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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.

I'm not an engine guy but I'm not grasping how low rev's would be harmful in that situation, even if accelerating wide open. I'd think there would be much more stress on con rods etc at high RPM.

Imagine riding a bicycle uphill in high gear. It's hard to push the pedals, right? Now, the engine does the same via conrods and crankshaft. Imagine hitting the pedals with a hammer if they're hard to push. In the engine the hammer strike would be the fuel explosion in the cylinder.

I've heard about these stresses you've mentioned however I don't quite understand why they're bigger at low revs. If they are then if you cruise the engine is not idling but it is putting some torque through the drivetrain so these stresses smaller than during acceleration but still present.

I would also add the greater risk of knocking at low revs.

dragvorl wrote:Imagine riding a bicycle uphill in high gear. It's hard to push the pedals, right? Now, the engine does the same via conrods and crankshaft. Imagine hitting the pedals with a hammer if they're hard to push. In the engine the hammer strike would be the fuel explosion in the cylinder.

But the power of that explosion is similar at any revs and the engine is designed to cope with that power. Unless you could also say that it's harmful to the engine to accelerate quickly and you should do it gently to save it. That's why I don't quite understand the reasoning behind that low revs harmfulness.

I also ride a bike a lot and I push hard all the time as I am quite an enthusiast of biking. Using wrong gear may be inefficient but it doesn't harm the bike's pedals or crankarms.

The pedal lever would bend if you could act with a force high enough on the pedals with your legs, right? When you act with a force on the piston top, and it won't go down easily, the force has to act somewhere else...

No, the crankarm won't break nor bend doesn't matter how hard I try because it is designed to cope with the maximal force which any rider can exert on it. It's the same (I hope) with the conrods in the engine, they can cope with the force from the pressure after the combustion.

Well, of course, you get the point. Now, there are some bearings in the conrod/crankshaft which may not last that long if they're subjected to heavy load for prolonged time periods, I guess.

Remember that the engine has alot less power at low rpm's. The explotions is not as powerfull at low rpm's as at high rpm's.

I dont see why it should be harmfull to run at low rpm's.

Average cylinder pressures are highest in the highest torque range as far as I know.

That is true. But a 1.6 N/A engine with 100 hp does not have its peak torque at 1200 rpm's. Thats for sure.

Yep, but we can broaden the discussion to other engine configurations too.

dragvorl wrote:Average cylinder pressures are highest in the highest torque range as far as I know.

Yes, I'd imagine cylinder pressures would have to be highest where the engine is making the most torque.

But keep in mind, the engine is not quite like riding a bicycle. I believe there are considerable inertial forces on the con rods from increasing acceleration as the RPM's increase. I believe valve train dynamics as well become increasingly difficult to manage as RPM's go up. Those I see as more destructive to an engine than high cylinder pressure.

The problems I have experienced with very low revs were damaging vibrations.
With aero engines with reduction gears on the airscrew this was found to be sufficiently bad to strip gears.
There is a similar effect on the timing gears, belts or chains on car engines.
With the airscrew it is the low rpm aerodynamics working against the slow firing pulses of combustion resulting in a jerky movement.
With the timing gears, it is the cam lobes working against the pulses giving a similar although not so pronounced effect.
This happens at tick over just above engine stall.

The worst case is on cold start when there is little oil pressure at low rpm as well as these opposing loads.
The result is increased wear to bearings and the cam and cam components.
The answer is to start the engine and run it at 2,000 rpm for 20 seconds to prime it with good oil pressure and not to set the tick over to low on a road car.
However I always set a very low tick over on race and rally car engines to facilitate a more rapid gear shift.
The rpm would decay faster during a racing shift with a lower tick over set.
You will find that with a good dog ring box set to a low tick over, a skilled driver of the 'old' school can easily shift faster than any modern layshaft box no matter what gimmicks for shifting it uses.

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.