel system changes the amount of fuel injected according to this principle. There is a measuring hole at the fuel inlet of the injector of the system, and its size cannot be changed after being selected. The time that the fuel flows through the orifice is mainly related to the speed of the diesel engine and varies with the speed of the engine. Changing the fuel injection amount is mainly achieved by changing the fuel inlet pressure of the injector.
Composition of the PT fuel system
the composition of the PT fuel system is shown below. Among them, the gear type oil pump, the voltage regulator, the diesel filter, the oil shut-off valve, the throttle valve and the governor are integrated, and the combination is called a PT fuel pump. Generally, only the PTG two-pole governor is installed in the automobile, and the mechanical variable speed full-speed governor (MVS) and the variable speed full program are installed on the construction machinery (such as the generator set) or the vehicle with frequent load change. Governor (VS) or dedicated full speed governor (SVS). when only the PTG two-pole governor is installed, the throttle valve is connected to the speed control handle, and the adjustment handle can rotate the throttle valve to change the cross-sectional area of the throttle valve. If the MVS, VS or SVS full-speed governor is installed, the throttle valve will remain in the fully open position, and the MVS, VS or SVS governor will be regulated in the range of speed at which the PTG governor does not function.
When the engine is working, the diesel oil is sucked out of the diesel tank by the gear type oil pump, and the impurities in the fuel are filtered out by the diesel filter 2, and then the pulsation of the fuel pressure is eliminated by the regulator, and then sent to the diesel filter 5. The filtered diesel is divided into two ways, one enters the PTG two-pole governor and the throttle valve, and the other enters the MVS (VS, SVS) full-program governor. The pressure is adjusted by the governor and the throttle valve. The fuel injector s supplied through the oil out valve. After the diesel fuel is metered and pressurized in the injector, it is periodically injected into the cylinder. Excess diesel is returned to the diesel tank via the return line. The fuel injector drive mechanism includes an injection cam, a swing arm, a injector rocker arm, and a fuel injector push rod. The injection cam is coaxial with the valve of the valve. The electromagnetic shut-off valve is used to cut off the fuel supply and stop the diesel engine.
Main feature of Cummins generator set PT fuel system
Compared to conventional plunger fuel system, Cummins generator set PT fuel system has the following advantages:
1. In the piston pump fuel system, diesel high pressure, timed injection and oil quantity adjustment are all carried out in the fuel injection pump, in the PT fuel system, only the oil quantity adjustment is carried out in the PT pump, while the diesel produces high pressure and tuned injection. It is then completed by the PT injector and its drive mechanism. There is also no need to adjust the injection timing when installing the PT pump.
2. The PT pump operates at a lower pressure, with an outlet pressure of approximately 0.8 to 1.2 MPA, and the high pressure fuel line is eliminated, and there are no various failures due to pressure fluctuations in the high pressure system of the plunger pump. In this way, the PT fuel system can achieve high injection pressures, which improves spray quality and high speed. In addition, the drawbacks of high-voltage loopholes are basically avoided.
3. In the piston pump fuel system, almost all of the diesel fuel sent from the fuel injection pump to the injector in the form of high pressure is injected, only a small amount of diesel oil leaks from the injector, and in the PT fuel supply system, the fuel is injected from the PT injector. Diesel only accounts for about 20% of the PT pump’s oil supply. The vast majority (about 80%) of the diesel oil is recirculated by the PT injector. This part of the diesel fuel can cool and lubricate the PT fuel injection, and it may exist in the oil circuit. Take away the bubbles. The returning fuel can also bring the heat in the injector directly back to the float tank, which can act to heat the fuel in the tank when the temperature is relatively low.
4. Since the PT pump governor and the oil supply are all adjusted by the oil pressure, the wear amount can be automatically compensated by reducing the amount of bypass oil to a certain extent, so that the oil supply of the PT pump is not reduced, thereby reducing maintenance. The number of times.
5. In the PT fuel system, the fuel supply of all PT injectors is completed by a PT pump, and the PT injectors can be replaced separately, without the need to adjust the oil supply uniformity on the test bench like a plunger pump.
6. The PT fuel system is compact in structure and simple in piping arrangement. Only one precision coupler in the injector is used in the whole system, which is much smaller than the piston pump fuel system. This advantage is more obvious in diesel engines with a larger number of cylinders.
Compared with the traditional plunger fuel system, Cummins generator set PT fuel system also has short comings:
1. The PT fuel system is equipped with a PTG governor and an MVS governor (or VS governor). the turbocharged diesel engine is also equipped with an AC controller, which is structurally complicated.
2. The PT injector is adjusted by the torque method. If the adjustment is improper, the fuel atomization mat be poor, the exhaust gas may be black smoke, the power may drop, and sometimes the needle valve may cause the injector head to be broken, causing the injector nozzle to fall off.
3. The PT fuel pump and the PT injector need to be commissioned on their own dedicated test bench before they can be installed, which is also troublesome to assemble.