Xin Q. Diesel Engine System Design

862 Diesel engine system design

BSFC. Figure 13.3 shows the typical behavior of air system control factors

and their design directions in the engine speed–load domain.

13.2 Overview of low-emissions design and air

system requirements

In the base engine design with a given displacement, there are ve factors

directly related to air system performance in terms of volumetric efciency,

heat rejection, pumping loss, and mechanical friction. The factors are: (1)

number of cylinders; (2) stroke-to-bore (S/B) ratio; (3) valve ow area related

to number of valves and valve size; (4) valve overlap height; and (5) the

‘K-factor’ used to measure the proportion of air available for combustion.

The K-factor refers to the ratio of piston bowl volume to total combustion

chamber volume at the TDC. Fewer cylinders result in fewer parts and lower

material cost, but they increase the reciprocating mass of each cylinder. The

reciprocating inertia force is high at high speeds. Using more cylinders is an

option to reduce the inertia force. Increasing the number of cylinders also

increases the surface-to-volume ratio and heat losses. A larger S/B ratio

reduces piston ring circumference and the potential air leakage during cold

start and may also reduce the surface-to-volume ratio for the TDC volume

and heat losses. A smaller S/B ratio allows bigger intake or exhaust valve

size to achieve higher volumetric efciency and lower pumping loss. Engine

friction and inertia force increase as the mean piston speed increases, which

Simulation by using same heat release

rate and start-of-combustion timing,

with different turbine nozzle area

(or wastegate opening) and efﬁciency

BSFC deterioration

due to combustion, engine

friction, etc.

(non-pumping loss items)

Engine delta P (bar)

EGR = 25%

EGR rate = 35%

Measured BSFC

Calculated

BSFC penalty

due to pumping

loss increase

20%

15%

10%

5%

0%

Percentage of BSFC increase

0 0.5 1 1.5 2

13.2 Effect of engine delta P on BSFC at rated power.

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863Diesel engine air system design

is proportional to the S/B ratio. The K-factor affects air system design

because it is related not only to the tolerances of the piston assembly, the

cylinder head, and the gasket, but also to the size of the valve recession

or the piston cutout and hence the valve overlap height at the TDC. The

greater the K-factor, the lower the dead volume in the combustion chamber.

Valve overlap is a key design parameter for engine breathing performance

and valvetrain dynamics. Reducing the clearance volume can increase the

K-factor, reduce soot emissions, and improve the smoke limit. Competitive

engine data analysis indicates that there seems to be a trend for the K-factor

to be a function of the S/B ratio. As the S/B ratio increases from 1.0 to 1.2,

the K-factor range increases from 0.7 to 0.8 approximately. The optimum

system design concept should be based on the best trade-off among the S/B

ratio, the K-factor, and the valve overlap height.

The prevailing cylinder head design in modern diesel engines uses a four-

valve head. Compared with the two-valve head design, it has the following

benets: (1) higher volumetric efciency; (2) enabling a centralized vertical

fuel injector; (3) the feasibility of shutting off one intake port to regulate the

swirl; and (4) a lower valvetrain weight and better valvetrain dynamics. In

intake port design, the appropriate swirl ratio needs to be determined based on

0.8

0.7

0.6

0.5

0.4

0.3

0.2

45

40

35

30

25

20

15

10

5

15

10

5

0

–5

60

50

40

30

20

10

0

Engine brake torque (ft.lb)Engine brake torque (ft.lb)

Engine brake torque (ft.lb)

The design objective is

to expand the low EGR

restriction region to the right

The design objective is to

expand the low engine delta

P region to the right

The EGR valve closing is

caused by the high engine

delta P due to the small

turbine area

The design objective is to

minimize the use of intake

throttle in speed–load domain

Engine speed (rpm)

Engine speed (rpm) Engine speed (rpm)

EGR valve opening c

d,EGR

Engine delta P (back minus boost) (inch Hg)

Intake throttle opening (duty %)

HP turbine wastegate opening (mm)

13.3 Engine air system control parameters in speed–load domain.

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864 Diesel engine system design

the following considerations: (1) matching with the fuel injection parameters

for good penetration and mixing; (2) matching with the combustion chamber

shape for proper air motion and utilization; (3) matching with the cylinder

bore size to control wall impingement of the fuel spray and evaporation of

the fuel lm on the cylinder wall; (4) an appropriate compromise with port

ow discharge coefcient or volumetric efciency; (5) a proper balance

between the swirl levels at low speeds and high speeds; and (6) an acceptable

cylinder coolant heat rejection loss, which is affected by the in-cylinder air

turbulence level. The current trend in heavy-duty diesel engine design is to

decrease the intake swirl but to increase the fuel injection pressure, number

of nozzle holes, and combustion chamber diameter. Diesel engine cylinder

head design is reviewed by Gale (1990).

