Lugging a diesel engine?

[sixdogs]

I cut and pasted this info below from another thread regarding lugging a diesel engine and would like to get more feedback about it. We are very careful about "lugging" an engine and we all know what serious lugging is and sounds like. However, could some of us be lugging an engine and not know it? Are there degrees of lugging? Any more thoughts on this?


"......There is one firm rule that every equipment operator and truck driver memorizes on their first day: NEVER LUG A DIESEL ENGINE! With the throttle wide open under load, keep the RPMs above 80% of the maximum RPMs. For example, if your maximum engine RPM is 2600, never let it lug below 2000. When you get below that you will drop off the power curve anyway, and lugging a diesel will hammer the rod bearings right out of the engine....."

 

[Soundguy]

if the engine is laboring at a certain load and throttle.. increase the throttle till it is smooth. If you get to a point that increasing the throttle does not increase rpm.. you are lugging it.

soundguy

 

[donn12]

I would also like more info...I thought I understood but maybe not.

 

[Builder]

I cut and pasted this info below from another thread regarding lugging a diesel engine and would like to get more feedback about it. We are very careful about "lugging" an engine and we all know what serious lugging is and sounds like. However, could some of us be lugging an engine and not know it? Are there degrees of lugging? Any more thoughts on this?


"......There is one firm rule that every equipment operator and truck driver memorizes on their first day: NEVER LUG A DIESEL ENGINE! With the throttle wide open under load, keep the RPMs above 80% of the maximum RPMs. For example, if your maximum engine RPM is 2600, never let it lug below 2000. When you get below that you will drop off the power curve anyway, and lugging a diesel will hammer the rod bearings right out of the engine....."

I let my diesel truck engines drop below 2000 RPM all the time. Lugging creates excessive EGT's and is not good for your engine. You'll know when you're doing it. Engine gets loud, rough, might see black smoke, no throttle response. Drop a gear and see if it clears-up.

 

[pat32rf]

90% of the time my JD4300 is running between 1500-2000rpm and not under "load". Just carrying a bucket of something or dragging logs. About the only time it see's 2600 rpm is when I'm road running, mowing or snow blowing. (well, maybe to climb a steep hill as well)
If the rpm starts to drop, I'll either lift the "go-petal" or increase the throttle till I crest the hill.
Its worked for me for the past 4000hrs...

 

[Chilly807]

"Lugging" any engine is not good for it, however I don't buy into the 80 % rule. Provided the engine is providing smooth power delivery and isn't on the verge of stalling you're not doing any damage.

Case in point: Some of the stuff I work on is multi-cylinder, high horsepower, medium speed engines. The best example is a 20 cylinder engine fitted in a frigate as "economical" cruise alternative to gas turbines. It develops about 10,000 horsepower at 1050 rpm maximum. Because of the gearbox configuration it runs under load anywhere from 400 to 1050 rpm depending on the speed demand. Most of the ones we have are between 12,000 and 24,000 hours into their life cycles, so I'd say things are ok in general.

If you drop below idle speed under load, that's not good. As I said before, if it's running smoothly and not labouring, you're not hurting it. As Soundguy said, if pressing the accelerator doesn't produce an increase in speed you're either at the top end of the governed speed range or you're on the verge of being overloaded. Drop a gear or otherwise reduce the load.

A diesel doesn't require an increase in "throttle" setting to deal with an increased load, the governor deals with that. There is no "throttle" as such, just a governor that tries to maintain a given engine speed within the power limits of the engine. In fact, there are no throttle plates at all, the engine has full air supply at all times, only the amount of fuel being delivered varies.

Sean

 

[First one]

While de-rocking my blueberry field, my tractor's RPM's would die back while trying to get out big rocks ,the bucket wouldn't even move, I'd let off and the RPM's would come back up .I have the L 40 series with load sensing, so does this prevent any damage to it because the engine won't stall? or should I not be forceing it that hard?

 

[SPYDERLK]

"Lugging" any engine is not good for it, however I don't buy into the 80 % rule. Provided the engine is providing smooth power delivery and isn't on the verge of stalling you're not doing any damage.
If you drop below idle speed under load, that's not good. As I said before, if it's running smoothly and not labouring, you're not hurting it. As Soundguy said, if pressing the accelerator doesn't produce an increase in speed you're either at the top end of the governed speed range [or you're on the verge of being overloaded]. Drop a gear or otherwise reduce the load.
Sean

While I agree with most of what youve said one cannot generalize to term loss of rpm at full throttle to be lugging. Lugging is rpm and throttle related. When running at full rpm setting and load causes rpm to fall below that where maximum torque occurs then it is time for a downshift ... soon if load is not being reduced. Altho, as you said, the engine is not lugging until rpm falls well below that.
larry

 

[Chilly807]

I'm going to go a little deeper into this one, I hope it'll clarify things. Diesels and governor operation are different enough from gas engines typically found in cars or trucks that I think it's worth digging deeper.

