Exhaust Gas Recirculation

Hannu Jääskeläinen, Magdi K. Khair

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Abstract: Exhaust gas recirculation (EGR) is an effective strategy to control NOx emissions from diesel engines. The EGR reduces NOx through lowering the oxygen concentration in the combustion chamber, as well as through heat absorption. Several configurations have been proposed, including high- and low-pressure loop EGR, as well as hybrid systems. EGR is also used in gasoline engines, primarily in order to reduce pumping work and increase engine efficiency.


Exhaust gas recirculation (EGR) is an emission control technology allowing significant NOx emission reductions from most types of diesel engines: from light-duty engines through medium- and heavy-duty engine applications right up to low-speed, two-stroke marine engines. While the application of EGR for NOx reduction is the most common reason for applying EGR to modern commercial diesel engines, its potential application extents to other purposes as well. Some of these include: imparting knock resistance and reducing the need for high load fuel enrichment in SI engines, aiding vaporization of liquid fuels in SI engines [2471], as an enabler for closed cycle diesel engines [2472][2473], for improving the ignition quality of difficult-to-ignite fuels in diesel engines [2474] or for improving the performance of SCR catalysts [3578][3579]. While NOx reductions had been reported with EGR as early as 1940 [2475], the first engine experiments to investigate the NOx reduction potential of EGR appeared to be carried out in the late 1950s in SI engines [2476]. By the 1970s, EGR was being seriously considered as a NOx control measure for diesel engines [2477][2470].

From 1972/73 to the late 1980s EGR was commonly used for NOx control in spark-ignited gasoline fueled passenger car and light-duty truck engines in North America. After the early 1990s, some gasoline fueled applications were able to dispense with EGR. Following the early gasoline application, EGR was also introduced to diesel passenger cars and light-duty trucks and then heavy-duty diesel engines. While there were applications to heavy-duty diesel dating back to the 1970s, it was not until the early 2000s that cooled EGR became very common in heavy-duty diesel engines in North America [367]. It was this heavy-duty application that attracted the most attention to EGR, due to the more difficult technical challenges compared to the earlier light-duty applications. After 2010, the application of EGR into spark ignited engines was expanded—not for NOx control but for fuel economy purposes. It was applied not only to light-duty gasoline but heavy-duty gasoline, natural gas and propane fueled engines. For SI engines, EGR can reduce pumping losses, improve combustion efficiency, improve knock tolerance and lessen the need for fuel enrichment [2478]. A potential non-NOx reducing application EGR for modern diesel engines is to combine it with other engine control measures to increase exhaust gas temperature and facilitate the regeneration of diesel particulate filters [379].

The NOx emission benefit of EGR comes at a cost: other measures are usually required to avoid unacceptable increases in fuel consumption, emissions of PM, HC, and CO, engine wear and reductions in engine durability. In order to address these trade-offs in commercial diesel engine applications, engine manufacturers have had to simultaneously adopt a range of other technological changes such as:

More than one technical route exists to meet a given NOx limit, and EGR can sometimes be used as one of several alternative technologies. Such competition exists, for example, between cooled EGR and urea-SCR technology in heavy-duty Euro IV, Euro V and US 2010 diesel engines. However, to meet more stringent NOx emission limits, it may be necessary to use EGR in combination with NOx reduction catalysts. Commercial applications of EGR on diesel engines are summarized in the following table. On several occasions, small scale EGR applications occurred earlier than indicated in the table, typically driven by various voluntary incentive programs.

