Exhaust Gas Recirculation (EGR) complete guide – architectures

This article is focusing on the types (architectures) of Exhaust Gas Recirculation (EGR) system. For more details about why EGR is needed on an internal combustion engine, how it works and what components an EGR system contains, read also the following articles:

• Exhaust Gas Recirculation (EGR) complete guide – introduction
• Exhaust Gas Recirculation (EGR) complete guide – components

Exhaust Gas Recirculation (EGR) systems are widely used in diesel engines with the purpose of reducing the formation of nitrogen oxides (NO_x), by lowering the combustion temperature and the amount of oxygen into the cylinders.

Depending on whether the exhaust gas is recirculated before or after the turbocharger, there are two types of EGR systems:

• High pressure EGR
• Low pressure EGR

High pressure EGR

High pressure EGR, compared with a low pressure EGR system, is referred to as the “classic” EGR architecture. This architecture is the most widespread and has been used on diesel engines starting with Euro 2 pollutant emission limits. In a high pressure EGR system, the exhaust gas is taken before the turbine and reintroduced into the intake manifold after the compressor, thus in the high pressure zone of both exhaust and intake manifolds.

1. internal combustion engine
2. intake manifold
3. intake throttle
4. EGR cooler bypass
5. EGR cooler
6. EGR cooler bypass valve
7. EGR valve
8. intercooler (turbocharger)
9. turbocharger
10. intake air-mass meter
11. diesel oxidation catalyst (DOC) and diesel particulate filter (DPF)

The EGR valve is placed on the exhaust manifold side for several reasons:

• keeps the additional volume of gas in the exhaust manifold, due to extra ducts, as minimum as possible in order to minimise the influence on the pressure waves; this preserves the kinetic energy of the exhaust gases, which translates into faster response of the turbocharger
• reduces the risk of blockage of the EGR valve due to deposits; particulate deposits usually form when the temperature of the exhaust gas is below 150 °C, keeping the EGR valve on the exhaust side, where the temperatures are high, reduces the risk of deposits
• protects the other components of the EGR system (cooler, intake throttle) from the pressure waves of the exhaust gases, especially at high engine load

In the majority of the operating points of the engine (speed and torque), there is enough pressure difference between exhaust (higher) and intake (lower), which allows the exhaust gases to flow into the intake manifold, when the EGR valve is open. If the pressure difference is not enough, the intake throttle is closed and the pressure in the intake manifold will drop, which will allows the exhaust gases to flow from the exhaust manifold.

A high pressure EGR system is able to provide high EGR rates due to low volume, high dynamics of the EGR gas circuit. This allows fast adjustments of the EGR rate function of the operating point of the engine. Also, because the exhaust gas is mixed with the intake air after the compressor, there is no impact of the particulate matter on the compressor wheel. For this reason, the reliability of the turbocharger is not affected and can operate with nominal performance through its entire service life. However, the EGR cooler must withstand the damaging effects of the high pressure and high temperature of the exhaust gases.

Low pressure EGR

Euro 6 standard reduced the NO_x emissions limit to only 80 mg/km in the NEDC cycle, compared with 180 mg/km for Euro 5. This significant drop has imposed an increase of the EGR rates and the introduction of low pressure EGR systems.

In a low pressure EGR system, the exhaust gas is taken after the turbine and reintroduced into the intake manifold before the compressor, thus in the low pressure zone of both exhaust and intake manifolds. The first passenger car automotive application with low pressure EGR system was marketed on the VW Jetta with a 2.0 liter TDI engine, compliant with Tier 2 Bin 5 emission limits for the US market (end of 2008).

1. internal combustion engine
2. intake manifold
3. intercooler (turbocharger)
4. turbocharger
5. intake air-mass meter
6. diesel oxidation catalyst (DOC) and diesel particulate filter (DPF)
7. exhaust throttle
8. EGR valve
9. EGR cooler

In a low pressure EGR architecture, the EGR valve is positioned after the diesel particulate filter (DPF). This way the soot does not end up in the EGR valve, cooler or turbocharger. Another advantage is the lower temperature of the exhaust gases which reduces the thermal stress on the EGR components (valve, cooler, throttle). Due to lower temperatures, a low pressure EGR system is more efficient in reducing NO_x emissions, compared with a high pressure EGR system.

The main disadvantage of the low pressure EGR system is the higher inertia of the exhaust gases. All the ducts and components are relatively far from the engine and can not respond quickly in a change of the EGR rate. For this reason the EGR rates of a low pressure EGR are lower, compared to a high pressure EGR, in order to avoid excessive exhaust gas recirculated back into the engine.

The advantage and disadvantages of a low pressure EGR, compared with a high pressure EGR, are summarised in the table below.

Low pressure EGR vs. high pressure EGR
Advantages	Disadvantages
has natural pressure difference between exhaust and intake manifolds, which allows the exhaust gas to flow allows conservation of the exhaust gas energy before entering the turbine more suitable be used at full engine load generates better exhaust gas-air mix before entering the cylinders cleaner exhaust gas (if taken after DPF) protects the EGR components from deposits	the increase of the intake gas temperature can cause thermal failure of the compressor (this risk can be mitigated by cooling down the exhaust gases) water condensation can cause erosion (physical damage) of the compressor wheel higher gas inertia due to longer EGR circuit, lower response time in adjusting the EGR rate

Hybrid (combined), dual loop EGR

Hybrid (combined) EGR system integrates both high pressure EGR and low pressure EGR on the same engine. This type of EGR architecture is also called dual loop EGR system.

The advantage of this architecture is that combines the benefit of both low and high pressure EGR systems, switching between them depending on the operating point of the engine (speed and torque). The hybrid (combined) EGR allows the turbocharger to operate in a region of high efficiency at any operating point of the internal combustion engine.

1. air filter
2. hot-film air mass meter
3. VNT turbocharger
4. cylinder pressure sensors
5. piezo common-rail fuel injection system
6. charge air cooler (intercooler)
7. throttle valve
8. high pressure EGR valve
9. low pressure EGR valve
10. EGR cooler
11. diesel oxidation catalyst (DOC) and diesel particulate filter (DPF)
12. lean NO_x trap (LNT)
13. exhaust throttle valve
14. hydrogen sulfide (H₂S) catalyst

The disadvantage of dual loop EGR is the additional cost, complexity and space requirement, given by the high number of components and potential issues with controlling the EGR rate depending on the engine operating point. The control algorithm becomes quite difficult since several actuators (high/low pressure EGR valve, intake/exhaust throttle and turbine vanes/waste gate) need to be controlled to get the required amount of air and exhaust gas into the cylinders.

Dedicated EGR (D-EGR)

In spark ignitions engines, high levels of EGR rates (more than 15%) increase fuel efficiency and reduce pollutant emissions. The drawback is that high EGR rates can cause incomplete combustion and increased hydrocarbons (HC) emissions. To overcome this, the combustion stability can be improved by supplementing the air charge with either pure hydrogen (H₂) or reformed fuel, which consists of a mix of carbon monoxide (CO) and hydrogen (H₂).

The effects of high level of EGR rates on a spark ignition engine are summarised in the table below:

High levels of EGR rates
Advantages	Disadvantages
reduce exhaust temperature, which reduces or eliminates the need for enrichment for engine cooling at high loads reduce engine knock, allows engine to operate with compression ratios higher than 12:1 reduce CO and NOx emission due to reduction of combustion temperature reduce pumping losses, which translates in higher engine efficiency	misfire, combustion instability increased hydrocarbon (HC) emissions

Southwest Research Institute (SwRI) has developed a Dedicated EGR (D-EGR) which is an EGR system that uses in-cylinder fuel reformation to improve fuel economy and reduce emissions. The entire exhaust of a sub-group of power cylinders (dedicated cylinders) is routed directly into the intake. These cylinders are run fuel-rich, producing hydrogen (H₂) and carbon monoxide (CO), with the potential to improve combustion stability, knock tolerance and burn duration.

HEGO – heated exhaust gas oxygen sensor
UEGO – universal exhaust gas oxygen sensor
TWC – three-way catalyst
WG – waste-gate
WGS – water-gas shift catalyst

The fuel reformation process occurs inside a power cylinder that is operated with excess fuel. Rich combustion leads to the formation of large amounts of hydrogen (H₂) and carbon monoxide (CO), which are then recirculated back into the engine. Since the fuel reformation occurs in a power-producing cylinder and all the combustion products are recirculated, the normal losses associated with fuel reformation in an external device are avoided.

Because the fuel rich cylinders also generate high levels of carbon monoxide (CO), there is the potential to install a water-gas shift (WGS) catalyst in the EGR system to further increase the level of hydrogen (H₂) in the exhaust gas. The WGS catalyst converts carbon monoxide (CO) and water (H₂O) into hydrogen (H₂) and carbon dioxide (CO₂).

In a nutshell, D-EGR technology delivers cooled recirculated exhaust gas and reformed fuel in the same package.

The dedicated EGR concept, which uses individual cylinders dedicated to EGR production, was tested on a 2.4 litre naturally aspirated engine at both low and high load conditions. The engine was configured to run at an ultra-high compression ratio (14:1) with cooled EGR. In this application, one cylinder out of four was used as the dedicated EGR cylinder, providing a constant 25% EGR to the engine. The results show that running the dedicated EGR cylinder rich generates sufficient hydrogen (H₂) and carbon monoxide (CO) to significantly improve the burn rate and EGR tolerance of the engine.

The improvement in EGR tolerance and combustion leads to a reduction in fuel consumption of up 10% at light loads. In addition, hydrogen (H₂) addition reduces the quench distance of the mixture and improves dilution tolerance, which promotes more complete combustion, leading to significant decreases in carbon monoxide (CO) and hydrocarbons (HC) compared to the baseline, naturally aspired case. NO_x emissions increased with improved combustion but, due to the 25% EGR, were still significantly lower than the undiluted engine.

References:
[1] Dedicated EGR: A New Concept in High Efficiency Engines, Terry Alger and Barrett Mangold, Southwest Research Institute, 2009.
[2] Technology today, Southwest Research Institute, Summer 2013.