Why is lugging the engine bad?

[ShawnD1]

Seems like a simple question. A gas engine in a car has max torque around 4000rpm and it sounds great. Flooring it at 4000rpm = no problem. Flooring it at 1000rpm puts out less torque but it sounds horrible and old people always warn against doing that.

What fails when the engine is running at a low rpm with an open throttle?

 

[Demo24]

Largely bearings, but other issues like 'pinging' (early combustion), and piston slap (piston hits the cylinder wall) can happen during this. You are putting a lot of load on the engine and the inertia of the pistons,crank, etc can't effectively keep up with the load. Not quite bad enough to actually stop the engine(which you could do), but the loads coupled with lower oil pressure = failures.

 

[ShawnD1]

Interesting. Thanks for the info :thumbsup:

 

[ShawnD1]

I started googling this quite a bit before starting the thread and I just now found something that sounds plausible.


http://ecomodder.com/forum/showthread.php/lugging-engine-good-bad-economy-3159-2.html

The crankshaft and connecting rod bearings in the engine are just plain bearings. They have no balls or rollers, just a thin metal shell insert with an oil groove which "rubs" against the crankshaft.

Lots of pics, here, look at the half-circle sheet metal looking things.
http://images.google.com/images?hl=e...h+Images&gbv=1


You know how your car tires will hydroplane when you go fast enough thru deep enough water. The bearings work on this hydroplaning principle. If the rotational surface speed of the bearing is fast enough (fast enough engine rpm) the crankshaft will hydroplane up on the oil film and never touch the bearing shell when the engine is running.

Otherwise, it will rub and wear the bearing. You will not know it is wearing for a long time, and then it will be too late and you will hear knocking noises when the engine idles. This noise is caused by the clearance in the bearing getting bigger due to wearing away the bearing shell. At this point people put in thicker oil and sell the car.

I would suggest not using full throttle at very low rpms for this reason. You combine high bearing loads with low bearing surface speeds.

This is also why all those "endurance" tests where a car is driven at 100 MPH for 500 K miles are such horse-pucky. The starter motor gets used once per oil change, the clutch 5 times per gas fillup and oil change, and the bearings in the motor get no wear at all, they are on the oil film almost all the time. And body rust? Forget about it.

So running the engine at low speeds is like running it with little to no oil. Wide open throttle at low speed ~ wide open throttle with no oil.

How do diesel engines get around this? It's not a problem to run a diesel at 1000rpm with the pedal on the floor.

 

[yh125d]

I started googling this quite a bit before starting the thread and I just now found something that sounds plausible.


http://ecomodder.com/forum/showthread.php/lugging-engine-good-bad-economy-3159-2.html


So running the engine at low speeds is like running it with little to no oil. Wide open throttle at low speed ~ wide open throttle with no oil.

How do diesel engines get around this? It's not a problem to run a diesel at 1000rpm with the pedal on the floor.

It is if theres a considerable load on the engine, and its in a higher gear. If you floor it in 5th at 1k, you're not gonna get anything but black smoke.

 

[DivideBYZero]

I started googling this quite a bit before starting the thread and I just now found something that sounds plausible.


http://ecomodder.com/forum/showthread.php/lugging-engine-good-bad-economy-3159-2.html


So running the engine at low speeds is like running it with little to no oil. Wide open throttle at low speed ~ wide open throttle with no oil.

How do diesel engines get around this? It's not a problem to run a diesel at 1000rpm with the pedal on the floor.

Where did you learn this?

 

[ShawnD1]

It is if theres a considerable load on the engine, and its in a higher gear. If you floor it in 5th at 1k, you're not gonna get anything but black smoke.

lol. True as that may be, I do see a lot of 18 wheelers pouring out huge clouds of black smoke when they get started from an intersection

 

[Demo24]

You can still lugg a diesel engine, its just that it will probably be a bit lower due to the torque differences. Diesel engines also tend to have thicker, stronger, and heavier parts (changing these days) which can mean more inertia in moving parts and more leniency for the forces produced.

 

[skyking]

It is true for my diesel or a truck low RPM diesel, but not a high revver.
It is relative. My engine runs from 600 to 2600 in stock form.

