Xin Q. Diesel Engine System Design

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36 Diesel engine system design

the design specications wide enough in tolerance (i.e., not over-constrained)

so that the manufacturing process variation has little or no impact on the

system/product performance.

The causes of failure modes may come from system design, component

design, material selection, prototype build, etc. The failure modes can be

classied into two categories: noise related and non-noise related. Each

category is handled by a specic robust engineering tool. The functional block

diagram and interface matrix for system FMEA development can be used

to identify the non-noise-related failure modes. The purpose of a functional

block diagram (Fig. 1.10) is to identify all the inputs and outputs of the

functional blocks as well as their interfaces in the forms of physical or force

connection, energy transfer, material exchange, and information or signal

exchange. The system interface matrix (Fig. 1.11) quanties the interfaces

in terms of strength and importance and their potential effects. It is a useful

tool for managing interfaces and the potential causes of failure resulting from

subsystem interactions in diesel engine system design. Interactions present

if the behavior of one subsystem depends on that of another subsystem.

The parameter diagram (i.e., P-Diagram, Fig. 1.12) is used to identify

the noise-related failure modes. The P-Diagram identies intended inputs

and outputs, noise factors, control factors, and error states. Noise factors are

unintended interfaces or sources of disturbing inuences that may cause a

deviation, disruption, or failure of the function during the mission time of

the component or the engine. They are the causes of failure modes. Noise

factors are uncontrollable (i.e., impossible, impractical or expensive to control)

in product design or in-use operation. Generally, noise factors include the

following ve sources:

∑ Piece-to-piece or unit-to-unit variation (e.g., manufacturing variability

in component geometry or material properties, variations in product-

controlled parameters such as engine compression ratio, valve timing,

turbocharger variance, fuel injection timing, injector variance or drift,

tolerances in the controllable variables).

∑ Internal environment noise (also called system interaction noise or

proximity noise, i.e., the unwanted effect of one subsystem on another, or

subsystem interactions due to the variation of the input from neighboring

subsystems or from the in-vehicle system operating environment, for

example, variability in sensor signals, variance and drift of the air ow

sensor, variability in the exhaust gas temperature or engine-out emissions

composition).

∑ External environment noise (e.g., ambient temperature, humidity, altitude,

road surface condition).

∑ Customer usage (e.g., accidental or foreseeable misuse and abuse of the

product, real world usage duty cycle or load, different types of fuel or

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Gas ﬂow

Fuel ﬂow

Coolant ﬂow

Power

Control signal

Fuel system

Engine controls

and calibration

Power cylinder &

piston assembly

Waste heat

recovery system

Accessories

Crankshaft

Hybrid powertrain

Cylinder head

Valvetrain

Intake manifold

Exhaust manifold

Aftertreatment

Turbocharger

EGR system

Engine brake Vehicle drivetrain

Cooling system

(Optional)

1.10 Robustness tool

– functional block

diagram of diesel

engine system.

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Diesel engine system

Effects (to)

Vehicle

drive-

train

Hybrid

power-

train

Hybrid

power-

train

Power

cylinder

& piston

Power

cylinder

& piston

Waste

heat

recovery

Waste

heat

recovery

Cylinder

head

Cylinder

head

Valve-

train

Valve-

train

Connect-

ing rod

Connect-

ing rod

Crank-

shaft

Crank-

shaft

Turbo-

charger

Turbo-

charger

EGR

system

EGR

system

Fuel

system

Fuel

system

Cooling

system

Cooling

system

Engine

brake

Engine

brake

Acce-

ssories

Acce-

ssories

Engine

controls

Engine

controls

After-

treat-

ment

After-

treat-

ment

Engine

cali-

bration

Engine

cali-

bration

Intake

manifold

Intake

manifold

Exhaust

manifold

Exhaust

manifold

Vehicle

drive-

train

Causes (from)

System FMEA interface matrix

P: Physical connection

E: Energy transfer

M: Material exchange

I: Information exchange

2 = Necessary for function (i.e., required)

1 = Beneﬁcial but not absolutely necessary for functionality (i.e., desired)

0 = Does not affect functionality (i.e., indifferent)

–1 = Causes negative effects but does not prevent functionality (i.e., undesired)

–2 = Must be prevented to achieve functionality (i.e., detrimental)

1.11 Robustness tool – interface matrix and subsystem interaction of diesel engine system.

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39The analytical design process and diesel engine system design

1. Piece-to-piece

variation (e.g.,

tolerance)

2. Internal noise

due to neighboring

subsystems

3. external

environmental noise

(e.g., climate, road)

4. Customer usage

proﬁle and duty cycle

5. Changes over time

Iterate

Noise factors

(uncontrollable

due to variations)

Robust design process:

• Analyze the sources of noise and magnitudes

• Analyze the effects of noise factors or their

combinations on error states (strong/weak/no

interaction)

• Assess the robustness of the design

• Generate noise factor management strategy

• Parameter design, tolerance design, design for

variability

• Reliability conrmation test

Robust design (a link)

Error states

(failure modes

entered in FMEA)

Input

Control factors

Engine system or

subsystem

Output response or

quality characteristic

(ideal or intended

function)

1.12 Robustness tool – parameter diagram (P-Diagram) and robust

design process.

lubricant or coolant used in an engine, accumulation of soot in the DPF

and the associated increase in the back pressure).

