Exhaust gas recirculation is a technique used to reduce nitrogen oxide emissions in both petrol and diesel engines.
These 'EGR' systems are used in conjunction with other emissions control devices, such as catalytic converters, in order to help minimise harmful exhaust gases.
What does an exhaust gas recirculation system do?
The ambient air ingested by an engine during the intake stroke contains oxygen, which allows the fuel supplied to the combustion chamber to burn.
However, the air also contains a high proportion of nitrogen; some 21 per cent of the air consists of oxygen, while over 78 per cent is nitrogen. When this nitrogen is pressurised and heated during the combustion cycle, in the presence of excess oxygen, nitrogen oxides are formed. These, when exposed to sunlight in the atmosphere and other chemicals, can cause the formation of smog and acid rain.
To combat unwanted nitrogen oxide - NOx - emissions, exhaust gas recirculation systems are used. These systems of sensors, valves and pipes recycle a portion of the engine's exhaust gases back into the engine's intake.
This inert exhaust gas dilutes the fresh air being supplied through the intake system to the combustion chamber. This means that less of the cylinder's contents are suitable for combustion - which reduces the speed, temperature and pressures experienced during the combustion cycle, greatly lowering the amount of NOx created.
The exhaust gas present can also absorb some of the heat of combustion, lowering cylinder temperatures and pressures even more; this further helps cut NOx emissions.
Predictably, reducing the quantity of oxygen available to burn with fuel also potentially limits the power produced during that stroke. To avoid any unwanted power loss, EGR systems will subsequently only run at certain engine speeds and loads.
In a diesel engine, for example, EGR will typically only take place when the load is comparatively light and the crank speed is below 3,000rpm - with the aim being to ensure that just enough oxygen is present in the chamber to ensure complete combustion, nothing more.
What does an EGR system consist of?
The EGR valve, which directs exhaust gas back into the intake, is the key part of an EGR system. It is often pneumatically controlled by a standalone system that monitors engine vacuum and load in order to best regulate the operation of the valve. These valves can, however, be entirely electrical in operation.
In some cases, a cooler is fitted to lower the temperature of the exhaust gas before it is recirculated into the engine. This helps avoid dumping excessively hot gas into the combustion chamber which would otherwise increase the temperature, resulting in excess NOx production and potentially causing knocking.
A manufacturer, in turbocharged applications, may use 'high-pressure' or 'low-pressure' exhaust gas recirculation systems - or a combination of both. High-pressure systems, as the name suggests, draw and feed exhaust gas from and into high-pressure zones; waste gas is extracted from the exhaust manifold, upstream of the turbocharger, and recycled into the intake downstream of the intercooler.
In a low-pressure EGR system, exhaust gas is tapped off downstream of the turbocharger and fed into the intake tract before the turbocharger's inlet. There are advantages and disadvantages to both; low-pressure systems, for example, recirculate exhaust gas that has been through the emissions control systems. This means that particulate matter isn't being fed into the engine, which can contaminate the oil.
A low-pressure system can be slower to respond than a high-pressure system, though, due to the pipework involved - resulting in the production of unwanted NOx during engine load changes. This is why some manufacturers use both, allowing the overall EGR system to better cope with a wider range of conditions.
How beneficial is an exhaust gas recirculation system?
According to manufacturers, the use of EGR systems can cut diesel NOx emissions by upwards of 50 per cent - while petrol NOx emissions can be cut by over 40 per cent
There are also other benefits to EGR; in diesels, the recirculation of the exhaust gases helps cut particulate and hydrocarbon emissions while also fractionally reducing the volume of the exhaust. In petrol engines, on the other hand, EGR can also reduce carbon dioxide emissions.
As a result, EGR often forms a key part of an engine's emissions control hardware and helps manufacturers to meet increasingly stringent emissions regulations.