|
EGR – Exhaust Gas Recirculation
 EGR is a NOx (nitrogen oxide and nitrogen dioxide) reduction technique used in most diesel applications.
This process is to reduce oxides of Nitrogen, and there are two forms of EGR:
- Low pressure:
- High pressure: typically new engines using such technology are fitted with high pressure EGR
EGR works by re-circulated a portion of an engine's exhaust gas back to the engine cylinders. Inter mixing the incoming air with re-circulated exhaust gas dilutes the mix with inert gas, lowering the flame temperature in diesel engines, in turn, reducing the amount of excess oxygen. The exhaust gas also increases the mixing and lowers the peak combustion temperature. NOx is primarily produced when a mix of nitrogen and oxygen is subjected to high temperatures within the engine, and because NOx formation progresses much faster at high temperatures, EGR helps limit the generation of NOx 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 re-circulated gas; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (when idle) which aid in controlling NOx emissions.
Due to the fact that diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does in gasoline engines. 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. This aids the diesel engine's efficiency by reduced heat rejection and dissociation. There are trade offs however, and 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 method of PM reduction is particulate filters in the exhaust system that can result in a reduction of fuel efficiency up to 3%. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen, these particulate filters need to function properly to burn off excess soot, DET has considered injecting fuel and air directly into the exhaust system to keep these filters from plugging up as this is a common fault especially with Low pressure EGR techniques. At DET, accepting the faults of EGR systems has lead us push our combination DPF and SCR system as it is more effective and less susceptible to plugging and many other problems.
|
