Really basic capped PJ

[m1tch]

Hi again, just saw this design on this site:



I have seen a lot of designs with the usual intakes pointing backwards, and the U shaped ones, but I don't see many simple capped ones, is there any reason for this?

An upgrade to this design could be to mount the sparker in the middle of the cap at the front, and then use the actual fuel lines as the spacers to hold out the cap?

Blue = spark plug
Red = Fuel - there will be 4 nozzles round to support the cap



I have found a few off the shelf parts that I might be able to use - as I can't weld lol im guessing that aluminum would melt?

[Mike Everman]

Yes, aluminum is not ever, ever going to take the temp. Capped designs have major flaws, mostly because of skin drag and the fact that it's very difficult to make the annular area constant (or known exactly.) Also, boundary layer effects makes it so that eccentricity of one diameter to the other actually changes the effective area of that annulus. Worse still is that the difference in diameter is very small, and the cc grows substantially when it heats up, so changing this area substantially; so the outer shroud diameter needs to be bigger so that the area comes in when it all heats up. NOT a great place to start, and if you are to get it running (after much frustration and "cut and try"), it will almost certainly be not so good a motor, and considering the manufacturing limitations available to most experimenters, one motor will not act like the next. It is far better to have an intake tube that is small and will not change much.

[m1tch]

Ah so this design is more of a 'good idea on paper and not in practice' kind of engine, due to the inability to maintain the intake size due to the added heat?

I managed to find a few off the shelf parts made in mild steel, but I will try another design - perhaps the no weld ugly stick design, but with mild steel ducting and clamps rather than the plumbing parts as they seem to be difficult to get hold of!

[Ghrey]

Another problem, is that the inside "layers" if you will, have effectively NO cooling, this can distort and destroy even the most durable materials. The annular design is not impossible but it is NOT for the average beginner.

And yes I am still working on one, Glutton for punishment I guess....

[Mike Everman]

At least it gets cool air some of the time. Some other designs are far worse. Don't get me wrong, you can make it work. And if it worked well, I'd say put it on the mantle, cause if you look at it sideways it might not work a second time anyway. Good or bad, no one around here would be able to tell you why, too!

[m1tch]

Amazingly this pulse just looks almost exactly like Ghrey's metal jam jar pulse jet lol just with a hole rather than a sleave over it lol would look awesome on a mantle peice lol

Should I try and make that as my 2nd pulsejet? (still actually need to build one lol)

[Ghrey]

That odd little, noisy, bit of scrap, has given me an idea for a capped tube that would not be annular.

I will try to get a better camera....