Xin Q. Diesel Engine System Design

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498 Diesel engine system design

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503

Diesel aftertreatment integration and

matching

Abstract: This chapter addresses one of the central tasks in diesel engine

system design, engine–aftertreatment matching. It begins with an overview

of aftertreatment requirements on engine system design, including the

performance of diesel oxidation catalyst (DOC), urea-based selective

catalytic reduction (SCR), solid ammonia storage and release (SASR),

lean NO

trap (LNT) and diesel particulate lter (DPF). The chapter then

discusses exhaust thermal management and aftertreatment calibration,

followed by a description of DPF regeneration requirements for engine

system design. The chapter is concluded by an analytical approach of

engine–aftertreatment integration.

Key words: engine–aftertreatment matching, aftertreatment calibration,

diesel oxidation catalyst (DOC), selective catalytic reduction (SCR),

solid ammonia storage and release (SASR), lean NO

trap (LNT), diesel

particulate lter (DPF), DPF regeneration.

8.1 Overview of aftertreatment requirements on

engine system design

Current design and calibration strategies for the reduction of diesel engine-

out emissions are approaching their practical limits. The diesel particulate

lter (DPF) is required for the US 2007 model year, and NO

aftertreatment

may be considered in the future. Diesel aftertreatment devices exhibit several

characteristics which inuence the engine design and operation: (1) performance

variation (e.g., the change of DPF pressure drop as soot builds up); (2) the

need for regeneration and exible engine controls; (3) performance being

closely related to vehicle duty cycle; and (4) the interactive chemistry nature

between the different aftertreatment devices. There is a large body of literature

on aftertreatment performance itself. However, there is very little literature

on either the approach of engine–aftertreatment matching or the analysis of

aftertreatment integration in engine system design and calibration.

Diesel exhaust gases exhibit several unique characteristics compared to

the gasoline engine: lower HC and CO; higher particulate matter (PM); lower

exhaust temperature due to higher air–fuel ratio; and oxidizing exhaust gas

chemistry suppressing the chemical reduction of NO

. Modern diesel exhaust

aftertreatment devices consist of a diesel oxidation catalyst (DOC), a diesel

particulate lter (DPF) and possibly a NO

aftertreatment device such as

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504 Diesel engine system design

selective catalytic reduction (SCR) or lean NO

trap (LNT, also known as

adsorber). The mufer can usually be replaced by a wall-ow DPF in

commercial vehicles, or it may be used to further reduce the exhaust noise.

The emissions formation mechanisms in the diesel engine are provided in

detail by Eastwood (2000, 2008 and 2010) and Majewski and Khair (2006).

Exhaust aftertreatment control technologies are reviewed by Khair and

McKinnon (1999), Johnson (2000a, 2000b, 2001, 2002, 2003, 2004, 2006,

2007a, 2007b, 2008a, 2008b, 2009a, 2009b and 2010), Graves et al. (2001),

Corro (2002), Blakeman et al. (2003), Edgar et al. (2003), Eastwood (2000,

2008), and Majewski and Khair (2006).

8.1.1 Diesel oxidation catalyst (DOC) performance

The DOC reduces HC (including PAH), CO, SOF in PM, and diesel exhaust

odor by converting them to H

O and CO

. The de-NO

capability of the

DOC in the presence of HC is insignicant so that the NO

level is almost

unaffected. The level of nanoparticles is not affected either. The combustion

of a high-sulfur diesel fuel produces SO

, which is converted to SO

and a

large amount of sulfate particulate in DOC at high engine loads under high

exhaust temperatures. Therefore, the SOF removal ability of the DOC may be

compromised by the addition of the sulfate particulate matter. Using a low-

sulfur diesel fuel can reduce the sulfate particulate matter and avoid catalyst

poisoning. Because NO

may assist the DPF regeneration, the oxidation of

NO to NO

by O

and the reduction of NO

by HC in the DOC need to be

optimized. DOC performance was investigated by Phillips et al. (1999), Khair

and McKinnon (1999), Nieuwstadt et al. (2005), and Kozlov et al. (2010) and

reviewed by Majewski and Khair (2006). A real-time-capable DOC model

was developed by Wahiduzzaman et al. (2008) and Wenzel et al. (2009). An

engine CO emission model was developed by Bagal et al. (2009).

