Xin Q. Diesel Engine System Design

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Table 2.1 System performance parameters as durability design constraints

Transmission and

drivetrain

Cylinder head

Piston assembly

Cylinder liner

Valvetrain (e.g.,

valves, cam)

Connecting rod

and crankshaft

Bearings

Crankcase

Intake manifold

Exhaust manifold

Turbocharger

EGR system

(valve and cooler)

Charge air cooler

Fuel injector

Aftertreatment

(e.g., DPF)

Engine brake torque x

Mean piston speed x x x x

Peak cylinder pressure x x x x x

Peak cylinder temperature and

cylinder heat ﬂux

x x x x

Exhaust manifold temperature x x x

Exhaust manifold pressure x x x

Compressor outlet air

temperature

Compressor pressure ratio x

Turbocharger speed x

Heat rejection and engine

outlet coolant temperature

x x

Cooler outlet gas temperature x x x

Piston slap kinetic energy x

Valvetrain loading x x

Engine-out soot loading x x

Oil soot percentage x x x x

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117Durability and reliability in diesel engine system design

A typical example is the peak cylinder pressure that is determined by the

required power density, emissions level and optimum engine compression

ratio. Another example is the need to increase the design limit for the

maximum allowable exhaust manifold pressure for achieving high air–fuel

ratio in order to reduce soot at rated power. A third example is that the EGR

cooler outlet gas temperature cannot be designed too low due to concerns

about corrosion and hydrocarbon fouling. The system engineer needs to

understand the reliability consequences of the proposed limits.

Secondly, many loads are related to each other inherently via engine

thermodynamic processes and therefore require system-level coordination.

For example, when the air–fuel ratio is increased, the peak cylinder pressure

increases but the exhaust manifold temperature may decrease. Another

example on a system-level balance between different durability constraints

dated back to the 1970s when Zinner (1971) compared the design solutions

of increasing BMEP and mean piston speed in order to increase the diesel

engine power for a given engine displacement. For automotive, industrial

and marine applications, the increase in continuous power output is always

desirable. The engine components are subject to increased mechanical and

thermal loads at higher power. Based on a simplied analysis, Zinner (1971)

concluded that increasing BMEP is a simpler, cheaper and more reliable

approach than increasing the mean piston speed to meet the higher power

requirement.

Thirdly, the overall design and sizing of many subsystems need to be

conducted and coordinated at a system level in order to ensure the entire

system is optimized and the loads for each subsystem or component are

well controlled. One example is the cam prole design and the valve spring

load selection. Cam prole affects engine performance. The corresponding

valvetrain load matched for the cam needs to be determined by the system

engineer. Another example is to select the size of the inter-stage cooler

for a two-stage turbocharger in order to control the air temperature at the

high-pressure-stage compressor outlet. The cooler sizing is the result of the

overall coordination of comparing and balancing different design solutions

related to the cooler, the turbocharger and EGR in order to control the air

temperature.

2.2.2 Iterative design of system performance and

durability

There are usually trade-offs among performance, durability, packaging, and

cost, as shown in Fig. 2.1. For example, the engine structure can always be

designed strong enough to sustain a very high cylinder pressure, but the penalty

is heavy weight and high cost. If the maximum allowable system loads can

be specied accurately at the early stage of the design, it will be easier to

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118 Diesel engine system design

close the loop of re-iteration from the system level to the component level

to ensure the loading parameters match well with the component strength

at affordable cost and packaging.

System performance and durability engineers need to ensure the loads

designed at the system level are the reasonable and sustainable design targets

to be cascaded to each subsystem to be realized through detailed component-

level design. A thorough understanding of how engine durability issues

are generated, analyzed, and resolved is required for a system engineer in

order to determine or propose appropriate design limits. Engine durability

validation usually requires prolonged experimental testing at the late stage

of the development program. A detailed component-level nite-element

structural analysis usually requires long computational time to simulate the

failures and estimate the lifetime of the components. In order to consider

structural robustness and reliability at the early engine system design stage,

analytical models need to be developed to determine the appropriate limits

of durability design constraints. A computationally efcient system-level

durability and reliability model is highly desirable in order to estimate the

impact of system loads on engine lifetime. Due to the extreme complexity of

durability issues, the values of durability design constraints (e.g., maximum

allowable cylinder pressure and exhaust temperature) to be used in engine

system design as design limits often have to rely on empirical experience.

