Xin Q. Diesel Engine System Design

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882 Diesel engine system design

∑ There should be no turbine choke. In pulse turbocharging, it is necessary

to size the turbine frame large enough to account for the highest

instantaneous ow rate.

∑ Appropriate turbine efciency is desired, being neither too high nor too

low, to deliver the turbine power to match the engine needs of air–fuel

ratio and EGR rate. Occasionally, the turbine or compressor is deliberately

matched in the low-efciency region for the purpose of holding down

the boost pressure and hence the maximum cylinder pressure below the

design limit at high engine speeds, or for the purpose of creating a high

engine delta P with a small turbine area to drive EGR at low speeds.

∑ Some special EGR congurations require the turbocharger efciency to

be as high as possible, and in such circumstances the turbocharger may

become the limiting factor in engine development.

∑ There should be no size mismatch or speed mismatch between the

compressor and the turbine for good aerodynamic performance. Driving

a large amount of EGR at low engine speeds requires a small turbine

area to build up a high engine delta P, and the corrected turbine ow

rate may be much lower than the corrected compressor ow rate. On

the other hand, the requirement of high air ow rate at rated power may

demand a large compressor. Matching a relatively large compressor

with a very small turbine will give poor turbocharger performance due

to their speed mismatch. If their size difference is too large, the turbine

efciency may decrease, and the turbocharger bearings may wear due to

excessive axial loading. Possible counteraction includes balancing trim

size (turbine exducer or compressor inducer ow area) and impeller size

between the turbine and the compressor (Arnold, 2004).

∑ For fast transient performance, the turbine size (or A/R ratio) or the

moment of inertia should be as small as possible in order to reduce

turbocharger lag, for example by using light ceramic turbine wheels.

∑ Pulse turbocharging with proper turbine entry may be considered to

better utilize the exhaust energy to increase the air–fuel ratio at part

load and improve transient acceleration performance.

∑ The turbine power split between the stages in two-stage turbocharging needs

to be optimized for EGR driving, BSFC, compressor outlet temperature

control, and the minimum risk of compressor surge or choke in the two

stages. The transition between the two stages at different engine speeds

needs to be smooth and should not cause a high engine delta P.

∑ Turbine durability life needs to be acceptable. The steel turbine wheel

and iron housing must withstand the exhaust manifold gas temperature.

The temperature limit is governed by the creep and scaling properties

of the turbine wheel and housing materials.

Turbine design is introduced in Moustapha et al. (2003), Okazaki et al.

(1982), Hussain and Bhinder (1984), Spraker (1992), Baines (2002, 2005a),

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883Diesel engine air system design

Rodgers (2003), and Zhang et al. (2007). Turbine losses are discussed in

Spraker (1987) and Keshavarz et al. (2003). Turbine pulsating ow unsteady

performance is addressed in Bhinder and Gulati (1978), Ehrlich et al. (1997),

Gu et al. (2001), Macek et al. (2002), Lam et al. (2002), Winkler et al.

(2005), Capobianco and Marelli (2006), and Macek and Vítek (2008). VGT

aerodynamic performance modeling is discussed in Kessel et al. (1998) and

Luján et al. (2006).

13.4.4 Turbocharger performance at extreme ambient

conditions

The turbocharger performance at high altitude can be explained by the four

‘core equations’ of the air system, 4.40, 4.44, 4.47, and 4.57 (or combined in

equation 13.1). As the air density and air mass ow rate reduce, the turbine

inlet temperature increases due to the lower air–fuel ratio. As ambient

pressure falls, the turbine pressure ratio increases. Therefore, the turbocharger

speed and the compressor pressure ratio increase so that the inlet air density

reduction is partially compensated to alleviate fueling/power derating. At hot

ambient, the air–fuel ratio decreases, and the operating limits are usually

due to smoke and turbine inlet temperature. At cold ambient temperatures,

the maximum cylinder pressure or compressor surge may be the limiting

factors. In summary, large variations of ambient temperature or altitude level

may cause problems in turbocharging such as compressor surge, choke,

over-boosting, compressor over-speeding, excessively high exhaust manifold

gas temperature, low air–fuel ratio, and high smoke. The situation is further

complicated by the EGR strategy used at various ambient conditions. For

example, the compressor may choke when the EGR valve is fully closed at

high altitude. Moreover, turbocharger operation at high altitude affects the

strategy of engine fueling derating.

