Computers and Control Systems: Description and Operation
"P" SYSTEM FUEL INJECTION 12 CYLINDERDESCRIPTION
The 'P' type electronic fuel injection system is an indirect injection system which incorporates solenoid operated low pressure injectors intermittently spraying fuel into the inlet port of each cylinder. A digital electronic control unit (ECU) with an integral manifold pressure sensor, governs the amount of fuel injected by controlling the duration of injection. The manifold pressure and speed signal, derived from ignition pulses, provide the main control for the fuel injected. Additional sensors are used to monitor engine temperature, inlet air temperature and throttle position, etc, thereby ensuring optimum fuelling for all engine operating conditions.
Fuel Metering
Fuel metering is obtained by controlling the length of time the injectors are held open during each engine cycle. The pulse duration is varied by the ECU (Fig 1) according to inputs from the engine and chassis mounted sensors.
The control parameters fall into two groups. The first consists of intake manifold pressure and engine speed.
Information on engine speed received by the ECU is obtained from the ignition coil negative terminal via a 6.8 ohm resistor located inside the AB14 ignition amplifier (Fig 2).
The intake manifold pressure is sensed by a transducer located inside the ECU and linked to the intake manifold by a pipe. The transducer is an integral part of the ECU and cannot be replaced without recalibrating the electronic circuits.
The fuelling information for the engine is stored in a preprogrammed memory so that for any combination of manifold pressure and engine speed, the memory gives out a number which is proportional to the amount of fuel required by the engine. The injectors will be energized for a time proportional to the number computed plus what is sensed by the secondary control parameters.
The second control parameters consist of engine coolant temperature, inlet air temperature, throttle movement, closed loop and battery voltage.
Air Temperature Sensor
The quantity of fuel supplied per cycle is adjusted according to the air intake temperature, so that it varies approximately with the density of the air aspirated. At low ambient temperatures, the density, and thus the weight of air drawn into the engine is higher, therefore the air/fuel ratio would become leaner. In order to compensate for this, an air intake temperature sensor is incorporated in the air intake system. The electrical resistance of the device alters with the changes of air temperature. This signal is monitored by the ECU and used to ensure the air/fuel ratio stays correct.
Battery Voltage
The electrical system voltages change with the battery state of charge, electrical load etc, and this in turn affects the amount of fuel delivered. The ECU compensates for this by constantly monitoring the system voltage and generating a correction factor to ensure the pulse sent to the injectors is independent of system voltage.
Coolant Temperature Sensor
The coolant temperature sensor located in the coolant thermostat housing controls the warm-up enrichment delivered by the ECU. In addition to the enrichment generated during warm-up, further enrichment is required during engine cranking. The cranking signal to the ECU is via a connection from the starter relay.
When the engine fires and the starter is released, the start enrichment decays slowly to the normal fuelling level temperature as determined by the engine coolant.
This over fuelling is known as the "after start enrichment". The "normal" fuelling level is determined by the engine coolant temperature, such that the fuelling level increases as the temperature decreases.
Throttle Potentiometer
To ensure the vehicle road performance is satisfactory, with good throttle response, acceleration enrichment is necessary. Signals are provided by the throttle potentiometer, which is mounted on the throttle spindle and indicates the throttle position to the ECU. When the throttle is opened the fuelling is enriched and when closed the fuelling is weakened. When the throttle is opened very quickly, all the injectors are simultaneously energized for one pulse.
This ensures that there is enough fuel available at the inlet ports for the air admitted by the sudden opening of the throttle. The duration of this extra pulse is controlled by the engine temperature signal, and is longer with a cold engine.
Lengthening the normal injection pulses is done in proportion to the rate at which the throttle is opened and it takes a short time to decay when the throttle movement stops. Enrichment in this way is also varied according to the engine temperature. The fuel cutoff function is controlled by the throttle potentiometer, and the conditions under which it occurs are programmed into the ECU memory.
Injectors
Each fuel injector (Fig 1) consists of a solenoid operated needle valve with the movable plunger rigidly attached to the nozzle needle. In the closed position, a helical compression spring holds the needle against the valve seat.
