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Exhaust Gas Recirculation


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Name:

Exhaust Gas Recirculation

 

Also known as:

EGR

 

What is it?

Popular misconception:

EGR "cleans" the exhaust gas of unburnt hydrocarbons by running it through the engine a second time.

 

What it really does:

EGR manages the tradeoff between two different types of exhaust emissions: HC and NOx (unburnt hyrdocarbons and oxides of nitrogen) by controlling the speed of the combustion process within the cylinder.

 

A fast burn of the fuel / air mixture inside the combustion chamber will tend to produce NOx emissions. A slower burn will tend to produce unburnt hydrocarbons. In order to control NOx, we need to have a way to artificially slow down the combustion process, which would normally proceed at its own rate according to the conditions inside the conbustion chamber.

 

Exhaust gas, although it can contain a degree of unburnt fuel, is essentially inert. Trying to re-burn the exhaust gas from an engine with correct fuelling is akin to trying to re-light a burnt-out fire.

 

The burn time of the combustion process is linked, amongst other things, to the physical distance between the fuel molecules. This rate of combustion is measured in Mass Fraction Burn time (the time taken for a certain percentage of the mass of fuel and air to be consumed).

 

By inserting molecules of an inert gas in between the combustable fuel / air mixture, the progression of the flame front can be artifically slowed down and the MFB time increased. This significantly reduces the level of NOx emissions.

 

The entire system consists of a valve and some metal tubing, which weighs approximately 1kg.

 

Pros:

Lower NOx emissions levels.

 

Cons:

The EGR system generally feeds a small proportion of the exhaust gas into the intake charge via a long tube and a valve. During the passage, the gas hopefully loses a degree of heat. However some heat will be retained, which is undesirable. Many tuners advocate removal of the entire EGR system because of the supposed intake charge temperature increases, and the charge dilution effects. However, EGR generally only operates in the low speed / low load area of the map (where emissions tests are performed) and so should not be a concern for WOT performance.

 

-Digsy

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