What causes an engine to lug?
 

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?

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

I'll expand the thread with another question.

Is lugging an engine detrimental to it?

Yes it is.

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.

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.

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.

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

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. :thumbup:

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?

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.

You could probably lug any modern engine all day long above 1000 RPM and not hurt it.
Bucking and jerking happens at extremely low RPM, like after nearly stalling when dumping the clutch at <500 RPM. If you are constantly running in the <500 RPM range, something is wrong with you.

By the time damage is done it might be too late to correct it. As has been mentioned, there are potential detrimental effects that can take many forms. It's hard to predict which symptom might arise.

Ecomodders tent to want to be in the highest gear and at the lowest RPM's, and that can lead toward lugging the engine. But you can feel the effects of the engine lugging when it is happening, so it is avoidable.

An ounce of prevention is worth as pound of cure... so IMHO it is best to avoid doing it. ;)

Precisely stated and I agree 100%

regards
Mech

Lugging was much worse in carbs with an accelerator pump, which if you floored it, put the same amount of fuel in regardless of the engine speed. When done at too low a speed it was way too much gas and usually atrocious spark knock, which fuel injection does not really allow all the way down to idle speed.

My 37 Ford idle was 350 RPM. To check it you drove the car in top gear with your foot off the gas and it read 7 MPH, you were good. The engine was not "lugging" on flat ground at 7 MPH, but if you touched the gas pedal any more than just a smidgen, you were lugging. It was possible to pull away in 3rd from an idle but it took like 5% throttle to avoid lugging, which also restricted the accelerator pumps injection of fuel.

Redline on that flathead was 3800 RPM or 82 MPH in top gear depending on the rear axle ratio. That 37 weighed the same as my Fiesta, around 2500#, and the old flathead was maybe 217 cubes.
Good old torque monster.
No problem driving in modern traffic until you had to stop really fast!

regards
Mech

If you can tell us the make and model we can do wonders for you. In general either reduce load or increase rpms. Maybe inflate the tires another ten psi? Under drive pulley for the accessories? Turn off the AC? Check for vacuum leaks, do a general tune up.

My example with my sidekick seems to like to be around 3 thousand rpms. Since going up 3 tire sizes I find situations where I need to go from 5th to 4th where as I use to leave it in 5th. The benefits of the bigger tires well out weigh that issue.

Its all theoretical discussion. No specific model in mind.

Downshift... ;)

A much tighter definition of "lugging" is needed.
 

Seat of the pants definitions where vibration and noise are the measure do not adequately tell you if you are "lugging" an engine. Many engines are fine being run in low, heavy loads while others fail very quickly.

I won't go into engine design theory of peak pressure to brake-mean-effective pressure, but, Old Tele Man's description pretty much sums things up. Assuming your engine holds together, the point at which the cylinder pressure causes displacement of your lubricating film is when things go south. Low speed, high power demands result in increased cylinder pressures. Coupled with most engines which use mechanical lubrication pumps who's output follows engine speed, there is a point at low revolutions where the oil film is displaced and damage occurs.

Of course, even if an engine does not incur damage at high loads and low speed, it can feel awful. The bucking and vibration can damage motor mounts and other tertiary components. The configuration and design of an engine has much to do with how violent "lugging" can feel. The 3 cylinder Suzuki/Geo Metro engines are inherently rough at lugging speeds whereas the Daihatsu CB90 engine of the same time period is much happier being lugged with its balanced shafts.

For heavy duty towing, I have always preferred the inherent smoothness of the straight 6 configuration if the engine has 7 large bearings. I worked with an old 70's Dodge pickup with the venerable slant 6. We lost first gear, and for about half a year, we would start out in second gear using the the column shifter. The engine seemed happy to pull our 3500 pound trailer, though the second gear starts wore out the clutch pack. Even after such abuse, the engine ran for a couple more decades.

I still have straight sixes in my current Dodge trucks, but of course, they are both diesels. They are designed to be "lugged" from the get go.

You can try reclutching it. This is where you press the clutch and rev higher and try engaging it again.

For off road vehicles you have a low range and most of those can be modded for even more reduction. In the event you stall on a sticky situation many 4wd have a clutch over ride switch so you can start in gear and roll off.

Automatics "reclutch' by unlocking the torque converter and using the 2 to 1 ratio of the converter on top of selecting a lower ratio from the gearing. Many cases they use the knock sensor on top of the MAP to do this in relation to speed.

When I would use cheap gas in my supercharged tacoma it loved to change gears. When I ran 93 octane with a bottle of 104 on top of it once it got to over drive it stayed there and mashing the throttle just added BOOST. :eek:

I remember being told that lugging the motor pounds the crank bearings (especially the mains?). This is 70's era knowledge though.

As soon as my current tires wear out I'm going up 4.3% in tire diameter and my biggest concern is that, in situations where I used to be able to be in 5th gear, I may now have to be in 4th to avoid lugging the motor. My hope is that the benefit from highway driving offsets this situation. I'm more often in a speed range where I can comfortably be in 5th so I think it's worth a shot.