There are several different types of governor available to the engine manufacturers. Cost and accuracy are directly related, as is the degree of complexity.

Electronic and hydraulic governors have a very high degree of accuracy, usually within 1 to 2 rpm speed variance under all load conditions within the capacity of the engine. They are normally only seen in power generation units where that kind of accuracy is needed, or in the case of the hydraulic controls where more output force is needed to control the fuel rack (as in the case of the frigate engine I mentioned earlier, which is electronic control over a hydraulic actuator.)

In our world of tractor and automotive type diesels, we don't need that kind of accuracy in speed control. PTO speed isn't that critical even when running a PTO generator, unless you want to run co-generation with the electric utility, which is highly unlikely.

So, we typically have a mechanical flyweight type governor. For anything a tractor was intended to do they are perfectly suitable, and far more cost-effective.

They do have one inherent characteristic that impacts on this discussion, in that they will reduce the speed setting as load on the engine increases. This isn't a design goal, it's a limitation of that particular governor type.

So for example, imagine you are running a brush hog at a steady 2300 rpm engine speed, over a sparsely grown field. Near the edge of the field is a heavily weeded, densely grown area. As the tractor and brush hog moves into this area, the rpm drops slightly as the load on the engine increases.

I'm not talking about an initial drop than recovery back to 2300 rpm, I mean it drops down to maybe 2100-2200 and stays there until you come back into the lightly grown areas and the load on the engine decreases. The governor still has control over engine speed, and the engine isn't overloaded.

It's hard to see unless you're using the throttle lever and not a foot pedal, our natural reaction from years of operating cars or trucks is to press harder on the pedal to bring the speed back up.

This doesn't mean the engine is "lugging" or overloaded, just that the governor is doing it's job as best it can within it's mechanical limitations. With a hydraulic or electronic governor (set for "0" speed droop) you would see no drop in rpm, just a deepening of the engine exhaust note as more fuel was added to compensate for the increase in load. Exhaust temperature, turbo speed, and cylinder pressure also rises.

Lugging occurs when the ignited fuel mixture has no where to go, the piston is moving too slowly to allow the combustion gases to expand at their normal rate. You typically get jerky power delivery, a loud fuel knock from the engine, and very high forces on the con rods and bearings. To make matters worse, the engine is turning slowly and has a harder time maintaining an effective oil wedge in the bearings to carry the load.

Sean

 

[SPYDERLK]

I'm going to go a little deeper into this one, I hope it'll clarify things.

[SNIP]

Lugging occurs when the ignited fuel mixture has no where to go, the piston is moving too slowly to allow the combustion gases to expand at their normal rate. You typically get jerky power delivery, a loud fuel knock from the engine, and very high forces on the con rods and bearings. To make matters worse, the engine is turning slowly and has a harder time maintaining an effective oil wedge in the bearings to carry the load.

Sean

A very good treatment of the subject. Thank you!

Our L2550 has a hyper active governor that acts close to the zero droop you describe ... esp when the engine is set to full speed. That speed is held quite close throughout large load changes. Could it have to do with a good fueling curve .. good governor mechanics .. both.. ? I wish all my tractors had so good a torque / power response characteristic.
larry

 

[RoyJackson]

90% of the time my JD4300 is running between 1500-2000rpm and not under "load"...

Yep, run about the same here. That's where the hand throttle is set and I'll use the foot throttle for upping the RPM, when necessary.
Even with no load, I idle the engine at 1500 RPM...never below 1000.

I'm not enthusiastic about the "80%" rule" either, although the quote (quoted from another thread or forum) may have made a lot of sense in it's original context.

 

[milkman]

In trucking, we were told by Cat, Cummins, and Detroit that they were meant to pull down to 1000 RPM before downshifting, that's for the 4 stroke diesel, not the old 2 stroke Detroits. All the fleet trucks were driven that way and I'm not aware of any bottom end bearing problems. In 2,000,000 on the two that I owned, neither Detroit had an engine bolt touched. They max out at 2100 or 2200 RPM so that's more that 50% rpm drop. If they pulled down to 1100 RPM and were able to maintain that, then they run that till the end of the pull.