∑ Changes over time or vehicle mileage (i.e., time-dependent deterioration

or aging, e.g., component wear, corrosion, fatigue, component strength

reduction, catalyst’s physical or chemical degradation in conversion

efciency, build-up of impurities in recycled materials such as soot in

engine oil, accumulation of ash in the DPF and the associated increase

in the back pressure, EGR cooler fouling).

In system design or reliability verication, several noise factors may be

combined to form a worse or the worst case noise level so that the number

of tests can be reduced. When noise sources exist the pieces or units that

marginally meet the minimum acceptable functional performance will suffer

from a loss of function that may cause a failure. For example, a borderline

component with maximum fuel injection quantity may fail at an extreme

ambient condition. It should be noted that for a multi-cylinder engine, the

cylinder-to-cylinder variation (for example, variations in EGR distribution,

peak cylinder pressure, temperature, heat ux, exhaust runner pressure and

temperature) is not a noise factor. The cylinder-to-cylinder variation is a

controlled variable which can be improved by design. Usually, the cylinder’s

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40 Diesel engine system design

worst parameter should be selected as the characteristic parameter to represent

the whole engine in probabilistic design. Moreover, the malfunction or

catastrophic failure of a component or system is usually a noise factor. For

example, when the EGR valve malfunctions to fully close at rated power,

all the exhaust gas ows to the turbine and the compressor may over-speed.

Although the turbocharger matching in engine system design should be

targeted for normal operation rather than such a malfunctioning failure mode,

the effect of such a failure mode needs to be checked as part of a reliable

and robust design.

The control factors are the measures that can be changed in design to

affect the mean response of the component or system to reduce variability.

The response refers to the output of the component or system in the form

of force, energy, material, signal, etc. For example, the set point of VGT

vane opening is a control factor, but the tolerance of the VGT opening due

to turbine actuator errors is a noise factor. Changing control factors can

make the system function more robust (i.e., less sensitive to the inuence of

noises). The error states identify the failure modes. The error states reect

the deviation of the intended function, and they are potential failure modes.

There are seven types of failure modes, namely omission of action, excessive

action, incomplete action, erratic action, uneven action, action too slow,

and action too fast. For example, the error states for a diesel aftertreatment

system may include excessive tailpipe emissions, excessive back pressure,

increased BSFC, decreased time between DPF regeneration cycles, etc.

At the end, a robustness checklist can be developed to manage the noise

factors and failure modes. Each failure event is assessed by the frequency of

occurrence of the cause (O), the severity of the effect (S) and the ability of

detection (D, Fig. 1.9). To quantify the risks, each failure mode may have

a risk priority number, calculated as the product of occurrence, severity,

and detection. Design changes are usually required in order to reduce the

occurrence and the severity and consequently the risk.

1.3.3 The concept of performance

Performance is the most important attribute for diesel engine systems. It

represents the functionality of the engine. As shown in Fig. 1.8, performance

is a major composition of the product quality. Engine performance includes

the following six categories of characteristics:

1. displacement, velocity and acceleration (e.g., engine transient response

of speed, vehicle acceleration, drivability)

2. force and torque (e.g., engine ring torque, vibration)

3. energy and energy rate (e.g., engine power, coolant heat rejection, exhaust

gas exergy)

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41The analytical design process and diesel engine system design

4. uid ow (e.g., the pressure, temperature and ow rate of ambient air

ow, engine gas ows, fuel ow and coolant ow, as well as the ow

rate and concentration of emissions species)

5. noise (e.g., combustion noise, exhaust noise, piston slap noise)

6. electronic control signals.

It should be noted that the derived parameters by using the above parameters

usually also belong to the performance attribute, for example, fuel economy,

or BSFC (dened as the ratio of fuel ow rate to engine power). Both quality

and reliability of the engine are measured by performance, durability, and

packaging. For example, if the engine has a problem of power loss in service,

it is a reliability problem in performance, rather than a quality problem due

to durability.

