Exhaust Gas Recirculation

[damionclaymore]

I understand that it is used in automotive engines to reduce the NOx emissions of the engine. What happen to the carbon dixiode as it going through another combustion cycle? Does it become something different? Does it do anything beside take up volume in the cylinder chamber that could be better filled by oxygen?

 

[GregLocock]

I imagine that the CO2 remains as an inert gas during the second combustion. You might want to check that by finding out the temperature of dissociation for CO2 and typical combustion temperatures. Certainly by the time it runs out the exhaust it is CO2 again.

In effect the exhaust (which is largely nitrogen by the way) dilutes the reaction and gives the fixed quantity of fuel more stuff to heat up, so it reduces the combustion temperature. NOx forms at high temperatures.

Cheers

Greg Locock

 

[enginesrus]

Since it is drawn through the intake valve-port.It also displaces freash air ---> Nitrogen and Oxygen.
Less O2 means less heat.

 

[enginead]

Normally EGR on gasoline engines is introduced after the throttle at part load and thus is in addition to the air charge and thus doesn't displace any air. On an unthrottled diesel engine however it does displace fresh air ( in fact this is how it is usually controlled). It works as GregLocock suggested by introducing more mass for the fuel energy to heat up, thus lowering combustion temperatures. In asnswer to the original question the CO2 doesn't do anything, it is essentially inert at these temperatures.

 

[turboice]

I am admitedly not as familar with this as I would like to be but here is a couple documents on the topic:

http://www.damek.kth.se/~borje/Artiklar/SAE 982476.pdf

http://www.damek.kth.se/~borje/Artiklar/SAE 1999-01-3505.pdf

My real interest in both of these papers is whether or not EGR is really just an alternative type of water injection since steam is a primary component of the exhaust gas.

Though I am interested in following this thread for any information on the possibility of induction charge displacement from the other inert gases in the EG that is being recirculated.

Ed.

http://www.turboice.net

 

[SBBlue]

At least for diesel engines, there have been a variety of studies done trying to resolve the exact mechanism of Exhaust Gas Recirculation.

The biggest effect seems to be the decreased oxygen decreases peak flame temperature.

 

[BB78]

Yes, EGR systems introduce a certain quantity of inert gases in combustion chamber, lowering combustion temperatures and so reducing NOx formation. This effect is comparable to that of water injection, or to the use of a fuel/water emulsion, such as the so called (in Europe) "white Diesel oil". EGR recirculation is used especially for Diesel engines and lean-burn gasoline direct injection engines, where the high air/fuel ratios at wich these engines operate would lead to a very poor reduction efficiency of the trivalent cathalythic converters. Typically, on these engines are used bivalent cathalythic converters (oxydation of CO and HC to CO2) associated with an EGR system.
Recently, throttled Diesel engines have been tested: using an electronic controlled throttle valve (ETC), it's possible to lower the intake manifold pressure and increase the fraction of recirculated gases, being their mass flow related to the exhaust/intake pressure ratio. This, in future, can contribute to the NOx reduction in Diesels, always a critical emission of these engines. To increase volumetric efficiency is also widely used an EGR cooler, a gas/liquid heat exchangers using engine's cooling water.

 

[geoffreyjm]

My view of the inert gas introduction would be that it does not affect the combustion temperature (not much anyway, it would slow the combustion, giving a slower pressure rise and lower peak pressure and temp), but it reduces the number of oxygen atoms available, thus reducing the formation of oxides of nitrogen.
Any throttling of the diesel inlet gases would eat into their efficiency, better to displace the air with inerts.