Table 1
Commercial application of EGR systems on diesel engines
Emission LegislationNOx LimitAreas of EGR Application
Light-Duty Vehicles
Euro 1/2 (1992/96)NOx+HC = 0.97-0.7 g/kmIntroduced in DI and larger IDI Euro 1 engines, EGR (non-cooled) became the main NOx reduction strategy in nearly all Euro 2 vehicles.
Euro 3/4 (2000/05)NOx= 0.5-0.25 g/kmCooled EGR introduced in larger size Euro 3 engines, and became the standard in Euro 4 and later diesel passenger cars and light trucks.
Heavy-Duty Engines
US 2004 (2002-04)NOx ≈ 2 g/bhp-hrCooled EGR introduced on heavy-duty truck and bus engines by most manufacturers (Cummins, Volvo/Mack, DDC, International). Miller-type intake valve timing was the alternative technology to EGR (Caterpillar).
Euro IV (2005)NOx = 3.5 g/kWhEGR introduced by some manufacturers of heavy-duty truck and bus engines (Scania, MAN); used in competition to urea-SCR technology.
Japan 2005NOx = 2.0 g/kWhEGR introduced by some manufacturers of heavy-duty truck and bus engines (Hino, Isuzu); used in competition to urea-SCR technology.
US 2007NOx ≈ 1 g/bhp-hrEGR used on heavy-duty truck and bus engines by all manufacturers.
Euro V (2008)NOx = 2 g/kWhEGR continues to be used in some products by several OEMs (Scania and MAN), however, no OEM relies solely on EGR. Urea-SCR is still the competing technology.
US 2010NOx = 0.2 g/bhp-hrEGR combined with NOx credits allows one heavy-duty diesel engine manufacturer (Navistar) to certify engines to a 0.5 g/bhp-hr NOx level. All other manufacturers rely on a combination of EGR and urea-SCR.
Euro VI (2013)NOx = 0.4 g/kWhMost manufacturers use a combination of EGR and urea-SCR. The competing technology is urea-SCR without EGR (Iveco and some Scania engines).
Nonroad Engines
US Tier 3 (2006)NOx = 4.0 g/kWhCooled EGR engines introduced by John Deere. A number of other manufacturers used internal EGR.
US Tier 4i / EU Stage IIIB
NOx ≈ 2 g/kWhCooled EGR introduced by a number of nonroad engine manufacturers; used in competition to urea-SCR technology.
IMO Tier III (2016)NOx = 3.4 to 1.96 g/kWhEGR will be used in some two-stroke low-speed marine diesel engine applications (MAN Diesel & Turbo). Ammonia-SCR is an important competing technology.

Light-Duty Engines. The introduction of EGR technology to diesel passenger cars in the 1990s went almost unnoticed and was not considered a major breakthrough for several reasons. Because the required NOx reduction was quite modest, the system allowed little EGR back into the cylinder and there was no need for EGR cooling. Typical passenger car engines operate mostly at part load conditions where temperatures are relatively low. It was only the Euro 3/4 legislation that created higher demands on EGR systems and triggered the introduction of increasingly more sophisticated, electronically controlled cooled EGR systems on light-duty engines.

Heavy-Duty Engines. Heavy-duty applications of EGR date back to at least 1977 when the technology was used on some naturally aspirated engines—such as Caterpillar’s 3208—to comply with California’s 5 g/bhp-hr NOx+HC limit for heavy-duty diesel engines. However, through the 1980s and 1990s the use of EGR on heavy-duty engines remained limited—EGR was not required to meet regulatory emission standards and the application of the technology was driven primarily by incentives such as the US EPA voluntary “low emission vehicle” certification program.

The wide scale launch of cooled EGR on heavy-duty engines that attracted a lot of attention to the technology took place in late 2002 in the North American market. This momentous introduction was in part triggered by the Consent Decrees and the political upheaval that surrounded the issue of “dual-mapping” that led to the 1998 settlement between the US EPA, the Department of Justice and the heavy-duty diesel engine manufacturers [1129]. The Consent Decrees advanced the implementation of the EPA 2004 emission standards by 15 months, to October 2002, putting heavy-duty engine manufacturers under extreme pressure to quickly select a technology capable of achieving the new NOx limits of approximately 2 g/bhp-hr. High pressure loop cooled EGR was the most expedient in-cylinder NOx reduction technology that could achieve this emission level [365]. In October 2002, several heavy-duty engine manufacturers introduced their new EPA-certified engines equipped with EGR systems. There was a considerable apprehension in the field regarding the performance, fuel economy, and the durability of these new engines. While initial statements from fleet managers appeared to praise the new technology [1125], some users have complained of the increase in fuel consumption. For EPA 2007, EGR continued to be the primary NOx reduction technology and allowed a number of engine makers to reach about 1 g/bhp-hr NOx. For EPA 2010, the 0.2 g/bhp-hr NOx limit proved to be too low to be effectively reached with EGR alone and additional help from NOx aftertreatment was required. Navistar—the only manufacturer to temporarily use EGR without aftertreatment for EPA 2010—was able to do so only by certifying engines to 0.4-0.5 g/bhp-hr NOx and making up the difference with credits.

