In internal combustion engines, exhaust gas recirculation ( EGR ) is a technique of reducing nitrogen oxide emissions (NO x ) used in gasoline/diesel and diesel machine. EGR works by recirculating a portion of the engine exhaust gas back to the engine cylinder. This melts O 2 in the incoming air stream and gives the inert gas to the combustion to act as a combustion heat sink to reduce the cylinder's peak temperature. NOT x is produced in narrow waves of temperature and high cylinder pressure. Another major benefit of external EGR valves on spark ignition engines is increased efficiency, because charge dilution allows for greater throttle positions and reduces the associated pumping loss.
In a gasoline engine, this inert muffler replaces the amount of combustible material inside the cylinder. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Since NO x is formed especially when the mixture of nitrogen and oxygen is subjected to high temperatures, the lower combustion chamber temperatures caused by EGR reduce the amount of NO x generated combustion (though at some loss engine efficiency). The reintroduced gases of the EGR system will also contain close equilibrium concentrations of NO x and CO; a small fraction initially in the combustion chamber inhibits this total net production and other pollutants when sampled at the time average. The chemical properties of various fuels limit how much EGR can be used. For example methanol is more tolerant to EGR than gasoline.
Video Exhaust gas recirculation
Histori
The first EGR system is raw; there is as simple as the orifice jet between the drain and the intake that recognizes the flue to the intake channel whenever the machine is working. Difficult start, rough idling, and reduced performance and fuel economy produced. In 1973, the EGR valve controlled by the vacuum manifold was opened or closed to receive the exhaust to the intake ducts only under certain conditions. Control systems grow more sophisticated when automakers gain experience; "Coolant Controlled Exhaust Gas Recirculation" of the 1973 Chrysler system is an example of this evolution: the coolant temperature sensor is blocked vacuum to the EGR valve until the engine reaches its normal operating temperature. This prevents driveability problems due to unnecessary waste induction; NOx forms in high temperature conditions generally do not come with a cold engine. Moreover, the EGR valve is controlled, in part, by a vacuum extracted from the venturi carburetor, which allows more precise restrictions of EGR flow only under machine loading conditions where NOx will be formed. Then, a backpressure transducer is added to the EGR valve control to further adjust the EGR flow to the engine load condition. Most modern machines now require exhaust gas recirculation to meet emission standards. However, the latest innovations have led to the development of machines that do not need them. The Chrysler Pentastar 3.6 engine is one example that does not require EGR.
Maps Exhaust gas recirculation
EGR
The exhaust gases, added to the fuel, oxygen, and combustion products, increase the specific heat capacity of the cylinder contents, which decreases the adiabatic flame temperature.
In a fire engine (SI) engine, 5% to 15% of the exhaust gas is returned to the intake as EGR. The maximum amount is limited by the need for the mixture to maintain the front of the fire continuously during combustion events; Excessive EGR in poorly regulated applications can result in shootouts and partial burns. Although EGR performs slow burning measurably, this can largely be compensated by advancing spark time. The impact of EGR on engine efficiency relies heavily on specific machine designs, and sometimes leads to a compromise between efficiency and NOx emissions. A properly operated EGR can theoretically improve the efficiency of a gasoline engine through several mechanisms:
- Reduce throttling loss . The addition of an inert exhaust gas to the intake system means that for a given power output, the throttle plate should be opened further, thus increasing the manifold pressure of the inlet and reducing the throttling loss.
- Reduce heat rejection . Low peak burning temperatures not only reduce NOx formation, but also reduce the loss of heat energy to the surface of the combustion chamber, making more available for conversion to mechanical work during expansion expansion.
- Decrease in chemical dissociation . Lower peak temperatures produce more of the remaining released energy as reasonable energy near the TDC (Top Dead-Center), rather than being bound (at the beginning of the expansion step) in the dissociation of the combustion products. This effect is small compared to the first two.
EGR is usually not used at high loads because it will reduce peak power output. This is because it reduces the intake charge density. EGR is also removed at idle (low speed, zero load) because it will cause unstable combustion, resulting in a rough idle.
