# Why don't car engine piston-rods (or similar parts) fail as often as other car parts?

Anyone who has owned a car has ultimately dealt with failure of a sensor (parking), or A/C failure, battery shutdown or a tyre puncture. Part breakdowns like these are pretty common, at least relatively.

In a car however, the most stress (mechanical and thermal) is faced by the engine and engine-parts. If you think about it a little more, you could see that the most stress is faced by the piston-rod and connecting shaft. Those and related parts face constant tensions and compressions, on the order of many times a second, and not to mention the higher temperatures.

Despite that, I rarely hear of such failures (in proportion to other common failures mentioned above). What could be the reason for this? Are those engine parts over-engineering to never fail (almost, 'cause nothing is certain), over the lifetime of the vehicle?

• Crank and con-rod design has had over 100 years of work but parking sensors are relatively recent so have not been perfected to the same level... – Solar Mike Jun 22 '18 at 5:50
• When is the last time your piston rod ran over a nail and punctured? This is as silly as why do I need to replace my house roof every 20 year but not the foundation. – paparazzo Jun 22 '18 at 13:10
• Automotive engineers are fully aware of the stresses that con-rods must endure & have designed them to be durable. – Fred Jun 23 '18 at 9:22

The main reason from a designer's point of view is the consequence of a failure. Almost always, something like battery failure or a puncture doesn't cause any collateral damage - you just replace the failed part. (Even a tyre blowout when driving at high speed rarely causes much secondary damage - and that is a rare type of "tyre failure" compared with the more common slow puncture, which doesn't even permanently damage the tyre if you regularly check the tyre pressures.)

On the other hand, failure of a piston or conrod usually destroys the whole engine, or damages it beyond economic repair compared with the cost of replacing it - much more expensive than plugging a hole in a tyre.

You could design a tyre that would have the same life as the car (say 15 years and 250,000 miles) but the cost would be huge, and many car users would never get the benefits simply because they would trade in the car long before "wearing out" such a tire. Would a car buyer want to pay say \$10,000 for such a tyre, compared with say \$100 for a "normal" one? For most people, the answer is rather obvious.

The failure occur in piston rod or connecting rod is less as compare to the other parts (battery, tyre alternator, etc) because the material used for manufacturing a connecting rod are usually steel which has lower thermal conductivity and the other properties like yield strength (659 MPa), tensile strength (855 MPaa), elastic modulus (201 GPa) are high and the hardness value of connecting rod is 276 vh due to which failure occur less in connecting rod. Now days aluminium alloys or titanium is used as a material for connecting rod so that it becomes light in weight and the performance of the engine increases. The main aim of connecting rod is to connect piston to a crank shaft because of the connecting rod the motion in piston and crank shaft occur so if connecting rod fails engine will not work that's why the designing of connecting rod is done like that the chances of failure occur less.

Its not like that connecting rod does not fails. The connecting rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and being compressed with every rotation, and the load increases as the square of the engine speed increase. Failure of a connecting rod usually called throwing a rod which is a most common cause of catastrophic engine failure in a car and the failure occur on connecting rod due to many reason like at the end of exhaust stroke it fails because the tensile force being at the highest at tdc. You can know the connecting rod fails by the knocking noise from engine low oil pressure. Similarly for the other parts of engine like piston, push rod, valves etc failure is less compare to the other parts.

• How does the thermal conductivity of a connecting rod affect its failure? – Solar Mike Jul 22 '18 at 13:00
• Actually thermal conductivity is a property of material to conduct heat it means heat transfer occur low in a material which have low thermal conductivity and steel (connecting rod material) has low thermal conductivity and in ic engine combustion take place so the material which has low thermal conductivity deformation will be low – Rahul Maheshwari Jul 22 '18 at 13:24
• Steel has a low thermal conductivity? compared to what? Steel is not used as an insulator... And given the range of temperature that the con rod works in how much deformation will be due to temperature and how much to the inertial forces created by the piston and crankshaft? – Solar Mike Jul 22 '18 at 13:29
• This talks around the question but never actually answers the question - why aren't thrown rods a more common occurrence like tire or battery issues are? – user16 Jul 22 '18 at 23:12