Xin Q. Diesel Engine System Design
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© Woodhead Publishing Limited, 2011
Overall process of engine development program
Overall process of diesel engine system design
Iterate and repeat the processes till successful validation
Engineering
program
management
(work plans,
budget,
financial
control, etc.)
Plan & set
up attribute-
driven
systems
engineering
processes
Product
definition
System
FMEA
Emissions
certification
Production
sign-offs
Analysis and simulation
(virtual engine)
Design (robust, BOM, models,
drawings, prototype build,
manuals)
Testing, validation and reliability
(performance, emissions, NVH,
drivability, durability, reliability)
Advanced
technology
and concept
design
Competitive
analysis
for system
design
targets
Advanced
concept
analysis
Reliability-
based robust
design to set
targets and
tolerances
for engine
system
design
Diesel engine
system
specification
design
Follow
through the
systems
engineering
processes
1.35 Overall process of diesel engine system design.
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© Woodhead Publishing Limited, 2011
∑ Performance models for virtual sensors
∑ Evaluation of engine control strategies and algorithms.
1.8.4 Work processes of diesel engine system design
Engine system performance analysis is characterized by large-scale complex
data handling across a wide range of technical specialties. The following
are ve key factors for successfully implementing system design analysis
in the engine product development process:
BSFC
Heat rejection
or intake
manifold
temperature…
Analytical system design:
Simulate and optimize
engine configuration and
design specifications to
meet emissions with
minimum BSFC in the
‘system capability’ domain
(A/F ratio vs. EGR rate).
A/F ratio
Turbine size
decreases
EGR circuit
restriction
reduces
Peak
torque
Rated
power
Intake throttle
closes
EGR rate
Step 1: Understand the current engine’s performance
• List all design constraints
• Analyze performance test data and subsystem characteristics
• Analyze emissions test data
• Calculate heat release rate
• Build well-tuned simulation model
• Identify transient problems by analyzing test data
Step 2: Check sensitivity and define new engine system
specification
• Dene engine performance functional targets
• Explore parametric performance sensitivity by simulation
• Generate system design specication and obtain consensus
• Issue specication to subsystem teams and suppliers to start
design
• Simulate to check proposed design hardware and controls
• Analyze test data of engine hardware and calibration
• Revise functional targets and iterate system specication analysis
PCP 2900 psi
3500 psi
Wastegate
closes
Different
cooling system
configurations,
or else
EGR
valve
opens
1.36 Tasks in diesel engine system design.
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108 Diesel engine system design
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1. Effectively communicate technical data across different functional areas
to ensure timely information sharing.
2. Establish analysis task categories and corresponding standard analysis
procedures to ensure quality and consistency.
3. Develop advanced data processing templates to maximize efciency.
4. Provide effective training for system design engineers.
5. Conduct organized research in all system design areas to develop advanced
analysis methodologies.
The essence of diesel engine system design is to reveal the fundamental
parametric dependency among various design and attribute parameters in
order to derive an optimized system solution. Figures 1.35–1.36 summarize
the processes and tasks involved in executing system specication design.
1.9 References and bibliography
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113
2
Durability and reliability in diesel engine
system design
Abstract: Consideration of durability and reliability is necessary at the
earliest stage of system design. This chapter presents the theory and analysis
methods of durability and reliability in diesel engine system design. It begins
by describing engine durability issues, followed by an elaboration on the
relationship between performance and durability through the discussions on
system-level loading and durability design constraints. It then provides a
systematic introduction on the fundamentals of thermo-mechanical failures
and the applications on diesel engine cylinder head, exhaust manifold,
valvetrain, piston, turbocharger and aftertreatment devices, followed by
discussions on cylinder liner cavitation, engine wear, and EGR cooler
durability. An integrated analysis approach on system durability–reliability is
nally summarized.
Key words: durability, reliability, thermo-mechanical failures, damage,
fatigue, wear, safety factor, probabilistic design, system durability–reliability
optimization model.
2.1 Engine durability issues
Engine durability is very important since customers demand longer service
intervals and engine lifetime. Increasing demands on higher engine power
density, lower emissions, and longer lifetime impose great challenges on
durability for modern diesel engines due to increased mechanical and thermal
loading. Engine structural durability issues are important constraints in system
design for performance. The durability issues can be mainly classied into
the following categories based on failure modes:
∑ fracture (acute rupture and chronic cracking)
∑ buckling
∑ thermo-mechanical failures, mainly including ablation, excessive thermal
deformation, fracture and fatigue at high temperatures, creep, oxidation,
and corrosion
∑ cavitation in cylinder liners, fuel system, cooling system, etc.
∑ wear and lubricant oil degradation at all tribological contacts (piston
assemblies, bearings, valvetrain, fuel injector, etc.)
