Lugging the engine - good or bad for economy?
 

I was just reading another thread on here, where the final drive ratio was being discussed, and several people said that lugging the engine is bad, and will cause a drop in MPG.

What do people here think? Is Lugging the engine a useful tool for hypermiling or not?

lower rpm is good from an internal friction and lower stress point of view, but if it is run too low where the engine is bucking then that isn't so good :) I don't think hypermilers lug their engines.

Anything above the jerky-jerky point is ok, if you're gentle on the throttle. Realize that there's not much power available and act accordingly. I'll run mine as low as 30mph in 5th (1700 rpm) and there's still enough for some gentle acceleration.

From a dead stop, low rpm (just above lugging) and gentle throttle is the best way to get up to speed.

I have to say that lugging is very underrated among most hypermilers. I can't wait to get a manual car and start lugging again. I got 240% above EPA by using lugging and engine-off coasting in my last car :)

Your engine is not running at its best efficiency when it is lugging. You'll see better results from higher rpms. Take a look at this thread for more info on why this is.

I think we have to get our terms straight.

Landspeed, when you say "Lugging", I think many people are thinking of that mode of operation where the engine is literally bouncing back and forth because the rpm is too low for the given load. I suspect that isn't what you are talking about.

lugging is hard on the motor.. low rpm thats smooth isnt lugging..

OK!

When I say Lugging, I mean high load, low RPMs, at the highest possible load to avoid the engine shaking / struggling. I was using about full throttle between 600 and 1200rpm in 5th gear (!), then lifting off to 65% throttle between 1200 and 1500rpm, then going to 40% or so throttle between 1500rpm and 1800rpm, before EOCing and coasting for a long time. (I was lifting off the throttle to avoid fuel mixture enrichment).

I think it may have been because my engine was low compression that it let me do this 'lugging' (although importantly it wasn't 'shaking', it was quite smooth). It started shaking below 600rpm.

Using this technique together with EOCing, I got about 54mpg over a whole tank which was around 240% of EPA IIRC, so about 140% above EPA (the car was a low-compression 1.8 petrol turbo with no aerodynamics at all, and it weighed about 1300kg (2860 lbs). No other method got better economy - and I was EOCing for up to 66% of the whole journey :)

yep lugging is when when youre engine is "jerking" or working harder.

i usually drive in a "higher" gear than many people i know, for instance about 1000rpm 4th gear 25 mph.

th eimportant point is that if it seems like it is hard for your vehicle to do the work it is probably not good (i.e. no accel power, jerking engine, hard to maintain speed etc) but it is good to keep the rpms low and the engine running smoothly. i mostly drive in the city and get pretty decent gas mileage with this technique

Lugging is more of a term of art than science ... so defining it as folks have done above helps.

Traditionally, Europeans think lugging is letting the RPMs go below 3,000. A friend of a friend ate up his Porsche's flat six driving like that ... because his mechanic told him to do it. :rolleyes:

I tend to drive at a very low RPM ... just above where the engine would struggle. My minimum speeds in gears for my stock '06 Civic are as follows (from memory):

5th 36mph
4th 24mph
3rd 13mph
2nd 7mph

I'll have to see where the RPMs are in these speeds 'n gears.

landspeed -

I think of it as being similar to baseball's "Mendoza Line", i.e. every car has a "Lug Line". For me, that definition is the point where :

function(MPH, Transmission Gear) = engine RPM below the normal idle RPM.

The above assumes your are on the flat and no one is charging up your butt. At this point, if you are a feather-light on the throttle, you can accelerate gently as needed.

Lugging is not efficient from an engine POV, but I think that in many cases, the lower RPM X @ MPH Y usually beats the "engine operating at peak efficiency" argument.

If you are driving at the "Lug Line", external conditions may change (hill, need to speed up) such that you will have to drop into a lower gear.

CarloSW2

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. :D

I know this fact :) I decided to risk it anyway - and would (and will) do it again.

I had a car with the same engine (but the non-turbo carb version), and managed to run it dry on oil a few times, including times where I would accelerate around a corner and the oil pressure light would come on, and other times (twice) where it would 'squeak' due to no oil (I topped it up at this point). This all happened due to a large oil leak it had. This car lasted from 150,000 up to 230,000 miles like this, then died due to an unrelated problem.

My last car (1.8 Turbo Bluebird) went from 120,000 to 176,000 miles, with a lot of that being extreme hypermiling. The car was almost certainly 'clocked' (and had been abused from some things I found). It was more worn than my 240,000 miles Bluebird mentioned above. I drove thousands of miles with repeated engine-off coasting, sometimes shutting off and restarting the engine a few times in each mile (as long as no-one was behind me), and dropping to 20mph before going into 5th gear and lugcelerating :) up to 35mph before EOCing back down to 20mph. I had an oil pressure gauge which helped.

