|MadSci Network: Engineering|
I think the place to start with this question is to review how diesel engines work. All internal combustion engines generate power by creating explosions using fuel and air. These explosions occur inside the engine's cylinders and push the pistons down, which turns the crankshaft. Some of the power thus produced is used to prepare the cylinders for the next explosion by forcing the exhaust gases out of the cylinder, drawing in air (or fuel-air mixture in non-diesel engines), and compressing the air or fuel-air mixture before the fuel is ignited.
There are several differences between diesel engines and non-diesel engines. Non-diesel engines combine a fuel mist with air before the mixture is taken into the cylinder, while diesel engines inject fuel into the cylinder after the air is taken in and compressed. Non-diesel engines use a spark plug to ignite the fuel-air mixture, while diesel engines use the heat created by compressing the air in the cylinder to ignite the fuel, which is injected into the hot air after compression. In order to create the high temperatures needed to ignite diesel fuel, diesel engines have much higher compression ratios (which is the volume of the cylinder pre-compression divided by the volume post-compression) than gasoline engines. Because diesel fuel is made of larger molecules than gasoline, burning diesel fuel produces more energy than burning the same volume of gasoline. The higher compression ratio in a diesel engine and the higher energy content of diesel fuel allow diesel engines to be more efficient than gasoline engines - they can run further on a gallon of fuel.
The same factors that cause diesel engines to run more efficiently than gasoline engines also cause them to run at a higher temperature. This leads to a pollution problem - the creation of nitrogen oxides (NOx). You see, fuel in any engine is burned with extra air, which helps eliminate unburned fuel (another class of pollutants) from the exhaust. This air, like air anywhere, is approximately 79% nitrogen and 21% oxygen. Some of the oxygen is used to burn the fuel, but the extra is supposed to just pass through the engine unreacted. The nitrogen, since it does not participate in the combustion reaction, also passes unchanged through the engine. Normally, this is true. However, at the high temperatures found inside an engine, the nitrogen and excess oxygen can react to form a class of compounds called nitrogen oxides. These compounds are one of the chief constituents of photochemical smog.
Now we come to the purpose of exhaust gas recirculation in a diesel engine. The goal of exhaust gas recirculation is to reduce the amount of NOx produced. There are three ways to do this: first, reduce the concentration of nitrogen in the fuel-air mixture prior to combustion; second, reduce the concentration of oxygen in the fuel-air mixture; and third, reduce the temperature, since the NOx-producing reactions proceed more slowly at lower temperatures. Exhaust gas recirculation does two of these by taking some of the exhaust gas and mixing it into the engine inlet air.
First, exhaust gas recirculation reduces the concentration of oxygen in the fuel-air mixture. By replacing some of the oxygen-rich inlet air with relatively oxygen-poor exhaust gas, there is less oxygen available for the combustion reaction to proceed. Since the rate of a reaction is always dependent to some degree on the concentration of its reactants in the pre- reaction mix, the NOx-producing reactions proceed more slowly, which means that less NOx is formed.
In addition, since there is less oxygen available, the engine must be adjusted to inject less fuel before each power stroke. Since we are now burning less fuel, there is less heat available to heat the fluids taking place in the reaction. The combustion reaction therefore occurs at lower temperature. Since the temperature is lower, and since the rate of the NOx-forming reaction is lower at lower temperatures, less NOx is formed.
For more information on diesel engines (as well as a lot of other everyday things), check out the How Stuff Works page on how diesel engines work.
I hope this helps answer your question.
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