∑ EGR cooler fouling, boiling, and corrosion
∑ fouling and deposits
∑ hydrogen embrittlement.
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Note that the durability issues related to engine electronics such as
overheating are not covered here. Among the above mechanisms, fracture,
thermo-mechanical failures, wear, and cooler fouling are closely related to
the design constraints or limits used in diesel engine system design, hence
will be explained in detail in later sections. The rest are briey introduced
here for completeness. Buckling is a failure of elastic components in terms
of instability subject to high compressive stresses at the two ends of a long
and slender object such as a pushrod or a long connecting rod. The buckling
stress is less than the ultimate compressive strength. Cavitation is a damaging
phenomenon caused by the high energy explosion from the ruptured vapor
bubbles which are originally formed by the undesirable dynamic pressures
in the uid system. Fouling and deposits may occur in situations such as
injector coking, carbon soot deposits built up in the combustion chamber and
at the valve seat, and deposits in the piston top land resulting in associated
cylinder bore polishing. Hydrogen embrittlement refers to a situation where
the metal material becomes more brittle when the hydrogen generated from
water decomposition enters the metal, for example during the welding process.
The material becomes less capable of resisting shock loading, deformation,
and low cycle fatigue.
There are numerous failure modes for all the engine components. Most of the
failures in modern diesel engines usually occur in the following components:
fuel injection system, turbocharger, EGR system, aftertreatment, cylinder
head, piston assembly, exhaust manifold, and valvetrain. Sometimes failures
may also occur in electronic sensors, lubrication and cooling systems, etc.
Examples of durability failures include: the cracks in the piston, turbocharger,
cylinder head, exhaust manifold, diesel particulate lter, rocker arm fulcrum
plate, and crankcase; valve head and spring breakage; piston ablation and
scufng; bore polishing on the cylinder liner; VGT vane sticking; thermal
distortion of the EGR valve; turbine blade–casing interference due to high
temperature creep; pushrod buckling; injector spool valve and coil failures;
fuel injector coking (soot deposit formation); valve sticking and leaking
caused by carbon deposits on the valve seat and valve stem; leakages at the
gasket, water jacket, turbocharger, fuel line, and oil cooler; liner cavitation
erosion; unusual wear in the piston ring and bearings; fuel dilution in engine
oil; EGR cooler fouling due to carbon soot and unburnt hydrocarbons; EGR
cooler lm boiling; and corrosion in the intake manifold.
Engine durability issues can be either an acute catastrophic failure or a
chronic mild failure. The engine components subject to normal wear need
to be replaced after a scheduled service period. Engine durability life is
usually dened by the engine overhaul point, the life-to-overhaul. When an
engine has excessive wear, oil consumption or blow-by, the engine needs
an overhaul. Scufng, scratches and unusual wear patterns and discoloration
are signs of chronic durability problems.
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2.2 System design of engine performance, loading,
and durability
2.2.1 Engine system-level loading and durability design
constraints
In general, all durability problems are related to four elements: loading,
component structural design, material, and manufacturing. Failures occur
when either the loading is too high or the structural design and material are
not sufciently strong. While the structural design and material strength
are mostly component-level topics, the loading is usually a system-level
parameter. The system design engineers determine the load that is then
cascaded to the component design teams for the selection of a structural
design with the suitable material in order to sustain the load. The load here
refers to any mechanical, thermal or ow-related parameters in a general
sense. For instance, cylinder pressure and valve seating impact force are
mechanical loads; cylinder head heat ux is a thermal load; and engine-out
soot is a ow-related load to the DPF.
The following engine performance parameters in system design are related
to the load and subject to durability design constraints or limits:
∑ engine brake torque
∑ mean piston speed and piston-assembly inertia load
∑ peak cylinder pressure, temperature, and heat ux
∑ exhaust manifold pressure and temperature
∑ compressor outlet air temperature and compressor pressure ratio
∑ turbocharger speed
∑ coolant heat rejection and engine outlet coolant temperature
∑ charge air cooler and EGR cooler outlet gas temperatures
∑ valvetrain load
∑ piston slap kinetic energy
∑ engine-out soot load for lubricant oil and DPF regeneration.
The major design constraints are listed in Table 2.1. Other miscellaneous
components or subsystems that are not directly related to system design loads
are not shown, for example electronics and sensors, other components in
the fuel, cooling and lubrication systems, pumps, seals, breathers, etc. The
engine-out soot ow rate affects the lubricant oil degradation and component
wear. It also affects the regeneration frequency and durability of the DPF.
The exhaust manifold pressure limit refers to both the cycle-average and the
maximum instantaneous pressure pulsation.
The load is a system parameter for three reasons. First of all, most of the
loads acting on different components are essentially produced by the engine
gas-side performance requirements (e.g., gas pressure, temperature, and ow
rate). Therefore, they are determined at the design stage of system sizing.
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