2.2.3 The role of system durability engineers

In the iterative design process to achieve performance and durability, a

key issue between the system and component engineers is to determine the

reasonable loading parameters or system design constraints/limits. As shown

in Fig. 2.1, the system engineer selects the point on the curves as a system

Durability life of each

component

Loading 1

Design 1

Design 2

Constraints of loading,

steady state maximum

Design constraints, level of loading,

steady state maximum (peak cylinder

pressure, exhaust manifold pressure

and gas temperature, etc.)

Loading 2

Cost

Packaging

(size,

weight,

etc.)

Loading 1

Performance

Designed for same

durability life

2.1 Durability analysis for engine system design.

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119Durability and reliability in diesel engine system design

design specication to cascade to the component engineer. If the loads are not

reasonable from a durability standpoint, they need to be revised iteratively

with the component engineers during the course of design.

In order to conduct system optimization, the performance engineer needs

design maps of durability as a function of loads or design constraints (Fig.

2.1). The map should include parametric sensitivity variations of key design

and material parameters, preferably for each failure mode and each major

component. Similar demands exist for the attributes of packaging and cost

since the system performance engineer needs the maps from all the attributes in

order to integrate them together. One major requirement for a system engineer

is to generate the durability constraint maps of various components based on

experimental data and numerical simulations, and then assemble a durability

map for the whole engine system through appropriate integration.

Currently, the engine system design in the performance area has reached

a level of highly precise on-target design, under the assumption of pre-

selected durability design constraints. The validity and accuracy of the

design constraints naturally become an issue for further examination. If the

durability design constraints corresponding to satisfactory reliability are

wrongly determined, the precise design capability in the performance area

cannot ensure good design outcome in the durability/reliability area. Such a

mismatch may become the bottleneck for the overall engine system design

from the standpoint of design for reliability.

In the early stage of the design it is unlikely that detailed nite element

analysis or extensive experimental work will be conducted to search for

appropriate maximum allowable design constraints. Fast and effective

heuristic modeling and benchmarking analysis are required to fulll the above

demand. Although very challenging, better predicting engine durability and

reliability is the future direction for improving the quality of diesel engine

system design.

2.3 The relationship between durability and

reliability

As discussed in Chapter 1, the relationship between durability and reliability can

be summarized as follows. Reliability is a probability reecting the possibility

of failure of any quality issue such as durability, performance, packaging, or

manufacturing after the product is put in service. The probability of failure

can be caused by the variations in production population or environmental

changes. Durability is a product attribute reecting structural endurance. If

a device never functions properly, it is a quality issue. If the device works

well once but fails at the second time, it is a reliability issue. The issue can

be related to performance (e.g., device not functioning due to excessive

friction or heat without structural failure) or durability (e.g., device failing

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120 Diesel engine system design

due to rupture or fatigue). Reliability can be assessed by using customer

service data and/or engine development data in the design/validation stage.

Durability development needs to screen and solve most, if not all, of the

problems that would otherwise be encountered in service. A durability issue

sometimes may be addressed with only one prototype in the engine test cell

or may be studied with many samples and environmental noise factors in

terms of probability of failure. When a performance or durability issue in

the design stage is studied by using the statistical probabilistic approach,

it essentially reects the concept of ‘design for reliability’, which is the

ultimate design goal.

Durability study focuses on how to solve the structural problems in the

design stage, while reliability focuses on counting how many problems still

show up in service after the durability development. For example, if the

work is to revise a piston design to prevent premature rupture, it is called

durability work. On the other hand, if the focus is piston failure rate in

production population with different customer usage after one million miles,

it is a reliability topic. This difference is reected by the fact that in the

structural durability area the deterministic approach using safety factors has

been convenient and acceptable (although simplied), while in the reliability

area the probabilistic approach has always been required.

2.4 Engine durability testing

Engine durability testing is the most important development work to validate

the design after the prototype is available. Heavy-duty vehicle endurance

testing was summarized by Murphy (1982). Diesel engine durability testing

was introduced by Goshorn and Krodel (1978) and Alcraft (1984). The

fundamentals of material properties of the automotive engine components

were introduced by Yamagata (2005).

