Exhaust Gas Recirculation System
The EGR system reduces NOx production by circulating small amounts of exhaust
gases into the intake manifold where it mixes with the incoming air/fuel charge.
From: Innovations in Fuel Economy and Sustainable Road Transport, 2011
Related terms:
Biodiesel, Exhaust Gas Recirculation, Diesel Engine, Turbines, Hydrogen, Nitrogen,
Fuel Injection, Turbocharger, Exhaust Gas
View all Topics
Experimental study on two-cylinder direct injection diesel engine for BS-III
emission compliant
S. Kaleemuddin, ... S. Bhattacharya, in Innovations in Fuel Economy and Sustainable
Road Transport, 2011
4.3 Exhaust gas recirculation (EGR) on upgraded engine
The engine EGR system is designed to reduce the amount of oxides of Nitrogen
produced by the engine during operating periods that usually result in high combustion temperatures. NOx is formed in high concentrations whenever combustion
temperatures nearing adiabatic flame temperature. The EGR system reduces NOx
production by circulating small amounts of exhaust gases into the intake manifold
where it mixes with the incoming air/fuel charge. By diluting the air/fuel mixture
under these conditions, peak combustion temperatures and pressures are reduced,
resulting in an overall reduction of NOx output. Usually EGR flow should match
engine operating conditions such as higher EGR flow is necessary during cruising
and mid-range acceleration, when combustion temperatures are typically very high.
Low EGR flow is needed during low speed and light load conditions and no EGR
flow should occur during engine warm up, idle and full throttle condition so that
EGR operation could not adversely affect engine operating efficiency or vehicle
drivability. To suit this condition EGR mapping was done with respect to throttle
position and engine speed on vehicle when it runs on ECE -EUDC cycle. Optimum
over all EGR operating zone with 15% EGR below 90% engine load and 30% EGR
below 75% load was frozen with respect to engine and part load condition of engine
as shown in Fig.17.
Fig. 17. EGR Operating Zone.
> Read full chapter
Diesel engine air system design
Qianfan Xin, in Diesel Engine System Design, 2013
13.3.1 Classification of EGR systems
EGR systems can be classified into external and internal EGR systems. The internal
EGR, usually uncooled, refers to the trapped in-cylinder residue of combustion
product and the reverse gas flow from the exhaust manifold/port to the cylinder.
Cooled external EGR is generally more effective than uncooled internal EGR for
emissions reduction and fuel economy, although the heat rejection in the external
EGR system needs to be handled by the cooling system. The external EGR can be
further classified into high-pressure-loop (HPL) EGR, low-pressure-loop (LPL) EGR,
and hybrid (or dual-loop, i.e., HPL plus LPL) EGR systems (Fig. 13.4).
13.4. Turbocharged EGR engine systems.
Diesel engine EGR mixing has been researched by Siewert et al. (2001) and Partridge
et al. (2002). Diesel engine EGR systems have been extensively investigated by
Akiyama et al. (1996), Baert et al. (1996, 1999), Kohketsu et al. (1997), Mattarelli et
al. (2000), Graf et al. (2000), Lundqvist et al. (2000), Luján et al. (2001), Osborne and
Morris (2002), Andersson et al. (2002), Chatterjee et al. (2003), Maiboom et al. (2008),
and Shutty (2009).
> Read full chapter
Operation examples of emission control systems
Masaaki Okubo, Takuya Kuwahara, in New Technologies for Emission Control in
Marine Diesel Engines, 2020
4.8.2 Pilot-scale example of exhaust gas recirculation in marine
diesel engines
EGR systems are classified into low- and high-pressure exhaust gas recirculation
(LP EGR and HP EGR) systems [25]. These EGR technologies can be applied to
two-cycle and four cycle engines. In the subsection the case of two-cycle is explained.
In general, for marine diesel engines, an EGR scrubber, a demister, and a water
treatment system for PM and SOx reduction in recirculated gas should be integrated
into the EGR system because the engine is damaged by introducing uncleaned EGR
gas. The difference of the position at which exhaust gas is taken in for recirculation
exists for LP EGR and HP EGR. Fig. 4.8.2 shows a principle of a schematic diagram of
the LP EGR system for marine diesel engines with an EGR scrubber, a demister, and
a water treatment system. The water treatment system consists of collecting tank and
centrifugal separators. In the EGR scrubber, which is a combination of a Venturi type
and a packed tower type, SOx is reduced by caustic soda (NaOH solution), and PM is
also removed. NaOH solution is supplied to the EGR scrubber. The mist is removed
by the demister and returned to the collecting tank. The solution in the collecting
tank is cleaned by a pair of centrifugal separators. pH of solution is regulated. In the
LP EGR system, the EGR gas exits from the outlet of the turbine of the turbocharger
and enters the inlet of the compressor of the turbocharger as controlled by the EGR
valve. Fig. 4.8.3 shows a principle of a schematic diagram of the HP EGR system
for marine diesel engines with an EGR scrubber, a demister, and a water treatment
system. Although the structure is almost the same as that in Fig. 4.8.2, in the HP
EGR, the EGR gas enters the inlet of the turbine of the turbocharger and flows into
the outlet of the compressor of the turbocharger as controlled by the EGR valve.
