After the diesel engine is pressurized, the fuel supply of the fuel injection pump is increased, which makes it easy to generate black smoke during low speed, large load or accelerated working conditions. When operating under low speed, high load conditions, the exhaust turbine operates at low engine exhaust energy, and the compressor operates in a low efficiency zone, resulting in insufficient air supply, causing the exhaust to emit black smoke. When the load suddenly increases and the fuel supply suddenly increases, the supercharger speed cannot rise immediately, so that the amount of air entering the cylinder cannot keep up with the rapid increase of the fuel quantity, resulting in incomplete combustion and black smoke. To this end, the earlier-produced Cummins supercharged diesel engine also installed a vacuum air-fuel ratio controller (smoke limiter) on the PT pump, which can change the fuel entering the cylinder as the amount of air entering the cylinder changes. The amount of fuel is bypassed and the excess fuel supplied to the injector is bypassed to return it to the fuel tank, so that the air-fuel ratio is well controlled to match the amount of intake air to achieve fuel consumption and emissions.
The Cummins supercharged diesel engine produced in recent years uses a new air-fuel ratio controller. It can supply oil at any time according to the amount of air entering the cylinder, thus replacing the vacuum-type air-fuel ratio controller used in the early stage to limit the exhaust by means of fuel cut-off and residual oil splitting.
The air-fuel ratio controller is installed between the throttle valve and the fuel cut-off valve in the PT pump (as shown in Figure 5-84). In the PTG-AFC fuel pump, after the fuel leaves the throttle valve, it passes through the AFC unit and then reaches the shut-off valve at the top of the pump body. In the PTG fuel pump, fuel flows directly from the throttle valve through a passage to the shut-off valve.
The structure and working principle of AFC are shown in Figure 5-85. The fuel enters the AFC after it exits the governor and passes through the throttle. When the air pressure supplied by the turbocharger is not applied, the plunger 13 is in the upper end position, so that the plunger closes the main fuel circulation circuit, and the second passage of the position control of the regulating valve 6 without the inflation is supplied with fuel, such as Figure 5-85 (a). The regulator valve is installed directly on the throttle shaft in the throttle cover.
As the intake manifold pressure increases or decreases, the AFC plunger acts to proportionally increase or decrease the fuel supplied. When the pressure increases, the plunger descends, the gap between the plunger and the plunger increases, and the fuel flow increases, as shown in Figure 5-85(b). Conversely, when the pressure is reduced, the plunger gap is small and the fuel flow rate is reduced. This prevents the fuel-air mixture from becoming too rich and causing the exhaust to excessively emit black smoke. The position of the AFC plunger is determined by the interaction of the intake manifold air pressure acting on the piston and diaphragm with the proportionally moving spring.