How an EGR (Exhaust Gas Recycling valve) works - in layman's terms
I am sure that if I asked 100 diesel vehicle owners in the street if they knew what an
EGR valve was, let alone how it works, a significant majority would not. You might
find this interesting or even scary but I hope to explain how it works. I have copied a
diagram from Google images to help with the explanation.
In a diesel engine clean air is drawn through the air inlet by the help of the
compressor inlet of the turbo charger (want to know more about the workings of a
turbo see here – link)
The inlet air, warmed by the turbo system during uptake, will cause the carbon
dioxide (CO2) molecules to expand and therefor cause a imbalance with the air to fuel
ratio but it is then passed through the charge air cooler (also called a intercooler)
which then cools the inlet air. Cooling the air therefore will increase the molecules
thus allowing for more clean air to burn.
The clean cooled air then enters the cylinder via the inlet valves. When the inlet
valves closes the piston then compresses the trapped air and causes the air
molecules to expand resulting in a cylinder temperature of over 400°C. Atomized fuel
is then injected and combustion takes place which is the explosion known as the
power stroke (want to know more see here – link)
The exhaust valve opens allowing the burnt gases to be forced through the turbo
outlet to the exhaust. Flow of the exhaust gases causes the turbo to spin. The
spinning of the turbo draws clean air into the compressor inlet of the turbo.
This sounds good because this maintains two individual passages for different
airflows; the clean inlet air passage and the dirty exhaust for burnt gasses.
However there is a problem because although the engine runs efficiently, it also
produces more Nitrogen oxide (NOx) gases that are harmful to the ozone layer and
are a known greenhouse gas.
As a result of this the engine manufactures have thought of a way to reduce the NOx
emissions by reducing the oxygen levels in the cylinder, when efficient fuel
combustion is not required, i.e. when the vehicle is idling or at low rpm.
This led to the birth of the EGR valve. Here dirty, used, warm air is redirected into
the clean inlet of the cylinder from the exhaust. This results in less oxygen being
drawn into the cylinder, reducing the combustion temperature and resulting in less
NOx emissions.
The down side is that the EGR valve and the cylinder inlet valves get heavily coated in
carbon deposits either causing EGR failure or a reduction in the amount of quality
clean air being drawn into the engine. This creates an incorrect air to fuel ratio which
results in black exhaust smoke, poor running and increased fuel consumption.
Another problem with increased carbon emissions is that it has an effect on the diesel
particulate filter (DPF), which is something else that has been developed to help
reduce emissions from the exhaust. I will discuss that later.
Below is a picture of an inlet manifold that to be efficient would be clean and free of
carbon deposits. However over a period of time the EGR valve has allowed burnt
gasses and carbon to enter the inlet manifold thus restricting the ports. This would
cause an imbalance in the air to fuel ratio resulting in poor fuel consumption and
more carbon to be drawn back in to the inlet via the EGR valve.