Secondary air injection
Secondary air injection (SAI) is an emissions control technology employed in spark-ignition internal combustion engines to reduce harmful exhaust pollutants by injecting oxygen-rich ambient air into the exhaust manifold shortly after engine startup.[1] This process promotes the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO₂) and water (H₂O) through secondary combustion reactions in the hot exhaust stream.[1] SAI is especially vital during the cold-start phase, when the engine and three-way catalytic converter are below their effective operating temperature of 300–350°C, a period accounting for up to 80% of a vehicle's total HC and CO emissions over a typical drive cycle.[1] Introduced in the late 1960s as part of early U.S. federal efforts to curb urban air pollution under the Clean Air Act, SAI systems were among the first aftertreatment technologies mandated for new automobiles to meet tightening hydrocarbon and carbon monoxide standards.[2] Initially featuring belt-driven "smog pumps" to force air into the exhaust ports, these systems evolved with electronic controls and electric pumps to improve reliability and integration with engine management.[3] In contemporary applications, SAI activates for approximately 100 seconds post-startup, controlled by the engine control unit (ECU) via temperature sensors and oxygen feedback, until the catalytic converter achieves light-off and assumes primary emissions reduction duties, enabling over 90% conversion efficiency for HC and CO in three-way catalysts.[1] SAI remains relevant for proposed Euro 7 regulations expected in 2026.[4] SAI systems comprise key components including an air pump (electric or belt-driven), air filter, combination or change-over valves for flow regulation, check valves to prevent backflow, and associated relays and solenoids monitored by the ECU for diagnostic fault detection.[1] Active SAI uses powered pumps for precise air delivery, while passive variants rely on exhaust pressure pulses for aspiration, though active systems predominate in modern passenger vehicles and motorcycles to comply with stringent standards like Euro 6 and U.S. Tier 3.[1][5][6] Common faults, such as pump failure or valve blockages, can trigger the malfunction indicator lamp and increase emissions, underscoring the need for regular maintenance in emissions-compliant fleets.[1]Fundamentals
Purpose and emissions reduction
Secondary air injection (SAI) is an emissions control strategy that introduces fresh, oxygen-rich air into the exhaust stream to facilitate the oxidation of unburnt hydrocarbons (HC) and carbon monoxide (CO), thereby reducing their release into the atmosphere.[1][7] This system primarily targets the cold-start phase of engine operation, during which incomplete combustion occurs due to rich fuel mixtures and cold engine components, leading to elevated HC and CO emissions.[1][7] By promoting post-combustion oxidation in the exhaust manifold or near the exhaust ports, SAI helps mitigate these pollutants before they reach the tailpipe.[8] A key benefit of SAI is its role in accelerating the warm-up of the catalytic converter to its light-off temperature, typically around 300–400°C, which enhances overall emissions conversion efficiency during the initial 1–2 minutes of operation.[8] The exothermic oxidation reactions generate additional heat, shortening the time required for the converter to become effective and thereby minimizing cold-start emissions.[8] Quantitative studies demonstrate SAI's effectiveness, with reductions in cold-start HC emissions ranging from 46% to 88% and CO emissions from 37% to 93% over the first 25 seconds of operation, depending on air flow rates and engine conditions.[9] These improvements occur independently of three-way catalytic converters, providing a complementary mechanism for early emissions control.Operating principles
Secondary air injection introduces fresh oxygen-rich air into the exhaust manifold or ports immediately after the engine's combustion chamber, facilitating the post-combustion oxidation of unburnt fuel components present in the exhaust gases. This process primarily targets carbon monoxide (CO) and hydrocarbons (HC), converting them into carbon dioxide (CO₂) and water (H₂O) through exothermic reactions that also generate heat to accelerate catalytic converter warm-up. The key chemical reactions are the oxidation of CO:$2CO + O_2 \rightarrow 2CO_2
and for hydrocarbons (generalized for typical exhaust species like ethylene):
C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O
or more broadly represented as supplying oxygen for HC oxidation to harmless byproducts, with the injected air providing the necessary O₂ under oxygen-deficient conditions. Physically, the injection occurs when exhaust gas temperatures are between 200–400°C to ensure sufficient thermal energy for reaction initiation without quenching the oxidation process due to excessive cooling from the ambient air. The system's operation is timed for 20–120 seconds following a cold start, allowing enough duration for effective HC and CO reduction while preventing overheating of the catalytic converter, which could otherwise lead to thermal damage.[1][10][11] This mechanism integrates with engine operation during cold starts, when the air-fuel mixture is intentionally rich (lambda < 1) to ensure reliable ignition and stable combustion in a cold engine, resulting in elevated levels of unburnt HC and CO in the exhaust. By supplying secondary air, the system oxidizes these rich-mixture byproducts in the exhaust stream, tying directly to emissions targets for HC and CO reduction during the non-operational phase of the catalyst.[1][12]