Fuel Delivery and Air Induction: Description and Operation
LH-Jetronic 2.4 (LH 2.4)
The designation '2.4' denotes the latest version of the LH-Jetronic fuel injection system with which the B 234 F engine is equipped. (LH is the acronym for the German term 'Luftmassenmesser-Hitzdraht'.) Electronic like its predecessors, the new system employs a number of sensors whose signals are processed by a control unit to vary the duration of the injection period.
The main difference between LH 2.4 and earlier versions is that the control unit functions are integrated more closely than ever before with those of the ignition system control unit. The systems share a common diagnostic unit which greatly facilitates fault tracing. In the case of the fuel system, the diagnostic unit is capable of storing up to three fault codes in its memory.
Other fault tracing measures may then be carried out using the functional test and control test systems.
LH 2.4 features an adaptive (or self-correcting) Lambda emission control function, a new design of idling control valve and an EVAP function - a system designed to recover the vaporized fuel produced in the fuel tank.
The functions of the system components may be summarized below (the numbers referring to the illustration on the following page):
1 - Control unit
The microcomputer in the control unit has the following main functions:
Inputs:
- Signal from oxygen sensor (Lambdasond�) indicating residual oxygen content of exhaust gases.
- Speed and position signals from ignition system control unit.
- Engine temperature information from temperature sensor.
- Signal from throttle switch indicating throttle status (closed or wide open).
- Load signal from air mass meter.
- Current signal from [1][2]system relay indicating battery voltage level.
Control functions:
- Energizes the system by earthing the [1][2]system relay.
- Disconnects the [1][2]system relay earth at excessively low engine speed (engine stopped), thereby preventing battery discharge and continued operation of the fuel pumps when the engine is at rest.
- Earths the injectors to ensure that they open at the correct instant and for the correct duration.
- Transmits load signals to ignition system control unit.
- Controls constant idling valve (CIS).
- Transmits control signal to air mass meter for cleaning of platinum wire.
A special program, which first delivers additional fuel and then reduces the flow to prevent over- enrichment, is used when starting.
When driving, the duration of the injection period is controlled on the basis of factors including coolant temperature and air mass.
The injection period is extended when accelerating.
Knock enrichment is introduced when knock persists although the ignition system knock control function has retarded the timing in all cylinders by a certain number of degrees. Since knock raises the combustion temperature, the control unit increases the fuel flow to reduce the temperature - a measure which also suppresses the tendency to knock.
At full load, the air/fuel mixture is enriched to reduce the thermal stresses on the engine and catalytic converter.
Overspeeding is prevented by a speed limiting function which closes the injectors.
When decelerating, the fuel supply is cut off at speeds above approx. 1800 rpm in all gears. The supply is restored between 1400 and 2000 rpm depending on engine temperature.
An emergency program intervenes in certain circumstances (such as when the hot wire in the air mass meter burns out) to impose the 'limp home' mode. Under these conditions, the duration of injection is preset to enable the car to be driven at low speed to the nearest workshop for attention.
2 - Air mass meter
The device measures the mass of air supplied to to the engine. The measured value is influenced by factors including ambient temperature, relative humidity and atmospheric pressure (i.e. height above sea level) - all of which vary the density. The air mass meter incorporates a wire which is heated to a temperature 100°C above that of the air. The wire is cooled as the flow increases, requiring a higher current to maintain it at the correct temperature. In other words, the current is a measure of the airflow.
When the engine is stopped, deposits are burned off by heating the wire to a temperature of over 1000°C for about one second. Dirt on the wire will cause erroneous signals to be supplied to the control unit, resulting in an incorrect air/fuel ratio.
The previous air mass meter incorporated a CO adjusting screw. However, since the emission control function is now self-correcting, this has been omitted in the new system.
3 - Oxygen sensor (Lambdasond�)
A ratio of 14.7 kg of air to 1 kg of fuel is usually quoted as the ideal value. The oxygen content of the exhaust gases is measured as a means of achieving these conditions. Since the device functions only above a certain temperature, it is electrically heated to ensure that the specified value is reached quickly and maintained. Current is supplied to the heating element when the [1][2]system relay operates.
4 - Catalytic converter
The three-way catalytic converter is used to purify the exhaust gases of unburned residues of
carbon monoxide (CO)
hydrocarbons (HC)
nitrous oxides (NOx)
by chemical reaction i.e. combustion with unburned oxygen.
