30/11/2002
What is an Air Injection System?
An air injection system, often referred to as a secondary air injection system, is a crucial emissions control technology designed to reduce harmful pollutants released from internal combustion engines. This system works by pumping fresh air into the engine's exhaust ports. The primary goal is to facilitate a secondary combustion process for unburnt hydrocarbons (HC) and carbon monoxide (CO) that are present in the exhaust fumes. By introducing oxygen into the hot exhaust stream, these harmful gases are oxidised, converting them into less harmful substances like water (H₂O) and carbon dioxide (CO₂).
The concept of air injection dates back to the early days of emission control, where it served as one of the first add-on devices to combat rising pollution levels. It was a relatively simple yet effective method to meet the increasingly stringent emission requirements of the time. While modern vehicles employ more sophisticated emission control strategies, the fundamental principle of air injection remains relevant.
The information provided also touches upon Rudolf Diesel's pioneering work, suggesting an early link to fuel injection technologies where both air and fuel were injected into the cylinder. In such systems, the air pressure required for fuel injection could be substantial, often around 70 bar or more. In the context of modern internal combustion engines, fuel injection is a mechanism for precisely combining fuel with air, aiming for optimal combustion to achieve various functional goals, including power output, fuel efficiency, emissions performance, and overall engine smoothness and reliability.
How Air Injection Systems Work
The operation of a basic air injection system is quite straightforward. When the engine is running, an air pump, typically driven by the crankshaft, draws in ambient air through an external filter. This air is then pressurised and directed towards the engine's exhaust ports via connecting hoses. The air is propelled through a diverter valve and a check valve before reaching the air injection manifold, which then distributes it to each exhaust port.
The injected air mixes with the hot exhaust gases within the exhaust manifold. This mixture, containing unburnt fuel components (HC) and carbon monoxide (CO), undergoes a secondary oxidation process. The presence of fresh oxygen allows these pollutants to combust further, transforming them into water vapour and carbon dioxide. This process effectively cleans up the exhaust gases before they exit the vehicle.
The Role of the Diverter Valve
A critical component in many air injection systems is the diverter valve, also known as an anti-backfire valve or gulp valve. During normal engine operation, it allows air to flow into the exhaust ports. However, during engine deceleration, when the engine management system might create a fuel-rich mixture, unburnt fuel can enter the exhaust manifold. If air is continuously injected during this phase, it can lead to a combustion of this unburnt fuel within the exhaust system, causing a 'backfire'. This can potentially damage components like the muffler. The diverter valve prevents this by temporarily shutting off or diverting the air supply during deceleration, thereby preventing backfires.
The Importance of the Check Valve
The check valve is another vital component that ensures the system operates correctly and prevents damage. Its primary function is to act as a one-way valve, preventing the reverse flow of hot exhaust gases from the engine back into the air pump. If the exhaust gas pressure were to exceed the air injection pressure, or if the air pump were to fail, the check valve would close, sealing off the air injection lines. This protects the air pump and associated components from being damaged by the high temperatures and pressures of the exhaust stream. On inline engines, a single check valve is often used, while V-type engines might have one per cylinder bank or a single unit.
Components of an Air Injection System
A typical air injection system comprises several key components:
| Component | Description |
|---|---|
| Air Supply Pump | Belt-driven from the crankshaft, this pump draws in fresh air, filters it, and pressurises it for injection into the exhaust ports. |
| Air Manifolds and Nozzles | These distribute the air from the pump to the individual exhaust ports. Early systems used external manifolds with injection tubes, while larger engines might have integrated internal manifolds within the cylinder head or exhaust manifold. |
| Anti-Backfire Valve (Diverter Valve) | Prevents backfires during deceleration by temporarily stopping or diverting the air supply when manifold vacuum is high. |
| Check Valve | A one-way valve that prevents exhaust gases from flowing back into the air pump. |
Additional Components in Secondary Air Injection Systems
More advanced secondary air injection systems, often found in modern vehicles, may include:
- Air Filter: Ensures the air pumped into the system is clean.
- Secondary Air Pump: The core component that supplies the air.
