The Scotch Yoke, part 2

Wednesday, July 09, 2014

Tags :  con-rods

In the previous article, there was a picture of a very basic Scotch Yoke mechanism, and the mechanically astute among you would have noticed some obvious problems with such a basic implementation of the principle behind it. The two glaring ones are high contact pressure in the contact between the crankpin and the slot in the con rod, and the lack of stiffness in the rod.

If we deal with the first of these, this is solved relatively easily. If we interpose a bearing block between the crankshaft and the rod then we can reduce contact pressures; the block is ‘racetrack shaped’ – that is, a round-ended block – with a cylindrical bore. Both of these contacts are conformal, and the contact between the crankpin and bearing block bore is much as it would be in a conventional con rod, while the contact on the outside of the block is simply two flat surfaces in sliding contact.

It would be relatively easy to assure ourselves via calculation that our bearings can generate a proper oil film. In the case of the block running in the slot, we need to take account of the fact that the bearing block is reciprocating in the slot and comes to a stop when the crankpin is at ±90° relative to top dead centre. Coming to a stop means there is no velocity to generate an oil film, and this could lead to wear. However, with a suitably arranged feed of pressurised oil, it should be possible to preclude any wear problems.

The stiffness of the ‘big end’ of the rod is also a concern, as is its strength. In order to make the big end of the rod stiff enough, a lot of detailed design and analysis would be required, especially if the design was of the type that is split through the big end, as is the case with most four-stroke engines. The bolts would need to be large, as the bending moment would be considerable.

When the inertia loads are at their maximum, the distance from the bolts to the line of action of the applied load is much greater than in a conventional rod. However, this is mitigated to an extent by the fact that there are no secondary inertia forces, owing to the fact that the motion of the piston is simple harmonic motion. Also, because there are no concerns over high secondary forces or high side thrust loads – both of which are associated with short articulating con rods – the Scotch Yoke rod can be made as short as is practical, thereby minimising the reciprocating mass and the inertia forces associated with it.

The concept lends itself best to 180° vee engine architectures, because on such flat engines two pistons and the rod assembly operate on a single axis – that is, there is no bank offset – and some of the mass penalty of a Scotch Yoke mechanism is shared with another cylinder. However, the flat engine is not widely used in motorsport where bespoke engines are concerned. Granted, there are a great many Porsches competing in all kinds of racing, and Subarus have long been a favourite for rallying, but the unique packaging challenges of a flat engine means that the most advantageous engine configuration for the Scotch Yoke would present its own disadvantages in most race vehicles.

Written by Wayne Ward

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