I think the important thing is that when I went back into 5th gear each time, I would bring the revs up with the ignition switched off so oil pressure came up. Also the engine was slightly lower compression than usual. Finally, by avoid actual 'lugging', the bearing loads were kept within reasonable limits.

With petrol prices as they are, my UK car was worth about 450 pounds, but petrol is probably about 65 pounds per tank now. So, the car is worth 6-7 tanks. Once I have hypermiled 12 tanks at 100% above EPA, then the car has paid for itself. I hypermiled so many tanks in that car that it paid for itself several times over.

For aerodynamic air bearings this is certainly a true statement, but I do not know that it is a primary concern for plain bearings with sufficient oil pressure. Pressure in a fluid is applied evenly, more or less, to the surfaces.

Also I can guarantee that for a given torque level, there are a lot greater forces at work and stress at higher rpms.

There exist two types of air bearings: ones that do and ones that do not require an air supply. This second type lifts off when you spin it.
The oil pressure does not lift the crankshaft off the bearing shell. Oil pressure (or more accurately, flow) is needed to replace oil that leaks out the sides of the bearing shell and drips back into the oil pan.

Yes. And the higher rpms also cause and increase the strength of the hydroplaning (hydrodynamic) effect of the crankshaft riding up on the oil film. So the bearing is protected.

That is not true or relevant here. The oil is introduced into each bearing shell thru only one hole. A car engine is not a hydrostatic lubrication system, it is a hydrodynamic lubrication system. A system where fluid pressure was applied evenly all around the bearing would have a net sum force of zero, and would support nothing. You might as well drop the shaft into a pond of oil and watch it sink for all the support it would get. But it would have even fluid pressure around it, more or less, as it sank.

More info here:
http://www.bearingsindustry.com/aboutbearing/jb.htm

landspeed,
I am glad it worked out for you. :D Of course your results may not be reproducible for everyone else

"And the higher rpms also cause and increase the strength of the hydroplaning (hydrodynamic) effect of the crankshaft riding up on the oil film."

I believe you, but could you just show me where it says that the RPM effect applies to journal bearings in situations appropriate to what landspeed described please?

http://www.engineersedge.com/lubrica...l_bearings.htm

http://www.machinerylubrication.com/...?articleid=779

I did look at both those site prior to asking,

re #1, they state the minimum thickness increases with surface speed, and decreases with load. My point here is that load increases in an engine with rpm as well, having to accelerate the pistons and overcome internal resistance. At lower RPMs the engine might not be making that much torque even.

re: #2, they did mention using an external oil pressure source to center the journals. Might be nice to have a valve and an accumulator for worrysome hypermilers to use just prior to restart :)

my old '88 toyota had a internal gear "gearoter" oil pump, and being positive displacement, had output pressure directly proportional to engine speed. So while engine loads decrease with RPM, so did oil pressure. It lugged a lot more than my mazda does now....

now after reading these articles, i realize that the lugging "jerk" at low RPM/high throttle is litterally the crank being thrown into the side of the bearing. ...not good.

OK, so now i guess i'll feather it past the "lug line", but have more confidence lugging around below it. slow and easy, not slow and WOT.

Centering the journal with oil pump pressure is very involved. You would need oil holes with restrictors all the way around the crankshaft at every bearing. It is not going to happen in a car engine.
Accumulators however do exist for people who want to build oil pressure before they start the engine.
http://www.google.com/search?hl=en&q...or&btnG=Search


Why do you think the engine is not producing torque at low rpm? You could have wide open throttle and significant cylinder pressure, which translates to lots of torque, and large bearing loads, but the rpm is not high enough to produce enough HP to speed up the vehicle due to air drag at the speed you are currently going, or you are going up a hill, etc.

You will recall that (if you do not recall see this: http://www.vettenet.org/torquehp.html)
HP = (Torque * RPM)/5252. (which by the way is why every graph you see of power in horsepower and torque in foot-lbs cross at the same rpm of 5252)

Or to put it another way, you can get the same HP at these two different engine operating conditions :
100 Ft-lbs of torque at 200 rpm, or 10 Ft-lbs of torque at 2000 rpm.
The bearing load on the crankshaft is lower at 10 Ft-lbs. torque.

As far as engine inertial loads being higher at high rpm, the loads increase as the square of the RPM. Thus they are only significant near the red-line rpm. At "normal" engine rpms the cylinder pressure loads dominate.

Another reason you would want an rpm high enough to get good oil flow volume out of the oil pump is that the bearings are COOLED by the flowing oil. The slower the oil goes thru, the hotter and thinner it gets in the bearing.

