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Paper II (i) Design and Analysis: A Perspective for the Future
Abstract
The continuing evolution of the internal combustion engine, together with required improvements in component design lead times and reliability, places increasing demands on improved design and applications engineering techniques. This paper charts the evolution of predictive design techniques for selected components within the engine and outlines the anticipated trends and developments necessary to meet the challenge of future design requirements.
Energy saving is an important task today. Friction reduction in reciprocating engines is a way to reduce fuel consumption and to improve engine endurance. To find the value of frictional losses of every component is necessary in order to reduce engine losses. The piston-liner system, valve train and bearings are the most important components for mechanical friction losses in reciprocating engines. In this paper both theoretical and experimental works published within the last ten years about the losses related to the motion of pistons, valves and bearings have been reviewed. Some known design improvements to reduce friction in these components are briefly reported and indications for future developments are pointed out, related in particular to the numerical approach, capable of more realistic simulations due to new software and hardware, and to the new experimental and material technologies.
The fixed 'offset' analysis of piston ring lubrication presented in Part II of this series was extended to include cyclic variations of the angular distortions of the ring. A calculation of the forces and moments acting on the ring at each point in the engine cycle is used in an iterative procedure which satisfies both elastic and hydrodynamic considerations. The significance of the forces and moments due to gas pressures, hydrodynamic normal and shear forces and reaction and friction forces at the ring/groove pivot is considered. The effect of the resulting angular distortions and ring axial movements on oil film thickness and nett oil transport is demonstrated for an experimental 2-stroke Diesel engine. It was shown that while large angles of ring twist are undesirable, the improvement in the geometry of the ring/liner conjunction that may accompany small ring angular displacements can improve oil film thicknesses, particularly in the region of the top dead centre. The location of the effective ring/groove pivot position is crucial to adequate ring performance and the anlaysis shows that the deformed profile of the groove has a significant effect on the dynamics of the ring. In particular, curvature of the groove is shown to exert a strong influence on film thickness and nett oil transport, which emphasises the practical significance of groove wear. Thus novel designs of ring grooves can be used to optimize the ring performance in reciprocating engines. (A)
Although the piston ring is a critical component in a reciprocating engine and although its successful functioning depends upon adequate lubrication, very little work has been done on the analysis of piston ring operation as a phenomenon of hydrodynamic lubrication. This paper outlines briefly the theoretical and experimental work that has been done, and produces a new analysis particularly suited to solution by computer.
The analysis assumes:
that the shape of the piston ring face is parabolic, but not necessarily symmetrical; that the lubricant viscosity is constant under the ring, but may vary in any fashion over the piston stroke; that oil film disruption may be represented by calculating oil pressure assuming no disruption and then discarding negative pressures; that the pressure upstream and downstream of the ring and the load on the ring may vary cyclically over the stroke; that there is circumferential symmetry; and that the motion of the ring parallel to the cylinder face is that of the piston, while the motion perpendicular to the cylinder face satisfies the force balance.
Using the simple example of the symmetrical ring face, isoviscous lubricant, constant load, zero upstream and downstream pressures, and sinusoidal piston motion, it is shown that there exists a parabolic shape which maximizes the minimum oil film thickness. Comprehensive results are presented for this case. The use of the more general analysis is illustrated for a diesel engine design for which the ring face shape has been measured.
The Newton-Raphson algorithm was used in conjunction with Murty's algorithm and the finite-element method to analyze the elastohydrodynamic lubrication of a journal bearing under dynamic loading. Cavitation boundary conditions were used. A realistic compliance matrix and load schedule were used in the illustrate example. Solutions for the film pressure, the film thickness and its rate of change with time were obtained as functions of the crank angle.
Based on the 30-year history of finite element practice a view of the future of that practice and the unresolved problems associated with it is presented. The process of computer aided engineering is chosen as the vehicle for the outlook, and within it such issues as adaptive mesh refinement, optimum design, education of the user, and the legal aspects of CAE practice are unresolved areas, e.g., shell analysis, nonlinear transient analysis, and software integrity, are also discussed.