Secondary air injection

The secondary air system (short SLS) is used in gasoline engines to comply with the emission standards and laws. It consists mainly of the secondary air pump and the secondary air valve. The system is activated after a cold start in the warm-up phase of the engine, to minimize the exhaust gas constituents CO and HC. The secondary air pump supplies ambient air, which is blown by bypassing the motor through the valve into the exhaust system. There, the air reacts exothermically with unburned exhaust components ( thermal oxidation ), whereby the catalyst is supported to come up to operating temperature faster. Depending on the position of the catalyst, the air injection is carried out just after the exhaust valves just before the catalyst.

Operation

To guarantee during the engine warm-up, a "round " engine running and to ensure sufficient engine power with no dropouts in case of need, the mixture adjustment is made here to reach the operating temperature even with an excess of fuel, that is substoichiometric. This has, inter alia, the consequence that a portion of the hydrocarbons of the fuel leaves the combustion chamber unburned and thus may reach the environment. Another part burns incompletely to toxic carbon monoxide.

Normally, both materials were made ​​both hydrocarbons (HC ) and carbon monoxide (CO) oxidized in the catalyst and harmless. In the phase of engine warm-up, however, the catalyst itself is not yet ready for use. Moreover, in setting fat mixture lacks the oxygen in the exhaust gas in order to initiate oxidation. The atmospheric oxygen for post-combustion so can not be provided by the mixture itself available, but must be supplied from outside.

The operation of the secondary air injection is timed in the control. The catalyst is ready in a few seconds after a cold start, so that then also the secondary air can be stopped. The mixture is then adjusted from stoichiometric and regulated at λ = 1.

In the past, mechanical pumps have been used for the secondary air, which were driven by the engine. Today, only electrically driven air pumps are used. The pump extracts the air to be conveyed to the filter housing and conveys it to the secondary -air valve. The promotion is continuous, one adapted to the frequency of discontinuous gas exchange promotion brings no benefits.

The air delivery rate of the pump is not controlled according to the engine operating point. The air ratio in the exhaust system is established as a chance result of unregulated air injection and in-cylinder air-fuel ratio. In most cases, the injection of air takes place in parallel in all the discharge channels, that is relatively far removed from the catalyst. The pump is powered by unregulated constant voltage. The valve opens either the air path for the secondary air to be blown or blocks the air path that in order to prevent return flow of exhaust gas into the air conduit. Some systems are supplemented by sensors for diagnosis.

Was only driven in the past the valve pneumatically by the interplay of under-or overpressure, for some years an electromagnetic actuator used. The advantages lie in the higher speed when opening and closing and the lower space requirements.

Often passive SLS ( ' Pairc ') are used, which make do utilizing sub -pressure phases in the exhaust flow without a pump.

Benefits

  • By the exothermic reaction of the exhaust gas with the additional oxygen, the catalyst more rapidly achieved after a cold start the Idle state.
  • Due to the rich mixture setting, the formation of nitrogen oxides is almost impossible.
  • The rich mixture setting ensures a smooth engine running after a cold start.
  • The issue of limited pollutants during an exhaust measurement cycle, for the first few seconds are crucial after a cold start, is significantly reduced.
  • By the rapid achievement of the operating temperature of the catalyst may be installed under conditions further away from the discharge passage in order to increase its service life.
Statute
721838
de