Compression

Discussion in 'General Engine Discussion' started by Gordon, May 20, 2019.

Help Support HMEM by donating:

  1. May 22, 2019 #21

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    I thought about O rings but the engine runs hot so O rings may be short lived. Also there is more friction with O rings and as marginal as this engine is the additional friction may be too much.

    Basically the LH piston is just a a cylinder head but movement is required because it also draws in the fuel mixture.
     
  2. May 22, 2019 #22

    dsage

    dsage

    dsage

    Well-Known Member

    Joined:
    Jan 1, 2011
    Messages:
    283
    Likes Received:
    59
    Well....???
    There is no reason for this engine to run hot. It has almost no compression. Plus there are lots of much more capable engines using O-rings with no issues. It doesn't have to last a 100,000 miles or anything. If you use the right O-rings (forgot the material name) they'll take it. Friction.... well that's a matter of having the right sized ring in a good cylinder so it makes a good seal.
    You've tried a lot more difficult fixes and you've gotten nowhere. So IMHO it's worth a try.
    Just my 2cents worth.
     
  3. May 22, 2019 #23

    jimsshop1

    jimsshop1

    jimsshop1

    Steamman70

    Joined:
    Nov 23, 2016
    Messages:
    112
    Likes Received:
    25
    I gave up on mine a long time ago. Same issue as Gordon. Looks nice sitting on a shelf and I get a lot of questions like "what is that?" Good luck Gordon.

    Jim
     
  4. May 23, 2019 #24

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    I have pursued this far beyond what a reasonable person would do. Pretty soon it becomes a contest where I become obsessed with trying to get this stupid thing to run. After all there are YouTube videos of others who have gotten this thing to run so why can't I get it to run. I have built over 20 model engines and have always gotten them to run after some fine tuning. At this point I am wondering about the folks who have gotten this engine to run. I can get it to run for anywhere from 1 minute to 15 minutes so I guess that I could make a two minute video and declare success. At this point I do not believe that it is a matter of my poor skills. It is a matter of this engine is never going to run successful unless it is facing south west and the moon is in the third quarter and Mars is aligned or whatever the magic formula may be. In order to get the engine to run a carburetor adjustment of just a fraction of a degree or ignition timing of a degree or two makes the difference between running and not running. It will run for a period of time then suddenly quit and refuse to start for the next 15 minutes and then suddenly start again.
     
    EnginePaul likes this.
  5. May 23, 2019 #25

    dsage

    dsage

    dsage

    Well-Known Member

    Joined:
    Jan 1, 2011
    Messages:
    283
    Likes Received:
    59
    Have you tried a thermal insulating gasket between your carb and the engine block. Something like a piece of Delrin, maybe 1/16 to 1/8 thick. Maybe the heat from the engine is vaporizing the fuel (vapor lock). It is a problem on a lot of models. You also said a small adjustment of the carb throws it off. Maybe a more tapered needle would help. Just grasping at straws.
     
  6. May 23, 2019 #26

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    I have tried paper and teflon carburetor gaskets and the engine seems to only start after it is warmed up and when it darn well please. That consists of hooking it up to an electric motor and turning it over for 5 minutes or so. The needle on the carburetor is a sewing needle. Hard to get more tapered than that. There seems to be no rhyme or reason to why or how it starts.
     
  7. May 23, 2019 #27

    dsage

    dsage

    dsage

    Well-Known Member

    Joined:
    Jan 1, 2011
    Messages:
    283
    Likes Received:
    59
    I wasn't aware there was so much trouble getting it to go. I thought it was just a problem keeping it going. Obviously the design is seriously flawed and you are lucky to have gotten as far as you have.
    FWIW if you look carefully sewing needles are not a straight taper. They are sort of curved. With a bit of careful file work on the lathe you can make a gradual straight taper on the end of a screw. I use sewing needles as well just because they are easy. But I also find them tough to get a fine adjustment with.
    As for the gasket, the intent is as insulation so the thicker and more insulating the better (thickness within reason of course otherwise you mess up the carb action).
    All that said it sounds like you might have reached the limit of the design.
    Best of luck.
     
  8. May 24, 2019 #28

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    I would consider it successful if I could get it to start consistently and run for 10 to 15 minutes. This thing is never going to be anything but marginal. The action is intriguing to watch but there is no chance that this engine was ever going to be useful.
     
  9. May 25, 2019 #29

    bluejets

    bluejets

    bluejets

    Well-Known Member

    Joined:
    Apr 22, 2014
    Messages:
    122
    Likes Received:
    49
    I've mentioned this before as many have had problems. There is a design flaw as a mate of mine found out a few years ago when he built one and had similar outcome.

    What he did was redrew the complete unit on ProDesktop and with the help of the "birdcage" view, he was able to determine there was very little time for compression, the spark plug was in the wrong location, and the timing was way out.
    As others have done, it now sits on a shelf with the appropriate signage to warn others.

    As a side note, congratulations to George for becoming the winner of this year's Duke of Edinburgh award in the UK.
    No easy feat in itself plus the transportation of his magnificent tractor from Aus and back , all within 3 weeks or so.
    I'm certain he will post images here later.
    Fantastic achievement for a life long commitment to model engineering.
     
