"LUGGING?" - why not?

[R.E. Miller]

Just a reminder folks,

The cummins engines are designed to be operated within certian rpm ranges, more importantly, max torque is reached at different rpms for different engines. The electronic engines offer unique features with torque, and what rpm it comes on at. The owner's manual is your friend, an it is important to follow what they say about torque and rpm. Likewise, the trucks that are modified with fueling increases and different turbocharging, might be able to hit peak torque at different numbers than before; I would still recommend not going below the rpm that the manual states. Under load, there should be few excuses that would warrent running at WOT below 1600. I dyno cummins diesel engines on a chassis dyno at work, and never take them below 1700, you shouldn't need to unless the specs for that engine are otherwise. Finally, for the electronic guys, believe in the rpm that the manual tells you, because the ECM doesn't lie to the manual, and you will see that proven on both the dyno, and while pulling heavy loads. . 02 cents



Russell

 

[Diesel Freak]

Re: Engine Lug

Originally posted by HWhite

IMHO I feel that as long as the oil pressure is up, there's no way you can have any problem with engine comp.

you are quite mistaken.



it is very possible to have bearing to journal contact when lugging an engine, and it is quite apparent when the engine is torn down and the main and rod big end bearings are examined.

 

[speedracer]

I drive my truck between 12-1700rpm regularly, but that's unloaded and at light throttle. When loaded I rarely go below 1700rpm. I also rarely have to use full throttle.

 

[Steve M]

I like higher rpm when towing our 5th wheel. That's on my fourth Dodge Cummins I opted for another 4. 10 ratio. Above 2k rpm the engine is far less likely to lug and the transmission doesn't downshift.

The oil not only lubricates the bearing surfaces, it also carries heat away. Air not only burns the fuel, it carries heat away from the combustion chambers, heads and turbo. Higher rpm not only flows more air and oil in a period of time, it also lessens the time the connecting rods are pushing on each load of oil in the bearing journals during the power stroke.

1400rpm is great if the load allows it. 1100rpm is even better. Peak power is at 2900rpm in the '03. On the uphill grades I'll use peak HP even if O/D will pull it at 1750rpm. With my '95,'98 and '01. 5, noise was the only drawback. The '03 is quiet at peak HP full load.

Just my ever so humlbe opinion.

 

[Big_Daddy_T]

noise was the only drawback. The '03 is quiet at peak HP full load.

Drawback? I love to hear her sing no matter what time of day it is.

Revving mine a little higher has seemed to help me keep my transmission much longer than I had anticipated. It has 180k on it and some of that was around 500hp flywheel. I shifted manually the whole time and it had no problem as long as I kept the rpm up. It likes it best around 2200 judging by the response I got from it. So thats what I gave it. I can tell you that most should respond well to that rpm if you are torturing it. Good luck.

 

[GeneralMachine]

Re: Re: Engine Lug

Originally posted by Diesel Freak

you are quite mistaken.



it is very possible to have bearing to journal contact when lugging an engine, and it is quite apparent when the engine is torn down and the main and rod big end bearings are examined.

If The Oil Pressure Is Up And the oil is worth A I TRIED TO BY-PASS THE CUSSING FILTER It will never touch.

 

[JyRO]

Lugging...

Mine lugs every time I let off the throttle. Especially at idle. Use to at idle it would only run at 750 ~ 800 RPM. Man, it must've really been lugging there, from the load of the mass of the engine alone. So now I add Dawn concentrated dishwashing liquid to the engine oil, at a 50-50 ratio to the oil. Any time the engine's cranked, I hold my foot to the floor to make sure I have enough rpms to not lug. And the Dawn keeps the oil film strength up.



Lastly, I run mine in 5th gear instead of 6th, on the interstate to keep the RPM in the "more fuel efficient" range. Since I keep my right foot on the floor, once I've sped up too fast I just push in the clutch until it slows back down again.



I don't think the burn rate of fuel versus the speed of the piston has anything to do with lugging. Naaaw.



- JyRO

 

[Champane Flight]

Tell this

Tell this to the "Great smoke story" boys... .

