Xin Q. Diesel Engine System Design

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A design problem in four-dimensional space

Hardware

conﬁguration

Engine speed

and load

At critical speed load modes, under normal and extreme

ambient temperature and altitude, produce parametric

sensitivity design charts or optimization charts, choose

design spec to meet constraints or at optimum trade-off

for all hardware conﬁguration options

Set design constraints/limits (maximum cylinder P, turbo

speed, compressor outlet T, exhaust manifold T, coolant

heat rejection; emissions recipe requirements; BSFC target)

Ambient

temperature

and altitude

Hardware

design spec

Compare BSFC, hardware

cost and complexity, transient

performance at

ﬁxed emissions

Others

(valve,

etc.)

EGR conﬁgurations

Cooling conﬁgurations

Aftertreatment

Turbo conﬁguration

Screen to choose

best conﬁguration

Expand design spec

to entire speed–load

domain at all ambient/

altitude (conduct virtual

calibration)

Repeat the above for all ambient and altitude conditions

At critical speed

load mode 3

(driving part load)

The overall

winning

conﬁguration

At critical speed

load mode 2

(peak torque)

At critical speed

load mode 1

(rated power)

Optimized

design spec

for each

conﬁguration

Hardware conﬁgurations

Hardware design and

calibration specs

Hardware design and

calibration specs

Hardware design and

calibration specs

Hardware conﬁgurations

15.1 Design space and design process of engine system speciﬁcation.

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943Diesel engine system speciﬁcation and subsystem interaction

and cooling. Sensitivity design charts are produced at this stage. At each speed

and load mode, the DoE factors may include some or all of the following:

exhaust restriction, charge air cooler size, EGR cooler size, compressor or

turbine efciency (if turbocharger maps are not used) or efciency multiplier

(if turbocharger maps are used), turbine effective area, turbine wastegate

opening, EGR circuit ow restriction coefcient, certain design factors

affecting engine volumetric efciency (e.g., valve size, valve timing, port

ow discharge coefcient), and start-of-combustion or start-of-injection

timing. The analysis output includes critical instantaneous parameters

such as the in-cylinder details and the gas ow pulsation in the pipes. The

output also includes all the cycle-average parameters describing the entire

engine system performance such as air–fuel ratio, EGR rate, oxygen mass

fraction, BSFC, peak cylinder pressure, peak cylinder bulk gas temperature,

pumping loss, engine delta P, volumetric efciency, manifold pressures and

gas temperatures, heat rejections, cooler effectiveness and ow restriction,

turbocharger ow rate, pressure ratio and temperatures, and turbocharger

efciency (if turbocharger maps are used).

In the concept design phase, technology evaluation is important in

order to choose the right path for the engine product development. Diesel

engine technology reviews and evaluations are provided by Merrion (1994),

Regueiro and Chen (1997), Chen et al. (1997), Khair (1997), Hikosaka

(1997), Schindler (1997), Hountalas (2000), and Conley and Taylor (2002).

Diesel engine research program reviews are summarized by Rickeard et al.

(1996), Eberhardt (1999), Singh et al. (2000), and Thompson et al. (2004).

System-level design and optimization techniques are described by Page and

Edgar (1998), Burtt and James (2004), Berard et al. (2000), and Fussey et

al. (2001).

15.2 Roadmap of fuel economy improvement

In the efforts to improve fuel economy, the role of engine system design

is to lead the way in dening a reasonable roadmap and cascade it to each

subsystem to implement design changes. The roadmap, as shown in Fig.

15.2 (for improving a non-optimized high pumping loss engine), usually

consists of six areas: (1) aftertreatment; (2) combustion and fuel system; (3)

mechanical design of cylinder, valvetrain, cylinder head, and manifolds; (4)

turbocharger, EGR, and waste heat recovery systems; (5) mechanical friction

and parasitic losses of accessories; and (6) vehicle drivetrain matching.

Improvements can be made either through incremental change or by adopting

new technologies. Figure 15.3 shows a simulation analysis on the methods

to improve BSFC by improving the design or calibration parameters in the

air system, cooling system, and the combustion system.

