Exhaust gas recirculation
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide () emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure. Another primary benefit  of external EGR valves on a spark ignition engine is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses. Mazda's turbocharged SkyActiv engine uses recirculated and cooled exhaust gases to reduce combustion chamber temperatures, thereby permitting the engine to run at higher boost levels before the air-fuel mixture must be enriched to prevent engine knocking. In a gasoline engine, this inert exhaust displaces some amount of combustible charge in the cylinder, effectively reducing the quantity of charge available for combustion without affecting the air-fuel ratio. In a diesel engine , the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of the combustion generates. Gases re-introduced from EGR systems will also contain near equilibrium concentrations of and CO; the small fraction initially within the combustion chamber inhibits the total net production of these and other pollutants when sampled on a time average. Chemical properties of different fuels limit how much EGR may be used. For example methanol is more  tolerant to EGR than gasoline. History. The first EGR systems were crude; some were as simple as an orifice jet between the exhaust and intake tracts which admitted exhaust to the intake tract whenever the engine was running. Difficult starting, rough idling, and reduced performance and fuel economy resulted. By 1973, an EGR valve controlled by manifold vacuum opened or closed to admit exhaust to the intake tract only under certain conditions. Control systems grew more sophisticated as automakers gained experience; Volkswagen's "Coolant Controlled Exhaust Gas Recirculation" system of 1973 exemplified this evolution: a coolant temperature sensor blocked vacuum  to the EGR valve until the engine reached normal operating temperature. This prevented driveability problems due to unnecessary exhaust induction; forms under elevated temperature conditions generally not present with a cold engine. Moreover, the EGR valve was controlled, in part, by vacuum drawn from the carburetor's venturi, which allowed more precise constraint of EGR flow to only those engine load conditions under which is likely to form. Later, backpressure transducers were added to the EGR valve control to further tailor EGR flow to engine load conditions. Most modern engines now need exhaust gas recirculation to meet emissions standards. However , recent innovations have led to the development of engines that do not require them. The 3.6 Chrysler Pentastar engine is one example that does not require EGR. EGR. The exhaust gas contains water vapor and carbon dioxide which both have lower heat capacity ratio than air. Adding exhaust gas therefore reduces pressure and temperature during the isentropic compression in the cylinder, and lowers the adiabatic flame temperature. In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the  mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and emissions. A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms: EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the  intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle. Since the EGR system recirculates a portion of exhaust gases, over time the valve can become clogged with carbon deposits that prevent it from operating properly. Clogged EGR valves can sometimes be cleaned, but replacement is necessary if the valve is faulty. In diesel engines. In modern diesel engines, the EGR gas is usually cooled with a heat exchanger to allow the introduction of a greater mass of recirculated gas. Uncooled EGR designs do however exist, often  referred to as hot gas recirculation (HGR). Unlike spark-ignition 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 emissions. Exhaust recirculated back into the cylinder can increase engine wear as carbon particulates blow by the piston rings and into the oil. Since modern diesels often have a throttle, EGR can reduce the need for throttling eliminating losses in the same way that it does for SI  engines. Reduced throttling also reduces the problem with engine oil being sucked past the piston rings into the cylinder causing carbon deposits there. Exhaust gas—largely nitrogen, 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 emission increases caused by EGR. The most common is a diesel particulate filter (DPF) in the exhaust system which cleans the exhaust but causes a constant minor reduction in fuel efficiency due to the back pressure created. The nitrogen dioxide component of emissions is the primary oxidizer of the soot caught in the DPF at normal  operating temperatures. This process is known as passive regeneration. Increasing EGR rates cause passive regeneration to be less effective at managing the PM loading in the DPF. This necessitates periodic active regeneration of the DPF by burning diesel fuel in the oxidation catalyst in order to significantly increase exhaust gas temperatures through the DPF to the point where PM is quickly burned by the residual oxygen in the exhaust. By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of . This makes combustion less efficient, compromising economy and power. The normally "dry