You will find the figures mentioned in this article in the German issue of MTZ 06I2007 beginning on page 432.

Brennverfahren und Abgasnachbehandlung im

Mercedes-Benz-Bluetec-Konzept

Combustion Process and

Exhaust Gas Aftertreatment in the

Mercedes-Benz Bluetec Concept

Authors:Hermann Breitbach, Joachim Schommers, Ralf Binz, Bernd Lindemann, Andreas Lingens and Stephan Reichel

In recent years, Mercedes-Benz has frequently reached important emission control milestones in diesel engines: In 1985, the world’s first serial production particulate filter for passenger cars, in 2003 the first modern particulate filter with additive-free regeneration, and, in October 2006, the United States market launch of the E 320 Bluetec. This vehicle is the first serial production diesel pas-senger car to exceed the requirements of U.S. Tier II Bin 8 standards. The Blue-tec concept is based on several building blocks: the base engine with signifi-cantly reduced engine out emissions, the oxidation catalyst and particulate fil-ter, the nitrous oxide aftertreatment, and the associated system control.

1 Introduction

Mercedes-Benz has a long tradition as a pio-neer in diesel technology: in 1936, the world’s first production diesel automobile was presented at the Berlin Auto Show. Since then, Mercedes-Benz has regularly intro-duced decisive innovations in the diesel en-gine, like four-valve technology, turbocharg-ing, direct injection or the common-rail in-jection system. These technologies have transformed the diesel into a high-torque, powerful and yet economical power unit [4], and have facilitated its extraordinary growth. As a result, diesel powered passen-ger cars account for about 50% of new regis-trations in Europe. It is primarily American emissions regulations that have prevented a comparable success in the United States.

For the first time ever the E 320 Bluetec, the latest Mercedes-Benz innovation in die-sel technology, meets the U.S. Tier II Bin 8 standards as a serial production passenger

car. This vehicle enables Mercedes-Benz to offer diesel powered passenger cars in the US market.

The development of a diesel engine that complies with these limits needed the re-duction of particulate emissions as a first step. The world’s first particulate filter in a production automobile was introduced in 1985. In 2003, Mercedes-Benz became the first manufacturer in the world to bring a modern, additive-free particulate filter into serial production [2]. This system is so effi-cient that it can meet every particle emis-sion standard in the world, both current and anticipated.

But one final, key challenge remained: nitrous oxide emissions. Figure 1 shows how nitrous oxide emission limits will decrease in coming years. Especially the US have tight limits. Mercedes-Benz has taken up the challenge of nitrous oxide reduction, and with Bluetec technology, has now de-veloped a solution.

2 MTZ 06I2007 Volume 68 2

2 The Bluetec System

Figure 2 shows the building blocks that make up the system. The central element of Bluetec is the emissions-optimized en-gine itself. Various internal engine design features have been adopted to reduce en-gine out emissions. In addition a diesel oxi-dation catalyst with particulate filter has been added, a component that is standard in modern diesel engines. The particulate filter in the E 320 Bluetec is an additive-free, low maintenance system.

Nitrous oxide aftertreatment is another component of Bluetec. This can be realized in two variants: either with an advanced NOx storage catalyst (advanced NSC) or with an SCR system using AdBlue, a urea solu-tion that releases ammonia as an active component [3].

Finally, the control of these three com-ponents, with its associated algorithms and application is equally a very important element of Bluetec. Its complexity is not to be underestimated.

3 Internal Engine Design Measures to Reduce Engine Out Emissions

For the derivation of the engine out emis-sion targets, the potential of exhaust after-treatment technologies had first to be esti-mated in comprehensive testing. With the OM 648 inline six-cylinder diesel of the 2005 E-Class as a base, Figure 3 shows the potentials of the particulate filter, the NOx

storage catalyst and the SCR urea system in the FTP test cycle.

For premium-class passenger cars, U.S. Tier II Bin 8 values appear to be attainable with an advanced NOx storage catalyst while the urea SCR system is required for Tier II Bin 5. To achieve Tier II Bin 8 emis-sion limits with a NOx storage catalyst, the target for engine out NOx emissions had to be lower than 0.3 g/mile. As a target the ni-trous oxide emissions of the new V6 CDI engine had to be reduced by about 40% compared to the predecessor engine. Par-ticulate emissions had to be reduced by about 20%. Even with the particulate filter, the particulate emissions have to stay be-low 0.4 g/mile, since otherwise the frequen-cy of particulate filter regenerations be-comes too high.

Regarding these emission targets, the engine components and application of the V6 CDI engine were extensively modified as compared to various Euro4 applications. In this process, special attention was paid to retain the good fuel consumption values of

the Euro4 engine and of the existing U.S. E 320 CDI (Bin 10 – application).

