BIG DAMAGE By Lugging at LOW RPM? Eco Driving Warning! (Manual Gears)

[Ecky]

I'm aware of a recent Honda Insight engine that failed... at 995,000 miles. It was driven most of its life at very low RPM - 1000-1500rpm mostly - with 7,500 mile oil changes using the factory recommended 0w20, and the engine had extensive start-stop use for its entire life. The rings were pretty shot by the time it failed (lots of blowby) but they weren't actually what did it in and it was still delivering fair fuel economy. The failure was in one of the cam journals, caused the cam to seize and snap. Possibly more oil pressure could have helped this, but at nearly a million miles, I don't think that engine owned anyone anything.

Can low RPM damage engines? Sure, but I'd say it probably has as much to do with the engine's design as anything. Low RPM can kill an engine. High RPM can kill an engine. Staying at a fixed RPM can kill an engine. Cold starts kill an engine. Getting too hot kills an engine. Infrequent oil changes kill engines, but so can flushes. Too much oil can kill them, as can too little. Oil too thick, or oil too thin. What wears out first will be in large part due to engineering.

[Isaac Zachary]

Originally Posted by Logic

Yoh! There's a lot to write down here, but yes you are correct, till you 'floor it!' at low rpm:

Back to piston ~at TDC where the condrod and crank are all in at straight line.
At low load throttle opening there's very little Air/Fuel being ignited so little load trying to push the crank straight down.

Not enough to force the oil layer out of small end, big end and main bearings,
so no metal to metal contact.

The/a turbo is not spooled up and turbo engines have a low compression ratio by nature, till the turbo spools up at higher RPMs providing that extra pressure/compression...

With today's engines being 'clever' the spark is likely retarded so that combustion occurs at a more advantageous crank angle.

Also the oil pump is not ;producing all the oil pressure it does at higher rpm.

But 'floor it!' (without downchange) and all that changes and you get the wear/damage discussed.

ie: Low rpms are OK at low load and its only the; more mixed and boundary lubrication and less oil pressure that might increase wear some.
But apply enough pressure to the bearing surfaces by flooring it and the oil layer is squeezed out and that's where the problem/s is/are.

The 'knock' discussed is not really the normal knock which occurs from too much spark advance or compression or hotspot pre-ignition; It's more that the crank has not rotated enough.

What you're describing sounds like something that happens at 400 RPM, not 1,100 RPM, in a typical engine. If there are recommendations from the manufacturer, or an automatic transmission that sets the example, of the engine being "floored" at X amount of RPM, then it is ok to floor it at X amount of RPM.

Everyone likes using ambiguous terms. "Don't floor it at low RPM." What is "low RPM?" I know people who won't let the RPMs go below 2,000, much less floor it under that. But regardless of what people say, if the owner's manual says that you can shift into 5th gear at 31 MPH and can run at any engine load at that speed and gear, and doing the math or looking at a tachometer, you see that you're at 1,000 RPM, then yes, you can floor it at 1,000 RPM in that car. The manufacturer says so.

[Ecky]

Originally Posted by Isaac Zachary

But regardless of what people say, if the owner's manual says that you can shift into 5th gear at 31 MPH and can run at any engine load at that speed and gear, and doing the math or looking at a tachometer, you see that you're at 1,000 RPM, then yes, you can floor it at 1,000 RPM in that car. The manufacturer says so.

Unless the manufacturer is European. Or American.

I jest, but it's also true that there have been instances of manufacturers giving pretty lousy advice. It doesn't inspire confidence in the other 99.9% of the time they're right.

[Isaac Zachary]

Originally Posted by Ecky

Unless the manufacturer is European. Or American.

I jest, but it's also true that there have been instances of manufacturers giving pretty lousy advice. It doesn't inspire confidence in the other 99.9% of the time they're right.

Perhaps. But when they're putting 10 speed automatic transmissions in cars that keep the RPMs to about 1,000 and then run high load and vary power by shifting gears...

Personally, I'm not sure of what lousy advice you're referring to.

[Ecky]

Originally Posted by Isaac Zachary

Perhaps. But when they're putting 10 speed automatic transmissions in cars that keep the RPMs to about 1,000 and then run high load and vary power by shifting gears...

Personally, I'm not sure of what lousy advice you're referring to.

A few of these are not advice per se, but some that come to mind:

-Ford's wet timing belts, which shed material into the oil and block drains
-Nissan's (seemingly) overly long CVT fluid change intervals, and high CVT failure rates
-Manufacturer "lifetime" fluids for transmissions (sometimes with no fill or drain ports) - which go against the manufacturers of the transmissions themselves (ZF/Aisin)
-Timing components which are known to fail early and have no replacement interval (I'm looking at you, BMW)
-Honda and Toyota trying out low tension piston rings, and finding they're far more prone to sticking due to longer OCIs, leading to high oil consumption and (in cases where people don't check their oil) engine failure

[Logic]

I knew this post would get a lot of kickback from people here, who want to save fuel by never revving their engine more than 200 rpm above idle, which is a big part of why I posted it:
The forum certainly needed some action!

