Xin Q. Diesel Engine System Design
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86 Diesel engine system design
© Woodhead Publishing Limited, 2011
ows, and instantaneous pressure or ow through valves or in manifolds. When
setting up an engine test, it is often benecial to conduct simulation before
testing to verify whether the test objectives can be met. Testing can conrm
engine system design specications or identify deciencies. Many factors
can render inaccurate system modeling results: model quality, assumptions,
incorrect input data of the component or subsystem, etc. Simulation model
tuning and comparison with experimental data are always necessary. Only
after a successful model validation can the model be used with condence to
produce system design results. The analysis and testing functions should be
partners reinforcing each other. The system design engineer needs to actively
participate in the planning of testing to help dene the objectives and procedures
of the testing in order to discover any problems as early as possible. Test
planning and analysis is a primary responsibility for system engineers.
Diesel engine system design is a new integrated function. The systems
engineering theory (Armstrong, 2002) believes that the system engineers
must release the system from the design departments. Because the majority
efforts of diesel engine ‘system design’ are placed on producing system
performance (or functional) design specications, it is natural to consider
the simulation analysis work conducted by a ‘system analysis’ engineer as
the diesel engine ‘system design’ work instead of ‘system analysis’ work.
In other words, most system engineers are design engineers, and they are
granted design authority for the entire system. They use advanced simulation
software as their design tools. They are not just the simulation or analysis
engineers in a supporting role. As the importance of diesel engine system
design emerges in product development, the job function of engine performance
analysis assumes a dominant role in the engine design process. The system
testing engineers run test cells to validate the whole engine system. Their
work complements the work of system design engineers.
Advanced analytical engine system design process
A work ow process of advanced analytical engine design and development
from concept to production is illustrated in Fig. 1.27. The input and output
of the functional areas are interactive and affect each other, as shown by the
double-headed arrows in the gure. The program management located in the
middle controls the specied program schedule and budget. The efciency
of the engine development process depends on how clearly technical
specications or design/development changes are organized and cascaded
by the system team to the component teams within a given organization.
Although the technical communication channels between different functional
areas are complex, overall there is an optimum ‘top–down’ process as shown
by the thicker block arrows. In the old era when technical specications were
generated with crude hand calculations, precise design and optimization were
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Customer
needs, vehicle
design
requirements,
engine
functional
objectives,
design
constraints
Technology
evaluation and
development:
combustion,
aftertreatment,
EPSI
Engine system concept
analysis (EPSI, structural,
design layout, cost)
Drivability Cost
Noise
Program
lead
NVH testing
Production design release
Manufacturing tooling development
Preproduction builds and production release
Mechanical
durability testing
Vehicle integration
testing
NO
x
Packaging
Fuel
economy
Design
criteria
Durability
PM
Mechanical design
Design analysis (EPSI,
CFD, structural, NVH)
Engine control
design
Combustion, performance,
aftertreatment, emissions
dyno testing and calibration
1.27 Simultaneous
engineering processes.
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88 Diesel engine system design
© Woodhead Publishing Limited, 2011
impossible. Today, with the effective use of computer simulations of engine
thermodynamic cycle performance, it is feasible to accurately predict air
system performance and subsystem interactions. Such a function of analytical
system integration needs to be placed at the very top of the development
chain. A common objective in engine development is to continue improving
the upstream analysis capability and minimizing the costs in the prototype
testing stages, as pointed out by Hoag (2006) in his detailed description of
the engine development path. Figure 1.28 shows a more detailed process of
analytical engine system design. Figure 1.29 illustrates the technical scope
and examples of diesel engine system design.
Figure 1.30 illustrates the process of automotive powertrain denition,
which links the engine system design to its upper level, the powertrain system
design. Planning is the rst step in a typical engine program. The vehicle-
level requirements are cascaded down to generate the design targets at the
engine level. After the engine system design specications are determined,
the design targets are then broken down further to the individual component
level. A wide range of criteria must be considered, including performance
(acceleration, fuel economy), emissions, cost, weight, packaging, and
reliability. Often there are trade-offs between different criteria, for example,
between 0–60 mph acceleration and fuel economy at different engine
displacement. A larger swept volume (displacement) usually gives better
naturally aspirated breathing capability so that the vehicle acceleration is
faster. But during the driving cycle the larger engine runs more frequently
at a lower BMEP level, hence the fuel consumption becomes worse. In this
‘top-down’ process, changing program targets or system design specications
will disrupt the development process and require extra modications outside
the agreed scope of modication freedom. Therefore, it is essential to dene
the program targets carefully by foreseeing future requirements and produce
engine system design specications accurately so that the extra modications
can be avoided as much as possible. Figure 1.31 shows an example of a
powertrain design decision tree.