Figure 4 shows the measures to achieve the engine out emission targets for the U.S. Tier II Bin 8 application of the OM 642. The Table explains the effects of engine meas-ures on nitrous oxide emissions and engine performance.

Figure 5 compares torque and power val-ues of the existing OM 648 engine with those of the new OM 642. The OM 642 reaches 500 Nm already at 1400 rpm, sig-nificantly earlier than the predecessor en-gine. At 1600 rpm a peak torque of 540 Nm is achieved and maintained over a broad speed range. Peak engine power rises from 150 kW to 155 kW, and is now reached as early as 3400 rpm, so that excellent drivea-bility is achieved.

4 Oxidation Catalyst and Particulate Filter

The oxidation catalyst is standard on all modern Mercedes-Benz diesel engines for some years now [2]. Rapid light off of the oxidation catalyst is particularly important for good conversion efficiency. With the E 320 Bluetec this is achieved with a small oxidation catalyst installed very close to the engine.

Figure 6 shows the layout in the vehicle. The NOx storage catalyst is located directly behind the oxidation catalyst. Since it uses a precious metal coating, it also functions as an oxidation catalyst. The two catalysts complement each other ideally: at engine start and under tight loads, the small cata-lyst heats up quickly and ensures suffi-cient conversion of hydrocarbons and car-bon monoxide, while at high loads and temperatures the large NOx catalyst func-tions with good efficiency, even at high flow velocities.

When the particulate filter was initially introduced in North America in 1985, Fig-

ure 7, Mercedes-Benz was at the cutting edge of this technology. The modern, addi-tive-free particulate filter has been offered by Mercedes-Benz since 2003. With its cata-lytic coating and thermal regeneration, the system functions without additional fuel additives and requires little maintenance. At the end of 2005 this particulate filter was available for the whole range of Mer-cedes-Benz models.

With the E 320 Bluetec, the particulate filter has now been introduced in the Unit-ed States and in 2006 in Japan with the E 320 CDI, thus, the technology is now available worldwide.

As noted earlier, the particulate filter helps in the trade-off between NOx and par-ticles. Nevertheless, it remains important to reduce particulate emissions to a low level. The particulate filter must be regu-larly regenerated and these regenerations negatively affect fuel consumption. When a certain particulate loading is reached, the deposited soot particles must be heated up until they react with oxygen in the exhaust gas. This reaction is initiated at tempera-tures between 600° and 650°C.

Since the system does not use catalytic fuel additives and is regenerated thermally, the amount of ash deported in the filter is low. Consequently, exhaust gas back-pres-sure rises only slowly over lifetime, which brings advantages in fuel consumption. Moreover, service intervals are significantly longer.

5 NOx Aftertreatment with a

NOx Storage Catalyst

In the E 320 Bluetec, a NOx storage catalyst (NSC) is used for nitrous oxide aftertreat-ment. The operating principle of a NOx storage catalyst is based on two successive steps [1].

In a first step the NO from the exhaust gas is oxidized to NO2 through the precious metal catalytic coating. The NO2 is then stored on storage components in the cata-lyst as nitrates, and cleaned up from the exhaust gas. As the amount of nitrates in the catalyst increases, the free storage ca-pacity of the catalyst decreases. Before a critical NOx loading is reached, the engine controller will start a NOx-regeneration. To regenerate, rich exhaust gas will be gener-ated by the engine. In the NOx-storage cata-lyst, the rich exhaust gas will reduce the stored nitrates to nitrogen. Figure 8 shows different operating modes for NOx-storage catalyst and particulate filter regeneration and the desulfurization of the NSC.

Supplying rich exhaust gas from a diesel engine for the NSC regeneration is a chal-lenging task, since it normally is a lean burn engine. It is solved through an intel-ligent control strategy in the engine man-agement system, using the oxygen sensor signal and exhaust gas recirculation, post injection and inlet throttling as control pa-rameters. Obviously, all these measures have to be taken without the driver notic-ing any change in power, torque, driveabil-ity or engine noise.

Figure 9 shows, how the injection strate-gy is modified in the NSC regeneration mode. The two standard pilot injections

MTZ 06I2007 Volume 68 3

COVER STORYMercedes-Benz Bluetec Concept

are moved to an earlier timing, quantity is slightly increased. Main injection is also moved to earlier injection timing, whereas the injected quantity is significantly de-creased. Additionally, a post injection is added. Further engine measures as dis-cussed above are taken, and thus a rich ex-haust gas is generated, without excessive particulate emissions.

