Why is Lugging Engine Bad?

Read the FAQ and still not sure about something? Want to shift faster? Post here.
Prodigal Son
Master Standardshifter
Posts: 3079
Joined: Fri Jun 23, 2006 11:20 pm
Cars: 2006 Jetta, 2004 Miata
Location: Ottawa

Post by Prodigal Son »

Thanks Tony, that's helpful.
Just some guy on the Internet. Heed with care.
Tinton
Master Standardshifter
Posts: 3086
Joined: Sat Nov 04, 2006 8:01 pm
Cars: 91 MR2t, 88&86 Fieros
Location: GA

Post by Tinton »

vios wrote:
Prodigal Son wrote:
Tinton wrote:I haven't been given any definite reasons for lugging to be bad either.
In some sense this is putting a strain on it, but really all you can do to an engine is dump fuel and air into it, and if that generates enough torque it goes and if it does not, it doesn't go. What parts exactly are being strained?
Assuming the same amount of air-fuel mixture, then the same amount of energy is produced. If the engine is at the correct revs, then the energy of detonation is transferred by pushing down the piston, the piston moves at the appropriate speed such that the burn is complete by the time the piston is at its lowest point. The cylinder volume is at maximum, and the cylinder pressure drops.

If the revs are too low, then the downward movement of the piston may not be fast enough such that the burn is complete but the piston hasn't gone down completely. The cylinder volume is not at its maximum, so there is build up of pressure. That pressure has to be transferred somewhere (presumably to the cylinder walls, etc) or at least converted as more heat.
That, I presume, is the concern for damage when dumping the gas at low revs.

But as you mentioned, modern cars have intelligent ECU's. So it would either adjust the air-fuel ratio or cut off the fuel completely when the throttle is mashed at low revs.. The engine either goes, hesitates or stalls. But I don't think it would lug that significantly. Maybe with old cars, this is a concern.

Tony
Thanks! Nice technical response, you're great! :o
Friend
Junior Standardshifter
Posts: 28
Joined: Sat Sep 30, 2006 4:38 am

Post by Friend »

vios wrote:
But as you mentioned, modern cars have intelligent ECU's. So it would either adjust the air-fuel ratio or cut off the fuel completely when the throttle is mashed at low revs.. The engine either goes, hesitates or stalls. But I don't think it would lug that significantly. Maybe with old cars, this is a concern.

Tony
Thanks for such a visual explanation. Now I finally understand what lugging is doing to the engine. So with the ECU's I don't think I did any damage to my car. Especially since I did not even dump a lot of gas. Just stayed at the higher gears and accelerated very slowly. Of course my car is broken in and I no longer do this. I normally shift at 3,000.

Thanks again.
- V35 -
DW10+BE4/5L
Senior Standardshifter
Posts: 149
Joined: Thu Sep 07, 2006 2:08 pm

Post by DW10+BE4/5L »

Usually in an Otto engine the combustion takes about 20 to 30 crank degrees. It depends mostly on engine speed. This is a known fact, it's been verified in experimental engines by photographing the combustion chamber through a window. So by this argument, the pressure would depend mostly on the mass of the charge. The detonation phenomenon takes place, according to the most credible theory I've heard, when the pressure rises too high. The unburned mixture in front of the flame front is then compressed to so high a pressure by the expanding burned mixture that it ignites all at once. Because the suddenly igniting mixture is in the periphery of the combustion chamber, pressure waves (shock waves?) start to travel in the combustion chamber and make the loud sound. The pressure waves are what breaks the engine.

In homogeneous charge compression ignition (HCCI) combustion the entire mixture is ignited at once without a spark plug. It's almost like detonation, but there aren't any shock waves because it all happens at once in the entire combustion chamber.

Prodigal Son, good point about the more easily felt low frequency vibrations, I never thought about that. But that diesels would last longer than gasoline, I don't know. Why would the manufacturers give an advantage to diesels? After all, they are all trying to make the cars with as little as money as possible. Are there any studies on this?
User avatar
vios
Senior Standardshifter
Posts: 103
Joined: Fri Nov 03, 2006 8:55 am
Location: Philippines

Post by vios »

Friend wrote:
vios wrote:
But as you mentioned, modern cars have intelligent ECU's. So it would either adjust the air-fuel ratio or cut off the fuel completely when the throttle is mashed at low revs.. The engine either goes, hesitates or stalls. But I don't think it would lug that significantly. Maybe with old cars, this is a concern.

