Super low RPM running/lugging mods?

[stillsearching]

This is a topic potentially crossing a number of areas, but the 'mother' topic is what kind of effects an engine has when run at very low RPM's (lets say everything under 1800rpm at the absolute maximum, and mostly focusing on the 500-1200rpm range as a target for actual cruising) including potential engine damage ("lugging" is called bad, why? Can you modify around what makes it harmful to handle it?), efficiency benefits, problems from slow turning pistons making flamefronts not run well under load, etc. Both gas and diesel.

If you think about it, engine efficiency is nothing more than cubic inches of displacement times rpm per mile in a way. To get 30hp to cruise on you are pumping X amount of air normally (in gas/stoichometric engines, diesels are different) and it doesn't matter whether thats a 250cc motor at 14,000rpm or an 8 liter at idle. Using a smaller engine is not automatically more efficient, but when your air pump moves less air it uses less fuel (in gas engines), has less friction and other benefits. Lots has been done trying to put small engines in large cars, often with forced induction to make back up the power, sometimes with less than stellar results. I'd like to push things the other way - big engines turning ridiculously slowly and modified for slow running.

There's a couple reasons for this:
- one of them is actually exploring cheap alternative power stuff. You can get used car engines cheaper than you can get used garden tractor engines alot of the time, they are just ridiculously overpowered. The only choices are deactivating certain cylinders (something i'll explore in a separate topic) and turning the engine SLLOOOOOOWWW.
- long engine life is also a side benefit. The Lister 6hp diesels built since the early 1900's were known for running _50-100k hours_ before needing a rebuild. (6-12 years of nonstop 24/7 service, producing power, working in oilfields or whatever) Part of that was because they turned at 600rpm and were overbuilt so that a 6hp engine weighed more than a big block chevy. (something like 890lbs?)
- noise, i've always been annoyed by the BUUZZZZZZ of small engines at high rpm, whereas the slow putt-putt-putt-putt of some of those late 1800's and early 1900's engines is almost relaxing. For stationary quiet genset power and such it's alot easier to muffle.
- plus just plain other projects of interest where ultraoverdrives could make large engines (like in a hot rod) actually efficient, I just don't know of many people who have even TRIED to cruise under 1200rpm on the highway... I mean, what even happens?
- some fuels have a slower flame front more suited to slow turning engines (producer gas is I think one, whereas atomised gasoline burns fast by comparison, and some of the thicker straight vegetable oils run great in a 600rpm Lister but will screw up your Veedub apparently)


Now it's possible there are diminishing returns below a certain speed, otherwise like big truck engines would already be capable of lugging like this... yet certain much larger engines DO lug like this - train/rail stuff is like 800rpm if I remember whereas the bunker fuel used in container ships maybe runs 90rpm direct drive to the screws. And as they do so start quoting phenomenal efficiency numbers. (exceeding 50% in large ship diesels)

Now i'm not desiring to run dirty as hell bunker fuel, i'm not sure if it would work the same under road diesel in a large marine diesel or not, i'm just trying to increase efficiency by turning slower, or to find optimum efficiency rpm's for different types of fuels especially various alternative and natural fuels. Even if there is an efficiency loss turning slower than ideal, it may be worth doing so for economic reasons at times.

I'm a part of another project elsewhere on the web (related to/branched off from the global village construction set stuff, look it up the original on TEDtalks and such) trying to help mechanize the third world with better solutions, lower pollution than what they are forced to do now, using available local/renewable fuels, on a very low budget like in places where financing is not often available. A $200 junkyard diesel for a utility tractor which is only 80% as efficient (turning real slow to produce 30shp) as a $5000 industrial diesel turns into a form of self financing - you use the cheaper engine the first year, until you can afford to buy the more efficient industrial diesel. Or if the industrial diesel ever breaks since you probably cant afford to repair it til after harvest, you can use the $200 diesel to get in this year's harvest. Both are supported, run whatever engine you have available, modified to work as well as possible, while also having an ideal efficiency version as well.

Sooo.... yeah. Part of my volunteering was to find ways to find the "market-cheapest" engines and derate them down to the desired shaft powers (typically 18-50hp constant) with the best efficiency possible by turning slower or deactivating cylinders, while also being understressed. (a 6.2 GM diesel is preferred to a 1.0L lupo engine because the former will run for decades at 40shp but i'm not sure the lupo would, it's also available in junkyards and cheaper) Other projects aimed at generators or stationary would like to lower it to the 4-10shp level, with the smallest car engines turned as slow as possible, maybe with 1-2 cyls shut off but mostly just with low rpm mods if doable. New industrial diesels usually cost at least $2000 in that HP range, and new 3600rpm gasoline engines have too short of a lifespan running that fast.


So that's the why to try... can someone maybe an engineer or experimenter tell me why I can't do it, or maybe if someone else has tried and found problems?

[ksa8907]

It can be done and im sure has been done, you'll need a long stroke to bore ratio. My 3.2 v6 is 81mm stroke by 92mm bore and has a hard time climbing small hills below 1500 rpm.

[stillsearching]

Thats because youre running out of power below 1500rpm. :^) If it were in a motorcycle it'd probably climb hills fine.

Thats what i'm trying to figure out, 1500rpm really seems to be about the floor. (though I think the Dodge viper can cruise at 1200rpm as the single sole exception) Yet the drop from 1500 to 750 is potentially half the fuel usage... IF the power is enough i'm wondering what else stands in the way? Were I to do this in a car i'd be doing it on a hot rod with a 10 liter engine. For my more practical and useful tractor projects the engine will be pretty oversized for the load. A heavy flywheel would be added to help maintain smoothness as you engage the load. Whether additional mods to restrict flow or optimize torque peaks for even lower in the curve would be done is to be researched.

