What is exhaust gas recirculation? PH Explains

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.

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Comments (17) Join the discussion on the forum

  • Welshman Adam 18 Sep 2018

    Really enjoy these articles.

    My question is this - why do some people block off the EGR valve? What's the benefit? It was a common mod on the Focus 1.8 TDCi I used to run, but never did it myself.

  • TooMany2cvs 18 Sep 2018

    Welshman Adam said:
    My question is this - why do some people block off the EGR valve? What's the benefit?
    Because it clogs in short order, and causes the engine management to have a huff. So block/disconnect and remap to remove the whinge, all is happy. Well, apart from a minor emission downside...

    What it doesn't mention is how long EGR's been about. I've had it on a 1990 Saab 900T16, and currently have it on a 1997 Citroen 1.9XUD turbo, and a 1980 2.25 petrol Land Rover. It's been disconnected or clagged solid on every single one before I bought 'em - despite the XUD and Landy both being ~40k from new - but they're all old enough not to whinge about it.

    Edited by TooMany2cvs on Tuesday 18th September 12:17

  • ingrowtn 18 Sep 2018

    I know exactly what it is - a monumental and expensive pain in the behind necessetating numerous stints back in the garage workshop to try and figure out excatly what bits of it are not working properly. Sadly the design of my engine is such that apparently it is not possible to blank off or disengage.

    Having chaged the turbo (and associated pipework), turbo actuator, vacuum pipes, EGR valve and the MAF sensor (diagnosed from code readings) it still doesn't work.

    I would also define excfhause gas recirculation as 'if fitted, walk away and buy a different car'.

    Only my experience and no doubt others will never have had the issues I have had; but for me never, ever, again.

  • Turbobanana 18 Sep 2018

    PH said:
    ...in diesels, the recirculation of the exhaust gases helps...
    ...create prodigious amounts of soot to be deposited when the DPF reaches a certain level, normally just after I turn left onto Childs Way, MK, on my way to work.

    TooMany2cvs said:
    I've had it on a 1990 Saab 900T16...
    I have one of these, albeit a 1991. Is it standard fit? How do I know if I've got it? Would love to remove it if I have. Car is pre-1992, so doesn't need a cat smile

  • TooMany2cvs 18 Sep 2018

    Turbobanana said:
    TooMany2cvs said:
    I've had it on a 1990 Saab 900T16...
    I have one of these, albeit a 1991. Is it standard fit? How do I know if I've got it? Would love to remove it if I have. Car is pre-1992, so doesn't need a cat smile
    Yep, standard. Look at the front of the engine, between bellhousing and distrib. Is there a pipe wrapped in a shiny metal spring, going from the exhaust manifold round to inlet, with a valve connected to one of the myriad of vac hoses? That's the EGR.

    Removal just needs both ends bunging, and the now-spare vac blocking off at the plenum.

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