Tony
Thanks for such a visual explanation. Now I finally understand what lugging is doing to the engine. So with the ECU's I don't think I did any damage to my car. Especially since I did not even dump a lot of gas. Just stayed at the higher gears and accelerated very slowly. Of course my car is broken in and I no longer do this. I normally shift at 3,000.

Thanks again.
What I described is what can happen within the combustion chamber at low rpms, a lot of fuel and high load. It may or may not be lugging. The explanation is simplified, and may not be 100% correct.

Anyway, as long as you are not demanding anything from the engine, staying at low revs is relatively safe. Think of the formula:

H = P x L x B x R

where: H = horsepower, L = stroke length, P = cylinder pressure, B = bore or area, R = RPM

You can't do anything much with the stroke length or the bore, so you are left with P and R.

During low hp conditions (creeping, cruising, neutral, first gear), both R and P can be low. It is only when you require more HP (ie dumping gas at low revs and a high gear) that cylinder pressure becomes a concern, ie if H needs to be high, and R is low, then P needs to be high. The excess cylinder pressure may not exert immediate damage, but it might eventually cause wear and tear in the long run.

Here are some possible conditions of what can happens if you floor the gas:

1. correct revs, any gear = good
2. any rev, neutral = 0 load = 0 HP
3. low revs, 1st gear = good (the 1st gear decreases the load, and therefore the HP requirement)
4. low revs, high gear - may not be good (high HP required, low revs, resulting in excessively high cyclinder pressure)
5. high revs (redline), any gear - maybe nothing will happen as cylinder pressure may actually drop past redline compensating for the increased RPM. You'll just overheat or blow the engine, but then you have a rev limiter.

Note, the presumed possible damage may be pure speculation, these car engineers are very good. They have designed the engine to operate safely and durably within the operating limits of the engine, or prevent it from operating outside parameters whatever driving conditions you throw at it (well, mostly).

Tony
User avatar
vios
Senior Standardshifter
Posts: 103
Joined: Fri Nov 03, 2006 8:55 am
Location: Philippines

Post by vios »

DW10+BE4/5L wrote:Usually in an Otto engine the combustion takes about 20 to 30 crank degrees. It depends mostly on engine speed. This is a known fact, it's been verified in experimental engines by photographing the combustion chamber through a window.
Correct. You want to ignite the mixture when the piston is already moving. It's easier to push. That's why it's hard to launch from a full stop.
So by this argument, the pressure would depend mostly on the mass of the charge. The detonation phenomenon takes place, according to the most credible theory I've heard, when the pressure rises too high. The unburned mixture in front of the flame front is then compressed to so high a pressure by the expanding burned mixture that it ignites all at once. Because the suddenly igniting mixture is in the periphery of the combustion chamber, pressure waves (shock waves?) start to travel in the combustion chamber and make the loud sound. The pressure waves are what breaks the engine.
Right, it is usually the increased cylinder pressure that causes predetonation. Gasoline will self ignite given sufficient amount of pressure and temperature. That's why if you hear a knock caused by predetonation, it usually happens at low revs or high load. Both conditions may cause excessively high pressure and temperature. You can even have multiple detonations (one part is predetonated, and another part detonated by the spark plug, effectively producing multiple flame fronts). When you have uneven detonation, the pressure inside the cylinder is not symmetrical, so you may actually push the piston head against the cylinder wall (the knock). Worse is when gasoline predetonates during the compression stroke. Imagine the forces produced when gasoline ignites and the piston is moving upwards.

Tony
DW10+BE4/5L
Senior Standardshifter
Posts: 149
Joined: Thu Sep 07, 2006 2:08 pm

Post by DW10+BE4/5L »

From a fatigue point of view it is of course understandable that high pressure implies high amplitude stress. But at low speed you will also get fewer cycles. My car's owner's manual says that you should select the gear so that the engine runs smoothly but there are no exact specifications. Conclusion: the more revolutions at high pressure, the faster the fatigue of the engine. On the other hand the each revolution wears all sliding parts. So why don't we all start growling our engines and report back in this thread in a few years if the engines or transmissions fail. That, I guess, is the only way to find out what really happens. Besides, I kind of like the growl. I wish I had a 10.4 liter Cummins, that growl at 1100-1200 RPM is like music. I used to drive a truck that had one and I just couldn't get enough of it.
User avatar
vios
Senior Standardshifter
Posts: 103
Joined: Fri Nov 03, 2006 8:55 am
Location: Philippines

Post by vios »

