The dangers of lugging your engine

[rationull]

I'm a software guy and I'll be the first to admit that I don't have any experience with ICEs aside from driving and simple maintenance (fluid changes, fuel injector replacement, etc). The downsides to over-revving an engine are pretty clear IMO; higher speed comes with higher ring and bore wear, and if you run your engine way too fast you can get valve float and valve contact w/ the pistons (ignoring rev limiters).

But I've always been cautioned to avoid lugging an engine and that's given be a little bit of irrational fear of running the engine at too low a speed. I've got some understanding of the reasoning behind this but there are lots of holes. In my detail-obsessive post of the week, I'd like to clear some of those up. What are the problems connected with running an engine at too low a speed, and how exactly does the term "lugging" apply?

One clear problem is that there's a point at which the engine is spinning just fast enough to maintain adequate oil pressure. This point must vary based on engine/oil pump design, and to run the engine below that point would cause lubrication problems. It's not clear to me that this point should have too much correlation with displacement, so presumably it would be close to the same whether you're talking about a 6 liter V8 or a 1.5 liter 4 banger. Correct?

Depending on engine displacement and tuning, every engine has a unique torque curve. Thus, depending on the weight and shape of the vehicle surrounding it, every engine has a range in every gear where it's got to "work harder" (i.e. requires excessive throttle opening) to maintain speed or accelerate. Below that point the vehicle will slow down despite a wide throttle opening. Because of common torque curve characteristics, this point will tend to be lower on larger displacement engines and higher on smaller ones (I realize I'm generalizing here). Thus a heavy Suburban with a 454 ci can cruise along at 2000 rpm at highway speed while a Civic with a 1.8 liter cruises around 2800-3000 rpm instead. But what consequences does attempting to cruise at a lower engine speed have? Is that civic engine being damaged trying to pull up a hill on the highway at 2500 RPM at 60 mph? If lubrication is OK, will this harm the bearings, rods, and other load-bearing components? Clearly this is "running the engine hard" but is it really harmful?

Wow, this turned out longer than I thought it would...

In short, what exactly does "lugging" mean, and what damage does running an engine at higher load/lower rpm actually do?

 

[01rangerxl]

From what I have heard, lugging is really hard on the connecting rods.

 

[GROUCHO MARX]

I wouldn't worry about any mph at 2500 rpm.

 

[Quest]

lugging puts the most stress on the bottom end of your engine: from con rod bearings to cranks (although crank bearings usually can take a lot more beating than that of the con rods).

 

[Drew99GT]

Lugging is like when you come to a stop light and you end up going about 2 mph, and instead of putting it 1st, you go to second. RPMs drop to say, 400 or 500 and when you give it gas, it makes a knocking sound.

Or, like how a guy I used to work with drove our work trucks. Would hardly never go above 2K and would constantly end up below 1K rpms after a shift in higher gears. Motor sounded like a rod was gonna knock right out the side of the block.

2500 rpm? That's not lugging.

It also centers on what kind of transmission. You can't lug an engine with an auto, only a manual.

 

[Chris142]

The old diesels were designed to run @ 2100rpm which was usually the maximum rpm. Running one below 1700 or so fully loaded would make the replacable cylinder liners bounce around in the block. This bouncing would wear the block where the liners and the block fit together and cause coolant to leak into the oil pan.

Called "Sucked a liner" as in "Billy Bob was too lazy to downshift on that big hill and sucked a liner".

I consider lugging to be say under 1500 rpm , foot to the floor and losing speed. As long as your able to accelerate your not lugging it.

 

[rpn453]

I've always considered lugging to be running the engine at an rpm that is too low for the engine to run smoothly at. Obviously this varies by engine design and how much power you're giving it, but I wouldn't consider anything under 1500 rpm to be lugging on anything I've driven. I avoid giving my car any power below 1500 rpm in 4th and 5th, but it seems fine down to about 700 rpm in 1st, 1000 rpm in 2nd, and 1200 rpm in 3rd, though I generally go easy on the throttle anytime I'm below 1500 and not in 1st.

I'm in 5th gear running just over 1500 rpm every time I'm cruising steadily at 40 mph, and the engine doesn't seem to complain at all, so I guess I'll eventually find out if that's actually a problem!

 

[chrome]

Before the days of electronic ignition and knock control, this advice made a lot of sense as pulling the vehicle in too high a gear for the speed generally will result in engine knock. This doesn't happen (well, almost never) in cars with knock control.

Perhaps a good way to visualize "lugging", is picture an engine on a bench dyno, connected to a load. Run it to it's peak torque rpm for load x. This may require throttle opening of say, 70%. Now, increase the load to y, which will result in the rpm dropping. Open the throttle to compensate to maintain the original rpm. Keep increasing the load and very soon you will have 100% throttle and the rpms decreasing (load is greater than the torque output of the engine). Load it up till the engine speed drops to 1000rpm at 100% throttle.

What are the implications on the engine? The load on the piston, conrods, crank and bearings are proportional to the load applied to the engine. Perhaps the component getting it worst, would be the conrod bearings should the oil film separation fails at those loads.

 

[Black Bart]

While it is critical to maintain lubrication and this will very from engine to engine due to not only the oil pump but the size of the bearings the combination of bearing size and oil pressure must keep the bearings from contacting the crank
Technologically speaking lugging can be at any RPM

Lugging can accrue at higher rpm than what you think.
If you are at WOT and it don't accelerate it is lugging

If you are climbing a hill in a 4 cylinder and it is not able to accelerate in high gear the engine is lugging regardless of rpm
but normally no damage will accrue unless the rpm is low enough to make metal contact but technically any time an engine can not gain rpm it is lugging.

