Cummins PT Injection System
Below figure is an overview of the Cummins PT system. This is a mechanical fuel injection system which evolved into the electronically controlled HPI fuel injection system.
While the plunger is raised (position shown in above figure), fuel from the rail enters the injector through Orifice A and proceeds to the base of the plunger. Most of this fuel is returned to the tank through drain Orifice B. Fuel returned to the tank represents about 80 percent of the fuel introduced into the injector through Orifice A.
The main purpose for pumping this large amount of fuel and letting it flow through the injector is to cool it. The remaining portion of the fuel entering through Orifice A is then metered through metering Orifice C into the injector cavity D. The pressure is controlled according to the engine speed and load requirements to correctly meter the amount of fuel entering cavity D through metering orifice C.
The metered quantity in cavity D is then injected into the cylinder at the right time through the action of a large cam that pushes the plunger down, thus forcing the fuel through the small nozzle holes for a fine spray (mist) into the combustion chamber. The entire system operates at relatively low pressure except for the high pressure that is generated exclusively within the injector sac when the plunger is pushed down.
