Secondary air system

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The secondary air system ( SLS for short ) is activated in gasoline engines after a cold start in order to minimize the exhaust gas components HC and CO in the warm-up phase. It essentially consists of the secondary air pump and the secondary air valve. The fan delivers ambient air, which is blown into the exhaust system through the valve, bypassing the engine. There the air reacts exothermically with unburned exhaust gas components ( thermal post-combustion ), which helps the catalytic converter to reach operating temperature. Depending on the position of the catalytic converter, air is injected shortly after the exhaust valves to shortly before the catalytic converter.

Mode of action

In order to guarantee a "smooth" engine run during warm-up and, if necessary, to guarantee sufficient performance without misfires, the mixture is set with excess fuel, i.e. under stoichiometric, until the operating temperature is reached. That has u. a. As a result, some of the hydrocarbons in the fuel leave the combustion chamber unburned and could get into the environment. Another part burns incompletely to form toxic carbon monoxide.

Schematic representation of the entire "secondary air" system

Normally, both hydrocarbons (HC) and carbon monoxide (CO) would be oxidized in the catalytic converter and rendered harmless. In the engine warm-up phase, however, the catalytic converter itself is not yet ready for operation. In addition, with a rich mixture setting, there is no oxygen in the exhaust gas to initiate oxidation. The oxygen in the air for afterburning cannot be made available by the mixture itself, but has to be supplied from outside.

The secondary air injection process is usually time-controlled. The catalytic converter is ready for operation in a few seconds after a cold start, so that the secondary air injection can then also be stopped. The mixture is then adjusted to be stoichiometric and regulated to λ = 1.

In the past, mechanical pumps powered by the engine were used for secondary air injection. In the meantime, only electrically driven air pumps are used. The pump takes the air to be conveyed from the filter housing and conveys it to the secondary air valve. The delivery takes place continuously, a discontinuous delivery adapted to the frequency of the gas exchange brings no advantages.

The pump is operated with constant electrical voltage. The air ratio in the exhaust system is a random result of the unregulated air injection and the internal engine air ratio . In most cases the air is blown in parallel into all outlet ducts, i.e. relatively far away from the catalytic converter. The valve either opens the air path for the secondary air to be blown or shuts off the air path in order to prevent exhaust gas from flowing back into the air line. Some systems are supplemented by sensors for diagnosis.

In the past, the valve was only actuated pneumatically through the interplay of negative and positive pressure, but electromagnetic actuation has also been used for several years. The advantages are the higher speed when opening and closing and the lower space requirement.

Often passive SLS ("PAIRC") are also used, which manage without a pump by utilizing negative pressure phases in the exhaust gas flow.

advantages

  • Due to the exothermic reaction of the exhaust gas with the additional oxygen in the air, the catalytic converter is ready for operation more quickly after a cold start.
  • The rich mixture setting means that the formation of nitrogen oxides is almost impossible.
  • The rich mixture setting ensures that the engine runs smoothly after a cold start.
  • The emission of limited pollutants during an exhaust gas measurement cycle, for which the first seconds after a cold start are decisive, is significantly reduced.
  • As the operating temperature is reached more quickly, the catalytic converter can under certain circumstances be installed at a greater distance from the exhaust duct in order to increase its service life.

Other aspects

  • The operation of the secondary air pump can only be adapted to a limited extent to different start-up situations during the warm-up.
  • In the case of direct-injection gasoline engines, the catalytic converter can also be heated up by post-injections, so that there are usually no secondary air systems installed in such systems.

literature

  • Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 28th edition, Verlag Europa-Lehrmittel, Haan-Gruiten 2004, ISBN 3-8085-2238-0 .

Individual evidence

  1. ^ Bosch: Kraftfahrtechnisches Taschenbuch. Vieweg-Verlag, 24th edition, 2002, p. 564.
  2. ^ Hallgren, Heywood: Effects of Substantial Spark Retard on SI Engine Combustion and Hydrocarbon Emissions. SAE paper 2003-01-3237, 2003.
  3. ↑ Paffrath, Panhans: Secondary air injection - a component of low-consumption, low-emission concepts. In: MTZ 12/2010, p. 878.