Turbocharged diesel engine performance at high altitude is analyzed by

Dennis (1971), Wu and McAulay (1973), and Bi et al. (1996). The aerodynamic

performance of centrifugal compressors at high altitude is investigated in

Wiesner (1979) and Strub et al. (1987).

13.5 Exhaust manifold design for turbocharged

engines

Exhaust manifold design is an important part of diesel engine air system

design, along with the designs of valvetrain and EGR system as well as

turbocharging. The following topics need to be considered in the exhaust

manifold design for modern EGR engines: (1) exhaust manifold congurations;

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884 Diesel engine system design

(2) turbine entry (divided or undivided); (3) EGR pickup location and EGR

driving potential. The exhaust manifold congurations include divided and

undivided manifolds, and the manifolds used for pulse turbocharging and

constant-pressure turbocharging. The exhaust manifold design analysis is

usually conducted by using the simulation software of one-dimensional

gas wave dynamics coupled with engine cycle simulation. High exhaust

energy utilization, minimum pumping loss and good part-load and transient

performance are key objectives in exhaust manifold design.

The efciency of the turbocharging system is characterized by

= h

cyl

13.5

where h

cyl

is the efciency of the energy transfer from the cylinder to

the turbine inlet, and h

is the turbocharger efciency, which is equal to

TC,mech

. Usually, at the same engine speed, h

cyl

decreases as the engine

load decreases; and at the same engine power, h

cyl

decreases as the engine

speed decreases. Pressure loss is often a more convenient parameter to use

than h

cyl

in engine design. The following measures can increase the efciency

of pulse energy utilization and h

cyl

1. Use fast exhaust valve opening, large exhaust valve diameter, and

high valve lift. During the exhaust stroke, it is necessary to reduce the

throttling loss of the supersonic ow across the exhaust valve and the

ow restriction or friction in the exhaust manifold in order to reduce

the pumping loss.

2. Use appropriate exhaust runner grouping and turbine entry design.

3. Optimize the cross-sectional area of the exhaust manifold pipe for the

best effect of pressure wave dynamics.

4. Use short exhaust runner and manifold piping without sharp turns or

abrupt changes in the cross-sectional area.

Divided exhaust manifold together with pulse turbocharging has been used

for some non-EGR engines and EGR engines. Design simplications to the

undivided exhaust manifold systems often require evaluation of their impact on

EGR driving and BSFC, for example, comparing a divided exhaust manifold

of an inline six-cylinder engine having three larger exhaust pressure pulses in

the manifold with an undivided manifold having six smaller pressure pulses.

Other examples are to compare the pulse difference caused by divided and

undivided turbine entries, or to compare the optimum EGR pickup location

for different types of exhaust manifolds and turbine entries. The following

aspects need to be considered in the exhaust manifold design.

1. The effect of the exhaust pressure pulses on volumetric efciency and

engine delta P.

2. The effect of the exhaust pressure pulses on transient corrected turbine

ow rate and turbine efciency.

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885Diesel engine air system design

3. The relationship between the pressure pulses and the EGR driving

capability. This topic needs to be analyzed for two scenarios: with and

without a reed valve. For low-EGR engines, at the peak torque condition,

the EGR rate can be so low that the gas pressure pulses at the EGR circuit

inlet (exhaust manifold back pressure) in the crank angle domain cross

over with the intake manifold pressure. In this case, the use of a reed

valve may harvest more EGR ow by eliminating the reverse ows at

the crank angle locations where the back pressure pulse is lower than

the boost pressure. The greater the exhaust pulse, the more effective the

reed valve. However, for high-EGR engines, a greater cycle-average

pressure differential is required to drive the high EGR ow rate so that

the back pressure and boost pressure become far apart from each other

and do not cross over in the crank angle domain. In this case, the reed

valve cannot help to obtain more EGR and hence does not affect the

comparison between the two exhaust manifolds.

4. The margin between the maximum peak of the exhaust pressure pulse

and the allowable design limit. The undivided exhaust manifold, having

smaller (less violent) pressure pulses, can reach the design limit pressure

with a higher cycle-average manifold pressure than the divided manifold,

resulting in a higher air–fuel ratio capability at rated power.