The efciency of the catalytic converters is related to air–fuel ratio,

light-off temperature and light-off time, space velocity, ow restriction

characteristics and the durability features. The light-off temperature is dened

as the temperature at which 50% conversion efciency is achieved. The

space velocity is the ratio of the normalized exhaust volume ow rate to the

aftertreatment component volume. The ow restriction can be estimated by

using the curve of ‘pressure drop vs. ow rate’. The light-off temperatures

of HC and CO become lower when the air–fuel ratio is higher. The three-

way catalytic converter used in the gasoline engine has a high conversion

efciency above 80% simultaneously for CO, HC, and NO

only when the

excess air–fuel ratio stays around 1 (i.e., at the stoichiometric ratio) with a

narrow uctuation window about 0.01. Diesel engines operate under the ‘lean

burn’ conditions with a much higher air–fuel ratio due to the heterogeneous

nature in diesel combustion and the high PM emissions produced. In the

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505Diesel aftertreatment integration and matching

oxidizing exhaust environment, the reductants such as HC and CO are

more reactive to the excess oxygen rather than to the NO

. Therefore, the

three-way catalyst used in the gasoline engine usually cannot be used in the

diesel engine although some research was conducted on using a three-way

catalyst on diesel engines to explore the potential of emissions control (e.g.,

Sung et al., 2009; Simescu et al., 2010). Urea-based SCR, solid ammonia

storage and release, and LNT are three promising technologies for diesel

aftertreatment with up to 80–90% NO

reduction. They all require

reductants such as urea (CO(NH

)

), ammonia (NH

), or hydrocarbons.

8.1.2 Urea-based selective catalytic reduction (SCR)

performance

The urea-based SCR is believed by many people in the industry to be more

suitable than the LNT for heavy-duty applications. Urea is used to generate

ammonia by thermal decomposition. The aqueous urea is sprayed into the

exhaust line, where under ideal conditions it evaporates and decomposes

completely to ammonia and CO

. In the SCR, ammonia is less reactive to

oxygen than the hydrocarbons, and can be mainly reduced by NO

rather

than the other oxidant, O

. Therefore, the NO

conversion efciency of the

SCR is greater than the active de-NO

catalytic reduction using hydrocarbons.

In the SCR, NO

is reduced to nitrogen and water. The dosing rate of the

urea must match the temperature, the exhaust composition, and the space

velocity of the exhaust ow. Precise dynamic dosage control is required

for the SCR in order to prevent excessive slip of the toxic ammonia during

engine transients in automotive applications. A second DOC may be added

downstream of the SCR to convert the slipped ammonia to nitrogen. The

freezing point of the urea in aqueous solution at –11°C could be a problem.

The ammonium sulfate accumulated in the SCR needs to be purged at

high temperature periodically. Moreover, the SCR is effective in removing

HC and some SOF. Urea-based SCR performance is elaborated by Müller

et al. (2003), Scarnegie et al. (2003), Klingstedt et al. (2006), Hosoya et

al. (2007), Iitsuka et al. (2007), and Hirata et al. (2009). Urea-based SCR

control has been investigated by Song and Zhu (2002), Willems et al. (2007),

Devarakonda et al. (2008), Wang et al. (2008), and Herman et al. (2009).

Urea-based SCR modeling has been conducted by Chi and DaCosta (2005)

and Kim et al. (2007).

8.1.3 Solid ammonia storage and release for NO

reduction

The urea-based SCR is an aqueous solution that has a number of issues for

automotive applications, for example, large storage volume, freezing at cold

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506 Diesel engine system design

climate, injector dosing reliability, risk of deposits with urea hydrolysis at

low exhaust temperatures, and inconvenience for the end user. Another novel

and promising technology for reducing the NO

from the exhaust of diesel

vehicles is to use solid ammonia storage and release (SASR). It does not have

the above-mentioned shortcomings of the urea-based SCR. It uses compact

solid metal ammine complexes (e.g., M

(NH

)

, Elmoe et al., 2006) as

a source to safely store ammonia and provides a controlled release in gas

phase on demand for NO

reduction. It can store a large amount of ammonia

in the solid metal ammine complex with a high volumetric density that is

very similar to pure liquid ammonia (NH

), and hence provide long lasting

ammonia storage on the vehicle. It requires much smaller storage volume

than the urea-based SCR. Upon heating, the solid complex uses dynamic,

accurate, and controlled dosing release of pure ammonia gas directly to the

exhaust line of the engine, and does not require an injector for the dosing.

This technology can perform reasonably well at all ambient temperatures

and exhaust temperatures. It does not have the freezing problem and the

issue of low urea-conversion rate at low exhaust temperatures as occurring

with the urea-based SCR. More information on solid ammonia storage and

release can be found in Elmoe et al. (2006), Johannessen et al. (2009), and

Chakraborty et al. (2009).

