tegwin
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| posted on 31/1/13 at 01:39 PM |
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Resonance wear on an engine, a brain experiment
Ok... So.. my musings have often led me to consider this scenario... (and someone was moaning earlier about the lack of technical content on here...so
lets get technical)
You have a well designed, perfectly balanced, brand new engine. Well set up to run through all rev rangers. You run for 100,000 miles on the
motorway.... So the engine sits at 4000RPM for its entire life.
Obviously the cylinders/bearings etc will wear as the parts slide over each other but... does the RPM and therefore vibration frequencies (caused by
that RPM) within the components effect the way they wear?!
I guess my question is... If you took two identical engines, run one at 4000 RPM for 100,000 miles and run the other with a good all rounded cycle of
rev ranges for 100,000 miles would both engines wear in the same way?
Will the engine sat at 4000RPM develop a resonant "sweet spot" where the engine runs smoother or perhaps more efficient....
Discuss 
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MikeRJ
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| posted on 31/1/13 at 01:57 PM |
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Crankshafts suffer from torsional vibration that can hit a resonance within the working RPM range. This can cause fatigue failure of the crank very
quickly, and is why it's usually a bad idea to replace harmonic balancers with alloy pulleys.
In terms of bore wear, an engine run at a constant low (ish) speed will eventually wear a lip in the bore where the top ring reaches. At high engine
speed the piston actually moved further up the bore due to the higher forces in the bearings etc. which can cause the rings to break if the lip was
formed from constant lower speed running.
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mark chandler
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| posted on 31/1/13 at 03:12 PM |
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At low res you get much less wear as the transitions and moving speeds are lower, big low revving diesels as example with 1,000,000 miles are not
uncommon. Modern oils do help, but ultimately loads are far less.
As above a piston stopping at the end of its stoke carries inertia and will stretch the con rod pulling on the crank etc.
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coyoteboy
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| posted on 31/1/13 at 07:16 PM |
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I don't think it's something you'd ever be able to conclusively prove without lab engines and control variables.
Undoubtedly though, yes. You've differing load scenarios, differing flex in parts, differing resonances (though I suspect most parts are not
seeing excitation near their natural frequencies so not sure it's really relevant).
Comparing large low revving HGV diesels to a normal car diesel and their lifespans is apples and oranges really, but there are plenty of highway
mileage yank versions of euro cars (with the same engines) that last 3-4x as many miles, which I suppose is indicative.
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TheGiantTribble
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| posted on 31/1/13 at 08:06 PM |
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ok, not well (daughter given me her cold bless her), and my head is hurting
but my thoughts are
Since engine ware with regard to RPM is not a straight line, so if your engine was running at 8000 it would be waring
at more than double the engine at 4000
I conclude (and have no proof of this just 'gut feeling' and faulty intuition)
the engine that sits at 4000 all it's life will last longer than the one which uses various revs.
Of course a lot depends on what revs the engine was designed for, what proportion of time is spent at what revs.
In real life I used to regulary change gear while sitting at 70 MPH in my Toyota Tersel delibratly to stop a ridge forming
in the cylinders...then a Toyota Mechanic told me I was being an idiot 
Yes all bow to the Tribbles genius 
Or as more likly shoot me down in flames 
Nice thought experiment.
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alistairolsen
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| posted on 3/2/13 at 08:08 PM |
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I think we need to separate "resonance" from the rest of the discussion.
If you mean which will be worse, 4000rpm continuous, or an average of 4000rpm, but continuously varied, then that will depend on the tolerance above
and below 4000rpm, as another poster highlighted, it's not a linear relationship, so 4000 +/- 300rpm isnt not the same as 4000 +/- 3000rpm.
If you mean will an engine which is run continuously at it's resonant frequency wear prematurely, then yes, it will, which is why most
manufacturers arrange this not to occur at common cruising speeds and then fit all manner of vibration dampers to mitigate it. It's almost
certain this wont be 4000rpm however!
My Build Thread
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spiderman
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| posted on 3/2/13 at 09:16 PM |
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A generator run at 3000rpm will do a lot more hours of work than a car engine run at an average of 3000rpm, I believe it is more to do with varying
loadings and the harshness of the change between the different loads, that's why engines attached to a auto box tend to be in better condition
and last longer than engines attached to manual gearbox.
Spider
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ianclark1275
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| posted on 3/2/13 at 10:49 PM |
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i did 100k miles in a Focus 1.8TD 2004 in about 2.5 years.
it mostly did motorway miles generally flat out.
full ford servicing
it lasted 105K miles and the big ends went...
the forces in a turbo engine versus a normal asp mean it will be subject to higher forces.
i think the "wear" on the engine is not necessarily the Revs used but the torque range used (ie the way you drive it)
old deaf people who drive in second gear at 5000 revs everywhere i suspect dont have a worn engine, whereas my brisk driving probably bust the engine
prematurely..
i also did 24 tyres and 4 set of discs and 2 TPS.....
i actually liked that car
IC
measure twice, cut once, scrap it, start again.
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mcerd1
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| posted on 4/2/13 at 09:11 AM |
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if an engine has a real problem with resonance anywhere in its normal rev range then its been badly designed
forming a proper ridge in the bores at low rpm could happen but in the real world it will only really affect generators, ship engines, etc.... even
crusing on the motorway a car engines rpm will vary a bit (and you've got to get upto speed in the first place), besides how long can you drive
without stopping for fuel / sleep ?
I guess the nearest thing you could get is a car with a CVT....
btw tractor engines sit at ~2000 rpm for most of there life and don't seem to suffer for it (althought they are designed to run at that
speed)
fatuge and other related effects are much more likley to shorten the life of the engine
that could be a simple as a crack growing in a rod untill it lets go
or 'impact' damage on the bearing surfaces
cyclic loading will 'wear out' /fail parts much faster than a constant load as every cycle ads another bit of microsopic
'damage'
obviously even at low constant rpm there is a fair bit of cyclic loading, but if this could be made worse by cycles that ocasionally involve very high
stesses
also I know that these are the loads that cause engine designers the biggest headaches
e.g. KV6 prototypes used to shear 1/2 of there main bearing bolts even though they should have been more than strong enough (a stress cocentration at
the wrong spot made them fatuge and fail very quickly)
quote:
Originally posted by ianclark1275
i did 100k miles in a Focus 1.8TD 2004 in about 2.5 years.....
....it lasted 105K miles and the big ends went...
well the old NA version of that engine in my dads fiesta was still going when the chassis had rotted out completly - with more than 220k on the clock
the extra stress from the turbo (and the way you drive it) is bound to have more impact on its life than sitting at constant revs
just my 2p's worth
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