Two Types of Fuel Pump (2)

In article “Two Types of Fuel Pump (1)”, we have mentioned two fuel pump types: plunger fuel pumps and PT fuel pumps. Here we will focus on PT fuel pumps.

PT fuel pumps

The PT fuel system is a unique fuel supply system of American Cummins Corporation. It works on the principle of hydraulic pressure. The first letter P stands for pressure, and the second letter T stands for the metering time for the PT injector to control the entry of fuel. The PT fuel system makes the fuel supply metering and timing injection meet the ideal working conditions of the engine through the coordination of pressure and time. PT fuel system is composed of PT fuel pump and PT injector, which has the advantages of simple structure, reliable use, convenient maintenance, small size and light weight. Taking the PT(G) VS AFC fuel pump as an example, the following passage is going to analyze the working process and the function of each device of it.

The working process

The PT fuel pump directly transports the fuel to the oil passage of the cylinder head, and each injector bears the oil pressure supplied by the PT fuel pump, and the fuel passes through the injector inlet filter, the measuring hole plug, the internal oil passage and the check ball valve, etc. until nozzle head. Since the movement of the plunger is driven by the engine itself, the time when the plunger rises and the fuel enters the nozzle head for metering varies with the speed of the engine. The high speed will lead to a short fuel-intake time, while the low-speed fuel-intake time will be long.

The cam has a precise shape, when it makes one revolution, the crankshaft makes two revolutions. The 4 strokes of intake, compression, work and exhaust of the piston in the cylinder just correspond to the 4 phases of the cam. At small radii, the plunger is at its highest position and the nozzle tip meters oil. When the cam is at a large radius, the plunger is at its lowest position. At this time, the high-temperature and high-pressure gas in the cylinder will not enter the fuel injector, and the fuel will continue to flow from the bypass oil passage to cool the fuel injector. The oil injection of the injector is when the cam rotates to a certain angle before the compression top dead center in the corresponding cylinder, which depends on the ejector rod and the corresponding position of the piston, which can be checked with special instruments. If it does not meet the requirements, the thickness of the gasket can be adjusted.

The function of each device

Firstly, we need to make it clear that the letter G in the PT(G) VS AFC fuel pump represents speed regulation (Govern), V represents Variable, S represents Speed, A stands for Air, F stands for Fuel, and C stands for Control. The oil supply pressure and flow rate of the gear pump itself are proportional to the speed of the pump itself, which is not in line with the ideal characteristics of the engine, so there is a flying block auxiliary plunger and spring, which function to increase the fuel pressure at low speed. The effect of the high-speed torque spring is to reduce the high-speed fuel pressure, which makes the characteristics of the engine more ideal. Another important function of the PTG governor is to control the minimum and maximum speed of the engine.