In another operating mode, the particu-late filter is regenerated. Regeneration is scheduled model based [2] about every 500 to 1000 km through increasing the exhaust gas temperature.

Unfortunately, the NSC does not only absorb nitrates as described, but similarly it also absorbes sulfates. The chemical bonding of the sulfates to the metal compo-nents of the NSC is even stronger then the bonding of nitrates. Thus, fuels with a sul-fur content below 15 ppm are a must for the operation of the NSC. But even with such “sulfur free” fuels, the absorption of sulfates leads to a decrease of the NSC free storage capacity for nitrates. In conse-quence, from time to time a desulfuriza-tion has to be run. This desulfurization needs even higher temperatures as the NOx regeneration, and a combination of tem-perature, stoichiometry and duration of desulfurization has to be carefully deter-mined in order to not damage the NSC thermally, while at the same time, suffi-ciently regenerating the sulfates [7, 8]. What helped the development of such re-generation strategies was also the develop-ment of new NSC technologies in close col-laboration with Umicore. Finally, for the E 320 Bluetec, it was possible to combine desulfurization and particulate filter re-generation. After the exhaust gas tempera-ture has been raised for the DPF regenera-tion, the engine is now operated a few times with rich exhaust gas for a few sec-onds each time. Sulfates are reduced and will be removed from the NSC.

With the many different operation modes of the Bluetec-system, it becomes clear, how important the accurate control of engine, oxidation catalyst, particulate filter and NOx aftertreatment is. In princi-ple, the engine controller software is struc-tured as Figure 8 indicates: different oper-ating modes of the engine and aftertreat-ment are controlled by different modules of the software.

Part of the software controls the engine with the normal engine combustion. For air path and exhaust gas recirculation, model based controls are used. Other mod-ules take over operation during nitrous ox-ide regeneration, particulate filter regen-

eration and desulfurization. Change of op-erating modes is torque-based. A torque-neutral transition between the different operating modes is key for an engine opera-tion, where changes in operating mode cannot be perceived.

Figure 10 shows both the trade-off be-tween fuel consumption and NOx and MN-HC and NOx. The dotted curves show the trade-off for engine internal, the solid curves those for NSC application optimi-zation.

For internal engine measures initially significant reductions in nitrous oxide emissions are achieved with only minor penalties for fuel consumption and hydro-carbon emissions. But below a NOx value of about 0.3 g/mile, consumption and hydro-carbon emissions progressively worsen. Starting from an acceptable value for inter-nal engine emissions, the NSC is used with different application strategies. Significant NOx reductions are achieved even with very restrained use of nitrous oxide regenera-tion and desulfurization, with only minor penalties in consumption and hydrocarbon emissions. However, to meet Tier II Bin 8 emissions regeneration and desulfuriza-tion frequencies had to be increased. These changes in application finally allow to achieve emission targets.

As Figure 10 shows, the Tier II Bin 8 hy-drocarbon emission targets are also met.

However, the European limit for hydro-carbon emissions is lower, and Figure 10 graphically depicts the challenge to the in-troduction of the Bluetec system in Europe. European HC/CO limit values include methane as an exhaust gas component, but it is not included in the Tier II Bin 8 HC/CO limits. Thus the European HC/CO standard is considerably tougher to meet than the EPA limits. To fit Bluetec for European re-quirements, further development has to be done, for hydrocarbon emissions particu-larly. The same holds true for fuel con-sumption. In addition as said earlier, sul-fur-free diesel is a necessary precondition for the introduction of the Bluetec system, and sulfur free fuel is not yet available in Europe.

6 NOx Aftertreatment with

an SCR Catalyst

For more demanding emission require-ments such as those of the CARB (Califor-nia Air Resource Board) or post-new-long-term Japanese regulations, Mercedes-Benz develops a NOx aftertreatment based on SCR technology using AdBlue – a urea solu-

tion [5, 9]. A typical system layout is shown in Figure 11 in a Vision GL 320 Bluetec. The figure shows, as already described for the E 320 Bluetec, a closed coupled oxidation catalyst, the particulate filter, which now moves forward into the position of the NSC, a dosing valve immediately behind the particulate filter, and the SCR catalyst in the vehicle underfloor.

Also shown is the AdBlue tank under-neath the vehicle luggage compartment. As already discussed in [5], the AdBlue tank is dimensioned so that one filling suffices to cover the AdBlue requirement between two service intervals. Thus normally the system does not have to be serviced by the customer, since the AdBlue is replenished during regular maintenance.

When AdBlue is injected into the hot ex-haust gas, it first decomposes into water and urea. At temperatures around 200°C, the urea is transformed into ammonia, the active component in this process. The injec-tion nozzle, the mixing path between noz-zle and catalyst, and the flow into the con-verter, must all be designed to achieve a uniform distribution of ammonia in the SCR catalyst.

