What is an Air Injection System?
An air injection system is an emission control technology designed to reduce harmful pollutants such as unburned hydrocarbons (HC) and carbon monoxide (CO) emitted from an internal combustion engine. It achieves this by introducing fresh, ambient air directly into the engine’s exhaust ports or manifold, facilitating the oxidation of residual HC and CO in the exhaust gases. This process transforms these pollutants into less harmful substances like water (H2O) and carbon dioxide (CO2), thereby significantly lowering emissions.
The concept of air injection originated from Rudolf Diesel’s innovative ideas. In early engine designs, the system’s basic configuration involved pumping air into the exhaust stream, which helped maintain cleaner emissions and comply with stricter environmental regulations. Typically, the system comprises key components such as the air pump, diverter valves, air distribution manifolds, and check valves, all working in unison to optimize emission reduction during various engine operating conditions.
Diagram of an Air Injection System
While a detailed schematic helps visualize the system’s components and flow paths, the core principle involves the controlled delivery of air into the exhaust system, especially during cold starts, to promote complete combustion and catalytic converter efficiency.
Core Components of an Air Injection System
Air Supply Pump
This pump is typically belt-driven from the engine’s crankshaft and positioned at the front of the engine compartment. Its primary role is to draw in fresh air through an external filter and deliver it under low pressure to the exhaust ports via a network of hoses. The injected air reacts with HC and CO present in the hot exhaust gases, facilitating their oxidation into less harmful compounds.
Air Manifolds and Nozzles
Early versions of the system employed external air manifolds to supply air directly to each exhaust port using injection tubes, especially in smaller engines. Larger engines, on the other hand, utilized internal air manifolds integrated within the cylinder head or exhaust manifold, distributing air efficiently near each exhaust valve to maximize oxidation during emissions control.
Anti-Backfire Valve
During deceleration, high vacuum conditions in the intake manifold can cause the air-fuel mixture to become excessively rich, leading to unburned fuel in the exhaust. Injected air can ignite these unburned gases, resulting in backfires that damage the muffler and other components. The anti-backfire valve, also known as a diverter or gulp valve, prevents this by shutting off airflow during deceleration, thereby protecting the exhaust system from backfire-induced damage.
Check Valve
This one-way valve ensures that exhaust gases do not flow back into the air pump. Positioned between the air manifold and diverter or gulp valves, it closes when exhaust pressure exceeds the pressure of injected air or if the air pump malfunctions. On inline engines, a single check valve is common, while V-type engines often incorporate two, one for each bank, to maintain system integrity.
Additional Components of Secondary Air Injection System
- Air Filter
- Secondary Air Pump
- Engine Control Unit (ECU)
- Control Relay
- Change-Over Valve
- Combination Valve
Working Principle of Air Injection System
The operation of this system is straightforward yet highly effective. When the engine runs, the air pump’s vanes rotate, drawing in ambient air and pushing it through the diverter valve. If the vehicle is not in a deceleration phase, the system directs this air through check and diverter valves, into the exhaust manifold or ports, where it mixes with hot exhaust gases. This promotes oxidation of unburned HC and CO, especially during cold engine starts when the catalytic converter has not yet reached optimal operating temperature.
During cold starts, the secondary air injection significantly reduces emissions by ensuring sufficient oxygen is available for the catalytic converter’s reactions, which convert pollutants into less harmful substances. The system is typically active for about 100 seconds after cold engine startup, after which it deactivates as the catalytic converter warms up and becomes more effective.
In passive systems, pressure oscillations in the exhaust help draw in additional air automatically, whereas active systems employ a dedicated pump and electronic controls to regulate airflow precisely. The primary goal is to elevate the catalytic converter’s temperature rapidly and maintain high efficiency during the initial warm-up phase, thereby minimizing environmental impact.
Advantages of an Air Injection System
- Enhanced Emission Reduction: Effectively oxidizes HC and CO, aiding compliance with environmental standards.
- Improved Combustion Efficiency: Promotes more complete fuel burning, leading to better fuel economy.
- Cost-Effective Fuel Usage: Allows the use of lower-grade fuels without compromising emission standards.
Disadvantages of an Air Injection System
- Complexity and Maintenance: The system’s mechanical and electronic components add to the overall engine complexity and require regular maintenance.
- Increased Engine Weight: Additional components contribute to a heavier engine assembly, impacting vehicle dynamics.
- Power Drain: The operation of the air pump consumes engine power, which may slightly reduce overall BHP (Brake Horsepower).
Frequently Asked Questions (FAQs)
What exactly is an air injection system?
It is an emissions control strategy where fresh air is injected into the exhaust stream, especially during cold engine operation, to facilitate complete combustion of unburned gases, thereby reducing emissions.
What is the primary function of an air injector?
The air injector’s main role is to supply additional oxygen to the exhaust gases, promoting oxidation of pollutants like HC and CO, and assisting the catalytic converter in reaching optimal operating temperatures faster.
What occurs if the air injection system fails?
A malfunctioning system typically does not impair engine performance but can lead to increased emissions. It may trigger a check engine light and could result in failed emissions tests, necessitating repairs or system inspections.
Is it harmful to drive with a defective air injection pump?
While driving with a faulty pump won’t usually cause immediate engine damage or safety issues, it can lead to higher emissions, potential increased fuel consumption, and may cause the vehicle to fail emissions testing. Repairing or replacing the pump is advisable to maintain optimal vehicle performance and compliance.
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