Fuel Delivery and Air Induction: Description and Operation

Fuel Control System (Turbocharged):

GENERAL DESCRIPTION
The fuel injection system delivers a precise amount of highly atomized fuel to the intake system to ensure optimum performance and emissions compliance.

The Fuel System consists of three major sub-systems:
- The fuel storage system,
- Fuel injectors with the supporting fuel injection control system, and
- The fuel delivery and return system.

FUEL STORAGE
The fuel storage system consists of a fuel tank, fill spout and filler cap. The filler caps primary function is as an evaporative emissions control device and is therefore covered in detail in Emission Control Systems. The remaining components are covered in this section by name.

FUEL INJECTION AND CONTROL
The Injectors are solenoid operated shutoff valves designed to deliver fuel in an appropriate spray pattern to promote total fuel atomization in the intake air stream

The Fuel Injection Control System consists of the Engine Control Module (ECM) and all of the engine and operating condition sensors. Using the information from the sensors to determine the proper fuel amount and delivery time, the ECM varies the injectors "ON" time to adjust the amount of fuel delivered during each individual firing cycle.

For a more detailed description of the fuel injection system operation, refer to Computers and Control Systems. For all Fuel System mechanical component descriptions, refer to this section by component name.

TURBOCHARGERS AND CONTROL
The turbochargers are used to increase engine output by providing a denser air/fuel charge to the engine than could normally be attained through natural aspiration.

Each turbocharger utilizes spent exhaust gas heat and pressure to drive a turbine rotor, which is connected to and drives an impeller wheel in the intake air piping. The impeller compresses the air in the intake manifold according to the amount of turbine revolutions. Only air is compressed, as the fuel is injected at the intake ports by the fuel injection system. However, when the fuel atomizes with the pressurized air, a larger volume "charge" can be forced into the cylinders. Because engine output depends on the amount of air/fuel mixture ignited per unit of time, supplying a larger mixture amount can substantially increase output, without an increase in engine speed.

The turbine speed increases with exhaust gas flow, (approx. equivalent to engine speed times throttle opening), causing intake manifold air pressure to increase, thus increasing engine output (performance). The turbocharger has an operational range of approximately 20,000 to 110,000 rpm. The Turbocharger turbine/impeller shaft rides on two full floating bearings, which are lubricated with crankcase oil under pressure from the engine oil pump. The housing also has coolant passages, which allows engine coolant to cool the turbo.

If Turbocharger pressure gets to high, predetonation (pinging) may occur, causing engine damage and poor performance. To limit turbo pressure, a Wastegate Valve opens in the exhaust passage, of each turbocharger, and bypasses exhaust flow around the turbine. This serves to slow the rotation of the turbine and therefore the impeller. The Wastegate Valve is opened by a pressure diaphragm that senses the pressure in the air intake, prior to the throttle valve. To insure that fuel injection can be metered accurately, the turbocharger is positioned in the piping after the Airflow Meter, therefore data sent to the ECM concerning intake air volume/temperature will not be affected by intake charge pressurization.

The turbochargers also utilize a pair of intercoolers (heat exchangers) designed to increase intake air density by cooling the intake air charge.

FUEL DELIVERY AND RETURN
The fuel delivery components consist of fuel supply lines, a high pressure fuel pump (mounted in the fuel tank), a fuel filter assembly, a fuel pressure accumulator and a fuel pressure regulator. The delivery systems function is to deliver fuel at a pressure and quantity high enough to allow the fuel regulator to maintain a constant fuel pressure in the fuel delivery rail (in reference to the intake manifold pressure) under all driving conditions. Additionally a check valve in the fuel pump holds pressure against the system for a period of time after engine shutdown to ensure adequate pressure is available during warm engine re-start conditions.

The Fuel return portion of the fuel system consist of lines and couplings and therefore is addressed, jointly with the fuel supply lines, under the heading of FUEL/VAPOR LINES. The function of the return line is to recover excess fuel vented by the fuel pressure regulator (mounted on the fuel rail) and return it to the fuel tank. The fuel return line is low pressure and returns directly to the fuel tank.

The Vapor recovery system also consists of lines and couplings addressed under the heading of FUEL/VAPOR LINES in this section. The function of the vapor lines are to provide a route for the recovery of fuel vapors (from the fuel tank) either for storage in the charcoal canister, or for evacuation through the purge control system. Additionally contained in the vapor recovery system are two components, an overfill limiter and a fuel check valve. These components primary functions are as evaporative emissions control devices and are therefore covered in detail in Emission Control Systems.