The ammonia is first stored in the cata-lyst. Exhaust gases containing nitrous ox-ides flow through the catalyst, and the ni-trous oxides are reduced by the ammonia. This catalysis occurs at temperatures lower than for the decomposition of urea into ammonia. AdBlue is dosed in discrete injec-tions, and a SCR catalyst ammonia storage ensures the ammonia supply, so that suffi-cient ammonia is available between injec-tions. Dosing of AdBlue is model-based, so that the ammonia buffer in the SCR cata-lyst is always sufficiently filled. The control has to be accurate so that overdosing and ammonia slippage are avoided. A NOx sen-sor is used to adjust the model based con-trol.

7 Summary

Primary target for Mercedes-Benz diesel en-gine development is to design powerful, high torque, yet fuel efficient diesel en-gines, that at the same time meet the high expectations of Mercedes-Benz customers with regard to NVH and driveability. This is primarily achieved by using innovative en-gine technologies, in order to already mini-mize engine out emissions as much as pos-sible. This is imperative both from the cost and fuel consumption aspects. Two after-treatment systems are alternatively availa-ble, to be used depending on market re-

COVER STORY

MTZ 06I2007 Volume 68 4

Mercedes-Benz Bluetec Concept

quirements, to reduce nitrous oxide emis-sions below engine out values. The system with NOx storage catalyst is the more eco-nomical solution. The SCR system is overall more efficient, and the superior solution with regard to fuel consumption, hydrocar-bon emissions and engine power.

The Bluetec system with NSC is in serial production in the United States since Octo-ber 2006 in the E 320 Bluetec. It is the world first diesel powered passenger car to meet the Tier II Bin 8 emission limits. With outstanding power and torque, very good fuel economy [6] and excellent driveability, the E 320 Bluetec underlines Mercedes-Benz’s claim to technological leadership in diesel engine development. The Bluetec system with SCR catalyst will mid term be another important milestone towards the goal “The diesel engine as clean as the gasoline engine”.

References[1] Breitbach, H.; Schön, Chr.; Leyrer, J.: Potenziale und

Grenzen der Abgasnachbehandlung durch NOx -

Speicherkatalysatoren [Potential and limits of ex-

haust-gas after-treatment using NOx storage cata-

lysts], 14th Aachen Colloquium on Vehicle and Engine

Technology, Aachen, 2005

[2] Schommers, J. et al: Das neue Mercedes-Benz Diesel-

partikelfilter-Konzept für Pkw in Verbindung mit der

Abgasstufe Euro 4 [The new Mercedes-Benz diesel

particulate filter concept for passenger cars in relation

to Euro 4 exhaust gas limits], 25th Vienna International

Engine Symposium, Vienna 2004

[3] Mikulic, L.; Schommers, J.; Breitbach, H.: Emissions-

strategie Dieselantriebe [Diesel power emissions

strategy], VDA Technical Congress 2006, Munich,

March 2006

[4] Mercedes-Benz E 320 CDI vs. Lexus GS 450h, Shukan

Post [Japan], April 28, 2006

[5] Enderle, C.; Breitbach, H.; Paule, M.; Keppeler, B.: Se-

lective Catalytic Reduktion mit Harnstoff – Der effek-

tive Weg zur Stickoxidminderung am Pkw-Dieselmotor

[Selective Catalytic Reduction with urea – the effec-

tive path to nitrous oxide reduction in passenger car

diesels], 26th Vienna International Engine Symposium,

Vienna 2005

[6] EPA Fuel Economy Label Values, www.fueleconomy.

gov, June 2006

[7] Theis, J.; Göbel, U.; Kögel, M.; Kreuzer, T.; Lindner, D.;

Lox, E.; Ruwisch, L.: Phenomenological Studies on the

Storage and Regeneration Process of NOx Storage

Catalysts for Gasoline Lean Burn Applications, SAE

Tech. Paper 2002-01-0057 (2002)

[8] Eberhardt, M.; Riedel, R.; Göbel, U.; Theis, J.; Lox, E.

S.: Fundamental Investigations of Thermal Aging

Phenomena of NOx Storage Components, Topics in

Catalysis, 30/31(1-4), (2004), 135-142

[9] Lepperhoff, G.; Schommers, J.: Verhalten von SCR-

Katalysatoren in dieselmotorischem Abgas [Behavior

of SCR catalysts in diesel exhaust gas].In: MTZ 49

(1988), No.1, pp. 17-21

MTZ 06I2007 Volume 68 5

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