OBSERVATIONS ON MODES OF INCREMENTAL CHANGE IN DESIGN

OBSERVATIONS ON MODES OF INCREMENTAL CHANGE IN DESIGN McMahon, Christopher A., Journal of Engineering Design, 09544828, 1994, Vol 5, Issue SUMMARY This paper presents a framework for the classification of modes of historical development of normal design, and for tracing this development as products mature Normal design involves the incremental development of an existing design principle Designs are defined in terms of sets of explicit and implicit attributes, and the framework is based on a model in which design involves searching within a design space, subject to the requirements of a product design specification Five modes of design change are identified design parameter space exploration, improvement in understanding of design attribute relationships; change in product design specification; modification of the feasible design space; and adoption of a new design principle These modes of change are illustrated by considering the development of automotive engine piston design Introduction This paper aims to provide a framework for the classification of modes of historical development of designs in normal design and for tracing this development as products mature Normal design comprises the improvement of established design solutions, or their application under new or more stringent conditions [ 1, 2] It is also termed 'conceptually static design' [ 3] or 'incremental design' [ 4], and it encompasses the variant and adaptive design modes described by Pahl and Beitz [ 5], in which new designs are respectively generated either by varying a standard design, or by incrementally adapting a known system to a changed task In normal design, the adopted solution principle is essentially static, but changes take place at subsystem and detail level as time progresses For example, many of the solution principles adopted in automotive engine design have been known and routinely applied for much of this century Even where designs are regularly produced simply by varying the dimensional parameters of a prototypical design (in 'variant' or 'fixed-principle' design [ 5]), the rules and algorithms for determining the dimension values change with time The emphasis in this paper is on the classification of engineering activities in this development, and the identification of the circumstances in which conceptual changes are appropriate The situations in which the design process may change to a 'radical' (or 'original') design mode are also briefly discussed The view which will be presented is that the design process leads to the definition of product designs in terms of parameters, such as form, dimension and tolerance, which must by explicitly defined for the product to be made Designs are developed within a feasible design space, in response to a product design specification The process also leads to judgements of the suitability of the emerging designs in terms of performance, cost, durability and other factors which are considered to be implicit in the design and the environment in which it operates Given this model, designs may evolve in a number of ways These include the development of an improved understanding of the relationship between the performance factors and the explicitly defined parameters; changes in emphasis within the product design specification; and local conceptual modification to allow constraints on performance improvement to be removed This paper will first review related work on the classification and evolution of designs, and on the growth of technical knowledge A design attributes model will then be introduced and used in the identification of development modes, and these will be illustrated with examples from automotive diesel engine piston design Design Classification and the Development of Designs Related work in this area may be grouped into two broad categories: classification of design context, and observations on the nature of the development of technical knowledge in particular, in the incremental development of designs 2.1 Classifications of Design Context A recurring theme in the design literature is the classification of the design context according to the degree of conceptual originality However, there is little agreement on the terminology to be used to describe the different cases We are concerned here with the design activity involving the development of established designs Constant [ 2] used the term 'normal technology' to describe "what technological communities usually do", in direct analogy to Kuhn's concept of normal science [ 6]; Vincenti [ 1] used the term 'normal design' to describe the design involved in such normal technology It describes those cases where the engineer knows at the outset how the device in question works, what are its customary features, and what is the likelihood of success in accomplishing the desired task [ 1] Vincenti drew his examples from aeronautical engineering Happold [ 7] adopted the same approach in the domain of structural engineering, and Konda et al [ 8] used the concept in their development of the notion of 'shared memory' as a theme for design research and practice They stressed that normal design does not imply that it is not creative Normal design corresponds broadly to the conceptually static mode of design of Pugh [ 3], to the variant and adoptive modes