Simon Bulk Posted October 16, 2023 Share Posted October 16, 2023 Interesting, thanks Jon. So, seems there is reason in special combustion design (such as hemi heads)… I pretty more get the impression that all 4C engines -despite of their specific design features- work equal and react very similar to certain changes in fuel composition etc. That means: Running a Saito on high nitro % serves as inner coolant, allows a slightly richer needle setting at peak power without loosing too much rpm (due to the additional oxygen by the nitro component). That works especially well on high compression ratio as the Saito’s have. Same applies to Laser, but they’re less critical due to the lower compression ratio. Detonation (knocking) is mainly caused by overheating and not by too much nitro in the fuel. Nitro allows cooler running and even prevents knocking. Shimming lowers compression ratio and allows furthermore higher nitro % resulting them to run cooler. anything wrong here in that theory? best Simon Quote Link to comment Share on other sites More sharing options...
David Davis Posted October 16, 2023 Share Posted October 16, 2023 (edited) 40 minutes ago, Jon - Laser Engines said: ... To summarise. All standard 2 and 4 stroke engines from any brand use 5% nitro and 15% synth oil. Its literally the only fuel in the world as far as i care.... ...except for new (vertical plug?) Laser engines which are quite happy on 5% nitro and 7% oil? Edited October 16, 2023 by David Davis Quote Link to comment Share on other sites More sharing options...
Jon H Posted October 16, 2023 Share Posted October 16, 2023 24 minutes ago, Simon Bulk said: Interesting, thanks Jon. So, seems there is reason in special combustion design (such as hemi heads)… I pretty more get the impression that all 4C engines -despite of their specific design features- work equal and react very similar to certain changes in fuel composition etc. That means: Running a Saito on high nitro % serves as inner coolant, allows a slightly richer needle setting at peak power without loosing too much rpm (due to the additional oxygen by the nitro component). That works especially well on high compression ratio as the Saito’s have. Same applies to Laser, but they’re less critical due to the lower compression ratio. Detonation (knocking) is mainly caused by overheating and not by too much nitro in the fuel. Nitro allows cooler running and even prevents knocking. Shimming lowers compression ratio and allows furthermore higher nitro % resulting them to run cooler. anything wrong here in that theory? best Simon We are in danger of falling too far down the rabbit hole here and are into the realms of personal research for the individual. Although discussing cars, this offers a simple overview of combustion chamber types http://www.secondchancegarage.com/public/585.cfm Saito use hemi, Laser and Enya use wedge, OS and its clones use bathtub. There is more to the story though with bore/stroke ratio, valve size, carb size, valve timing, valve lift, valve duration, pumping losses, squish area....How they all interact is not always predictable and our 180 for example has a really high compression ratio, but other factors mean its still a fairly laid back engine. Also what prop are you using? Revving its rod off will warm it up a treat, but using a 2 foot paddle will also have an impact. If were were designing a race engine this would all make a difference, but we are playing with toy aeroplanes and none of this really matters. If you compare a saito 82, laser 80, and OS81, all of which use different figures for everything listed above, and you run them on the same prop and same fuel its doubtful they will perform by more than a few % different despite all these major differences. I did this with an OS, Enya and Laser 155 and there was very little to choose between them. The winner was dictated by the specific prop it was running and how that worked with that engine. Run 10 different props and each engine would score a few wins, a few 2nd places and a few 3rds. The Laser tended to perform best on bigger props, and the Enya best on smaller props, but with only a few hundred rpm between them on any prop did it matter much? Not really. Its not like changing from one brand to another would transform the performance of the model. Also the Enya and Laser are the most similar in terms of design concept and yet they performed the furthest apart. While i applaud any efforts to learn more about the hobby and understand the granular detail of certain subjects, there can be a tendency to overthink it if you arent too careful. 51 minutes ago, Simon Bulk said: Detonation (knocking) is mainly caused by overheating and not by too much nitro in the fuel. Nitro allows cooler running and even prevents knocking. ehhhh not exactly. Knocking is pre ignition, ie the fuel charge fires too early in the rotation of the engine. The cause could be heat, the plug, the load on the engine, the rpm of the engine, many factors play a part and blanket statements are not really possible. 1 Quote Link to comment Share on other sites More sharing options...
