Sunday, November 20, 2011

Exhaust gas recirculation flow what does this mean?

it's the flow of exhaust gasses back into the cylinder for re-burning|||your e g r valve cirrculates your exuast back through your cylinders ity can get gunked up with carbon resin which causes your car to sputter|||It is not technically correct to say that the exhaust gas is "re-burnt". Indeed it may no longer have enough oxygen or hydrocarbons present to enable combustion.

EGR is actually used to lower the temperature inside the cylinder during combustion. When fuel is burnt in the engine it is combined with oxygen. However, air is approx 21% oxygen, 78% nitrogen and 1% CO2, other gases etc. If conditions exist that result in both high combustion temperatures and the fuel air ratio has an excess of oxygen, then the nitrogen in the air combines with the excess oxygen to form oxides of nitrogen. These combine with water in the atmosphere to form acid rain. In order to prevent this, the air that is drawn into the engine has some of the oxygen replaced with an inert gas. The exhaust gas is used for this purpose, as it is freely and readily available. It is introduced to the inlet via the EGR valve which is a variable opening between the exhaust and the inlet manifold controlled by the engine ECU. By 'diluting" the incoming air in this way the combustion chamber temp can be lowered when at part throttle. There is also less excess oxygen availble so the combination of these 2 factors means that the combustion process is now less conducive to the formation of oxides of nitrogen. At full throttle the engine ECU shuts the EGR valve to ensure that the engine is able to deliver full power. The engine ECU is able to monitor EGR flow via the MAF, as intake airflow reduces when the EGR valve is open. EGR is more common on, though not exclusive to,diesel engines as they have a greater excess of oxygen during the combustion process when at part throttle than do petrol engines.

To summarise, EGR flow is a measure of the amount of exhaust gas being introduced into the intake system.The engine ECU "map" will have predetermined values for this , stored in a look up table. If the actual values differ from that which are expected during EGR operation then a DTC (diagnostic trouble code) will be generated

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