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972 Diesel engine system design
© Woodhead Publishing Limited, 2011
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974 Diesel engine system design
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975Diesel engine system specification and subsystem interaction
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976
16
Concluding remarks and outlook for diesel
engine system design
Abstract: This chapter summarizes the key points of each earlier chapter
and provides an outlook on the critical research topics in diesel engine
system design. It emphasizes the importance of durability prediction, system
reliability analysis, emissions prediction, engine transient modeling, air
system integration, and virtual engine calibration. It is concluded by a vision
of system integration for future diesel powertrain design.
Key words: diesel engine system design (DESD), engine performance and
system integration (EPSI), durability prediction, system reliability, emissions
prediction, transient modeling, air system integration, virtual engine
calibration.
The stringent emissions regulations and ever-increasing customer demands
on power, fuel economy, NVH, and reliability will impose greater challenges
on future diesel technologies. Different aspects of engine design including
direct fuel injection, advanced combustion and air systems, aftertreatment,
electronic controls, and powertrain need to be seamlessly integrated. Fast-
paced engine product development requires a precise emissions-driven system
design approach to integrate different technical areas in order to reduce engine
development time and cost. Being an emerging technical eld, diesel engine
system design (DESD), particularly in the area of engine performance and
system integration (EPSI), will become increasingly important.
This book summarizes the theory and analytical approaches of diesel
engine system design, and elaborates its relationship with other related
technical elds. It also provides a new perspective regarding the traditional
mechanical design areas from the viewpoint of engine system design.
Diesel engine system design requires a systems engineering approach to
coordinate different design attributes and subsystems. Because the primary
application of diesel engine system design is industrial product design,
reliability is the ultimate goal and needs to be accounted for at the system
design stage. This requires the system designer to consider a systems
engineering approach together with a robust engineering and reliability
engineering philosophy to properly handle the variability and reliability
optimization problems. Design for just a nominal target is not sufcient as a
system design solution. Design for variability and design for reliability must
be considered. The four attributes (performance, durability, packaging, and
cost) in system design need to be balanced. These concepts set the logic and
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977Concluding remarks and outlook for diesel engine system design
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development path for diesel engine system design from a systems engineering
perspective, and they are the topics addressed in Chapter 1.
Engine system design is a performance-driven activity subject to durability
constraints. Properly understanding how the durability constraints are derived
in order to maximize their potential is critical for a system engineer. The
system design solutions are often produced right on the edge of the durability
constraints in order to maximize the engine performance capability. The
research on analytical and experimental analysis for durability and reliability
issues should not be overlooked. This is the topic of Chapter 2.
Advanced optimization techniques are used by system engineers in their
daily work to address the complex interactions between engine subsystems.
Single- and multi-objective optimizations, deterministic and nondeterministic
(probabilistic) optimizations (i.e., DoE RSM and Monte Carlo simulation),
and variability/reliability-based optimization are all required. These topics
are discussed in Chapter 3.
The four cornerstones in diesel engine system design are static design,
dynamic design, and the rst and second laws of thermodynamics. Chapter 4
lays out a foundation to explain the parametric relationship between the system
design parameters in the engine in-cylinder cycle process. It also elaborates
the engine gas ow network with the focus on pumping loss, which is the
key for modern high-EGR turbocharged diesel engines. Chapter 4 provides
the system-level mathematical formulae (e.g., the four core equations, 4.40,
4.44, 4.47, and 4.50) that are helpful for understanding the interactions
between subsystems and other topics addressed in the following chapters. The
fundamental theory presented in Chapter 4 can be used to explore the options
of low-pumping-loss engine design and model-based engine controls.
The three central tasks in engine system design are engine–vehicle matching,
engine–aftertreatment matching and engine–turbocharger matching. The
matching is surrounded by many boundary conditions such as the requirements
from combustion, emissions, cooling, structure, etc. Engine–vehicle is the
rst design interface a system engineer needs to consider in a ‘top-down’
approach. Chapter 5 emphasizes this concept by illustrating the parametric
relationship between the vehicle/powertrain level and the engine level. The
vehicle integration theory in Chapter 5 can be used to build a truly ‘top-
down’ optimization design approach targeting real-world driving proles.
As a part of the considerations for vehicle and engine system performance,
engine brake needs to be considered at the engine system design stage. This
topic should not be ignored in engine air system hardware sizing. Chapter 6
summarizes the performance design topics for all the retarders, including vehicle
braking performance, engine brake design and performance optimization,
braking thermodynamic cycles, and the interactions with valvetrain, VVA,
and turbocharger. The technical foundation of engine brake performance is
based on thermodynamic cycle processes and engine valve ow characteristics.
Thoroughly understanding engine brake performance and its cycle simulation
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978 Diesel engine system design
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will provide a valuable learning experience for a system engineer in order
to comprehend the related simulation and design techniques, at least before
the more sophisticated combustion/emissions topics come into the play.