 

[DocStressor]

I've been lurking on this interesting thread for a while. I don't normally chime in on these things unless I have something to add. Here goes:



1. Gary did a nice job of describing in non-technical terms how lugging affect engine wear. It accelerates main and connecting rod bearing and journal wear by promoting oil film break through. The result is a transformation from hydrodynamic (metals separated by an oil film) to boundary (metal to metal contact) lubrication conditions. The effect is greatest at low rpm, since oil film thickness increases with rpm.



2. Higher oil viscosity does afford increased protection under “lugging” conditions by maintaining better film thickness under load. The only point I disagree with is Gary’s contention that 5W-40 oils have lower film strength or thickness than 15W-40 oils. Actually, the viscosity of the oil under high temperature high shear (HT/HS) conditions is what matters. Film strength is simply a function of viscosity and 40 weight is 40 weight.



3. Drive train components don’t know that an engine is lugging. Gear lubes contain high amounts of extreme pressure additives and are designed to operate under mixed hydrodynamic/boundary conditions. The 5th gear nut problems is made worse by vibration, which can result from low speed operation, but it really doesn’t have anything to do with lubricant film thickness.



4. As several folks have pointed out, if you apply full throttle and cannot accelerate, you are lugging the engine. As you increase the pressure on the oil film, the viscosity increases greatly. While it may be hard to believe, the viscosity of the oil film exceeds that of the metal bearing material and the bearing distorts. On a large diesel engine, the metal distortion can be to the order of a couple of millimeters with an oil film thickness of maybe 1 micrometer (1/1000 of a millimeter). Distortion causes an increase in surface area and more friction and heat. If you continue to operate under these conditions, the heat accumulation causes the oil film to break down. Then you get the accelerated wear. That’s why Cummins advises against running at full throttle for more than 30 sec below maximum torque rpm. If you want your engine to last forever, don’t stomp on the go peddle until you hit say 1600 rpm. Above that engine speed the oil film is strong enough to better handle the load.



5. The oil pressure you read on the dash gauge is not the same as the thickness of the “oil wedge” in a rotating bearing. As long as you have enough oil pressure generated by the oil pump for oil to enter the bearing, rotational velocity and oil viscosity determine the film thickness.



Sorry for the long post. Here is a link to a semi-technical explanation of hydrodynamic lubrication - http://www.lubrizol.com/LubeTheory/theory.htm

 

[Hohn]

corrections, input, etc...

Lugging is much less a phenomenon of oil and lubrication dynamics than it is of the amount of load and how it is applied to the engine.



Follow me through here.



The VOLUME of oil delivered to bearing surfaces is supplied by an engine-driven pump, thus oil flow is directly proportional to RPM. That also means that for a given engine revolution, the same volume of oil will be pumped to the bearings.



Now when we add a load to those bearings, time becomes a factor. The longer time we ask the bearing to support a given load, the more oil volume it requires to do so. Unfortunately, oil volume goes DOWN with RPM, so this is not favorable.



So the ratio of load to time becomes more favorable as we increase RPM. Unfortunately, load increases exponentially from RPM, and the ratio soon becomes unfavorable again (pumping losses in the oil system also play a role).



But these scenarios are true whether you are "lugging" the engine or not. I guess what i am trying to say is that the lubrication element is AN ASPECT of lugging, but not the DEFINITION.



LSMITH's definition is the best, simple description of what lugging is.



Think of it this way: it takes a given amount of power (x) to move a given load (y). In a diesel, fuel is power. So to move a certain load, it takes a fixed quantity of fuel. We can deliever that fuel at varying RPMs, though. I can dump it all in at 1200RPM, or I can deliver it at 2400RPM. Logically, there is one half as much fuel per combustion event at 2400RPM than there is at 1200RPM. This has the effect of dividing the fuel into "bite sized" chunks for the engine to handle. Thus, 2400RPM is easier on the engine for MANY reasons:



1. More air relative to fuel means lower temps.

2. Engine harmonics shifted up- less strain of crank, rods, etc. .

3. The turbo boost increases the effect of #1.



It's also easier on the engine in a way that's hard to describe. Think of an impact wrench. It generates high torque using numerous, small, hammer-like blows. Imagine trying to generate the same torque on a lug nut all in one blow with a hammer on a wrench. Which do YOU think is harder on the fastener? Right-- less strain by dividing the torque generation into smaller units and distibuting it.