Compared to naturally aspirated engines, turbocharging with intercooling

can increase the air–fuel ratio and reduce soot. Soot can also be greatly

reduced by increasing fuel injection pressure. NO

x

is controlled mainly by

using EGR and retarding fuel injection timing. In order to satisfy different

air demands within a wide speed range in automotive diesel applications,

the xed-geometry turbine has gradually been replaced by the wastegated

turbine or the VGT. Turbocharging also enables EGR to be used because

engine delta P can be modulated by turbine area to drive EGR in a high-

pressure-loop EGR system. Alternatively, a compressor can be used to

pump EGR into the intake manifold in a low-pressure-loop EGR system.

Coordinated joint design of turbocharging, intake port and cam prole to

match with the EGR system is the key to fulll the air ow requirements

over a wide speed range. More information on the fundamentals of the air

system congurations and performance of turbocharged EGR diesel engines

can be found in Jacobs et al. (2003) and Hochegger et al. (2002).

Turbine outlet gas temperature and exhaust manifold gas temperature

are two critical parameters in air system design. The former is related to

aftertreatment regeneration. The latter affects turbocharger performance and

also has been used to gauge the thermal loading on engine components. When

the in-cylinder gas temperature increases, the exhaust temperature and the

heat ux to the power cylinder components also increase. The following

can lead to a decrease in exhaust manifold gas temperature: (1) increase

in air–fuel ratio or EGR rate; (2) fuel injection timing advance; and (3)

enhanced cylinder cooling. Previously, without an emissions-driven design

approach, turbocharging parameters were selected based on a target turbine

inlet gas temperature determined by the requirements of acceptable thermal

load and engine reliability. The required air–fuel ratio and large valve overlap

were then calculated to achieve the target exhaust manifold temperature.

Finally, the turbine ow area and pressure ratio were determined based on

the required intake boost pressure and turbocharger efciency. Now the

situation is different due to an emissions-driven approach. The turbocharger

Diesel-Xin-13.indd 864 5/5/11 12:04:42 PM

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865Diesel engine air system design

selection criterion is based on air–fuel ratio and EGR rate to meet emission

requirements, with the maximum exhaust manifold gas temperature as a

durability design constraint.

13.3 Exhaust gas recirculation (EGR) system

conﬁgurations

13.3.1 Classiﬁcation of EGR systems

EGR systems can be classied into external and internal EGR systems. The

internal EGR, usually uncooled, refers to the trapped in-cylinder residue of

combustion product and the reverse gas ow from the exhaust manifold/port to

the cylinder. Cooled external EGR is generally more effective than uncooled

internal EGR for emissions reduction and fuel economy, although the heat

rejection in the external EGR system needs to be handled by the cooling

system. The external EGR can be further classied into high-pressure-loop

(HPL) EGR, low-pressure-loop (LPL) EGR, and hybrid (or dual-loop, i.e.,

HPL plus LPL) EGR systems (Fig. 13.4).

Diesel engine EGR mixing has been researched by Siewert et al. (2001)

and Partridge et al. (2002). Diesel engine EGR systems have been extensively

investigated by Akiyama et al. (1996), Baert et al. (1996, 1999), Kohketsu et

al. (1997), Mattarelli et al. (2000), Graf et al. (2000), Lundqvist et al. (2000),

Luján et al. (2001), Osborne and Morris (2002), Andersson et al. (2002),

Chatterjee et al. (2003), Maiboom et al. (2008), and Shutty (2009).

