Quote:
Originally Posted by Daox
Its all theoretical discussion. No specific model in mind.
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Seat of the pants definitions where vibration and noise are the measure do not adequately tell you if you are "lugging" an engine. Many engines are fine being run in low, heavy loads while others fail very quickly.
I won't go into engine design theory of peak pressure to brake-mean-effective pressure, but, Old Tele Man's description pretty much sums things up. Assuming your engine holds together, the point at which the cylinder pressure causes displacement of your lubricating film is when things go south. Low speed, high power demands result in increased cylinder pressures. Coupled with most engines which use mechanical lubrication pumps who's output follows engine speed, there is a point at low revolutions where the oil film is displaced and damage occurs.
Of course, even if an engine does not incur damage at high loads and low speed, it can feel awful. The bucking and vibration can damage motor mounts and other tertiary components. The configuration and design of an engine has much to do with how violent "lugging" can feel. The 3 cylinder Suzuki/Geo Metro engines are inherently rough at lugging speeds whereas the Daihatsu CB90 engine of the same time period is much happier being lugged with its balanced shafts.
For heavy duty towing, I have always preferred the inherent smoothness of the straight 6 configuration if the engine has 7 large bearings. I worked with an old 70's Dodge pickup with the venerable slant 6. We lost first gear, and for about half a year, we would start out in second gear using the the column shifter. The engine seemed happy to pull our 3500 pound trailer, though the second gear starts wore out the clutch pack. Even after such abuse, the engine ran for a couple more decades.
I still have straight sixes in my current Dodge trucks, but of course, they are both diesels. They are designed to be "lugged" from the get go.