EXHAUST GAS RECIRCULATION
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Better fuel economy and higher power with lower maintenance cost has increased the popularity
of diesel engine vehicles. Diesel engines are used for bulk movement of goods, powering
stationary/mobile equipment, and to generate electricity more economically than any other
device in this size range. In most of the global car markets, record diesel car sales have been
observed in recent years. The exhorting anticipation of additional improvements in diesel fuel
and diesel vehicle sales in future have forced diesel engine manufacturers to upgrade the
technology in terms of power, fuel economy and emissions.
In recent year due to globalization and industrial development, transportation industries are
flourishing very fast. Such industries are very much responsible for atmospheric pollution which
is detrimental to human health and environment. Internal combustion engines are the main power
source for the automobile vehicles which is used by transportation industries. Mostly all the
diesel engines have high thermal efficiencies because of their high compression ratio and lean
air-fuel operation. The high compression ratio produces the high temperatures required to
achieve auto ignition and the resulting high expansion ratio makes the engine discharge less
thermal energy in the exhaust. Due to lean air-fuel mixture, extra oxygen in the cylinders is
present to facilitate complete combustion. Increasing diesel consumption increases the pollutant
that pollutes the atmospheric air. Thus good efforts are being made to reduce the pollutants
emitted from the exhaust system without loss of power and fuel consumption. Recent concern
over development in automotive technology is the low environmental impact. In fact, partial
recirculation of exhaust gas, which is not a new technique, has recently become essential, in
combination with other techniques for attaining lower emission levels. The development of a
new generation of exhaust gas recirculation (EGR) valves and improvements in electronic
controls allow a better EGR accuracy and shorter response time in transient condition.
Pollutants are because of the incomplete burning of the air-fuel mixture in the combustion
chamber. The major pollutants emitted from the exhaust due to incomplete combustion are,
i. Carbon monoxide (CO)
ii. Hydrocarbons (HC)
iii. Oxides of nitrogen (NOx)
If, combustion is complete, the only products being expelled from exhaust would be water
vapour which is harmless, and carbon dioxide, which is an inert gas and, as such it is not directly
harmful to humans.
MECHANISM OF FORMATION OF POLLUTANTS
i. Carbon Monoxide (CO)
Carbon monoxide (CO) is a colorless, odorless, and tasteless gas that is slightly lighter than air.
It is toxic to humans and animals when encountered in higher concentrations. CO is generally
formed when the mixture is rich in fuel. The amount of CO formation increases as the mixture
becomes more and more rich in fuel. A small amount of CO will come out of the exhaust even
when the mixture is slightly lean in fuel because air fuel mixture is not homogenous and
equilibrium is not established when the products pass to the exhaust. At the high temperature
developed during the combustion, the products formed are unstable and following reactions take
place before the equilibrium is established.
2C+O2 = 2CO
As the products cool down to exhaust temperature, major part of CO reacts with oxygen to form
CO2. However, a relatively small amount of CO will remain in exhaust.
ii. Hydrocarbons (HC)
Hydrocarbons, derived from unburnt fuel emitted by exhausts, engine crankcase fumes and
vapour escaping from the carburetor are also harmful to health. Hydrocarbons appears in exhaust
gas due to local rich mixture pockets at much lower temperature than the combustion chamber
and due to flame quenching near the metallic walls. A significant amount of this unburnt HC
may burn during expansion and exhaust strokes if oxygen concentration and exhaust temperature
is suitable for complete oxidation.
iii. Mechanism of formation of nitric oxide (NOx)
Oxides of nitrogen is produced in very small quantities can cause pollution. While prolonged
exposure of oxides of nitrogen is dangerous to health. Oxides of nitrogen which occurs only in
the engine exhaust are a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Nitrogen
and oxygen react at relatively high temperature. NOx is formed inside the combustion chamber
in post-flame combustion process in the high temperature region. The high peak combustion
temperature and availability of oxygen are the main reasons for the formation of NOx. In the
present of oxygen inside the combustion chamber at high combustion temperatures the following
chemical reactions will takes place behind the flame.
Calculation of chemical equilibrium shows that a significant amount of NO will be formed at the
end of combustion. The majority of NO formed will however decompose at the low temperatures
of exhaust. But, due to very low reaction rate at the exhaust temperature, a part of NO formed
remains in exhaust. The NO formation will be less in rich mixtures than in lean mixtures. The
concentration of oxides of nitrogen in the exhaust is closely related peak combustion temperature
inside the combustion chamber.
1.2 NOx EMISSION CONTROL
NOx emission is closely related to temperature and oxygen content in the combustion chamber.
Any process to reduce cylinder peak temperature and concentration of oxygen will reduce the
oxides of nitrogen. This suggests a number of methods for reducing the level of nitrogen oxides.
Among these the dilution of fuel-air mixture entering the engine cylinder with an inert or noncombustible
substance is one which absorbs a portion energy released during the combustion,
thereby affecting an overall reduction in the combustion temperature and consequently in the
NOx emission level. The following are the three methods for reducing peak cycle temperature
and thereby reducing NOx emission.