Since the EGR system recirculates a portion of the exhaust gases, over time the valve can be clogged with carbon deposition that prevents it from operating properly. Clogged EGR valves can sometimes be cleaned, but replacement is required if the valve is damaged.
In diesel engine
In modern diesel engines, EGR gas is cooled by heat exchanger to allow greater mass recognition of gas recirculation. Unlike spark-ignition engines, diesel engines are not limited by the need for adjacent flamefronts; Furthermore, because diesel engines always operate with excess air, they benefit from an EGR level as high as 50% (when idle, when there is a great surplus of air) in controlling NOx emissions. Exhaust that is circulated back into the cylinder can increase engine wear when carbon particles wash through the ring and into the oil.
Since the diesel engine is not littered, EGR does not reduce throttling losses in the way it does for SI engines. Exhaust gases - mostly nitrogen, carbon dioxide, and water vapor - have specific heat higher than air, so it still works to lower the peak burning temperature. However, adding EGR to the diesel reduces the specific heat ratio of the combustion gases in a 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 power stroke. This is evidenced by the increase in particulate emissions in accordance with the increase in EGR.
Particulate material (especially carbon) that does not burn in power stroke is wasted energy. Strict regulations on particulate matter (PM) require further emission control to be introduced to offset the increase in PM emissions caused by EGR. The most common is a diesel particle filter in a exhaust system that cleans the exhaust but causes a small constant reduction in fuel efficiency because the back pressure is made. The nitrogen dioxide component of the NOx emission is the main carbon oxidizer caught in the diesel particulate filter (DPF) at normal operating temperature. This process is known as passive regeneration. Increased levels of EGR cause passive regeneration to be less effective in managing PM loading in DPF. This requires regular periodic regeneration of the DPF by burning diesel fuel in an oxidation catalyst to significantly increase the temperature of the exhaust gas through DPF to the point where the PM is rapidly burned by residual oxygen in the exhaust.
By feeding the lower exhaust gases into the intake, the diesel EGR system lowers the combustion temperature, reducing the NOx emissions. It makes burning less efficient, sacrificing economy and power. The "dry" emission system of diesel engines now undergoes fouling of soot, unburned fuel and oil in EGR landings, which has little effect on airflow. However, when combined with oil vapor from the PCV system, it can cause sticky tar buildup in the intake manifold and valve. This can also cause problems with components such as flaps swirl, where installed. EGR Diesel also increased the production of soot, although it was covered in the US by the introduction of simultaneous diesel particulate filters. The EGR system can also add rough contaminants and increase engine oil acidity, which in turn can reduce engine life.
Although engine manufacturers have refused to release details of the EGR effect on fuel economy, the 2002 EPA regulations that led to the introduction of refrigerated EGR were associated with a 3% reduction in engine efficiency, bucking a 0.5% year trend on the rise.
See also
- Exhaust Gas Recirculation versus Selective Catalytic Reduction
Source
- Heywood, John B., "Basics of Internal Combustion Engine," McGraw Hill, 1988.
- van Basshuysen, Richard, and SchÃÆ'¤fer, Fred, "Internal Combustion Guides," SAE International, 2004.
- "Bosch Automotive Handbook," 3rd Edition, Robert Bosch GmbH, 1993.
- Alger, Terry (2010). "Clean and Cool" (PDF) . Technology Today . San Antonio, Texas: Southwest Research Institute. 31 (2): 10-13. ISSNÃ, 1528-431X . Retrieved 2017-04-08 .
- Sileghem, Louis; Van De Ginste, Maarten (2011). "Methanol as a Fuel for Modern Spark-Ignition Machines: Efficiency Study" (PDF) . Department of Flow, Heat and Combustion Mechanics. Ghent University . Ghent, Belgium. Archived from the original (PDF) in 2016-09-10 . Retrieved 2017-04-07 .
References
External links
- Lecture notes on fuel efficiency improvements that address the effects of specific heat ratios, University of Washington
- Solar cycle calculator that can be used to show the effect of specific heat ratios, Georgia State University HyperPhysics
- The Imperial Chrysler fan club describes the different EGR control mechanisms
- Do Not Block or Remove EGR Valves, It Saves You Money
Source of the article : Wikipedia
0 Comments