13.3.2 EGR engine design principles

Engine pumping loss reduction is the most important design objective in EGR

system design. The BSFC of modern diesel engine is largely dominated by

pumping loss (Fig. 13.2). Pumping loss control is critical for three reasons:

(1) to maintain good BSFC in a high-EGR engine within a wide speed range

from peak torque to rate power; (2) to increase air–fuel ratio to reduce soot;

and (3) to reduce peak cylinder pressure and exhaust manifold pressure to

alleviate their design challenges. One example of engine delta P distribution

in the engine speed–load domain is shown in Fig. 13.3. The control factors in

air system design include mainly turbine area and/or wastegate, EGR valve

opening, and intake throttle opening. Wastegating essentially means higher

pumping loss. Closing the EGR valve indicates an undesirable increase in

EGR circuit ow restriction. Closing the intake throttle results in a large

decrease in air–fuel ratio.

The design guidelines for the air system of high-EGR engines to achieve

good BSFC include the following:

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High-pressure-loop EGR and low-pressure-loop EGR system mechanisms

Air ﬁlter

Turbine

Compressor

EGR cooler

Engine

HPL-EGR system:

EGR is taken from the exhaust

manifold to the intake manifold

LPL-EGR system:

EGR is taken from the downstream

of the turbine to the compressor

inlet

Tailpipe

Air ﬁlter

CAC

DPF

DOC

Intake

manifold

Intake

manifold

Exhaust

manifold

Exhaust

manifold

DPF and SCR or

LNT

EGR

Exhaust

gas

Exhaust

gas

DOC

HPL EGR

LPL EGR

Intake

throttle

EGR valve

SCR or LNT

Air + EGR

EGR valve

EGR cooler

EGR

EBP valve

Air

13.4 Turbocharged EGR engine systems.

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867Diesel engine air system design

∑ achieve high EGR rate with low  ow restriction in the EGR circuit or

low engine delta P.

∑ eliminate local throttle losses (i.e., exergy losses or entropy increases)

at the EGR valve, the intake throttle valve, and the turbine wastegate

valve.

∑ use inexpensive ways to achieve high turbocharger ef ciency (e.g., shift

engine operating points to the high-ef ciency region on the compressor

map).

The design and calibration principles of EGR engine’s air system can be

explained by the four core equations introduced in Chapter 4, i.e., equation

4.40 (engine volumetric ef ciency), 4.44 (EGR circuit pressure drop),

4.47 (turbocharger power balance), and 4.57 (turbine  ow), summarized

(combined) as below:

h

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13.1

These four core equations can be used to solve for any four unknowns (e.g.,

intake manifold boost pressure p

2

, exhaust manifold pressure p

3

, turbine area

A

T

, EGR circuit  ow restriction coef cient C

d,EGR

) with a given target of

air  ow rate and EGR rate. Based on these equations, in any EGR system

design, two questions need to be considered:

1. Does the EGR system raise engine delta P unfavorably?

2. Which of the following causes more deterioration in BSFC: EGR pumping

work with a pump (if any) or engine pumping loss?

The answer to these questions depends on turbocharger ef ciency.

13.3.3 Advantages and challenges of HPL EGR system

The HPL EGR system has traditionally been used in EGR engines. The HPL

EGR system uses the engine delta P created by the turbocharger to drive

EGR from the exhaust manifold to the intake manifold. A higher EGR rate

Diesel-Xin-13.indd 867 5/5/11 12:04:43 PM

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868 Diesel engine system design