Figure 13.9 illustrates the pulsation characteristics of the exhaust manifold

pressure at different engine speeds and loads for a V8 heavy-duty diesel

engine. Engine exhaust manifold design and analysis are summarized by

Winterbone and Pearson (1999, 2000) and are researched by Tabaczynski

(1982), Capobianco et al. (1993), Bassett et al. (2000), Lakshmikantha and

Kec (2002), Fu et al. (2005), and He et al. (2006). The effect of turbine entry

design is studied by Hribernik (1997). Pulse converters were researched by

Bassett et al. (2000), Yang et al. (2006), and Zhang et al. (2008b)

13.6 The principle of pumping loss control for

turbocharged exhaust gas recirculation (EGR)

engines

As discussed in Chapter 4, pumping loss consists of the contributions from

engine delta P and intake manifold mixture volumetric efciency. Using

simulation data, Figs 13.10 and 13.11 illustrate the design effects on engine

delta P and volumetric efciency, respectively. In the gures, the EGR rate

is kept constant (unless especially mentioned) by regulating the EGR valve

opening. Figure 13.10 shows that the engine delta P is largely affected by

air–fuel ratio when VGT vane opening (turbine area) is regulated. In fact, if

the air–fuel ratio is varied by changing the turbocharger efciency without

changing the turbine area, the engine delta P is basically not affected.

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V8 simulation on the magnitude of exhaust manifold pressure

pulse in the left bank (after cylinder #8) at full load

V8 simulation on full-load lug curve

V8 simulation on the magnitude of exhaust manifold pressure pulse

in the left bank (after cylinder #8) at 2500 rpm, different load (fueling)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

500 1000 1500 2000 2500 3000 3500

Engine speed (rpm)

–90 0 90 180 270 360 450 540 630

Crank angle (degree)

1000 rpm full load 2000 rpm full load

2500 rpm full load 3300 rpm full load

2500 rpm 100% load 2500 rpm 75% load

2500 rpm 50% load 2500 rpm 25% load

Exhaust manifold gas

pressure (bar, absolute)

Engine brake power (hp)

Exhaust pulse increases

Exhaust manifold gas

pressure (bar, absolute)

400

350

300

250

200

150

100

3.5

2.5

1.5

0.5

Exhaust pulse increases

Cylinder-to-cylinder variation

of EGR rate

Magnitude

of exhaust

pressure

pulse

EGR valve opening

area (related to

pressure throttling)

EGR valve

location

(before or after

EGR cooler)

EGR

rate

Engine

delta P

Engine

speed

Engine

load

Exhaust

manifold

volume

Firing

order

13.9 Exhaust manifold pressure pulse and the impact of engine speed and load.

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887Diesel engine air system design

From Fig. 13.11 it is observed that with a given valvetrain and cylinder

head design, the volumetric efciency is strongly affected by engine delta P

because the in-cylinder internal residue fraction can be strongly affected by the

pressure differential between the exhaust port and the intake port, depending

on the valve overlap size. As the residue fraction increases, the volumetric

Baseline

3% higher

efﬁciency turbine

8% larger turbine

area

22% smaller

turbine area

2.2 inch Hg higher

exhaust restriction

5% lower EGR rate

Smaller charge air

cooler

Close VGT vane to

increase A/F ratio

Note: Fuel injection

timing and in-cylinder

heat transfer coefﬁcient

basically do not have an

impact on such a curve.

2500

2000

1500

1000

500

Engine delta P

(back minus boost; mbar)

17 18 19 20 21 22 23 24

A/F ratio

13.10 Design effects on engine delta P at rated power.

Baseline

EGR rate reduced by

half (15K colder IMT)

30K hotter intake port

Smaller EGR cooler

(10K hotter IMT)

8% smaller turbine

area

2mm larger intake

valve diameter

20 deg more advanced

IVC timing

20 deg more retarded

IVC timing

1 mm greater intake

valvetrain lash

Close VGT vane to increase A/F ratio;

residue fraction also increases

91%

90%

89%

88%

87%

86%

85%

Intake manifold mixture volumetric efﬁciency

0 500 1000 1500 2000 2500 3000 3500

Engine delta P (back minus boost, mbar)

13.11 Design effects on engine volumetric efﬁciency at rated power.

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888 Diesel engine system design

efciency decreases. Intake manifold gas temperature also strongly affects

the volumetric efciency. At the same intake manifold mass ow rate, a

hotter charge actually results in a higher value of intake manifold volumetric

efciency. The hotter charge can be produced by a smaller EGR cooler

or a smaller charge air cooler. Therefore, when comparing the volumetric

efciency values of different engines to try to assess the design effects of

the cylinder head, the valvetrain, and the manifolds, it is very important to

align the values at similar or comparable intake manifold gas temperatures

and engine delta P values in order to avoid misleading conclusions.

