What Is Secondary Air Injection System?
Secondary air injection (commonly known as air injection) is a vehicle emissions control strategy, wherein fresh air is injected into the exhaust stream to allow for a fuller secondary combustion of exhaust gases.
The Secondary Air Injection (SAI) system consists of three main parts, a pump, a valve and a solenoid, controlled by the engine computer.
The Secondary Air Injection (SAI) pump that creates positive pressure which is routed to a combination valve, or “Kombi Valve“. The SAI pump design is a vane pump turned by an electric motor. The pump’s air intake is filtered by a rotating screen or the vehicle air filter to exclude dirt particles large enough to damage the system. Air is delivered under light pressure to the injection point. A check valve prevents exhaust forcing its way back through the air injection system, which would damage the pump and other components.
The Kombi valve, as it’s called at times, is a pneumatically actuated shut-off valve that is integrated into the valve housing and allows the Secondary Air Injection (SAI) pump to add air to the exhaust gasses during start up and open loop to help with emission control. The early valves are vacuumed switched, while the later valves are electrically operated.
Finally a switching solenoid that routes engine vacuum when commanded to do so by the engine computer, to open the Kombi valve and allow air to flow into the catalytic convertor.
When the vehicle is cold, the engine computer commands on the SAI system motor to turn on, and create positive pressure. This pressurized air is then routed to the Kombi valve, directing the air into to the catalytic convertor. The computer also controls the Kombi valve, via engine vacuum and an electronic solenoid, to open and allow the air to flow into the catalytic convertor. This added pressurized air causes the convertor come on line quicker, clean up emissions, and prevent clogging of the expensive catalytic convertor.
The active secondary air system usually consists of an electric pump (see figure), the control relay, a pneumatic control valve, and a combination valve. The system is controlled by the engine control unit. While the system is working, the electric pump is switched on by the engine control unit via the control relay. The pneumatic control valve is actuated at the same time. The valve opens and the vacuum from the intake pipe operates the combination valve.
The vacuum causes the combination valve to open and the additional air conveyed by the pump is pumped into the exhaust pipe behind the exhaust valves. As soon as the lambda control becomes active, the secondary air system is deactivated. The engine control unit deactivates the electric pump and the pneumatic control valve. The combination valve is also closed, preventing hot exhaust emissions from reaching the electric pump and damaging it.
This design has been refined over the years, as emission control strategies grew more sophisticated and effective, the amount of unburned and partially burned fuel in the exhaust stream shrank, and particularly when the catalytic converter was introduced, the function of secondary air injection shifted. Rather than being a primary emission control device, the secondary air injection system was adapted to support the efficient function of the catalytic converter.
The original air injection point became known as the upstream injection point. When the catalytic converter is cold, air injected at the upstream point burns with the deliberately rich exhaust so as to bring the catalyst up to operating temperature quickly. Once the catalyst is warm, air is injected to the downstream location — the catalytic converter itself — to assist with catalysis of unburned hydrocarbons.
Increased emission values during the cold start and warm-up phases can be caused by a lack of post-combustion. The catalytic converter only reaches its operating temperature at a later point. Secondary air systems which are monitored by the engine control unit’s self-diagnosis function, cause the engine indicator lamp to illuminate in the event of fault.
The combination valve, or “Kombi Valve” as it’s called at times, is a pneumatically actuated shut-off valve that is integrated into the valve housing and allows the Secondary Air Injection (SAI) pump to add air to the exhaust gasses during start up and open loop to help with emission control. It serves as a one way check valve to allow the air from the SAI to flow, and prevent exhaust gases from entering and damaging the SAI pump.
A common failure is the Kombi valve, and If you are getting a fault code for secondary air injection system, it could very well be the Kombi valve that’s the problem. When these go bad, it can lead to malfunction in your SAI pump. Because these valves live in the exhaust system, they often can get carboned up over over time and when they do, they allow hot exhaust gases to enter the pump, condense into water, often filling and damaging the electronics on the SAI pump. As such a failed Kombi valve should be replaced right away to prevent damage the expensive SAI pump.
Not An EGR Valve
The Kombi valve is not EGR (exhaust gas recirculation) valve, both are one way check valves, but unlike the Kombi valve, the EGR valve recirculates unburnt exhaust fumes back into the combustion chambers (cylinders) to increase the quality of emissions.
An Ounce Of Prevention
If you’re an Atlantic Motorcar client, we’re going to be keeping an eye on this for you.
If you’re not, make sure it gets checked, kind of like that old bromide about voting, “early and often”. 😉
Our Goal For You and Your Car
Our goal is to save you money, not spend it. This is why each and every car that we service gets a free Courtesy Maintenance Inspection during its first visit. Our goal is to let you know about the small problems before they become big ones. Right now, we have a number of customer cars with well over 200,000 miles, and several approaching 300,000! And these cars are not just limping along – most look and drive pretty much the way they came out of the showroom.
Proper maintenance is an investment in the life of your vehicle.
Knowing, not just doing, but actually knowing and understanding the difference, makes all the difference.
Be sure your car is properly loved, our professionals will attend to both you and your car’s needs.
Atlantic Motorcar…Extraordinary Service for Extraordinary Cars, just a phone call away, (207) 882-9969.