Lugging the engine - good or bad for economy?

[cfg83]

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

[ttoyoda]

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 (!),

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.

[landspeed]

Originally Posted by ttoyoda

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.

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.

[dcb]

Originally Posted by ttoyoda

You know how your car tires will hydroplane when you go fast enough thru deep enough water. The bearings work on this hydroplaning principle.

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.

[ttoyoda]

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.

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.

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

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.

Pressure in a fluid is applied evenly, more or less, to the surfaces.

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. Of course your results may not be reproducible for everyone else

[dcb]

"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?

[ttoyoda]

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

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

[dcb]

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

[sickpuppy318]

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.

[ttoyoda]

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

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

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.

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