Is a ' Moment' harmful to an engine ?

[derekpara]

Driving to Holland recently I remembered being told that you should not drive at a constant engine speed for long periods because a 'moment' will develop which could be harmful.  Could any of you auto engineers describe what is a 'moment'  and does this theory still apply to modern engines ?

[d0gsb0dy]

I was told, many years ago, that driving long distances without varying speed could be harmful for an engine. Can't remember the actual reason, but I think it was something to do with a build up of heat under the bonnet.
Sorry I can't be of more help.

[Redcab]

I drive four times a year between homes in the UK and Finland (850 miles) in temperatures from +30C to -30C at a steady 80 kph (50 mph) where the limit allows. I find no problems, and I get fantastic mpg (around 53 in a 1994 diesel Renault Espace). A possible issue might be resonances from the bodywork at various speeds, but these don't threaten the engine and they can even be used as an alternative speedometer (in a given gear).

[derekpara]

Thanks, dOgsbOdy and Redcab. As I understand it a 'moment' is where a slight imbalance in moving parts of an engine causes centrifugal forces to build up in one particular point called a 'throw' which eventually places excessive stress on other parts of the engine to eventually cause a fracture. It couldn't happen over here where we have hills, traffic congestion etc resulting in speed variations, but somewhere like the routes through Europe where you could travel on clear level roads for hours on end, the problem ( if it still exists with today's advanced engineering) could easily arise, especially in worn engines. I hope the more knowledgeable ABers will have something to say on this.

[jake-the-peg]

I've never heard this before but looking around this sounds like a mix up of effects.

Firstly with an engine that is mounted logitudinally there is a slight tendancy for it to want to turn in the direction of rotation due to this "moment" - apparently this is most apparent in planes where it tries to yaw the plane, especially if you have a twin engine plane and gey one engine failing.

I believe the "throw" is the part of the crankshaft that sticks out from the central shaft.

I suspect that the actual problem is the potential to hit a resonant fequency in an engine which could cause it to shake itself to bits  (remember the tacoma narrows bridge?) but if you did hit a resonant frequency you'd soon notice and take tour foot of the gas and it would go away.

All sounds a touch unlikely to me. Were you told this a long time ago? 

[derekpara]

Thanks, Jake, I think you've got it spot on. A resonance frequency is the perfect definition and you are right about a 'throw'. That is something I remember from my metal turning days at school, I suppose !  And you are also correct about the ttime factor - it WAS a long time ago, but when you are buzzing along on a straight level road for hours on end the mind does tend to stray off line a little !   Thanks again. I knew someone would hit the target.

[Kev_UK]

I was told years ago to vary speed, but I think RPM (engine speed) is more important than MPH (travelling speed).
Most modern cars have CRUISE CONTROL which keeps a car at a constant speed but the rpm will vary with road conditions, gradient, wind resistance, etc