kinda sorta.
lugging an engine is driving in a high gear with low rpm's where you are under the power band. kinda like trying to go 20 mph in high gear. you can do it, but if you try to accelerate you are gonna be a slow pig and blow black smoke.

 

If your IDI(and pump) are tuned correctly, you might be slow, but shouldn't blow(much) smoke.

I did a few minutes worth of research on 'lugging', and what I found is that the term originated in carburated gasoline engines; if you loaded the engine down too much(especially at low RPMs), the gasoline would burn too fast and you would get pinging.
On our diesel engines, this isn't really a problem; timing is adjusted based on load and RPM, and injected pretty close to TDC, so you won't generally have the "pinging" problems. Also, diesels are meant to take a /lot/ more strain, due to the rapidly combusting diesel, so there's less worry about "overpressure" or anything like that.

There is also a worry about inadequate lubrication, but our engines are pretty well designed that way, what with the oil pressure regulator and all.

The Ford manual I saw didn't say 'lugging', but said to avoid fully loading the engine below peak torque RPM. How do you know if you are fully loaded? If you can press the accelerator and the engine produces more power, you aren't fully loaded. If you press it and nothing happens(or you /lose/ power), you are fully loaded.

Effectively, you should be able to drive(in some gear) down to basically idle. If you are maxing out the engine's output below ~1600 RPM or so(and not accelerating quickly), drop it a gear. Simple.

 

Lab tests are generally correct. Max efficiency is very close to torque peak. The misunderstanding revolves around people not understanding what that MEANS.

Torque peak will occur during full throttle, thus cruising at torque peak full throttle is impossible most of the time.

For real world, you amend your torque peak by shifting gears. At a lower engine speed peak efficiency is made with less fuel delivered.

Let me give you an example. You are in 4th driving at "torque peak" using minimal throttle. If you shift to 5th, and cruise at the same speed, you will get better fuel efficiency- even though overdrive has a parasitic geartrain loss. Why? Because pumping extra air not burnt in the combustion chamber robs a significant amount of horsepower. As do all the other friction losses in the engine.

This issue of loading pertains to ALL engines, not just diesels.

So in short, it's not that the data is wrong, it's that the majority of folks reading it misinterpret what it means.

Lugging... these engines are a bad example since they are so forgiving of low rpm loading. Why are they so forgiving? Because they run a low volumetric horsepower, a combustion process that is inherently gentle comparatively, and a LOT of rotational mass. The rotational mass smooths out torsional oscillations in the crank and the low hp limits them in the first place. Enough torsional flex can break parts in the engine (torsion changes injection timing), though the herky jerk usually stops the operator before it happens.

 

So here is my highly scientific test.

So if I had a dyno I'd park it on the dyno, but I don't. So what if I jack the rear end up, disconnect the lift pump into a 1 gallon fuel container and run it at a fixed rpm range and repeat in upward increments of 100 and plot the change. I know there will be no resistance but then again its relative. Now if I could get resistance on it I would have something.

Thoughts,
Devin

 

Or I could put one of these bad larry's on and do it on the highway. Then again i'd probably have to put another on the return line and subtract what goes back to the tank.

Devin

 

Why not get a precise scale(0.1lbs or better resolution) and a 5-gallon gas can.
Rig the supply and return lines into the gas can, get all the air out of the line, and weigh it.
Then drive for X miles at Y speed in Z gear, and weigh the can at that point.
Take the difference, convert to gallons.

Should take all the guessing factors out of it quite easily, and cheaply too.

 

Exactly what I was going to say. You don't generate your peak torque unless you are at full throttle. Just because you are driving at 1400/1800 RPM does not mean you are making peak torque.

I do agree that 1800-2000 RPM is a happy cruising RPM though.

 

There's tons of advice to run at peak torque. But there are also a lot of discussions among owner operator CDL's who discover that the method I describe garners better efficiency than running at peak torque. Usually followed by significant lamenting of wasted fuel over the years.

These IDI's are not the greatest example to demonstrate this though. They just *love* low rpm, and the low torque peak is often the spot to run anyway due to finite horsepower available.