Multiport Fuel Injection (MFI) System LH 2.4
Design and function
Multiport Fuel Injection (MFI) system LH 2.4
The MFI system can be divided into five sub-systems.
^ The control system controls fuel and air quantities In order to provide the optimum fuel mixture and correct Idle speed.
^ The sensor system gives information to the control system in order to provide optimum control.
^ The fuel distribution system is controlled by the control system and distributes fuel to the cylinders.
^ The evaporative emission (EVAP) system manages the gases evaporating from the fuel tank (not B 204 E/FT/GT).
^ The on-board diagnostic (OBD) system is common to the fuel system and DI system. It has three test functions to facilitate fault tracing. The OBD system is described at the end of the section Design and function.
Sensor system
Mass air flow (MAF) sensor
The MAF sensor measures the mass of air drawn into the engine. It automatically compensates for air pressure and temperature which affect the density of the air. There Is a wire In the MAF sensor which is heated to a temperature 150°C (270°F) higher than the intake air temperature (IAT). When the mass of air passing this wire increases it is cooled and It requires a higher current to keep the wire at the correct temperature. This current provides a measurement of the air passing the heated wire.
When the engine is stopped deposits are burnt off by electrically heating the wire to 1000°C (1832°F) for about a second. Were deposits allowed to collect the control module would receive false signals which would result in an incorrect fuel mixture.
The MAF sensor on the B 200 G, B 204 E, B 230 G, B 234 G and B 204 GT has an adjustment screw to set the idling CO content. This has been omitted on the other engines as the H026 control is adaptive throughout its range.
Coolant Temperature Sensor:
Engine coolant temperature (ECT) sensor
Sends signals to the control module so that it can control injection time and Idle speed in relation to engine temperature. It is mounted in the cylinder head and cooled by engine coolant.
Exhaust gas temperature sensor (B 204 FT/GT)
The exhaust gas temperature sensor consists of a thermal element in the exhaust manifold before the TC unit. It senses exhaust gas temperature and sends a signal to the fuel system control module, which gradually lengthens injection time when the exhaust gas temperature reaches a critical level (approx. 950 °C or 1740 °F). This contributes to a lower combustion temperature which in turn reduces the exhaust gas temperature. The increased fuel supply stops when the exhaust gas temperature has reached normal.
Throttle position (TP) switch
Sends signals to the MFI and DI system control modules that the throttle is closed or wide open.
In the B204FT/GT the TP switch also contains a potentiometer which sends the TC control system information about the throttle position.
TC models do not use the full load switch.
Heated oxygen sensor (HO2S)
Normally 14.7 kg air to 1 kg fuel is given as the ideal mixture for complete combustion. This mixture is called lambda = 1. The mixture can be gauged by measuring the oxygen content of the exhaust gases after combustion by using an HO2S.
The HO2S produces a voltage signal proportional to the oxygen content in the exhaust gases. This signal alternates from high to low at the ideal mixture of 14.7 kg air/1 kg fuel. A rich mixture produces a high voltage while a weak mixture produces a low voltage. This voltage varies between 0.1-0.9 volt. The control module bases the quantity of fuel injected on this signal so as to achieve the ideal mixture, lambda = 1.
The HO2S will only operate above a certain temperature, approx. 285°C (545°F). It is electrically heated so that it reaches operating temperature quickly. When the ignition is switched on a current is applied to a PTC resistor, the resistance of which increases with temperature. This provides a short warm up time and keeps the sensor at the correct operating temperature when the exhaust gas temperature is low.
In engines without a TWC (B 230 GT, B 204 GT), i.e engines for markets where unleaded gasoline is not fully available, an HO2S which can withstand leaded fuel must be used. However, this HO2S has a limited lifespan and must be replaced regularly.
Three-way catalytic converter (TWO)
The TWO converts 90-95 % of the harmful substances in the exhaust gases to nontoxic ones.
The TWO cleans by:
^ Oxidizing unburned hydrocarbons (HO) to steam (H20) and carbon dioxide (CO2)
^ Oxidizing carbon monoxide (CO) to carbon dioxide (CO2)
^ Reducing nitrogen oxides (NOx) to nitrogen gas (N2)
If the TWO is to work properly the H028 must be producing the correct signal so that the fuel/air mixture is optimal (lambda=1). The higher temperatures caused by air leaks in the exhaust system or unburnt fuel (if the engine is not firing on all cylinders for example) can damage the TWC.
