EGR in diesel engines
In modern
diesel engines, the EGR gas is cooled through a
heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, where there is otherwise a very large amount of excess air) in controlling NOx emissions.
Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines (see above). However, exhaust gas (largely carbon dioxide and water vapor) has a higher
specific heat than air, and so it still serves to lower peak combustion temperatures. There are trade offs however. Adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the
power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR. Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter(PM) call for further emission controls to be introduced to compensate for the PM emissions introduced by EGR. The most common is particulate filters in the exhaust system that result in reduced fuel efficiency[
citation needed]. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen these filters need to function properly to burn off soot, automakers have had to consider injecting fuel and air directly into the exhaust system to keep these filters from plugging up.
EGR deletion
EGR deletion in diesel engines is considered justifiable by a wide range of people,
including the environmentally conscious. Although deleting the EGR system results in
increased NOx levels,
hydrocarbon emissions, Particulates, Carbon monoxide and Carbon dioxide are drastically reduced. Furthermore, EGR deletion results in an
increase in fuel economy as high as 25%. Exhaust gas recirculated back into the cylinders
adds wear-inducing contaminants and causes an
increase engine oil acidity, which can result in an
inefficient, poorly running engine. The
increased level of soot also creates the need for
diesel particulate filters to prevent environmental contamination.
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