DW10+BE4/5L wrote:From a fatigue point of view it is of course understandable that high pressure implies high amplitude stress. But at low speed you will also get fewer cycles. My car's owner's manual says that you should select the gear so that the engine runs smoothly but there are no exact specifications. Conclusion: the more revolutions at high pressure, the faster the fatigue of the engine. On the other hand the each revolution wears all sliding parts. So why don't we all start growling our engines and report back in this thread in a few years if the engines or transmissions fail. That, I guess, is the only way to find out what really happens. Besides, I kind of like the growl. I wish I had a 10.4 liter Cummins, that growl at 1100-1200 RPM is like music. I used to drive a truck that had one and I just couldn't get enough of it.
Where the low rpm is depends on the engine. You have a high displacement engine, which means you need not rev your engine too much to develop the same amount of torque. So with your Cummins, growling at 1100 rpm is enough.

In contrast, low displacement engines can only develop the same amount of torque by revving high. So these drivers (like me) has no choice but to whine the engine to go.

H = P x L x B x R

Tony
Flash
Senior Standardshifter
Posts: 108
Joined: Wed Sep 05, 2007 5:51 pm

Post by Flash »

Prodigal Son wrote:
Hatchman wrote:
Prodigal Son wrote:Putting a load on an engine at low RPM will make it growl, but growling is not lugging. Lugging is irregular. If it growls and goes it is not lugging
Still, even this can't be too healthy huh.
I don't know. I've never heard a convincing explanation of why it would be bad for it though. Maybe those who say it is bad are confusing it with lugging?

The growl is simply the engine note at a lower frequency due to lower revs. Some may find the noise unpleasant, or even assoiciate it with an animal growl and assume (unconsciously) that if an animal is unhappy when it growls then a motor must be too.

Can anyone give a reasoned and authoratative argument as to why or why no it would be bad simply to run an engine at low revs (assuming no lugging)?
I can answer that.

You will simply damage the transmission. I have no idea about the engine, but if you go at 30km/h, put it in fifth and floor it, then you may want to look for a new transmission after a certain timespan.
hockeystyx16
Master Standardshifter
Posts: 6960
Joined: Tue Dec 26, 2006 3:54 am
Cars: 95 probe GT
Location: toledo ohio
Contact:

Post by hockeystyx16 »

lugging (along with the burning clutch smell) are unmistakeable. if you think you lugged your motor, you didnt. once you lug, you will know exactly what it is.

clutch chatter has nothing to do with lugging, but the car will feel like its lugging, but lugging is a lot more severe that clutch chatter

ask somebody to grab you around the neck and yank your head back and forth very fast for a few seconds. thats how your car will feel when your lugging. i did a launch in 3rd gear when i was way too tired to be awake, let alone driving. it lugged for about 10 seconds before rpms finally came up to idle and it smoothed out. i was like WTF. needless to say, i was wide awake after that.

if your above idle, you pretty much cant lug. the general lugging rpms is under heavy load between stalling and idle. AKA launching in 3rd gear, need mroe throttle to keep the car from stalling, more load on the motor, clutch out too fast, rpms dip below idle but not enough to stall (because you are already moving). once you lug, you will know exactly what it is.

growling when you floor the gas at low rpms, tahts because your letting more air in the cylinders, ECU accordingly sprays more gas, and you have more powerful combustion. thats why it growls.

and anybody who wonders why lugging is bad for the motor. if you ever lug, youll know.
spark plugs fire before top dead center (BTDC). in most cars the range is 8-15* BTDC.
now when idling or under light load, the pistons are moving up and down fast enough to where by the time the plug fires, the piston will just be reacing the top of its travel, the part of its travel where its barely moving. then as the crank keeps turning, another few degrees of a turn, and the piston is on its way down, and the explosion pushing down on it.

at the kinds of rpms you would lug at, your pistons are going up and down slow. you have to be under heavy load to lug, thus, throttle open wider than usual. this lets more air in, and more fuel. you have more fuel mixture in your cylinders.
when lugging, theres so much mixture in the cylinder, and the piston is going up so slow, it will diesel from the compression. as we all know, compressing something will increase its temperature. theres a lot of mixture to compress into a small space, so the piston going up will compress it a lot. boom. preignition from combusiton pressure flashpointing the gas. before the plug fires.
what happens now, is the combustion happened while the piston is still on its way up. combusion makes thousands of PSIs of pressure, and all that pressure is instantly pushing back down on the piston. if its bad enough, it could snap a rod right in half. most of our cars dont have cylinders big enough to make that much pressure.

