2500 rpms in any water-cooled engine is not lugging.
The dangers of lugging your engine
- Thread starter rationull
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Depends on what definition of lugging we go with. If we go with Black Bart's above, then 2500 RPM could be lugging depending on how much load is on the engine.
Technically speaking your engine CAN be lugging at 5000 rpm.
Any time you go WOT and the engine can not gain rpm it is lugging.
Will it damage the engine probably not it depends on the design of the engine how well it is tuned how good is the fuel their are a lot of variables.
At very low rpm you can damage the bearings this is the type lugging most think of when you talk about lugging but if at 4000 rpm and WOT you can not gain rpm the engine is lugging.
The oil temp will Begin to rise and the tip of the spark plugs will become extremely hot
If they get hot enough to cause pre-Ignition it will cause catastrophic failure.
Will this happen??? Probably not but Technically speaking the engine is lugging so if you want to get the maximum engine life you would downshift rather than remain at WOT when the engine can not gain rpm.
This is true of ALL engine regardless of size or number of cylinders but the small econo-boxes are more likely to be lugging on a hill due to the low torque output of a 1.8 as opposed to a 5.7 or larger
Any time you go WOT and the engine can not gain rpm it is lugging.
Will it damage the engine probably not it depends on the design of the engine how well it is tuned how good is the fuel their are a lot of variables.
At very low rpm you can damage the bearings this is the type lugging most think of when you talk about lugging but if at 4000 rpm and WOT you can not gain rpm the engine is lugging.
The oil temp will Begin to rise and the tip of the spark plugs will become extremely hot
If they get hot enough to cause pre-Ignition it will cause catastrophic failure.
Will this happen??? Probably not but Technically speaking the engine is lugging so if you want to get the maximum engine life you would downshift rather than remain at WOT when the engine can not gain rpm.
This is true of ALL engine regardless of size or number of cylinders but the small econo-boxes are more likely to be lugging on a hill due to the low torque output of a 1.8 as opposed to a 5.7 or larger
The engine does not know whether it is speeding up or slowing down. Bearing pressures will be identical in either case for a given throttle setting and rpm. The only difference is that if you are speeding up, you will stay in the lugging range for a shorter period of time.
I am skeptical that one can tell from the way an engine feels whether it is lugging or not. Vibrations are from power pulses and their interaction with the engine mounting systems. You could have heavy lugging-type vibrations but still have relatively low stress in the bearings. You could also have very smooth running, for example in a straight six, where the power pulses are evenly spaced and overlap, but be damaging the engine from lugging. I don't think there is any reliable way for the driver to tell what is happening in the bearings.
I am skeptical that one can tell from the way an engine feels whether it is lugging or not. Vibrations are from power pulses and their interaction with the engine mounting systems. You could have heavy lugging-type vibrations but still have relatively low stress in the bearings. You could also have very smooth running, for example in a straight six, where the power pulses are evenly spaced and overlap, but be damaging the engine from lugging. I don't think there is any reliable way for the driver to tell what is happening in the bearings.
The engine may not know it's speeding up or slowing down, but for a certain throttle setting, rpm, and load combination the engine can either make enough power to speed up or maintain speed or it cannot. It sounds like by Black Bart's definition of lugging, you are lugging if the engine cannot make enough power to speed up, regardless of throttle position, at a certain rpm and load combination. Presumably bearing pressures are greater under this condition.
EDIT: To clarify, I'm just trying to reason about this, not say anyone's wrong.
EDIT: To clarify, I'm just trying to reason about this, not say anyone's wrong.
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Whether it can make enough power to speed up, or can not, does not matter, which was really my point. I know it "feels" different, but it is our mistaken human impression that the difference we think we sense as a driver translates into an actual difference inside the bearings.
Certainly at a given engine speed+load combination having the throttle wide open but not being able to speed up indicates you're making as much power as you possibly can at that engine speed, right? So it is fair at least to say that you're stressing the engine as much as possible in those cases? (Whether you call that lugging or not is a different issue)
Yes, that's true. But again, you can be stressing the engine as much as possible and be accelerating, or decelerating (because of a hill or whatever), and the engine doesn't care about the difference.
The other thing is whether or not it is what we think of as "lugging." At top speed, for example, you are certainly stressing the engine, and you are obviously at WOT, but very few people would call that lugging. Lugging is a term that is normally reserved for over-stressing the bearings by being in too low a gear (too low rpm) for conditions. As I've written here before, the important thing to realize about lugging is this:
Hp = torque x rpm
To make a certain amount of hp, say the amount required to go up a certain hill at a certain speed, if you cut the rpm in half you double the required torque. So at a given load, torque goes up as rpm goes down, or as you pick a higher gear. Also, oil pressure and flow goes down as rpm goes down. So to go up this hill, or whatever, at a lower rpm in a higher gear, increases the torque the engine has to put out, at the same time as it decreases the oil flow to the bearing. At some point the torque being applied will exceed the ability of the oil film to protect the bearing, and I would think that would be the point at which you would define it as lugging the engine.