of Pahl and Beitz [ 5], and to the incremental and complex modes identified by Slusher et al [ 4] 2.2 Patterns of Development In each of the cases described, effort is concentrated on the incremental development of a limited number of design concepts Sahal [ 9] termed these essentially invariant design patterns 'guideposts', and suggested that there are two key determinants of technical progress: the process of learning or the acquisition of production skills; and the process of scaling or patterning of the design He suggested that innovations have their origin in this process of scaling (i.e the change in size, output or performance of some artefact, an example being the increase in output of turbine generator sets), in order to solve problems that arise from the new circumstances For Abernathy and Utterbeck [ 10], 'process innovation' was a characteristic of a mature product, for which the design objectives are well articulated The early stages of a product's life are characterized by diversity and uncertainty about product performance criteria, and hence product innovation For a design concept to survive as a guidepost, it must survive scaling and development without encountering fundamental limitations Gardiner [ 11] termed concepts which have this characteristic 'robust', and described the attributes of robust designs, such as an absence of narrow restrictions on the feasible range of dimensional parameters 2.3 Development as Evolution The development of designs has been regarded by some authors as an evolutionary process, where the term 'evolutionary' follows more or less closely the biological evolution metaphor Nelson [ 12] and Businaro [ 13], in particular, adhered closely to this approach Nelson proposed that many firms working with a relatively incomplete understanding of market demands will lead to competing designs being proposed, which are then tested for fitness for purpose by the market Businaro discussed evolution in terms of innovations being generated by applied research, stored and then released to the market when a change occurs in the environment in which the designs operate The biological analogy to the evolution of design also has been applied in architecture by Steadman [ 18], and French [ 19] has explored similarities between natural and artificial systems in a number of respects Hybs and Gero [ 20] presented a loose analogy with natural evolution in an evolutionary model for the design process (for a single artefact) They viewed design as an iterative cyclic process of generation or refinement of partial design solutions, which are evaluated using a model of the environment They used the notion of designs being tested in the market, but they did not consider the evolution of classes of designs 2.4 Growth of Design Knowledge Continuing the evolutionary theme, Vincenti [ 1] discussed the growth of design knowledge in terms of the 'blind-variation-and-selective-retention' model of evolutionary epistemology proposed by Cambell [ 14] The model suggests that new knowledge is introduced by 'blind variation', in the sense that it goes "beyond the limits of foresight or prescience", and that this involves mechanisms for introducing variation, such as trial and error, accompanied by consistent selection processes Vincenti proposed that the cumulative growth of knowledge alters the nature of the variationselection processes In particular, he stated that, in the design of mature products, variations are more likely to be tried out vicariously by analysis and experiment in place of direct trial In a more Popperian approach [ 15], Blockley and Henderson [ 16] proposed that engineering failures are the occasions when the designer's conjectures (in the form of designs) are 'falsified', and suggested as did Petrowski [ 17], that this is a significant mechanism in the growth of engineering knowledge Konda et al [ 8] described the accumulation of design knowledge in terms of the development of 'shared memory', the capturing of which they saw as a critical engineering resource As a basis for their evolutionary model, Hybs and Gero [ 20] offered a formal description of the design process based on design as 'transformations' With this exception, there is little formal basis for judging where effort should be expanded in normal designs, and for identifying the circumstances in which design should be more conceptually dynamic Design Attributes and Design Space A prerequisite for a formal model of modes of incremental change is a notation to describe designs and the design space which is explored and developed in their generation The proposal outlined below is based on the following notions: • • • design is driven by the requirements or demands placed upon it by the customer, expressed in the form of a design brief, or a product design specification [ 3, 5]; designs are defined within a design space bounded by constraints, and the design process is one of searching within the bounded design space [ 21, 22]; the design may be described by a collection of attributes, describing the modelled properties of the design [ 23, 24] Specifically, it is proposed that the design D of an artefact may be formally represented by D = < E, L, I > where we have the following definitions • • • E is the set of explicit attributes describing the design, such as its dimensional parameters, the