"LUGGING?" - why not?

[Gary - K7GLD]

Occasionally, discussions appear here concerning engine "lugging" - what it is and how to recognize it - but what I *haven't* seen, is any discussion on it's harmful effects, and WHY it should be avoided... SO, I'll start... .



"Lugging" is basically the act of operating an engine under a load at a lower RPM than it can properly function, or is designed to operate efficiently. We are all pretty familiar with the term "hydro-planing" - it is often used to describe a situation out on wet roads, where a vehicle reaches a speed where the tires no longer are in contact with the roads surface, but are actually "water-skiing" on top of the water - a dangerous, no traction scenario in this case!



Hydro-planing can be reduced or eliminated by increasing the WEIGHT of the vehicle, reducing SPEED so the tires can "settle" back down to the roads surface, or reducing the "contact patch" of the tire in relation to the weight applied to it...



Let's apply all the above to internal engine operation...



An engine is designed with a specific lubricant flow, and internal parts clearances that allow components to "hydro-plane" on an oil film - not actually allowing components to come in contact with one another. This hydro-planing action demands that certain conditions MUST be maintained - proper lubricant flow and density - viscosity - and relative SPEED of moving parts to maintain constant proper hydro-planing action.



IF relative engine component SPEEDS are reduced too far, the hydro-planing action is compromised, parts break thru the protective oil film, and metal-to-metal contact takes place - the same happens if the LOAD is too high, or the contact patch - or cushion - is too small - That's why LARGE connecting rods and main bearings are BETTER than SMALL ones! It permits a larger contact patch, and subsequent oil "cushion" to prevent contact between adjacent parts.



The engines in our trucks are designed to carry heavy loads at relatively LOW engine speeds - but as good as they are, they STILL must be operated at sufficient RPM as to maintain proper lubricant flow, and component speeds to maintain oil film strength - in other words, don't "lug" the engine down so low in speed or load as to compromise oils ability to isolate components from each other!



What suffers most in lugging situations?



Main bearings lose lubricant isolation, and begin contact with the crankshaft - pistons and rings begin rocking and wobbling under combustion pressure, contacting the cylinder walls and accelerating wear - similar activity also take place in the drivetrain it transmissions and differentials, where heavy loads at low speeds allow individual components to break thru the lubricants protecting film, and come in contact with each other... .



There - any clearer?

 

[Big_Daddy_T]

I like your analogy. Very easy to picture.

 

[csevers]

GARY-KJ6Q must be a "professor"? Or he does a lot of research.



Craig

 

[Gary - K7GLD]

"GARY-KJ6Q must be a "professor"? Or he does a lot of research. "



NAHHhhh - just OLD!



Spent most of my adult life working on, or around mechanical stuff - cars, customs, built hotrods from the ground up, engine swaps and such - most anyone HAS to pick up SOME knowledge if you do this stuff long enough...



Genius is definitely NOT required! :-laf :-laf

 

[Boldt's Wagon]

Your transmissions suffer. Fifth gear nut in manual. Clutches in automatics due to low hydraulic fluid pressuree, gears (OD) due to low flow. The low rpms will also cause hammering of the springs in a clutch. Marine gears can destroy the gear sets at low rpm.



Keep the engine over 1800 rpm for normal loads, over 2200 for heavy pulling.

 

[HBarlow]

When I bought a Dodge-Cummins less than two years ago it was my first diesel-powered vehicle. I knew only that driving techniques should be different than with a gasoline engine.



I purchased a glove compartment-sized book from Cummins titled, "Operation and Maintenance Manual ISB Engine" I was surprised to read some of the recommended driving techniques such as the following:



Page 1-79, Driving Techniques

"ISB engines produce maximum power at an rpm less than governed engine speed. Placement of maximum power has been changed on ISB engines to encourage operation in the most fuel efficient speed range. "



"To obtain optimum engine performance on a grade, allow the engine speed to load down to near peak torque before shifting. This will result in an engine operating speed in the maximum power zone after the shift is completed. "



Page 1-10. Engine Operating Range

"Do not operate the engine at full throttle operation below peak torque rpm (refer to engine dataplate for peak torque rpm) for more than 30 seconds. Operating the engine at full throttle below peak torque will shorten engine life to overhaul, can cause serious engine damage, and is considered driver abuse. "



"Cummins engines are designed to operate successfully at full throttle under transient conditions down to peak torque engine speed. This is consistent with recommended operating practices. "



"Operating the engine below peak torque rpm can occur during gear shifting due to the difference of ratios between transmission gears, but engine operation must not be sustained more than 30 seconds at full throttle below peak torque rpm. "



I copied the text quoted above into an e-mail and sent it to Cummins along with my questions. I got a reply back from the Cummins technical staff confirming that the guidance contained in their manual was accurate and applied to our Dodge Rams.



I explained that when pulling my TT (16k#GCW) at highway speeds in sixth gear overdrive (3. 54 differential gears) with the cruise control on, the engine will frequently pull down to about 1600 rpm and hold speed on a long grade. I usually don't downshift if the mighty Cummins keeps pulling and rpm doesn't drop below 1600 rpm. I told the Cummins reps I was concerned that this practice violated what I had read and been told by Dodge-Cummins owners but it seemed to be acceptable according to the Cummins operating manual. I was told that it will not hurt the Cummins ISB to run it at full throttle at or above peak torque rpm.



