Interesting information about compression ratio

[cfellows]

After a failed attempt to convert my large horizontal engine to run on the Lenoir cycle, I converted back to a normal 4-stroke operation with a sideshaft operating the valves.  One issue I've had repeated is getting the engine to idle in normal 4 stroke operation.  It continually misses a stroke and fires every 4 revolutions instead of 2 as it should.

I had read on another forum that running properly at low speed is hard to attain without a load.  Apparently closing the throttle down far enough to get a decent idle cause the engine to pull a partial vacuum on the intake stroke.  This effectively reduces the compression ratio since at least part of the compression stroke is used to equalize the internal pressure from the partial vacuum.  This, then, reduces the amount of travel left to compress the fuel/air mixture.

When I built this engine, I designed in a 4:1 compression ratio, which is pretty low to start with.  So, today, to try and fix the problem, I raised the compression ratio by fastening a 1/4" plug onto the top of the piston.  Lo and behold, when I started engine again, I was able to get it to run at low RPM and hit on every firing stroke.  I think this may have cured the problem.

I'll get a video up of it running a bit later today...

Chuck

[Don1966]

Chuck I can say one thing about you, your very persistent. I will be waiting to see the video.

 
Do

[cfellows]

Chuck I can say one thing about you, your very persistent. I will be waiting to see the video.

 
Do

Thanks, Don, I guess if I was really persistent, I would have seen the Lenoir conversion through to completion.  But, I did want to get this engine running properly in one form or another and I do like the 4-stroke, side shaft configuration the best. 

So, here is the video shot a few minutes ago.

<a href="https://www.youtube.com/watch?v=hfsal3ilczg" target="_blank">http://www.youtube.com/watch?v=hfsal3ilczg</a>

Next, I'm going to see exactly how slow it will idle.

Chuck

[Don1966]

Cool Chuck bet your pretty pumped up. She runs great bud and I like...............


 
Don

[sco]

Chuck,

You are da man - really love that!  Have you published plans for this one?

Simon.

[Johnmcc69]

Great engine Chuck! She runs nice! Interesting finding about the compression problem.

 John

[Dave Otto]

Hi Chuck

That is an interesting observation; I have always wondered why an un-loaded  engine will not fire properly? When the slightest load placed on the flywheel will smooth them out and fire like they should. The full sized throttle governed stationary engine exhibit this same behavior. I never understood what the load had to do with it, but what you are saying seems to makes since.

Dave

[ogaryd]

That's great info Chuck, I've experienced that on engines at idle and couldn't find any reason or cure for it. How much would you estimate the compression ratio was increased?                                                                                                 

Gary

[cfellows]

Thanks for the kind words, everyone.  Gary, the stroke is 1.75" with .5625" head space at the top of the stroke.  The bore is 1.25" so the cylinder volume at BDC is 2.84 and at TDC is .69.  2.84/.69=4.1:1 compression ratio.  I added a 1" diameter plug .25" long which has a volume of .196.  Subtracting .196 from 2.84 & 69 = 2.64 / .49 = 5.39:1 as the new compression ratio.  Does this all look right?

Chuck

[Rustkolector]

Hi Chuck,
Nice work on the 8 cycling issue. Big help! Did you get an accurate RPM reading on your slow speed capability with the higher C/R? Your results are timely as I am currently working on a slow speed engine and the pistons have not yet been made. It will run on propane, as do most of my engines. I find that propane eliminates a lot of liquid fuel issues.

I have one engine that requires a liquid fuel/oil mix that I believe still has fuel related issues. What version of your simple carb were you using for the slow run video?

Jeff