Control system
Control module
The control module is located behind the panel in front of the right hand door post. It contains a microcomputer which receives signals from the various sensors. From this information it calculates the time that the injectors should be open per engine revolution in milliseconds.
It controls the IAC valve so that the correct idle speed is obtained. It also controls other functions, cold start injectors and FPs for example. One of its important functions is to communicate with the DLC when fault tracing.
The control module is adaptive - it adapts its calculations to experienced values.
When starting a special program is run which gives two injections per revolution.
At very low engine temperatures (below approx. -16°C (5°F)) the cold start valve is operated as well.
The choke operates up to an engine temperature of approx. 60°C (140°F).
Under normal driving conditions the injection time is controlled by the signal from the MAF sensor and the HO2S.
When accelerating injection time is lengthened.
Knock enriching (does not apply to B 204 FT/GT, B 230 F) is carried out when the DI system knock controller retards all cylinders a number of degrees but knocking still occurs. The control module increases the amount of fuel if knocking (which raises combustion temperatures) occurs. This reduces combustion temperature which suppresses any tendency to knock.
At full load the fuel/air mixture is enriched so that the engine produces maximum power and to reduce the heat burden on the engine and the TWC.
Over-revving is prevented by an engine speed limiter shutting down the injectors.
When decelerating (TP switch in idle mode) the fuel injection is shut down at engine speeds of above approx. 2000 rpm. it restarts at approx. 14O~2000 rpm, depending on engine temperature. This function has been gradually omitted from the B 230 FB engine for 1992 models.
The control module runs an emergency program (limp home mode), if certain signals are faulty or missing. The program gives the signals a predetermined value which allows the car to be driven to the workshop.
Idle air control (IAC) valve
The IAC valve has two tasks. Firstly it must hold the engine idle speed constant no matter what the load from the automatic transmission, A/C, FC, power steering or GEN. Secondly it must supply air to the engine when the engine is braking the car so that the partial vacuum in the intake manifold is kept at a permissible level. The valve is only operative and controlled by the control module when the idle switch is closed. if the idle switch is open the valve goes into an open standby mode while still receiving signals from the control module.
To provide the IAC valve with the proper opening angle the control module utilizes information about engine speed, engine temperature and intake air mass. Then information from the speedometer is used to ascertain whether the car is moving or stationary, from FC at full speed (B 230 F/FT, B 234 F 1992-, B 204 FT/GT) from the A/C control when the A/C is switched on and from the A/C compressor when it starts and stops. On cars with automatic transmission the idle control also receives information about the gear engaged.
Idle speed is mainly determined by engine temperature. However, depending on variables, speed can be altered by the A/C being turned on, the FC switching to full speed or a gear being engaged on cars with automatic transmission.
The control module has safety functions which under certain circumstances will transmit a signal to the IAC valve which effectively corresponds to opening angle=0°. This prevents the idle speed from running too fast. This is the same as the valve operating without power when a spring Is used to pull it towards the mechanical stop.
In both cases the engine idle speed is 1000-2000 rpm. This safety function is often triggered if there is too great a flow over the throttle disc (throttle disc opening poorly adjusted).
Adaptive idle control
In time deposits and wear will affect the throttle so that it allows through more or less air. Instead of making adjustments based on a preprogrammed value the control module sends a signal based on Information It received the last time the car was driven. This signal opens the IAC valve by exactly the right amount to keep the engine idling at the correct speed.
Adaptive Fuel trim control
Using the adaptive fuel trim the control module can adapt injection times to compensate for engine wear, small air leaks, clogged injectors and similar allowing the HO2S to control engine functions optimally.
The adaptive fuel trim includes two types of function, additive and multiplicative compensation. Additive compensation acts on injection time quickly at low engine speeds. Additive control corresponds to the basic CO setting carried out by a potentiometer in other systems. Multiplicative control generally acts on longterm changes.
The ability of the HO2S to compensate for large faults is limited, The control module posts the code 2-3-1 or 2-3-2 depending on whether it is an additive or multiplicative limit that has been exceeded.
The [1][2]system relay
The [1][2]system relay is controlled by the control module and powers the FPs, the injectors, the IAC valve, the cold start valve, the MAF sensor, the H028 pre-heat resistor and some of the control module functions.
Fuses
The [1][2]system relay Is protected by a 25 A fuse. The FP and HO2S pre-heat resistor are protected by a 15 A fuse.
Fan control (FC)
(Applies to 1992 models with A/C)
The fan is controlled by half and full speed relays. The operation of these is based on ECT, vehicle speed and A/C system pressure.