Figure 4.8.2. Schematic diagram of low-pressure exhaust gas recirculation (LP EGR)
system with an EGR scrubber, a demister, and a water treatment system for marine
diesel engines [25].
Figure 4.8.3. Schematic diagram of high-pressure exhaust gas recirculation (HP
EGR) system with an EGR scrubber, a demister, and a water treatment system for
marine diesel engines [25].
A recent example of the use of the LP EGR system in a marine diesel engine is
Mitsubishi LP EGR system developed by Mitsubishi Heavy Industries, Ltd. and Japan
Engine Corporation [26,27]. Fig. 4.8.4 shows a schematic diagram of the system
[25]. The system can achieve NOx reduction with LP EGR and also PM and SOx
removal from the exhaust gas with an EGR scrubber installed downstream of the
EGR valve. A demister is installed between the EGR scrubber and the EGR blower.
The scrubber is a combination of a Venturi type and a packed tower type scrubber
[26]. Typically, the Venturi part cleans PM away, and the packed tower part removes
SOx. Furthermore, the water used in the EGR scrubber is recycled with a water
treatment system, in which water from the acid scrubber is collected in a collecting
tank after spraying into the exhaust gas, where the water is neutralized by controlling
the pH value. The pH control is performed by using a pH sensor and NaOH solution
(caustic soda). The PM in the neutralized water is removed by centrifugation leaving
sludge as the residue. Treated water is obtained from these processes and is recycled.
Miniaturization of the system is achieved, and the performance in engine operations
with a single-cylinder test engine (NC33; two-cycle, bore diameter of 330 mm,
Mitsubishi Heavy Industries, Ltd.) and a full-scale marine diesel engine (4UE-X3;
two-cycle, bore diameter of 600 mm, output power of 9970 kW, Mitsubishi Heavy
Industries, Ltd.) was investigated. An electronic control system, the Ecosystem, is
introduced to optimize the timing of the fuel injection, exhaust valve, etc., during
the evaluation of the system performance.
Figure 4.8.4. Schematic diagram of the low-pressure exhaust gas recirculation (LP
EGR) system for marine diesel engine [25].
The result of the scrubber performance test showed that, in the case of an engine
load of 100%, the SOx concentration is 135 ppm (SO2) at the scrubber inlet and
2.8 ppm (SO2) at the outlet, that is, SOx reduction of 98% is achieved. In the case of
an engine load of 75%, the SOx concentrations are 130 ppm (SO2) at the scrubber
inlet and 1.0 ppm (SO2) at the outlet, that is, SOx reduction of 99% is achieved.
Further, they investigated soot removal and the SOx rates against engine loads. The
resulting soot removal rates are 81% at a load of 25%, 91% at a load of 50%, 91%
at a load of 75%, and 88% at a load of 100%. The resulting SOx removal rates are
91% at a load of 25%, 96% at a load of 50%, 99% at a load of 75%, and 96% at a
load of 100%. Overall, typically SOx removal exceeding 95% is achieved.
The LP EGR system with an EGR scrubber can reduce the O2 concentration in the
combustion chambers and add inert gases such as H2O and CO2 to the exhaust
gas to increase the heat capacity. Therefore, the temperature of the combustion gas
can be suppressed and NOx generation is reduced. As a result of the NOx reduction
performance in EGR, in the case of an engine load of 75%, an increase in the EGR
rate led to a gradual decrease in NOx emission. NOx reduction of approximately 80%
is achieved at an EGR ratio of 35%. Further, they investigated the NOx reduction
rates against the EGR rates. For example, at an engine load of 100%, the resulting
NOx reduction rates are approximately 18% at an EGR rate of 5%, 30% at an EGR
rate of 10%, 52% at an EGR rate of 20%, and 80% at an EGR rate of 30%. The
deterioration rate of specific fuel oil consumption can be suppressed to within 1.5%
by introducing this system.
As a recent example of the use of the HP EGR system, Mitsui Engineering and
Shipbuilding Co., Ltd. developed the first unit of its large low-speed diesel engines
with a low sulfur fuel-type HP EGR system [28]. Fig. 4.8.5 shows a schematic diagram
of the system. The system can satisfy the Tier III NOx regulation and is installed in a
product tanker for the first time in Japan. For this class of marine diesel engines, the
Tier III NOx regulation requires NOx emission up to 3.4 g/kWh, which corresponds
to an 80% reduction compared with the Tier I regulation of NOx emission up to
17.0 g/kWh and 76% reduction compared with the Tier II regulation of NOx emission
up to 14.4 g/kWh. The EGR devices are integrated into the diesel engine. As a
result, a compact system is achieved. Therefore, the system has a lower impact
on engine room design among technologies for responding to NOx regulations.
It is announced in 2016 that this marine diesel engine with an HP EGR system
(6G60ME-C9-EGRBP, Mitsui-MAN B&W) will be installed in two oil product tankers.
The HP EGR system reduces NOx in emissions by recirculating part of the exhaust
gas from the engine to the scavenging pipe.
