Lambda control
The lambda control sets a desired combustion air ratio (λ) in the exhaust gas of an internal combustion engine or a burner .
Basics
In exhaust technology, the lambda (λ) symbol stands for the ratio of air to fuel compared to a combustion stoichiometric mixture. With the stoichiometric fuel ratio, there is exactly the amount of air that is theoretically required to completely burn the fuel. This is referred to as λ = 1. For gasoline this mass ratio is 14.7: 1, which means that 14.7 kg of air are needed to completely burn 1 kg of fuel. The ratio for ethanol is 9: 1 and for diesel fuel and heating oil 14.5: 1.
If there is more fuel, it is called a rich mixture (λ <1), and if there is excess air, it is called a lean mixture (λ> 1).
The lambda window (lambda = 0.995–1,000) is the area in which a three-way catalytic converter achieves the maximum cleaning performance of more than 99%. Whether the control adheres to this range of values is checked when the engine management and emission control system are examined.
The lambda control records the actual lambda value via a lambda probe in front of the catalytic converter and changes the amount of fuel so that the target value is set. This is necessary because, without re-measurement, neither the fuel can be dosed precisely enough nor the amount of air in the cylinder can be measured precisely enough. The accuracy can be further increased with an additional probe after the catalytic converter and a guide control.
Maintaining a certain lambda value has a major influence on the quality of the combustion and the possibility of complete catalytic exhaust gas cleaning .
The fuel consumption in a gasoline engine reaches a minimum at a value of λ = 1.2-1.5. Normal engines no longer have stable combustion in this area.
For a maximum engine torque, albeit with increased fuel consumption (incomplete combustion due to lack of air), a value of approx. Λ = 0.9 is optimal.
With a high engine output, rich engine operation and the resulting colder exhaust gas prevent overheating and destruction of exhaust gas components such as manifolds, turbochargers and catalytic converters. Modern engines achieve a lower exhaust gas temperature through design measures (such as water-cooled manifolds, direct injection or cooled exhaust gas recirculation ), so that enrichment of the mixture is not necessary or only necessary in a small area for reasons of component protection.
The control method used differs depending on whether a jump probe or broadband probe is used as the lambda probe .
Two-point lambda control
1.
Air mass meter 2. Pre-catalytic converter (three-way catalytic converter)
3. Main catalytic converter
4. Fuel injection valves
5. Lambda probe upstream of the catalytic converter (two-point lambda probe or broadband
probe ) 6. Lambda probe after the catalytic converter (two-point lambda probe, only if required)
7. Fuel supply
8. Air supply
9. Exhaust gas discharge
With a jump probe, regulation to λ = 1 is only possible:
If the engine mixture is lean and the probe voltage is therefore low (approx. 100 mV), the amount of fuel is increased until it falls below λ = 1 and the probe voltage rises (approx. 800 mV).
With this rich engine mixture, the amount of fuel is reduced again until λ = 1 is exceeded and the probe voltage drops again. The control cycle then starts all over again.
There is a periodic change between rich and lean mixture, high and low probe voltage. Due to the buffer capacity of the catalytic converter due to the oxygen-storing components, adherence to the target value λ = 1 is sufficient over time to ensure a high conversion of the harmful exhaust gas components.
Continuous lambda control
A constant lambda control aims to keep this value as stable as possible instead of oscillating around the setpoint. To do this, it is not only necessary to know whether λ <1 or λ> 1, as is the case when using a jump probe, but the exact value must be known in order to determine the deviation from the target value. This requires the measurement with a broadband probe. The correction can then take place, for example, with a PI controller . With the broadband probe, a value outside the stoichiomometric mixture can also be adjusted, e.g. B. during enrichment for component protection.
Diesel engine
While the lambda control in gasoline engines allows direct intervention on the amount of fuel injected, such intervention is not common in diesel engines. The reason for this is that in the diesel engine, the engine torque is controlled by changing the amount of fuel. The lambda control in the diesel engine takes place instead via the air system by regulating the exhaust gas recirculation rate. Extended additional functions such as smoke limitation for diesel are also possible in this way.
literature
- Jürgen Kasedorf: Motor vehicle engine test, gasoline engines. 7th revised edition, Vogel Buchverlag, 1997, ISBN 3-8023-0461-6 .
- Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition, Verlag Gehlen, Bad Homburg vor der Höhe 2000, ISBN 3-441-92250-6 .
- Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition, Holland + Josenhans Verlag, Stuttgart 1997, ISBN 3-7782-3520-6 .
Individual evidence
- ^ A b Robert Bosch GmbH (ed.): Kraftfahrtechnisches Taschenbuch . 25th edition. Vieweg Verlag, ISBN 3-528-23876-3 , pp. 662 ff .
- ^ Robert Bosch GmbH (ed.): Kraftfahrtechnisches Taschenbuch . 29th edition. Karlsruhe 2018, ISBN 978-3-658-23583-3 , pp. 655 .