The point at which that would happen would depend on the oil viscosity, temp, rpm, load and probably more, and obviously every engine would behave differently, since the bearings themselves would be designed differently, as would the oil pumps and everything else.
In general I think the best approach is to avoid heavy throttle at low rpm, especially for long periods of time. Accelerating from a stop under heavy throttle in first gear while engaging the clutch early would be just as hard on the engine as trying to go up a hill in full throttle in top gear at low speed, but for a much shorter period of time, and that is enough of a difference to make the former okay and the latter a bad idea.
Obviously there would be complications, for example detonation, engine management software that might react differently under different conditions, and who knows what. But mostly it's about the physics of combustion pressures and oil.
The other thing is whether or not it is what we think of as "lugging." At top speed, for example, you are certainly stressing the engine, and you are obviously at WOT, but very few people would call that lugging. Lugging is a term that is normally reserved for over-stressing the bearings by being in too low a gear (too low rpm) for conditions. As I've written here before, the important thing to realize about lugging is this:
Hp = torque x rpm
To make a certain amount of hp, say the amount required to go up a certain hill at a certain speed, if you cut the rpm in half you double the required torque. So at a given load, torque goes up as rpm goes down, or as you pick a higher gear. Also, oil pressure and flow goes down as rpm goes down. So to go up this hill, or whatever, at a lower rpm in a higher gear, increases the torque the engine has to put out, at the same time as it decreases the oil flow to the bearing. At some point the torque being applied will exceed the ability of the oil film to protect the bearing, and I would think that would be the point at which you would define it as lugging the engine.
The point at which that would happen would depend on the oil viscosity, temp, rpm, load and probably more, and obviously every engine would behave differently, since the bearings themselves would be designed differently, as would the oil pumps and everything else.
In general I think the best approach is to avoid heavy throttle at low rpm, especially for long periods of time. Accelerating from a stop under heavy throttle in first gear while engaging the clutch early would be just as hard on the engine as trying to go up a hill in full throttle in top gear at low speed, but for a much shorter period of time, and that is enough of a difference to make the former okay and the latter a bad idea.
Obviously there would be complications, for example detonation, engine management software that might react differently under different conditions, and who knows what. But mostly it's about the physics of combustion pressures and oil.
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Thanks for all that detail! I think I'm starting to understand it better. What you've written implies the following:
1) The problem isn't heavy throttle, but high torque load at low engine speed. Thus, climbing a hill or whatever at WOT at 2500 RPM is going to stress the engine but not cause lubrication problems for the bearings (realistically, but depending on engine design).
2) Because the cause of the lubrication problems is engine speed, the problem isn't necessarily more likely in low torque engines than in high torque engines.
For instance, suppose you have a two vehicles of the same weight/shape, one with a small gasoline engine and the other with a small turbo diesel with similar bearing and oiling system design (possibly not a realistic scenario). Both are attempting to climb the same hill at 1000 RPM in top gear at the same road speed. Suppose both engines have the ability to produce the required torque to climb the hill, but the gasoline engine is at WOT while the diesel is at half throttle. Since both are producing the same torque, both would be lugging despite the difference in throttle application.
Actually, the above implies a torque anemic engine is probably less likely to incur damage from lugging because it won't be able to pull itself up hills even at safe engine speeds, thus necessitating a downshift sooner than an engine that produces a lot of low end torque.
It also implies that cruising on the flat at a low engine speed is safe even in small engines, because of the very low hp requirements of such cruising.
1) The problem isn't heavy throttle, but high torque load at low engine speed. Thus, climbing a hill or whatever at WOT at 2500 RPM is going to stress the engine but not cause lubrication problems for the bearings (realistically, but depending on engine design).
2) Because the cause of the lubrication problems is engine speed, the problem isn't necessarily more likely in low torque engines than in high torque engines.
For instance, suppose you have a two vehicles of the same weight/shape, one with a small gasoline engine and the other with a small turbo diesel with similar bearing and oiling system design (possibly not a realistic scenario). Both are attempting to climb the same hill at 1000 RPM in top gear at the same road speed. Suppose both engines have the ability to produce the required torque to climb the hill, but the gasoline engine is at WOT while the diesel is at half throttle. Since both are producing the same torque, both would be lugging despite the difference in throttle application.
Actually, the above implies a torque anemic engine is probably less likely to incur damage from lugging because it won't be able to pull itself up hills even at safe engine speeds, thus necessitating a downshift sooner than an engine that produces a lot of low end torque.
