The and investigated continuously by researchers, allows to comply

The
Exhaust Gas Recirculation System (EGR) is based on the idea of re-entering
burnt gases from the combustion process in the engine intake to reduce
emissions and improve fuel economy. This technology, developed and investigated
continuously by researchers, allows to comply with current emission norms by
reducing flame temperatures and oxygen concentration, which results in a lower
emission of NOx particles. This is because exhaust gases tend to
tend to dilute the mixture between air and fuel and have a very high specific
heat, acting like a heat sink.

Nevertheless,
the EGR System also entails some penalties. For example, Zeng, Reader and
Hawley (2004) affirms that some of the most important penalties are, among
other, more production of particulate
matter due to low oxygen concentration, combustion instabilities, carbon
sediments and power losses. More disadvantages are listed in Cambustion.com, where
it can be found that, since the system works with a valve, it is impossible for
it to respond instantly, so there is a need to calibrate the transient
behaviour, which is very complex.

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Traditionally,
EGR systems have been associated with compression ignition engines due to its
natural tendency to produce nitrogen oxides. That is the reason why many
studies and papers of EGR are focused on this type of engine. On the one hand, Jain,
Parihar, Jain and Mulla (2013) carried out a study about the performance of
Exhaust Gas Recirculation on Diesel Engines by comparing different parameters
with and without EGR. With respect to Brake Power, it was observed that the
exhaust gas temperature, brake thermal efficiency and the emission of carbon
oxides and nitrogen oxides were reduced with EGR. Nonetheless, fuel consumption
and HC emissions were increased. Hence, EGR is a good way to reduce emissions
(except HC) and the exhaust temperature, but it is associated with a reduction
in thermal efficiency and therefore a higher consumption to achieve the same
power.

It is
important to see how EGR operates in different conditions of speed and load,
since an engine has different working conditions. To deal with this, Selim
(2002) studied what the effect of engine speed and load was on thermal
efficiency when using EGR on a dual fuel engine. It was necessary to do
different experiments, changing the load, the revolutions and the amount of
recirculated gas. In terms of speed, the efficiency increases with engine
speed. It was also seen that the efficiency is slightly increased when a low
rate of EGR is used, especially at low speeds. This could be due to a higher
inlet temperature, which can lead to a better combustion of natural gas. However,
when the EGR rate increases, the efficiency is reduced in all cases. In terms
of load, the more engine load, the greater efficiency. The thermal efficiency
increases again with low EGR rates but drops rapidly for any EGR rate value
greater than 5%.

Jacobs,
Assanis and Filipi (2003) also examined the impact of EGR on emissions and
performance of a diesel engine, comparing different operating conditions. It
was seen that the reduction of NOx emissions as EGR increases is
much more important under low air to fuel ratio. Besides, it was also seen that
flame temperature drops when using EGR despite that the bulk temperature of the
gas is higher because of the exhaust gases being recirculated. An increase in
the rate of EGR means also an increase in pumping work and less indicated work
obtained, and therefore a reduction in thermal efficiency. According to this
paper, to deal with this problem it is possible to change the injection timing.
This study also states that combustion deterioration is a factor to take into
account at low speeds and high loads.

Current
trends are mainly focusing on supercharged ignition engines and downsizing. Thus,
it is logical that many of the studies and papers published in the last four or
five years are about this matter and systems that can be coupled to this type
of engine. Takaki, Tsuchida, Kobara, Akagi, Tsuyuki and Nagamine (2014)
conducted a study to see how fuel consumption could be reduced in a downsized
turbocharged gasoline engine using EGR. It was clearly seen that to reduce fuel
consumption there were four key points: suppress knocking, reduce pumping
losses (less important in turbocharged engines), reduce exhaust temperature and
increase the specific heat ratio. EGR system was seen as a good solution to
solve these problems and they implemented and compared three types of EGR: a
low-pressure loop system, a high-pressure loop system and a mixed-pressure loop
system. The best system to improve fuel economy was seen to be the low-pressure
loop in all cases, providing a better suppression of knocking and being able to
better reduce the fuel enrichment zone, as well as keeping a constant rate of
EGR when the valve is fixed. The effect on fuel consumption reduction can be
around a 5% as maximum when the turbocharger is running and in steady state.

In
addition to the findings of the previous study, Rehan (2017) examined a
Dedicated Exhaust Gas Recirculation System (D-EGR) in spark engines. This
system is based on the idea that a few cylinders could create EGR for all of
them. In the case of a four cylinder engine, the exhaust gas of one cylinder
would feed the intake of all. This new system could solve many limitations
related with traditional EGR, like control and tolerances, in order to increase
the potential improvements of EGR. As the exhaust of the dedicated cylinder
will never leave the engine without passing through it again, this cylinder can
be run in richer conditions, producing more H2 and CO that will act
as a heat sink in a more effective way than the traditional system and will
also reduce knocking. The D-EGR can lead to a stable rate of EGR, and in the
case of this study it was 25% because the whole exhaust gases from a single
cylinder were directly connected to the air inlet. This system showed that as
D-EGR values were increased, the thermal efficiency was improved in all cases.
Besides, a lower HC and CO emissions as well as a lower Brake Specific Fuel
Consumption can be achieved through improved combustion and better knock
tolerance.

The EGR
has several possibilities associated to implement it, depending on where the
exhaust gases are extracted and where they are reintroduced. Chao, Lu, Hu,
Deng, Wu and Li (2017) conducted a study to compare high and low-pressure EGR
systems in order to reduce fuel consumption on a downsized gasoline engine. According
to this study, the effect of a low-pressure system could be stronger under the
same rate of EGR compared to a high-pressure system. However, a greater
improvement in fuel economy could be achieved with a high-pressure system as it
has a better response under transient conditions, what means a better control.

            The Exhaust Gas Recirculation is a potentially powerful
system, capable of reducing some harmful emissions from engines. Moreover, in
the new types of engine that are being studied in recent years, supercharged gasoline
and downsized engines, it has been proven that it can also be an effective way
to reduce fuel consumption. There are still many studies to be done related
with EGR and its possible improvements, since there are many penalties and
difficulties to be solved or minimized, and a lot of working conditions in
which the system has to be optimized.