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Lowest RPM and V6 engine or any engine can goHelpful Member!(2) 

hafele (Automotive) (OP)
20 Feb 04 9:12
Does anyone know the lowest RPM that a combustion engine can go?
ivymike (Mechanical)
20 Feb 04 12:22
there are very large engines with rated speeds of 100rpm or so - I'm sure they can run at part-speed, at least temporarily.
76GMC1500 (Mechanical)
20 Feb 04 19:54
Well, you said any combustion engine so that doesn't limit us to internal combustion.  A steam engine is a form of combustion engine and reciprocating steam engines can make torque at 0 rpm.

I'm sure you meant internal combustion, though, so those 100 RPM slow speed 2-stroke diesels would be the slowest I can think of.  They are design rated to run at 100 rpm which means they can overspeed to 110-120 rpm at the most.  They don't idle because they are direct drive, but I'm sure they operate at speeds as slow as 50 rpm or maybe slower.
xr7755 (Mechanical)
20 Feb 04 23:02
I suppose if you had a large enough flyweight, you could run that V-6 pretty slow too. But you also would need the valve timing to match.
malbeare (Automotive)
21 Feb 04 4:04
The Beare sixstroke prototype4 based on a Yamaha TT 500 single produces very usable power at 500 RPM. So much so that it can smoke the rear tire at that RPM with the front wheel against an imoovable object.

evelrod (Automotive)
22 Feb 04 13:32
My old 1937 Buick inline 8 would idle all day at ~200 rpm and pull easily from that setting without a 'bobble'!

Several of the natural gas/methane V-12 and V-16 Co Gen engines used in the generation of electricity could run in the 100 rpm range but were usually set up to operate at closer to 600 rpm in turbo charged form, at least the units in Orange County and in Santa Barbara County, the ones I worked around.

Helpful Member!  GregLocock (Automotive)
28 Feb 04 1:24
How fast do you think your engine is turning on the starter motor?

Well OK, I'll tell you: 150 rpm, so it /has/ to be able to run at least that slow, reliably. There are practical installation reasons (driveline resonances etc) why automotive engines are not run more slowly, but in principle there isn't really any particular reason why even quite standard engines could not run very slowly.

One very useful thing would be to be able to run engines up from idle to red line and use ALL of that speed range. This is feasible for a prototype, but would need a lot of changes in various sub-systems.


Greg Locock

Helpful Member!  JRW261 (Mechanical)
28 Feb 04 11:22
Greg, I disagree.  yes a starter might only turn an engine over at 150RPM but the engine doesn't run at 150RPM ussually.  The minute one cylinder fires the engine, it  accelerates up to at least 500RPMs from the rich condition provided by choke or cold start mode.  Yes, some possibly can run at 150RPMs but most cannot.  At that low of an RPM the engine has internal friction and loads (waterpump,oil pump, alternator, acpump, ect) that it will loose rotational velocity between power strokes and start to shudder badly if it manages to stay running at all.  When it starts to shudder it is very bad for an engine over along period of time.  Im not saying all cars cannot idle at 150rpms, Im just saying most cars new and used cannot.  I personally think less than 4-500rpms is below its design operation and MAY experience oiling problems below such RPM.
GregLocock (Automotive)
28 Feb 04 17:27
I took the trusty Toyota out, warmed it up, stuck it in first, and then braked it down to 300 rpm. Pickup was fine from 300 rpm.

If a 2 hp starter motor can overcome the waterpump, oilpump and alternator friction at 150 rpm then I don't see why a 100 hp engine operating at 1/40th of its rated speed cannot do so.

Really, these things aren't hard to test for yourself, why not do the test before writing notes like that?


Greg Locock

patprimmer (Publican)
28 Feb 04 19:51
In my observations things that really help low idle speed are, HEAVY flywheel, small ports, If fuel injected, injecting close to the inlet valve, and sequential, if carburetored, a straight, downward shot from the carby base to the valve and small carby and manifold, short duration cam, minimal free play or backlash in ignition and cam timing devices, NO air leaks, warm manifold.

These observations are from working on pre 1920 cars, which sometimes need to be crank started, and often only have a 2 speed gearbox, and from working on ski boats with dog clutches, and tow cars.