Figure 4.8.5. Schematic diagram of the high-pressure exhaust gas recirculation (HP
EGR) system for marine diesel engine [28].
> Read full chapter
Corrosion of aluminium Charge Air
Coolers in exhaust gas condensates –
application to Exhaust Gas Recirculation systems
E. Szala, ... K. Vieregge, in Vehicle Thermal Management Systems Conference and
Exhibition (VTMS10), 2011
2 STATE OF THE ART
Exhaust Gas Recirculation (EGR) systems, especially cooled EGR, are used in modern
vehicles to satisfy current emission regulation based upon reductions in emissions
of particulates and pollutants, in particular nitrogen and sulphur oxides. Since the
demands on exhaust gas purification are becoming stricter, greater exhaust gas mass
flow rates are required, which are difficult to obtain with conventional high pressure
EGR systems.
In view of current emission regulations, some diesel engine manufacturers propose
low pressure EGR systems [1–4] as an alternative to the more conventional high
pressure EGR systems [5].
Figure 1 shows diagrams representing low and high pressure EGR systems.
Figure 1. schematic of Exhaust gas recirculation systems: (a) High pressure EGR; (b)
Low pressure EGR
In high pressure EGR systems, the exhaust gas flow is recirculated back into the
charge air flow downstream from the Charge Air Cooler (CAC). In low pressure
systems, the recirculated exhaust gas flow is mixed with the charge air flow upstream
of the CAC. For typical engine systems, under the majority of engine and environmental conditions, some water vapour from the intake air and the recirculated
exhaust gas will condense due to the CAC cooling the mixture of intake air and
exhaust gas below the dew point of the mixture. The condensate is acidic due to
the formation of nitric and sulphuric acid from the components in the exhaust gas
therefore corrosion of the material becomes an issue in the CAC flow passages that
are wetted by the mixture of condensed intake air and exhaust gas.
Most of corrosion resistant alloys selected for CAC application in EGR systems are
stainless steel alloys [6, 7]. In order to reduce the weight of the CAC with high
corrosion resistance properties, CAC have been designed with a bi-metal tube alloy
(Nickel plating or clad aluminium core with stainless steel) [8, 9]. Alternatively, some
manufacturers design more complicated systems which do not allow the aluminium
CAC to be in contact with the acidified condensate media [4].
In order to reduce the costs of the components and increase heat transfer, aluminium CAC become more attractive to industry. So far, due to the lower corrosion resistance of aluminium in comparison to stainless steel, only two patents recommend
the use of aluminium alloys for CAC in EGR systems [2, 10]. Modine developed in
collaboration with Alcoa a 5-layer long life alloy to withstand the harsh corrosive
environment whilst Behr proposes to select a 3-layer Long Life alloy. The Alcoa 5 layer
tube alloy provides long life properties through the presence of the ‘Dense Band of
precipitates’ [11] with increased level of Ti and a higher Mg content (~0.3 wt%) to
increase the strength but also the corrosion resistance of the core. To enable good
brazing to the turbulator or inner fin, Mg should not diffuse to the joining surfaces
during brazing; this is obtained due to addition of AA1145 alloy on both sides of
the core as interliner. Behr proposes to select a higher strength Long Life alloy with
AA4343 brazed clad on both side of the core as developed in the patent [12].
> Read full chapter
Application of exhaust gas recirculation
of NOx reduction in SI engines
Dhinesh Balasubramanian, ... Kasianantham Nanthagopal, in NOx Emission Control Technologies in Stationary and Automotive Internal Combustion Engines, 2022
6.3 Stratified form of EGR
The homogeneous exhaust gas recirculation system reduces laminar flame speed,
burning speed, cycle-to-cycle variations, and rise in hydrocarbon emission. It also
makes it difficult to achieve steady combustion [6, 7]. During the suction and
compression strokes, highly stratified EGR will diversify the air-fuel mixture, and
it will quickly overwhelm the above complications. In a stratified EGR system, it
separates fresh air and EGR inside the cylinder. Flame propagation is improved when
a lower concentration of exhaust gas exists near the spark plug, and the compatibility
of EGR is also increased. Due to the complex flow inside the cylinder, it is difficult
to separate the exhaust gas and air. The second challenging task is to achieve EGR
stratification at the suction stroke and maintain it during the compression stroke.
The various opportunities of stratified exhaust gas, such as lateral stratification,
radial stratification, and axial stratification, are shown in Fig. 6.3. In lateral stratification, the separation happens as a mixture of air and fuel exists at the combustion
chamber's intake valve and EGR exists at the exhaust valve. This system requires a
large intensity of solid tumble to keep the momentum in the vertical flow direction
[8]. In radial stratification, air and EGR would swirl in the same direction to reach
the required force balance due to the conservation of angular momentum at the
location of the interface. In axial stratification, the combustion cylinder is split into
two different regions–one at the top with air and the other at the bottom with EGR.
The injected air first pierces into the pure air zone, then fuses with EGR, and then
again would be ignited and burned. A flow in the vertical direction, such as squeeze,
mixes the gases thoroughly.
Fig. 6.3. Stratified EGR techniques.
