Under "running losses", one understands the separation or evaporation of fuel, oil, washer water, underfloor protection or engine fumes. In the USA, lawmakers demand a strict examination of vehicles in this respect. Carmaker must therefore construct and fit all fluid containers, pipes etc. in such a way that no vapours can escape.
The new 3/2-way valve reduces the heating up of the fuel and therefore the evaporation of the fuel. By using the 3/2-way valve, two fuel circuits, one smaller and one larger were created. The large fuel circuit is required in the start-up phase. In this phase, the fuel injection strip is washed with the entire quantity of fuel supplied (possible vapour bubbles washed out). When the engine has been running for approximately 20 seconds, the valve is de-activated, allowing the small circuit to be utilised.
Regardles of the pollutants created by the combustion within the engine, a vehicle will emit considerable amounts of unburned hydro-carbons. These hydro-carbon emissions can stem from leaks in the fuel system, but also from an insufficiently large tank bleed system (activated carbon filter becomes permeable).
For this reason, a further OBD II requirement refers specifically to the fuel system and the fuel tank bleed system. The maximum permissible level of escaping fuel fumes has been determined anew. Moreover, leaks larger than 1 mm in the fuel system must be recognised by the DME.
To this end, the following measures have been implemented in BMW vehicles:
- Fuel temperature reduced through new fuel circuit with 3/2-way valve
- The activated carbon filter has been reshaped
- New activated carbons with improved absorption capability
- Inclusion of a tank bleed system diagnosis with the DME, using an AKF cut-off valve and a fuel tank pressure sensor
The tank bleed system diagnosis i sperformaed automatically in pre-defined cycles. It is only performed with the engine running. In order that leaks in the fuel tank and the tank bleed system be detected, the entire system must be sealed air-tight. This is done by the cut-off valve (AAV) on the activated carbon filter (AKF).
Depression sytem:
The fuel tank bleed valve (TEV) is opened. This remains open until a depression of 5-10 mbar in the entire system has been generated by the intake system. The depression in the fuel tank system is measured by the fuel tank pressure sensor.
The fuel tank bleed valve is closed when the required depression is attained.
Now, the fuel tank bleed valve and the AKF cut-off valve are both closed together. In this state, the DME control unit uses the fuel tank pressure sensor to monitor the previously generated depression in the fuel tank and fuel tank bleed system. If the depression drops below a given threshold within about 10 seconds, the DME will recognise a leak.
Pressure release system:
The LDP (leak detection pump) is switched on, the fuel tank bleed valve closed. The LDP remains on until a pressure of 5-10 mbar in the entire system has been generated. The pressure in the tank is measured by the reed switch in the LDP.
The LDP is switched off when the required pressure has been reached.
Now, the fuel tank bleed valve and the AKF cut-off valve are closed together, the LDP is switched off. In this state, the DME control unit uses the fuel tank pressure sensor to monitor the previously generated pressure in the fuel tank and fuel tank bleed system. If the pressure drops below a given threshold within about 10 seconds, the DME will recognise a leak.
A second Lambda probe (the monitor probe), installed behind the catalytic converter, monitors the problem-free operation of the catalyst. To recognise the functional ability of the cytalytic converter, the DME control unit compares the signals from the control probe (before the catalyst) and the monitor proble (after the catalyst).
the operation of the control Lambda probe is similarly monitored. Incorrect Lambda probe operation, caused for example by the use of leaded petrol, causes a change in the Lambda control frequency, which is recognised as a fault by the DME control unit.
An inductive impulse sender on the increment gear to measure the speed of rotation (rpm) of the engine. Moreover, the smooth running of the engine is also monitored (misfire detection) as a measure of the engine speed.
To detect misfires, the increment gear is divided (by the control unit) into 3 segments according to the ignition separation (e.g. 3 sparks per crankshaft turn on a 6-cylinder engine). Within the control unit, the periodic duration of the individual increment gear segments is measdured and statistically evaluated. For each point on the characteristic map, the maximum permissible rough running values are stored as a function of engine speed, load and engine temperature.
If these values are exceeded, the cylinder registered as faulty is stored in the fault memory.
The diagnosis programme offers a test step, "smooth running measurement". Here, the smooth running values of the individual cylinders are indicated in a bar chart.
OBD II demands that the operation of the secondary air system be monitored. To do this, the operation of the secondary air injection and of the cut-off and air switch-over valves are monitored each time they are activated. The secondary air injection serves as an exhaust gas aftertreatment during the engine warm-up phase. Here, fresh air is blown directly into the exhaust manifold.
About 10 seconds after the engine is started, the secondary air pump is activated by the SLP relay. The time until it is switched on is dependent on the following fringe conditions:
- Engine temperature
- Load signal
- Engine speed
During the activation of the secondary air pump, the Lambda probe voltage is monitored in the DME control unit. During problem-free operation of the secondary air system, the Lambda probe voltage is primarily in the lean range.
At regular intervals (every 20ms), the Lambda probe voltage is registered within the control unit. Each measurement in which the Lambda probe voltage is registered as being in the lean range is counted by an internal counter. If this count exceeds a pre-defined threshold, the system is recognised as being fully operational. If this threshold is not reached, the DME recognises a fault within the secondary air system. An entry is made in the fault memory.
As has been commonplace on BMW vehicles for some years now, almost all electrical and electronic components of the engine management system are monitored by the DME control unit. In the event of electrical, or in some cases even mechanical malfunctions occuring, these are recognised and stored in the DME fault memory.
In the event of malfunctions occuring in components whose failure or incorrect operation would affect the exhaust gas composition, the "CHECK ENGINE" warning lamp lights up in the instrument cluster in addition to the entry being made in the fault memory.