on a pressure-time principle. A gear-driven positive displacement low-pressure fuel pump naturally supplies fuel pressure. The time for metering is determined by the interval that the metering orifice in the injector remains open. This interval is established and controlled by the engine speed, which determines the rate of camshaft rotation and consequently the injector plunger movement.

Since Cummins engines are all four-cycle, the camshaft is driven from the crankshaft gear at one-half of engine speed. The fuel pump turns at engine speed. Because of this relationship, additional governing of fuel flow is necessary in the fuel pump.

A flyball type mechanical governor controls fuel pressure and engine torque throughout the entire operating range. It also controls the idling speed of the engine and prevents engine overspeeding in the high- speed range. The throttle shaft is simply a shaft with a hole; therefore, the alignment of this hole with the fuel passages determines pressure at the injectors.

A single low-pressure fuel line from the fuel pump serves all injectors; therefore, the pressure and the amount of metered fuel to each cylinder are equal.

The fuel-metering process in the IT fuel system has three main advantages:

1. The injector accomplishes all metering and injection functions.

2. The injector injects a finely atomized fuel spray into the combustion chamber at spray-in- pressures exceeding 20,000 psi.

3. A low-pressure common-rail system is used, with the pressure being developed in a gear-type pump. This eliminates the need for high- pressure fuel lines running from the fuel pump to each injector.

FUEL PUMP. - The fuel pump (fig. 5-29) commonly used in the pressure-time system is the PTG- AFC pump (PT pump with a governor and an air-fuel control attachment). The "P" in the name refers to the actual fuel pressure that is produced by the gear pump

Figure 5-29. - Pressure-time (PT) gear pump.