Fuel Supply System
Fuel Delivery System:
The fuel delivery system (illustrated above) have been used for all AJ6 4.0 litre installations.
A recirculating fuel system provides a continuous supply of pressurized, cooled fuel to the fuel rail. There is sufficient fuel flow to allow full load engine operation at all times.
Purpose and Operation
An electrically driven Fuel Pump Module (FPM) mounted in the fuel tank delivers fuel to the solenoid operated FI via the fuel rail, the fuel pressure is controlled by a fuel pressure regulator.
Actual fuel pressure however, varies between 2.4 to 3.0 bar depending on the inlet manifold vacuum acting on the fuel pressure regulator diaphragm at the time. The differential pressure across the FI nozzle therefore remains constant and the quantity of fuel injected for a given duration of injector ON (open) time is thus maintained constant.
The FI is a solenoid valve operated by an electrical pulse. The duration of the pulse, and hence the quantity of fuel injected, is determined by the PCME on a basis of intake air flow (engine load) and engine speed, information derived from the MAFS and CKPS.
This information is used to access a mapped look-up table, containing injector pulse durations pertaining to sixteen engine loads at sixteen different speeds. Between load and speed sites, the values in surrounding sites are used to calculate the correct pulse duration.
Correction factors are imposed on the basic injector pulse to compensate for varying conditions. The resultant pulses are then normally applied to the FI twice per engine cycle, i.e. once per crankshaft revolution, with only half the required amount of fuel being injected at each injector ON time.
FI pulse adjustments are necessary to provide:
- Cranking enrichment during starting.
- After-start enrichment during warm-up.
- Temperature enrichment during starting and warm-up.
- Demand corrections during idle, full power and acceleration.
- Voltage corrections for variations in the electrical system voltage.
Air Fuel Ratio (AFR) corrections for emission control as follows:
Cranking enrichment:
Provided appropriate to the engine temperature when the starter motor is activated. This is achieved by increasing the injector operating frequency from one pulse to three pulses per crankshaft revolution and is implemented by the PCME in response to the speed signal from the CKPS. Cranking enrichment terminates at an engine speed of 250 RPM above a coolant temperature of 42°C (107°F) and at 500 RPM below a coolant temperature of 42°C (107°F).
After-start enrichment:
Provided appropriate to the engine temperature. The PCME increases the injector ON time above the fully warm requirement and decreases the amount of additional fuel supplied incrementally with engine revolutions.
Temperature enrichment:
Provided during starting and warm-up. This is achieved by increasing the injector ON time above the fully warm requirement and is implemented by the PCME in response to an input from the ECTS.
Demand corrections:
Provided for idle, acceleration (rate of throttle movement) and full power demands. This is achieved by increasing the injector ON time above that of basic requirements and is implemented by the PCME in response to an input from the TPS.
Voltage corrections:
PCME monitors the electrical system voltage, i.e. the battery voltage. The time taken for an injector to open is affected by supply voltage and results in a corresponding change in the quantity of fuel delivered. The PCME compensates for battery voltage by adjusting the injector ON time accordingly.
Air Fuel Ratio corrections:
On catalyst equipped vehicles exhaust pollution is reduced to a minimum by monitoring the oxygen content of the exhaust and correcting the air/fuel mixture to maintain an intake ratio of approximately 14.7:1. This is achieved by the PCME in response to an input from the HO2S mounted in the exhaust downpipe.
On USA specification vehicles additional air is also introduced into the exhaust system by a relay activated AIRP and Secondary Air Injection Solenoid Vacuum Valve (AIRP). The AIR is controlled by the PCME for a variable time period depending upon the engine starting temperature. e.g. cold engine, a few minutes; hot engine, a few seconds.