Look at this:
http://www.performance-auto-parts.co...nnecting-rods/
Also read this, look at the chart. Film thickness is zero at 180 rpm. They need to speed up that crankshaft!
http://130.15.85.212/proceedings/Wor...apers/A542.pdf
Finally here are pretty pictures of bearings that have been lugged.
http://www.mlcmotorfactors.co.uk/Tro.../BearingTT.pdf
http://www.hadmac.com/technical_info.htm

The engine load is an exponential function of rpm, but I don't know that the increase in the oil layer also offers exponential cushioning. There is a graph there somewhere which would answer your concern for landspeeds bearings, but I don't know if it is a valid concern without seeing it.

re: accumulator, I was just thinking upside down 2-liter and a solenoid tapped into the oil pump with a restriction and check valve to fill it :)

Again the term lugged has surfaced, from a murrican referring to an english document. What do you suppose the article meant by "lugged"? I don't think landspeed mentioned anything about 180RPM :)

Oh, and reality check, last I checked a set of bearings are like $20. Us murricans need to be more self sufficient anywho.

And "the engine might not even be making that much torque at low RPM" part means just that. Here's a picture, it is a pretty steep climb until 3000RPM for this engine:

http://www.solsticeforum.com/forum/a...n_sky_dyno.jpg

Yeah, but at 1250 rpm it's already making more than my car's peak horsepower. More than enough. Step up to about 1500 to account for it being a heavier car.

I would be fascinated to see the results if you go ahead and do that. Please post pictures of before and after. If you could take a video of the "during" and put it up on youtube that would be great.

No, it goes up as the square. It does not go up as e^rpm.
"exponential" does not mean the same as "squared", so you did not bother to read the links.

LOL, you have never put a set of bearings in. If you had ever done the job, you would know that the amount of labor time makes the cost of the bearings unimportant.

Look at your chart. The minimum RPM the data is presented at is 1100. Why do you think that might be?

Tell you what, though. You just go ahead and run your engine how you see fit. My work here is done. :thumbup:

Pull that sucker on the ramps and drop the pan. I bet HE could rebuild that whole bottom end in about an hour. Not me, i'm not that good.

jerk
 

sickpuppy,I think your's was the first use of the term "jerk",which I understand as an "instantaneous acceleration." Under some of the extreme "lugging" scenarios mentioned,I understand that if jerk occurs,the bearing journal can actually displace the oil film,achieving "high-point",where there exists momentary metal-to-metal contact between the journal and babbit material.Instantaneous friction-welding and then fracture-release can occur,and this is basically the beginning of the end for both surfaces.I have been told that the switch to automatic transmissions for Diesels,has happened in part do to broken crankshafts due to "jerk",when operators let rpm's drop too low.Evidently,the torque-converter on an automatic will stall before rpm's can drop to a level where jerk would occur,protecting the crank.

Well you your driving around at 500 RPM and giving yourself whiplash you'd have to expect engine damage... I think more relevant to this thread are the less apparent effects of "half-lugging". What about 1300 RPM? My car does this low shudder that sounds and feels like a powerfull 18 inch subwoofer at 25Hz. Besides for the new squeeks, what kind of engine damage have i done????

Anyway, I did 800rpm WOT (and even got some positive turbo boost at about 950rpm :) ). And repeatedly did this for 10,000s of miles.

The reason my car survived this, as well as rapid EOCing (must have been 30,000 EOC cycles easily), may be due to the fact that the CA 8-valve engine is VERY tough anyway, and also that it was low compression, so the cylinder pressure loads were not that high.

On my 1.0 Micra, I did maybe 1000rpm at 60% throttle. My rule is, if the engine feels comfortable (apart from the fact that the power delivery is less smooth as it is at about 15Hz at 800rpm), then lug all you want. If the engine starts jerking at all stop, because that is when you can damage the crank and so on. I never did that in my car.

Another random point - when EOCing, bump-starting with ignition on is, in my opinion VERY bad for the engine. Doing this revs the engine straight up to 1500rpm+, and you are often accelerating at that point, with no oil in the bearings!. Better is what I did - ignition off, and gently tap the clutch up a few times, so RPM goes up to 80, then 150, then 300, then gently lift the clutch till you reach engine speed. Then, 0.5 seconds later, switch on the ignition. This is what I did, and the whole cycle took 1 second or so, once it became second nature.

Lugging the engine is essentially opening the intake/throttle as wide as it gets (assuming heavy pedal pressure) with no response. Also is a nightmare of stress on your main bearings, which in a small 4cyl, are not that strong to begin with. better off redlining the engine than lugging it - MPG not considered

I realize I'm dragging up an old thread, but I need some clarification, and am hoping to get some here! Aerohead, can you provide a source for this information? I'd like to do some more reading on this idea.

I've been wondering what causes the "shaking" feeling when you lug an engine - this momentary welding and fracturing or the crank journal-to-bearing-surface would certainly explain it. Simple bearing contact would not!

Thanks!
-Mateospeed