Continuous RPM vs. Intermittent in a piston engine.

[Shaft109]

This is a question relating to piston engines in a car but have seen a similar phrase in a POH.

In the Manual 6800rpm is stated as the maximum continuous but allows up to 7200 intermittent (does not specify time though).

Why is this? I understand it to mean that heat dissipation is the reason as the reciprocal parts should be able to spin all day long at those rpm's, otherwise they'd fail anyway above 6800 (either that or bearing pickup would occur).

FYI it's a 2 litre 16v 4cyl. 86mm x 86mm.

It's just a curiosity question

Shaft

[Intruder]

Metal fatigue from cyclic stress is an exponential function of temperature, force, duration, and repetition rate. When the forces and/or temps are higher, failure can be expected after fewer repetitions. At the extreme ends of the force and temperature spectra, the number of repetitions until failure can be expected drops dramatically. As the pistons pound up & down harder/faster, and the shafts spin faster, more stress is placed on them.

The "intermittent" characterization of the limitation likely indicates there is a higher risk of failure as well as a significant probability the engine life will be reduced below normal or warranted time. Where the projected life cycle of an airliner's engines can be predicted with reasonable fidelity (e.g., a number of start, taxi, takeoff, climb, cruise, descent, approach, taxi cycles), and therefore engine parts can be designed to a specific expected lifetime, that is not the case with car engines. There is too much variation in the number and type of usage cycles. OTOH, the airplane engine is more likely to have a specific time limit at extreme temps and pressures, so the reliability can be better predicted.

[rigpiggy]

Read an article several years back, specifically piston speed had the most to do with reliability. very basically piston stroke*2*rpm I forget the number that they figured but somewhere near what you would get in a lycoming/continental.