1.7 Engine system design specifications
1.7.1 Overview of engine design specifications
Design documents include schematic diagrams, functional block diagrams,
computer simulation results, reports, drawings or graphics, bill of materials,
traceability matrices, engineering specications, material specications, etc.
Engineering design specication refers to a description of the subject – what
it does, how it works, and how it is built. Engine design specications include
a complete set of information about how the engine is designed, operated,
maintained, and repaired. Technical publications include operations manual,
maintenance manual, service manual, diagnostics manual, etc.
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Output &
data flow
Output &
data flow
Output &
data flow
Output &
data flow
Each subsystem
(Air and turbo system, EGR system, fuel system, combustion system, cooling system, aftertreatment
system, cylinder head, valvetrain, power cylinder, engine brake, etc.)
Output &
data flow
Output &
data flow
Output &
data flow
Output &
data flow
Output &
data flow
Subsystem
design,
simulation
and testing
Engine
system
design
Advanced
combustion
and
aftertreatment
Input
Input
Aftertreatment
concept
selection
Combustion system
design with simulation
(combustion chamber)
Fuel injection characteristics, fuel–air–
combustion systems matching and
hardware screening/searching test
Advanced combustion
concept evaluation
and development
Input Input Input
Output
EGR
system
Exhaust
restriction
Heat
rejection
Turbo
matching
EBP, intake
throttle
Cyl. head,
manifolds
Valvetrain
Engine
brake
Input Input Input Input Input Input
Iterate with supplier designs
and testing results
Iterate with combustion recipes,
supplier designs and testing results
Output: Emissions target & recipe (A/F ratio, EGR rate, IMT, HRR, intake air swirl), combustion system design)
Emerging technologies
Output
Test data analysis,
engine system modeling,
vehicle modeling
Vehicle performance:
• Engine lug curve shape
• Drivetrain conguration
& transmission
matching
• Vehicle driving cycle
simulation and design
points selection
• Fuel economy
• Transient and controls
Aftertreatment analysis:
• Design & performance
• Regeneration controls
(soot, ash, frequency,
impact on engine/vehi.)
• Thermal management
• Calibration simulation
Engine system design and integration
optimization analysis (steady state and
transient; baseline & new; vehicle in-use)
• Hardware conguration (incl. competitive)
• Performance design specication
• Entire engine speed and load domain
• Ambient temperatures and high altitude
Output: optimized engine system configuration
and system performance specification:
• Steady-state design specication
• Steady-state virtual calibration
• Steady-state vehicle in-use simulation
• Transient engine spec simulation
• Transient vehicle in-use simulation
Fuel economy & aftertreatment analyses
1.28 Analytical diesel
engine system design
process.
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© Woodhead Publishing Limited, 2011
1.29 Illustration of diesel engine system design.
300 hp rating 350 hp rating
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91The analytical design process and diesel engine system design
© Woodhead Publishing Limited, 2011
Define vehicle applications
and driving cycles based on
marketing and vehicle design
Define vehicle performance
targets
• maximum vehicle speed
• maximum gradeability
• 0-60 mph acceleration
• overtaking acceleration
• driving cycle fuel economy
Analyze required engine firing and braking
performance characteristics and technology
• specic power (hp/liter)
• engine torque curve
• rated power and rated speed
• peak torque and its speed
Analyze vehicle system performance with
driving cycle simulation:
• drivetrain conguration and parameters
• engine-transmission matching
• performance, fuel economy and emissions
Define drivetrain technology,
packaging & cost
• hybrid or conventional
• transmission and efciency
• drive axles
• tire size, etc.
