What is an EGR Valve?

03rd Dec 2019

Over recent years, climate change has been a big concern in the UK. Exhaust emissions from road transport contributed to 21% of UK greenhouse gas emissions (GHG emissions) in 2017, according to Office for National Statistics - UK Environmental Accounts.

Emissions regulations are becoming increasingly strict to combat the amount of GHG emissions emitted from road transport. Car manufacturers are having to make adaptations to the designs of their vehicles to meet the stringent limits. 

One method of reducing exhaust emissions is the use of after-treatment devices. The majority of modern diesel vehicles are designed to incorporate an Exhaust Gas Recirculation (EGR) system. But, what is an EGR...?

What is an EGR Valve?

Diagram of an EGR system

Within modern, diesel internal combustion engines, Exhaust Gas Recirculation (EGR) is a method to control Nitrogen Oxides (NOx) emissions, produced as a byproduct during the combustion process. Air from the environment is mostly a combination of Oxygen and Nitrogen. When the air combines with fuel and ignites inside the combustion chamber, temperatures increase and produce NOx emissions.

The EGR system works by returning a small portion of an engine's exhaust gas to the engine's combustion chambers through the intake manifold, lowering combustion temperatures and therefore reducing the amount of NOx emitted.

The EGR valve is the main component of the EGR system and it's normally closed. It connects the exhaust manifold to the intake manifold and is controlled by either a vacuum or a built-in electric step motor. The function of the EGR valve is to control the flow of exhaust gas being recirculated depending on the engine load. 

Nitrogen Oxide (NOx)

Black smoke coming out of car exhaust

Nitrogen oxides are emissions produced as a byproduct from the process of combustion, Nitrogen and Oxygen gases in the air react during combustion, especially at high temperatures.

NOx is a major component of smog and can have detrimental affects on human health as well as well as ecosystems and agricultural crops. Therefore, incorporating EGR systems into the design of a car is important with regards to lowering harmful emissions to save the environment and have a positive impact on human health.

How Does an EGR Valve Work?

Diesel fuel gauge

The majority of modern diesel vehicles incorporate EGR valves into their design to reduce NOx emissions and therefore meet stringent emissions regulations. EGR systems recycle a portion of the exhaust gas back into the combustion chamber, where it combines with fresh intake air.

This lowers the Oxygen content and increases the water vapour content to the combustion mixture which reduces peak combustion temperature. Because more NOx is created as peak combustion temperature rises, the EGR valve effectively reduces the amount of NOx produced by the engine. 

The EGR valve begins working once the engine is started, attains the correct operating temperature and speed increases. Gradually, the EGR valve regulates the flow of exhaust gases. Once you slow down and the engine stops, the EGR valve will return to its closed position and prevent the flow of exhaust gases.

Problems with an EGR Valve

Fuel pumps

A frequent problem with the EGR valve is sticking due to a build up of carbon deposits. In worst cases, the EGR valve and EGR passages can be completely blocked, preventing the process of recirculating the exhaust gases. 

Clogged EGRs are often the cause of black smoke escaping the exhaust, in addition to increased fuel consumption or reduced performance. If the EGR valve fails to open or close, the 'check engine' light will illuminate on the dashboard. 

A fuel odour smelt from inside the vehicle is also a sign of a failing EGR valve as more hydrocarbons will be emitted through the exhaust due to the increase in fuel consumption. The smell is easily noticeable due to its irritating nature as it can lead to human health issues. 

EGR Valve and an MOT Test

EGR valve

In the past, car owners attempted to remove EGR valves and Diesel Particulate Filters (DPFs) from their vehicles to avoid costly repairs. However, for Euro 6 Emissions Standards compliant vehicles it has now been stated by the Department for Transportation that removing the EGR valve or DPF is illegal as the vehicle will no longer be compliant with the road vehicle regulations.

Vehicles will also fail their MOT test due to emission levels emitted and owners can face an astounding £1,000 fine for removing the EGR valve or diesel particulate filter. 
An EGR valve is definitely a positive addition to vehicles in terms of reducing harmful NOx emissions, despite their expensive repair costs. If you think you're experiencing a faulty EGR valve you can book in for a free vehicle health check at your local Evans Halshaw retailer.