It also implies that cruising on the flat at a low engine speed is safe even in small engines, because of the very low hp requirements of such cruising.
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Originally Posted By: rationull
1) The problem isn't heavy throttle, but high torque load at low engine speed. Thus, climbing a hill or whatever at WOT at 2500 RPM is going to stress the engine but not cause lubrication problems for the bearings (realistically, but depending on engine design).
I would say that is exactly correct. To some degree what you call "lugging" might be a matter of semantics, but heavy throttle at a normal running rpm is among the things the engine is designed for, and the oiling system should be able to handle those stresses without problems.
Originally Posted By: rationull
2) Because the cause of the lubrication problems is engine speed, the problem isn't necessarily more likely in low torque engines than in high torque engines.
Both the torque and the oiling system will determine the rpm at which stress exceeds the ability of the engine to handle it without damage. I'd expect that a high torque engine would be designed to handle the higher bearing forces that the torque output would make it capable of, with larger bearings or higher-flow oiling, but that really depends on how the engineers have done their job. What you say in the last three paragraphs is all true, I believe, but it all depends on the engineering decisions made in the engine's design. You could make an engine relatively lug-proof, or highly susceptible to damage from lugging, regardless of whether it's a high- or low-torque engine, diesel or gas, four or eight cylinder, or whatever. In reality, I really don't know how it plays out in actual real-world engines. It has always felt to me that low-torque four-cylinders are very susceptible to lugging, and that the straight-sixes that I prefer are almost lug-proof, but it could very well be that I'm being deluded by faulty impressions of what is actually happening inside the engine. It is for all those reasons that I say I'm not sure a driver can be a good judge of whether an engine is lugging harmfully, or not, because the cues of sound and vibration and perceived engine stress that we use as drivers might not be giving us good information. Obviously it seems like a good idea to avoid the type of operation that gives us those cues, but I often wonder with my car (straight six), which feels just fine going up a hill in fifth at 1200rpm and heavy throttle, whether I am lugging it harmfully even though it is not giving me those cues.
1) The problem isn't heavy throttle, but high torque load at low engine speed. Thus, climbing a hill or whatever at WOT at 2500 RPM is going to stress the engine but not cause lubrication problems for the bearings (realistically, but depending on engine design).
I would say that is exactly correct. To some degree what you call "lugging" might be a matter of semantics, but heavy throttle at a normal running rpm is among the things the engine is designed for, and the oiling system should be able to handle those stresses without problems.
Originally Posted By: rationull
2) Because the cause of the lubrication problems is engine speed, the problem isn't necessarily more likely in low torque engines than in high torque engines.
Both the torque and the oiling system will determine the rpm at which stress exceeds the ability of the engine to handle it without damage. I'd expect that a high torque engine would be designed to handle the higher bearing forces that the torque output would make it capable of, with larger bearings or higher-flow oiling, but that really depends on how the engineers have done their job. What you say in the last three paragraphs is all true, I believe, but it all depends on the engineering decisions made in the engine's design. You could make an engine relatively lug-proof, or highly susceptible to damage from lugging, regardless of whether it's a high- or low-torque engine, diesel or gas, four or eight cylinder, or whatever. In reality, I really don't know how it plays out in actual real-world engines. It has always felt to me that low-torque four-cylinders are very susceptible to lugging, and that the straight-sixes that I prefer are almost lug-proof, but it could very well be that I'm being deluded by faulty impressions of what is actually happening inside the engine. It is for all those reasons that I say I'm not sure a driver can be a good judge of whether an engine is lugging harmfully, or not, because the cues of sound and vibration and perceived engine stress that we use as drivers might not be giving us good information. Obviously it seems like a good idea to avoid the type of operation that gives us those cues, but I often wonder with my car (straight six), which feels just fine going up a hill in fifth at 1200rpm and heavy throttle, whether I am lugging it harmfully even though it is not giving me those cues.
Thanks again for all the feedback, it's cleared a lot up for me. I know what you mean about straight sixes. I used to drive a 280zx with a 2.8 liter I6, great engines. It would seem logical for them to commonly be more "lug resistant" than other engines, I suppose, given that the power output is being spread over more main bearings than an I4/V8 or a V6.
I remember reading somewhere that the forces on the bearings went up by a factor of the rpms squared..... I'm not saying this right, brain cells aren't working. But if you graphed it the low end of the curve would suddenly go way up when lugging occurs, even higher than what it is at high rpms. It is all about stress.
Doesn't make sense to me that the bearing stress would go up with higher engine speeds since higher speeds don't always correspond to higher power or torque output (but maybe I'm just misunderstanding). Maybe you're thinking of wear to the rings and cylinder bores? Or maybe even wear on the bearings, but not force?