BUT
The main point I am trying to get across here is this:
Look at the middle Top Dead Center picture:

Wack the top of that piston with a big hammer; which way does the crank turn?
Clockwise? Anti-clockwise..?
Fact is; neither: ALL the force is straight down!
So ALL the force provided by burning fuel at TDC is wasted!

Worse than that; all your fuel is doing is pressing the lube oil out of conrod and main bearings, causing extra friction and wear!

In the right hand pic the situation is not much better:
Lets say the line from the crank center down to the '6 deg' arrow represents the downward force from the piston.
THEN
if there was a short line between the 2 arrows, THAT would represent how much of the downward force was actually rotating the crankshaft!

ie: The long line down to 6 represents fuel burned.
The short distance between the 2 arrows represents how much of that fuel actually is doing what you want!

As fuel (/air) burns at the same rate, the lower the rpm; the less the crank has rotated before the main burn; the more the fuel is wasted causing wear on said bearings.

That's it; The point!
____________________________


Going further; the more downward force there is on the piston when the conrod is at 90 degrees to the crank; the more of your fuel actually goes to doing what you actually want; turning the crank!

ie: it's a crying shame we cant retard the crap out of ignition so the burn is at that 90 degree point!
But then there is just too much (elastic, compressible air) volume for the explosion to expand into without pushing down on the piston.
Trial and error for each individual engine design has fond that the burn is best (compromise) at around 10 to 20 deg after TDC.

The CV engine linked earlier 'fixes' this, by making the conrod to crank angle 90 deg at ~20 deg ATDC, but tooth wear !

Cylinder offset is a simple, elegant way of ameliorating this fundamental issue:
https://youtu.be/gwtZkHoVMso
But causes a LOT of engine vibration that cannot be counteracted by the normal means.

With 2 counter rotating engines, in the same casting, the vibrations cancel each other out exactly:
https://youtu.be/oYSequFtDwA

[Isaac Zachary]

Originally Posted by Logic

I knew this post would get a lot of kickback from people here, who want to save fuel by never revving their engine more than 200 rpm above idle, which is a big part of why I posted it:
The forum certainly needed some action!

BUT
The main point I am trying to get across here is this:
Look at the middle Top Dead Center picture:

Wack the top of that piston with a big hammer; which way does the crank turn?
Clockwise? Anti-clockwise..?
Fact is; neither: ALL the force is straight down!
So ALL the force provided by burning fuel at TDC is wasted!

Worse than that; all your fuel is doing is pressing the lube oil out of conrod and main bearings, causing extra friction and wear!

In the right hand pic the situation is not much better:
Lets say the line from the crank center down to the '6 deg' arrow represents the downward force from the piston.
THEN
if there was a short line between the 2 arrows, THAT would represent how much of the downward force was actually rotating the crankshaft!

ie: The long line down to 6 represents fuel burned.
The short distance between the 2 arrows represents how much of that fuel actually is doing what you want!

As fuel (/air) burns at the same rate, the lower the rpm; the less the crank has rotated before the main burn; the more the fuel is wasted causing wear on said bearings.

That's it; The point!
____________________________


Going further; the more downward force there is on the piston when the conrod is at 90 degrees to the crank; the more of your fuel actually goes to doing what you actually want; turning the crank!

ie: it's a crying shame we cant retard the crap out of ignition so the burn is at that 90 degree point!
But then there is just too much (elastic, compressible air) volume for the explosion to expand into without pushing down on the piston.
Trial and error for each individual engine design has fond that the burn is best (compromise) at around 10 to 20 deg after TDC.

The CV engine linked earlier 'fixes' this, by making the conrod to crank angle 90 deg at ~20 deg ATDC, but tooth wear !

Cylinder offset is a simple, elegant way of ameliorating this fundamental issue:
https://youtu.be/gwtZkHoVMso
But causes a LOT of engine vibration that cannot be counteracted by the normal means.

With 2 counter rotating engines, in the same casting, the vibrations cancel each other out exactly:
https://youtu.be/oYSequFtDwA

Yes, I get that. And when I'm teaching a guy how to drive a commercial truck and he keeps lugging the engine because he doesn't want to double clutch and downshift because "it's too hard", it's not only irritating, but also bad for the engine.