- Engine Control Unit (ECU): Manages the operation of the secondary air system based on engine parameters.
- Control Relay: Controls the power supply to the secondary air pump.
- Change-over Valve/Combination Valve: Directs air flow, often to bypass the catalytic converter or shut off the system when not needed.
The Secondary Air Injection System and Cold Starts
A significant application of air injection technology is in managing emissions during cold starts. When an engine is first started, it operates with a richer fuel-air mixture, and the catalytic converter, which is essential for reducing emissions, has not yet reached its optimal operating temperature (typically 300-350°C). During this warm-up phase, a substantial portion of a vehicle's total emissions can be released.
The secondary air injection system plays a vital role here. By injecting air into the exhaust stream during the cold start phase, it provides the necessary oxygen to oxidise the unburnt hydrocarbons and carbon monoxide. This secondary reaction helps to reduce HC and CO levels significantly. Furthermore, the heat generated by this secondary combustion process helps to warm up the catalytic converter more quickly, allowing it to become effective sooner. In vehicles with three-way catalytic converters and lambda control, the secondary air system is typically active for a limited period, often around 100 seconds, after which it is deactivated once the catalytic converter reaches its operating temperature.
The supply of secondary air can be either active or passive. A passive system utilises pressure oscillations within the exhaust system to draw in additional air through a timed valve. An active system, on the other hand, uses a pump to actively blast air into the exhaust stream, offering greater control over the air supply.
Advantages and Disadvantages of Air Injection Systems
Like any automotive technology, air injection systems have their pros and cons:
Advantages:
- Reduced Emissions: The primary benefit is the significant reduction in HC and CO emissions by facilitating secondary combustion.
- Improved Fuel Atomisation: In some injection systems, the air can contribute to better fuel atomisation, leading to more efficient combustion.
- Higher BMEP: More complete combustion can result in a higher Brake Mean Effective Pressure (BMEP) compared to some other injection methods.
- Fuel Flexibility: Some systems may allow for the use of less expensive fuels due to the improved combustion process.
Disadvantages:
- Complexity: The addition of components like pumps, valves, and manifolds increases the complexity of the engine system.
- Increased Weight: The extra components add weight to the vehicle.
- Power Draw: The air pump requires engine power to operate, which can slightly reduce the engine's overall Brake Horsepower (BHP).
Applications Beyond Automobiles
While most commonly associated with vehicles, air injection technology has broader applications:
- Wastewater Treatment: Air is injected to provide oxygen for aerobic bacteria, which are crucial for breaking down organic waste.
- Aquaculture: Used to maintain optimal dissolved oxygen levels in fish farms, promoting fish health and productivity.
- Industrial Processes: Employed in combustion, incineration, and gasification to improve efficiency and reduce emissions.
- Soil Remediation: In situ techniques like bioventing use air injection to introduce oxygen into contaminated soil, promoting the breakdown of pollutants by beneficial bacteria.
Frequently Asked Questions (FAQs)
- What is the air injection system?
- The air injection system is an emissions control strategy where fresh air is injected into the exhaust stream to promote a secondary combustion of unburnt gases, thereby reducing pollutants like HC and CO.
- What is the function of the air injector?
- The air injector's role is to deliver the pressurised air from the air pump into the exhaust ports, enabling the oxidation of unburnt fuel components.
- What happens when an air injection system fails?
- A failure in the air injection system typically won't directly impact driving performance or safety. However, it will lead to increased emissions and will likely trigger the 'Check Engine' warning light.
- Is it bad to drive with a faulty air injection pump?
- Driving with a faulty secondary air pump is not immediately dangerous, as it's not a primary engine intake component. However, it can lead to issues like reduced power, rough idling, or stalling during cold starts, and will negatively affect your vehicle's emissions output.
In conclusion, air injection systems, particularly the secondary air injection variant, have been instrumental in reducing vehicle emissions, especially during critical cold-start phases. While they add some complexity, their contribution to cleaner air and meeting environmental standards is undeniable.
If you want to read more articles similar to Understanding Air Injection Systems, you can visit the Emissions category.