values of the properties of the materials from which the artefact is constructed, etc They are termed explicit attributes here, because they must be explicitly defined for the artefact to be made L is the set of design values of external effects on the artefact, such as the applied loads (these will often be maximum loads that the design has to support, or 'worst case' effects) I is the set of implicit attributes which describe the characteristics and behaviour of the artefact subjected to the external effects L The implicit attributes describe the functional performance of the artefact, including such parameters as strength and durability The term is used here, because the attributes are considered to be implicit in the design of the artefact They may be estimated from the explicit attributes and the external effects They may also, in some circumstances, be regarded as relationships between the explicit attributes and the external effects Individual artefacts are examples of designs produced within a design space S, defined by S = < F, R > where F is the set of relationships defining feasible values for the explicit attributes For a particular attribute, this may comprise specified single values, limiting values or sets of discrete values for the attribute, together with limiting relationships for attributes or between explicit values Also, R is the set of relationships relating implicit attribute values to explicit values and to the external effects on the artefact A particular design will be produced to a product design specification DS, described by DS = < FR, SI, L, C, U > where FR is the set of functional requirements for the design; SI is the set of specified values of implicit parameters which the design is to achieve (these specify inter alia how well the design is to meet the functional requirements); L is the set of specified values of external loads and effects which the design is to support (this is normally identical to the term L in D); C is a set of constraints on the values of explicit parameters (these in effect reduce or qualify the more general constraints and relationships R in the design space for the class of attributes); and U is an expression or expressions defining the utility of the design, which allows trade-offs to be made (for example, concerning the cost-weight trade-off for an aircraft) Utility functions have been applied in design evaluation with multiple attributes by Thurston [ 25] Therefore, a particular design D will be selected from the attribute space for that particular class of designs (S), in response to a product design specification DS A particular design solution is feasible if there exists in S a design D which satisfies a particular DS For a given product design specification DS, the feasible values F will map to a feasible set of implicit parameters The design is optimal if there is no scope for improvement in the value of the design utility for a given DS, without going outside limits imposed by S or DS Under these circumstances, the design is at a constraint bound This bound may be the result of a single attribute imposing a constraint on the design, or there may be multiple-bounded attributes 3.1 Handling Uncertainty In most cases, design is undertaken under conditions of uncertainty In particular, imprecision may result in an effective design space which is smaller than the nominal space, through scattering of feasible attribute values (e.g in a dimensional or material parameter); through uncertainty in the constraining relationship between values; or through uncertainty in the relationships relating implicit to explicit attributes The effective design space may be written Se = < Fe, Re > There will generally be many methods for estimating the value of particular implicit attributes of a design These may range from the application of rules of thumb, through analytical techniques of various descriptions to prototype and field tests These methods will have various levels of precision and will require different levels of detail in the explicit attribute set For example, the strength of a structural part may variously be estimated by classical solid mechanics, by finite element and boundary element analysis at different levels of precision, and by using elastic-plastic material models Imprecision and uncertainty are also found in the models of the external loads and effects (L) and the utility function U Indeed, a large part of engineering activities is aimed at reducing the uncertainties that exist concerning the desired qualities of artefacts and the properties of the materials and manufacturing processes used for their construction [ 1] Modes of Design Change The initial establishment of an accepted design in a given domain is usually the result of a wide examination of alternative approaches Preferred design approaches may draw together successful features from a number of example designs Once a 'guidepost' has been established, there are a number of modes in which it may be developed These include the following (1) Parameter space exploration This mode of design change involves the variation of explicit attribute values within the limits imposed by the feasible explicit attribute set Fe or F This may be with the aim of optimizing the design but often, because of complexities and uncertainties in the problem, it is with