A secondary air system mixes fresh air with the exhaust gas in order to increase the oxygen level present. This is necessary in the case of spark-ignition engines which tend to run with a rich mixture, in order to achieve effective two-way operation of a catalytic converter.
At Blackthorn, we usually design and supply secondary air systems which rely either on the pulsing nature of the exhaust gas, or on the Coanda effect. There are several factors which will influence the choice, and in either case some development and testing will be required. Further details of the two approaches follow.
Pulse air valves
This is a type of valve (usually a reed valve) which is open when the pressure inside the exhaust is below atmospheric, and closed when it is above atmospheric. Because it relies on pressure pulses it is most suitable for single cylinder engines or for multi-cylinder engines where the design of the exhaust permits a separate valve to be installed for each cylinder. We can design and supply catalytic converters incorporating pulse air valves for single cylinder engines, or exhaust manifolds with pulse air valves for multi-cylinder engines.
Coanda systems
The Coanda effect is partly responsible for the ability of aircraft to fly, and can also be used to draw secondary air into an exhaust system, even if the pressure inside the exhaust is above atmospheric. Because it relies on the movement of the exhaust gas rather than its pressure pulses, it is ideally suited to multi-cylinder engines. Blackthorn can design and supply Coanda air induction systems either as separate units for inclusion in the exhaust upstream of the catalytic converter, or alternatively we can incorporate them into catalytic silencers.
