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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 sufficiently 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 flow-related parameters in a general sense. For instance, cylinder pressure and valve seating impact force are mechanical loads; cylinder head heat flux is a thermal load; and engine-out soot is a flow-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 flux
∑ 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 flow 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 flow rate). Therefore, they are determined at the design stage of system sizing.
Table 2.1 System performance parameters as durability design constraints
Tra nsm iss io n a
nd drivetrain
Cylinder head Piston assembly Cylinder liner Val vet rai n ( e.g
., valves, cam)
Co nn ect in g r
od and crankshaft
Bearings Crankcase Intake manifold Exhaust manifold Turbocharger EG R s yst
em (valve and
cooler)
Charge air cooler
Fuel injector
Engine brake torquex Mean piston speedxxxx Peak cylinder pressurexxxxx Peak cylinder temperature and cylinder heat fluxxxxx Exhaust manifold temperaturexxx Exhaust manifold pressurexxx Compressor outlet air temperaturex Compressor pressure ratiox Turbocharger speedx Heat rejection and engine outlet coolant temperaturexx Cooler outlet gas temperaturexxx Piston slap kinetic energyx Valvetrain loadingxx Engine-out soot loadingxx Oil soot percentagexxxx
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 simplified 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 profile design and the valve spring load selection. Cam profile 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 specified accurately at the early stage of the design, it will be easier to
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 finite-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 efficient 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 Loading 2
Cost B
B A
A
Packaging (size, weight,
etc.) Loading 1
Performance
Designed for same durability life
2.1 Durability analysis for engine system design.
design specification 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 finite 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 fulfill the above demand. Although very challenging, better predicting engine durability and