Simon Bulk Posted October 16, 2023 Share Posted October 16, 2023 (edited) I widely agree with all mentioned above. Especially with the tendency to overthink things instead of enjoying them. Perfectionism sometimes is not very beneficial in that matter. I still believe that our glow engines are still in parts an undiscovered world because the market is just not big enough… I mean we do not speak about commercial engines that are sold millions of times worldwide, big companies spending billions of $ in R&D to entirely understand and perfection their design. I still believe RC IC engine market is mainly based on professional try and error by chaps pursuing their vocation. Nevertheless the outcome are really good running engines and that is the most important part of it. Back to the topic: I still don’t get the following point: If nitro helps to cool the combustion process, why does the use of highly nitrated fuels several times come along with shimming the engine? That’s a bit agains the theory I understood. Another interesting thing I thought about is the following: A very popular German Professor back in the 80s published several books about model engines. He described that our 4C engines need a certain intake manifold length to condensate the fuel vapor correctly on the cold intake side. Too short manifolds will not work, he stated out. Even though Laser carbs manifolds have a certain length (the part that get stuck in the head) they seem quite short … but nevertheless it works perfectly despite of being “sooo short”. Edited October 16, 2023 by Simon Bulk Quote Link to comment Share on other sites More sharing options...
Jon H Posted October 16, 2023 Share Posted October 16, 2023 4 minutes ago, Simon Bulk said: Back to the topic: I still don’t get the following point: If nitro helps to cool the combustion process, why does the use of highly nitrated fuels several times come along with shimming the engine? That’s a bit agains the theory I understood. If you are asking what i think you are (shimming high nitro engines that knock) then there is more to it than just the fuel. The more fuel you are burning the more the compression increases as the fuel is incompressible so you need to shim it to bring the comp down a bit. I believe top fuel dragsters run almost on the verge of hydraulic lock to get maximum power. 6 minutes ago, Simon Bulk said: A very popular German Professor back in the 80s published several books about model engines. He described that our 4C engines need a certain intake manifold length to condensate the fuel vapor correctly on the cold intake side. Too short manifolds will not work, he stated out. Even though Laser carbs manifolds have a certain length (the part that get stuck in the head) they seem quite short … but nevertheless it works perfectly despite of being “sooo short”. If i understand this correctly, You do not want the fuel to condensate into drops. You want the finest vapour you can get mixed well with air, so i am a bit confused by this. Unless i am misunderstanding the wording here it makes no sense at all. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted October 16, 2023 Share Posted October 16, 2023 (edited) Our friends in Formula 1 can teach us a thing or two and they agree with Jon. Their experimental fuel intake arrangement involves a mixing area, prior to the piston cylinder, which mixes air and fuel, producing the said vapour, then ( in my understanding ) that vapour is ignited, rendering the intake charge Hot into the cylinder. We can but dream for the time being. Found the article here The traditional spark plug is replaced with a jet ignition chamber. Most of the fuel (97 percent) is injected as normal into the cylinder when the piston is about 60 degrees away from top-dead-center. (Top-dead-center is when the piston is at the very top of its travel within the cylinder). But within the jet ignition device, there's a small pre-chamber that includes a spark plug, and it's into this pre-chamber that the remaining three percent of the fuel is added and then ignited, when the piston is at 22 degrees from top-dead-center. At this point it becomes a high pressure jet of hot gases that enter the cylinder via a series of tiny holes, igniting the rest of the (now well-mixed) fuel and air. Edited October 16, 2023 by Denis Watkins Quote Link to comment Share on other sites More sharing options...