Each engine manufacturer has developed proprietary durability test cycles

based on their experience to validate structural components and the engine

system. A durability test is conducted to let the engine or the component

undergo sufciently high mechanical and thermal loads (stresses) and a

sufcient number of fatigue cycles (e.g., hundreds of hours). Durability

tests are custom made and depend on engine specic requirements. Typical

engine durability tests include full-load test in the lab, over-fueling test, load

cycle tests, eld test in vehicles, etc. Specic cycles include low-temperature

environment, thermal cycling, deep thermal shock, resonance cycle, overload

cycle, high load factor in combination with superimposed thermal cycle,

engine brake cycle, etc. For example, thermal fatigue cycles are run from

no load to full load repeatedly for several hundred hours to validate the

sealing capability of the cylinder head gasket joint and the durability of the

exhaust manifold. The test is conducted by exposing the engine components

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121Durability and reliability in diesel engine system design

under deep thermal gradient conditions. Thermal shock tests are conducted

by quickly changing the rate of temperature change in thermal cycles. Note

that many durability tests for alternative fuels have been run according to

the cycles suggested by the EMA (Engine Manufacturers Association) to

determine the difference in wear and carbon deposits.

Moreover, modern diesel engines are equipped with EGR systems and

many control valves. Those new devices need to be validated with respect to

long-term durability. Some of the tests for those devices need to be conducted

at low speeds and low loads (e.g., soot accumulation test, DPF regeneration

test) instead of the traditional high-speed and high-load tests.

Depending on failure mechanisms, different engine components may

encounter their worst durability conditions at different engine speeds/loads.

The peak stress may not necessarily occur under the steady-state full load

or maximum power condition. For instance, the thermal load may be higher

at peak torque than rated power. Maximum cam stress might occur at the

cranking speed rather than rated speed. The oil control problem may become

worst at low idle and part load. The worst crack due to low cycle fatigue of

the cylinder head may occur after a thermal cycle instead of steady-state full

load. In other words, different driving cycles may produce different durability

issues because the failure mechanism is dependent on engine speed, load,

and cycle history.

It is important to investigate the correlation between the engine dynamometer

test cycles and the real world usage proles in order to avoid over-design or

under-design of the engine. The load cycles used in dynamometer durability

testing need to be representative of real world usage in service. In order to

save testing time, sometimes accelerated durability tests are conducted in the

lab to accelerate the wear and structural failures by intentionally increasing

the engine speed, load or loading cycle frequency.

In-vehicle eld durability tests are conducted under various climate

conditions such as normal ambient temperature, hot humid, hot dry, cold

humid, cold dry, sea level, high altitude, etc. The tests need to represent real

world driving conditions including the worst case scenarios such as highway,

full load and low load. In vehicle eld tests, some important engine parameters

need to be recorded, including vehicle speed, fuel economy, oil consumption,

used oil data (for wear analysis), engine speed and torque, accelerator pedal

position, fuel injection pressure and quantity, exhaust manifold pressure,

turbine outlet pressure, EGR valve duty cycle, turbocharger control duty

cycle, and uid temperatures (fuel, oil, coolant, and air).

Component bench testing, metallurgical analysis and engine oil analysis are

also important in determining design acceptance related to material selection,

wear, and reliability. Regular oil sampling and analysis can prevent major repairs

and catastrophic failures. At the end of the engine durability test, acceptance

is determined based on experience and design margins are validated.

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122 Diesel engine system design

It is always important to correlate the following three sets of data in order

to predict reliability: the predicted durability life, the predicted reliability, and

the collected reliability service data. Durability assessment and prediction

rely heavily on experimental endurance tests, either in engine laboratory with

specially designed accelerated testing or on vehicle in the eld. A durability

target of B10 or B50 life can be derived by using a Weibull diagram which

depicts the failure rate (usually on the vertical axis of the Weibull chart) as a

function of product life, number of running cycles or vehicle distance traveled

(on the horizontal axis of the Weibull chart). The failure characteristics of

the components (e.g., early failure, random failure) can be evaluated based

on the beta slope of the Weibull curve.

