Lugging diesels = bad or newfangled myth?

[getut]

Ok... I'm a new tractor owner, but I'm not completely new to tractors since I grew up in a farming community. My family did not farm, but I helped out a few of our neighbors for fun (and occasionally some spending money) during my childhood and teens. I never owned my own tractor until now and my question deals more with the diesel engine.

Nearly every time people on this forum answer questions, I see quotes like "but don't lug the engine", or "keep the RPM up", "designed to run that fast". It was always my understanding that diesels are all about lugging and torque and slow RPM pulling. In the community that I grew up in, it was nothing to see 20-40 year old tractors putting along at barely more than an idle pulling a trailer load of hay stacked 15' high and really lugging up the hills. If the diesles weren't designed for that, it sure seems like those would have gave up the ghost long before then. Is there a difference in todays engines? Too much brain and not enough brawn in the designs? or is all a myth.. and todays tractors really do just fine when lugged at slower RPMS. Someone who is in the know on this, please answer.

Getut

 

[bandit67]

I'm no expert, so I can only comment on my experiences.

First, many of today's diesels are smaller and more powerful than in years past. In the world of motors, including diesels, the old saying "there's no replacement for displacement" still holds true. Although a newer, smaller diesel may have a higher hp rating than an older larger displacement motor, the newer motor meets that hp level at a higher rpm. In addition, I've found that the newer, smaller, higher hp rated diesels seem to make slightly less torque per hp than their older, larger "relatives".

Lugging a diesel will cause it to bog down and smoke more. If your motor can handle lugging without much problem, that is, if it can do the task at hand while lugging, then there is nothing wrong with that. However, if your diesel motor lugs and is operating less efficiently because of it, the operator can normally feel it.

You just need to learn what the sweet spot is with your motor and the task you are doing, and that comes with trial and error.

Even with a larger diesel, like my 7.3L in my truck, when it is lugging too much, it blows a lot more black smoke, and the Exhaust Gas Temperatures rise quite quickly - a simple downshift, and the smoke clears, EGT's drop, and the motor starts pulling better. For example, the mfr. claims the highest TQ rating is at 1800 rpm, but both my real world mountain hill towing and tests on chassis dyno's have shown me that the sweet spot for the hp and TQ are closer to 2600 rpm.

The same goes for the diesel tractor motors - if you're lugging it too much, it will bog down and not do the job, so a simple adjustment to the throttle will take care of everything.

 

[lynxpilot]

Good question. I've often wondered the same. Found THIS link. Essentially says if you are operating WOT when RPM is too low to accelerate the engine, you are pumping raw fuel in that can cause some serious EGT's. Seems to me it would only be an issue with a turbo.

 

[To20Chris]

One of the biggest problems with lugging any engine, gas or deisel, is damage to the con rod bearings. When you lug an engine, the piston fires, there is a lot of resistance to turning the crank, and the piston is near TDC (not much angle). If the film of oil is not strong enough, then the force will drive the bearings together, and they will wear fast. A low RPM engine will have big diameter bearings to increase the surface area, and an oil pump geared to work at low RPM. If you run it too fast, the large diameter of the bearing will create heating problems for the oil, and the pump will not be at its best operating point. For the engines in these CUTs, keep the RPMs up high enough to get good oil presure, film strength, and flow - you don't need to run flat out for that. It all comes down to operating it in the RPM range it was design to work at.

 

[MicroPilot]

So in a nut shell it's OK to lug the engine on occasion, but it's not a condition that you want to be running the engine in most of the time. The worst way to lug an engine is at full throttle for more than a minute or so.

 

[Degree]

Hmm.. That thing about letting a diesel idle past 10 minutes is a bunch of bull. ive had truck drivers keep there trucks running overnight(8+ HOURS) and they are still going.

 

[getut]

I'm not saying this to fire up the weight wars again, but I do tend to believe that most of todays designs are OVER designed. Nearly nothing manufactured today lasts as long as the same item manufactured 30 - 40 years ago. There is no substitute for mass. There is too much emphasis on weight and material savings, so expensive alloys are used, stress testing is simulated and mocked up and the costs go through the roof.

There is something to be said for maintaining strength with lighter materials when it HAS to be done (i.e. aircraft, race cars, etc). But I'm not a big fan of new manufacturing methods just for the sake of material reduction alone. Give me the beef (and the HP).

I took a round about path to get into computer networking and systems administration (my current career path)... I came up through a CAD (computer aided drafting) environment, so I know better than most about the methods that are used to achieve all of this, I just don't agree with it. No matter how much people tout the new methods, history just does not bear witness to the wisdom of material reduction. I hope our diesel engines (especially MINE because I can't afford a replacement until after the kids are gone) is/are not a victim of over-design to the tune of fewer years of serviceable life.

I want my tractor to still be around in 40 years.

Getut

 

[getut]

To provide and FYI to my own post before too many people get bent out of shape....

I am aware that "BEEF" also is not a substitute for a technically sound design. More material just pushes a weak point to somewhere else in a system if it is not designed properly. Keep in mind that my statement in that previous post is meant as a broad statement. For two PROPERLY designed systems, history shows the beefier of the two to last much longer.

If my diesel or any other can't handle lugging, then the engineer who designed it failed in his task, IMHO.

Getut

 

[Bob_Skurka]

Micro, I'd say you summed it up reasonably well.

We have, in the past had problems with lugging diesels in our truck fleet. We use Cat and Navistar units. Granted our trucks are much larger than a typical CUT. Lugging them has caused us a few problems, lack of fuel efficiency is probably the largest visibile problem and as Bandit pointed out causes a lot of excessive smoking. Also premature wear and carbon fouling of the injectors. We've rebuilt a few too early and replaced a few too.

In terms of CUT, constantly running the engine at PTO speed puts the engine below the maximum RPM speed and at/near the top of the torque rating so lugging short term is not usually going to lead to any of the above issues.

An engine can be lugged at much lower RPMs, and also cause issues (although it was not noted in the Cummins link) where the engine is producing far less than the rated HP and something like a mower deck, tiller, or rotary cutter is used and that overloads the engine. Under those periods it would be very similar to running a truck in too high of a gear while hauling a heavy load. If the operation continues, the load on the engine will cause it to stall. Under that condition there is an issue of premature wear as well as serious injector clogging. The engine is not getting lubricated properly at the lower rpms which downwardly spiral toward stop and the carbon smoke build up as the engine is choking can clog the injectors. If this happens occasionally there is no real likelyhood of serious damage, if someone thinks they can run their engine at much lower speeds while simultaneously working the engine hard then they are looking to premature rebuild for their engine. When using an implment that is PTO powered it can become an issue if that practice is continued over time.

I think the biggest chance for problems comes in with repeated and or prolonged periods of misuse. Occasional events are not really too much issue.

Regarding long periods of idle, we see this commonly occur in the winter to run the heaters in our cabs, we also see idle periods to run the refrigeration units of cargo boxes. Typically what is actually happening is not a sustained idle (or perhaps what should not be happening). What should be happening is that the engine should be running at idle during periods of time, and should be run up a bit, ideally automatically, when a load is placed on the engine to run the compressors for the refrigeration units, etc. We instruct our drivers NOT to allow their trucks to idle for more than an hour, we also believe that idling can create carbon build up which can clog the injectors, especially in cold weather conditions.

 

[Charlie_in_TX]

I am no expert, but I did stay at a Holiday Inn last night.

I think you have an additional issue. The cooling system can't handle the high EGT and the motor overheats. I know when I start to lug the motor the temp start going up.