Harvey

 

[GeneralMachine]

Engine Lug

IMHO I feel that as long as the oil pressure is up, there's no way you can have any problem with engine comp. looseing lubracation. eng lug damage is when your rpms are down to where you can't build any boost and then you start gettin high ex temp's, I feel as long as the mighty Cummin's will still pull as you smash the go pedal you will be just fine. Harv

 

[John Berger]

Our engines make rated power at 1600 rpm, correct? So I don't see a problem with oil as long as the engine is functioning within its rated power band. Personally, I would worry more about the transmission. I've recently had the fully splined mainshaft installed, so I'm good to go with the 5th gear issue, however, I like to try to keep the engine above 2000 rpm when shifting while towing my 5'ver. The engine sounds fine, it's the way the rest of the driveline sounds below 2000 that gets the puckering started.

 

[Pit Bull]

LSMITH gives the best definition I have read on this site "Lugging is anytime you add fuel (pedal) and the engine cannot increase speed. Regardless of RPM, load, or conditions, that is the ONLY definition of lugging the engine. You can lug the engine at 2500 RPM, just the same as 1000 RPM. "



I like the simple easy to understand definitions like this one by LSMITH

 

[Snow man]

Good post Gary, and one that needed to be written. It seems a lot of the CTD owners,especially older ones,dont want to hear the motor revving much above 2000RPM,so they shift up to the next gear,even though that puts them below the minimum RPM they should be at. They then act baffeled whe there trans gets wiped out,or they lose 5th gear. All theg uys that drive it like they stole it,and keep here wound tight have much less problems with the entire drivetrain than those who baby it with a heavy load,thinking they are helping it when they are lugging the motor and pounding the drivetrain.

 

[tgbol]

Gary-Well said.

Pit Bull is right on too!



Lugging an engine is any time it is loaded down so that it will not accelerate, no matter what speed.

The damage done to engine from lugging usually starts at pounding the rod bearings out. No matter what the oil pressure is the load on the bearing breaks through the oil film. It is true that damage can occur more quickly at lower RPM.

 

[GeneralMachine]

Originally posted by tgbol

Gary-Well said.

Pit Bull is right on too!



Lugging an engine is any time it is loaded down so that it will not accelerate, no matter what speed.

The damage done to engine from lugging usually starts at pounding the rod bearings out. No matter what the oil pressure is the load on the bearing breaks through the oil film. It is true that damage can occur more quickly at lower RPM.

I just can't see the bearing load getting greater if you are lugging the engine, for you have less boost, less power, the piston's are feeling the heat, from the fuel with not as much air, as long as it will pull out it arnt in no trouble. my thinking is when you are @ max boost & rpm the rod bearings are feeling the most pressure. Harv

 

[Pit Bull]

Tell me if I'm in trouble

The other day I loaded the truck up with one of my heavier loads. 2- 50# bags of black oiled sunflower seeds and a 40# bag of dog food (did not even scuff my LineX bed liner). When pulling out of the Hardware store I let the rpms slip to 1200 while changing gears on level surface. It seemed to accelerate smoothly but I know I was not in the meat of my torque band. I bought this 3500 4x4 ETH/DEE because of these types of heavy loads . You guys think I might be in trouble and lugging her down? :-laf



I know my wife thinks I'm nuts too

 

[Gary - K7GLD]

MOST lugging stress occurs at low RPM - where there is ALSO reduced oil pressure, which reduces available flow and film protection - sure, you CAN lug the engine at higher RPM - but most cases involve low speed operation...

 

[drees1]

With K&N air filter and stock turbo, boost falls off below 1750 rpm pretty fast and you should not go much below that. For best economy you want to be near peak torque rpm or slightly above it. This engine is truely so over built and well designed by anyone's standards that it is practically impossible to harm it short of outright abuse. The driveline issues are valid. I run mine a little lower rpm than most, but I use the Gear Vendors to stay out of trouble, then go right back into double over drive when the hill is crested. My best towing mileage is cruising at 1800, 67mph, drop 1/2 gear for the big hills and lug back to 1700 - 1750. I have only had to back more than 1/2 gear once or twice. The fuel mileage is 1 - 1-1/2 mpg better than keeping it wound up. I usually tow a flatbed gooseneck with a 8 x8 x 25 ft square steel container, Total gross wt around 23k, including truck.



Doug Rees

 

[GeneralMachine]

Pit I don't think it can stand much more of that. :-laf :-laf :-laf

 

[Pit Bull]

Why don't we all go out and see at what RPM we lug at in third gear in a empty truck? Then report back. Should be level ground I bet it is under 1,000 RPM's

 

[Pit Bull]

Originally posted by HWhite

Pit I don't think it can stand much more of that. :-laf :-laf :-laf

I know I need this truck like I need another hole in the head :-laf I could get by with one of them Luv Trucks But as we all know the CTD gets in your blood. and the dually wheels look

 

[athompson]

Mine doesn't lug in any of the 7 gears, even at idle, on level ground. In fact, it doesn't lug on slight inclines, either. The best mileage I get is running around town at 1100 rpm or less.

 

[Gary - K7GLD]

"Why don't we all go out and see at what RPM we lug at in third gear in a empty truck? Then report back. Should be level ground I bet it is under 1,000 RPM's"



It would certainly seem logical that the manual transmission trucks are most suceptable to lugging - autos can better adjust automatically to changing loads and RPM - and with the manuals, pulling heavy loads at shift points can easily place load spikes on the engine and drivetrain. Thinner weight oils - such as the 5/40 stuff, once the engine is hot and loaded must surely be more likely to lose film strength than the standard weight stuff - lightweight oils might be great for cold weather starting and down-the-road fuel economy, but *I* sure have reservations about the durability they provide over the long haul in working situations.



Otherwise, the driving impression the low end torque the Cummins provides seems to generate a sense of omnipotence - that the engine is capable of just about any form of low end loading without strain - and that gets even worse after adding power mods that enhances low end power - but power mods do NOTHING to provide better lubrication flow and protection at low engine speeds - and the lugging issue just gets more pronounced and potentially damaging.