Mark, I know it looks like I havent utilised the area available on the valveplate very well for intake area but I have made the valve plate bigger to allow for a flange which allows easier dismantling. The flange projects 20mm around so the combustion chamber ID is 117mm. This leaves approx 10mm between the tips of the valves and the inside edge of the combustion chamber. I am going to run 2 tests in the morning. One with the original valveplate and a second petal valve behind the first and another with the second valveplate and the same valve plate. I will report back on the results.
The propane is being regulated through an industrial regulator at the tank end which goes as far as 4bar. The fuel is being injected directly into the chamber between 2 valve retainer plates. Im afraid I cant change that design now because I just dont have the time. I would be happy now just to get it pulsing for about 30 seconds. This is a completely static test engine so I will never need to run it for any length of time. This way the chilling of the tank should not pose a problem.
Starting Problems
Moderator: Mike Everman
Re: Starting Problems
By the way, does anyone know the relationship between valve area and exhaust area? Is it the same as Tharratts 0.2Ae? This is the only problem with writing engineering reports... I need to reference everything I use.
Re: Starting Problems
What is the shape of your pulsejet? You valves are fine if the reeds are only 10mm from the outer edge of the combustion chamber. Are you working from some plans or what page are you on in what article or book you are working from???
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Re: Starting Problems
Somebody should not have mentioned that idea. Don't bother with it. A second layer of petal valves increases static stiffness, like making it harder to blow air into the jet, but dynamically it has changed little if anything at all. IOW, both reeds will vibrate at a single's frequency.motthomas wrote: I am going to run 2 tests in the morning. One with the original valveplate and a second petal valve behind the first and another with the second valveplate and the same valve plate. I will report back on the results.
You're going to have to make some valve retainers, or just washers since you're pressed for time, of varying OD's , in order to shorten the free length of each petal valve thus increasing its frequency, or vice-versa.
Re: Starting Problems
"Somebody should not have mentioned that idea. Don't bother with it. A second layer of petal valves increases static stiffness, like making it harder to blow air into the jet, but dynamically it has changed little if anything at all. IOW, both reeds will vibrate at a single's frequency.
You're going to have to make some valve retainers, or just washers since you're pressed for time, of varying OD's , in order to shorten the free length of each petal valve thus increasing its frequency, or vice-versa."
There are a few designs that use double or even triple reeds for a reason. From Tharratt's article ... "FIG 24a shows an exploded view of a typical reed valve consisting of spacers, reeds and seats, held together by tie bolts and end plates. The reeds can be clearly seen to consist of a rubber reed sandwiched between two metal reeds per set. The composite construction being necessary to give added stiffness whilst retaining, as closely as possible, the response characteristics of a single metal reed." The article goes on to talk about preloading the reeds for stiffness ... and continues ....
"Another method which has found wider favour, is to incorporate double or even treble reeds. Utilizing the fact that the reed stiffness increases in proportion to the number of reeds while the dynamic response characteristics varies only slightly from that of a single reed."
Going by his information that the inside diameter is 117mm and "this leaves approx 10mm between the tips of the valves and the inside edge of the combustion chamber," that makes his valve 97mm in diameter.
Now if you were to hold in your hand a petal valve 97mm in diameter with 10 petals and a thickness of .006ths, that's a relatively big petal valve for each port and at that diameter there is WAY more surface area to press upon than the little 23/64 ports of a Dynajet, recall his ports aren't even circular. A retainer would help, but a thicker reed would be helpful too. I've got a huge amount of .006ths reed material and I have an eighteen petal valve reed made of it that's amazingly 97mm diameter, the same as our topic at hand. My 18 petals are a bit bigger than the Dynajet's though at 1/2 inch in diameter.
It takes very little force to bend a long reed of this thickness, the stuff is very flimsy. And maybe a double reed would be too thick for his engine, but it really doesn't hurt to try things because often that's how you really know something.
http://www.pulsorohr.de/xp90.html
You're going to have to make some valve retainers, or just washers since you're pressed for time, of varying OD's , in order to shorten the free length of each petal valve thus increasing its frequency, or vice-versa."