2.5 Accelerated durability and reliability testing

Accelerated durability or reliability testing is a fast growing area because it

enables the designer to determine the durability or reliability of a product

more quickly. The goal of controlled accelerated laboratory testing is to

achieve the results that are representative of real world usage at a lower testing

cost and reduced testing time. By exposing a design to a combined set of

amplied stresses, multiple failure modes and their sequence and distribution

may be obtained in a short time. The same failures which typically show

up in service over time at much lower stress levels show up quickly in the

short term overstress condition in the lab. However, it should be noted that

generally the greater the acceleration, the less realistic the test result.

Engine durability accelerated testing is very complex due to multiple types

of loading and multiple failure modes involved. It is difcult to decompose

them to simpler cases or cycles. Appropriate acceleration factors need to be

determined to use in the test. Engine durability development often employs

accelerated testing by using specially designed durability cycles and load

factors. For the need of engine system design, it is desirable to nd out the

correlation between the accelerated testing and the real world usage scenarios

for critical system design constraints. For example, if an accelerated durability

test concludes that 200 bar peak cylinder pressure needs to be used as the

design constraint, the question to conrm is the level of condence that the

200 bar is representative in the non-accelerated real world. It is desirable to

possess a certain level of understanding on how accelerated tests are designed

and how the results are interpreted.

For the fundamental theories on accelerated testing, the reader is referred

to Nelson (2004), Dodson and Schwab (2006), Escobar and Meeker (2006),

Klyatis and Klyatis (2006), and Klyatis (2010). More detailed information

on accelerated testing in engine or automotive applications was provided

by Indig and Williams (1984), Goshorn and Krodel (1978), Krivoy et al.

(1986), Dystrup et al. (1993), Vertin et al. (1993), Davis and Christ (1996),

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123Durability and reliability in diesel engine system design

Kestly et al. (2000), Niewczas and Koszalka (2002), Feng and Chen (2004),

Evans et al. (2005), and Franke et al. (2007).

2.6 Engine component structural design and

analysis

The objective of engine component structural design and analysis is to achieve

light and durable design by the optimization of shape and the selection of

material with low manufacturing cost. General design guidelines to ensure

satisfactory thermo-mechanical structural durability include the following:

increase structural stiffness, avoid high gradients in stiffness and temperature,

avoid high strains, and reduce temperatures by cooling. General guidelines for

good wear durability include reducing the load and promoting hydrodynamic

lubrication.

Engine structural analysis usually includes the following areas:

∑ Static analysis of deection, plasticity, stress, and strain

∑ Thermo-mechanical fatigue

∑ Wear

∑ Cavitation

∑ Fatigue life analysis

∑ Multi-body dynamic vibration and modal analysis

∑ Transient structural analysis

∑ Fluid-structure interaction.

Finite element analysis (FEA) (Haddock, 1984) coupled with advanced

constitutive material models is the routine analysis tool used for structural

design. It can handle complex component geometry and identify local stress

concentration. Safety factors have been widely used in structural design. Safety

factors are calculated at every node in the FEA model based on the stress

calculated so that a spatial distribution of the safety factor can be obtained

in order to view the most critical location at a given loading condition.

The capability of today’s structural analysis in component design has

reached an advanced level so that it can predict reasonably well the critical

failure area and the fatigue life. The trend of analysis is to integrate individual

components and individual failure modes to achieve a full system validation,

and also to include a probabilistic approach for estimating the structural

reliability.

2.7 System durability analysis in engine system

design

With the system loads used as input, durability analysis in system design

prior to the detailed component-level design serves three purposes: to provide

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124 Diesel engine system design

preliminary structural sizing and strength evaluation on stress and strain for

major engine components; to estimate durability/reliability life; and to help

the performance area determine the durability design constraints/limits. A

detailed component-level design analysis is certainly too time-consuming

hence not realistic at the early stage of the system-level design. The system

durability calculation normally does not require a full sophisticated nite

element analysis. Therefore, the detailed information of stress concentration

due to complex geometry is generally ignored. The analysis has two types

of approaches: the deterministic approach with the concept of safety factor,

and the probabilistic approach with the concept of reliability or failure rate.