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Aftertreatment

(total: 1.5%)

Reduce EGR circuit

restriction (0.5%)

Turbocompounding

(>2%)

Reduce EGR rate in

engine calibration

or combustion (1%)

Increase

EGR cooler

effectiveness (1%)

Cooling fan power

reduction by using

electric fan (3%)

Reduce valvetrain

friction with

softer valve

springs (0.3%)

Increase turbocharger

efﬁciency to reduce

pumping loss (3%)

Advanced

thermodynamic

cycles (Rankine, or

Brayton, or Sterling)

for waste heat

recovery (6%)

Reduce exhaust

restriction (0.2%)

Reduce DPF

regeneration frequency

and fuel consumption

penalty with more

passive regeneration

and larger ﬁlter

capacity (1%)

Reduce DPF

regeneration NO

penalty (0.3%)

Turbocharger, EGR and waste heat

recovery systems (total 13.5%)

Mechanical friction and

parasitic accessory loss

reduction (total 8%)

Reduce piston

skirt and ring

friction by 10–20%

with optimum ring

proﬁle and tension

design and lower

cylinder

bore distortion (2%)

Power reduction

of water pump,

fuel pump and oil

pump, or smart

cooling (0.7%)

Change to low-

viscosity oil (2%)

Vehicle and drivetrain

matching (total 9%)

Increase drivetrain

efﬁciency, change

tire size, reduce

tire-road rolling

friction coefﬁcient,

reduce vehicle

frontal area,

reduce vehicle

aerodynamic drag

coefﬁcient (1%)

Reduce ROA and

recirculation (1%)

Change transmission

shift map (5.5%)

Vehicle braking

energy recovery

Change rear axle ratio

and transmission gear

ratios (1.5%)

Total vehicle fuel

economy improvement

in typical driving cycle

53%

Notes:

(1) Data in parentheses indicate fuel

economy improvement percentage.

(2) Need to minimize the penalty on

drivability and transient acceleration

Combustion and fuel

injection systems (total: 4%)

Engine-out soot

reduction to reduce

DPF regeneration

frequency (0.5%)

Swirl reduction to

reduce cylinder heat

rejection loss (0.5%)

Optimize calibration

to advance fuel

injection timing (1%)

A/F ratio reduction

to reduce pumping

loss through better

combustion bowl,

fuel injection

nozzle, higher fuel

injection pressure,

better or variable

swirl ratio (2%)

Mechanical design of cylinder,

valvetrain, cylinder head and

manifolds (total 17%)

Reduce displacement (3%)

Cylinder deactivation (8%)

Bigger intake and exhaust valves (1%)

Variable compression

ratio via VVA (2%)

Increase intake port

ﬂow coefﬁcient (1%)

Lower exhaust port

heat rejection (1%)

Less restrictive

exhaust manifold

(0.5%)

Aggressive cam proﬁles

for higher volumetric

efﬁciency (0.5%)

15.2 Example of fuel economy improvement roadmap.

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945Diesel engine system speciﬁcation and subsystem interaction

A/F ratio

Overall two-stage turbocharger efﬁciencyOverall two-stage turbocharger efﬁciency

Overall two-stage turbocharger efﬁciency

HP turbine effective area (cm

)

Overall HP turbocharger efﬁciency

0.49

0.48

0.47

0.46

0.45

0.44

0.43

0.42

0.41

0.4

0.49

0.48

0.47

0.46

0.45

0.44

0.43

0.42

0.41

0.4

0.39

0.5

0.495

0.49

0.485

0.48

0.475

0.47

0.495

0.49

0.485

0.48

0.475

0.47

0.465

0.495

0.49

0.485

0.48

0.475

0.47

0.5

0.495

0.49

0.485

0.48

0.475

0.47

0.495

0.49

0.485

0.48

0.475

0.495

0.49

0.485

0.48

0.475

2.95

2.9

2.85

2.8

2.75

2.7

2.65

2.6

2.55

0.47

0.46

0.45

0.44

0.43

0.42

0.41

0.4

0.52

0.5

0.48

0.46

0.44

0.42

BSFC (lb/(hp.hr))

25 30 35 40

EGR rate (%)

0.4 0.45 0.5 0.55 0.6 0.65 0.7

EGR cooler effectiveness

0 1 2 3 4 5 6

Start-of-combustion timing (degree ATDC)

6.5 7 7.5 8 8.5 9 9.5

LP turbine effective area (cm

)

0.6 0.7 0.8 0.9 1 1.1

Total exhaust restriction (inch Hg)

0.18 0.2 0.22 0.24 0.26

Overall LP turbocharger efﬁciency

Simulation at variable

emissions

Simulation at

ﬁxed emissions

Simulation at ﬁxed

emissions

Simulation at

ﬁxed emissions

Simulation at

ﬁxed emissions

Simulation at

ﬁxed emissions

15.3 Summary on the impact on BSFC at typical part-load operation

(2000 rpm 30% load).