requires higher engine delta P to drive in HPL EGR. Modern diesel engines

need to drive a high EGR rate and sufcient air–fuel ratio from peak torque

to rated power in order to meet the stringent emissions requirements relating

to NO

x

and soot. If a xed-geometry turbine is used, with the lowest level of

EGR system ow restriction (i.e., with the EGR valve fully open), the turbine

area is usually sized small enough to drive the required EGR rate at peak

torque. The turbine usually needs to be wastegated at rated power in order

to avoid over-boosting the engine on peak cylinder pressure and/or exhaust

manifold pressure. The smaller the turbine area, the more wastegating is

required at high speeds. Compared to a VGT (without wastegating) to deliver

the same boost pressure, wastegating has to use a higher turbine pressure

ratio which compensates for the loss of the turbine mass ow rate in order

to maintain the same turbine power. This is the reason why wastegating

causes higher engine delta P and BSFC than VGT. Therefore, by the nature

of engine–turbocharger matching with a xed turbine area, there is a trade-

off between low-speed and high-speed performance in pumping loss and

system efciency. VGT may solve the dilemma by using exible turbine area

at different engine speeds, but there are often penalties in turbocharger cost

and durability as well as a reduction in turbine efciency at the very closed

or wide open VGT vane opening. The role of intake throttle is to reduce

excessively high air–fuel ratio at peak torque or high speed and low load, or

slightly increase EGR rate at a penalty of throttling loss. An excessively high

air-fuel ratio may cause high NO

x

and high engine delta P. This is usually

because the turbocharger efciency is too high. It should be noted that in

most applications the air–fuel ratio at peak torque is short compared to the

ideal requirement instead of being excessively high.

In summary, when the engine delta P is insufcient to drive EGR at low

speed and high load, the following measures can be considered: (1) reducing

the ow restriction of the EGR valve and the EGR cooler (e.g., using parallel

instead of serial EGR coolers); (2) using a check valve or a reed valve to

harvest EGR ow pulsation and prevent EGR backow loss; (3) using a

small turbine area or a VGT to raise the engine delta P; (4) using the intake

throttle to lower the intake manifold pressure; (5) using a venturi device to

locally reduce the static pressure at the EGR merging location in order to

induce the EGR ow into the intake manifold; or (6) using an EGR pump.

The weakness of the HPL EGR system in heavy-duty diesel engines is

that a large EGR ow rate often needs to be driven by the engine delta P

at peak torque. An improved EGR-driving system needs to provide a better

way to pump EGR in order to reach a lower engine delta P and higher

turbocharger efciency. One possible solution is to use the LPL EGR or a

dual-loop EGR system to help EGR driving at peak torque or low speeds

so that a larger turbine area can be selected in order to avoid high pumping

loss at high speeds. However, it should be noted that an excessively large

Diesel-Xin-13.indd 868 5/5/11 12:04:43 PM

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869Diesel engine air system design

turbine area may not be able to deliver the required air–fuel ratio at high

altitude.

13.3.4 Design principles of LPL EGR system

In a clean LPL EGR system (Fig. 13.4) the EGR ow is driven by the

pressure difference between the EGR pickup location (usually downstream

of the DPF) and the compressor inlet. EGR cooling is needed in order to

prevent the compressor outlet gas temperature from exceeding the allowable

limit. The EGR circuit ow restriction needs to be minimized because any

articial increase in the EGR-driving pressure differential beyond the ‘naturally

available free pressure differential’ provided by the system would lead to a

large increase in pumping loss. Such an articial increase in the LPL EGR

circuit pressure differential includes closing an EBP valve installed after the

DPF or closing an intake throttle valve at the compressor inlet. The latter

option usually incurs a larger pumping loss. Equation 13.1 shows that raising

the exhaust restriction (p

4

) or reducing the intake restriction (p

1

) in order to

drive EGR in LPL EGR actually cause a large increase in engine delta P.

A LPL EGR system may be promising under the following four

conditions:

1. There is a sufciently large ‘naturally available free pressure differential’

(i.e., free of extra pumping loss penalty) to drive the EGR ow. Some

examples include the high local vacuum created by the suction near the

compressor wheel at the compressor inlet, or the high local pressure

created by a diffuser at the EGR pickup location in the exhaust pipe.

2. The turbine area can be sized large enough to reduce the engine delta P

while still delivering sufciently high air–fuel ratio at peak torque and

high altitude.

3. The engine operating points can be shifted from the low efciency

region to the higher efciency region on the turbocharger map (e.g.,

Fig. 13.5).

4. The power consumption of any EGR pump can be more than offset by

the BSFC benet resulting from a lower engine delta P in the LPL EGR

system.

It should be noted that the LPL EGR system has a higher turbine ow

rate than the HPL EGR system. Therefore, the LPL EGR system demands

a larger aftertreatment device in order to maintain an acceptable level of

exhaust restriction. The LPL EGR system also requires a larger compressor

size (due to increased compressor ow rate), a higher compressor outlet

temperature, a higher heat rejection of the charge air cooler, a longer EGR

piping route, and worse transient response due to longer EGR purging time

and larger turbocharger inertia.