The tumble flow is simply collapsed by turbulent flow when the piston transports to
the top dead center. But in radial stratification, the arrangement seems to easily
maintain the tumble flow. In this type, the mixture of air and fuel exists at the
center of the cylinder, and EGR exists in the cylinder's outer area. In the compression
stroke, the piston moves upward, and the cylinders are compressed in the axial
direction. To reach a new equilibrium stage, the two regions would rearrange their
location boundary by maintaining the angular momentum when both EGR and air
are swirling in the same direction. Radial stratification will withstand considerably
extended time toward the end of the compression stroke [9]. Due to this, radial
stratification is the most suitable technique for intake and exhaust stratification.
> Read full chapter
A methodology for evaluation of charge
air coolers for low pressure EGR systems with respect to corrosion
B. Grünenwald, ... C. Saumweber, in Vehicle Thermal Management Systems Conference and Exhibition (VTMS10), 2011
1 INTRODUCTION
The application of low pressure exhaust gas recirculation (LP-EGR) systems becomes
increasingly popular to meet Euro 6 emission requirements in case of Diesel engines
and to achieve fuel consumption benefits for gasoline motors. In a LP- EGR system
the exhaust gas is mixed with fresh air upstream of the compressor. As a result the
charge air cooler downstream of the compressor is exposed to a mixture of air and
exhaust gas (Fig. 1). Due to the need of low charge air cooler outlet temperatures
parts of the exhaust gas will condense in a wide range of operating points forming
aggressive liquids which contact the charge air cooler wall surfaces. One of the
consequences for the charge air cooler is the necessity of a corrosion robust design
with respect to exhaust gas components.
Figure 1. Cooling circuit schematics and charge air cooler types
Due to the fact that LP-EGR systems implicate longer EGR routings and hence a
certain lag in response times, it can be attractive for OEMs to use indirect (charge
air to coolant liquid) instead of direct (charge air to air) charge air coolers. A direct
charge air cooler (CAC) needs to be assembled in the frontend of the vehicle, whereas
an indirect charge air cooler (iCAC) can be mounted on a more or less arbitrary
position in the engine compartment, thereby significantly shortening the charge
air routings, which reduces the overall charge air volume and therefore improves
engine response times (Fig. 1). With respect to corrosion, another benefit of the
iCAC in combination with LP-EGR is the possibility of controlling the charge air
temperature via the coolant flow rate, which allows engine designers to minimize the
amount of exhaust gas condensate forming inside the charge air cooler. Still however
condensation of exhaust gases cannot be fully avoided also for iCACs. Therefore it is
mandatory to enhance the robustness with respect to corrosion caused by exhaust
gas condensates for iCACs as well.
Both, direct and indirect charge air coolers, usually consist of an Aluminium heat
exchanger core which is basically a stack of air fins (in case of direct charge air
coolers) or water fins (in case of indirect charge air coolers) combined with tubes
or flat plates with turbulators on the charge air side (Fig. 1). It is the primary goal of
the development to avoid leakage on the tubes or flat plates ducting the charge air
when designing a corrosion robust charge air cooler suitable for LP-EGR systems.
Since the turbulators exposed to the mixture of air and exhaust gas play a key role
for the structural capability of the core, it is essential to avoid extensive damage due
to corrosion for these components as well. In order to achieve the aforementioned
targets, a procedure needs to be defined which allows the designer to assess the
corrosive attack of the charge air cooler components exposed to exhaust gas in a
LP-EGR system.
> Read full chapter
Exhaust emissions and control
Malcolm Latarche, in Pounder's Marine Diesel Engines and Gas Turbines (Tenth
Edition), 2021
Exhaust gas recirculation
Exhaust gas recirculation (EGR) is a method of modifying the inlet air to reduce
NOx emissions at source, an approach widely and successfully used in automotive
applications. Some of the exhaust gas is cooled and cleaned before recirculation to
the scavenge air side. Its effect on NOx formation is partly due to a reduction of
the oxygen concentration in the combustion zone, and partly due to the content of
water and carbon dioxide in the exhaust gas. The higher molar heat capacities of
water and carbon dioxide lower the peak combustion temperature, which, in turn,
curbs the formation of NOx.
Some of the early work on EGR considered it more practical for engines burning
cleaner bunkers such as low sulphur and low ash fuels, alcohol, and gas. Engines
operating on high sulphur fuel might invite corrosion of turbochargers, intercoolers,
and scavenging pipes. However, EGR has emerged as the favoured method for
reaching Tier III levels albeit in conjunction with SCR on some engine types.
For ships with a dual-fuel engine operating on oil fuels, Tier II levels can be reached
but to reach Tier III would require SCR. However, the same ships operating on LNG
or methanol can reach Tier III without SCR. Any ship planned for operation outside
of an ECA or built before any ECA it operates in was established need only ever reach
Tier II standards.
The main components of an exhaust gas recirculation system comprise:
•
a high-pressure exhaust gas scrubber fitted before the engine turbocharger
•
a cooler to further reduce the temperature of the recirculated gas
•
a water mist catcher (WMC) to remove entrained water droplets
•
a high-pressure blower to increase recirculated gas pressure before reintroduction to the engine scavenge air
automated valves for isolation of the system
•
A Tier III engine has two emission cycle operating modes: Tier II for operation outside
NOX emission control areas and Tier III for operation inside NOX emission control
areas.