There are a few designs that use double or even triple reeds for a reason. From Tharratt's article ... "FIG 24a shows an exploded view of a typical reed valve consisting of spacers, reeds and seats, held together by tie bolts and end plates. The reeds can be clearly seen to consist of a rubber reed sandwiched between two metal reeds per set. The composite construction being necessary to give added stiffness whilst retaining, as closely as possible, the response characteristics of a single metal reed." The article goes on to talk about preloading the reeds for stiffness ... and continues ....
"Another method which has found wider favour, is to incorporate double or even treble reeds. Utilizing the fact that the reed stiffness increases in proportion to the number of reeds while the dynamic response characteristics varies only slightly from that of a single reed."
Going by his information that the inside diameter is 117mm and "this leaves approx 10mm between the tips of the valves and the inside edge of the combustion chamber," that makes his valve 97mm in diameter.
Now if you were to hold in your hand a petal valve 97mm in diameter with 10 petals and a thickness of .006ths, that's a relatively big petal valve for each port and at that diameter there is WAY more surface area to press upon than the little 23/64 ports of a Dynajet, recall his ports aren't even circular. A retainer would help, but a thicker reed would be helpful too. I've got a huge amount of .006ths reed material and I have an eighteen petal valve reed made of it that's amazingly 97mm diameter, the same as our topic at hand. My 18 petals are a bit bigger than the Dynajet's though at 1/2 inch in diameter.
It takes very little force to bend a long reed of this thickness, the stuff is very flimsy. And maybe a double reed would be too thick for his engine, but it really doesn't hurt to try things because often that's how you really know something.
http://www.pulsorohr.de/xp90.html
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Re: Starting Problems
Duh, ... what do you think I said above?"Another method which has found wider favour, is to incorporate double or even treble reeds. Utilizing the fact that the reed stiffness increases in proportion to the number of reeds while the dynamic response characteristics varies only slightly from that of a single reed."
Re: Starting Problems
Good news! (Well sort of...) I got a result. Its not much but its a step in the right direction. I tested the engine again using the original valves but using the second valve plate in the pdf I attached in an earlier post. Thats the plate with the bigger valve area. The engine fired no problem and appears to be pulsing but the burning is not very energetic and the pressures being built up in the combustion chamber dont seem to be that high at all. It still doesnt sound like a proper pulsejet as the level of noise is not very high. I used a camera to look up the tailpipe into the engine and I can see the burning. However, the flame colour is very yellow which suggests a rich mixture or perhaps inadequate mixing of fuel/air as somebody here already suggested?
I also tried a second test using the same valveplate but including a second petal valve along with the original, as suggested also. The result was worse than the 1st test but still showed signs of some form of pulsing combustion occurring and sustaining without the spark.
Webpilot; I read more into your thread on "dynamic modelling of a strip valve" and you have given me some food for thought... The graphs you show of f/fn are quite interesting! based on the 2 tests I ran today, I would like to increase the stiffness of my petals so the dfr is raised closer to the magic 0.59 you mentioned. As you say above, the easiest way to do that is decrease the effective length of the petals by including a washer of a certain size to restrain the movement of the valves. This is a simple modification which I should be able to make up and test tomorrow.
Another thing I would like to do, as somebody already mentioned is try and move the point of injection for the propane further forward into the intake diffuser in an attempt to aid mixing. I have an idea for a relatively simple component which I should be able to make up quickly and thread on between the existing fuel jet and the propane hose.
I also tried a second test using the same valveplate but including a second petal valve along with the original, as suggested also. The result was worse than the 1st test but still showed signs of some form of pulsing combustion occurring and sustaining without the spark.
Webpilot; I read more into your thread on "dynamic modelling of a strip valve" and you have given me some food for thought... The graphs you show of f/fn are quite interesting! based on the 2 tests I ran today, I would like to increase the stiffness of my petals so the dfr is raised closer to the magic 0.59 you mentioned. As you say above, the easiest way to do that is decrease the effective length of the petals by including a washer of a certain size to restrain the movement of the valves. This is a simple modification which I should be able to make up and test tomorrow.
Another thing I would like to do, as somebody already mentioned is try and move the point of injection for the propane further forward into the intake diffuser in an attempt to aid mixing. I have an idea for a relatively simple component which I should be able to make up quickly and thread on between the existing fuel jet and the propane hose.