 

[Demo24]

lol. True as that may be, I do see a lot of 18 wheelers pouring out huge clouds of black smoke when they get started from an intersection

That's also potentially due to a bad tune as black smoke equals too much fuel and not enough air, or something else is off.

 

[KIAman]

For a long block, I6 for example, the crankshaft is long enough to crankshaft flex when enough of a load is applied.

Clipped from here.

Because of the six cylinder engine's smooth characteristic, there is a tendency for a driver to load the engine at low engine speeds. This can produce crankshaft flex in four main bearing designs where the crank spans the distance of two cylinders between main bearings. This distance is longer than the distance between two adjacent main bearings on a V6 with four mains, because the V6 has cylinder bores on opposite banks which overlap significantly; the overlap may be as high as 100%, minus the width of one connecting rod (1.00" or so). In addition, modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crankthrows to have greater support from adjacent bearings, so it is now customary to design straight-sixes with seven main bearings.[7]
Many of the more sporty high-performance engines use the four bearing design because of better torsional stiffness (e.g., BMW small straight 6's, Ford's Zephyr 6). The accumulated length of main bearing journals gives a relatively torsionally flexible crankshaft.

Translation: Load at low RPM enough and you will kablewy your engine.

 

[yh125d]

lol. True as that may be, I do see a lot of 18 wheelers pouring out huge clouds of black smoke when they get started from an intersection

Thats because they need maintenance. Any properly running big rig will not smoke, except perhaps a small amount upon hitting the accelerator to get up a hill

 

[Eli]

Thats because they need maintenance. Any properly running big rig will not smoke, except perhaps a small amount upon hitting the accelerator to get up a hill

All diesel engines produce copious amounts of smoke under heavy loads.

 

[yh125d]

All diesel engines produce copious amounts of smoke under heavy loads.

Not if they're tuned properly and in the right gear

 

[Eli]

Not if they're tuned properly and in the right gear

What? All diesel engines produce large amounts of black smoke when heavily loaded.

In fact, from what I've seen, tuned diesels produce MORE black smoke than stock.

 

[yh125d]

What? All diesel engines produce large amounts of black smoke when heavily loaded.
In fact, from what I've seen, tuned diesels produce MORE black smoke than stock.

Ok, you just lost all credibility there. Its pretty obvious you don't pay attention to the diesels you see. Modern diesels barely smoke at all. A bit on a cold start which quickly goes away (if its there at all). A puff when a big rig downshifts on the hwy to go up a hill. A bit when pulling up from a stop, but they're not constant smoke machines billowing out blackness all over the place.


And by properly tuned I mean by a mechanic to run properly. Not those aftermarket tuning chips people put on to turn their perfectly good trucks into smoke machines.

 

[Eli]

Ok, you just lost all credibility there. Its pretty obvious you don't pay attention to the diesels you see. Modern diesels barely smoke at all. A bit on a cold start which quickly goes away (if its there at all). A puff when a big rig downshifts on the hwy to go up a hill. A bit when pulling up from a stop, but they're not constant smoke machines billowing out blackness all over the place.


And by properly tuned I mean by a mechanic to run properly. Not those aftermarket tuning chips people put on to turn their perfectly good trucks into smoke machines.

I said while under heavy load, not all the time.

Actually I was paying attention while we were on our road trip this last weekend. I DID notice the puffs while downshifting, I thought it was interesting.

But you're right, modern big rigs don't billow black smoke like they used to. I chalked this up to modern emissions regulations - soot traps, etc. I wouldn't be surprised if it was due to advances in diesel engine technology itself though. Or both.

I still somewhat stand by my statement that all diesels will produce black smoke when heavily loaded though. That's been my experience.

Of course, I've never owned a diesel, so...

But I see what I assume to be stock diesel trucks lay a big fat cloud all the time when they're accelerating hard from a stop. Buses do it too, though their clouds don't seem as thick.

 

[yh125d]

I said while under heavy load, not all the time.

Actually I was paying attention while we were on our road trip this last weekend. I DID notice the puffs while downshifting, I thought it was interesting.