the aim of what Simon called "satisficing" producing a solution that is satisfactory rather than optimum (but at a higher utility than previous designs) [1, 26] (2) Improved understanding of explicit-implicit attribute relationships The design also may change by exploiting an improved understanding of the relationships Re relating implicit to explicit values (i.e expanding the effective design space) For example, improved analytical techniques may allow a more precise calculation of stresses in a component, and reduced factors of safety (or uncertainty) perhaps may be used in the analysis The understanding also may come from empirical results from experiment; from the application of computer-based analytical techniques; from improved modelling or mathematical methods; from examination of competitive product data, etc (3) Change in product design specification If the market conditions change, then this may be reflected in a change in the product design specification DS, particularly through the following (i) A change may occur in the specified values of implicit or explicit attributes or external factors that the design must meet For example, a change in legislation may reduce the emissions criteria which automotive designers have to meet (ii) A change may occur in the utility function for the design An example of this is a change in emphasis in automotive design from performance to economy in the mid-1970s, or for aircraft design from performance to cost during the 1980's (In general, points (i) and (ii) will be closely intertwined.) (iii) The set of functional requirements L that the design has to meet may be extended An example is the addition of anti-lock requirements to braking system design criteria (4) Modifying the feasible design space If the feasible explicit attribute space Fe may change, then the design may develop by retaining the design principle but modifying the design space for example, by expanding the feasible region Examples of this include changing the feasible attribute values by manufacturing process change (such as reducing the minimum wall thickness for a casting, or increasing the maximum dimension of an extrusion), or by material change (such as the selection of an alloy with improved hot strength) (5) Changing the design principle The final mode of change involves the adoption of an alternative design principle with different design space In principle, this always may be done but considerations of design risk often may mean that companies will retain established design configurations as long as an acceptable design is feasible using invariant design principles 4.1 Design Evolution Much design activity falls into classifications ( 1)-( 4) above For example, the fundamental principles used in automotive engines, suspensions and transmissions have changed little in the past 25 years Designs are often developed such that they are as close to known constraints as the designer risks making them Where conceptual changes have been made to designs, it is contended that these have been made principally to overcome or to reduce constraints placed on the design This constraint alleviation activity (which will be of type ( 5) above) is usually concentrated on specific areas where designs are perceived to be weak at any particular time, such as in safety, in economy, in emissions, etc We will see in the example in the next section that this concentration leads to apparent phases in the development of particular designs We also will see that the changes which are introduced often are not necessarily novel they may have been developed first in some other related area, or perhaps may be the result of some earlier research or invention, the incorporation of which may finally be justified Occasionally, a new overall design principle may emerge which offers the prospect of substantially higher utility through the alleviation of some very significant constraint The complete replacement of the design principle is a 'revolutionary design mode' (the term is used here by analogy with Kuhn's revolutionary science, although Constant [ 2] used the term 'radical' in design, implying chat it is somehow less traumatic than revolution) Examples of such changes are the replacement of carburation by fuel injection, or of the reciprocating engine by the gas turbine in aircraft applications In each of these cases, it is worth noting chat there is considerable overlap between original and new technologies, with the displacement taking decades in each instance, and remaining incomplete Example of the Product Maturing Process The approach chat has been outlined may be illustrated by considering the development of automotive diesel engine pistons during the last 50 years During this period, the solution principle for pistons has been largely as shown in Fig (a) Indeed, this form is broadly as shown in Otto's patent drawings of 1876! The gas pressure within the cylinder acts against the piston crown, and the load is transmitted to a pair of bosses chat support the piston pin that links the piston to the connecting rod The piston is guided in the cylinder bore mainly by the lower part or skirt, and the gas pressure in the cylinder is sealed by piston rings above the piston pin A further ring (or rings) serves to scrape excess oil from the cylinder wall Figure l(b) shows some of the dimensional