8.1.4 Lean NO

trap (LNT) performance

LNT converts NO to NO

through an oxidation catalyst when it is exposed

to the oxidizing exhaust. Then, it adsorbs NO

as nitrates with alkaline earth

compounds such as Ba(NO

)

through acid–base neutralization reactions.

The LNT is periodically and briey regenerated to restore its NO

reduction

capability by using a reductant or electric heating. The working temperature

window of the LNT is bounded by NO light-off on the lower side and

by the nitrate instability on the upper side. The LNT regeneration in the

diesel engine is more challenging than that in the lean-burn GDI gasoline

counterpart because a rich air–fuel ratio cannot be utilized in the diesel

engine due to concerns about smoke production. The LNT performance is

very sensitive to sulfur content in the fuel, even as low as 3 ppm. Therefore,

desulfation or other sulfur management is needed. The desulfation usually

lasts several minutes with a rich air–fuel ratio at high temperatures. Other

sulfur management includes using an ultra-low sulfur fuel or a sulfur-free

fuel or sulfur traps. The LNT performance has been researched by Sluder and

West (2001), Parks et al. (2002), Takahashi et al. (2004), Hinz et al. (2005),

Theis et al. (2005), Hu et al. (2006), Nam et al. (2007), McCarthy Jr and

Holtgreven (2008), Ottinger et al. (2009), and McCarthy et al. (2009). LNT

modeling has been conducted by Kim et al. (2003) and He (2006).

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507Diesel aftertreatment integration and matching

8.1.5 Diesel particulate ﬁlter (DPF) performance

Achieving high ltration efciency and low ow restriction with small

dimensions and low costs is a challenging task in DPF design. The exhaust

restriction at the turbine outlet refers to the pressure drop through all the

aftertreatment components plus the exhaust pipes. Exhaust restriction depends

on hardware design and the ow restriction characteristics of the components.

The pressure drop of the DPF increases as the soot and the ash accumulate

in the lter. It also varies with the amount of soot deposits in the lter before

and after the DPF regeneration. Exhaust restriction also can be regulated

by using an exhaust back-pressure valve, such as a ap valve installed at

the turbine outlet. Closing the back-pressure valve reduces the exhaust ow

rate or may assist engine braking. Exhaust restriction has a large impact on

turbocharged engine performance.

DPF technologies are reviewed and summarized in three comprehensive

books by Majewski and Khair (2006), Johnson (2007b), and Eastwood

(2008). There is no need to elaborate on them here. The literature on DPF

research areas is organized as follows:

∑ The DPF selection for retrotting vehicles is discussed by Mayer et al.

(2001). DPF design effects were studied by Konstandopoulos et al. (1999,

2005b), Nikitidis et al. (2001), Merkel et al. (2001), Konstandopoulos

and Kladopoulou (2004), Soeger et al. (2005), Yamaguchi et al. (2005),

and Ido et al. (2005).

∑ DPF general performance was studied by Khair (2003), Khair and

McKinnon (1999), Toorisaka et al. (2004), Herrmuth et al. (2004), Cutler

(2004), and Kapetanovic et al. (2009).

∑ DPF pressure drop was researched by Taoka et al. (2001), Konstandopoulos

et al. (2001b), Stratakis et al. (2002), Konstandopoulos (2003),

Cunningham et al. (2007), and Ohyama et al. (2008).

∑ DPF regeneration performance was experimentally investigated by Tan

et al. (1996), Gantawar et al. (1997), Park et al. (1998), Bouchez and

Dementhon (2000), Salvat et al. (2000), Gieshoff et al. (2001), Locker

et al. (2002), Hiranuma et al. (2003), Flörchinger et al. (2004), Mayer

et al. (2005), Kong et al. (2005), Ogyu et al. (2007), and Ootake et al.

(2007).

∑ DPF regeneration control algorithms were developed by Brewbaker and

Nieuwstadt (2002), Nieuwstadt and Trudell (2004), Birkby et al. (2006),

and Bencherif et al. (2009).

∑ DPF modeling has been carried out mainly by the following groups:

 A group of researchers primarily at Aerosal & Particle Technology

Laboratory in Greece (review papers by Konstandopoulos et al.,

2000, 2005a; Konstandopoulos and Kostoglou, 1999, 2004; Masoudi

et al., 2001; Konstandopoulos et al., 2001a, 2002, 2003a, 2003b,

2004; Kladopoulou et al., 2003).

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