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Re: Starting Problems
I am so freekin' happy to read you got a +tive result.motthomas wrote:Good news! (Well sort of...) I got a result. Its not much but its a step in the right direction. I tested the engine again using the original valves but using the second valve plate in the pdf I attached in an earlier post. Thats the plate with the bigger valve area. The engine fired no problem and appears to be pulsing but the burning is not very energetic and the pressures being built up in the combustion chamber dont seem to be that high at all. ...
I realize you're not "out of the woods", yet, ... but ... congratulations!
Where's some pics? Even a pic taken with a cell phone camera will do.
You'll find it differs considerably from what Tharrat wrote.motthomas wrote:Webpilot; I read more into your thread on "dynamic modelling of a strip valve" and you have given me some food for thought... The graphs you show of f/fn are quite interesting! based on the 2 tests I ran today, ...
Re: Starting Problems
Secondly, the reeds might not have enough "snap-back" with this larger engine, that is on the inflow phase, they aren't going to have much resistance and might swing open for a longer period of time than the engine likes. I had an engine that would backfire nicely, but would never catch, and when I limited the play of the reed, it sprang to life.
Above is what I wrote earlier. If you weren't using a retainer at all, no wonder the jet isn't starting, especially with .006ths reeds. All of the reeds I have limit the play to around a quarter of an inch. I wouldn't go any higher than a third of an inch for your design at most. The Helmond Team in some of their valves use triple reeds, the second and third each reduce in diameter and I'm sure in a way act as a "secondary" retainer. With the outboard motor V valve reeds I have, even though the reeds in some cases are well over an inch long, the point at which they start to bend is about the halfway mark. And in one large outboard motor design there is no retainer at all, because once it bends open a quarter of an inch, the direction of lift can't exceed the angle of the air flow, the pyramid angle so sharp. The very wide outboard motor reeds are .010ths. And unlike the Dynajet, they don't flex from the base where the retainer holds them down but up higher. Also the outboard V valve with the odd, silvery retainer allows a mere one eighth inch of play, but the valves are approaching 3/4 inch wide, and again only 1/8 inch play!
The Dynajet reed also flexes about a quarter of an inch if slightly more, only at the very tip can it achieve this height. There is this "larger" 9 petal design pictured using .006ths I made for my 3 inch diameter pulsejet. But again the petals are only allowed a quarter inch of lift over a half inch diameter hole. The ports are larger than my 18 petal design. It's a good running engine and valve. If you make the ports too much bigger using .006ths , and the jet backfires hard, it is likely the reeds will deform or get hammered to death. I can feel the slightest bit of flex when I push a petal down with my finger firmly over the 1/2 inch port.
Above is what I wrote earlier. If you weren't using a retainer at all, no wonder the jet isn't starting, especially with .006ths reeds. All of the reeds I have limit the play to around a quarter of an inch. I wouldn't go any higher than a third of an inch for your design at most. The Helmond Team in some of their valves use triple reeds, the second and third each reduce in diameter and I'm sure in a way act as a "secondary" retainer. With the outboard motor V valve reeds I have, even though the reeds in some cases are well over an inch long, the point at which they start to bend is about the halfway mark. And in one large outboard motor design there is no retainer at all, because once it bends open a quarter of an inch, the direction of lift can't exceed the angle of the air flow, the pyramid angle so sharp. The very wide outboard motor reeds are .010ths. And unlike the Dynajet, they don't flex from the base where the retainer holds them down but up higher. Also the outboard V valve with the odd, silvery retainer allows a mere one eighth inch of play, but the valves are approaching 3/4 inch wide, and again only 1/8 inch play!
The Dynajet reed also flexes about a quarter of an inch if slightly more, only at the very tip can it achieve this height. There is this "larger" 9 petal design pictured using .006ths I made for my 3 inch diameter pulsejet. But again the petals are only allowed a quarter inch of lift over a half inch diameter hole. The ports are larger than my 18 petal design. It's a good running engine and valve. If you make the ports too much bigger using .006ths , and the jet backfires hard, it is likely the reeds will deform or get hammered to death. I can feel the slightest bit of flex when I push a petal down with my finger firmly over the 1/2 inch port.