The fan operates at half speed if:
ECT Is above 102°C (216°F)
The A/C system pressure Is greater than 17 bar (247 psi) on the high pressure side and the vehicle speed is less than 100 km/h (60 mph).
The fan operates at full speed if:
ECT is above 115°C (240°F) or the pressure in the A/C system high pressure side exceeds 22 bar (320 psi).
The fan always starts at half speed for 15 seconds before it can operate at full speed. It always runs at half speed for at least 5 seconds when it stops operating at full speed. If the ignition is turned off when the fan is operating at full speed it will continue at half speed for 5 seconds.
So as not to overload the engine there is a delay which prevents the fan from starting for 9 seconds after the engine has been started, no matter what the engine temperature or A/C system pressure.
To cool the engine and avoid overheating, the fan will continue to operate at half speed for three minutes if engine temperature exceeds 105°C (221°F) when the ignition is switched off.
Fuel distribution system
The fuel distribution system consists of
- Tank pump
- FP
- Fuel filter
- Fuel injection manifold
Fuel tank pump
An electrical impeller pump maintains pressure in the line to the main FP to counteract the partial vacuum in the main pump's low side.
The tank pump has a coarse strainer filter.
Fuel pump (FP)
An electrical roller pump which is cooled by the fuel running through it. It has a non-return valve and overflow valve which opens if the pressure becomes too great.
Both tank and FPs are connected and operate when the starter motor or engine are running. If the engine stops running and the ignition is on, the control module cuts the power to the pumps so as to minimise the risk of fire in the event of an accident.
Fuel filter
The fuel filter has a paper filter with a strainer to catch particles which could drop oft the paper filter.
Fuel injection manifold
The fuel inlet line, pressure regulator, injectors and cold start injector are connected to the fuel injection manifold.
Pressure regulator
Controls the fuel pressure In the injector lines. A vacuum line connected to the engine intake manifold maintains the fuel pressure at a constant 300 kPa (43 psi) above the pressure in the intake manifold. The pressure drop across the injectors is always constant irrespective of throttle position. The quantity of fuel injected is therefore only dependent on how long the injectors are open. Surplus fuel is returned to the fuel tank via a return pipe.
Injectors
These are fitted with an overdrive solenoid and fuel metering needle which open and close a nozzle. The control module grounds the injectors via a suppressor relay for a certain time so that the valves open, Injecting atomized fuel. Injection occurs twice per engine revolution when the starter motor is operating (engine cold), and once per revolution when the engine is running. Injection takes place in the Intake manifold near the intake valves.
Injectors may only be tested using a special fluid as gasoline spray is extremely explosive.
Cold start injector (2-valve engines)
The cold start function was dropped from certain engines during 1991 and was dropped completely for the 1992 models.
When the engine is being started from cold a great deal of fuel condenses into droplets on cold surfaces. A separate cold start injector Improves cold starting characteristics. It is located further from the engine than the injectors and supplies fuel in a more vaporized form rather than in droplet form. It starts to operate at about -16°C (5°F) and at engine speeds below 900 rpm. It disengages completely when the engine speed has exceeded this boundary value once.
Evaporative system (EVAP) system
EVAP stands for Evaporative Emission and Is a system which deals with the gases evaporating from the fuel In the fuel tank. It prevents them being emitted into the atmosphere.
Fuel vapor passes through a system of hoses from the fuel tank tiller opening through a roll-over valve into a carbon filter canister. This absorbs the fuel vapor and prevents it from venting Into the atmosphere.
Carbon filter, canister
Fuel vapor from the tank is led into the top of the carbon filter and bonded to active carbon. Air is pushed out through a channel In the base of the canister. Depending on the temperature etc. the carbon filter can adsorb about 90 g of fuel.
Roll-over valve
This closes when the car tilts sideways more than 45° and prevents fuel leaking out In the event of an accident.
Evaporative emission (EVAP) valve (vacuum)
This is located on the top of the carbon filter and is closed when the engine is not running. It is also closed when the engine is idling when it could interfere with the automatic IAC system and the engine idle characteristics. The valve is controlled by a vacuum from the intake manifold which is connected to the positive side of the throttle.
When engine load Increases the EVAP valve opens allowing fuel vapor to flow from the carbon filter to the intake manifold. Air is drawn in at the same time through the channel in the base of the filter. Normally the carbon filter is emptied within 15-20 minutes.
Evaporative emission (EVAP) valve (electrical)
On the B234F the EVAP vacuum valve has been replaced with an electrical valve located between the canister and the Intake manifold. It works in the same way as the vacuum valve.