Originally Posted By: John K
I remember reading somewhere that the forces on the bearings went up by a factor of the rpms squared..... I'm not saying this right, brain cells aren't working. But if you graphed it the low end of the curve would suddenly go way up when lugging occurs, even higher than what it is at high rpms. It is all about stress.
There is a very important distinction to be made between rotational forces, which like almost all circular motion are proportional to the square of the speed (for example cornering force = v^2/r); and combustion forces, which are proportional to combustion pressure. Both forces exist at any given time in an engine and both have different effects and can even act in opposite directions (for example, for a short time after ignition at high advance, combustion forces will be pushing down on the piston while rotational forces will be pulling it up). At low rpm, rotational forces are low but combustion forces can be very high. At high rpm rotational forces are much higher but combustion forces are not that high.
I remember reading somewhere that the forces on the bearings went up by a factor of the rpms squared..... I'm not saying this right, brain cells aren't working. But if you graphed it the low end of the curve would suddenly go way up when lugging occurs, even higher than what it is at high rpms. It is all about stress.
There is a very important distinction to be made between rotational forces, which like almost all circular motion are proportional to the square of the speed (for example cornering force = v^2/r); and combustion forces, which are proportional to combustion pressure. Both forces exist at any given time in an engine and both have different effects and can even act in opposite directions (for example, for a short time after ignition at high advance, combustion forces will be pushing down on the piston while rotational forces will be pulling it up). At low rpm, rotational forces are low but combustion forces can be very high. At high rpm rotational forces are much higher but combustion forces are not that high.
I should mention that combustion forces are linked to torque and not to power. Peak combustion forces are always at lower rpm than peak power, and drop off as peak power is approached. The reason you get more power is that you have less energy provided per revolution, but more revolutions per second. This is the origin of the hp = torque*rpm/constant formula.
Every component in drivetrain has elasticity (and a bit of clearance). If force input is lower than the threshold level to overcome resistence at the end of the drivetrain the sum of the elasticity bounces (some of the) force back. This is lugging. Drivetrains elasticity is being worked-out twice, both ways alternatingly as force wave. This jams the designation.
This is why it tend to occur more at heavier gears and lower rpms. Higher rpm oscillation modes are further away form the drivetrain's natural elasticity modal (low frequency oscillation) so the extra safety reserved for the higher rpms where it could be most dangerous.
One could demolish a building by sound frequencies... Every oscillation has to be proved against resonance possibilities for long term operational compatibility. Prooving is much harder on 6 and 8 cyl. engines. These are harder to lug repectively because in-a-way some of the job done by the mechanical engineering on behalf of the operator by balancing out more cylinders and quantatively much more probable combustion sequences. By the way, equal combustion timing has nothing to do with lugging directly. At times unequal sequences are even preferred, like spacings of the cooling fan blades. Same for the quantitiy of the main bearings, just don't space them at peaks of the sine vaves, bearings does not stop the force waves.
Resonance frequenciences of complex bodies are not I-0, they gradually phase in and out, so narrower operational ranges benefitical. On a good design imperceptible lugging is not present, however on trucks and busses this "good design" may not be possible because of the greatly varying load.
This is why it tend to occur more at heavier gears and lower rpms. Higher rpm oscillation modes are further away form the drivetrain's natural elasticity modal (low frequency oscillation) so the extra safety reserved for the higher rpms where it could be most dangerous.
One could demolish a building by sound frequencies... Every oscillation has to be proved against resonance possibilities for long term operational compatibility. Prooving is much harder on 6 and 8 cyl. engines. These are harder to lug repectively because in-a-way some of the job done by the mechanical engineering on behalf of the operator by balancing out more cylinders and quantatively much more probable combustion sequences. By the way, equal combustion timing has nothing to do with lugging directly. At times unequal sequences are even preferred, like spacings of the cooling fan blades. Same for the quantitiy of the main bearings, just don't space them at peaks of the sine vaves, bearings does not stop the force waves.
Resonance frequenciences of complex bodies are not I-0, they gradually phase in and out, so narrower operational ranges benefitical. On a good design imperceptible lugging is not present, however on trucks and busses this "good design" may not be possible because of the greatly varying load.
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At higher engine speeds one typcially has better oil pressure and assumed flow, and provided the oil isn't too light due to viscosity choice or excesive heat. Turbos do fine provided there is enough oil flowing. At low speeds, especially with higher compression negines like a diesel there can be 'issues.
We had a rather lively discussion about lugging over at http://www.fordsix.com sometime back.
Link:
http://fordsix.com/forum/viewtopic.php?t=34563&highlight=lugging
Link:
http://fordsix.com/forum/viewtopic.php?t=34563&highlight=lugging
Thanks! I'll have to read that after work tonight. BITOG (and the internet in general) really are unmatched in their ability to help people to easily feed their obsessions!
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