Define target vehicle attributes
• vehicle weight
• frontal area
• aerodynamic drag
• tire-road friction coefcient
Define required engine displacement
and key performance parameters (engine
weight, cost and performance assumptions)
Modify
powertrain
targets and
iterate
Revise
engine
assumptions
and
iterate
No
No
Yes
Finish
Modify engine
design and iterate
Performance
& fuel economy targets
achieved? Assumptions
satisfactory?
Generate engine system design
specifications
Define engine technology, packaging & cost
• engine-out emissions target;
aftertreatment
• naturally aspirated or turbocharged
• cylinder arrangement, bore, stroke
• four-valve-head or two-valve-head engine
• overhead cam or pushrod valvetrain
• cylinder deactivation, etc.
1.30 Powertrain system definition process.
System functional specication refers to a statement describing completely
and concisely all the functions of the system to fulll its operational
requirements. The specication, as a denition of the system, should include
mission, concept of operation, conguration, system interfaces, functions,
system targets, requirements, hardware/software performance characteristics,
and a history of document revision and methodology used. Design specications
ensure technical disciplines in the processes and coordination between different
functional areas. The system design specications cascade from the system
level down to the subsystem and component levels, and they evolve during
the course of engine development. Engine design process is a process to
generate, implement and validate the design specications continuously and
iteratively (Fig. 1.32).
The system specications include identication and description of all
functions to be provided, along with the associated quantitative requirements
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Conventional
• Vehicle weight
• Frontal area
• Aerodynamic drag
• Tire-road friction
• Tire size
• Drive axles
• Transmission
• Power take-off
Hybrid
Vehicle
powertrain
Engine displacement
Design
variants
in each
subsystem
Design
variants
in each
subsystem
• Engine displacement
• Electric motor
• Battery
• Vehicle weight
• Frontal area
• Aerodynamic drag
• Tire-road friction
• Tire size
• Drive axles
• Transmission
• Power take-off
• Aftertreatment
• Fuel system
• Combustion system
• Cylinder head
• Power cylinder
• Turbocharger
• EGR system
• Valvetrain
• Intake manifold
• exhaust manifold
• Crankshaft
• Cooling system
• Engine brake
• Accessories
• Electronic controls
• Aftertreatment
• Fuel system
• Combustion system
• Cylinder head
• Power cylinder
• Turbocharger
• EGR system
• Valvetrain
• Intake manifold
• Exhaust manifold
• Crankshaft
• Cooling system
• Engine brake
• Accessories
• Electronic controls
Design
variants
in each
subsystem
Design
variants
in each
subsystem
Design
variants
in each
subsystem
Design
variants
in each
subsystem
1.31 Powertrain design decision tree.
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93The analytical design process and diesel engine system design
© Woodhead Publishing Limited, 2011
Requirements
Design
specifications
Manufacturing
specifications
Product
Performance
specification
Durability
specification
Packaging
specification
Cost
specification
System design
specifications
Subsystem
hardware design
specifications
Subsystem
software design
specifications
1.32 Design specifications.
to be met by each subsystem (Kossiakoff and Sweet, 2003). Another
classical denition of general system specications was given by Armstrong
(2002):
System specications state the technical and functional requirements for
a system, allocates requirements to functional areas, documents design
constraints, and denes the interfaces between and among the functional
areas and other systems. This specication will also identify necessary
performance requirements and test provisions necessary to ensure that
all requirements are achieved. The requirements for use of any existing
equipment will also be identied.
The fact is that there is no commonality or consistency with regard to
deliverables or standard design documents (including system specications)
for engine products within the industry or even within the same organization.
This raises the demand for a common process with identied metrics or
deliverables.
Diesel engine development programs usually start with a document of
functional objectives that describes the system requirements. The functional
objectives need to be maintained up-to-date and followed by all the parties
throughout the program. The functional objectives include the basic denitions
of the engine architecture (e.g., displacement, Vee or inline conguration,
valvetrain type), performance targets (e.g., rated power, fuel economy,
vehicle acceleration, emissions, noise), overall packaging geometry and
weight, durability target (e.g., B10 life, peak cylinder pressure) and overall
cost target of the engine.
