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1.7 Engine system design specifications .1 Overview of engine design specifications
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 definition, which links the engine system design to its upper level, the powertrain system design. Planning is the first 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 specifications 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 specifications will disrupt the development process and require extra modifications outside the agreed scope of modification freedom. Therefore, it is essential to define the program targets carefully by foreseeing future requirements and produce engine system design specifications accurately so that the extra modifications 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
Output & data flowOutput & data flowOutput & data flowOutput & 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 flowOutput & data flowOutput & data flowOutput & data flowOutput & data flow
Subsystem design, simulation and testing Engine system design
Advanced combustion and aftertreatment InputInput
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 InputInputInput
Output EGR systemExhaust restriction Heat rejectionTurbo matching EBP, intake throttleCyl. head, manifolds ValvetrainEngine brake InputInputInputInputInputInput
Iterate with
supplier designs and testing
results Ite
rat e w ith co mb ust io n r eci pes
, results and testing supplier designs
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 configuration & 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 configuration (incl. competitive) • Performance design specification • Entire engine speed and load domain • Ambient temperatures and high altitude Output: optimized engine system configuration and system performance specification: • Steady-state design specification • 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.
1.29 Illustration of diesel engine system design.
300 hp rating350
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
• specific 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 configuration and parameters
• engine-transmission matching
• performance, fuel economy and emissions Define drivetrain technology,
packaging & cost
• hybrid or conventional
• transmission and efficiency
• drive axles
• tire size, etc.
Define target vehicle attributes
• vehicle weight
• frontal area
• aerodynamic drag
• tire-road friction coefficient
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 specification refers to a statement describing completely and concisely all the functions of the system to fulfill its operational requirements. The specification, as a definition of the system, should include mission, concept of operation, configuration, system interfaces, functions, system targets, requirements, hardware/software performance characteristics, and a history of document revision and methodology used. Design specifications ensure technical disciplines in the processes and coordination between different functional areas. The system design specifications 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 specifications continuously and iteratively (Fig. 1.32).
The system specifications include identification and description of all functions to be provided, along with the associated quantitative requirements
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 controlsDesign variants in each subsystem Design variants in each subsystem
Des variants in eac subsystem Des variants in eac subsystem 1.31 Powertrain design decision tree.
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 definition of general system specifications was given by Armstrong (2002):
System specifications state the technical and functional requirements for a system, allocates requirements to functional areas, documents design constraints, and defines the interfaces between and among the functional areas and other systems. This specification 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 identified.
The fact is that there is no commonality or consistency with regard to deliverables or standard design documents (including system specifications) for engine products within the industry or even within the same organization.
This raises the demand for a common process with identified 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 definitions of the engine architecture (e.g., displacement, Vee or inline configuration, 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.
The necessity for the system design specifications 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 defined design document as the design target. In diesel engine system design, the functional objectives are translated into more detailed measurable specifications 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 flow 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 specification 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 NOx 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 specifications can be classified into four areas: performance, packaging, durability, and cost. They are introduced in the following sections.
1.7.2 System performance specifications
Introduction of system specifications for hardware and software
The design specification 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 flow rate, air flow rate, BSFC, air–fuel ratio, EGR rate, intake manifold pressure and temperature, exhaust manifold pressure and temperature, engine delta P, volumetric efficiency, 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 efficiency), turbine area (affecting air–fuel ratio and EGR rate), EGR cooler flow 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
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 specifications
3. root cause analysis of particular problems.
Specifically, the system specification refers to a predicted list of all critical steady-state and transient engine performance and emissions parameters for a given concept configuration 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 specification also defines 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 specification 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 configuration concepts or justifying design specifications 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 specification should develop from optimization.
The engine system specification needs to cover the following five aspects:
1. Steady-state performance design specification.
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 specification 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, insufficient EGR flow at engine peak torque is caused by inadequate engine delta P due to an excessively large turbine nozzle
area. Another example is an excessively high exhaust manifold gas temperature caused by cooler failure or low air–fuel ratio. Generally, the need of root cause analysis should be minimized as much as possible through successful up- front specification design with system optimization. The system performance specification is usually the most important of the four system specifications in engine design (performance, durability, packaging, and cost).
During the process of generating engine system design specifications, often off-the-shelf solutions or design need to be considered in order to simplify the product design. The off-the-shelf design usually has been fully tested on a specific engine. Its cost and manufacturing method are also known.
Engine system design needs to verify the selected off-the-shelf solution prior to testing validation.
1.7.3 System durability
The durability design specification is expressed in terms of both ‘stress’ and
‘strength’. The stress refers to any general loading, usually coming from the performance specification (e.g., peak cylinder pressure, cylinder heat flux, exhaust manifold temperature, compressor outlet temperature). The strength refers to the desirable structural design parameters representing the capability of the system or component. The strength is also used by the performance area as a design constraint for iteratively refining the performance specification.
Examples of preliminary engine structural calculations were provided by Makartchouk (2002) and Delprete et al. (2009).
1.7.4 System packaging
System packaging addresses the issues with weight, size, shape, component location, and clearance between the components. Good engine packaging requires low weight, compactness, suitable shape fit in the engine compartment, reasonable relative locations and clearances between the components without functional or geometrical interferences. Engine packaging design is handled mainly by three-dimensional solid modeling.
Engine weight can be estimated by using the volumes of the components.
The representative dimensions of major components can be classified into two categories: (1) the dimensions determined by the basic design parameters of the engine such as cylinder bore diameter and engine stroke; and (2) the dimensions determined by the durability requirements that are related to engine operating speed and cylinder pressure loading. Weight, size, and cost are especially important for NVH design when seeking an acoustically-effective solution to package. Schuchardt et al. (1993) introduced the concepts of packaging quantity and quality and the relationship between packaging and performance in their study on engine intake noise control.
Engine and component size and the overall shape are strongly affected by the engine configuration (i.e., number of cylinders, inline, Vee, or opposed arrangement), bore, stroke, connecting rod length, valvetrain type (pushrod or overhead cam), turbocharger type (single-stage, two-stage, or twin-parallel), EGR cooler size, and FEAD. Component size is also related to performance. This is especially true for diesel aftertreatment devices. The device size is usually limited by the available spacing of the under-floor for a given vehicle. A smaller aftertreatment component has higher space velocity and may have lower operating efficiency. For example, there is a trade-off between LNT size and fuel consumption penalty. A smaller size gives higher fuel consumption penalty.
The design of relative locations and clearances between the components deserves great attention. The performance and durability characteristics of the engine components are often subject to the harsh thermo-mechanical boundary conditions. They are affected by the heat transfer and vibration of the neighboring components. A primary focus of system packaging work is to optimize the relative positions of all the subsystems or components in order to minimize the negative impact of all thermo-mechanical effects as a whole.
Lastly, it should be noted that in modern diesel engine system design, design for manufacturability and design for serviceability are two important trends in packaging design. They represent the processes to optimize the relationships among design function, manufacturability, ease of assembly, and ease of maintenance.
1.7.5 System cost
Engine system cost can be estimated by using bill of materials and operating cost. The attributes of performance, durability and packaging all directly affect cost. The engine system cost specification plans the capital cost and operating cost of the engine technologies adopted. It also coordinates between the subsystems on the maximum allowable cost for each subsystem in order to control the total engine cost.