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Re: Starting Problems
Thomas,
For now, use the K.I.S.S. philosophy ... keep it simple stupid ... and stick with the driving frequency ratio adjustment.
The backing plate is only there to reduce root stresses thus lowering the fluctuating stresses to at or below the endurance limit of the material - but right now, you are not concerned with longevity.
For now, use the K.I.S.S. philosophy ... keep it simple stupid ... and stick with the driving frequency ratio adjustment.
The backing plate is only there to reduce root stresses thus lowering the fluctuating stresses to at or below the endurance limit of the material - but right now, you are not concerned with longevity.
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Rocket Man
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Re: Starting Problems
I can see you have several problems.
The inside diameter of your tail pipe should be 2.330" and it should be 25.630" long. Cross sectional area = 4.263. To find thrust 4.263 x 3.6 = 15.3468 lbs of thrust.
The combustion chamber should be 3.375" inside diameter and 6.365" long.
The cone should be 8.581" long.
Air intalk for 100% throttle should be the tail pipe cross sectional area times .5. For 80% throttle air intake cross sectional area time .4.
The air intalk has its own frequency. If the air intake is too short the engine will be hard to start and flame out easy. Air intake should be no shorter than 2.250" long.
The propane fuel injector should be 1.5" from the reed valves and located in the center of the combustion chamber. Fuel injector orfice diameter should be .093". The engine will start easy on 2 psi of propane and throttle up to full throttle at about 8 psi. If your using the double disk method injection the propane between the 2 disks that is a lot of extra work for nothing. There is a better much simpler way and the engine is 85% throttable.
The reed valve retainer should not let the reed valves open more than .200" and it should cover the entire reed valve to protect it from combustion chamber heat.
The spark plug should be 1.750" forward or infront of the fuel injector.
You need to be using .010" thick blue spring steel for the reed valves. The reed valve diameter needs to be 2.500" with 10 pedals. Each pedal is .875" long. The center of the flat section of the reed valve retainer is .750" diameter.
Pedal valve air intake holes should be 10% of the total air intake of the engine. Air intake hole diameter or width should never be more than .375". You can put several air intake holes under one pedal valve to get the required air intake. Holes can be slotted.
This engine produces 15.3 lbs of static thrust running on propane at 100% throttle. Ram air from forward speed of the engine allows the engine to throttle up to 20 lbs of thrust this is about 140% throttle. I have run this engine for 2 hours and the reed valves look as if they have not been used.
Here is a video of the engine running. http://www.youtube.com/user/Lowracerman ... VNyTsUT2Xg
Read the Data the Germans did for the V1 engine. Their data is correct. A pulse jet engine is no different than a car engine if you run a car engine wide open as hard and as fast as it will go it will self distruct very quick. Run a pulse jet the same way it will self distruct too. The Germans started their engine at low throttle and it had an air speed indicator that automatically throttled the engine up the faster it went until it reached full throttle.
The inside diameter of your tail pipe should be 2.330" and it should be 25.630" long. Cross sectional area = 4.263. To find thrust 4.263 x 3.6 = 15.3468 lbs of thrust.
The combustion chamber should be 3.375" inside diameter and 6.365" long.
The cone should be 8.581" long.
Air intalk for 100% throttle should be the tail pipe cross sectional area times .5. For 80% throttle air intake cross sectional area time .4.
The air intalk has its own frequency. If the air intake is too short the engine will be hard to start and flame out easy. Air intake should be no shorter than 2.250" long.
The propane fuel injector should be 1.5" from the reed valves and located in the center of the combustion chamber. Fuel injector orfice diameter should be .093". The engine will start easy on 2 psi of propane and throttle up to full throttle at about 8 psi. If your using the double disk method injection the propane between the 2 disks that is a lot of extra work for nothing. There is a better much simpler way and the engine is 85% throttable.
The reed valve retainer should not let the reed valves open more than .200" and it should cover the entire reed valve to protect it from combustion chamber heat.
The spark plug should be 1.750" forward or infront of the fuel injector.