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946 Diesel engine system design

15.3 Critical mode design at various ambient

conditions

15.3.1 System design constraints

In engine system specication design there are two categories of limits,

usually occurring at full load: the hardware design limit and a lower limit

used for the calibration test setting. The necessity for the two limits is based

on the statistical distribution of hardware variations. The ‘nominal’ engine

design or calibration set point (e.g., intake manifold pressure or exhaust

manifold gas temperature) under the standard normal ambient condition

should be lower than the ‘calibration limit’ by a certain margin. The full-load

conditions of the hardware in the system specication need to be designed

below the ‘design limit’ in order to cover the worst ambient conditions in

emissions certication and real-world driving. Different design limits may

occur at different ambient conditions or engine speeds, for example (also

illustrated in Section 3.4.5):

∑ In order to be compliant with the US EPA NTE emission requirements,

the EGR cooler and the charge air cooler need to be sized for sea-level

altitude and hot ambient (e.g., 38°C or 100°F) to reach a target intake

manifold gas temperature.

∑ The compressor inter-stage cooler in a two-stage turbocharger needs

to be sized with a sufcient cooling capacity and an appropriate cooler

sink temperature (i.e., the cooling medium temperature) to ensure proper

operation of the high-pressure-stage compressor under high ambient

temperatures at sea level or high altitude conditions. Specically, the

outlet air temperature of the compressor needs to be below the material

limits of the compressor wheel, the housing, and the charge air cooler

inlet tube.

∑ The engine coolant heat rejection and the associated EGR strategy need

to be designed to ensure that at high altitude and hot ambient (e.g., 38°C)

the engine outlet or radiator inlet coolant temperature does not exceed

the durability limit. High altitude refers to, for example, 1676 meters

or 5500 feet – the US EPA NTE limit, or 10,000 feet in practical real-

world applications.

∑ The turbocharger control and the EGR strategy need to be tailored for

high altitude and hot ambient conditions to ensure that soot emission is

still below the smoke limit with a lower air–fuel ratio. Additionally, the

exhaust manifold gas temperature should be within the limit imposed

by the durability of the cylinder head and the turbine.

∑ The compressor needs to be sized large enough to ensure the turbocharger

speed is still below the maximum limit at high altitude without fueling

derating.

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947Diesel engine system speciﬁcation and subsystem interaction

∑ The maximum cylinder gas pressure and temperature as well as the peak

heat ux need to be designed below the combined limits of mechanical

and thermal stresses for the power cylinder components.

∑ The turbocharger and EGR controls need to be designed to ensure that

the peak cylinder pressure along the full-load lug curve does not exceed

the structural limit in a cold climate. Moreover, any compressor surge

at cold ambient should be avoided. The full-load lug curve refers to US

SET modes A100, B100, C100, peak torque and rated power.

∑ During the ring and engine braking operations in both steady state and

transient, the exhaust manifold pressure should be below the structural

limit. The engine delta P should be kept below the point where the

exhaust valve oats off the valve seat.

∑ For high EGR engines in extremely humid climates, the intake manifold

gas temperature needs to be designed not to exceed the durability limits

of condensation control and corrosion resistance.

∑ All the hardware design constraints need to be satised during the

maximum engine braking operation.

15.3.2 Design from extreme ambient to standard lab

condition

The ambient conditions used in hardware sizing usually include the

following:

1. Sea-level altitude and normal ambient temperature (e.g., 25°C or

77°F).

2. The US EPA’s heavy-duty emissions NTE limits (sea-level altitude,

from cold ambient to 38°C or 100°F hot ambient).