In order to meet combustion and emissions requirements, there is a precise

design target of air–fuel ratio and EGR rate at each engine speed and load

in steady state. Ideally, the EGR valve should be set fully open all the time

with minimum ow restriction in order to minimize the pumping loss. But

in reality at some speeds and loads the EGR valve has to be set partially

closed for the following reasons: (1) insufcient turbocharger efciency

and the consequent use of a small turbine area and the resulting high engine

delta P; (2) necessary smooth transition to EGR shut-off conditions; (3) long

actuator response time of the EGR valve when the VGT vane is suddenly

closed during the fast transient. If the turbocharger efciency is not high

enough, the exhaust manifold pressure must be made high by using a small

turbine area to produce a sufcient turbine pressure ratio and turbine power

to deliver the target air–fuel ratio. This results in a high engine delta P that

forces the EGR valve to be partially closed (throttled) undesirably. On the

other hand, if the turbo efciency is too high, the exhaust manifold pressure

must be lowered by using a larger turbine area to reduce the turbine power

to prevent over-boosting the air–fuel ratio. In that case, the engine delta P

may become too low and cannot drive sufcient EGR. The root cause of the

inappropriate engine delta P is the fact that turbocharger efciency has its

inherent characteristics in the turbomachinery design. The efciency cannot

match engine needs exibly at every speed and load. Generally, closing

the EGR valve results in a decrease in the EGR rate and an increase in the

air–fuel ratio. Closing the VGT vane or the turbine wastegate may result

in an increase in both the EGR rate and the air–fuel ratio. Increasing the

turbocharger efciency results in an increase in the air–fuel ratio basically

without changing the EGR rate. Closing the exhaust back-pressure valve

or the intake throttle results in a large reduction in the air–fuel ratio and a

small increase in the EGR rate. Figures 13.12 and 13.13 use engine cycle

simulation data to illustrate the ability of exibly controlling the air–fuel

ratio and the EGR rate with different air system options.

When evaluating how many ‘knobs’ are needed in air system design

or how many controls need to be tuned to achieve air and EGR ows, the

mathematical systems formulated in Table 4.1 can be used in any exible

combination to count the number of hardware control parameters to match

with the number of unknowns in the system of equations. In this way, the air

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889Diesel engine air system design

system control does not become over- or under-constrained. Moreover, the

relative effectiveness of each ‘knob’ (control parameter) can be compared

by analyzing the equations in Chapter 4. For example, to reach a given pair

of air–fuel ratio and EGR rate values, with low EGR circuit restriction (e.g.,

EGR valve fully open) and the fully closed turbine wastegate, the required

turbine effective area A

and turbocharger ef ciency h

can be solved as

two unknowns by using the equations in system 10. In other words, two

design ‘knobs’ are needed: A

and h

T/C

. However, the turbocharger ef ciency

normally cannot be adjusted freely in the entire engine speed–load domain

due to the turbocharger design constraints. Nor can the exhaust restriction

due to the aftertreatment be freely adjusted. In that case, system 2 has to

be used to solve for the required turbine area (e.g., VGT vane opening)

and EGR valve opening in order to reach a pair of target air–fuel ratio and

EGR rate values. That is the situation usually encountered by a calibration

engineer during the engine calibration stage once the air system hardware

has been  nalized. It should be noted that closing the EGR valve raises the

engine delta P and results in a high pumping loss.

There are two reasons for using a turbine wastegate: (1) the required

turbine effective area computed with system 10 at a low speed is smaller

than that at a high speed; (2) the fast transient response demands low turbo

inertia, which requires the turbine not to be sized too large so that at high

speed a wastegate may be used to bleed off the exhaust. System 3 can be

used to solve for



or essentially the wastegate opening. In system 13, the

0.386

0.384

0.382

0.380

0.378

Exhaust restriction

pressure drop increases

25 30 35

EGR rate (%)

WG: turbine wastegate

A/F ratio

Change

and WG

Turbo efﬁ ciency

increases

Close VGT

Change

WG and c

d-exh

Further open VGT vane

or open turbine

wastegate

BSFC (lb/(hp.hr))

Exhaust

restriction

pressure drop

decreases,

or close

EGR valve

Block arrow

means

extending

boundary

Line arrow

and Italic

notes

mean

moving the

whole

region of

data to that

direction

13.12 Air system capability to control air–fuel ratio and EGR rate with

different hardware.

