Question for the experts: Some time last year I came across a picture on the Ultima Naltio website (the guys producing the KAP Arctic adapters), showing a Cobra burner cross section. I can't find it there anymore to provide a link, so I attach it. To the mods: Delete it if it violates forum rules. You can clearly see additional holes on top of the inner cap and at the upper end of the main tube leading into the inner cap. I'm wondering about their function, it looks quite sophisticated. But what for? Gas flow should go through the main tube entering the space between inner and outer cap, then to the jet holes. So why have an additional gas pathway? Anyone can shed some light on this? Thanks juergen
Hi, I believe that Bill is correct on the reason for the holes in certain inner caps and spigots. Have a look at this link for earlier examples: https://classiccampstoves.com/threads/8449 Best Regards, Kerophile.
I'm with Bill. As I understand it; when underburn starts in the top of the central tube the burnt gasses (exhaust) go up into the space between the caps, then some of it goes down thru' the holes in the top of the inner cap & thru' the holes in the side of the central tube where they end up at the point of underburn and deprive the underburn of oxygen, so it ceases. All this happens almost instantaneously as soon as underburn tries to start. If this is wrong, no doubt someone will enlighten us by the by.
That's true; there was a lot of discussion on these pages a couple of years ago on the purpose of the holes in the tube.
To save weight It is interesting. I believe it has to do with under-burning like you guys have said. I will have to run some tests with a inner cap with no holes and one with holes. Maybe it improves simmering???? Jeff
(Please forgive the long post, I just wanted to write down what was jumbling around in my brain even if it is right or wrong.) I've been thinking about this all day today. The burner science has been on my mind and I couldn't stop thinking about it. After studying the pictures and reviewing comments, I've come to one conclusion about the holes in the inner cap. First, I went with the theory that the air flows as shown in the pictures with the arrows. Here's how that went: Seems like the holes would improve the air/fuel mixture as well. Looking at the picture with arrows, as the jet stream passes up the main air way tube and into the mixing chamber, it would mix with fresh air and air/fuel mix coming from the inside of the inner cap. The main part of the jet stream would continue upward, possibly better mixed than it would have if there were no holes to begin with, hit the top of the outer cap and out the flame holes. While this is going on, some of the air/fuel mix would be pulled into the inner cap (maybe more air than fuel), possibly heated from being in the main mixing chamber, and then proceeding on through the holes in the main tube to be mixed again more thoroughly. This cyclic action also disrupts the jet stream as it travels up the main tube before it hits the top of the outer cap for better mixture. If you've ever seen vaporized fuel traveling upward from a burner jet, it comes out in a straight line about a millimeter in width until something disrupts it such as burner plate or the underside of an outer burner cap, AND/OR in this case the holes in the main tube. I've seen depictions showing the vaporized jet coming out as a spray from the jet but it doesn't appear to be so. As I've done some experimenting, I've been able to play with the jet stream and run a jet pricker alongside it pretty far up the burner (Primus 51 & Optimus No. 2) without touching it and without disrupting the jet's stream and travel. I've found that it stays quite narrow until it runs into something or something runs into it. In this case it could be the air coming in through the holes in the main tube, causing an even better air/fuel mix. So, I guess my conclusion with this theory is a better air/fuel mix than if there were no holes in the inner cap and no holes in the main tube. What do I think about underburn? Well, again, this is just my opinion and theory. I could very well be wrong here. I've found that in building silent burner caps, that if you drill too many flame jet holes or drill them too big and the pressure of air/fuel mix going into the mixture chamber is not high enough to push the mixture out the flame holes, the flames will actually recede back into the mixing chamber and start underburning. I'm not sure the holes in the inner cap have anything to do with reducing underburn (now remember, this is my opinion). My 111T has underburned and it has the holes in the inner cap, but it only did it when I turned the stove down low after it had been on high and it only has ever done it a few times out of the numerous times I've used it. So it can happen. If you look at BernieDawg's silent burner cap design, he's created a venturi system that keeps the pressure up high in the mixing chamber before the air/fuel mix enters the flame jet chamber. I've experimented with it and found that it can tolerate really low flames, which is excellent design and shows that pressure inside the cap is important to keeping underburn out. Combined with the right sized flame jet holes, underburn can be kept to a minimum or eliminated. Other factors exist, but these are the main ones. As a final comment, I respect the opinion that the holes in the inner cap could possibly prevent underburn. I also say there is no definitive proof of that. I'm not saying that my opinion is correct either. There is no definitive proof of this either. It is only a theory that I wanted to spout off on it. Thanks to all, sam