3. The 1676 meters or 5500 feet altitude, from cold ambient to 30°C or

86°F, with a humidity range.

4. The real-world driving environments (e.g., 5500 feet on a hot day, a

very high altitude level such as 8000–10,000 feet on a hot day, and a

cold ambient condition at sea level, for example –18°C or 0°F).

It is also necessary to consider the following factors: hot air recirculation,

turning on air-conditioning, and the rise-over-ambient elevated compressor

inlet air temperature due to in-vehicle underhood heating.

Figure 15.4 shows an example of the system design sequence from

the extreme ambient and in-vehicle conditions to the standard lab engine

condition. The EGR rate required at the hot ambient or high altitude is

affected by the emissions requirement, intake air temperature, air–fuel ratio,

and oxygen concentration. The difference in the required EGR rate between

the sea level and the extreme ambient condition depends on emissions

regulation level, particular engine design, and the fuel system capability.

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948 Diesel engine system design

The achievable EGR rate and air–fuel ratio at the same altitude but different

ambient temperatures are determined by the engine control strategy of the

air system. For example, the VGT vane opening at sea level hot ambient

can be controlled by exhaust manifold pressure or intake manifold pressure.

In the ‘lambda control’, air–fuel ratio is maintained at a constant level at a

given altitude and is controlled by regulating the EGR valve opening.

Figure 15.5 shows typical behavior of engine performance at different

ambient conditions at the full-load lug curve. Note that the exhaust manifold

gas temperature and compressor outlet air temperature are higher at the high

altitude and hot ambient condition than the sea level normal ambient condition.

The engine delta P can also be higher if the EGR is reduced and the turbine

wastegate is closed in order to increase the air–fuel ratio at high altitude.

The higher BSFC at the extreme ambient conditions is caused mainly by a

reduction in the indicated power in the compression and expansion strokes

due to the decrease in air–fuel ratio and secondarily affected by the increase

in engine delta P or pumping loss.

15.3.3 Design for varying exhaust ﬂow rate

In addition to the variations in ambient pressure and temperature conditions,

hardware matching is also complicated by the large variations in the EGR

ow and the exhaust restriction in the aftertreatment system. For instance,

15.4 Illustration of engine system design from extreme ambient to

standard lab condition.

Compressor pressure ratio

Compressor map

Surge line (not to scale)

Change wastegate and

EGR valve opening

5500 ft, 86°F ambient,

15°F ROA Emissions

recipe to meet uS EPA

NTE: A/F = 19, EGR =

27%, IMT = 135°F

Sea level, 77°F

ambient, 0°F

ROA. Recipe to

meet uS EPA

FTP:

A/F = 20,

EGR = 32%,

IMT = 130°F

Sea level, 100°F

ambient, 15°F

ROA. Recipe to

meet NTE: A/F =

20, EGR = 29%,

IMT = 150°F

‘lambda control’

(change wastegate

and EGR valve

opening)

Compressor mass ﬂow rate

Rated power

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Heavy-duty diesel engine system design at full load under different ambient temperature and altitude conditions

1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1000 1200 1400 1600 1800 2000

Engine speed (rpm)

1000 1200 1400 1600 1800 2000

Engine speed (rpm)

BSFC (lb/(hp.hr))

Engine delta P (back minus

boost, mbar)

HP compressor outlet

temperature (°F)

Exhaust manifold gas

temperature (°F)

0.40

0.39

0.38

0.37

0.36

0.35

0.34

0.33

0.32

0.31

0.30

1500

1000

500

450

400

350

300

250

1300

1250

1200

1150

1100

1050

1000

950

900

Sea level, 77°F

ambient, zero

rise-over-ambient

Sea level, 100°F

ambient, 15°F

rise-over-ambient

5500 ft altitude,

100°F ambient,

15°F rise-over-

ambient

Sea level, 77°F

ambient, zero

rise-over-ambient

Sea level, 100°F

ambient, 15°F

rise-over-ambient

5500 ft altitude,

100°F ambient,

15°F rise-over-

ambient

Sea level, 77°F

ambient, zero

rise-over-ambient

Sea level, 100°F

ambient, 15°F

rise-over-ambient

5500 ft altitude,

100°F ambient,

15°F rise-over-

ambient

Sea level, 77°F

ambient, zero

rise-over-ambient

Sea level, 100°F

ambient, 15°F

rise-over-ambient

5500 ft altitude,

100°F ambient,

15°F rise-over-

ambient

15.5 Engine system design at standard lab condition and extreme ambient conditions.

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950 Diesel engine system design

EGR rate control may vary from zero in very cold conditions or during fast

transients to very high at the normal ambient. Moreover, large uctuations in

the exhaust restriction may occur due to soot loading changes in the DPF. The

compressor size needs to be large enough to cover all these variations.