Jon H Posted October 16, 2023 Share Posted October 16, 2023 more info here https://www.enginelabs.com/news/f1-tech-what-is-turbulent-jet-ignition/ However, we should probably start an engine theory thread to deal with all this as we are well of base now. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted October 16, 2023 Share Posted October 16, 2023 6 minutes ago, Jon - Laser Engines said: more info here https://www.enginelabs.com/news/f1-tech-what-is-turbulent-jet-ignition/ However, we should probably start an engine theory thread to deal with all this as we are well of base now. Apologies Jon it was just supporting " vapour " Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted October 16, 2023 Author Share Posted October 16, 2023 A lot of good sense posted here, especially what Jon says, personally I use an oily mix as I don't want to make a mistake and run my helicopter engine on 10% oil, also I use 7.5% and 10% fuel mix on everything depending on what I can get my hands on, as for shimming, I did that on my Enya .90 and 120 and also a troublesome prop throwing second hand Saito that I got hold of which helped a lot. As for the long or short inlet, I have experimented on an old Laser .90 and there isn't a real différance in running, only positive note is fuel tank installation when inverted. Quote Link to comment Share on other sites More sharing options...
Jon H Posted October 16, 2023 Share Posted October 16, 2023 nah its fine. The whole theory section is technically off topic. A little is fine, but we are getting too far into the weeds now Quote Link to comment Share on other sites More sharing options...
Simon Bulk Posted October 16, 2023 Share Posted October 16, 2023 (edited) 55 minutes ago, Jon - Laser Engines said: If i understand this correctly, You do not want the fuel to condensate into drops. You want the finest vapour you can get mixed well with air, so i am a bit confused by this. Unless i am misunderstanding the wording here it makes no sense at all. If memory serves, the theory was as following: The spray bar is only able to create bigger drops, which are not very ignitable. Engines need to run finest vapor, highly ignitable because it’s well mixed with air. The inner surface of the intake manifold during running is colder than the surrounding and lets the big drops (coming from the spray bar) condensate there into finer ones. The rushing airflow through the intake takes these finer drops and converts them into a fine vapor which is needed for the combustion. Therefore you need a minimum length of the intake manifold to make that magic happen. If this minimum length is not given you just flood the engine with the big drops coming from the spray bar. That’s what the whole story is about. I don’t really believe that. Why? Because during starting process we all choke the engine and (until a certain point) we flood it and it starts… Let me check if I find the book. Then you’ll get the official version. Edited October 16, 2023 by Simon Bulk Quote Link to comment Share on other sites More sharing options...
Don Fry Posted October 17, 2023 Share Posted October 17, 2023 That’s a very old conjecture.Note well, not a theory. Theory is backed by experimental results, mathematical reasoning, top of the shop jobs in science. Conjectures are stuff like “ the Lord made the heavens and the earth in seven days” Both might be right, but one has more meat. in this case, if a Laser is lacking length, why does it run. Quote Link to comment Share on other sites More sharing options...
Jon H Posted October 17, 2023 Share Posted October 17, 2023 (edited) I have asked several times for the deep dive theory chat to be moved elsewhere. I am only posting this to deal with clear errors i do not want picked up as being potentially correct. 20 hours ago, Simon Bulk said: The spray bar is only able to create bigger drops The spray bar has one job, and it is literally to create the smallest fuel drops possible. This is aided by the pressure drop in the carb causing forced evaporation. This forced evaporation requires heat energy and this is stolen from the carb body etc which is why its cold. I am sure i covered this at school when i was about 13. 20 hours ago, Simon Bulk said: The rushing airflow through the intake takes these finer drops and converts them into a fine vapor which is needed for the combustion Much of the fuel/air mixing is done within the cylinder so this assumption too is incorrect. 20 hours ago, Simon Bulk said: The inner surface of the intake manifold during running is colder than the surrounding and lets the big drops (coming from the spray bar) condensate there into finer ones. Absolutely not. At about 8 years old we were all taught that water vapour in the air condenses into tiny water droplets as it cools and these are what we see as clouds/mist/fog etc. Methanol behaves no differently and any condensation of the vapour due to cooling will result in larger drops, not smaller. condensate will also tend to cling to the surface cooling it, not unlike the bathroom window after a shower, and then form even larger drops. This is far from ideal and the opposite of your claimed result. However, as we are not cooling the fuel mixture through the carb and the mixture is actually cooling the carb from within due to pressure drop/forced evaporation, this whole claim makes no sense as its completely backwards. The only condensation will be water on the outside of the carb, as this is being externally cooled. 20 hours ago, Simon Bulk said: during starting process we all choke the engine and (until a certain point) we flood it and it starts… Yes, we can flood the combustion chamber and cause the engine to start. Some of the methanol will vaporise naturally and offer a mixture that will fire. The rest is blown out of the exhaust all over your arm. Unless the German book chappy never went to school i would say he is either straight up wrong, i have misunderstood, or the point he was trying to make has been lost in translation somewhere and misreported here causing the confusion. Hopefully its the latter, as the fundamental principals here is basic pre GCSE/O level school stuff and if the book really contains this as reported then thats pretty shocking. So i am done with this now and will be asking the mods to deal with any follow up posts on this subject. If a deep dive engine theory thread is required that can be set up elsewhere as the whole thing is being completely over thought. Its another 'leave it alone' moment, just fly it. Edited October 17, 2023 by Jon - Laser Engines 3 Quote Link to comment Share on other sites More sharing options...