EGR is a suitable NOx reduction method for ships using fuels with almost any level
of sulphur likely to be supplied for marine engine use. However, to prevent sulphur
and particles from damaging the engine, cleaning of the recirculated exhaust gas
is required. This is performed in a combined cooling and cleaning process by a
prespray and an EGR cooler spray in the EGR string, using recirculated water. In
order to maintain the ability of the water to clean, cool, and neutralize the exhaust
gas, a water handling system (WHS) is needed.
The system must ensure the removal of accumulated particles and neutralization
of sulphuric acid in the water and ensure the delivery of water at a sufficient
pressure and supply rate to the EGR unit. In addition, the WHS handles the bleed-off w
ater, which is the surplus of water accumulated in the system from the combustion
process.
MAN Energy Solutions similar to other engine makers has designed a system
capable of dealing with different levels of sulphur that may be encountered. The
principle of the WHS is shown in Fig. 4.4.
Fig. 4.4. Layout of the WHS system.
The receiving tank, circulation pump, and control valve are part of the receiving tank
unit (RTU), placed on the engine below the EGR unit. The supply pump, the NaOH
pump, and the supply of freshwater are part of the supply unit (SU) installed in the
engine room. The water from the EGR unit is drained to the receiving tank and
recirculated to the EGR unit by the circulation pump. Part of the water is directed
to the buffer tank by the control valve and returned to the EGR unit by the supply
pump. In this string, an appropriate amount of NaOH is supplied to neutralize the
sulphuric acid in the system. The bleed-off water is discharged to the drain tank by
an overflow pipe on the buffer tank. The water accumulated in the drain tank may
be discharged to the sea provided the specified discharge criteria set by the IMO are
met. Any sludge that is collected must be disposed of ashore.
The WHS is designed according to the specified sulphur limits for the fuel oil used,
when the engine is running in EGR mode:
•
•
LS-WHS for EGR designed for low-sulphur fuels (max 0.5% S): In case the EGR
system is designed for fuels of max 0.5% sulphur, the production of particles
is ignorable, and cleaning of the recirculation water will not be required.
However, the neutralization of the recirculated water will still be required. For
LS-WHS, the flow of water in the SU loop and the amount of NaOH is relatively
low.
HS-WHS for EGR designed for high-sulphur fuels (noncompliant fuels): In
case the EGR system is designed for fuels not complying with the SOX requirements, the production of particles is significant, and the recirculated water
will need cleaning to prevent damage to the engine. A water treatment system
(HS-WTS) is installed in the buffer tank loop to reduce particulate matter in
the water. For HS-WHS, the flow of water in the SU loop and the amount of
NaOH is relatively high.
The buffer tank is part of the WHS and required for control of the amount and quality
of water in the EGR process. The tank size is defined by the volume of water needed
in the EGR system during start-up. The buffer tank is arranged with an overflow pipe
for automatic discharge of bleed-off water, which accumulates in the system due to
condensation of exhaust gas.
•
LS buffer tank (max 0.5% S)
In case the EGR system is designed for low-sulphur fuel, cleaning of the recirculated
water will not be needed. However, to prevent the accumulation of particulate matter
of high density at the tank bottom, the overflow pipe (low overflow) is connected
to the lowest possible point of the tank. In addition, to avoid trapping of foam,
particulate matter, and similar low-density substance in the top of the tank, an
overflow pipe (high overflow) with a slightly higher discharge level is also arranged.
Due to the ship movements at sea, bleed-off water will occasionally be discharged
this way.
•
HS buffer tank (noncompliant fuels)
In case the EGR system is designed for high-sulphur fuel, the recirculated water will
need cleaning by a water treatment system, HS-WTS. The outlet for the WTS is taken
from the lowest possible point of the buffer tank to ensure that particulate matter
of a high density is removed from the system. The treated water is returned to the
buffer tank at a level similar to the water inlet from the EGR circuit. Bleed-off water
including foam, particulate matter, and other low-density substance is discharged
by an overflow pipe, similar to the high overflow at LS buffer tank.
MAN Energy Solutions now incorporates EGR into all of its two-stroke engines.
Three different matching methods are used for the EGR systems:
•
•
•
EGR with bypass, configured with only one turbocharger and used for engines
of bore 70 or less.
EGR with TC cut-out matching, configured with two or more turbochargers
and used for engines of bore 80 or greater.
EcoEGR, configured as EGR with bypass. EcoEGR is a fuel optimized engine,
which lowers the SFOC in both Tier II and Tier III mode using EGR to meet
the relevant NOx limit.
An EGR system configured with bypass matching is shown in Fig. 4.5. Two strings,
the main string and an EGR string, are available to direct the scavenge air into the
scavenge air receiver:
Fig. 4.5. An EGR system configured with bypass matching.
•
•
The main string, with the capacity to lead all the scavenge air through the
turbocharger compressor and the scavenge air cooler.