But when those of us who have hypermiled, shifting into the next gear at as low of an RPM the car can allow without actually lurching and shaking, and we drove our cars like that until they have over 500,000 miles on it, and we didn't stop driving them because of engine bearing, cylinder or piston failure, I don't see the problem here.

Yes, lugging is bad. But define low RPM, and what engine are we talking about.

I had some 700,000 miles in my last car and I shifted into top gear at around 30 mph, and the engine still was fine. The reason I stopped driving it was because the uni-body was rusting away.

[Logic]

Originally Posted by Isaac Zachary

Yes, I get that. And when I'm teaching a guy how to drive a commercial truck and he keeps lugging the engine because he doesn't want to double clutch and downshift because "it's too hard", it's not only irritating, but also bad for the engine.

But when those of us who have hypermiled, shifting into the next gear at as low of an RPM the car can allow without actually lurching and shaking, and we drove our cars like that until they have over 500,000 miles on it, and we didn't stop driving them because of engine bearing, cylinder or piston failure, I don't see the problem here.

Yes, lugging is bad. But define low RPM, and what engine are we talking about.

I had some 700,000 miles in my last car and I shifted into top gear at around 30 mph, and the engine still was fine. The reason I stopped driving it was because the uni-body was rusting away.

Ah.
I cant define too low, lugging rpm.
But we have all 'felt it' and geared down because we 'just know' a lower gear will require less throttle for the same desired speed or acceleration.

Hmm! Maybe that's a definition!?

[Ecky]

Originally Posted by Logic

I knew this post would get a lot of kickback from people here, who want to save fuel by never revving their engine more than 200 rpm above idle, which is a big part of why I posted it:
The forum certainly needed some action!

BUT
The main point I am trying to get across here is this:
Look at the middle Top Dead Center picture:

Wack the top of that piston with a big hammer; which way does the crank turn?
Clockwise? Anti-clockwise..?
Fact is; neither: ALL the force is straight down!
So ALL the force provided by burning fuel at TDC is wasted!

Worse than that; all your fuel is doing is pressing the lube oil out of conrod and main bearings, causing extra friction and wear!

In the right hand pic the situation is not much better:
Lets say the line from the crank center down to the '6 deg' arrow represents the downward force from the piston.
THEN
if there was a short line between the 2 arrows, THAT would represent how much of the downward force was actually rotating the crankshaft!

ie: The long line down to 6 represents fuel burned.
The short distance between the 2 arrows represents how much of that fuel actually is doing what you want!

As fuel (/air) burns at the same rate, the lower the rpm; the less the crank has rotated before the main burn; the more the fuel is wasted causing wear on said bearings.

That's it; The point!
____________________________


Going further; the more downward force there is on the piston when the conrod is at 90 degrees to the crank; the more of your fuel actually goes to doing what you actually want; turning the crank!

ie: it's a crying shame we cant retard the crap out of ignition so the burn is at that 90 degree point!
But then there is just too much (elastic, compressible air) volume for the explosion to expand into without pushing down on the piston.
Trial and error for each individual engine design has fond that the burn is best (compromise) at around 10 to 20 deg after TDC.

The CV engine linked earlier 'fixes' this, by making the conrod to crank angle 90 deg at ~20 deg ATDC, but tooth wear !

Cylinder offset is a simple, elegant way of ameliorating this fundamental issue:
https://youtu.be/gwtZkHoVMso
But causes a LOT of engine vibration that cannot be counteracted by the normal means.

With 2 counter rotating engines, in the same casting, the vibrations cancel each other out exactly:
https://youtu.be/oYSequFtDwA

Most modern engines do have offset crankshafts to mitigate this. In my particular example, when the piston is at TDC, the crankshaft is already 12 degrees past vertical. In addition, at low RPM, you see positive ignition timing - which is to say, the spark plug fires even later than that.

As you've said, in a traditional engine, you might fire the spark plug as much as 45-60 degrees before TDC, because combustion is slow, and it takes time to build pressure, and you want to position peak pressure to be most advantageous. In a modern fast combustion chamber (again my case) Honda isn't firing the charge until the crankshaft is 22 degrees (+/-) past the top - and that's when pressure starts building. Hence my point about engineering around issues. If you are burning your fuel 80 degrees later than in older engines, you're moving all forces entirely past the damage zone. Which is already being done. The key is to be able a to make combustion fast enough that little to none of it needs to happen before the crank is vertical.

[Piotrsko]

If it was that bad to floor the gas pedal from idle or any low rpm, engines would be constant speed. Spent many years idling at the Christmas tree waiting for a green then stomp in 4,6,8 bangers that still went at least 150,000 miles or 240K clicks for metric fans MTBF

Forgot to mention sidestep the clutch, also

Trannies, diffs and pressure plates were my issues