In Europe, a couple of heavy-duty on-road engine makers introduced EGR-only engines at the Euro IV stage, with the remainder relying solely on urea SCR. By Euro V, the engine makers using EGR also adopted urea SCR to supplement their EGR strategies, while most manufacturers continued to rely on urea SCR only solutions. For Euro VI, the use of EGR became more widespread and many engine manufacturers introduced some combination of EGR and urea SCR. However, the use of EGR was often minimized and in some cases, restricted to certain low load conditions with no EGR used at higher load operating conditions such as highway cruise. Examples of the later approach are the Volvo D11 and D13 engines launched in 2013 that utilized uncooled EGR to control NOx and increase the exhaust temperature for the SCR catalyst at low engine loads. Some manufacturers, including Iveco and Scania, also introduced SCR-only Euro VI engines, without EGR.

Advances in urea-SCR aftertreatment systems have made it possible to produce low-NOx engines without EGR. This strategy requires an SCR conversion efficiency of higher than 94-96%, to enable engine-out NOx of as high as 7 g/kWh to hit Euro VI NOx limits of 0.4 g/kWh. Since the release of the initial Euro VI product lines, systems with over 99.5% NOx conversion have been under development that could enable engine-out NOx levels of over 12 g/kWh to meet Euro VI limits. The advantages of eliminating EGR include weight, cost and complexity reduction. On the negative side, SCR-only engines have higher urea consumption and require a larger urea tank. The on-board diagnostic (OBD) detection threshold requirements could also be a significant challenge in some markets if very high SCR conversion efficiencies are required to meet regulatory limits—a very small decrease in SCR conversion efficiency would need to be detected. Therefore, non-EGR engines were first deployed in the nonroad sector [2527] (there are no OBD requirements for nonroad engines) and in some Euro VI engines where the OBD requirements for NOx are less severe than in North America.

Nonroad Engines. EGR technology was also adopted by nonroad engines. Some of the first nonroad application of cooled external EGR were the US Tier 3 John Deere PowerTech Plus engines. The first cooled EGR John Deere engine was the 6 cylinder 9 L model (6090) launched in 2006, followed by the 6.8 L and 13.5 L models. These engines used high pressure loop cooled EGR and a variable geometry turbocharger. It is interesting to note that the 6090 EGR engine used in the John Deere 8000 series tractor in 2006 had the lowest fuel consumption of all time (as determined by the University of Nebraska Tractor Test Laboratory), demonstrating that the fuel economy penalty associated with EGR can be overcome by skillful engine design. Other applications of cooled EGR for Tier 3 included Komatsu’s 11.0, 15.2 and 23.2 L ecot3 engines [2479] and Deutz’s TCD 2012 [1988]. An alternative to cooled external EGR, internal EGR, was used by a range of engine manufacturers for US Tier 3. Cooled EGR became more widely adopted in US Tier 4i and EU Stage IIIB nonroad engines. For the final Tier 4/Stage IV emission standards, most nonroad engine manufacturers chose urea-SCR technology—with or without cooled EGR. By 2012, the idea of using an SCR-only approach for Tier 4 final seemed to be spreading [2527].

Marine Engines. Low-speed marine engine applications of EGR are perhaps the most challenging technically. Prior to the implementation of the IMO global sulfur cap on marine fuel of 0.5%, these engines were designed to burn heavy-fuel oil (HFO) that produces exhaust gas heavily laden with metals, sulfur and other components that must be removed before the exhaust gas is re-introduced into the engine. While EGR had been considered by some to be unsuitable for engines burning HFO because of the cleaning challenges, by about 2010, all major manufacturers of low-speed marine engines (MAN Diesel & Turbo, Wärtsilä and Mitsubishi) were considering EGR for HFO IMO Tier III applications. MAN Diesel & Turbo announced commercial orders as early as 2011. Some simplifications in the water treatment system for applications burning fuel with less than 0.5% sulfur are possible [4066].

Future Trends. Many advanced combustion concepts under development—for instance low temperature combustion (LTC)—utilize very high EGR rates for emission control. This is likely to put even more demand on future EGR systems and their components if the application of LTC over a significant portion of the engine operating map becomes commercial.

Urea-SCR aftertreatment will continue to be an alternative NOx reduction technique competing with EGR. Depending on the stringency of the respective emission standards, on the progress in NOx conversion efficiency and durability of SCR catalysts, and on the relative costs of diesel fuel and urea, three main NOx reduction technology pathways can be used in modern diesel engines:

This paper covers the theoretical background of the EGR technique and EGR configurations. Effects on combustion parameters and emissions are discussed under Effect of EGR on Emissions and Engine Performance. Commercial implementations of EGR, system components—including EGR valves and EGR coolers—and other practical issues are discussed in EGR Systems & Components, while electronic control of EGR is covered under EGR Control Strategy.