The EGR string, with the capacity to lead up to 40% of the exhaust gas through
the prespray and the EGR unit (EGR cooler and WMC) to a mixing point in the
main string.
Two modes are available for bypass matching: Tier II and Tier III modes.
In Tier II mode, only the main string is in operation. The valves in the EGR string
(SOV/BTV) and the cylinder bypass (CBV) is kept closed. In this mode, the exhaust
gas bypass (EGB) is fully open at high loads and partly open at low loads to balance
the turbocharger. However, on engines with a bore of 40 or less, the exhaust gas
bypass will be closed at high loads and the EGR string open, to obtain sufficient
scavenge air pressure while meeting restrictions on the turbocharger speed.
In Tier III mode, the EGR string is activated by opening the EGR shut-off valve and
the blower throttle valve (SOV/BTV). The exhaust gas is led through the prespray
and the EGR unit to the mixing point and scavenge air receiver, forced by the EGR
blower. The EGR ratio is controlled by changing the flow of the EGR blower. The
cylinder bypass (CBV) is active in this mode to increase the scavenge air pressure
and thereby reduce the SFOC. The exhaust gas bypass (EGB) is closed.
An EGR system with TC cut-out matching is shown in the diagram in Fig. 4.6. Three
strings, the main string, a cut-out string, and an EGR string, are available in the
system to direct the scavenge air into the scavenge air receiver:
Fig. 4.6. An EGR system with TC cut-out matching.
•
•
•
The main string leads up to 70% of the scavenge air through the basic
turbocharger and the scavenge air cooler.
The cut-out string leads up to 40% of the scavenge air through the cut-out
turbocharger and the EGR unit (EGR cooler and WMC) before entering the
scavenge air receiver through the balance pipe.
The EGR string leads up to 40% of the exhaust gas through a prespray and
EGR unit to a mixing point in the main string, forced by one or more EGR
blowers. In this case, the cut-out string is closed.
On some larger engines, a configuration with more than two turbochargers will be
needed. The principle is unchanged although the number of turbochargers and EGR
units is increased.
Three modes are available for TC cut-out matching:
•
Tier II mode
In Tier II mode the main string and the cut-out string are in operation. The TC
cut-out valves (TCV/CCV) and the blower by-pass valves (BBV) are open, while the
EGR string is kept closed by the EGR shut-off valve and the blower throttle valve
(SOV/BTV). In this mode, the EGR cooler works as a normal scavenge air cooler.
About 40% of the scavenge air is passed through the cut-out string, the remaining
60% through the main string. The cylinder bypass (CBV) is kept close in this mode.
•
Tier II mode—TC cut-out
The cut-out string gives an opportunity to run the engine in Tier II mode at low loads
with a TC cut-out and the SFOC could thereby be reduced. In this case, only the main
string will be open, while the cylinder bypass (CBV) is kept closed.
•
Tier III mode
In Tier III mode the cut-out string is closed (TCV/CCV). The EGR string is opened
by the EGR shut-off valve and the blower throttle valve (SOV/BTV). The exhaust gas
is led through the prespray and the EGR unit to the mixing point and the scavenge
air receiver, forced by the EGR blowers. The EGR ratio is controlled by changing the
flow of the EGR blower. The cylinder bypass (CBV) is partly active in this mode to
increase the scavenge air pressure and thereby reduce the SFOC.
The EcoEGR system operates permanently in EGR state regardless of the sailing
area. The EGR rate is adapted to the NOx limits in the area i.e. Tier III mode inside
NECA and Tier II mode outside NECA. The SFOC reduction is around 2.5% in Tier
II mode and 1.0% in Tier III mode.
An additional Tier II fall back mode is available to maintain Tier II compliance in case
a serious failure to the EGR system occurs. The EcoEGR system is configured similar
to the EGR bypass concept and may be installed with one or two turbochargers, as
shown in Figs 4.6 and 4.7.
Fig. 4.7. EGR process diagram—EcoEGR configured with two turbochargers.
Three modes are available for EcoEGR:
•
EcoEGR Tier II mode
In EcoEGR Tier II mode the engine operates with a low EGR rate, around 10%–15%.
The EGR shut-off valve (SOV) and the blower throttle valve (BTV) are fully opened,
while the cylinder bypass valve (CBV) is closed. Except at high loads, the exhaust gas
bypass (EGB) is also closed.
•
EcoEGR Tier III mode
In EcoEGR Tier III mode the engine operates with an EGR rate 25%–45%, similar to
EGR Bypass matching. The EGR shut-off valve (SOV) and the blower throttle valve
(BTV) are fully opened. The cylinder bypass valve (CBV) is opened except at low loads,
while the exhaust gas bypass valve (EGB) is closed.
•
EcoEGR Tier II fall back mode
The fall back mode, which is only relevant in case of EGR break down, operates
without EGR. Accordingly, the EGR shut-off valve (SOV) and the blower throttle valve
(BTV) are closed. Except at high loads, the exhaust gas bypass (EGB) is also closed.
EGR units cannot be described as small, but they do fit well within the envelope
of the main engine and in themselves are not excessively space consuming as can
be seen for the image below combining an EGR unit, a basic turbocharger, and a
cut-out turbocharger (Fig. 4.8).
