Idle RPM of single cylinder engines

[Brian Rupnow]

I have now built 8 or 9 i.c. engines. About half of them are hit and miss 4 cycle with "atmospheric" intake valves, and about half are overhead valve 4 cycle engines. The side valve engine I just finished is a single cylinder 4 cycle engine with two cam driven valves. I can get it to idle down to 700 rpm, but that is about the lower limit. I just spent half an hour doing an internet search to see what an "average" idle speed is for single cylinder 4 cycle engines with two conventional cam driven valves, but it was a case of "too much information". What about you other i.c. engine builders out there, with access to tachometers? What idle speed do you achieve with single cylinder 4 cycle, conventionally valved engines? I know that an enormous flywheel or two flywheels will make a difference, but I am talking about engines with single flywheels that fall into a more or less "normal" size range. I would be interested in hearing what low rpm consistent idle you are able to achieve.----Brian

[GWRdriver]

Brian,
I'm not an IC person other than having rebuilt a car and motorcycle engine or two in my time, but FWIW I recall reading years ago that the lower limit for idle rpm of a conventionally aspirated and ignited 4-cycle IC engine was about 600rpm.

[Brian Rupnow]

GWDriver. ---That may well be true for multi cylinder engines where there are only a few degrees of rotation between firing cycles of various cylinders. I think the game changes somewhat with a single cylinder engine, because it only fires every 720 degrees of rotation. The slowest idling engine I ever remember seeing was an old 6 cylinder flathead Dodge car. It ran so quietly and vibration free that you actually had to look at the crankshaft pulley to know the engine was running.

[philjoe5]

Brian,
I know this isn't exactly what you asked for but my Bonzer with atmospheric intake, vapor carby stalls at around 600 rpm as measured with a laser tach.  It only runs that slowly with the spark retarded to TDC.  With a some advance it screams along at 800 rpm

Cheers,
Phil

[BillTodd]

GWDriver. ---That may well be true for multi cylinder engines where there are only a few degrees of rotation between firing cycles of various cylinders. I think the game changes somewhat with a single cylinder engine, because it only fires every 720 degrees of rotation. The slowest idling engine I ever remember seeing was an old 6 cylinder flathead Dodge car. It ran so quietly and vibration free that you actually had to look at the crankshaft pulley to know the engine was running.

My brother has a V8 Daimler (2.5l) it'll tick over at <100rpm (you can see each cylinder fire as the crank pulses around

I suspect a single's lower limit is mainly down to compression, friction and flywheel mass. If the rolling friction is low, all you'd need is enough moment in the flywheel to over-come compression (and valve spring tension) to keep it ticking. If you could throttle the output down enough , I'm sure a couple of hundred RPM would be possible

Bill

[b.lindsey]

Brian, I will try to check the Briggs tomorrow, single cylinder, 1" bore and stroke, but it has a large brass flywheel as you can see from the plans. I suspect it will get down to 400 rpm but need to verify that. Will let you know.

Bill

[Brian Rupnow]

Thanks Bill---I was hoping you would chime in here.---Brian

[chuck foster]

not exactly what brian was asking but i thought it was kind of cool 

[youtube1]https://www.youtube.com/watch?v=s1ukguJxy1E[/youtube1]

chuck

[b.lindsey]

Well, since that is my one and only IC engine to date, I certainly don't consider myself an expert, but I can add that data point to the thread at least.

Bill

That is cool chuck, clearly under 60 rpm.

[Brian Rupnow]

Chuck--You're right. That is neat. Kind of a "How slow can you go"!!!---Brian

[PStechPaul]

There are "slow tractor races", where the winner finishes last. Here's one in "my neck of the woods":
http://articles.baltimoresun.com/2001-09-07/news/0109070069_1_farm-tractor-antique-tractors-garden-tractor

<a href="https://www.youtube.com/watch?v=quDigLDXSgA" target="_blank">http://www.youtube.com/watch?v=quDigLDXSgA</a>

It probably has hydrostatic drive. An electric tractor could also move as slowly as you might want, and you can always add multiple reduction drives.

As to low speed idling, I wonder if the addition of some water vapor may help to slow the combustion process for a more gradual and extended expansion in the cylinder. Something else I was wondering about is adding a spring-loaded piston in the cylinder to absorb the energy and then release it slowly. Maybe something like a spring-loaded connecting rod, or an air spring cylinder.

[b.lindsey]

Brian,
 I got the little Briggs cranked up this morning after almost 9 months of sitting on the shelf in my office and checked it with a portable strobe-a-tach. As close as I could get to a reading (freezing the starter hub) was 475. It was a little difficult hand holding the butterfly valve and adjusting the strobe but with a little help from a colleague that seems to be in the ballpark. Not quite as slow as I had expected but hope that helps anyway.

Bill

[Brian Rupnow]

Bill---Thank you. That is a low rpm, and I am sure it is a function of the flywheel size. Thank you again for taking the time to measure that for me.----Brian

[Brian Rupnow]

Here we are at "maximum slow".
I played a bit more with the needle valve setting on the new carburetor and was able to get the engine idle down to 670 rpm, but it simply won't go any slower than that.
<a href="https://www.youtube.com/watch?v=CQ0vm7rpf4U" target="_blank">http://www.youtube.com/watch?v=CQ0vm7rpf4U</a>

[Jasonb]

You will probably have to start playing with valve timing and bore/stroke ration if you wnat to get a slower engine.

Are you making any adjustments to the air bleed as you drop the revs? a bit more air will help keep the fuel/air mix nearer optimum

J