The Secondary Air Injection System (SAIS) is part of a vehicle's exhaust system. Manufacturers began introducing it in the 1960s to help reduce hydrocarbon and carbon monoxide emissions, both of which resulted from incomplete combustion. Toyota and Lexus SAIS systems are only commanded to operate after a cold start, which is when the engine has been sitting for at least seven hours.
The SAIS injects oxygen-rich ambient air into the exhaust manifold downstream of the exhaust valves. It does this for approximately 30 to 90 seconds to allow the engine to get to operating temperature. The SAIS then shuts off.
Petrol-powered engines aren't 100% efficient. As a result, there is a portion of unburned fuel (hydrocarbons) that doesn't get converted to engine power. This is particularly true on cold starts for two reasons. First, for more reliable starting, the fuel/air mixture goes richer in fuel. Secondly, it takes a couple of minutes for the engine and catalytic converters to get to operating temperature.
These two factors combine to increase the amount of unburned fuel after a cold start. Thus, the purpose of the secondary air injection system is reducing the hydrocarbon and carbon monoxide emissions that result from cold start conditions.
There are several components that comprise the secondary air injection system. They include air injection drivers, air pumps, air switching valves, and pressure sensors. The location of these components varies depending on year, model, and engine size. Subaru and V8 Toyota & Lexus vehicles will tend to have 2-Bank SAIS configurations, whilst 4- & 6-cylinder Toyota and Lexus vehicles will usually come from the factory with a 1-Bank SAIS configuration. This determines the number of air injection drivers and air pumps that are part of the SAIS.
Most of the air sent through the induction chamber for primary combustion is depleted, leaving exhaust lacking oxygen. That said, the SAIS reintroduces oxygen-rich air into the exhaust, allowing unburned fuel to continue burning before being expelled into the atmosphere. This process is carried out by the various components of the SAIS.
On cold start ignitions, the air injection drivers command the air switching valves to open and the air pumps to pump air through the air switching valves. The air is then injected into the exhaust manifold upstream of the catalytic converters. When the air/oxygen provided by the SAIS mixes with the post-combustion exhaust, oxidation occurs. Now replenished with oxygen, hydrocarbons and carbon monoxide can oxidise into less harmful emissions like water and carbon dioxide.
The continued burning of fuel also increases the temperature of the exhaust gas, helping the catalytic converters heat up to operating temperature quicker. It typically takes 30 to 90 seconds to get to operating temperature, after which air from the SAIS is no longer needed and the SAIS stops operating. The air pumps turn off and the air switching valves close. If an air switching valve does not close, exhaust can leak into the SAIS and cause additional issues.
The secondary air injection system only affects post-combustion operations, so a non-functioning SAIS doesn't impact engine performance or fuel economy. Therefore, the sole function of the SAIS is emissions reduction. That said, the SAIS will only run on a cold start when the vehicle has been sitting for at least seven hours. Even then, it will only run for 30 to 90 seconds to allow the operating temperature to be reached.
Practically speaking, the SAIS reduces emissions "at most" once every seven hours for 30 to 90 seconds. Outside of that specific time period, the SAIS is inactive. Consequently, if the secondary air injection system is not working properly, the exhaust will be richer after a cold start until operating temperature is reached. Unfortunately, there are certain secondary air injection system error codes that can turn on the "Check Engine Light" and put the vehicle into "limp mode," where power and acceleration are significantly limited.
Eurotune have a range of Secondary Air Injection System (SAIS) solutions starting from just $395+GST. The solutions for resolving Secondary Air Injection System issues range from repairing, replacing, deleting or disabling. Call us today.
Contact your nearest Eurotune dealer to discuss your vehicle's secondary air injection problems.
