Hi. Pulsing, particularly by the SVEA123 and 8R petrol stoves seems to be a problem that regularly crops up on this forum. I've observed it on my stoves and after some thought arrived at the following theory. The stoves in question operate by using the pressure in the sealed fuel reservoir to deliver liquid fuel to the jet, where the already burning fuel heats the jet and and chamber behind it, causing the fuel to vapourise before it leaves the jet and enters the burner. The vapourisation of the fuel before it leaves the jet causes an increase in pressure within the jet chamber, which as well as producing the pressure to force the vapour through the jet, will act as a 'back pressure', acting against the pressure within the fuel reservoir. If the pressure within the tank is sufficiently low, this back pressure interrupts the flow of fuel to the jet, such as to reduce the amount of fuel entering the chamber and leaving the jet to be burnt. When the flow of fuel to the jet chamber is interrupted, the pressure of vapour within the jet chamber falls, allowing the pressure within the tank to restore the flow - the cycle repeating. The system is essentially a 'tuned' system, the frequency of operation being largely determined by the _volume_ of trapped air in the reservoir - the smaller the volume, the higher the frequency, and vise versa. The wick would have the effect of damping the pulsing, and is probably the cause of pulsing dying away altogether as the pulse frequency drops, but pulsing may remain, but at a sufficiently low frequency and 'amplitude' as to be unnoticeable. As the fuel reservoir heats up, the pressure generated is sufficient to minimise the effect of back pressure at the jet, and again the pulsing will die away. ( However, it also seems to me possible that _all_ heathily running stoves are pulsing - it's just that the rate is sufficiently high that it's unnoticeable, and even is responsible for the characteristic 'growling' roar of these stoves.) It is important to note, that the pressure within the fuel reservoir is not related to the surface area of the fuel/air interface, as is often suggested, but to the temperature of the fuel. The surface are of the fuel/air interface within the reservoir _is_ related to the volume of air, and thus the 'tuned' frequency of the system. I believe that the above gives a satisfactory explanation for the phenomenom of pulsing, and would be interested in any comments! Jim Ford
It sounds like you have really thought about this. I guess your position is that all these stove pulse, some more, some less. Also, the pulsing decreases when the pressure in the fuel tank reaches some specific value. I?m just glad to know that my new 8R is behaving the way it should and I don?t have to send it back to Sweden. Would using a midi pump eliminate the pulsing?
Increasing the tank pressure with a midi pump should have the effect of overcoming the back pressure and reduce the pulsing, but as I mentioned, perhaps all these stoves pulse - it's just that when they're running 'properly' the pulsing is the normal purr we've have come to love! Having written down 'my theory', I'm already begining to refine it in my mind - I've got to make some more observations! Jim Ford
I'd come to more-or-less the same reasoning to explain pulsing in Tilley (and other) lanterns / lamps. I think the crude pulse jet on V1 flying bombs worked on a similar principle. The forward pressure in the combustion chamber shut flaps at the air intake. The pressure dropped and the forward motion of the bomb caused air to open the vents again. More fuel burned and the backward pressure pushed the bomb forward whilst the forward pressure shut the flaps again. This explains why the bombs needed to be launched by a catapult to get sufficient airspeed before the pulse jet took over. Having said all that, it's just occurred to me that this may explain why one of my stoves tried to destroy my house recently - the Monitor Vergeltungswaffen 1 flying stove obviously
Although my experience is with pressure lamps, my view is that you're nearly right. Where I differ is in regard to the reduction caused by back pressure; I tend to think that the pulse is due to a temporary increase in pressure over the tank pressure, rather than a reduction. I cannot see that the pressure in the vapouriser can be materially less than that in the tank; I accept that this can occur with a wave motion, but not to any real extent in a stove or lantern. The vapouriser contains both liquid and gaseous forms of the paraffin. As the vapour passes through the jet, the underlying liquid rises into the hotter part of the tube and is turned to gas; this tends to be a stop/start action due to the latent heat required for vapourisation - a bit like the large bubbles of steam in a boiling kettle. The fuel expands greatly as it turns to gas. If there were no obstruction in the vapouriser to tank route, there would be a modest, temporary increase in the overall pressure of the tank and system, the amount depending on the total volume of the air and vapour (i.e the fuel level). However, there are obstructions between vapouriser and tank. The liquid fuel in the vapouriser and delivery pipe cannot be pushed back into the tank in an instant, it has considerable inertia. Therefore there is a significant rise in the pressure in the top of the vapouriser and jet until it is reversed and/or expelled through the jet, reducing to "standard" pressure and allowing more liquid fuel to be vapourised. This leads to the pulse. I'm less sure about the next bit, but Boyle's law or something says that the proportional increase in pressure by the addition of a fixed volume will be less as the starting pressure rises. I think this might mean that the pulsing effect will be less when there is more pressure in the tank. Any correction or observation most welcome Ian
I don't know why or how the pulsing happens, but I sure love the sound of it. In the crisp morning air the sound of a 123 pulsing under a pot of coffee will bring more then a few campers wondering what that sound is. Then the education begins. It is amazing how many campers don't know about liquid stoves other then Colman. Jim the theory sounds good to me. Paul
Hi, A very basic issue I know the problem specially by petromax lantern which are converted for running with meths. Here is the Problem that the meths don?t vapourise steady, there the pulsing gets away by put in a "long-spring" in the vapouriser which spread the heat. Jim Ford wrote: Im unsure if a Backpressure exists I rather blame a too fast vapourisation : On Svea/Hunter the fuel gets transported by the wick in the and trough the vapouriser at least comes out ot of the burner gasiform. On this way from tank to jet the fuel gets warmer and warmer until it vapourise This process has to be "consistent". When the fuel get?s too fast vapourised than the flow rate rises up very fast. Too fast thus that fuel supply don?t comes afterwards , the flame gets lower up to the time the following fuel vapourises and so on. So I explain the difference between the Hunter an Svea: My svea don?t pulse but my hunter, I assume that on svea the fuel gets heaten more consistent like on hunter. After I reinstalled the wick on hunter the pulsing gets lower, so maybe a wick that don?t sitting deep enough in the steam could be a factor for pulsing svea?s. Interesting for me is: On which stoves do you have had pulsing problems up to know ? I assume mostly on self pressurised because on the tripod ones there the Burner design allows a permanent heating Regards, Christoph