Outrunner Posted October 17, 2023 Share Posted October 17, 2023 21 hours ago, Simon Bulk said: If memory serves, the theory was as following: The spray bar is only able to create bigger drops, which are not very ignitable. Engines need to run finest vapor, highly ignitable because it’s well mixed with air. The inner surface of the intake manifold during running is colder than the surrounding and lets the big drops (coming from the spray bar) condensate there into finer ones. The rushing airflow through the intake takes these finer drops and converts them into a fine vapor which is needed for the combustion. Therefore you need a minimum length of the intake manifold to make that magic happen. If this minimum length is not given you just flood the engine with the big drops coming from the spray bar. That’s what the whole story is about. I don’t really believe that. Why? Because during starting process we all choke the engine and (until a certain point) we flood it and it starts… Let me check if I find the book. Then you’ll get the official version. Just what are you trying to prove with all that twaddle? 1 Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted October 28, 2023 Share Posted October 28, 2023 On 12/10/2023 at 16:36, Don Fry said: I don’t understand all this interest in motors. Install correctly, feed it nice fuel, and feed it a glow plug every couple of years, thereafter you don’t have to take much notice, unless you actually have to dig it out of the ground, why the interest. Not interested in chain saw motors, car engines either. They're living breathing things that appeal to all the senses. The purr at idle to the scream/roar at full power, vibration, smell, exhaust coming out the back, taste too, if you had to suck the fuel through (not recommended). Get something like an open rocker four stroke and you've got the valve gear to watch too. The same reason people go to steam rallies and heritage railway lines. I used to start my Indian Mills 1.3 in my hand, just to feel the thing come alive. BTW, if you're going to do this, don't fill the tank, as the engine gets rather hot! Quote Link to comment Share on other sites More sharing options...
Don Fry Posted October 28, 2023 Share Posted October 28, 2023 3 hours ago, Robin Colbourne said: They're living breathing things that appeal to all the senses. The purr at idle to the scream/roar at full power, vibration, smell, exhaust coming out the back, taste too, if you had to suck the fuel through (not recommended). Get something like an open rocker four stroke and you've got the valve gear to watch too. The same reason people go to steam rallies and heritage railway lines. I used to start my Indian Mills 1.3 in my hand, just to feel the thing come alive. BTW, if you're going to do this, don't fill the tank, as the engine gets rather hot! Love it. I did wonder of you were talking about ladies, and the spell checker had got a dose of gender inclusion. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 30, 2023 Share Posted October 30, 2023 (edited) On 16/10/2023 at 15:46, Denis Watkins said: We can but dream for the time being I'm a bit late for the discussion here... Most current generation petrol engines in our cars are very very close to that F1 arrangement already, with a totally dry intake path and direct cylinder injection. Since mid 2000s this has become common. (see https://en.wikipedia.org/wiki/Gasoline_direct_injection) A world apart from our basic spraybar carbs. Diesel engines have been direct injection for many years of course. Edited October 30, 2023 by Nigel R Quote Link to comment Share on other sites More sharing options...
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