You need to be using .010" thick blue spring steel for the reed valves. The reed valve diameter needs to be 2.500" with 10 pedals. Each pedal is .875" long. The center of the flat section of the reed valve retainer is .750" diameter.
Pedal valve air intake holes should be 10% of the total air intake of the engine. Air intake hole diameter or width should never be more than .375". You can put several air intake holes under one pedal valve to get the required air intake. Holes can be slotted.
This engine produces 15.3 lbs of static thrust running on propane at 100% throttle. Ram air from forward speed of the engine allows the engine to throttle up to 20 lbs of thrust this is about 140% throttle. I have run this engine for 2 hours and the reed valves look as if they have not been used.
Here is a video of the engine running. http://www.youtube.com/user/Lowracerman ... VNyTsUT2Xg
Read the Data the Germans did for the V1 engine. Their data is correct. A pulse jet engine is no different than a car engine if you run a car engine wide open as hard and as fast as it will go it will self distruct very quick. Run a pulse jet the same way it will self distruct too. The Germans started their engine at low throttle and it had an air speed indicator that automatically throttled the engine up the faster it went until it reached full throttle.
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WebPilot
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Re: Starting Problems
What is your time deadline, Thomas?motthomas wrote:The propane is being regulated through an industrial regulator at the tank end which goes as far as 4bar. The fuel is being injected directly into the chamber between 2 valve retainer plates. Im afraid I cant change that design now because I just don't have the time. I would be happy now just to get it pulsing for about 30 seconds. This is a completely static test engine so I will never need to run it for any length of time. This way the chilling of the tank should not pose a problem.
Fig. 1. GRIM's EVG-043 valve plate head Note improved flow due to
V-shaped valve plate, and NO valve back side retainer. Valves showed
no signs of burn or fracture.
Re: Starting Problems
It is difficult to help without dimensions.
Thanks for sharing Rocketman, I think the reason Rocketmans engine is able to be injected directly into the CC is the length of the CC is relatively long. His injection point would fall within most jets venturi.
There will be no doubts when you are close to dialed in.
From what I gather your cone is shorter than Erics calculator engine?
Thanks for sharing Rocketman, I think the reason Rocketmans engine is able to be injected directly into the CC is the length of the CC is relatively long. His injection point would fall within most jets venturi.
There will be no doubts when you are close to dialed in.
From what I gather your cone is shorter than Erics calculator engine?
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Re: Starting Problems
Havent updated ye in the past few days because I havent done any more testing on the engine Im afraid. That is being left until tomorrow when I will test a new propane injector, a new valve retainer plate which will limit valve tip travel to about 7-8mm and new, thicker valves (0.008" and 0.010"). Ill be running individual tests on these obviously so I can gauge the effect each modification has on the performance.
Thanks very much for the input rocketman. But due to already having made up the jet body and having time constraints, I am sticking with the 3" tailpipe and trying to make the engine work in its current configuration regardless of the thrust I will eventually get out of it. Webpilot, I have about a week left to improve the performance as much as I can before I will have to call it a day and write the report based on the work Ive done so far.
Im attaching a drawing of the jet body so you can see the dimensions easily. Plus Ive eventually got around to sticking up a few photos for ye to see! Feel free to criticise... constructively of course!
Thanks very much for the input rocketman. But due to already having made up the jet body and having time constraints, I am sticking with the 3" tailpipe and trying to make the engine work in its current configuration regardless of the thrust I will eventually get out of it. Webpilot, I have about a week left to improve the performance as much as I can before I will have to call it a day and write the report based on the work Ive done so far.
Im attaching a drawing of the jet body so you can see the dimensions easily. Plus Ive eventually got around to sticking up a few photos for ye to see! Feel free to criticise... constructively of course!
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- After a few tests and with the new and much improved ignition system.
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- Before the very first test. Note the intact paint on the tailpipe!
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- The backup valve plate is a variation of the original and was designed to allow me to vary the intake area if needed by using different size washers placed in front.
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- This is the original valve plate designed to be 0.2Ae and allowing for 50% efficiency estimate
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WebPilot
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Re: Starting Problems
I don't see a combustion chamber ... er, I guess I should say "your cc is too small."
Nice work otherwise.
Nice work otherwise.