For reference I had a 1962 chevy 235 inline six that would idle at under 200rpm. It was so quiet you couldn't tell it was running standing by the hood if there was the slightest wind noise.

[Frank Lee]

It isn't rpm so much as it is piston speed. Too low a piston speed and efficiency drops because there's too much time for the heat of combustion to escape through the combustion chamber into the coolant. Too fast a piston speed and efficiency drops from increased internal engine friction.

Those super low rpm engines also have long strokes... figure out the piston speed and you will find it to be similar across many engine sizes and types.

Gasoline 4-strokers like 1000-1200 ft/mn. IIRC diesel 2-strokers like 1600 ft/mn.

I have postulated that a heavier flywheel would help me when short-shifting though.

[ksa8907]

Originally Posted by stillsearching

Thats because youre running out of power below 1500rpm. :^) If it were in a motorcycle it'd probably climb hills fine.

Thats what i'm trying to figure out, 1500rpm really seems to be about the floor. (though I think the Dodge viper can cruise at 1200rpm as the single sole exception) Yet the drop from 1500 to 750 is potentially half the fuel usage... IF the power is enough i'm wondering what else stands in the way? Were I to do this in a car i'd be doing it on a hot rod with a 10 liter engine. For my more practical and useful tractor projects the engine will be pretty oversized for the load. A heavy flywheel would be added to help maintain smoothness as you engage the load. Whether additional mods to restrict flow or optimize torque peaks for even lower in the curve would be done is to be researched.

For reference I had a 1962 chevy 235 inline six that would idle at under 200rpm. It was so quiet you couldn't tell it was running standing by the hood if there was the slightest wind noise.

In town I can accelerate quite well below 1500rpm, but at42mph and 1500rpm, there is a long 3-4% grade that really makes the engine work. Keep in mind its an automatic and if I press too hard it will unlock the converter and downshift, etc.

You really want to be at least square in stroke to bore or even a little longer stroke. That will give better low end torque.

[IsaacCarlson]

I have been "lugging" my engine for a couple of years now when driving in town.

I have noticed that the rod bearings are making noise. I believe it is from the very low oil pressure at idle and the fact that I am pushing close to 300 ft lbs just above idle. That is a ton of wear when there is not enough oil pressure.

I will be tearing the engine down to put new bearings in it and I hope the crank is still ok.

As of late, I have been trying to avoid lugging.

[Big Dave]

Hard lugging is rough on an engine because it sets up a resonant torsional vibration throughout the entire drive train. You’ll rapidly find the weak link. BTW don’t overbuild that weak link. It was put there deliberately to fail before more expensive components. A lot of lifted 4x4 guys oversized their U-joints when they failed only to find that axles and transfer cases broke. Beaucoup expensive.

But (at least for turbodiesels) the sweet efficiency spot is at a high load/low RPM just above a lug. Lugging occurs in arrange of load and RPM. It is possible to lug an engine turning 14,000 RPM if you load it up too much. If your engine is lugging, try reducing load (pull your foot out of it).

My F-350 (7,000 lb empty, International T444E engine (mostly stock), ZF 6-speed, Gear Vendor overdrive, 3.08:1 axle) rolls down the highway at 50 MPH at 950 RPM. That is juuuuust over a lug. But at 70 MPH and 1325 RPM lugging is nowhere in sight despite the fact my aero drag HP went up by 175%.

The strategy works well for diesels as they operate at excess air. Gas engines are married to a narrow band of fuel-air mixture ratios. Slowing the engine down reduces the pumping loss and you really need not speed up until she smokes.

[stillsearching]

So how can we reengineer an engine to lower the "lug"/resonant vibration? What kind of changes would need to be done? That's what i'm trying to figure out. (it's possible it's beyond the simplicity of having it worth doing and just down to selecting an engine with a low natural lug point too)

Thanks for the heads up of piston speed, that makes sense (although i'll note F1 engines have phenomenally high piston speed and are apparently 20% MORE efficient than our most efficient gasoline engines because the pistons are going away close enough to the speed of the flame front that it couples very nicely) so that means looking at undersquare engines and possibly modifying further could be interesting. Also possible that alternative fuels (producer gas/woodgas) with slower flame fronts might prefer a slower speed..?

By the way short rods long stroke might be worth considering too - the geometry near TDC means the short rod pulls it away more quickly, piston speed is not one speed but varies along with rod geometry. Long rods are often recommended for durability at higher speeds but it's possible a slow RPM torquer could be better served by short?

[euromodder]

I once read a report on how higher speed requires more gas - and the most economical steady speed was 30 kph (20 mph).

I was very sceptical about that, as it was performed with a car like mine, though I never could get anywhere near decent mileage at that speed - in fact, slow speed killed my average every day.

Later I found out that 30 kph in 3rd on CC works a lot better.
Still not anywhere near the claims though.

Then I tried 30 kph in 4th, at idle (around 800 rpm) and the magic happened.
As long as you're on level ground, the mpgs are simply stunning.
Doing this on a circuit as in the test, it's easy to see where they got their results !


It doesn't work in any other gear though ...
1,2,3 means you're going too slow to cover the miles and still get good mpg.
5 can't efficiently propel the car at idle - though it will, while lugging hard and guzzling diesel.

[serialk11r]

Combustion chamber geometry matters more at low speed, like people said you need a big stroke and relatively small bore. If you increase per cylinder displacement, I imagine the stroke (and thus crank, rods, and overall engine height) can get out of hand and your engine will be physically too large.

To slow down flame speed you can use cooled EGR on a spark ignition engine, and ceramic coatings for the heads and pistons can reduce the amount of heat that is rejected to the cooling system, but I wouldn't be surprised if these aren't enough to get your typical light vehicle engine to work happily below 1000rpm.