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1.3 The concepts of reliability and robust engineering in diesel engine system
1.3.4 The concept of durability
The definition of durability
Engine durability or endurance is the other major attribute that affects the quality and reliability of the engine. It is usually confused with reliability in the literature. According to the definition given in the APQP (advanced product quality planning) manual developed jointly by Chrysler Corporation, Ford Motor Company and General Motors Corporation, durability is ‘the probability that an item will continue to function at customer expectation levels, at the useful life without requiring overhaul or rebuild due to wear-out’.
The APQP manual also defined reliability as ‘the probability that an item will continue to function at customer expectation levels at a measurement point, under specified environmental and duty cycle conditions’. Another definition was given by O’Connor (2002), ‘durability is a particular aspect of reliability, related to the ability of an item to withstand the effects of time
(or of distance travelled, operating cycles, etc.) dependent mechanisms such as fatigue, wear, corrosion, electrical parameter change, etc. Durability is usually expressed as a minimum time before the occurrence of wear-out’.
The above definitions of durability are somewhat ambiguous. Engine durability is better defined as the probability related to the hardware structural ability of an item to withstand the effects of time-dependent or non-time-dependent thermal, mechanical or chemical mechanisms such as fracture, fatigue, wear, corrosion, creep, deformation, fouling, plugging, electrical parameter change, etc. The time-dependent in the above refers to the accumulated service time. The non-time-dependent refers to the situations that are irrelevant to the accumulated time such as an abrupt rupture caused by over-load.
Durability is a part of quality before the engine product is released to the customer. It evolves to a part of reliability after the product is in use. In other words, a structural failure occurring in the engine test cell is called a durability problem rather than a reliability problem; and after the engine is released in service, a structural failure is called a reliability problem associated with a durability attribute. Such a definition clearly distinguishes between durability (as a design attribute) and reliability. The latter is a characteristic of the ‘overall quality’ extended into the time-in-service domain.
It should be noted that the concept of reliability covers the failures from all three attributes: performance, durability, and packaging. Therefore, it is not appropriate to assume equivalency between durability and reliability.
Durability is usually expressed as a minimum time or vehicle mileage before the occurrence of any major type of structural failures (e.g., wear-out). For example, a B10 durability life is the expected life (e.g., 20,000 hours or one million miles) at which 10% of the population fails. A B50 durability life is the expected life at which 50% of the population fails.
Stress–strength interference model
The concept of structural durability is illustrated in the stress–strength interference model shown in Fig. 1.13. The figure shows a random probability distribution diagram of component strength and stress. The stress represents the load, and the strength represents the component’s structural capability to resist the load. The stress and strength are random parameters and have probabilistic distributions corresponding to the variability in noise input factors. For example, the peak cylinder pressure load acting on the cylinder head can be regarded as a type of ‘stress’. It has a probabilistic distribution due to the manufacturing tolerance in engine compression ratio, the variation in intake manifold boost pressure caused by the tolerance of turbocharger wastegate controls, the variation of exhaust restriction due to the change in DPF soot loading, the variation in ambient temperature, etc. The noise
factors affecting the structural strength of the cylinder head may include the manufacturing tolerances in cylinder head deck waviness, flatness and surface finish, the material properties of the head and gasket, and so on. A failure occurs when the stress is higher than the strength. The overlapping area between the stress and strength distribution curves in Fig. 1.13 indicates the probability of failure. The stress–strength model is elaborated in Chapter 2 for in-depth discussions on durability and reliability.
The role of durability in engine system design
Durability and performance are inter-related. Durability limits are used as design constraints in engine system design in order to determine the maximum achievable performance or appropriate hardware sizing. Figure
Stress distribution
Stress and strength
Strength, S(t)
Stress or load, s(t)
Time (t) Failure occurs
Reliability in time-in-service domain Strength distribution
Strength after aging
The overlap area indicates failure rate
Probability distribution at specific time t
Time to failure t = 0
1.13 Stress–strength interference model in durability and reliability.
1.13 shows that in order to control the failure rate, either the strength needs to be increased (i.e., move the strength probability curve to the right or reduce its distribution range) or the stress has to be reduced. In the example of peak cylinder pressure, in order to reduce the pressure for better durability, the control factors such as engine compression ratio, intake manifold boost pressure or fuel injection timing need to be modified, and this affects engine performance and emissions. The durability analysis on the stress and strength distributions helps determine the maximum design limit and the nominal design/calibration target of design parameters (e.g., peak cylinder pressure and exhaust manifold gas temperature) that can be used for a durable system design. These limits or nominal targets can ensure the engine will not be overloaded and the structural strength is designed sufficiently strong.