 

[Doug Hillary]

Hi,
with my Class 8 trucks (and in my Driver training programmes for Road Train drives etc) we classed "lugging as follows;

1) when the engine cannot be accellerated by applying full throttle and/or

2) When the pyrometer and coolant/oil temperatures were rising and oil pressure was less than normal for the revs

When all of those indicators were "stabilised" the engine was not under stress!

Modern (last 20 years) high speed diesel engines have been designed to hold revs longer and sometimes lugging is confused with using the engine's torque curve for the best result

This is due to a large degree to the engines technology, cooling and lubrications systems and their application specification.

Many of my trucks would run at 1200rpm fully loaded on say a 4% climb into a head wind for up to half an hour or more

MB diesels have long developed 80% of maximum torque at not much more than idle revs (800)

It is strictly a case with trucks as Chris142 has indicated of knowing how to use it correctly. In my case it was always driven by maximising fuel economy!

 

[Black Bart]

Originally Posted By: Doug Hillary
Hi,
with my Class 8 trucks (and in my Driver training programmes for Road Train drives etc) we classed "lugging as follows;

1) when the engine cannot be accellerated by applying full throttle and/or

2) When the pyrometer and coolant/oil temperatures were rising and oil pressure was less than normal for the revs

When all of those indicators were "stabilised" the engine was not under stress!

Modern (last 20 years) high speed diesel engines have been designed to hold revs longer and sometimes lugging is confused with using the engine's torque curve for the best result

This is due to a large degree to the engines technology, cooling and lubrications systems and their application specification.

Many of my trucks would run at 1200rpm fully loaded on say a 4% climb into a head wind for up to half an hour or more

MB diesels have long developed 80% of maximum torque at not much more than idle revs (800)

It is strictly a case with trucks as Chris142 has indicated of knowing how to use it correctly. In my case it was always driven by maximising fuel economy!
While everything you say is true you can not compare a modern turbo diesel to a civic.
One is designed to operate at heavy torque loads and the other is not. But as you point out even with the Cummins if it can not gain rpm it is considered lugging this is true of all engines.
Most believe that lugging can only accrue at a very low rpm

 

[Turk]

My Pick-up "Lugs" when it is in Overdrive and only going abouty 40 MPH. Worse is that condition & going up a hill!

So, most of the time, if I don't forget, will shift it down to 3rd Gear and the lugging will cease.

Hopefully it will allow the engine & tranny last longer.....

 

[Anduril]

I used to drive an air-cooled Volkswagen. Because of that design, "lugging" that engine was pretty much anything under 2,000 rpms. Because the only thing cooling that engine was air blowing over it from a large (engine-powered) fan, driving at too low of an engine speed posed a very real risk of overheating.

It turned about 4,000 at 75 mph on the highway, but people who changed their gearing to get lower engine speeds often got overheating as an unintentional consequence.

 

[rationull]

Thanks for all the replies.

So it sounds like the guideline is if the engine can accelerate smoothly then you're loading it but not lugging it, and the damage occurs when the load is too high for the lubrication system. Presumably the con-rod bearings etc are built to withstand being loaded but I guess theoretically driving for too long at high load could cause them to fail? Doesn't sound like something to worry about too much though.

One less I'm learning from thinking about this is that the speeds at which an engine can run comfortably doesn't really technically have anything to do with 4 cyl vs V8 or large vs small displacement, but with the torque curve of the engine and the weight it's carrying. Of course in practice there will be a correlation because typically highly loaded applications will have bigger engines. Does this sound correct?

And Anduril, if you're talking about a beetle, my hat's off to you. I've only driven one (a '74) and the suspension was as old as the car so it may not have been a good example but man, driving that thing at 75 (at least on the Los Angeles area highways) would've been pretty scary!

 

[Anduril]

Yeah, it was a '69 Beetle. I can guarantee you the suspension wasn't original, though.

 

[1sttruck]

One can see how an auto version of the vehicle shifts to get an idead on what the maker thinks is a good lower rpm. As an example I have a manual tranny on mine and watching other Dodge diesels with autos that friends have the auto seems to keep the RPMs above about 1500, which is close to the torque peak at around 1600 RPM. This means that I shift so that whan I try to catch the next gear it doesn't drop below 1500.

 

[bmph8ter]

Originally Posted By: Anduril
I used to drive an air-cooled Volkswagen. Because of that design, "lugging" that engine was pretty much anything under 2,000 rpms. Because the only thing cooling that engine was air blowing over it from a large (engine-powered) fan, driving at too low of an engine speed posed a very real risk of overheating.

It turned about 4,000 at 75 mph on the highway, but people who changed their gearing to get lower engine speeds often got overheating as an unintentional consequence.


I was actually going to post about the ACVW. Lugging the engine in one of those is probably one of the fastest ways to kill it. Huge load due to being out of the 'powerband'+no fan speed=you walk.

 

[1sttruck]

"I used to drive an air-cooled Volkswagen. Because of that design, "lugging" that engine was pretty much anything under 2,000 rpms. Because the only thing cooling that engine was air blowing over it from a large (engine-powered) fan, driving at too low of an engine speed posed a very real risk of overheating."

Use to have a 69 bug and use to work on them on the side for extra cash. The best thing for an older bug was a larger external oil cooler mounted on the fan housing and a full flow oil filter which added a qt to the meager sump capacity. The biggest problem with them seemed to be the fuel hose to the carb, which when old would leak, spray gas all over the engine, and result in someone being stranded with a nicely blackened engine compartment.

 

[John K]

Try riding a multi-speed bicycle up a hill in too high a gear and that is a good way to think of lugging. You'll kill your knees and have to downshift. Something has to give.

 

[rationull]

Understood. The main question being are the con-rods (unlike the knees) built to put up with this or should it be avoided at all costs? Not really talking about extreme conditions here of course, just the 2500 RPM hill climb type example.