and other parameters that together form the explicit attributes E for a piston The functional requirements F, together with the implicit parameters (SI) by which the design of a piston is judged and the external loads L it is designed to support, are shown in Table I Table II lists the general sources of constraints on the explicit parameter sets chat appear in the design space or the product design specification, and the factors that contribute to the utility function From the constraint list, we will consider in particular two factors in the overall proportions of the piston First, the sizes of the bearing areas of the joint between the piston, piston pin and connecting rod are broadly determined by the bearing capacity of the various materials, and the maximum firing pressure of the engine For high firing pressures, a large-diameter piston pin is used, but, as the firing pressure increases, a point is reached at which no further increase in bearing capacity is possible A second critical area is the top ring groove So long as the temperature at this point stays below a certain limiting value, the top ring is unlikely to stick in the ring groove Above this limiting value (which is determined in part by the geometry of the ring, and in part by the capacity of the lubricating oil to withstand high temperatures), the oil that reaches the top ring may oxidize and gum, causing the ring to stick 5.1 Historical Development of the Piston The modes of development outlined in Section are seen in piston development as follows Mode ( 1) change: design space exploration Prior to the availability of computational tools for the analysis of design variations, only a very limited design space would be explored in the development of individual designs design essentially involved a propose-test-develop sequence (However some design exploration using, for example, electrical analogues to heat flow have been reported [ 27], and the production of many design variations led to a cumulative design space exploration) A limiting factor in the exploration is the effort required to test each variant Early applications of computational techniques improved design understanding but did not allow widespread space searching Recently, computer programs for piston design have been reported which use automatic finite element mesh generation, with some 30-fold improvement in model preparation time, allowing some parameter optimization [ 28] Mode ( 2) change: improved understanding of attribute relationships While it is only in recent years that design space searching techniques have been widely applied, the understanding of piston behaviour has been the subject of extensive research for many years The purpose of such work was initially to obtain an improved understanding of the operation of the piston and of the reasons for failure Design rules and design analysis were still based largely on empirical relationships from service experience More recently, analytical techniques have been developed, supported by experimental study, which have allowed design configurations to be explored analytically An example here is in the selection of top land dimensions to ensure adequate cooling of the top ring Simple relationships relating the dimension to the cylinder bore and engine duty were used into the 1970's, although a considerable body of work investigating temperature distributions within the piston has been carried out since the 1950s [ 29, 30] This work initially used small, inserted plugs of fusible alloys of known melting point, or the hardness-temperature relationship of the piston or of steel plugs was used [ 28] By the 1960s, a number of developments in telemetry and in articulated connections allowed thermocouples to be used directly [ 31] These techniques allowed an understanding of the operation of pistons, and enabled the performance of a new design to be more closely predicted from test bed results However, the lengthy test procedures allowed only limited investigation of alternative arrangements More recently, analytical techniques have developed to the point that they allow the thermal performance of different piston forms to be predicted They have been used in particular to study the effect of the shape of the cooling channel in the piston crown (see below) on durability [ 32, 33] A survey of piston research from the 1940s to the present day reveals that, naturally, effort has been concentrated on aspects of the components that were known to be troublesome or amenable to improvement As problems have come to light, so research has been carried out which provides knowlede or understanding to underpin design thinking and to "rude innovation Thus, we can see in the 1940s study of methods for improving the wear resistance of the top ring groove, and for understanding piston and ring friction [ 34] (also associated with this work was a reduction in the number of piston rings fitted) Increased performance ratings in the 1950s led to the extensive research into thermal loading of the late 1950s and 1960s [ 31] Combustion design has been a theme throughout the period, but especially since the mid-1970s [ 35] The 1970s also saw extensive study of the effect of piston design on noise [ 36], and on emissions [ 35]; this work has continued to the present day Very high maximum firing pressures have led to current