Connecting rod geometry, particularly center-to-center length, can have a material influen
It is generally acknowledged that connecting rod geometry, particularly center-to-center length, can have a material influence on a variety of engine conditions. These include specific relationships to valve timing (camshaft design), cylinder pressure history, spark ignition timing requirements and torque output, the latter with respect to the actual shape of torque curves. We'll touch on the more important of these a bit later.
Depending upon specific applications, connecting rods are perhaps some of the most highly stressed parts in an engine, particularly those intended for racing. From the high loads experienced at and just beyond TDC piston position during combustion to the tensile and unsymmetrical loading caused by offset piston pin axis, loads that are actually opposite to combustion pressure loads and stresses set up by lateral inertia, connecting rods become virtual "whips" that mechanically join pistons to the crankshaft.
Further complicating the issue are vibratory loads caused by oscillatory motion of a crankshaft, rotating about its axis while spinning in a normal direction. Visualize this set of load conditions in very slow motion. Each firing impulse intended to accelerate crankshaft rotation is applied as a force delivered in a span of time. Because of its inertia, a crankshaft can't immediately increase its speed and, therefore, is momentarily deflected in the same direction as its rotation. This deflection is local to the crank pin to which the load-delivering connecting rod is attached. Then, because of its elasticity, the crankshaft (at that pin location) will spring back against its direction of rotation, continuing this back-and-forth oscillatory motion until the next firing pulse is delivered to that particular crank pin. The connecting rod is thereby required to absorb what amounts to a series of tensile and compressive loads caused by oscillations of the crank pin, during primary crankshaft rotation.
Keep in mind that we've just provided a very simplistic description of the load dynamics experienced by the connecting rod for only one operational cylinder. The complexity of this varying load environment is increased by orders of magnitude when you add another seven cylinders and turn up the wick on rpm. So, when you think about connecting rods as "shock absorbers," several issues come to mind.
For example, consider cylinder pressure loads not as "hammer blows" to a piston but very rapid pressure rises that are influenced by combustion flame rate and net combustion pressure development. We also know that this pressure "history" is not constant or uniform as it is applied to a piston. Plus, whatever auxiliary forces are applied to a piston are also transferred in some way into the connecting rod. Rods can be designed too stiff, thereby transferring combustion pressure too aggressively to rod bearings and crank journal bearings. They can also be too flexible, and neither condition is acceptable. But in any case, rods need to absorb load spikes and minimize pressure transfer loss to prevent a waste of torque that's ultimately produced by the crank.
Perhaps one area of concern where connecting rod stiffness is important deals with vibratory loads produced by the torsional stiffness of a connecting rod's beam section, as piston weight is reduced. As you might expect, the reduction of rotating and reciprocating mass in an engine's crankshaft assembly can become a trade-off to the absorption of gas and mechanical loads by sheer mass alone. Visualize throwing a medicine ball to a 150-pound person and then to a 250-pounder and you may understand this more clearly.
Of course, to minimize the rotational resistance of a crankshaft assembly, reducing the weight of pistons and rods is a time-honored approach. However, compromising weight for strength and durability is the fulcrum about which this issue pivots. Perhaps one exception to this "rule" was in the early design of composite connecting rods (the so-called "poly motor" of years past), in which first-design rods were inordinately stiff and caused rod bearing failures for a lack of load absorption capability. On the other hand, lightweight materials that offer strength and low mass may be too costly to market, even in the average racing engine. So while other considerations must be included, the fundamental objectives should include strength, low weight, and durability.
In speaking with leading connecting rod manufacturers, you often hear that a high percentage of rod failures don't occur during the high pressure of combustion. Rather, it's during the exhaust stroke that a rod gets "yanked" away from TDC. This sudden movement of the piston causes abnormally high tensile loads in the rod's beam and leads to a fracture in this area, typically somewhere just below the piston pin end.
Also, failures can occur during either valve float or conditions of over-revving the engine. What happens is that the open valves (and lost combustion pressure) don't provide any sort of a cushion for pistons heading toward TDC. So when they pass through TDC, there's nothing to stop them from being "pitched" at the cylinder heads, often leading to another cause of tensile fracture in the beam section. In fact, the "effective" or dynamic weight of a piston passing through TDC under these conditions can be far in excess of its actual static weight. Multiple times, in fact.