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870 Diesel engine system design

13.3.5 Comparison between HPL and LPL EGR systems

At high speeds and high loads with a high EGR rate, HPL EGR is usually

superior to LPL EGR due to its lower turbine power and pumping loss. At

low speeds or low loads or a low EGR rate, LPL EGR may exhibit certain

advantages if its compressor is better matched at a higher efciency. Hybrid

EGR combines the advantages of both HPL and LPL and provides the lowest

pumping loss but at the price of increased design complexity and cost. The

need for a hybrid EGR system rises in two scenarios. First, if the turbocharger

efciency is low and a small turbine area has to be used to compensate in

order to achieve a required boost pressure, any resulting engine delta P then

becomes a free pressure differential which can be utilized to drive EGR. In

this case, HPL EGR needs to be used, and the balance of the EGR rate can

be supplied by a LPL EGR system. Secondly, exibly switching between

the LPL and HPL EGR operations at different engine speeds/loads may be

able to keep the engine operating points in the high efciency region on the

compressor map by regulating the compressor mass ow rate.

Figure 13.6 shows a simulation comparison between HPL and LPL EGR at

rated power for a heavy-duty diesel engine. In the LPL EGR system analyzed,

a large articial increase in the LPL EGR circuit pressure differential is

created by using an EBP valve to drive EGR. Therefore, there is a large

BSFC penalty in the LPL EGR system. This demonstrates that the LPL EGR

system, if not designed correctly, may incur BSFC penalty. Also note that

turbocharger efciency has a large impact on the comparison between the

EGR systems.

13.5 Regulate engine operating points on compressor map by

controlling EGR.

HPL EGR

LPL EGR Dual-loop EGR

80%

60% 60% 60%

HPL

rated power

LPL

rated power

HPL

rated power

HPL

peak torque

LPL

peak torque

LPL

peak torque

m˙

comp,corr

m˙

comp,corr

m˙

comp,corr

p

2

p

1

p

2

p

1

p

2

p

1

Diesel-Xin-13.indd 870 5/5/11 12:04:44 PM

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871Diesel engine air system design

13.4 Turbocharger conﬁgurations and matching

13.4.1 Turbocharger conﬁgurations

Turbocharger fundamentals

In general, exhaust turbocharging (SAE J922, 1995) is superior to mechanical

supercharging in terms of thermodynamic efciency. Turbocharging with

intercooling may increase intake air density, engine power, and fuel economy,

while reducing combustion noise and emissions. Automotive turbocharger

design has evolved from the early xed geometry turbocharger to today’s

electronically controlled wastegated turbocharger and VGT with high turbine

efciency of around 80%. The turbine ow passage is designed to cause the

exhaust gas to impinge on the turbine blades at an optimum angle and velocity.

The compressor diffuser is designed to maximize the total pressure at the

compressor outlet. The efciency of centrifugal compressors is dependent

on the design of the blade and the diffuser.

On the compressor map, there is a surge limit curve on the upper left

side and a choke limit curve on the right side. The limit curves represent

the boundaries between stable and unstable operations. Surge is a self-

sustaining but unstable oscillation in ow rate and pressure ratio initiated

by a ow reversal within the boundary layers on the compressor blades.

Audible coughing and vibration occur when surge happens. Compressor

choke happens when the air velocity reaches the speed of sound at the inlet

HPL EGR, turbocharger efﬁciency 51%

HPL LGR, turbocharger efﬁciency 47%

LPL EGR, turbocharger efﬁciency 51%

LPL EGR, turbocharger efﬁciency 47%

Using EBP valve

At rated power, air–fuel ratio ﬁxed at 21,

EGR cooler effectiveness ﬁxed at 77%,

LPL-EGR charge air cooler is 40% bigger

than HPL-EGR charge air cooler

Not using

EBP valve

Gradually close

EBP valve

0 5 10 15 20 25 30

EGR rate (%)

50%

45%

40%

35%

30%

25%

20%

15%

10%

5%

0%

Percentage of BSFC increase from baseline

13.6 Performance interaction between EGR system and

turbocharging.

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Xin Q. Diesel Engine System Design

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