What is good for small mechanisms is very good for very big ones. Above is the major reciprocating parts of a rally engine. At top right is the force applied to the top of the piston that mimics the combustion process. Below it are the lateral and vertical forces at the piston pin. Note that combustion provides only a small contribution to the total force transmitted at the pin, inertia contributes the largest component Modelling the loads on a complex moving mechanism Engine Parts 1

The components are modelled faithfully, but practically within a few % of their production physical properties 2

Maximum bending of the connecting rods occurs when they are half way between top and bottom dead centres (top left). Maximum tension and compression occurs at top and bottom dead centres (right) 3

Each machine part is divided into elements (finite elements). The stress strain relationship within each element may be easily predicted but not the behaviour of all elements together with respect to the load and to each other. Forces and constraints are defined according to how the loads applied and the reactions are taken. (Matrices, vectors, their multiplication and inversions are heavily involved - Matlab The parts are discretiesed loads and constraints are defined 4

A rod in compression and then in tension showing similar stress areas but different deformations 5

Above, the rod in bending At left, the stresses within a preloaded bolt and its flanges. (This is not the sort of work that a drafting CAD system could be used for) 6

An H section conrod is shown at top. Example cross-sections of the I and H stems of contrasting designs. Both can be made to work, the H is made often available as an expensive aftermarket upgrade for competition engines – but is it really the more efficient design ? 7

8 Above - vertical force in kN plotted against crank angle rotation in radians. Inertia force due to piston con-rod assy mass of 1.8 kg, stroke 115, con-rod lengthen 180 mm, at 8000 rpm. Left - max con-rod force at crankpin in kN plotted against rpm

At left, a permanently bent connecting rod, the outcome of over-revving an engine due to a missed gear change. Above, the exaggerated deflections that take place within a crankshaft during normal operations. These deflections do not exceed the elastic limit of the shaft material hence may just contribute to vibrations and possibly wear. 9

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Tube of max practical diameter bar or any section I beam tube or barrel (max dia) Axial torque axial tension bending moment axial compression Appropriate sections, shown as elements of equal cross-section The load type:

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