Specic engine performance parameters can usually be found from engine

program functional objectives, for example:

∑ ambient operating conditions (e.g., altitude range, temperature range,

humidity range)

∑ engine speed range (e.g., maximum rated speed, maximum governed

speed, high idle speed, maximum over-speed)

∑ engine power and torque output and speed (usually dened by full-load

torque curve)

∑ engine friction and parasitic power losses (e.g., motoring friction power,

power consumption of FEAD accessories)

∑ transient driving cycle emissions (e.g., HC, CO, NO

or NMHC + NO

NMHC, particulate matter, formaldehyde, smoke during acceleration or

at full load)

∑ white smoke level

∑ exhaust odor

∑ fuel economy (e.g., BSFC at rated power, peak torque and typical part-

load modes)

∑ NVH (e.g., full-load noise, no-load noise, engine idle noise, turbocharger

noise, 1/3 octave band center frequencies, maximum displacement of

the powerplant mount, engine balance, resonances)

∑ engine startability (e.g., crank-to-start time at different cold ambient

temperatures, hot restart time, glow plug’s wait-to-start time, the

minimum cranking speed for unaided cold start, cold cranking torque

requirement)

∑ engine idle quality (e.g., hot curb idle speed and its maximum variation,

misre)

∑ fuel system (e.g., sulfur level, specications of the diesel fuel or the

alternative fuel, maximum fuel ow delivery capacity, fuel pressure,

maximum fuel pump resistance, maximum return fuel temperature)

∑ air induction system (e.g., intake restriction, maximum air ow rate,

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42 Diesel engine system design

maximum rise-over-ambient temperature at the compressor inlet, maximum

compressor outlet air temperature, charge air cooler restriction)

∑ exhaust system (e.g., maximum exhaust manifold gas temperature,

maximum EGR flow rate, maximum exhaust flow rate, exhaust

restriction)

∑ aftertreatment system (e.g., engine torque variations during DPF

regeneration and LNT lean-rich modulations)

∑ cooling system (e.g., coolant type, heat rejections, water pump ow,

maximum engine inlet and outlet coolant temperatures, maximum engine

oil temperature, thermostat control set points)

∑ cab heater (e.g., heater core ow).

In the functional objectives the following are also stipulated but they are

not performance attributes:

∑ engine durability life (e.g., B10 life in terms of the number of miles

traveled)

∑ maintenance intervals (e.g., oil and lter change intervals, DPF service

interval, valve lash adjustment; these are durability attributes)

∑ engine and aftertreatment weight (these are packaging attributes)

∑ engine architecture (e.g., displacement, cylinder conguration, bore,

stroke, bore spacing, ring order, rotation direction, compression ratio,

mean piston speed, package size in terms of length, width and height,

congurations of valvetrain, combustion system and turbocharging).

It is observed from the above that the engine functional requirements consist

primarily of performance parameters. Therefore, the core of this book is

focused on engine performance in system design.

1.3.4 The concept of durability

The denition of durability

Engine durability or endurance is the other major attribute that affects the

quality and reliability of the engine. It is usually confused with reliability

in the literature. According to the denition given in the APQP (advanced

product quality planning) manual developed jointly by Chrysler Corporation,

Ford Motor Company and General Motors Corporation, durability is ‘the

probability that an item will continue to function at customer expectation

levels, at the useful life without requiring overhaul or rebuild due to wear-out’.

The APQP manual also dened reliability as ‘the probability that an item

will continue to function at customer expectation levels at a measurement

point, under specied environmental and duty cycle conditions’. Another

denition was given by O’Connor (2002), ‘durability is a particular aspect

of reliability, related to the ability of an item to withstand the effects of time

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43The analytical design process and diesel engine system design

(or of distance travelled, operating cycles, etc.) dependent mechanisms such

as fatigue, wear, corrosion, electrical parameter change, etc. Durability is

usually expressed as a minimum time before the occurrence of wear-out’.

The above denitions of durability are somewhat ambiguous. Engine

durability is better dened as the probability related to the hardware

structural ability of an item to withstand the effects of time-dependent or

non-time-dependent thermal, mechanical or chemical mechanisms such as

fracture, fatigue, wear, corrosion, creep, deformation, fouling, plugging,

electrical parameter change, etc. The time-dependent in the above refers to

the accumulated service time. The non-time-dependent refers to the situations

that are irrelevant to the accumulated time such as an abrupt rupture caused

by over-load.

Durability is a part of quality before the engine product is released to

the customer. It evolves to a part of reliability after the product is in use. In

other words, a structural failure occurring in the engine test cell is called a

durability problem rather than a reliability problem; and after the engine is

released in service, a structural failure is called a reliability problem associated

with a durability attribute. Such a denition clearly distinguishes between

durability (as a design attribute) and reliability. The latter is a characteristic

of the ‘overall quality’ extended into the time-in-service domain.

