air injection system

Understanding Air Injection System

Air injection is a technique for lowering exhaust emissions that involves injecting air into each of an engine’s exhaust ports, where it mixes with the hot exhaust and oxidizes HC and CO. H2O and CO2 are formed. It was simple to achieve the required emission requirements by air injection in the early days of emission control. One of the first add-on devices to oxidize HC and CO in the exhaust was air injec­tion, also known as the air pump system.

In this article, you’ll learn the definition, components, diagram, working, advantages, and disadvantages of an air injection system.

air injection system


What is an air injection system?

To lower HC and CO emissions, an air injection system pumps fresh air into the engine’s exhaust ports. Unburned and partially burned fuel can be found in the exhaust fumes of an engine. This fuel continues to burn due to oxygen from the air injection system. The air pump, diverter valve, air distribution manifold, and air check valve are the main components of the system.

Rudolf Diesel was the one who came up with the idea. Figure 1 depicts the system’s configuration. During the supply of gasoline, both air and fuel are injected into the cylinder in this arrangement. The air pressure required for fuel injection is around 70 bar or more.

In an internal combustion engine, fuel injection is a mechanism for combining fuel with air. Fuel injection systems have a variety of functional aims, but they all have one thing in common: they feed fuel to the combustion process. There are various competing goals, including Power production, fuel consumption, emissions performance, dependability, smooth operation, startup, ongoing costs, and maintenance costs are all factors to consider.

Components of an air injection system

The following are the major components of an air injection system:

  • Air supply pump with filter.
  • Air manifolds and nozzles.
  • Anti backfire valve.
  • Check valve.
  • Connecting hoses.

Air supply pump:

The air pumps are belt-driven from the crankshaft and are located at the front of the engine. The pump pulls fresh air in through an external filter and sends it through connecting hoses to each exhaust port at low pressure. When this extra air is added to the heated HC and CO emissions in the exhaust manifold, they oxidize, converting these elements to H2O and CO2.

Air manifolds and nozzles:

To feed air to the engine’s exhaust system, early air injection systems used one of two methods:

  • On smaller engines, an external air manifold supplies air to the exhaust port near each exhaust valve through injection tubes.
  • Through tunnels in the cylinder head or the exhaust manifold, an internal air manifold transports air to the exhaust port near each exhaust valve on larger engines.

Anti backfire valve:

A high-intake manifold vacuum permits the air-fuel mixture to become fuel-rich during engine deceleration. Fresh air injected into the exhaust manifold during deceleration mixes with unburned gasoline in the exhaust, resulting in engine backfires. This backfire is caused by the quick combustion of unburned gases, which can destroy a muffler. An anti-backfire, or backfire suppressor, valve is used to prevent this by shutting off the airflow during deceleration. To stop airflow, both the gulp valve and the diverter valve are utilized.

Check valve:

A one-way check valve prevents the reverse flow of exhaust gases from the engine through the air pump. Between the air manifold and the diverter valve or gulp valve is the check valve. The check valve spring closes the valve to block the reverse flow of exhaust when exhaust pressure exceeds air injection pressure or if the air pump fails. On in-line engines, a single check valve is often placed, but on V-type engines, two valves (one per cylinder bank) are typically installed. Some V-type engines, on the other hand, only have one check valve.

The secondary air injection system has some additional parts:

  • Air filter
  • Secondary air pump
  • Engine control unit
  • Control relay
  • Change-over valve
  • Combination valve.

secondary air injection system

Diagram of a secondary air injection system:

Modern air injection systemWorking Principle

The working of an air injection system is less complex and can be easily understood. in its working, the air pump’s spinning vanes propel air into the diverter valve when the engine is running. The air is driven through the diverter valve, the check valve, the air injection manifold, and into the engine if the vehicle is not decelerating. The exhaust valves are blown by fresh air. The diverter valve prevents air from entering the engine exhaust manifold during deceleration. This avoids a potential backfire, which could harm the vehicle’s exhaust system. The diverter valve will release surplus pressure in the system when it is needed.

The secondary air injection system:

During the cold start phase, when the catalytic converter is not yet functioning, this system decreases the HC and CO levels even more. In stoichiometrically driven gasoline engines, a 3-way catalytic converter achieves a conversion rate of over 90%. During a cold start, up to 80% of a vehicle’s emissions are released. However, because the catalytic converter is only effective between 300°C and 350°C, emissions must be reduced during the cold start phase utilizing a variety of methods.

The secondary air system’s job is to do just that. When there is enough residual oxygen in the exhaust system and the temperature is high enough, the HC and CO react to generate CO2 and H2O in a secondary reaction. When the mixture is highly rich during the cold start phase, the air is injected into the exhaust flow to ensure there is enough oxygen for the reaction. The secondary air system is turned off after approximately 100 seconds in vehicles with a three-way catalytic converter and lambda control.

The heat created in the secondary reaction immediately raises the working temperature of the catalytic converter. An active or passive supply of supplementary air is possible. The passive system makes use of pressure oscillations in the exhaust system. Due to the vacuum caused by the exhaust pipe’s flow rate, additional air is brought in via a timed valve. The secondary air is blasted in by a pump in an active system. This system gives you more control.

Watch the video below to learn more about the air injection system:

Advantages and disadvantages of an air injection system


Below are the benefits of an air injection system in its various applications:

  • It allows for improved fuel atomization and distribution.
  • The BMEP is higher than with other types of injection systems because the combustion is more complete.
  • It enables the use of less expensive fuels.


Despite the good advantages of an air injection system, some limitations still occur. Below are the disadvantages of an air injection system in its various applications:

  • The compressor’s operation necessitates a complex mechanism.
  • The engine’s weight increases.
  • Because a portion of the engine’s power is needed to operate the compressor, the engine’s B.H.P. is diminished.

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That is all for this article, where the definition, components, diagram, working principle, advantages, and disadvantages of a secondary air injection system are been discussed. I hope you learn a lot from the reading, if so, kindly share with other students. Thanks for reading, see you around!