Fuel Injection
PRINCIPLE OF OPERATION, FUEL INJECTION
Fuel Injectors
There are two types of injector, one for normally aspirated engines and one for turbocharged engines. The injectors for turbocharged engines have higher flow capacity and lower electrical resistance.
The injectors are of solenoid type with needles and seats. They open when current flows through them and close by means of a powerful spring when the current is turned off.
To obtain optimal combustion and consequently cleaner exhaust emissions, the injectors are provided with 4 holes to ensure good distribution of the fuel.
The fuel jets are set with a high degree of accuracy (two jets for each inlet valve). This calls for extreme precision in the mounting of the injectors and to ensure their exact positioning they are fixed in pairs in special mounting brackets between cylinders 1, 2 and cylinders 3, 4
The injectors are supplied with current from the main relay and the control module grounds them as follows:
- Injector 1 is connected to ground via pin 3
- Injector 2 is connected to ground via pin 4
- Injector 3 is connected to ground via pin 5
- Injector 4 is connected to ground via pin 6
Pre-injection
When the ignition is switched on, both the main relay and fuel pump relay operate for a few seconds. When fuel pressure has built up, fuel is injected simultaneously by the 4 injectors. This initial injection of fuel is dependent on the engine coolant temperature and ensures that short starting times are achieved.
If the engine is not started, this function will be blocked for 15 minutes. If the engine is started and switched off again shortly afterwards, fresh pre-injection will be obtained after waiting for at least 45 seconds with the ignition switched off.
Calculation Of Fuel Injection Time
To determine how much fuel should be injected into each inlet pipe, the control module calculates the air mass that is sucked into each cylinder.
The calculation is performed as follows: (each cylinder in the B204L engine has a capacity of 0.5 liters): 0.5 liters of air having a certain density has a definite weight.The density is calculated on the basis of the pressure and temperature in the intake manifold.
The control module keeps the injector open precisely long enough for the correct amount of fuel to be injected in relation to the measured air mass.
Fuel injection time is corrected by means of the oxygen sensor so that Lambda = 1.0 is obtained. Lambda control is disconnected during heavy acceleration and the full-load enrichment system is activated for maximum performance at wide open throttle. A richer mixture is produced for acceleration (throttle open) and a leaner mixture for deceleration (throttle closed). When starting from cold and during engine warm-up before Lambda control is activated, a richer mixture dependent on engine coolant temperature is produced.
When the engine is warmed up and battery voltage is normal, the fuel injection times vary between about 2.5 and 3.5 ms at idling speed, depending on the engine version, and about 18 ms at wide open throttle.
Lambda Control
For the catalytic converter to function correctly, the fuel-/air mixture must be stoichiometric.
In other words, the mixture must be neither too rich nor too lean but consist of precisely 14.7kg of air to 1 kg of fuel (Lambda = 1).
For this reason the system is equipped with an oxygen sensor mounted in the front exhaust pipe section.
This sensor is connected to pin 23 of the control module and grounded to the exhaust pipe.
Reference ground from grounding point G7S is connected to pin 47 of the control module enabling it to measure the voltage signal from the oxygen sensor with an extra high degree of accuracy.
The engine exhaust gases pass the oxygen sensor and their oxygen content is measured through a chemical process. The output voltage of the oxygen sensor is proportional to the actual oxygen content at any given time.
IMPORTANT: The oxygen sensor obtains reference oxygen from the ambient air via the connecting cables. For this reason, contact cleaner and grease must not be used on the oxygen sensor connector.
If the engine is running rich (Lambda below 1), the output voltage of the sensor will be about 0.9 V.
If the engine is running lean (Lambda above 1), the output voltage of the sensor will be about 0.1 V. The sensor's output voltage changes very rapidly when Lambda passes 1.
In order to produce an output voltage signal as quickly as possible after the engine has started, the oxygen sensor must be preheated. The preheater is supplied with battery positive voltage from the fuel pump relay via fuse 28 and grounded via pin 50 of the control module. The control module estimates the exhaust gas temperature on the basis of engine load and rpm. At high exhaust gas temperatures the preheater is deactivated.
Lambda control is activated after about 1200 combustion sequences (engine coolant temperature and throttle position have a certain effect on the exact number).
Adaptation
The control module first calculates the injection time on the basis of intake manifold pressure and temperature.The injection time is then multiplied by a correction factor, obtained from the main fuel matrix, which is dependent on pressure and engine rpm. The injection time has to be corrected with this factor because the engine's fuel-air mixture changes with engine rpm.
Final correction is carried out with the aid of the oxygen sensor so that Lambda = 1 is obtained.
The Lambda control system can adjust the calculated injection time by 125%.
By means of the Lambda control system the control module can change the correction factors in the main fuel matrix so that driveablity, fuel consumption and exhaust emissions will be good even when Lambda control is not activated. This is called adaptation.
NOTE: Only two pressure columns are shown in the above figure.
Spot Adaptation
If the control module calculates the injection time as 8 ms but the Lambda control system adjusts it to 9 ms because the car's fuel pressure is somewhat low, the control module will "learn" the new injection time. It does this by changing the correction factor for the current rpm and load point in the main fuel matrix so that the result will be an injection time of 9 ms. The new correction factor in this example will be 0.90 x 9 /8 = 1.01 (see Fig. above).
Spot adaptation can change all points in the main fuel matrix by =L 25%, provided that Lambda control is activated, the engine coolant temperature is above 64°C and the EVAP canister purge valve is not in operation.
Global Adaptation
The control module scans the main fuel matrix 15 minutes after the ignition has been switched off to see whether any tendency can be discerned in the spot adaptation process. For instance, if the spot adaptation mean is 12.5%, all other values will be increased by 12.5%. This is called global adaptation.
Fuel Shut-off - Turbocharged engines:
Fuel shut-off takes place in all gears, regardless of the type of gearbox.
Fuel consumption signal
Fuel consumption signal goes out from pin 34 of the control module. Basically, this signal is identical to the output for injector No. 2 but is not in electrical communication with it.