The hand-calculation formulae used in concept-level structural evaluation

for engine components with the deterministic approach were summarized

by Makartchouk (2002). The structural analysis with the more advanced

probabilistic approach was introduced by Kececioglu (2003).

For the preliminary structural calculation of the connecting rod, crankshaft,

bearing and valvetrain, the structural loads can be calculated by using engine

cycle simulation and multi-body dynamics of piston assembly, crankshaft,

or valvetrain to obtain the cylinder gas pressure load, the component inertia

load, and the vibration load. In the thermo-mechanical fatigue problems

for the piston, the cylinder head, and the exhaust manifold, the mechanical

and thermal loads are calculated by using engine cycle simulation. The

boundary conditions of gas temperature and heat transfer coefcient can also

be calculated. A simple FEA can be conducted with the cycle simulation

software (e.g., GT-POWER) to obtain the metal temperature distribution in

the components. The fatigue damage models are detailed in the later sections.

For the cylinder liner cavitation problem, piston-assembly dynamics can be

used to calculate the piston slap kinetic energy (detailed in Chapter 11).

The wear at the piston ring, the cam and the valve seat can also be modeled

(also detailed in later sections). System durability modeling requires the

development of fast and simplied surrogate durability models to replace

the full FEA models.

Structural analysis usually starts with a deterministic model to analyze a

particular pair of stress–strength relationship and the safety factor. However,

it is not sufcient to only consider a nominal case (i.e., a combination of

mean material properties, average usage parameters, and pre-selected safety

factors). The engineer must also consider the nondeterministic probability

distribution of all the noise factors such as load changes, potential variations

in material properties, different customer usages and manufacturing

tolerances.

From a probabilistic standpoint structural failures can be illustrated by

the commonly used stress–strength probability distribution curve or the

stress–strength interference model (Fig. 1.13 in Chapter 1). Here the ‘stress’

refers to any type of load in general, and ‘strength’ refers to the maximum

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125Durability and reliability in diesel engine system design

structural capability of the component in general. Failure occurs in the over-

stressed area where the stress is higher than the strength. Mechanical failures

(e.g., fracture or fatigue) are caused by overload or strength degradation.

Overload can be caused by misuse or insufcient design limit adopted.

The strength degradation is caused by material property degradation, high-

temperature creep, fatigue, corrosion, or chemical attack. The probabilistic

analysis will be detailed in the later sections related to reliability.

2.8 Fundamentals of thermo-mechanical failures

2.8.1 Overview of thermo-mechanical structural

concepts

System durability analysis is an important part of diesel engine system

design. The failure mechanisms and the concepts in the area of structural

durability analysis are very complex. Most system engineers coming from

the engine performance area are not familiar with the structural analysis

concepts. Compared to other mechanical failure mechanisms such as wear

and cavitation, the theory of thermo-mechanical failures has a paramount

importance for engine system durability because two critical performance

parameters are directly related to them: peak cylinder gas pressure and

exhaust manifold gas temperature.

This section lays a foundation for system durability analysis by summarizing

the key concepts and the available analysis approaches related to thermo-

mechanical failures. Detailed explanation of the fundamentals can be found

in numerous textbooks (e.g., Draper, 2008) and will not be covered here.

The three most basic concepts are stress, strain and strength. They are

important because stress and strain are used differently in different failure

mechanisms as the control criterion. Strength is related to the structural

design limit selected. The next important concept is damage. In the damage

model the failures due to different mechanisms or types of loading cycles

can be combined in order to assess the reliability life. Thermo-mechanical

failure modes are introduced in this section, mainly including fracture,

fatigue and creep. Fatigue is elaborated, focusing on thermal fatigue, high

cycle fatigue (HCF), and low cycle fatigue (LCF). In-depth discussions

on structural durability analysis are presented in Rie and Portella (1998),

Nicholas (2006), and Yu (2002). For details of thermo-mechanical structural

analysis, the reader is referred to a series of research work conducted by

LMS International (Nagode and Zingsheim, 2004; Nagode and Hack, 2004;

Nagode and Fajdiga, 2006, 2007; Nagode et al., 2008, 2009, 2009a, Rosa

et al., 2007; and Seruga et al., 2009). SAE procedures also provide useful

information on durability and reliability (SAE J450, J965, J1099, J2816,

JA1000-1, and JA1000).

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