How do (too) low revs hurt the engine?

[xxChrisxx]

Low revs do not harm the engine directly, but may excite resonances in the powertrain mounting and driveline.

If you have an engine on a dyno that is rigidly mounted you can drag the speed of the engine right down to 100 rpm or so and back up again quite smoothly.

Beat me to it. People think its bad because its right where firing frequency sets off a load of body and powertrain modes.

[countersteer]

Two thoughts...
At full throttle, minimal rpm, would it not stand to reason that cylinder fill would be at its maximum subject to cam timing?
At what point in the rpm range are the bearing surfaces "saturated" with lubricant sufficient to ensure adequate lubrication?

Couple the two together, that is, big bang/minimal lube, would it not stand to reason that your're risking bearing damage by using Max throttle/max load @ minimal rpm?

edit. Clarify full throttle...

[Greg Locock]

I'd say that it is probably a good idea to stay above idle speed, a production engine will spend a lot of time at idle during a durability test so it will be functioning properly then.

On the dyno we used to run full throttle sweeps 1000 rpm through to redline without an issue.

[marcush.]

to my knowledge short piston skirts combined with some high performance alumium alloys do not like (high load) low rev scenarios -leads to piston slap ,or rock especially when engine is still cold.
These alloys have a higher thermal expansion rate so the running clearance (and sealing) is compromised -extended low speed idling and load application from low revs may lead to damage.

An engine design intended for mass production has to survive several "loaf about"validations before released to customers so one would think it is no issue there.

I was a bit surprised that Honda engines suffer broken valve springs when spending too much time in the limiter ....something you might experience on trackday driving ..

[SectorOne]

Clutch is what takes the beating. Like pulling away in 2nd gear is harder for the clutch then pulling away in 1st.
(normal takeoff from stop light etc)

[xxChrisxx]

Clutch is what takes the beating. Like pulling away in 2nd gear is harder for the clutch then pulling away in 1st.
(normal takeoff from stop light etc)

Why would that be the case?

Simply being at low rpm does nothing to the clutch. You can sit at idle all day rolling round with the clutch engaged.

The 'taking a beating' you are describing is from having to slip the clutch over a wider speed range. Which has nothing to do with lugging.

[SectorOne]

Pulling away in a higher gear will put more strain on the clutch.
I wasn´t talking about just cruising around with no throttle input in 2nd gear.

[xxChrisxx]

Pulling away in a higher gear will put more strain on the clutch.
I wasn´t talking about just cruising around with no throttle input in 2nd gear.

Indeed it does, but that's not what the thread is about.

Its a thread about lugging.

[SectorOne]

My mistake, i miss-understood the whole thread, sorry.

[olefud]

The problem with low revs has been pretty much solved with modern engines. In the days of long, flexible cranks with mains maybe every other pair of throws the crank could set up both torsional and longitudinal vibrations that would tear things up in the engine. If the engine pulls smoothly, you’re OK. But if it bucks, back off.

[g-force_addict]

An interesting topic concerning driving with low engine RPM emerged on another forum. So, the basic question is, do low revs (say 1200 or so in a ~100 hp 1.6 petrol, non turbo engine) hurt the engine during cruising, let's say on a level surface, that would be maybe in 5th gear and around 50-60 kph. Now I guess accelerating from those revs (lugging) in that gear puts lots of strain on conrods, crankshaft and bearings, but I guess driving w/o accelerating wouldn't really do too much harm to the engine.

The only thing I can think of would be lubrication as the oil pump output depends on rpm.
Maybe also for colling as the wáter pump is also rpm based.