Fig. 4.8. Side-mounted turbocharger and side-mounted EGR unit.
However, the peripheral components will add to the space requirements of the
system. These can be positioned where best suited although the NaOH supply will
need to be at or above the supply unit and drain tanks should naturally be below to
allow for gravity flow (Fig. 4.9).
Fig. 4.9. Typical EGR full system layout suited to high sulphur fuels.
> Read full chapter
CFD simulation of the fouling process
in EGR coolers
R. Stauch, ... J. Supper, in Vehicle Thermal Management Systems Conference and
Exhibition (VTMS10), 2011
ABSTRACT
A 3-D method for the simulation of the fouling behaviour of parts in the exhaust gas
recirculation system based on detailed CFD simulations is presented. The transport
of soot particles and unburned hydrocarbons within the exhaust gas and the deposition of the mentioned species are modelled by the presented method. The implemented transport mechanisms, like diffusion and thermophoresis, and deposition
mechanisms, like condensation, are described in detail. The simulation method is
validated by the comparison of numerical results of the thermophoretic deposition
efficiency of soot to experimental results. The usage of the simulation method
as a tool within the optimization process of individual parts of EGR systems is
demonstrated by performing a numerical analysis of the different fouling behaviour
of a winglet tube at two operating points. Applying this method it is now possible to
analyse the fouling behaviour of EGR systems virtually.
> Read full chapter
Control Technologies for Compression–Ignition Engines
Stephen J. Charlton, in Handbook of Air Pollution From Internal Combustion Engines, 1998
11.4.3.1 EGR Valve
As diesel engine emission standards for oxides of nitrogen (NOx) become more
stringent, an increasing number of engines are being equipped with exhaust gas
recirculation (EGR) systems. These systems return a fraction of the engine exhaust
gas to the intake manifold via corrosion-resistant tubing and a control valve, known
as an EGR valve. EGR control was discussed in an earlier section; this section will
discuss the valve itself and the different approaches adopted.
To the present day, the only diesel engines fitted with EGR systems have been for
light-duty applications, notably diesel engines for passenger cars in Europe, and
for light-duty trucks and vans in Europe and the United States. In Europe virtually
all diesel passenger cars have EGR systems, some also using EGR cooling to gain
additional NOx control. In the United States, the Dodge Ram pick-up truck powered
by the Cummins 5.9 L DI diesel engine has EGR to meet the California TLEV standards, and the GM 6.5 L light-duty diesel engine used in sport-utility and pick-up
truck applications also uses EGR for emission control [28]. In developing an emission
control strategy that uses EGR, the goal is to find the optimum trade-off between fuel
consumption, NOx control, and particulate control. In general, adding EGR allows
injection timing to be advanced for improved fuel consumption, but at some cost in
particulate control. When all of the trade-offs have been made, the emission control
strategy will usually require EGR to be scheduled as a function of engine speed and
load (or throttle position), as shown in Figure 11.17 for passenger-car application. In
addition, EGR may be scheduled during transient operations, although EGR is not
normally used during hard accelerations, during start-up or periods of motoring or
overrun.
The role of the EGR valve is to control the flow of EGR between no flow and
maximum flow according to the EGR rate demanded by the engine controller. This
requires that the valve is accurate and rapid in achieving the positions commanded
by the controller. It also demands a high level of repeatability and usually fine
resolution so that EGR rates can be controlled accurately. The accuracy required
depends on operating conditions and the emission stringency but a figure of +/–3
percent is not uncommon. Unwanted forms of valve behavior, such as hysteresis,
temperature drift, and dead bands, should also be minimized.
The most common configuration used for diesel engine EGR valves is the vacuum-actuated poppet valve, either inward or outward opening, as shown in Figure 11.32. In this design the poppet valve is actuated by vacuum to open, and is
closed by a return spring as the vacuum is removed. The other significant force
acting on the valve is the net gas loading force. This is generally greatest when the
valve is closed and depends on the net valve area exposed to the gases on either side,
and on the difference between exhaust and intake manifold pressures. The vacuum
is normally provided by a fixed-displacement vacuum pump driven by the engine or
by an electric motor. In order to limit the size of the EGR valve and provide positive
actuation, the design vacuum is often as low as 0.3 bar-abs. A typical vacuum circuit
is also shown in Figure 11.32. For fast opening of the valve, it is necessary to optimize
the displacement of the pump and minimize the volume of the vacuum tubing and
diaphragm chamber. The addition of a vacuum reservoir may also help response.
For rapid closing it is usual to add a vent valve to admit air to the vacuum line when
closing is required. This collapses the vacuum very quickly and enables the return
spring to close the valve. Typical response times are 100–200 ms for opening and
50–100 ms for closing.
Fig. 11.32. Vacuum-operated EGR valve with in ward opening poppet valve.
Although some applications do not require modulation of the valve between fully
open and closed, by far the majority of applications are modulated. This allows a
more flexible approach when calibrating the engine controller and enables EGR rates
to be compensated against system changes, such as increased intake or exhaust
restriction.