Chapter 7 establishes the system design logic from emissions to the overall
engine system especially the air system. It links the three sequential functions
in an engine development cycle: combustion/emissions development, engine
system design, and engine calibration. Emission strategies, emissions modeling,
combustion system design, combustion modeling, advanced combustion mode
(e.g., HCCI), combustion control, fuel system optimization, and calibration/
control strategies are among the general topics to consider. The calibration
function is the one usually ignored in a poor design practice. In fact, the
unique requirements of engine calibration must be considered in the early
stage of system design by using ‘virtual calibration’ to ensure the engine
system specications generated by the system design team will correctly reect
the engine reality down the road. Chapter 8 addresses engine–aftertreatment
integration and aftertreatment calibration to handle the engine-out emissions
produced by in-cylinder combustion.
The valvetrain is a unique subsystem having the dual characteristics of
air system and mechanical system. It is a part of the air system because it
controls engine gas ows, volumetric efciency and engine delta P. It also
possesses many design issues related to kinematics and dynamics. Chapter
9 starts to bridge the thermodynamic/thermo-uid topics addressed in the
earlier chapters with the mechanical design considerations such as dynamics
and vibration in order to provide system design engineers with a balanced
view on all the attributes. Valvetrain integration generally includes valvetrain
system congurations and performance/design optimization, VVA, cylinder
deactivation, activation systems and parasitic losses, cam design, and valvetrain
dynamics. VVA is very important for diesel engines. The Miller cycle, the
WR (wastegating reduction) or WE (wastegating elimination) intake VVA is
one of the very few measures that can reduce both NO
x
emissions and fuel
consumption. VVA performance is discussed in great detail in Chapter 9.
Engine friction is a typical performance-related topic based on mechanical
design considerations. Friction modeling is not only important for the
accuracy of an engine system design model, but also critical for fuel economy
improvement. Chapter 10 thoroughly elaborates the engine friction theory.
Noise, vibration, and hardness (NVH) (mainly noise), addressed in
Chapter 11, is a very important performance attribute in diesel engine system
design and it simply cannot be ignored. Engine noise is very critical for the
competitiveness of diesel engine products. There is a big potential for the
system design function to help optimize the NVH development process in
order to save cost and improve the NVH of the engine product. Although
very challenging from a modeling perspective, system NVH has the potential
to become one of brightest points in diesel engine system design. NVH and
friction represent the two most important mechanical topics a system engineer
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979Concluding remarks and outlook for diesel engine system design
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needs to understand. Sometimes they are related, for example, piston-assembly
dynamics covers both piston slap noise and piston friction.
Heat rejection is probably the last but one of the most important topics a
system design engineer needs to comprehend before he/she starts to conduct
steady-state system design by using engine cycle simulation tools. Accurate
heat rejection prediction is important for cooling system design. In-cylinder
heat rejection is one of the most challenging research topics in the history of
internal combustion engines. Chapter 12 presents a new view on the way to
handle heat rejection calculations from the system design perspective. The
theory is built based on the engine energy balance by using the rst law
of thermodynamics and a theoretical analysis of engine miscellaneous heat
losses.
Chapter 13 addresses a core area in diesel engine system design: the air
system, including the turbocharger, the manifolds, and the EGR system. It
outlines the design guidelines and illustrates the application of the pumping
loss theory. It points out the causes of engine subsystem interactions by
applying the mathematical theory developed in Chapter 4 to engine air system
design. Comprehensive analysis of the second law of thermodynamics for
modern turbocharged EGR diesel engines is also conducted in Chapter 13
to illustrate this powerful approach in engine system design.
Chapter 14 forms the foundation of dynamic system design which is
based on transient engine performance and electronic controls. The modeling
methodology used by the engine controls community is from a system
perspective due to the obvious needs to control the entire engine system. It
often requires fast computing speeds in the models. The system dynamics
approach is the methodology used by diesel engine system design for dynamic
system design, and it shares a common ground with the engine controls
community. In addition to the evaluation of engine hardware and control
strategies for transient performance, it is foreseeable that analytical controller
design may become an area where engine system design can make signicant
contributions. Chapter 14 also addresses virtual sensors and illustrates that
engine system design may greatly contribute to their model development.
Chapter 15 presents a large number of simulation examples of subsystem
interaction and optimization.
As outlined in each earlier chapter, in order to make simulations more
accurate and the system integration process more effective, key research
topics in diesel engine system design need to be identied not only within
each subsystem itself but also at all the interfaces between vehicle, engine,
aftertreatment, combustion, turbocharger, and EGR system. The key challenges
are summarized below.
1. Analytical durability prediction to rene the system design constraints
(addressed in Chapter 2) needs to be greatly promoted by developing
inter-disciplinary analysis tools.
