From: The Design and Tuning of Competition Engines

by Philip H. Smith,

C. Eng., F.I.Mech.E., M.S.A.E.

1967

 

Mechanical stresses

In stepping up power output, it is usual to increase the maximum crankshaft speed of the engine, and also to increase the rev/min at which it normally rotates, that is, at the cruising speed of the car, as this latter will naturally be higher than before. It will be instructive to consider first what effect this has on the working parts. The main significance of operating stresses in the conventional engine is in the reciprocating action, which in its working, sets up bearing loads quite unlike those encountered in purely rotating motion. These loads are particularly severe at the connecting-rod big-ends. In a conventional four- or six-cylinder engine, the main crankshaft bearing loads can be fairly well balanced, so that wear is distributed around the shell and journal. Inherent in the connecting rod and piston assembly, however, is the fact that its reversal of movement at top and bottom dead centres introduces an alternating tensile and compressive load in the connecting rod, which is of course reproduced as a shear stress at the gudgeon pin where it joins the piston bosses, and communicates a heavy load to the big-end eye at its bearing on the crankpin.

These inertia loads are independent of any others which may come from the actual pressures on the piston during the working cycle. If for the moment we consider the case of an engine being driven round by an electric motor, with all the sparking plugs removed, the reversal of forces at top and bottom dead centres will be virtually the only ones with which the reciprocating parts have to contend. It is important to realize that the loads caused in this manner increase in quite an alarming way with increase in revolutions. In fact, the increase is proportional to the square of the engine revs. For example, at 6,000 rev/min the loading is four times that at 3,000. It is not difficult, in view of the foregoing, to see why big-end failure may occur if the revs are pushed even just a little higher than normal, if at the " normal " point, the bearing construction is loaded fairly well to its safe limit. Thus, an engine that travels happily for mile after mile at 4,500 rev/min should not necessarily be condemned if bearing failure happens after a brisk bout at 5,000. The extra 500 revolutions has increased the big-end bearing loading by nearly 25 per cent. (See Plate 12.)



Power loading

The actual operational cycle of the engine has an effect on inertia stresses; for instance, on the induction stroke, the connecting rod assembly, for the first part of its movement, will be under tension caused both by inertia and by the suction " effect of drawing in the gases. For the remainder of the stroke, the assembly is being slowed down by the crank, as the reciprocating weight attempts to " over-run " the uniform crank speed. On the other hand, it is still subject to the suction effect, so that the degree of compressive load now coming on the connecting rod depends on throttle opening as well as engine speed.

On the compression stroke, the first loading is obviously compressive, but here again the latter part of the stroke has a powerful compression brake acting against the piston, so that in the changeover from compression to tension in the rod, the amount of tensile force depends on compression pressure (which again means throttle opening) and engine speed.

The firing or power stroke obviously puts a compressive load on the connecting rod, which opposes the tensile inertia load; again the extent to which the two forces act depends on throttle opening (that is, on the strength of the expansive force), and the engine speed. While the exhaust valve opens, the pressure is removed from above the piston, which is in any case now being slowed by the crank. Finally, on the exhaust stroke we can assume that the load changes from compressive to tensile, as in other strokes.

Complications of bearing loads

The foregoing has been no more than a very brief outline of what the big-end bearings and materials of reciprocating parts have to withstand, but sufficient has been written to show how very complicated is this combination of loading caused by inertia of the components and the engine operating cycle. It will now be interesting to see how driving conditions affect the loading.

It is true that generally speaking, bearing failure occurs at high revs and wide throttle openings. This is caused very often by conditions additional to the actual speed of the engine, or power being developed, as we shall see later. It is, however, not at all difficult to put serious loads on bearings when driving in what might be considered a very gentlemanly manner, and at relatively low engine revolutions. On an average engine, the maximum torque comes in at about half the maximum speed of which the engine is capable. It is at this point of full torque that the expansive pressure on the power stroke is highest, so that a considerable load, the result of this power production, is put on the big-ends. At these revolutions, the inertia force is moderate, so that there is less of this to balance the thrust of the power stroke. Furthermore, this full-torque effect persists at very low engine speeds, as there is plenty of time for the cylinders to become fully charged. All this adds up to the fact that " slogging " on a wide throttle at low engine revs is bad for bearings, and can lead to serious overloading if persisted in. As the revs build up, even on the same throttle opening, this, far from increasing the load, actually decreases it, due to the counteracting inertia forces.

At maximum engine revs, or over, the inertia forces can be said to take command, the very considerable load increase with engine speed having already been detailed. The torque is necessarily falling off at this point (unless the engine has an exceptionally efficient induction system), due to valve restrictions and impedance to mixture flow which is a consequence of the high speed of operation. Thus the balancing load on bearings is reduced, and they are subjected to their maximum stress.

A further factor which influences the liability of bearing failure at near maximum engine speed is oil temperature. Sustained high speeds obviously mean increased heat, and if the sump-oil temperature reaches an unusually high figure its viscosity will decrease, just at the time when it should be ample to maintain the oil film separating the highly loaded bearing surfaces. Any minor rupture of the oil film in a bearing will at once increase the heat generated in that bearing, so that a situation is created which very soon results in failure if the conditions are persisted in. This is the reason why short bursts at high speed are not harmful, even on a well-worn engine with low oil pressure, whereas indiscreet flogging for miles on end will wreck the bearings of the best maintained power unit.


Extra heating

Thus far we have considered simply the additional loading caused by higher revolutions plus extra piston thrust. The next effect is that caused by the fact that any measures taken to increase engine power by higher cylinder pressure must inevitably release more heat in the combustion chambers. A higher compression ratio does this, while the same applies in even greater measure when multiple carburetters or an improved manifolding arrangement, or both, allow the engine to inhale a larger quantity of mixture.

This extra heat can be catered for without much difficulty in the average engine, it being dissipated via the cooling water system and the engine oil to atmosphere. In some cases increased water and oil capacity may be desirable, or a modification of water pump speed, or other water circulation arrangements. It will, however, be appreciated that when additional heat is added to the loads already described, the total requirement in extra " toughness " may be quite appreciable.

It is possible, for example, for gasket failure to occur if flange areas and stud centres are not adequate to withstand the extra pressure and heat conduction requirements. Cylinder head castings may distort and actually lift between holding studs, under extreme conditions, but such troubles are nowadays confined to experimental units. Providing the necessary and logical modifications are made to components carrying extra stresses, there is no reason to suppose that stepping-up the power output of a basic design of engine need have any adverse effect.