Fuel Delivery and Air Induction: Description and Operation

FUEL SUPPLY
Fuel is supplied from fuel tank via two fuel pumps, to fuel pressure reservoir, through fuel filter to mixture control unit. Mixture control unit measures air flow and meters proper fuel mixture to individual cylinders. Excess fuel is returned to fuel tank.

The 911 Turbo is equipped with K-Jetronic Continuous Injection System (CIS) with oxygen sensor to provide air-fuel mixture.

A mechanical hydraulic injection system provides the basis for the Bosch K-Jetronic (CIS) fuel system. The mechanical portion of the CIS system consists of the mixture control unit (air flow meter and fuel distributor), primary pressure regulator, fuel accumulator, cold start valve, fuel injectors, pumps, filter, lines, tank and fuel cooler. In addition, electrical controls have been added for increased flexibility to meet todays more stringent performance and exhaust emission requirements. Refer to "COMPUTERIZED ENGINE CONTROLS" for a detailed description of electronic components.

Fuel Distributor With Oxygen Sensor:

To adapt fuel quantity injected to desired air-fuel ratio at lambda = 1, pressure in lower chambers of fuel distributor is varied. If pressure in lower chambers is reduced, differential pressure at metering ports is increased. This increases fuel quantity that is injected. To be able to vary pressure inside lower chambers, those chambers are separated from system pressure by a fixed restriction. Another restriction is provided to connect lower chambers to fuel return line. This restriction is variable. As long as restriction remains open, pressure inside lower chambers can drop. When restriction is closed, system pressure builds up inside lower chambers. If this restriction is opened and closed at a fast rate (pulsed), the pressure in lower chambers may be varied according to closing time vs. opening time ratio (pulse-duty factor). An electromagnetic valve (frequency valve) is used as a variable restriction device. It is controlled electrically by pulses emitted by oxygen sensor control unit.

AIR INDUCTION SYSTEMS

Exhaust System:

This model uses a turbocharger to enhance air flow through the engine. The turbocharger consists of two turbine wheels mounted on a common shaft and fitted inside a housing. One wheel is mounted in the exhaust system and is driven by exhaust gases flowing out of the engine.

As engine speed increases, exhaust flow also increases, causing the turbine wheel in the exhaust to spin faster. Because it is mounted to a common shaft, the turbine wheel in the intake side of the turbocharger also spins faster and will begin to compress the intake air, creating a positive pressure inside the intake manifold. This increases the volumetric efficiency in the cylinders and increases the engine power output.

The positive pressure in the intake, called "Boost Pressure", is generally limited to a specified amount. This is accomplished by use of a wastegate. This is a vacuum controlled valve, mounted in the exhaust stream. When specified boost is reached, the wastegate opens and excess pressure is diverted away from the turbocharger, into a separate exhaust pipe. This is done to prevent "Over-boosting" which can cause engine damage.