interest in the performance of the piston pin bearing, and to analytical study of piston stresses [ 37] Mode ( 3) change: altered product design specification The research and development effort noted above also has often reflected pressure from changes in the requirements placed on the piston Early designs were for naturally aspirated engines of low duty and refinement, so effort was expended on achieving adequate durability A gradual increase in required performance led to increased gas pressures and thermal loads, which brought pistons to the limits of simple designs In Diesel's early designs, the maximum cylinder pressure was about 40-50 bar By 1940, this had risen to about 60 bar and, with turbocharging, maximum pressures have increased from 90 bar in the early 1960s to the order of 120 bar in 1980, and 150 bar today There also have been extensive changes in requirements other than in engine performance terms The changes in requirements match and generally precede the research and development trends already noted, such that a number of demand phases may be identified In chronological order, these are as follows: • • • • • • 1940s improved wear resistance of ring grooves, and friction reduction; 1950s increased rating leading to pressure on thermal integrity; 1960s increased thermal and cylinder pressure ratings, with the wider use of turbo charging; 1970s noise reduction and reduced fuel consumption; 1980s reduction in emissions; 1990s performance improvement; life of re-entrant combustion chambers; very high cylinder pressures In each of these phases, the pressure for improvement from the marketplace took piston design beyond the established feasible limits Mode ( 4) change: modifying the design space For all the time that the market pressures were forcing design improvements, materials and manufacturing process technologies were changing to allow the design space to be expanded, in order to give improvements in performance This expansion may be seen particularly in the development of materials technologies for the piston itself and for the lubricating oil The almost universal use of aluminium alloys for automotive pistons was enabled by the development of low expansion coefficient, high silicon (13%) alloys These have good hot strength and adequate conductivity, but have poor wear resistance at operating temperatures, leading to high ring groove wear [ 38] Very high silicon ( 18-24%) alloys were used to give improved wear resistance, at the expense of thermal conductivity, and an alternative approach has been the incorporation of cast iron ring carriers, cast into the piston For many years, such combinations were adequate, but very high ratings have led to the recent exploration of squeeze casting techniques (a hybrid of casting and forging) [ 39], and to the re-examination of the feasibility of cast iron as an alternative to aluminium alloys [ 40] Developments in lubricating oils have been most significant in allowing increased engine ratings At the end of the 1950s, a suitable maximum temperature in the top ring groove was about 200DegreeC An engine for which the top ring groove temperature was at this limit could not have its power output increased, unless the piston ring was moved further from the piston crown by increasing the piston height Detergent oils allowed maximum temperatures of 230DegreeC or more in the 1960s, and maximum allowable temperatures today are of the order of 260DegreeC Each expansion of the design space by means of material and process changes allows an improvement in design utility For a critical component, such as the piston, this opportunity will usually be quickly exploited, such that the design will remain at or near the constraint bounds There is in effect a dynamic equilibrium between pressures for improvement in the performance of the design and the limitations imposed on performance by constraining factors Mode ( 5) change: modification to the solution principle Despite a greatly improved understanding of piston behaviour and of the changes made possible by material and process improvements, acceptable performance of the piston often cannot be achieved without a change in the design principle A number of examples of this may be observed in the history of piston design, such as the following examples (1) Despite developments in oils and an improved understanding of the temperature distribution in pistons, the thermal loading of turbocharged diesels is too great for adequate ring durability.The first innovation in this respect was in the development (in the mid-1930s for aircraft gasoline engines initially) of a ring form (tapered cross-section) which is less susceptible to sticking This allowed an increase in groove temperature of about 20DegreeC, but at the expense of more difficult ring manufacture The late 1950s and 1960s saw the development of a number of techniques in piston cooling by oil-using jets of oil on the underside of the piston by means of multiple part pistons, and by the manufacture of pistons with cast-in cooling passages (Fig 2) The implementation of the latter development required a process innovation, in the development of soluble salt cores in the casting process (2) As firing pressures increase, a point is reached at which either there is no space to enlarge the piston pin or the pin reaches such a diameter