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94 Diesel engine system design
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The necessity for the system design specications can be understood by the
fact that different groups working in various phases of the program need to be
coordinated by a detailed and clearly dened design document as the design
target. In diesel engine system design, the functional objectives are translated
into more detailed measurable specications for design implementation and
testing validation. For example, the functional requirements of engine power,
fuel economy and emissions can be translated to a set of parameters of engine
gas ow rate, pressure, temperature and heat rejection at different engine speed
and load. Those parameters will be used by each subsystem for hardware
sizing. The requirements of transient emissions and vehicle acceleration can be
translated to the required functions of engine controllers and control software.
The requirements of hybrid powertrain fuel economy and emissions can be
translated to functional block diagrams of supervisory control strategies. The
‘translation’ is conducted by simulation analysis or testing.
A design specication usually consists of a nominal target and an allowable
tolerance range (i.e., an upper limit and a lower limit). They ensure control
factors are designed properly with respect to their means and standard
deviations at the presence of noise factors. One example is the EGR rate at
a given engine speed and load mode. A nominal target of EGR rate needs
to be achieved in order to control NO
x
emissions. The EGR rate can vary
due to the noises in EGR valve opening and the pressure differential across
the EGR circuit. But the variation range needs to be controlled within a
certain tolerance range. Diesel engine system design specications can be
classied into four areas: performance, packaging, durability, and cost. They
are introduced in the following sections.
1.7.2 System performance specifications
Introduction of system specications for hardware and software
The design specication for diesel engine system performance is expressed
in terms of both performance parameters and hardware (or calibration)
parameters. The examples of performance parameters include the following:
engine speed, power, fuel ow rate, air ow rate, BSFC, air–fuel ratio, EGR
rate, intake manifold pressure and temperature, exhaust manifold pressure
and temperature, engine delta P, volumetric efciency, intake and exhaust
restrictions, and heat rejections. The hardware or calibration parameters are
used to achieve the performance. Examples of key hardware parameters at
the engine system level include engine displacement, compression ratio,
engine valve size and cam timing (affecting volumetric efciency), turbine
area (affecting air–fuel ratio and EGR rate), EGR cooler ow restriction
and effectiveness (affecting intake manifold gas temperature), aftertreatment
pressure drop (affecting exhaust restriction), etc. The calibration parameters
refer to the adjustable ones with electronic controls, and examples include
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95The analytical design process and diesel engine system design
© Woodhead Publishing Limited, 2011
VGT vane opening, EGR valve opening, fuel injection timing, etc. Examples
of engine concept layout analysis have been provided by Mikulec et al.
(1998) and Delprete et al. (2009).
Output of engine system design
The engine system design analysis generates three types of output. They
are:
1. performance (or attribute) sensitivity data and optimization results
2. system performance (or attribute) design specications
3. root cause analysis of particular problems.
Specically, the system specication refers to a predicted list of all critical
steady-state and transient engine performance and emissions parameters for
a given concept conguration in the entire engine speed and load domain,
and it is especially required at critical modes such as rated power, peak
torque and driving part load at various ambient temperatures and altitudes.
The specication also denes the data for turbocharging, EGR circuit design,
engine heat rejection and electronic controls to be used in each subsystem
design by suppliers and customers. The specication needs to cover both
target (for on-target nominal design) and limits/range (for variability/
reliability design).
The performance sensitivity and optimization refer to any steady-state and
transient simulation data of parameter sweeping or design-of-experiments
(DoE) optimization for comparing conguration concepts or justifying design
specications of hardware sizing. For example, the sensitivity study may be
conducted for the maximum achievable rated power, or the optimum turbine
nozzle areas in two-stage turbocharging, or transient vehicle acceleration
simulation as a function of time with different vehicle weights. The specication
should develop from optimization.
The engine system specification needs to cover the following five
aspects:
1. Steady-state performance design specication.
2. Steady-state virtual calibration.
3. Steady-state vehicle in-use simulation (e.g., variations in charge air
cooling, exhaust restriction, radiator performance and underhood thermal
conditions compared to engine test cell conditions).
4. Transient specication simulation.
5. Transient vehicle in-use simulation.
The above-mentioned ‘root cause’ analysis refers to a simulation on any particular
issue or failure. For example, insufcient EGR ow at engine peak torque is
caused by inadequate engine delta P due to an excessively large turbine nozzle
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