Effects of labouring or lugging an internal combustion engine - can anyone explain in more detail?

[Alittlecurious]

I have heard that load on engine parts (of an internal combustion engine) can be very high at lower revs, especially when the engine is being “labored” in a higher gear than is appropriate.

It makes sense to me as someone who often rides a push bike.

If I am in a higher gear and trying to ride up a hill, my legs move slow, however with a big strain, the chain will sometimes skip a sprocket.

If I am in a lower, more appropriate gear, the legs peddle faster however with a lot less effort.

I have also heard that on some engines, driving in low revs under load (aka “lugging”) will lead to the con-rod or other parts pushing through the side of the engine casing.

If anyone has more detailed experience and explanation about this, can you please share it?

Thanks in advance.

 

[Tmoose]

Probably ought to delete this post and any others, just leaving the post over in automotive engineering something or other.

regards,

Dan T

 

[Panther140]

Looks like this was accidently posted here, but still somewhat related to acoustics/vibrations because of combustion knock.

I am not sure what you mean by detailed as far as explanations go. Ask questions if you need. I'll answer them even if I don't know what I'm talking about . In essence, you're concentrating the work being done into fewer combustion cycles.

You demand that the engine supply X amount of work over a set amount of time.
When revving higher, the engine can divide X amount of work throughout more combustion events.
When revving lower, it has to do more work per combustion event.

When you do more work per combustion event, you are opening throttle more so that the engine can take in enough fuel and air to create higher pressures. Those higher pressures create more force pushing the piston and con-rod downward with each stroke to generate more torque. This creates more side loading on the piston, more compressive force on the con-rod, more structure borne vibration, and increased likelihood of pre-ignition or combustion knock.

Combustion knock is the major bane here. It increases peak cylinder pressure greatly, which amplifies everything that destroys engines.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[GregLocock]

Since we used to run WOT cycles on engines on the dyno from idle to redline I doubt that there is any inherent issue, I strongly suspect engine mounts and driveline are the reason it is discouraged.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[romke]

in some engines that are excessively loaded at low engine rpm it can happen that the load on connecting rod big end bearings get so high that the moving parts are no longer more or less continuously separated by a oil film. if that happens the result may be that the connecting rod bearing momentarily freezes with usually as result that the bearing halves are split apart and the connecting rod then "flies" through the engine block.

a failure like that in essentially a lubrication failure - or more precise: a failure to keep up a sufficiently thick oil film. lubrication of journal bearings in a engine is a rather complicated matter - because the direction of the load and its magnitude change continuously, there is no such thing as "steady state" - the lubricant film thickness and the area where this is the least varies with the combustion process in the cylinder above.

it also depends on the type of engine: bearings in a four cycle diesel engine have quite a different loading pattern then those in a two cycle diesel. in gasoline engines the bearings are most in danger at very high revs with practically no load.

bearing failures due to excessive lugging or overspeeding are nowadays a lot less common then say 30 years ago, due to better structural design and bearing materials (and maybe also better engine oils).

 

[Alittlecurious]

Thanks for the positive replies especially Panther140, about cause of extra load.

Any other replies welcome, I will certainly read.

 

[MikeHalloran]

In olden times, when I was young, if you lugged an engine, at some point the carburetor would get confused, and the engine would 'cough'. That event was often noticeably violent, and surely didn't do any good to the engine structure.

My daughter's new Jetta has a dash indicator that displays what gear you are in, and suggests what gear you should be in. Its 1.8l fuel injected turbo engine just powers up hills that would have required two downshifts for my first car, a 1.5l Hillman Minx. The Jetta's gear suggestor seems biased toward economy. I almost said it wants to chug up hills, but it doesn't chug, and it doesn't bog; it just goes, gracefully if you are in too high a gear, and smartly if you are in a lower gear.

I did manage to stall the Jetta on takeoff, quite a few times, but in my defense, I am used to a Z28, that is more difficult to stall. The Jetta also seems to have some kind of hill-holder built in, I'd guess associated with the ABS; it just doesn't roll back at all on an uphill launch, which saved me some embarrassment.

My advice: Don't worry, just drive.






Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

My two cents:

Purely from a gas dynamics point of view, high load, low speed is the operating condition where you often see flow reversals through the engine cycle. I.e. instantaneous flow may be in the opposite direction to mean flow. Maybe this can confuse transducers and simple control systems?

Steve

 

[dicer]

If you are driving a manual transmission vehicle with a gas engine and are lugging the engine at very low speeds or driving slow in a high gear, you will hear and feel vibration. That is crankshaft torsional vibration along with sympathetic vibration from the drive train. High cylinder pressures and very slow speed are going to cause more crank shaft torsional flex than at high speeds, unless of course the crankshaft was designed for the low speed pressures.
Personally I feel a lot of the crankshaft fatigue damage is caused by slow speed operation and lugging, especially in construction trucks.

 

[Tmoose]

Once upon a time highly tuned sports car engines had large carburetors and exhausts and camshafts best suited for high rpm service, The ability to carburet "cleanly" at low rpm was difficult, and low rpm and wide open throttle made it worse. When the spark plugs got covered in soot and worse from too much low speed running they would not fire correctly. A popular remedy was the probably no longer politically correct to say "Italian tune up" , which was a session of high revolutions and heavy throttle which would heat up the plugs and hopefully burn off the deposits.

The fanciest Porsche engines of the 50s had multi-piece crankshafts with roller bearings. Hirth made them and i guess they were pretty good. SPG made them for Porsche and VW that were more problematic. Legend has it any load at rpm under 2000 or so would hurt them.

I'd say with modern mixture control and lightning bolt ignitions and plugs good for >>50 kmiles "lugging" is not too likely to cause mechanical damage.

I drove some small rental car last year (Maybe a Chevy?).
The auto transmission jumped into high gears pretty early and even seemed to lockup. At city street speeds the cruising rpm were well below 2000 rpm, and slight increases in throttle would cause a bit of a low barely detectable shudder just before it decided to downshift.

One "definition" of lugging was when more throttle opening does not increase car speed, which would be max power at that rpm, so a shift down was indicated.

 

[Panther140]

"Thanks for the positive replies especially Panther140, about cause of extra load.

Any other replies welcome, I will certainly read. "

You're welcome. Be sure to pay attention to Greg's post about mounts and driveline components. He has a very important point. I wish he would elaborate.

Greg, I also run engines WOT from idle to redline on dynos. Certain engines will destroy certain couplings at the lower speeds and high loads (analogous to driveline abuse like you mentioned). I haven't seen an engine be destroyed from running WOT at low speeds, but I have seen signs of wear increase and shortened MTTF with certain duty cycles over long term tests.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin