Brian builds Atkinson Engine

[Brian Rupnow]

You will get greater "Oomph" with a larger bore because the volume of fuel ingested will increase. But unless I am missing something, the CR will not change. I think the calculation is:

(d+s)/d where d is the distance from the piston to the end of the cylinder when the piston is at TDC and s is the stroke.

both d and s vary as the square of the bore.

Jim

The compression ratio has to change. The cylinder bore is opening up--not the head. The head remains unchanged. I will be packing 26% greater volume of air/fuel mix into the head cavity than I was before enlarging the bore.

 

[dsquire]

Brian

Making a domed piston would also raise the compression.

Cheers

Don

 

[Jasonb]

I'd agree with jim, upping the bore will not alter the CR if you want to do that then the stroke has to go up or the head/top of cylinder needs skimming to reduce the unswept volume, a domed piston will also reduce the volume as said above.

You say its 0.875"bore x 1.666" stroke, lets say to make things easy the gap between head and piston at TDC is 0.340"

Volume at TDC 3.142 x 0.4375 x 0.4375x 0.340 = 0.204cu in
Volume at BDC 3.142 x 0.4375 x 0.4375 x 2.000 = = 1.203cu inc

1.203:0.204 = 5.89:1

Now with a 1" bore

Volume at TDC 3.142 x 0.5 x 0.5 x 0.340 = 0.267cu in
Volume at BDC 3.142 x 0.5 x 0.5 x 0.2 = 1.571 cu in

1.571 : 0.267 = 5.88:1

Near enough the same allowing for a bit of rounding up.

J

 

[jasonh]

Brian - for what it's worth my Gingery Atkinson uses a CM6 - and the spark gap ends up a fair way back in the hole... but still works ok. Of course - I have a 1.25 inch bore. Another possible idea - try model aircraft fuel with a bit of nitro methane (10%) and oil mixed in. It works well for me.

Keep on chipping away at it.
Jason H

 

[Jasonb]

A couple of other thoughts

You reduced the size of the flywheel which may not be enough to keep the engine firing and that is why it needs a bit of help from the drill.

Have you tried choking the intake to see if a richer mixture does any good, just a finger part way over the air intake is usually enough to try it out.

J

 

[rhitee93]

Huh! I would have fallen into the same thought trap that Brian did regarding the ratio. However, opening up the bore will increase the air/fuel charge, and therefore increase power, will it not?

 

[old-and-broken]

have you tried setting the spark gap closer together?

You have shorter arc, but with an increase in heat and current. Maybe you've tried this already and I missed it.

 

[Brian Rupnow]

Lets take another look at this compression ratio business. The actual displacement of the cavity in the cyl head is 0.118 cubic inches. The cylinder only comes flush with the end of the cylinder---it doesn't enter the cavity in the cyl head. If we are working with a cylinder length of 2.56 lg. minus 1.1 piston lgth, (the back end of the piston comes flush with the back end of the cylinder) that leaves a net cyl length of 1.46".------ .437 x .437 x 3.1416 x 1.46=0.877 cubic inches.
0.877:0.118=7.43:1 compression ratio.
Now if I bump the bore up to 1", then .5 x .5 x 3.1416 x 1.46=1.147 cu. in.
1.147:.118 = 9.72:1 compression ratio.
Remember---I'm not changing the bore in the head, because the piston never goes there.
It looks to me like the compression ratio is going to increase by 30.8% which is pretty signifigant.
1.308 x .877 =1.147

 

[Brian Rupnow]

Went out today and put a sizeable dent in the Rupnow fortune. Bought 15/16 and 31/32" Silver and Demmings drillbits, and a 1" reamer.---But I'm hesitant to use them. Methinks deep in my black little heart that there's something else going on here. I played some more with the engine tonight. Using transparent Tygon fuel line and the Traxxas carburator, it will suck fuel up the fuel line if I put my thumb over the carb air intake while turning the motor with my electric drill, but as soon as I take my thumb of the intake the fuel all runs back down the line into the tank. (Not the vapour tank, a different tank totally)) That shouldn't happen. The "tank" is about 1" below the carburetor. The fact that the fuel so readily runs back down the line means to me that there has to be an air leak somewhere between the carb body and the combustion chamber--(The fuel is running back under gravity--its not being blown back down the line.).When I do choke the engine, and it starts and runs but gradually slows and stops, I wonder if its actually running out of gas. The compression is good. Its not blowing back thru the carburetor like it would if the intake valve was leaking. There is only one bolted joint between the intake pipe and the cylinder head, but I have gasketed it and used gasket goo. Its almost impossible to quickly access the sparkplug and pull it out to check for fuel starvation or flooding. The vapour tank is a bit too "weird science" for me. If the engine is going to run, it should run on a carburetor. Once it runs on a carburetor, I'll try the vapour tank again.----And I did try some 20% nitro model aeroplane fuel tonight. Engine took right off and ran like a beast---for about 12 seconds, then laid down and died same as-same as.

 

[Cliff]

Brian
Still following along I'm taken back at the mechanics that go into building a engine I followed you when you built the webster and the issues you had with it I think at one time you thought about pulling your hair out but you worked through it and you will work through this. I will be waiting with bated breath to here it run. Cliff

 

[Jasonb]

Brian unless you have some sort of float valve in the RC carb then fuel will run back, there is nothing to shut of the needle valve so that is where the air gets in, don't forget the RC engines usually pressurise the tank with a pipe from the silencer.

If you don't want the fuel to run back then you need something like a Lukenheimer that I made for my domestic that only opens the fuel when there is a vacuum to lift the float.

Just on teh CR again, you say above that the max length of teh cylinder space is 1.46" but on the previous page you say stroke is 1.66" won't the piston hit the head. My calcs were based on a flat underside to teh head if there is a void then that will affect the figures.

 

[Brian Rupnow]

Damn!!! I've got it timed bass ackwards if this animation is right!!!
http://www.animatedengines.com/atkinson.html

 

[Cogsy]

Interesting. I'd have thought that the full length stroke would have been the compression/firing stroke, but then it does make sense to eject all the spent exhaust gas with the longer stroke as well.

This is one weird engine that I may just have to put on my wish list - provided you get it to run.

Good luck!

 

[Brian Rupnow]

I was so sick of engines that wouldn't run that I dug out my Webster, just to get that thrill that only a self built engine gives you---And IT wouldn't run. But a few minutes resetting the points (which I had removed 2 months ago to measure up for a model to use when designing the ignition on the Atkinson) and away it went, running like a trooper. It does run SO good!!! Now it seems while I was playing with the Webster that somebody called my attention to the fact that I probably have the Atkinson timed to fire on the wrong stroke, (If the animation is correct). I have emailed Jan Ridders to ask him for sure.

 

[Brian Rupnow]

Now my Kerzel Hit and Miss is setting out in the garage Popping away. I'm starting to feel almost human again.--And I sent an email off to Jan Ridders to sort out exactly what freakin stroke this Atkinson should be firing on!!!

 

[Brian Rupnow]

Just got the following email from Jan Ridders---which agrees with the animation.
Hello Brian,

OK, with the Atkinson the exhaust stroke is the longest, so at the end of the exhaust stroke the piston is the closed to the cylinder head. So the spark must not occur then. After te exhaust stroke the piston moves back again, sucking in the fresh gas mix; the intake stroke. Then the piston moves in the direction of the cylinder head again compressing the gas mix. The spark must occur at the end of this compression stroke. The distance of the piston to the cylinder head at the end of this compression stroke (when the spark must occur) is greater than at the end of the exhaust stroke.

I hope you understand my English.

Friendly greetings and success,
Jan

 

[AussieJimG]

So close!!! Alfred Hitchcock could not create suspense like this. It's just a gnats whisker away.

Jim

 

[Brian Rupnow]

Okay boys, thats it. Game's over. Re-timing didn't do squat!!! I have given up trying to get my engine to run in its present guise. I am going to bore the cylinder out to 1” from its present 7/8” diameter, and I am going to rotate the cylinder head 180 degrees, as the way it currently is according to Jan Ridders design, the sparkplug sets right on the bottom of the cylinder in a pocket, and if it doesn’t fire and run immediately, the pocket fills up with fuel and totally drowns the sparkplug. I know 72 different trick to making an i.c. engine work, and I've tried all of them---NADA. A rebore on the cylinder, some minor rerouting of the intake and exhaust, and perhaps a home brewed carburetor.----Brian

 

[rhitee93]

Hang in there Brian, you'll get it yet. After you bore it out, you might as well stroke it too. (Just kidding!)

 

[Brian Rupnow]

Now THATS a hole!!! Phase #1 of Atkinson recovery is under way. I have never yet failed to get one of my 16 models running. However, that being said, I've came mighty damned close. I thought the Kerzel hit and miss would be the death of me. This engine is certainly not going as planned, but then, Internal Combustion engines rarely do. This one COULD end up being a shelf monster, but Jeez, I hope not!!!