The injectors have a solenoid winding mounted in the rear section of the valve body, with a guide for the nozzle needle in the front section.
The injectors are operated in two stages, initially they are operated via a pull-in circuit, then when the injectors are open a change to a hold on circuit is made via current limiting resistors (Fig 2) for the remainder of the injection period as determined by the ECU in this way the heating effect on the output transistors of the ECU is reduced. It also ensures a rapid response from the injectors.
To open the injectors at the speeds required by the engine a fairly high current is needed. The ECU has an output stage to deliver this current, but to protect the output transistors of the ECU from injector faults a power resistor is wired in series with each three injectors. These resistors will limit current to a safe value, thus protecting the ECU. The power resistors (one for each group of injectors) are housed in a single unit to the right side of the engine valence by two screws.
The injectors are operated in two groups of six, which is further broken down into two sub-group of three, although each pair of sub-groups is operated simultaneously to make up the two groups.
A Bank B Bank
1A 3A 5A 1B 3B 5B
2A 4A 6A 2B 4B 6B
Auxiliary Air Valve
In order to maintain idling speed during cold start and warm up, more air than that supplied by the normal idle bypass is required.
This extra air is supplied by the auxiliary air valve which is temperature sensitive so that the idle speed can be controlled throughout the warm-up phase.The auxiliary air valve consists of a variable orifice, controlled by an expansion element. It is mounted on the engine cylinder head where it is responsive to coolant temperatures. By adjusting the profile of the variable orifice according to coolant temperature the engine idle speed can be controlled.
Supplementary Air Valve
An additional throttle bypass device is used to supplement the air necessary to support the engine idle speed when the air conditioning system is in use. This device is a solenoid operated valve which supplies supplementary air to the engine
Lambda Sensor (Where fitted)
The lambda (oxygen) sensor measures the oxygen concentration in the exhaust system. Excessive oxygen over a certain proportion indicates a weak mixture. Where as insufficient oxygen indicates a rich mixture. A signal is fed to the ECU to compensate for these variations by revising the applied injector pulse.
A service interval counter in conjunction with a warning light indicate when the lambda sensors must be renewed.
Fuel Pump
The fuel pump has a roller type cell driven by a permanent magnet electric motor. A small rotor is mounted on a armature shaft and rotates in an eccentric housing.
The rotor contains five metal rollers each one housed in a cutaway on the circumference of the rotor. When the armature rotates centrifugal force acting on the rollers forces them against the metal housing forming a seal. The trapped fuel is then forced to the pressure side of the system.
The pump is designed as a so called 'wet pump' which means the motor is full of fuel. However, the motor never contains an ignitable mixture, even when the tank empties.
The pump is controlled by the ECU via a relay and is energized for a short period when the ignition is switched on in order to pressurize the fuel rail. The pump will not operate again until a signal is received by the ECU from the starter relay or when the engine is running and the ECU receives speed signals from the ignition HT coil.
For safety an inertia switch is fitted to switch off the entire fuel system.
Fuel Pressure Regulator
The fuel pressure regulator is attached to the main fuel rail and consists of a metal housing containing a spring loaded diaphragm. When the pressure setting of the regulator is exceeded, the diaphragm moves, exposing an opening to an overflow duct which allows excess fuel to return to the fuel tank, causing a drop in fuel pressure. The reduced fuel pressure allows the diaphragm to move back to its original position, thereby closing the fuel return outlet. This sequence of events is repeated as long as the pump is running. In this way, fuel pressure is held constant as fuel demand varies. The pressure setting is adjusted to the correct value during production when the outer spring housing is compressed until the correct spring load is obtained. This is not adjustable in service.
The spring housing of the regulator is sealed and connected to the engine inlet manifold by a small bore pipe. By allowing it to sense inlet manifold depression, the pressure drop across the injector nozzle remains constant because the fuel pressure will alter as manifold depression alters. This arrangement ensures that the amount of fuel injected is only dependent on the the duration of injector open time.
System fuel pressure is 3 bar (43.5 lb/in2) above manifold pressure.
The three way vacuum valve is fitted to the fuel rail to prevent weak fuelling after a hot start.