in most cases of lug, the preignition will try to push the piston back down, and that will create a momentary stop in the drivetrain. insert violent jerk here. then it will continue turning, insert a violent jerk going the other way, and the piston has no choice but to keep compressing the already extremely compressed stuff. now it has nowhere to go but to force its way around the rings. theres your piston rings going bad.
then after all that, as the piston is coming up to where the plug would fire, it will fire, but theres nothing to burn, it already flashed over.

so to sum it up, it will diesel combust while the piston is on its way up (preignition) sending a severe shock going the wrong way thru the drivetrain
all that intense combustion pressure keeps getting squeezed some more by the piston thats still going up, and it has nowhere to go. it either blows by the rings, or it could blow your head gasket and get out that way.

something like that
-Roman
95 Probelem GT
Image
User avatar
jomotopia
Moderator
Posts: 10230
Joined: Fri Mar 24, 2006 1:01 pm
Cars: 04 STi, 05 Matrix XR (AT)
Location: AWD Turbo Nirvana
Contact:

Post by jomotopia »

Flash wrote:You will simply damage the transmission. I have no idea about the engine, but if you go at 30km/h, put it in fifth and floor it, then you may want to look for a new transmission after a certain timespan.
why? the transmission doesn't care what revs you're at or how your engine's combustion is happening. all it knows is this shaft turns that shaft through a gear. i would think that flooring it in your power band would be worse (though still not actually bad) for the tranny than running at low revs b/c you're putting more power through it.
2013 Subaru Impreza WRX in Orange
User avatar
DeHackEd
Senior Standardshifter
Posts: 122
Joined: Thu Apr 19, 2007 9:39 pm

Post by DeHackEd »

Yesterday I accidentally tried to stop while in 4th gear without clutching (could've sworn I had put it in neutral already). Car stared to shake pretty violently. I hit the clutch as son as I realized what the hell was going on. My car does have a knock sensor according to wikipedia, so don't think it makes you completely immune.

I think I now know what lugging is. Not trying that again.

Oh, and my sister was pretty mad with me as well. :oops:
2013 Mazda 3
Driving stick since mid-`07
Prodigal Son
Master Standardshifter
Posts: 3079
Joined: Fri Jun 23, 2006 11:20 pm
Cars: 2006 Jetta, 2004 Miata
Location: Ottawa

Post by Prodigal Son »

paul34 wrote: However, I still have yet to come up with a good term for the engine turning at very, very low RPMs and so am forced to use the term "lugging."
I think there are two terms for it, depending on whether you are under power or not. If not, its "idling". If yes, its "growling". At least, those are the terms I've always used. When people say "lugging" today, I think they mean "growling".

I think the main reason that people freak out when their engine growls is that you seldom, if ever, hear it in a slushie, which will just shift down. The scaremongers have told them that in a manual they are in danger of lugging their engine (a very real danger 30 years ago) and when they hear that unfamiliar sound they put two and two together. In this case, however, they get five. Growling ain't lugging.

In a manual you will tend to hear the engine growl if you take a corner in one higher gear than you should have done. You can't shift in the turn, so you listen to the engine growl for a few seconds. Usually by the time you are out of the turn and able to shift again, the engine has already picked up enough speed that there is no longer and need to shift down.
Just some guy on the Internet. Heed with care.
User avatar
DeHackEd
Senior Standardshifter
Posts: 122
Joined: Thu Apr 19, 2007 9:39 pm

Post by DeHackEd »

I've had slow RPMs. I do it a lot when launching. This was violent car shaking. Like the brakes said "stop running" and the fuel injectors said "Oh no you don't!" and I was caught in the middle. Maybe the injectors just dumped a lot of fuel in a vain attempt to keep the engine running against the brakes.

Well, either way it felt very unhealthy for the car. If it wasn't lugging, it was the next worst thing.

If you need need a name for really slow turning, I say sub-idling. Maybe drop the hyphen. :)
2013 Mazda 3
Driving stick since mid-`07
Johnf514
Moderator
Posts: 8574
Joined: Thu Feb 24, 2005 3:59 pm
Cars: '07 Mazda3, '06 Ninja 636
Location: Orlando, FL
Contact:

Post by Johnf514 »

When lugged, most cars will fight to keep the engine running and the computer will dump fuel in to prevent a stall. Sometimes it works, sometimes it doesn't.

Lugging isn't good for the vehicle, but the occasional lugging situation won't destroy anything. Just don't do it all the time.
2007 Mazda3
Mods: 15% tint, Eibach ProKit
2006 Ninja 636
Mods: NOS & sidecar
Post Reply