  10. May 28, 2019 #30

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    This is where I am at at this point. I finally gave up on trying to do it "right". Many folks seem to have successfully made this engine using CI Rings so I was aiming for that but finally just gave up and installed O-rings. The engine now has better compression and will run. One problem is that it only runs for short periods of time until the engine gets warmed up. I assume that the combustion chamber must require heat in order to vaporize correctly. I can choke the engine and it will run for 10-15 seconds for 8-10 times until it finally runs erratically until the engine is finally warmed up. Because of the still rather low compression the fuel mix is not really good. Another problem is that the carburetor adjustment is super fussy. Just an adjustment of a degree or so makes the difference between running and not running. I found that the taper on the end of the fuel jet which directs air flow across the jet face is absolutely necessary as is the distance that the jet extends into the air stream. Another problem is that the engine fires hard (explosively). I have tried to set the ignition timing BTDC, TDC, and ATDC but I still have not found the secret. Once the engine is running it seems to run for about 15 minutes until it seems to die. I assume that again this is a matter of heating the combustion chamber. It seems that unless everything is absolutely perfect this thing will not run.
     
  11. May 29, 2019 #31

    bluejets

    bluejets

    bluejets

    Well-Known Member

    Joined:
    Apr 22, 2014
    Messages:
    122
    Likes Received:
    49
    Hi Gordon,
    Seeing as you at least managed to get some life out of it, many of the problems you are having could possibly be overcome by using Methanol with a 10% Nitro mix and 4:1 castor oil (castrolM) or even any of the commercially available fuels with the 10% nitro content.

    Reasons being .........
    lower compression is essential for running nitro in rc engines......(plus 1)
    methanol is consumed at a much larger rate so much improved cooling. .....(plus 2)
    nitro added to fuel de-sensitises the needle adjustment so what may previously be 1 click now will become 1/4 turn or so. ..... (plus 3)
    Castor has a better lubrication effect AND adds to the already increase in cooling. (plus 4)

    Only negative is the cost of the fuel is much higher than pump fuel BUT I very much doubt you'll be using it in gallons anyhow.
    Just keep it sealed between use and stored out of sunlight.

    Hope this helps...cheers Jorgo
     
  12. May 29, 2019 #32

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    Sounds good but I am confused. I look for this and I find Methanol and Nitro which I assume are two different things and from what you are saying I should mix them. Where should I look for this? I see RC Fuel at 15%. Should I be looking at hobby shops or drag racers? Price seems to be all over the map. I have been using stove fuel (Naptha) with some Miracle Oil.
     
  13. May 29, 2019 #33

    bluejets

    bluejets

    bluejets

    Well-Known Member

    Joined:
    Apr 22, 2014
    Messages:
    122
    Likes Received:
    49
    Gordon,
    Methanol is a basic alcohol fuel used by aeromodellers, model cars and of course the drag bikes and cars.
    Can be ignited by spark or glow ignition.

    Nitromethane (or just Nitro) is the additive that gives oxygen boost to both the drag gear and small model engines.
    The ratio of 10% in the fuel part means ........1 part Nitro to 9 parts Methanol .......

    This then becomes your main fuel and you mix 1 part castrol_M ( castor oil) with 4 parts of the above Methanol/Nitro mix.

    I have found over the years that percentage of nitro mix works well at percentages of 10% ....... not much change at 15% ......... jump in performance at 20% ....... then not much changes until you hit 50%.
    The latter is usually reserved for the little Cox engines and others with "well built" bottom ends. :)
    So I would go with 10% if you can find some.

    Hobby shop prices may be the highest as they work in small quantities, usually 1 litre minimum BUT it would be the pre-mix fuel to avoid any mucking around.
    Pre-mix these days usually contains a synthetic oil which is ok for some, I just have a liking for castor given it's cooling and high ability to lube high clearances.
    Latter as an example where we would run 6 thou" big end clearance on 7.5cc pylon racing engines.( around 30,000rpm)
    Usually one can find an aeromodeller somewhere in the neighbourhood who would probably give you a small amount to try.

    Now I realise you don't need the performance mentioned above. The suggestion of nitro and methanol was for decreased needle sensitivity and better combustion for the nitro and cooling effect for the methanol and castor oil.
    You may be surprised with the result, most are.

    One final word, after running with nitro, give the engine a good cleanup.
    Burning Nitro creates a residual acid (nitric acid if my memory serves me correct) which will eat most metals if left to it's own resources.

    You can check some of the details here........
    https://en.wikipedia.org/wiki/Nitromethane
     
    Last edited: May 29, 2019
  14. May 29, 2019 #34

    Gordon

    Gordon

    Gordon

    Well-Known Member

    Joined:
    Jan 4, 2011
    Messages:
    562
    Likes Received:
    66
    I was out this morning so I stopped at a local hobby shop. They had the 10% nitro so I bought a gallon. It contains synthetic castor oil so I guess that I can give it a try. Thank you for the input. I am leaving for a few days so it will probably be next week before I get around to trying it.
     

Share This Page