Higher RPM is NOT easier on the reciprocating assembly with little or no load. remember that engine load due to its own mass increases exponentially with RPM. .



The long and short of it is that it is almost impossible to lug an empty CTD in any gear but the top two. Also, you can run very low RPM safely if you are tender with the fuel pedal. If you are going to tow a big load (Especially you BOMBERs with COMP boxes on KILL), know that your engine would LOVE to have the RPM (air) to burn all that fuel.



Fuel=load on the engine More load (towing) means more fuel



It's just that simple





HOHN

 

[Pit Bull]

Re: corrections, input, etc...

Originally posted by Hohn



LSMITH's definition is the best, simple description of what lugging is.



The long and short of it is that it is almost impossible to lug an empty CTD in any gear but the top two. Also, you can run very low RPM safely if you are tender with the fuel pedal.

HOHN

Very true Grasshopper

 

[Peterbilt379CJ]

I have not heard anyone talk about exhaust gas temps or high combustion pressures that are high enough to crack pistons. If someone did mention these things I appologize. But when lugging an engine your egt's can go through the roof in only a few seconds. Plus with the high heat inside the combustion chamber you can also burn the valves and crack the piston crowns.

 

[FAB]

Well, after reading all the learned posts about why not to lug, Peterbilt 379 finally hits on the real reason not to lug: the extreme EGT's! If 'lugging' (and lack of lubrication) was a real issue our trucks would have 4:56 gears as standard and no overdrive. So where are all these worn out 5. 9 connecting rod bearings and complaints? Did I miss something? I for one find myself looking for another gear at 70mph to lower the rpm's! If I wanted high rpm's, I would have bought a v8 gas(or diesel). My . 02

 

[GeneralMachine]

Peter Built If you go back and read the first of the post I brought up the fact that you were refering to, you hit the nail on the head with the exast temp doing the damage in a lug mode, I have thought this threw and I can't figure out how lugging could raise the rod load, or crank bearing load, the only way you could do that is to increase the compression and with the turbo speed down all you are doing is heating up the piston's, and the engine's not very happy. Harv

 

[TxDieselKid]

Originally posted by FAB

Well, after reading all the learned posts about why not to lug, Peterbilt 379 finally hits on the real reason not to lug: the extreme EGT's!

Even on a fairly BOMBed truck, EGTs will only raise above 1400-1500 for a moment, this is empty of course. The smoke will back most people off from lugging, and rasine the RPM nicly.



Mr. Lee Weber I think hits the nail on the head. Just because there is oil pressure, does not mean the engine is not straining.



Andrew

 

[Hohn]

It's funny to think that we all like to discuss the why and wherefores, but here's what Dad taught me:



1. Let the engine warm up before you work it

2. Let the engine have the RPM it needs to do the job

3. Don't ever-rev it.

4. Always move the pedal slowly, whether pushing it down or letting it up.





I think his "horse-sense" taught me as much about taking care of mechanical things as did all the techie stuff I have learned.



Hohn

 

[TxDieselKid]

Originally posted by Hohn

It's funny to think that we all like to discuss the why and wherefores, but here's what Dad taught me:



1. Let the engine warm up before you work it

2. Let the engine have the RPM it needs to do the job

3. Don't ever-rev it.

4. Always move the pedal slowly, whether pushing it down or letting it up.





I think his "horse-sense" taught me as much about taking care of mechanical things as did all the techie stuff I have learned.



Hohn

Isn't it funny how that works so much of the time.

 

[FAB]

TxDieselKid; I'm fully aware that momentary temps of 1400-1500 are OK. The only part of lugging that concerns me is high egt's from a sustained low rpm/ low boost condition where every thing else seems all right (no screws vibrating out of the dash, shifter knobs falling off etc. )