If you've ever listened to the sometimes raucous cold start-up sequence of a modern car, you may well have noticed that it suddenly quietens down after a minute or two.

Listen to the same car start up when warm and you'll no doubt further notice that this sequence doesn't repeat; instead, the volume isn't so antisocial and remains at a constant level.

The initial racket when cold isn't necessarily an attention-grabbing mechanism but, instead, it can often be the result of a system designed to help cut the car's cold-start emissions.

The set-up in question is called the 'secondary air injection' system, which pumps air into the engine's exhaust system when it is cold. The oxygen contained in this air allows the remaining fuel in the exhaust gases to combust, helping reduce emissions and warm up the catalytic converter.

How does secondary air injection work?

According to component manufacturer Pierburg, over 80 per cent of a car's driving cycle emissions are typically produced when the car is cold. Two factors contribute to this; the first is rich cold start mixtures, which cause excessive hydrocarbon and carbon monoxide emissions - both of which are a result of incomplete combustion.

The second issue is that the car's emissions-reducing catalytic converter doesn't become effective until it reaches around 300 degrees Celsius. This, in conjunction with the rich mixtures, causes high emissions in the initial phase of the driving cycle.

To help counter this, manufacturers fit secondary air injection systems. These feature an electrically driven air pump, which can be cycled on and off when needed, that supplies fresh air into the car's exhaust system. The air is injected upstream of the catalytic converter, where it can mix with hot waste gas from the engine's cylinders.

The presence of the oxygen-containing air in the hot exhaust gas allows for any remaining fuel to continue burning, which helps cut HC and CO emissions; the reactions that take place cause hydrocarbons to be oxidised into water and less harmful carbon dioxide, while carbon monoxide is oxidised into less harmful carbon dioxide.

Furthermore, the process increases the temperature of the exhaust gas. As the gas is hotter, it speeds the rate at which the catalytic converter warms up; when it is running efficiently, the air injection system can then be shut down - although there are sometimes variations in the way the systems operate.

A brief history of secondary air injection

In America, during the 1960s, regulations were introduced to combat high hydrocarbon and carbon monoxide emissions. These emissions were a result of incomplete combustion, often experienced when the mixture was excessively rich - such as during a cold start.

To alleviate these issues, air injection systems were introduced; Chevrolet, for example, unveiled its 'Air Injection Reactor' set-up in 1966. These systems, soon dubbed 'smog pumps' would use engine-driven pumps to deliver fresh air into the exhaust port or manifold of the engine, where the exhaust gases were hottest.

Other systems would rely on the pressure pulses in the exhaust to draw in fresh air but, in any case, the effect was similar - oxygen would be introduced to the exhaust stream and allow any remaining fuel to burn. However, these systems were often unreliable - partly because they relied on precise carburettor calibration - and many owners simply disabled them.

The introduction of catalytic converters and more accurate fuel metering systems, in the form of mass-produced electronic injection systems, also greatly reduced the need for air injection systems. That said, it was not uncommon to find air injection set-ups on large or high-performance engines which would produce excessive emissions when cold.

The 'S38' straight-six engine used in the E34 generation of M5 made use of such a system, as did the C4 generation of Corvette - and, when engaged, the system's operation would cause a pronounced change in tone. This additional volume would frequently be exacerbated when the pump itself was failing, which remains an issue in modern set-ups.

As emissions regulations continued to tighten and the control systems advanced, manufacturers again turned to air injection. This time, however, the aim of injecting air into the exhaust system was primarily to help warm up the catalytic converter more quickly.