Engine configuration has a minor effect, mainly as it effects port angle for a good downhill shot at the valve, hence tilted or slanted in line is better than upright in line, "V" configuration is better again and horizontal is best.

This applies mainly to carby engines as an injection nozzle aimed straight at the back of the valve will deliver all the fuel to the cylinder every time, no matter where the manifold runs.

The greater the number of cylinders, the less the need for flywheel weight, as 4 or more cylinders always has one on the power stroke, and 6 or more always has one in the effective part of the power stroke.

Longer stroke also helps as it increases piston speed at the specified rpm.

Just my ramblings, for what it's worth


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unclesyd (Materials)
28 Feb 04 20:11
Many years ago there were engines that were used in small inshore shrimp and fishing boats along the gulf coast that ran on "casing head gasoline" and diesels with very large flywheels that ran less than 100 RPM.  One diesel engine a "?Budda?" ran very slow and had no reduction gear only a reverse.  Small tugs also ran the same engine only a little larger.  On a four cylinder you could count the RPM by the firing
rmw (Mechanical)
28 Feb 04 21:23
I worked in a shop while in college, working on trenching machinery that was driven by air cooled Wisconsin engines.  We had one old rental machine, in particular, that was always around the shop, and when it was around, there was no greater fun than to crank it, (it had a very very slow idle) and as some unsuspecting visitor would walk by, I would grab the spark plug with one hand, and touch the innocent sucker walking by with my other hand.

Point is, the slow idle.  If this engine hadn't had such a slow idle, I could not have withstood the energy of the spark ignition in order to pull the trick on others.

Great fun for a college kid, and testimonial to how slow an internal combustion engine can actually idle.  (It wasby the way, a v-4 gasoline engine.)

GregLocock (Automotive)
29 Feb 04 3:26

we used to have a 4wd forklift on the farm. It had some galumphing great I8 in it.

To start it we decided which cylinder was at TDC, took the plug out, tipped some fuel in, put the plug back in, and switched the ignition on.


Greg Locock

evelrod (Automotive)
29 Feb 04 16:11
Greg, that is similar to the way we used to 'prop' the McCullogh engine in my cousins Benson Gyrocopter.  Pull a plug, squirt in a dab of fuel, replace the plug, advance the prop to bdc and flip it in reverse to top. Usually it would fire on the first try.  It was a 'pusher' and proping it to start always scared me silly as you had to stand astraddle the fueslage to reach the prop.

rmw---I 'sparked' my friends, unsuspecting passerbys. bugs on the fender, etc. with the same technique using a plug  on my old 1949 Mercury. I was only 18 and the 'jolting' wasn't too bad after the FIRST hit.  Crazy kids!  

Unclesyd---I saw a one cyl (Perkins, I think), similar to the Buddah you mention, on a fishing boat in Texas as a kid.  All I remember is it sounded like a "hit and miss" engine.  Just kinda 'chug, chugged' along.

ietech (Industrial)
29 Feb 04 18:00
Does any one remember the old "Johnny Popper" tractor. Huge flywheel.

I remember nosing it into a tree at idle, it would just stand there and --- chug -- pop - pop -- and just spin the rear wheels a partial rev at a time, with each pop. RPM? no guess probably pretty darn slow.

evelrod (Automotive)
29 Feb 04 20:33
Sure do, ietech.  I have been looking for a bargain "Poppin Johnny" ( Two cyl. John Deere tractor of the 30's) to restore and put in my wife's cactus garden out front.  So far I have only found a couple of later models but no 2 banger that is cheap enough.  I'll keep lookin'.

rmw (Mechanical)
29 Feb 04 23:20

I got a laugh out of your reply to my post.  You are right.  That first jolt was the hard one.  once you got your mind right, so as to be able to take hold of the plug wire initially, then the rest of it was all fun.

I would be afraid to do it at my age, now, fearing that it would act like some kind of pacemaker and do strange things to my ticker.

ietech (Industrial)
29 Feb 04 23:20

After my last post I did a Google search on the subject and found some really interesting sites. Seems I created some new curiosity in myself. Type in "Johnny Popper". Some of the links within the websites listed in the search results have items for sale.

