EGR Valves...

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EGR Valves...

Post by WTF on Fri Jan 30, 2015 11:59 pm

Exhaust Gas Recirculation Valve


An EGR or exhaust gas recirculation valve is used on gasoline and diesel engines to reduce the amount NOx or nitrous of oxides released into the atmosphere. When nitrogen and oxygen are combined with high cylinder temperatures this harmful NOx gas is formed.

The EGR valve reduces the amount of NOx by recirculating a small amount of exhaust into the intake. It is a controlled opening between the intake and the exhaust. By mixing this inert gas into the air fuel charge the cylinder temperature is reduced. Reducing the temperature reduces the amount of nitrous of oxides produced in the chamber. This prevents the engines cylinders from reaching damaging temperatures and protects the environment from NOx.

There are different types of EGR systems used on different engines. The vacuum operated EGR valve used in the illustration above is a typical design. The valve is opened by vacuum supplied through a duty cycle solenoid and closed by a spring. The solenoid controls the valve to open in proportion to throttle opening.
The valve remains in the closed position when the engine started, at idle and at wide open throttle. A faulty EGR valve or related component may leave the valve in either the stuck open or stuck closed position. The symptoms of a stuck open EGR valve are a rough idle with a lean air fuel mixture. The engine will behave as if it had a major vacuum leak. The symptoms of a stuck close EGR valve are an engine that has pre-ignition ping and knock. This is the result of high cylinder temperatures caused by the lack of the cooling exhaust gas introduced into the chamber by the EGR.

Duplicated under 'fair use' terms in the category of education

But they didn't say :

In modern diesel engines, the EGR gas is cooled with 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, when there is otherwise a large excess of air) in controlling NOx emissions.

Exhaust recirculated back into the cylinder can increase engine wear as carbon particulate wash past the rings and into the oil.

Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines. Exhaust gas—largely carbon dioxide and water vapor—has a higher specific heat than air, so it still serves to lower peak combustion temperatures. 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 a diesel particulate filter in the exhaust system which cleans the exhaust but reduces fuel efficiency.

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 inject fuel and air directly into the exhaust system to keep these filters from plugging up....

By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of NOx. This makes combustion less efficient, compromising economy and power.

The normally "dry" intake system of a diesel engine is now subject to fouling from soot, unburned fuel and oil in the EGR bleed, which has little effect on airflow, however, when combined with oil vapor from a PCV system, can cause buildup of sticky tar in the intake manifold and valves.

It can also cause problems with components such as swirl flaps, where fitted.

Diesel EGR also increases soot production, though this was masked in the US by the simultaneous introduction of diesel particulate filters.[

EGR systems can also add abrasive contaminants and increase engine oil acidity, which in turn can reduce engine longevity.

Though engine manufacturers have refused to release details of the effect of EGR on fuel economy, the EPA regulations of 2002 that led to the introduction of cooled EGR were associated with a 3% drop in engine efficiency, bucking a trend of a .5% a year increase


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