Exhaust Gas Recirculation (EGR)

Type:
Technical Option | Generic Example
Theme:
Energy | Transport

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Summary

Exhaust gas recirculation (EGR) is a nitrogen oxide (NOX) emission reduction technique used in petrol and diesel internal combustion engines. The technique works by re-circulating a portion of an engine's exhaust gas back to the engine cylinders. In a petrol engine, this inert exhaust displaces the amount of combustible matter in the cylinder. This means the heat of combustion is less, and the combustion generates the same pressure against the piston at a lower temperature. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.

Because NOX formation progresses much faster at high temperatures, EGR reduces the amount of NOX the combustion generates. NOX forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature. At 2,500oF or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produces a haze in our skies known commonly as smog.

Implementation

Usually, an engine re-circulates exhaust gas by piping it from the exhaust manifold to the inlet manifold. This design is called external EGR. A control valve (EGR Valve) within the circuit regulates and times the gas flow. Some engine designs perform EGR by trapping exhaust gas within the cylinder by not fully expelling it during the exhaust stroke, which is called internal EGR. EGR can also be implemented by using a variable geometry turbocharger (VGT) which uses variable inlet guide vanes to build sufficient backpressure in the exhaust manifold. For EGR to flow a pressure difference is required across the intake and exhaust manifold and this is created by the VGT.

Another method that has been experimented with is using a throttle in a turbocharged diesel engine to decrease the intake pressure, thereby initiating EGR flow.

Impact

The EGR valve re-circulates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are re-circulated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.

The EGR system of today must precisely control the flow of re-circulated exhaust. Too much flow will retard engine performance and cause a hesitation on acceleration. Too little flow will increase NOx and cause engine ping. A well-designed system will actually increase engine performance and economy.

Costs & Benefits

Benefits of using EGR technology over others include the absence of additional equipment (such as an additive tank as in the case of SCR), the lack of any need for subsequent processing of additives, no reduction in payload, and no added inconvenience for the driver. There is also no impact on maintenance regimes.

However, there are disadvantages also. Most notable perhaps is that any fall in the combustion temperature directly affects the performance of the engine. In addition, the much greater concentration within the engine of the recycled gases can result in the pollution of the lubricating oil, which in turn necessitates an increase in the capacity of this oil to disperse particles, which are processed downstream of the engine via a filter in the exhaust silencer.

Evidence & Reference

Modelling this Measure

The impact of EGR can be captured by accounting for the removal efficiency of the technology over a specified activity.


Site Entry Created by Policy Measures Admin on Oct 12, 2010

Reference This Source

Policymeasures.com (2017). Exhaust Gas Recirculation (EGR). Available:
www.policymeasures.com/measures/detail/exhaust-gas-recirculation-egr Last accessed: 12th December 2017

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