From: mshar@mentor.cc.purdue.edu (Mike Mshar)
Newsgroups: rec.autos
Subject: Re: Lugging an engine?
Date: 2 Oct 92 19:51:21 GMT
Distribution: usa
Organization: Purdue University Computing Center

In article <1992Oct1.180846.6230@lmpsbbs.comm.mot.com> shultz_ed@macmail1.rtsg.mot.com writes:
>I was listening to an auto talk show one time and they have a "question of the
>week" which people try to answer before the next week's show when they reveal
>the correct answer.  The question that I heard was:  Why is lugging an engine
>bad for the engine?  I was not around to hear the answer to this question so I
>was wondering if anyone knows the answer.  From what I gathered from the
>conversation, lugging an engine applies to manual transmission cars and means
>to drive the car in a higher gear than "recommended" for a given speed.  Does
>anyone know exactly what lugging the engine is (i.e. in terms of rpm) and why
>is this bad for the engine?

Lugging an engine is bad because it puts a great amount of stress on
the pistons and connecting rods. When the engine is in normal operation,
and you tromp on the gas, a fuel/air mixture near atmospheric pressure
enters the cyinders. When this mixture burns, it puts a force on the
top of each piston, which puts a force on the connecting rod, which 
puts a force on the crankshaft, etc. Thus, the engine revs up, and 
the car accelerates. 

When you open the throttle at a low, lugging rpm, the same thing
happens, but now you crankshaft is not capable of turning faster. Thus,
there is no `give` for this explosive force. This problem is compounded
by the fact that engines are more efficient at packing the cylinders 
with air at lower rpms than high rpms. So, you pack a cylinder chock 
full of air and fuel, it ignites, and then pushes the piston down.
But, the car is not accelerating, so the piston cannot accelerate the 
crankshaft. This force has to be dissipated some how, so the rods
and crank will start to flex. This is definitley not good. Keep in mind
that oil pressure is also low at low rpms, so the oil film between 
moving parts in the engine could be broken because of this excessive
force. This doesn`t do much for your main and connecting rod bearings.

Also, spark knock, detonation, and carbon build-up in the cylinders are
all more likely to occur in a lugging condition. As for what lugging
an engine actually is, well, if you tromp on the gas, and the car is in
such a high gear that it doesn`t accelerate any faster, you are probably
lugging the engine. It is more of a feel than a specific rpm.

For comparison, get out the trusty old ten speed, put it in the highest
gear, and try to start from a dead stop. Push as hard as you can. Now 
picture your engine doing this in your car. Not a pretty picture.

Whether this is the answer they gave on the show or not is hard to 
tell. It might have been something rather simplistic that I am not
thinking about right now. All I know is that lugging an engine is
not good, so I don`t do it. 


-- 
|       Michael G. Mshar        | My Life-Long Goal: To keep common sense    |
|   mmshar@mn.ecn.purdue.edu    | alive and keep the idiots from taking over.|
|  mshar@mentor.cc.purdue.edu   |     \\President, Nice Guys Anonymous//     |
|____>>> United States Steel, Mechanical Engineer, Technical Services <<<____|
Newsgroups: rec.autos