The valve shown in Figure 11.32 is modulated by controlling the vacuum above the
diaphragm to adjust the position of the valve. Vacuum is modulated by the vacuum
control valve, which usually consists of a solenoid valve operated by a pulse width
modulated signal, such that air is admitted to the vacuum line at a controlled rate to
modulate the vacuum. In this example, valve position is under closed-loop control.
A valve position sensor provides feedback to the controller so that valve position can
be controlled accurately. The position sensor is typically a linear potentiometer or an
LVDT.
Other types of EGR valve are being developed in an attempt to overcome the
limitations of the vacuum valve. The main limitations of vacuum technology are
speed of response, cost and complexity. The complexity derives from the need
to provide a vacuum system, and the distribution of vacuum around the engine
compartment. The most desirable systems from control and installation points
of view are actuated directly by electrical means. The candidate actuators include
stepper motors, proportional solenoid valves, and DC motors.
> Read full chapter
MPC Design for Air-Handling Control
of a Diesel Engine
Nassim Khaled, Bibin Pattel, in Practical Design and Application of Model Predictive
Control, 2018
10.2 Air-handling Control Survey
Light duty vehicle emission standards are getting more stringent every year as stipulated by US EPA Tier 2 standards and LEV III regulations proposed by CARB. Dual
loop EGR systems are known to have improved turbocharger operating efficiency,
charge air temperature reduction, improved BSFC at steady state, and the capability
to drive high rates of EGR while minimizing the impact on performance [4] and [5].
Therefore, dual loop EGR systems offer significant advantages to reduce emissions
and fuel consumption and can help meet strict emission requirements.
ATLAS is a Cummins program (Fig. 10.2) funded by the United States Department
of Energy. The main objective of ATLAS is to demonstrate high fuel economy with
significant emissions reduction for a small displacement engine (2.8 L) in a pick-up
truck. The air-handling system is complex with a VGT, low pressure EGR valve, high
pressure EGR valve, and an ET. Due to the higher complexity of the ATLAS engine,
the coordination of the four actuators presents a challenging control and calibration
hurdle for evaluating the transient performance of the engine.
Figure 10.2. ALTAS program.
Traditional control design techniques are typically based on standard PID controllers,
lookup tables, and logical switches, etc., which have been around for many decades
and are well known to both engineers and technicians. These techniques are relatively easy to use and implement, and may be ideal for control loops without
interactions and with simpler dynamics. However, as the complexity of engines
increase and the number of sensors and actuators grow, it becomes a difficult task
to design a feedback controller using traditional decoupled loops within a reasonable
timeframe. A more systematic approach using multivariable control techniques can
help to reduce the development time while ensuring the required performance and
emissions limits given by legislation.
Analysis of different EGR strategies can be found, e.g., in survey papers [6–8]. A
cascaded adaptive PI strategy with model based feedforward was proposed in Shutty
et al. [9]. A cascaded control structure of boost pressure served as a decoupling tool
for EGR rate and boost pressure. In Grondin et al. [10] the strategy of dealing with
HP and LP EGR separately by motion planning was introduced to control boost
pressure and EGR rate. A cooperative control strategy for dual-loop EGR system
was introduced in Yan et al. [11]. The authors used the fact that a LP-EGR loop
has a substantially larger volume and consequently slower dynamics than those of
the HP-EGR to decompose the original system into two separate subsystems with
different time scales. For each subsystem, Lyapunov based controllers were designed
and then collected together to control intake manifold pressure, temperature and
oxygen fraction. In Shutty et al. [12] a coordinated strategy of dual-EGR, VGT,
and intake throttle was shown with dynamic feedback of total EGR mass flow and
boost pressure with HP/LP EGR split strategy to minimize pumping losses while
maximizing the efficiency of the turbocharger.
Haber [13] designed a multivariable H∞ controller to manage the pressure, temperature, and oxygen concentration at the intake manifold. The controller commands
the VGT position to mainly control the intake manifold pressure. Furthermore,
the controller drives the LP and HP EGR valves to control the intake manifold
temperature and oxygen concentration. A mean-value model for the air-handling
system and air fraction estimator were developed in Wang [14].
Any multivariable approach mentioned above (i.e., H∞ and LQG) can be used to
develop a systematic procedure for designing flexible and configurable controllers
for air-handling systems. The main advantage of MPC is its ability to handle various
constraints. A MPC can be an extension of LQG control with systematic handling
of time-varying constraints. However, implementation of a MPC controller may be
more complicated than simpler control algorithms because it requires running an
optimization solver online at each sampling period. If the control problem is small
enough (in terms of the number of total constraints in the resulting optimization
problem) then the MPC control problem can be solved explicitly, where the resulting
online solver is simple. It is based on a set of static lookup tables. More details about
explicit MPC can be found in Bemporad et al. [15]. If the resulting problem cannot
be solved explicitly due to a large number of constraints (e.g., unacceptable amount
of memory to store all lookup tables) then it is still possible to implement some
alternative and very efficient solver that can be implemented in the electronic
control module (ECM) with reasonable CPU overhead, see, e.g., Borelli et al. [16].
> Read full chapter
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