that little increase in bearing area may be achieved Again, under these circumstances, innovation has been necessary A relatively early development in this respect was the tapered or stepped small end (Fig 3(a)), but lubrication of this arrangement can be difficult and manufacturing costs are high More recent developments include the shaping of piston bosses to relieve edge load on the bearing; the mechanical treatments of piston bosses; and the incorporation of bearing sleeves (Fig 3(b)[38]) These developments have allowed maximum bearing pressures to increase from about 450 bar to 700 bar over the past 30 years, matching the increase in cylinder pressure noted above These developments show that, although the general arrangement of pistons has remained broadly as used by Otto in the 19th century, their maturing has required continual innovation in materials, in experimental and analytical techniques, and in detail design Conclusions The designer working in an original design mode is well served with tools to assist in the design activity, but the paths to follow in the adaptive and variant activities of normal design can be much less clear This paper has proposed a classification of modes of development and has suggested that design is a dynamic process in which product designs are improved by one or more of these modes, in response to pressures for enhanced utility The key to successful design in conceptually static domains is perhaps an understanding of which mode of development is most appropriate in a given context In this respect, another key factor is an understanding of the dynamics of the demands on the design, and on the constraints imposed upon it The paper has concentrated on the product maturing process for component designs in the largevolume automotive sector Future work should examine whether or not a similar classification may be identified in system design, and in smaller volume sectors, and whether the classification allows insights into the differences between design approaches in different contexts Acknowledgements The author gratefully acknowledges the contribution made by Professors David Blockley and Roland Bertodo, Dr Bill Smith, Steve Culley and Jon Sims Williams, and anonymous reviewers in their comments on earlier drafts of this paper TABLE I Piston functional requirements, implicit parameters, and imposed loads and conditions Functional requirement Implicit parameter Transmit gas load to connecting rod with adequate life static strength[a]; fatigue strength[a,b]; wear resistance[b]; pin bearing performance[a]; scuff resistance[b] Seal gas pressure in cylinder with low fiction Compression pressure[c]; blow-by rate[c]; friction[c] Seal oil from cylinder Oil consumption[b]; particulate emissions[c]; smoke[c] Transmit heat from combustion chamber Temperature distribution in piston-ring[a,c]; ring durability[b] Absorb side-thrust of connecting rod with adequate durability and low noise Friction[c]; noise[c]; piston motion[c]; piston skirt bearing pattern[b] Form part of combustion chamber Engine performance[c]; emissions[c] Functional requirement Load and condition Transmit gas load to connecting rod with adequate life Cylinder pressure; loading frequency; piston velocity Seal gas pressure in cylinder with low fiction Cylinder pressure; piston velocity Seal oil from cylinder Oil temperature and viscosity; piston velocity Transmit heat from combustion chamber Thermal loading; coolant temperature; oil temperature Absorb side-thrust of connecting rod with adequate durability and low noise Cylinder pressure; power train dynamics Form part of combustion Cylinder pressure; chamber thermal loading; air motion; fuel injection [a] Measured by analysis [b] Measured by development or service experience [c] Measured by running engine TABLE II Piston design constraints and utility criteria Design constraints Utility function, contributory factors Manufacturing constraints: casting, machining Size constraints imposed by engine Material parameters: piston, rings, engine Engine oil Maintenance constraints Standards Utility is maximized by compact size low reciprocating mass adequate durability low emissions high combustion chamber performance ease of maintenance and repair low cost DIAGRAMS: FIG (a) General form of automotive diesel piston and (b) examples of its explicit attributes DIAGRAM: FIG Piston with integral cooling gallery DIAGRAMS: FIG Methods of improving piston boss bearing: (a) tapered small end; (b) bush in piston boss REFERENCES [1] VINCENTI, W (1990) What Engineers Know and How they Know It (Baltimore, MD, John Hopkins University Press) [2] CONSTANT, E.W (1980) The Origins of the Turbojet Revolution (Baltimore, MD, John Hopkins University Press) [3] PUGH, S (1990) Total Design (Wokingham, Addison-Wesley) [4] SLUSHER, E.A., EBERT, R.J & RAGSDELL, K.M (1989) Contingency management of engineering design, Proceedings of the Institution of Mechanical Engineers, ICED '89, Harrogate, pp 65-76 [5] PAHL, G & BEITZ, W (1984) Engineering Design (London, Design Council/ Springer) [6] KUHN, T S (1970) The Structure of Scientific Revolutions (Chicago, IL, University of Chicago Press) [7] HAPPOLD, E (1992) The nature of structural engineering, The Structural Engineer, 70, pp 349354 [8] KONDA, S., MONARCH, I., SARGENT, P & ESWARAN, S (1992) Shared memory in design: a