I. Water injection.
III. Exhaust gas recirculation (EGR)
I. Water injection
Nitrogen oxides NOx reduction is a function of water injection rate. NOx emission reduces with
increase in water injection rate per kg of fuel. The specific fuel consumption decreases a few
percent at medium water injection rate. The water injection system is used as a device for
controlling the NOx emission from the engine exhaust.
A copper catalyst has been used to reduce the NOx emission from engine in the presence of CO.
Catalytic converter package is use to control the emission levels of various pollutants by
changing the chemical characteristics of the exhaust gases. Catalyst materials such as platinum
and palladium are applied to a ceramic support which has been treated with an aluminium oxide
wash coat. This results in as extremely porous structure providing a large surface area to
stimulate the combination of oxygen with HC and CO. This oxidation process converts most of
these compounds to water vapour and carbon dioxide
III. Exhaust Gas Recirculation.
Exhaust Gas Recirculation is an effective method of NOx control. The exhaust gases mainly
consist of carbon dioxide, nitrogen etc. and the mixture has higher specific heat compared to
atmospheric air. Re-circulated exhaust gas displaces fresh air entering the combustion chamber
with carbon dioxide and water vapor present in engine exhaust. As a consequence of this air
displacement, lower amount of oxygen in the intake mixture is available for combustion.
Reduced oxygen available for combustion lowers the effective air–fuel ratio. This effective
reduction in air–fuel ratio affects exhaust emissions substantially. In addition to this, mixing of
exhaust gases with intake air increases specific heat of intake mixture, which results in the
reduction of flame temperature. Thus combination of lower oxygen quantity in the intake air and
reduced flame temperature reduces rate of NOx formation reactions .The EGR (%) is defined as
the mass percent of the recirculated exhaust (MEGR) in the total intake mixture (Mt).
EGR (%) = (MEGR ÷ Mt) × 100
From above three methods, EGR is the most efficient and widely used system to control the
formation of oxides of nitrogen inside the combustion chamber of I.C. engine. The exhaust gas
for recirculation is taken through an orifice and passed through control valves for regulation of
the quantity of recirculation.
HEAVY DUTY DIESEL ENGINE
In heavy duty DI diesel engines NOx emission decreases almost linearly with EGR rate. NO
emissions at full load remain almost constant when altering EGR temp. A small NO emission
increases only at high EGR rates. Formation of NOx is temperature and O2 sensitive. So that the
increase of EGR temperature is compensate by the reduction of air fuel ratio. For the same EGR
rate has no significant effect on NO for all engines speeds, but small NO emission increases only
at high EGR rates with speed on the other hand. The effect of EGR rate is slightly higher at low
engine speed. Emission of NO increases with increase of temperature of EGR (hot EGR)
compared to the cooled EGR. The increase of EGR temperature from 90°C to 240 °C results to
an increase of the mean gas temperature and the individual zone temperature during the main
combustion period and that create adverse effect on NO emission. Because of that, in heavy duty
DI diesel engine, EGR cooling is favorable to retain the benefits of law NOx emissions without
sacrificing the engine efficiency.
WEAR OF PISTON RINGS
The piston rings are one of the most important components in the engine, which are essential for
operation of the engine. Piston rings are subjected to high thrust imposed by combustion gases.
Rings are used to reduce the friction between cylinder liner surface and the piston. They are
made of very high strength material so that they can resist high temperature and high thrust of
combustion process and at the same time have very low wear. In the engine using EGR, top
compression ring faces lowest weight loss compared to other rings. The weight loss of top
compression ring is about 0.30% of the initial weight of ring. The oil ring faces highest amount
of weight loss in the engine using EGR. The amount of wear was approximately 0.90% of initial
weight. It has been observed that the extent of wear of top ring in the engine using EGR is lower
than normal operating engine. The possible reason of this may be the lower temperature of the
combustion chamber of the engine using EGR. However, the wear rate of second and third
compression ring and oil ring is comparatively higher for engine using EGR. The possible reason
for this may be presence of higher amount of soot and wear debris in the lubricating oil of the
engine using EGR.
Thus, as seen that using Exhaust Gas Recirculation Technique in engines, the emissions are very
much controlled due to lesser amounts of NOx entering the atmosphere. Exhaust gases lower the
oxygen concentration in combustion chamber and increase the specific heat of the intake air
mixture, which results in lower flame temperatures. Thus the emission levels to be maintained
are attained by the engines. It can be observed that 15% EGR rate is found to be effective to
reduce NOx emission substantially without deteriorating engine performance. As seen, Exhaust
Gas Recirculation is a very simple method. It has proven to be very useful and it is being
modified further to attain better standards. This method is very reliable in terms of fuel
consumption and highly reliable. Thus EGR is the most effective method for reducing the nitrous
oxide emissions from the engine exhaust. Many of the four wheeler manufacturers like Ford
Company, Benz Motors etc used this technique to improve the engine performance and reduce
the amount of pollutants in the exhaust of the engine.