Ignition System
The ignition system is operated electronically and is known as a Constant Energy System. In this system the distributor incorporates a standard automatic advance system, cover (1), rotor arm (2), anti-flash shield (3), reluctor (4)and a pick-up assembly (5). The reluctor is a gear-like component and is mounted on the distributor shaft in the place of a cam. The pick-up consists of a winding around a pole piece attached to a permanent magnet. The pick-up is prewired with two leads terminating in a two pin block connector.
When a reluctor tooth passes across the pick-up limb the magnetic field strength is intensified creating a voltage in the winding. The rise and fall of this voltage signal created by the reluctor and pick-up assembly is sensed by the amplifier causing the amplifier to switch the current flowing in the primary winding of the HT coil on and off.
Two HT coils are incorporated on the Vi 2 engine. The main coil primary winding is connected in parallel with the primary winding of the auxiliary coil.
The HT section of the auxiliary coil is not used and the HT outlet is sealed.
The auxiliary coil enables the ignition system to achieve the required performance at high engine speeds under load.The constant energy electronic ignition system employs output current limiting and variable dwell for optimum performance. A long dwell is provided at high speeds for adequate energy storage in the coil and a dwell is provided at low speeds for minimum power dissipation. The output current limiting function of the amplifier maintains the storage energy for spark, and the system open circuit voltage constant over a wide engine speed range.
It eliminates the need for a ballast resistor whilst ensuring correct current flow at all times even when the engine is cranking. No current flows through the HT coil when the ignition is switched on and the engine is stationary.
The amplifier assembly consists of a solid state electronic amplifier module (1), a Zener diode (2) to protect the amplifier in the event of a current surge, a moulding containing two resistors (3 & 4) and a suppression capacitor (5). The resistors provide two engine speed signals one for the tachometer and one for the ECU.
WARNING: THE AMPLIFIER IS A SEALED UNIT CONTAINING BERYLIA. THIS SUBSTANCE IS EXTREMELY DANGEROUS IF HANDLED. DO NOT ATTEMPT TO OPEN THE AMPLIFIER MODULE.
Relays
Four relays are used in the fuel injection system. They are the main relay which supplies battery voltage to the system, the pump relay which controls the fuel pump, the idle speed relay (certain markets only) and the lambda sensor relay (certain markets only).
Firing order with the cylinders numbered from the front: 1A 6B 5A 2B 3A 4B 6A 1B 2A 5B 4A 3B
The induction system is basically the same as that on carburetted engine: tuned ram pipe, air cleaners, plenum chambers and induction ports. Air is drawn through paper element cleaners to a butterfly valve for each bank and to individual ports for each cylinder leading off the plenum chamber. The injectors are positioned at the cylinder head end of each port so that fuel is directed at the back of each inlet valve.
Fuel supply
Fuel is drawn from a small tank (1) at the rear of the car by a fuel pump (2) via a non-return valve (3) to a fuel rail through an in-line filter (4) and a pressure regulator (5). Fuel is controlled so that the pressure drop across the injector nozzle is maintained at a constant 2.5 bar (36.25 lb/in2). Excess fuel is returned to the fuel tank via a fuel cooler (6). The twelve fuel injectors (7) are connected to the fuel rail (8) and are electro-mechanically operated to inject fuel into each inlet port. Fuel is also supplied to cold start injectors (9) which only operates during the starting of a cold engine.
Full load fuelling - Cars to USA
To obtain maximum engine power it is necessary to inhibit the "closed loop" system and simultaneously increase the fuelling level. This is obtained by using a vacuum operated switch, sensing inlet manifold depression and a micro switch operated by the throttle pulley spindle. These two switches wired in parallel, so that either or both can signal the need for full load fuelling.
Vacuum switch
The contacts of this switch are operated by a spring loaded diaphragm in a chamber.
This senses inlet manifold vacuum such that, when the depression falls below a certain value e.g. when the engine is operating at low speed, with part throttle near full load condition then the contacts close. The closing contacts causes the fuel system to go "Open loop" and simultaneously introduce a fuel enrichment of 12%.