Thank you Sam for sharing your thoughts. I have to confess that I'm not a fluid dynamics expert. But my initial thoughts were similar. All I can add are 2 observations. Gary's cap and the larger Optimus cap initially tend to some flame lift off at high output rates. Seems as if the head gets overloaded. After about 20 seconds, when the cap has reached operating temperature, lift off is gone. The main difference between the Optimus head and Gary's head is the distance between inner and outer cap. The inner cap of the original one touches the outer cap at the bottom, upwards the distance increases. Gary's caps have uniform distances between inner and outer. When simmering the original one looks like this: The head starts glowing read, looks as if burning starts in between. I could not get this effect with Gary's caps. Within several days I should receive a "hole" version inner cap, then I will check for the effect described above. Maybe the holes prevent underburn or prevent flame lift off, or both? I will try to find it out. Btw, so far I never managed to get underburn. juergen
I think Optimus should hire you guys and this site to design the new nova! Thanks Sam for your thoughts Interesting stuff. Jeff
Hi. I just noticed this thread. I am not an expert, although I have drawn that Cobra picture to my site. It's here: http://kotisivu.dnainternet.net/luontola/retkeily/winterstoves.htm It's modelled with Pro/E 3D cad program of my own Cobra burner without cutting or breaking it at all. So the innermost channels were guessed by myself. But all the details which I could see and measure are as accurately drawn as possible. Also I have wondered the holes in the burner. The construction is old as you know and I believe that those days they had to test several times with the trial and error method before it ran correctly. The result is very beautiful, isn't it? Lasse
Hi Lasse. Thank you for posting the direct link, I could not find it. Hope you don't mind that I attached your pic. Really nice work with PE I think in "these old days" the stove designers spent much more time on details. And I'm pretty sure they knew much more about stove design than our "economic design experts" nowadays. Otherwise current stoves wouldn't be that primitive as they actually are. Just look at the integrated vaporizing tubes and compare it with a modern Nova or Omnifuel. The Optimus Explorer was the last one build that way. Then silent burners became history. Only prices developed further juergen
Don't worry juergen, it's only a great honour to me that my work arouses interest here. I also wonder why nearly all new hiking burners are roarers. It seems that polar and other arctic explorers generally don't know other cookers than MSR, although they cause terrible noise and can't simmer at all. I don't believe that the reason is the high costs, because e.g. Coleman makes nowadays quite light, silent and fully adjustable cookers at a reasonable price. Fortunately there still are plenty of good old fashion cookers on the secondhand market. Real specialists buy them and leave the new roarers in the shop. Lasse
Lasse, I think it is really cost. Arctic "explorers"....well, have a look at their list of sponsors.... I tried out MSR once, and only once. Never again. Coleman: I had them on test in Finnish Lapland. Peak types and Exponent Fyrestorm. Without going into detail, they just can't compete with a modified silent head Nova, not to mention the old 111 variants or the Explorer. Best juergen
Yes, I agree. I have only one MSR, Whisperlite, for curiousity, but never on the trip. In summer I usually use Apex II, because it is very easy to use (lazy me). In winter (in Finnish Lapland, too!) I have a 111T or 111C, Explorer, original KAP Arctic or, of course, Ultima Naltio. Lasse
I'm with you. I usually go up north twice a year. Once in late autumn after Ruska (then it is more quiet) and during Kaamos when it's even more quiet in Finnish Lapland. I now only use my modified Nova, running on butane in autumn and on white gas in winter. juergen
Wow juergen, you are a real specialist, even in the best Finnish scale! I prefer myself seasons outside of kaamos time. Many times it's still cold enough (under –30°C) to test cookers in the winter tent. Tell more about your modified Nova. Perhaps it is my next stove to play with. Is it silent like 111?
Thanks for the compliment, but the older Finnish guys you sometimes even meet during Kaamos are the real experts Nova mods: https://classiccampstoves.com/threads/16247 But since this is a different topic we should switch to PT or continue discussion in the Ultimate KAP Arctic thread. Best juergen
Sorry guys, I have totally missed that Nova mod thread. Please go on, now I'll go to study the Garys caps. Thank you. Lasse
