Engines and Exhaust After Treatment Systems for Future Automotive Applications

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Engines and exhaust after treatment systems for

future automotive applications

Ulrich G. Alkemade , Bernd Schumann

Robert Bosch GmbH, Corporate Research, Applied Research Chemical Processes and Technology (CR/ARC), D-70049 Stuttgart, Germany

Received 4 January 2006; received in revised form 23 May 2006; accepted 29 May 2006

Abstract

Engine concepts for future automotive applications: Safe, clean and efficient engines will become more important in modern societies where we

will see higher levels of mobility on one hand and limited resources on the other hand. Gasoline engines for passenger cars have been developed to

generate more power and reduce emissions at the same time. Therefore the engine systems have become complex with a number of subsystems.

Because of its reliability and efficiency the diesel engine is classically operated in heavy duty vehicles, however in recent years because of its high

torque when used with a turbocharger it has become more popular for passenger cars and even sport vehicles as well. The development of the diesel

engine especially the direct injection as well as the common rail high pressure injection brought further improvement regarding power, efficiency and

emissions.In the future exhaust after treatment systems will be developed in order to complywith emission standards similar to those of gasoline engines.

Emission control systems with chemical and physical sensors: In order to meet the more and more stringent emission regulations gasoline as well as

diesel engines will need continuously improved exhaust after treatment systems.The options for the various applications are highlighted in the following.

Today exhaust gas of gasoline engines is typically treated with Three Way Catalysts (TWC). The catalyst converts the pollutants CO, NOXand Hydrocarbons into harmless compounds like CO2, H2O and N2 by chemical reactions. Lambda-Sensors control the air fuel ratio of the engine

and catalyst performance in order to get the best possible conversion of the pollutants.

Modern lean burn engines have other options. Here the pollutants in the exhaust gases are only partially converted by a TWC function i.e. COand Hydrocarbons. For the remaining NOX a so called NOX Storage Catalyst (NSC) is employed, which chemically stores NO and NO2 during

lean burn phase. For the conversion of stored NOXthe engine is periodically shifted to fuel rich operation. This more complex system is controlled

with the help of mathematical catalyst models and by Lambda-, Temperature- and optionally NO X-Sensors as well.

Diesel engine exhaust of heavy duty vehicles will be treated with ammonia by Selective Catalytic Reduction (SCR) to reduce NO Xadditionally

to the catalytic oxidation of CO and Hydrocarbons. The ammonia is generated on board of the vehicle using harmless precursors like for example

urea. For the control of this system Lambda-, Temperature- NOX- and optionally NH3-Sensors are employed.

In addition to gaseous pollutants the particulate emissions from diesel engines will be removed by Diesel Particulate Filters (DPF). The system

of oxidation catalyst and DPF is controlled by Temperature-, Pressure- and Particulate-Sensors.

The mentioned highlights show that all three goals safe, clean and efficient can be met in the future by both gasoline and diesel engines

combined with modern exhaust after treatment systems.

2006 Elsevier B.V. All rights reserved.

Keywords: Emission control; Exhaust treatment; Chemical sensors

1. Introduction

1.1. Emission control regulations

Due to increasing mobility especially in large cities environ-

mental protection has advanced to become a topic of central

concern. Emission control regulations have been introduced in all

industrialized countries in order to reduce the emissions of

Solid State Ionics 177 (2006) 22912296

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Corresponding author.

E-mail address: [email protected] (U.G. Alkemade).

0167-2738/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2006.05.051
mailto:[email protected]://dx.doi.org/10.1016/j.ssi.2006.05.051http://dx.doi.org/10.1016/j.ssi.2006.05.051mailto:[email protected]


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to low ppm levels provided the engine operates with the right A/F

mixture distribution, which is obtained by partially adjusting a

stoichiometric and modulated A/F ratio. Given the fact that it is not

always possible to operate the engine under optimal conditions, an

average pollutant reduction of more than 98% can be achieved [1].

2.2. NOX Storage Catalysts

On gasoline direct injection engines, the oxygen required tooxidize HC and CO is taken from the high proportion of oxygen

remaining in the exhaust gas. As a result, a TWC is not

sufficient, since the NOX cannot be converted under lean

conditions.

The catalytic layer of the NOX accumulator type catalytic

converter also contains substances which can store NOX (e.g.

barium and alkaline oxides). All conventional NOXaccumulator

coatings also have the properties of a TWC, with the result that

these catalysts operate like TWC at=1 conditions.

In lean stratified-charge operation, NOXis converted in threestages. In the storage phase, NOXis first oxidized to NO2, which

Fig. 3. Lambda-Sensor for the control and diagnosis of three-way-catalysts.

Fig. 4. NOX Storage Catalysts for lean burn applications.

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then reacts with the additives in the coating to become nitrates

(e.g. barium nitrate).

As the quantity of stored NOX increases, the ability to

continue binding NOXdecreases. At a predefined laden state, the

NOXaccumulator must be regenerated, i.e. the nitrogen oxides

stored must be removed and converted. For this purpose, theengine switches briefly to rich homogeneous operation (


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Sensors in reaction to a specific alteration of the set point value

of the lambda closed loop control.

Additionally the NOX accumulation capacity must be

assessed for the NSC. (Fig. 5) For this purpose, the actual

NOXaccumulator content resulting from the consumption of the

reduction agent during regeneration of the catalytic converter is

compared with an expected value [1,4].

2.3. Selective catalytic reduction systems

Selective catalytic reduction (SCR) is about to be introduced forthe reduction of nitrogen oxides in vehicle exhaust gas. For several

years this technology has been a proven means of removing

nitrogen oxides from exhaust gases of fossil fired power plants.

Ammonia (NH3) has proven to be the reducing agent with the

greatest selectivity in this respect. As ammonia is a toxic substance

the actual reducing agent used in motor vehicle applications is urea.

Urea is manufactured commercially and is both ground water-

compatible and chemically stable under ambient conditions.

The ammonia created by thermal hydrolysis primarily reacts

in the SCR catalytic converter with NO2 to form nitrogen and

water. Modern SCR catalysts can incorporate the function of

hydrolyzing catalyst so that there is frequently no need to use a

hydrolyzing catalyst.An oxidation catalyst, which oxidizes NO into NO2, brings

about an improvement in the conversion of NOX. This enables

Fig. 8. Pressure differential sensor for the control of diesel particulate filters.

Fig. 7. Operation of diesel particulate filters.

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the SCR catalyst to work at an optimum efficiency, especially at

low (

Engines and Exhaust After Treatment Systems for Future Automotive Applications

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