But you're right, modern big rigs don't billow black smoke like they used to. I chalked this up to modern emissions regulations - soot traps, etc. I wouldn't be surprised if it was due to advances in diesel engine technology itself though. Or both.

I still somewhat stand by my statement that all diesels will produce black smoke when heavily loaded though. That's been my experience.

Of course, I've never owned a diesel, so...

But I see what I assume to be stock diesel trucks lay a big fat cloud all the time when they're accelerating hard from a stop. Buses do it too, though their clouds don't seem as thick.

No, those like F-350s you see smoking are chipped. Its hard to make one smoke unless its got a load, up a hill, in too high of a gear.


Even my dads 1989 f-350 which badly needs a tuneup barely smokes, and only while loaded and accelerating.

 

[Eli]

No, those like F-350s you see smoking are chipped. Its hard to make one smoke unless its got a load, up a hill, in too high of a gear.


Even my dads 1989 f-350 which badly needs a tuneup barely smokes, and only while loaded and accelerating.

I was thinking more like Dodges, but okay.

Interesting. So what changed, exactly? Better engine management systems?

 

[EightySix Four]

Actually, the smoke from chipped diesels is a safety precaution from the tuners. Since they're making more power and likely never have physical access to the truck, they run really rich (hence the smoke) to make the extra power safe.

A diesel with a good custom tune will not smoke unless making HUGE horsepower with a large number of physical modifications (exhaust, intake, turbo, etc.), on HPT's forums we make fun of the smokers because they can't tune

 

[Eli]

Actually, the smoke from chipped diesels is a safety precaution from the tuners. Since they're making more power and likely never have physical access to the truck, they run really rich (hence the smoke) to make the extra power safe.

A diesel with a good custom tune will not smoke unless making HUGE horsepower with a large number of physical modifications (exhaust, intake, turbo, etc.), on HPT's forums we make fun of the smokers because they can't tune

Interesting. You learn something new every day. I mean, it makes sense.. but I'm just so used to seeing a puff of black smoke whenever I hear a diesel engine roar that I guess I don't notice when there isn't one....

 

[Turin39789]

I've seen it as common in my area for diesel pickup kids to tune/chip their car just to make it smoke.

 

[LTC8K6]

Was behind a Dodge Sprinter today. I was driving my Jeep with the Hemi, the Sprinter was a diesel. We were both behind some idiots passing with a .5mph speed difference. We were both in a hurry.

Finally the idiots cleared the left lane. I could hear the Sprinter punch it and take off. There was just the merest wisp of black smoke momentarily as he went from about 50 to 70 and got around the morons. No cloud of smoke at all, despite the max acceleration.

I then rapidly left them all in my wake.

 

[EightySix Four]

http://www.youtube.com/watch?v=z-1MTr8X_Z0

That guys is allowed to make smoke, street tuned trucks are not.

 

[Bartman39]

Guess this is where I should step in...? I own a diesel a Duramax and with no tune on it (stock) there is very little smoke because its choked down with a fuel curve that sucks... But with a good tune on it the fuel is there to prevent the higher boost pressures from leaning out the cylinders and burning pistons... It may smoke quite a bit from the start but the boost builds and it cleans up and is gone... I dont make but around 400HP with mine but there are guys that make tons more and yep they smoke like crazy... As for trucks not smoking its mainly due to the frick`in EPA and by manufacturers that want to promote their trucks as clean diesels for the public...

As for anybody saying I`m not allowed to make smoke well just fall in behind and find out if you can stop me... I dont smoke people out just for the fun of it though I do it only when someone is being an asshat and thinks my back bumper is a wind brake and yes I can make cars and headlights disappear day or night... BTW I`m not a kid at 50 but do have a truck that weighs a bit over 3 tons and turns a 14 second 1/4 mile but will drive anywhere and get 18-19mpg and tow 15,000+lbs (and still get 10-12mpg)...

A diesel with a good custom tune will not smoke unless making HUGE horsepower with a large number of physical modifications (exhaust, intake, turbo, etc.), on HPT's forums we make fun of the smokers because they can't tune

Your full of it...