The unit converts 90-95 % of these noxious substances into innocuous
water (H20)
carbon dioxide (C02)
nitrogen (N2)
The active surface area of the unit is 32 000 m2 and the noble metal content is 4 g of platinum/rhodium.
5 - Pressure regulator
The unit regulates the pressure in the injector fuel lines at a constant value 300 kPa above the vacuum in the induction manifold, using a connection from the manifold for the purpose.
In this manner, the pressure drop across the injectors is maintained at a constant value, regardless of the throttle setting, and the quantity of fuel injected is, therefore, a function only of the duration of injector opening. Excess fuel is returned to the tank through a return line.
6 - Injectors
The control unit supplies current through the auxiliary relay for a specified duration, opening the injectors to admit atomized fuel.
7 - Idling control (CIS) valve
The LH 2.4 system incorporates a new type of idling control valve of simpler design.
To compute the correct setting of the valve (thereby maintaining a constant idling speed), the control unit is supplied with signals indicating the engine speed and the quantity of air passing through the air mass meter. (These signals are supplied by the ignition system control unit and air mass meter respectively). The method also ensures that the valve is unaffected by air leaks or fouling of the throttle.
When the air conditioning system is switched on and off, the AC control transmits a signal to the control unit which acts to counteract the overshoot caused by switching the compressor in and out.
When the throttle switch is closed (i.e. when idling), current is supplied to the electric motor in the valve and the control unit maintains the idling speed at 850 ± 50 rpm.
In the absence of a current signal (i.e. when the throttle switch is open), the spring in the valve maintains an opening corresponding to an idling speed of about 1000-1100 rpm.
8 - Throttle switch
The switch supplies the fuel and ignition system control units with closed and fully-open throttle signals.
9 - Coolant temperature sensor
The sensor assists the control unit in regulating the injection period in response to engine temperature.
10 - EVAP System
EVAP (Evaporative emission control system) is a method of recovering the vaporized fuel produced in the fuel tank and preventing its release to atmosphere.
In this instance, the EVAP function is controlled efficiently by the fuel system control unit.
The fuel vapor is conducted by a system of hoses, through the roll-over valve, to a container (or canister) of activated carbon in which it is absorbed. When the engine is started, the vapor is drawn into the induction manifold and used in the combustion process.
The control unit is used to control the EVAP valve, to limit the effect of the vapor flow driveability or exhaust gas emission levels.
When the engine is running (above idling speed), the EVAP valve is opened for seven minutes and closed for two minutes on a cyclic basis. During the 'open' period, the valve operates at a frequency of 6 Hz, causing it to open and close six times per second.
The duration of the open/closed periods may be varied by the control unit.
When the engine is at rest, the fuel vapor is stored by the active carbon in the canister. Closure of the spring-loaded non-return valve in the EVAP valve prevents flow to the induction manifold.
The solenoid is energized when the engine is started, closing the outlet to the induction manifold.
When the engine temperature reaches 80°C (as indicated by the coolant temperature sensor), the solenoid is deenergized, enabling the vacuum in the induction manifold to open the non-return valve The vapor is drawn into the manifold and is burned in the engine.
11 - Tank pump
An electric impeller-type unit, the pump is used to maintain the pressure in the suction line to the main fuel pump, thereby counteracting vapor locking. The tank pump is fitted with a coarse strainer.
12 - Main fuel pump
The unit is an electric rotary pump which is cooled by the fuel itself. The pump is equipped with a non-return valve and a relief valve which opens if the pressure becomes too high. Both the tank pump and the main fuel pump are operated when the starter motor is turned or when the engine is running. However, the fuel system control unit interrupts the power supply to the pumps if the engine is stopped and the ignition remains on, to help guard against the risk of fire in the event of an accident.
13 - Fuel filter
Installed immediately after the fuel pump, the filter contains a paper element and a safety screen to trap any particles which may become detached from the element.
14 - Fuel injection relay
Operated by the fuel system control unit, the relay supplies power to items including the fuel pumps, air mass meter and interference suppression relay. The relay is mounted in the central electrical unit.
15 - Interference suppression relay
The relay is used to supply the pulsating and rapidly-operating injectors and idling control valve with current to prevent the propagation of radio interference to the rest of the electrical system.
16 - Diagnostic unit
The new fuel injection system incorporates a self diagnostic function and a functional test system. The diagnostic unit, which is also used for testing the ignition system, is dealt with in a separate chapter.