2. The design for variability and the design for reliability addressed in
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980 Diesel engine system design
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Chapters 1 and 3 with the Monte Carlo simulation need to be widely
applied in engine system design to supplement the design specication
data of using the design-for-target approach.
3. The engine system reliability theory (Chapter 2) and reliability-based
design optimization (Chapter 3) need to be fully explored and applied
to diesel engine system design areas.
4. Design of Experiments (DoE), neural network and the Monte Carlo
simulation are the standard data processing techniques used in system
design (Chapter 3). More advanced multi-objective optimization methods
need to be researched to better account for the trade-offs between
different design attributes for system design decision making.
5. Advanced and fast combustion models of heat release rate need to
be developed for better prediction of emissions, BSFC, and exhaust
temperature in engine cycle simulation models (Chapter 4). The
combustion model can also be used for effective prediction of combustion
noise (Chapter 11).
6. High-delity vehicle powertrain dynamics simulation for the transient
details will be helpful in the conventional and hybrid powertrain
performance modeling (Chapter 5). Transient corrections on the steady-
state engine maps (especially the emissions maps) used in vehicle
driving cycle simulations need to be accurately accounted for because
they largely inuence the prediction accuracy compared to real-world
transient driving events.
7. Innovative engine braking mechanisms (Chapter 6) need to be
continuously explored to (1) fully utilize the potential of compression-
release and braking gas recirculation (BGR) processes subject to the
design constraints of peak cylinder pressure and exhaust manifold gas
temperature; (2) reduce the braking component loading under high
cylinder pressure; and (3) minimize the compression brake noise from
the source of excitation.
8. Effective prediction of the difference between the steady-state emissions
and the transient emissions remains a great challenge. This particular
challenge has become very important for today’s low-emission engines
(Chapter 7).
9. Challenging research also remains in combustion, emissions (Chapter
7), and aftertreatment (Chapter 8) areas to develop the heuristic system
emissions models that are suitable for the needs of engine system design.
10. Complete system optimization of the entire valvetrain design needs to
be promoted to effectively handle the large amount of design parameters
involved in the conventional cam-driven valvetrain (Chapter 9).
11. Innovative system integration solutions via VVA and other air system
control valves need to be further explored, considering the interaction
with turbocharging, in order to reduce pumping loss and achieve superior
fuel economy with minimum system cost (Chapter 9). The importance
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981Concluding remarks and outlook for diesel engine system design
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of volumetric efciency and pumping loss should be emphasized. Their
relationship is not only affected by valve timing and engine delta P
but also complicated by manifold gas wave dynamics (e.g., Fig. 16.1).
Different engine congurations (e.g., I6 vs. V8) may have different
effectiveness of using a particular air system technology (e.g., wastegate
elimination intake-VVA). For instance, it is observed from Fig. 16.1 that
the I6 engine can achieve 26.4% pumping loss reduction due to its more
uniform pumping loops across all cylinders in the p-V diagram, while
the V8 engine can only achieve 13.9% reduction due to its drastically
different, large cylinder-to-cylinder variations in the pumping loops,
which are caused by manifold gas wave dynamics. Diesel engine system
design largely relies on such a type of engine cycle simulations.
12. Real-time capable and accurate engine system friction models with
crank-angle resolution are highly desirable for engine fault diagnostics
and transient performance prediction (Chapter 10).
13. Advanced high-delity (crank-angle-resolution) models need to be
further developed in engine system NVH research for component and
overall engine noise predictions (Chapter 11).
14. In the heat transfer area (Chapter 12), further research is required to
model EGR cooler soot fouling, transient, heat transfer and the exhaust
manifold gas temperature.
15. Innovative air system design solutions will continue to be one of the
hottest topics in diesel engine system design. It includes innovative air/
gas control valves in the engine gas ow network and their interaction
and integration with the turbocharger, the EGR system, and advanced
valvetrains. In order to achieve reliable and cost-effective engine system
designs, it is also important to evaluate the performance and durability
of different turbocharging systems, for example, single-stage vs two-
stage, wastegated vs VGT, and different types of VGT (i.e., VNT and
vaneless VAT) (Chapter 13).
16. A wide application of the second law of thermodynamics in diesel
engine system design will greatly enhance the quality of the work
and provide more profound insight into the problems in the energy
systems in the powertrain. It includes the system availability modeling
for hybrid powertrains (Chapter 5), waste heat recovery (Chapter 12),
air/EGR pumping and throttle losses in the air system (Chapter 13).
17. As to electronic controls (Chapter 14), owing to the need to accurately
simulate the engine load response, vehicle driving cycles and the
aftertreatment regeneration transients, engine system design will require
tremendous efforts in transient powertrain performance modeling
combined with developing model-based controls, despite the great
challenges in transient simulations. Dynamic system design theory will
result in a more advanced approach in analytical nonlinear controller
design for modern diesel engine transient performance (Chapter 14).
Diesel-Xin-16.indd 981 5/5/11 12:09:22 PM
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