unifying theme for research and practice, Research in Engineering Design, 4, pp 23-42 [9] SAHAL, D (1981) Patterns of Technological Innovation (Reading, MA, Addison Wesley) [10] ABERNATHY, W.J & UTTERBACK, J.M (1978) Patterns of industrial innovation, Technology Review, 80, pp 41-47 [11] GARDINER, J.P (1984) Robust and lean designs with state of the art automotive and aircraft examples, in: FREEMAN, C (Ed.), Design, Innovation and Long Cycles in Economic Development (London, Pinter) [12] NELSON, R.R (1987) Understanding Technical Change as an Evolutionary Process (Amsterdam, North-Holland) [13] BUSINARO, U.L (1984) Applying the biological evolution metaphor to technological evolution, in: FREEMAN, C (Ed.), Design, Innovation and Long Cycles in Economic Development (London, Pinter) [14] CAMBELL, D (1960) Blind variation and selective retention in creative thought as in other knowledge processes, Psychological Review, 67, pp 380-400 [15] POPPER, K.R (1963) Conjecture and Refutations: The Growth of Scientific Knowledge (London, Routledge and Kegan Paul) [16] BLOCKLEY, D.I & HENDERSON, J.R (1980) Structural failures and the growth of engineering knowledge, Proceedings of the Institution of Civil Engineers, 68, pp 719-728 [17] PETROSKI, H (1985) To Engineer is Human: The Role of Failure in Successful Design (London, Macmilan) [18] STEADMAN, P (1979) The Evolution of Designs (Cambridge, Cambridge University Press) [19] FRENCH, M.J (1988) Invention and Evolution: Design in Nature and Engineering (Cambridge, Cambridge University Press) [20] HYBS, I & GERO, J.S (1992) An evolutionary process model of design, Design Studies, 13, pp 273-290 [21] MEDLAND, A.J & MULLINEAUX, G.M (1989) The application of constraint modelling techniques to the description of the design process, Proceedings of the Institution of Mechanical Engineers, ICED '89, Harrogate, pp 621-636 [22] SAPOSSNEK, M (1989) Research on constraint-based design systems, Proceedings of the 4th International Conference on Artificial Intellegence in Engineering, Amsterdam [23] TJALVE, E., ANDREASEN, M.M & FRACKMANN SCHMIDT, F (1979) Engineering Graphic Modelling (London, Newnes-Butterworth) [24] SHAW, N.K., BLOOR, M.S & DE PENNINGTON, A (1989) Product data models, Research in Engineering Design, 1, pp 43-50 [25] THURSTON, D.L (1991) A formal method for subjective design evaluation with multiple attributes, Research in Engineering Design, 3, pp 105-122 [26] SIMON, H.A (1969) The Sciences of the Artificial (Cambridge, MA, MIT Press) [27] HOWARTH, M.H (1966) The Design of High Speed Diesel Engines (London, Chapman & Hall) [28] RUDDY, B.L., TRAVAILLE, J., KTNSELLA, F.H & BIRD, L.E (1990) Piston assembly design for total quality, Proceedings of T&N Technical Symposium, Wurzburg [29] MCCULLA, E (1958) Ratings of diesel engines depend on smoke, piston temperature, exhaust temperature, Society of Automotive Engineers (SAE), Preprint 64A [30] MANSFIELD, W.D (Ed.) (1964) Proceedings of Conference on the Thermal Loading of Diesel Engines (London, The Institution of Mechanical Engineers) [31] WESTBROOK, M.H & MUNRO, R.(1966) Electronic Instrumentation Techniques in the Development of Pistons and Rings, Proceedings of the Institution of Mechanical Engineers, 180, (3G), pp 54-69 [32] MUNRO, R & GRIFFITHS, W.J (1975) Diesel piston design and performance prediction, Proceedings of the 11th International Congress on Combustion Engines, CIMAC, Barcelona [33] REIPERT, P (1991) The computer-assisted design of pistons, Proceedings of Conference on Computers in Engine Technology (London, Institution of Mechanical Engineers) [34] HERSEY, N (1944) Bibliography on piston-ring lubrication, NACA Report TN956 [35] [35] MURRAY, E.J (1982) The light diesel piston and its performance, Proceedings of Conference on Diesel Engines for Passenger Cars and Light Duty Vehicles (London, Institution of Mechanical Engineers) [36] CHALLEN, B.J (1982) Techniques for engine noise control, in HICKLING, R & KAMAL, M.M (Eds), Engine Noise (New York, Plenum) [37] GRIFFITHS, W.J., BRUNI, L & MAURAT, P (1990) Pistons for highly rated diesel engines, Proceedings of T&N Technical Symposium Wurzberg [38] BAKER, A (1979) The Component Contribution: Engine Progress through the Specialist Manufacturers (London, Hutchinson Benham) [39] AVEZOU, J.C., COLE, A.T., GAZZARD, S.T & MUNRO, R (1990) The application of squeeze casting to piston technology, Proceedings of T&N Technical Symposium, Warzburg [40] MUNRO, R., DUBOIS, F & MATTEODA, P (1990) Ferrous diesel pistons for highway applications, Proceedings of T&N Technical Symposium, Wurzburg ~~~~~~~~ By CHRISTOPHER A McMAHON Christopher A McMahon, Department of Mechanical Engineering, University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR, UK  ... running engine TABLE II Piston design constraints and utility criteria Design constraints Utility function, contributory factors Manufacturing constraints: casting, machining Size constraints... Contingency management of engineering design, Proceedings of the Institution of Mechanical Engineers, ICED '89, Harrogate, pp 65-76 [5] PAHL, G & BEITZ, W (1984) Engineering Design (London, Design. .. using a model of the environment They used the notion of designs being tested in the market, but they did not consider the evolution of classes of designs 2.4 Growth of Design Knowledge Continuing