15.3.4 Critical mode design – rated power

When designing the maximum capacity of the hardware and comparing

different conguration options in heavy-duty applications, critical speed

and load modes are selected such as rated power, peak torque, and typical

part-load driving conditions identied by the vehicle matching analysis. In

light-duty applications, the rated power and peak torque conditions are rarely

encountered in real-world driving, and usually very little EGR is applied

under the full-load conditions for light-duty emissions certication. In light-

or medium-duty applications, two practical and frequently used operating

modes are: high speed and high load; and low speed and high load. They

both have high EGR demand and need to be added to the critical mode

analysis to size the hardware.

It should be emphasized that an important design target is to minimize

the engine pumping loss and fuel consumption for both steady-state and

transient operations in the proper speed–load region. The minimum pumping

loss is realized by a low engine delta P design in the EGR and turbocharger

systems, and an equally important high volumetric efciency design in the

valvetrain, ports, and manifolds. For the durability design targets, over-speed

and over-fueling may be simulated to determine the engine’s maximum

potential capability and determine the safety margins.

Successful design of the rated power condition is important for heavy-duty

engines. All the design constraints mentioned earlier apply to rated power.

Moreover, at high speed, the reciprocating inertia force is another constraint

for structure strength. The commonly used methods of designing a series of

engines with different power ratings or applications include the following: (1)

raising the rated speed; (2) using turbocharging to achieve different levels of

air density and the corresponding power ratings without changing the engine

cylinder bore and stroke; (3) increasing the cylinder bore diameter or stroke

to increase the engine displacement and the power rating; and (4) increasing

or decreasing the number of cylinders to achieve different power ratings.

Other methods of increasing the power rating include enhancing volumetric

efciency by valvetrain or port design, reducing mechanical friction, and

raising the peak cylinder pressure limit. Figure 15.6 shows the impact of

engine displacement and power rating on peak cylinder pressure.

At a given engine speed, there are trade-offs between the air–fuel ratio,

EGR rate, and power rating under a xed constraint of peak cylinder pressure.

Figure 15.7 shows these trade-offs.

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951Diesel engine system speciﬁcation and subsystem interaction

There is also another fundamental trade-off at a given speed/load between

the peak cylinder pressure and the exhaust manifold gas temperature, both

being design constraints. The trade-off is affected by the shape of the heat

release rate, fuel injection timing, and engine friction. The simulation data

in Fig. 15.8 show the measures needed to bring the cylinder pressure and

exhaust temperature under control at rated power (i.e., moving the curves to

the lower left direction). Reducing the engine compression ratio may reduce

the peak cylinder pressure at the expense of lower indicated thermodynamic

efciency and unaided cold start capability.

The rated power condition is used to dene the compressor and turbine

maximum ow ranges, the cooler size, the maximum heat rejection, the

intake restriction, and the exhaust restriction. In high-EGR engines, usually

the engine delta P at rated power has to be very high due to the small turbine

area sized for the required EGR rate and air–fuel ratio at peak torque. As a

consequence, the EGR valve has to be partially closed in order to prevent

excessive EGR. Therefore, the rated power condition should not be used to

dene the minimum ow restriction of the EGR circuit.

Heavy-duty diesel engine simulation at rated power

with bore-to-stroke ratio 0.8 at ﬁxed high EGR rate

Rated power = 475 hp Rated power = 525 hp

Rated power = 575 hp Rated power = 625 hp

11 12 13 14 15 16 17

Engine displacement (liter)

Peak cylinder gas pressure (psi, absolute)

4500

4000

3500

3000

2500

15.6 Effect of engine displacement and power rating on peak cylinder

pressure.

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