What causes an engine to lug?

[Daox]

What causes an engine to lug at low rpms?

I've noticed for different engines, they all handle low rpms differently. Some engines lug as high as 1500 rpm depending on load while others handle down to maybe 900 rpm without much if any vibration. Is it simply a matter of how balanced the internal components are?

[Frank Lee]

I think how much flywheel there is, how long the stroke is, how many cylinders there are, and camming would be the biggest factors..

[XYZ]

I'll expand the thread with another question.

Is lugging an engine detrimental to it?

[Frank Lee]

Yes it is.

[Baltothewolf]

Originally Posted by XYZ

I'll expand the thread with another question.

Is lugging an engine detrimental to it?

I remember someone saying that when an engine lugs it's also not getting enough oil pressure to properly lubricate it, he said something about having to re-sleeve the cylinder walls? Idk what that means but Yea.

[Cobb]

Lugging an engine causes a range of or can lead to other problems. Back in the day lugging an engine causes it to rattle as it wasnt turning fast enough to keep the layer of oil between the rods and journals. With the removal of lead from fuel you got preignition, ping and detonation to compound the issue.

With the above mentioned issues resolved as technology advances lugging an engine caused you to wipe the oil film from the compression side of the cylinder and caused it to wear in an egg shape.

Granted we have even better oil than 5 years ago and new coatings for cylinders and pistons this is still an issue. Honda is having several recalls for the v6 and vcm issues. the VCM is lugging the engine on too few cylinders causing excessive oil consumption. The solution is to upgrade or update the programming to not do that.

To help circumvent many of these issues this is why some races allow you to start from a roll.

Now due to the lack of a throttle plate of a diesel engine and the introduction of smoke maps this is the best way to operate one of them, within reason of course.

[darcane]

Technically speaking, lugging an engine is defined as operating it at WOT and decelerating (due to the load exceeding the available torque). This is most commonly done at low engine speeds in high gears, but you could lug an engine at high RPMs by towing a heavy load up a steep hill.

Lugging will cause very high loads on the crankshaft bearings, and very quickly build up heat in your engine. If the engine has suitable bearings and adequate lubrication, lugging, by itself, won't harm the engine.

Often associated with lugging is detonation and piston slapping, especially at low RPMs. Because of the high stress and heat when an engine is lugging, it is more prone to these conditions which are very damaging.

[Cobb]

I havent seen too many broke crank shafts, but they do occur.

My sidekick does all I described. If I go to 5th gear at 25 mph and try to accelerate I get all sorts of noises and the interior rattles. If I go up an over pass in 5th at 40 mph I cant accelerate, but I get that E flat tone from the exhaust that I need another gear.

[Daox]

So, not that I have this problem, but if you did have lugging issues, how could you fix them? Increase oil pressure? Increase flywheel mass?

[Frank Lee]

I'd say you not only don't want the damaging loads on the crank and rod bearings and cylinder walls, you don't want the bucking and jerking. That's tough on the motor mounts too.

Simplest thing is to operate above the lugging threshold. If you are gentle on the gas pedal you can even operate down in that lugging rpm range if you only lightly load the engine. So if you're in 5th gear at 20 mph and you step on it to accelerate and it starts bucking and protesting, back off on the gas. You may still be able to accelerate but at a very gentle rate until it reaches a higher rpm.

If I was certain I wanted to operate in the lugging rpm range and fix it, the first thing I'd do is add flywheel mass. I don't think adding oil pressure would help. And if this low rpm operation is happening because of a tall re-gearing, I'd look at whether a new cam could be ground to optimize the engine for this new low rpm range.