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890 Diesel engine system design

turbine area is known, and the turbine wastegate opening and the turbocharger

efciency are solved as unknowns for the target air–fuel ratio and EGR rate

values. If the turbine area, the turbocharger efciency, and the EGR circuit

ow restriction have to be treated as xed known inputs, turbine wastegate

and exhaust back-pressure valve (or intake throttle) need to be used (systems

11 and 12).

Intake throttle is sometimes used to induce more EGR or reduce air–fuel

ratio. It should be noted that if the intake throttle is the only ‘knob’ available

to adjust performance, either the EGR rate or the air–fuel ratio can be set as

a target, but not both. For example, in system 14, at peak torque, the EGR

valve is set fully open, the turbine wastegate is fully closed, and the turbine

nozzle area and the turbocharger efciency are xed known parameters. If

Turbine wastegate fully closed, c

d,EGR

= 0.8

(System 8 is good with wide-range controllability.)

Wastegate closed, EGR valve fully open, c

d,EGR

=0.8;

adjusting intake throttle and VGT vane opening

d,exh

=0.3. System 9 has good controllability.)

Turbine wastegate opening D = 10 mm, c

d,EGR

= 0.8

Wastegate closed, EGR valve fully open, c

d,EGR

=0.8;

adjusting exhaust restriction and intake throttle valve

(System 14 has bad with very narrow

controllability.)

BSFC (lb/(hp.hr))

0.386

0.384

0.382

0.38

0.378

0.387

0.386

0.385

0.384

0.383

0.382

0.381

0.38

0.388

0.386

0.384

0.382

0.38

0.378

0.388

0.386

0.384

0.382

0.38

A/F ratio

25 30 35

EGR rate (%)

25 30 35

EGR rate (%)

25 30 35

EGR rate (%)

25 30 35

EGR rate (%)

VGT vane=1

d,IT

= 0.07

d,exh

=0.2

d,IT

=0.07

d,exh

=0.2

d,exh

=0.6

d,exh

=0.6

d,exh

=0.2

d,IT

=0.4

VGT=0.7

d,exh

=0.6

d,IT

=0.35

13.13 Air system capability and controllability (BSFC contour maps).

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891Diesel engine air system design

the exhaust restriction C

d,exh

is also xed, then the only adjustable ‘knob’

is the intake throttle. Figure 13.14 provides an illustration to explain the

function of intake throttle.

From the above theory, changing any two of the following six parameters

can reach a given pair of values of target air–fuel ratio and EGR rate: (1)

EGR circuit ow restriction; (2) turbine effective cross-sectional area (or VGT

vane opening); (3) turbine wastegate; (4) turbine or compressor efciency;

(5) exhaust restriction or back-pressure valve opening; and (6) intake throttle

opening. However, their effects on engine delta P are different. A large

turbine area or wastegating is required in order to reduce the engine delta

P, meanwhile a less restrictive EGR circuit ow restriction is required in

order to maintain the same EGR rate. If the exhaust restriction cannot be

reduced, higher turbocharger efciency is required in order to maintain the

same air–fuel ratio. If EGR cooling is enhanced, the same emissions can be

achieved with a lower EGR rate, and this may reduce the engine delta P.

The effects of the coordinated air system controls (e.g., VGT and EGR

system, intake throttle) for optimizing emissions and performance are explored

by Boulouchos and Stebler (1998), Hawley et al. (1999), Chi et al. (2002),

Pfeifer et al. (2002), Senoguz et al. (2008), and Adachi et al. (2009).

Engine operation at peak torque

Compressor

Compressor Cylinder

Cylinder

Turbine

0.1 bar (not enough to drive EGR)

0.29 bar (enough to drive EGR, but

air–fuel ratio too low)

0.29 bar (enough to drive EGR,

smaller turbine to meet air–fuel

ratio). If the intake throttle is not

used, turbo efﬁciency must reduce

in order to keep 4.4 bar boost

pressure

Intake

throttle

open

Intake

throttle

closed

Intake

throttle

closed

4.4 bar

4.4 bar 4.5 bar

4.2 bar

4.0 bar 4.29 bar

4.58 bar

4.4 bar

4.69 bar

13.14 Illustration of the function of intake throttle.

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