It should be noted that the concept of reliability covers the failures from

all three attributes: performance, durability, and packaging. Therefore, it is

not appropriate to assume equivalency between durability and reliability.

Durability is usually expressed as a minimum time or vehicle mileage before

the occurrence of any major type of structural failures (e.g., wear-out). For

example, a B10 durability life is the expected life (e.g., 20,000 hours or one

million miles) at which 10% of the population fails. A B50 durability life

is the expected life at which 50% of the population fails.

Stress–strength interference model

The concept of structural durability is illustrated in the stress–strength

interference model shown in Fig. 1.13. The gure shows a random probability

distribution diagram of component strength and stress. The stress represents

the load, and the strength represents the component’s structural capability

to resist the load. The stress and strength are random parameters and have

probabilistic distributions corresponding to the variability in noise input

factors. For example, the peak cylinder pressure load acting on the cylinder

head can be regarded as a type of ‘stress’. It has a probabilistic distribution

due to the manufacturing tolerance in engine compression ratio, the variation

in intake manifold boost pressure caused by the tolerance of turbocharger

wastegate controls, the variation of exhaust restriction due to the change

in DPF soot loading, the variation in ambient temperature, etc. The noise

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44 Diesel engine system design

factors affecting the structural strength of the cylinder head may include

the manufacturing tolerances in cylinder head deck waviness, atness and

surface nish, the material properties of the head and gasket, and so on. A

failure occurs when the stress is higher than the strength. The overlapping

area between the stress and strength distribution curves in Fig. 1.13 indicates

the probability of failure. The stress–strength model is elaborated in Chapter

2 for in-depth discussions on durability and reliability.

The role of durability in engine system design

Durability and performance are inter-related. Durability limits are used

as design constraints in engine system design in order to determine the

maximum achievable performance or appropriate hardware sizing. Figure

Stress distribution

Stress and strength

Strength, S(t)

Stress or load, s(t)

Time (t)

Failure occurs

Reliability in time-in-service domain

Strength distribution

Strength after aging

The overlap

area indicates

failure rate

Probability distribution at speciﬁc time t

Time to failure

t = 0

1.13 Stress–strength interference model in durability and reliability.

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45The analytical design process and diesel engine system design

1.13 shows that in order to control the failure rate, either the strength needs

to be increased (i.e., move the strength probability curve to the right or

reduce its distribution range) or the stress has to be reduced. In the example

of peak cylinder pressure, in order to reduce the pressure for better durability,

the control factors such as engine compression ratio, intake manifold boost

pressure or fuel injection timing need to be modied, and this affects engine

performance and emissions. The durability analysis on the stress and strength

distributions helps determine the maximum design limit and the nominal

design/calibration target of design parameters (e.g., peak cylinder pressure

and exhaust manifold gas temperature) that can be used for a durable system

design. These limits or nominal targets can ensure the engine will not be

overloaded and the structural strength is designed sufciently strong.

1.3.5 The concepts of quality, robustness, and quality

loss function

Quality is probably the most commonly known but ambiguous term in

engineering system design. In order to dene the role of quality in diesel

engine system design, we need to review several important concepts in quality

engineering. The engineering method of quality improvement is referred to

as ‘quality engineering’ in Japan (due to the pioneering work by Dr Genichi

Taguchi) and as ‘robust design’ in the West.

Quality has usually been dened as the degree to which performance

meets expectations. The American Society for Quality (ASQ) provides a

denition of quality as: ‘Quality denotes an excellence in goods and services,

especially to the degree they conform to requirements and satisfy customers.’

ISO8402 standard denes quality as ‘the totality of features and characteristics

of a product or service that bear on its ability to satisfy stated or implied

needs’. Quality has been also sometimes loosely dened as conformance to

specications. Note that time dependency is not included in these denitions.

For example, a non-time-dependent inspector’s view of quality may be that

a product is assessed against a specication. The product either passes the

inspection or fails. When the product passes, it is delivered to the customer.

The customer knows that it might fail at some future time and accepts such

a ‘reliability’ risk over time. This approach provides no measure of quality

over a period of time.

Time-based concept of quality was discussed by O’Connor (2002). Also,

as pointed out by Rausand and Hoyland (2004), ‘the quality of a product

is characterized not only by its conformity to specications at the time

supplied to the user, but also by its ability to meet these specications over

its entire lifetime.’ However, to avoid a controversy between the concepts

of reliability and quality, Rausand and Hoyland (2004) again advocated that

‘according to common usage, quality denotes the conformity of the product

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