In addition I came accross some great sites regarding vintage/antique industrial engines that others posting here might be interested in reading about. Old LeGrange, Briggs, and others.

All very slow RPM's for those interested type antique engines or vintage engines.


patprimmer (Publican)
22 Mar 04 17:28
Also, longer stroke means higher piston speed and probably higher gas velocity in the ports and manifolds at a given rpm, as longer stroke, smaller bore often means smaller valves and ports

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PEW (Aeronautics)
23 Mar 04 8:43
When was a kid the Panther motorcycle was a reasonably common machine in UK; they were often used to pull a sidecar. The bikes had a very long stroke forward inclined single cylinder 600cc engine. In top they were so slow-revving that a common description was "one bang every lampost" .
franzh (Automotive)
28 Mar 04 13:25
Hate to stir up the mess, but when I was doing some research on air and fuel mixtures, one limiting factor was the air velocity entering the intake valve.  Liquid fuel tends to "drop out" of the air stream at slow speeds, whereas a vapor fuel tends to stay relatively homogenized, regardless of the air/fuel velocity.

Case in point:  I took an old irrigation engine, a Chrysler 413 (hey Evelrod, remember those?) and had problems running it down to 400 rpm without very erratic performance (no-load).  Installing a simple Propane carburetor (Impco 200 if anyone cares) and it easily idled down to 200 rpm, where I suspect the motoring friction losses exceeded the power production.

One thing more to add.  The engine rpm has little to do with spark intensity (contemporary distributor, either electronic or contact point primary, not the magneto system), as I read in “rmw’s” post above.  What you may have been feeling is one impulse every five or ten seconds.

bettonracing (Automotive)
31 Mar 04 8:39
Does the Carnot engine count?...

*exit stage left before bottles and tomatoes start flying*
evelrod (Automotive)
31 Mar 04 11:46
You did that on purpose!  How did you know that thermodynamic theory gave me bad headaches?

obanion (Automotive)
1 Apr 04 15:29
franzh, so you think gaseous (propane) fuel, since it doesn't suffer from drop out, would allow a engine to run slower?
franzh (Automotive)
2 Apr 04 8:55
Yes, done so many times.  In one of my former personal vehicles (import 4 cyl 2.4L w/ std) easily idles down to 300 rpm.  Low speed engine performance was also smoother than on gasoline.

Using an optical engine, and watching the air-fuel mixtures propagation and homogenization when changing from liquid fuels to vapor fuels also confirms this.  If you can change fuels in steady state mode, it is readily apparent.  In some cases, you can even hear the difference.

Next, using pressure transducers, vapor fuels show a lower overall pressure during combustion, but the combustion pressure remains longer on the power stroke than with gasoline.
obanion (Automotive)
3 Apr 04 20:27
Oh that third point I didn't know about (lower peak pressure, longer sustained avg pressure). That's a big bonus to me. Trying for lots of power, and lower peak pressures means a higher upper limit on hard parts breakage. Water injection does the same thing as well, which I am also using.
franzh (Automotive)
4 Apr 04 10:29
Without delving too deeply into combustion physics, using water injection and vapor fuels are two entirely different things.

Vapor fuels, LPG or Methane, are combustible hydrocarbons, water is not.  LPG and Methane have higher octane ratings than conventional gasoline.  It’s the octane that allows for a longer burn duration, thus a longer combustion pressure, albeit lower, than gasoline.  Both LPG and Methan have lower BTU’s than gasoline, thus a lower peak combustion pressure.

Water injection tends to suppress combustion temperatures and pressures.  It takes chemical energy to convert the liquid water droplets to steam.  There is relatively little (if any) energy derived from the steam expansion process.  The chemical energy required to convert water to steam is derived from the air-fuel mixture.  Consider the first and second laws of thermodynamics.

My experience has shown that if water injection is needed, there is a shortcoming somewhere in the engine dynamic process.  Either compression is too high for the fuel provided, the fuel is of an insufficient octane, the timing is too aggressive, the air-fuel mixture is incorrect, the combustion chamber is not set up properly, and so on.

Before I get lambasted for saying water injection is bad, there are circumstances where water injection can indeed prove beneficial, such as in stationary turbine power generation systems.


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