 

[RalphVa]

Lugging or the "lug" point depends on the engine size, hp & torque. We had a Fiat 124S sedan once. You didn't run its engine below about 2,000 rpm. It was only 1.4 liter. At the same time, I had a Triumph TR-3B. The engine on that car was from a tractor. It would do fine down to around 1,500 rpm (and quit revving at 4,500). On my VW Cabrio with 2.0 liter engine, the minimum rpm (stated in the owners manual and from operating experience) on it is 1,500. On our 2010 Jetta TDI, it will do fine down to around 1,300-1,400 rpm. It also has a 2.0 liter engine.

My 4010's lug point seems to be around 1,500 rpm in most cases. Going up hill and with a bit of load, the minimum for it to not strain or run unevenly is around 1,800 rpm. Running the bush hog in heavy grass? You better be at PTO speed, about 2,600.

Lugging is the point at which the engine appears to strain or possibly run unevenly. In the case of the Fiat, you didn't lug it because the oil pump just plain wouldn't put out enough oil flow/pressure to keep the bearings from not being fully lubed.

My grandfather once had a Plymouth V8 with 3 speed stick shift. He frequently drove it like the proverbial Indian or Singapore taxicab driver. 1,2,3 and maybe 10-15 mph he's in high gear. Dad took over the car and almost immediately had to replace main bearings on the engine. Grandpa's lugging had ruined the bearings due to too low oil flow.

Ralph

 

[SPYDERLK]

Lugging or the "lug" point depends on the engine

My 4010's lug point seems to be around 1,500 rpm in most cases. Going up hill and with a bit of load, the minimum for it to not strain or run unevenly is around 1,800 rpm. Running the bush hog in heavy grass? You better be at PTO speed, about 2,600.
Ralph

That is unusually high for a low reving engine. I would question the condition of an engine that acted thusly.
larry

 

[Eagleview]

Diesel engines in most applications use the engines torque produced down low and not so much the horsepower produced higher in the rev range . My New Holland TL100A produces maximum torque at 1400 revs and that's where I keep it when working hard . I use the 1000 rpm PTO position and dial the engine back too 1400 which gives me around 540 at the PTO . I would suggest you look at your tractor specs and see where it's maximum torque is made and run it there . Obviously if you only have a single speed PTO you have no choice in the matter .

 

[MicroPilot]

Isn't the idea of running the engine RPM above the torque band when using the PTO, so that when the engine gets a load it falls back into the max. torque band and has a chance to "power through" the load? If you run in the max. torque RPM band, if the engine gets loaded it has no "reserver" to fall back on.

 

[Eagleview]

You are correct when talking about a tractor doing extremely hard work .

For the type of work I do and that of hobby farmers etc , I see no need to run an engine at wide open throttle . If you look at this Dyno chart , it shows an engine's horsepower and torque curves . As you can see this particular engine can be run at 1700 rpm before the horsepower starts to drop off which is only about 10hp down from full throttle , but looking at the torque curve the engine will lug all the way back too 1200 rpm and is still making 800 ft/lb of torque which will carry it through a tough patch of grass etc. Torque = twisting force on a shaft and is what does the work , Horsepower figures in my opinion just sell tractors .

 

[irvingj]

I might be way off base here, but the OP's quote... that was referring to over-the-road trucks, yes? Aren't they just about all turbo-charged? Other than the oil issue, could that be the difference?

I know on my Jetta TDI, it's recommended the you "drive it like you stole it" --that is, much higher shift points/higher rpms than what I've been accustomed to-- to keep the turbo from carboning up.

For someone who prefers high-torque/low-rev engines, that took a bit of getting used to!

 

[sixdogs]

I might be way off base here, but the OP's quote... that was referring to over-the-road trucks, yes? Aren't they just about all turbo-charged? Other than the oil issue, could that be the difference?

The reference was to CUT and utility sized ag tractors. While I never purposely "lug" an engine as described here, I often just use the foot throttle since the work is light and there is no chance of lugging in the obvious sense. But should I be doing this?
Should I instead set the hand throttle at a higher RPM even though I have no need for the power created? Are there degrees of unintentional lugging even though it does not appear that I am "lugging" in the textbook sence?

 

[PHILIP8N]

You are correct when talking about a tractor doing extremely hard work .

For the type of work I do and that of hobby farmers etc , I see no need to run an engine at wide open throttle . If you look at this Dyno chart , it shows an engine's horsepower and torque curves . As you can see this particular engine can be run at 1700 rpm before the horsepower starts to drop off which is only about 10hp down from full throttle , but looking at the torque curve the engine will lug all the way back too 1200 rpm and is still making 800 ft/lb of torque which will carry it through a tough patch of grass etc. Torque = twisting force on a shaft and is what does the work , Horsepower figures in my opinion just sell tractors .

Better check that torue curve, something is way off, numbers dont add up. What engine is that for? Philip.