Hello and questions

[rocketbat]

hi, ive been browsing this forum for a while and have decided to create a valved pulsejet for my bicycle for my A level extended project.

i am unsure how much thrust will give decent preformance(>40mph <50mph) (any suggestions would be appriciated) but i am doing primary research into it. i have decided to make a valve grid similar to Bruce's 100lb pulsejet and the one pictured in Eric Beck's calculator only i will cast it out of aluminium using investment casting. i am going to create CNC cut reusable moulds for the wax in MDF but i am wondering if this is going to create a smooth enough site for the valves to sit upon in the aluminium?
Also, i was wondering . . . it would be a shame to use the moulds to just make one casting, would people be interested in purchasing cast aluminium valve heads (valves and retainers included) and if so, for how much?
im sure i will have plenty more questions but any help will be much appriciated
Joe
PS: i will keep this thread updated (with pictures) when i start manufacture soon

[Rocket Man]

I think this forum is dead very few people will reply. I think Bruce Simpson has never built a real working pulse jet engine that is large because he has NO videos of any large pulse jet engine running. He shows a pic of an engine he claims is 100 lbs of thrust but the physical size and demensions of that engine is wrong to be 100 lbs of thrust. But he has lots of videos of valveless engines running. 50 lbs of trust will push a bicycle over 70 mph. Pulse jets are fun to build and experement with but they are speed engines so what can you do with an engine that goes 200 to 400 mph.

[rocketbat]

thats quite a shame, i really enjoyed reading through the forum.
thanks for that, i had guessed at arround 50lbs so its a good thing i didnt carry that forward! i will be simply matching the thrust of the engine to the bikes (and engine's) resultant force that will give a certain terminal velocity that i will calculate to the best i can without access to a wind tunnell, i will probably thenn add a few more kilos of thrust so that hill climbing isnt out of the question.
thankyou for replying

[tufty]

This forum is far from dead, but it's true that the majority of stuff happening on the site as a whole is valveless. That said, you *will* get help with valved motors if you have specific questions.

Pulse jets are fun to build and experement with but they are speed engines so what can you do with an engine that goes 200 to 400 mph

This is just plain wrong. Indeed, (particularly valved) pulsejets open to ram air have a relatively low "terminal velocity", at which point they go into "poor quality burner" mode, or simply flame out. Not this this is likely to cause you any problems if you're thinking of attaching your motor to a bicycle. Well, hopefully not, anyway.

You *will* get usable thrust from a pulse jet at low speeds, and it's certainly feasible to build one or more valved jets to propel a bicycle, but unless you're ridiculously overpowered you probably won't get much acceleration.

50lbs of thrust is a helluva lot. All you actually need is to be able to overcome the rolling resistance + air resistance and a bit more. A pretty good guess at the drag on a bicycle and rider at 20mph is around 40N, or 9lb - a 50lb jet gives you 41lb / 180N, so assuming your bicycle is around 20Kg with jet and fuel, and rider at 70Kg or so, that gives you an acceleration of 2 m/s^2 or so. You won't hit the point at which drag equals thrust until - well - long before you quit due to brown trousers.

Here's Maddox's bike with a 50lb jet on it : http://gizmodo.com/391602/the-75-mph-bi ... jet-engine

Aim smaller than that. For a bicycle, I'd probably not consider anything more than a couple of 10-pounders (one either side, for balance, and that way they're small enough not to be too close to the pilot's legs and arse.

[vturbine]

When a 6 post guy says others aren't contributing enough, he really knows what he's talking about.

Hey, build something and post a log, photos, videos, and congratulate others on their projects. The forum is you, not somebody else.

A lot of people are focusing on their economic situation rather than pulsejets, right now by the way.

Everything has its cycle.

[rocketbat]

thanks a lot guys, i really appriciate the help, i understand it must get quite anoying as there are allways people asking the same questions so i will try to not waste your time!
the idea of two had not crossed my mind! if i were a little less far with my project i would have gone with that from the begining. when it comes to thrust/fuel consuntion would two be less eficient? i would imagine it would be roughly equal when you take into account the smaller chambers running at a higher frequency to produce equal thrust?
i will include some of my work so far to keep you updated . . . it seems i cant from collage, il have to do it when i next can at home.

[tufty]

Yeah, two will be less efficient. That's a downside, but then "efficiency" and "bicycle propelled by homemade pulsejets" aren't generally used in the same sentence anyway.

The upside is that a pair of smaller jets will be less likely to hit the "brown note". OK, the brown note is a joke, but the comment is only half-way there - being close to a large pulsejet whilst its running is an experience that will, quite literally, shake you to your bones. Sitting just above one - imagine sticking your arse in the bass bin at a drum&bass gig for an hour or so. There's also the issue of safe mounting, remember, you're dealing with something that's, again quite literally, red hot or hotter when running - even the radiant heat presents a real danger (although relatively easily dealt with), without considering the possibility of actually touching the damned thing by accident. Or falling off your jetbike and having it fall on you. And don't dare go near one (even a "smaller" one) without ear protection.

A guy called Mark built a bunch of Lockwoods from Eric's plans, amongst others, and has a 20lb lockwood-propelled mountain bike. there's a photo of it on this page, which is worth visiting anyway : http://www.beck-technologies.com/marksengines.html and I've linked it inline below. Note the heat shield to prevent it burning the tyre off the wheel.



And here's a 50lb or so Lockwood http://www.youtube.com/watch?v=hxfX3nbyffo

Okay, so Lockwoods are valveless, but you get an idea of the size of engine you're talking about.

[rocketbat]

thanks for the continuing help!
i suppose that because Eric's calculator gives information on the frequency i could get arround the pulsting thrust problems with some strong damping, the strength required coming from treating it as SHM?
i have just been doing some calculations for drag (i have ignored weel friction for now) but do not know the referance area from the frontal view of a bike, pulsejet and rider so i subbed in 65lbs of thrust gives a speed of 75mph from Robert Maddox's efforts, this gave me a referance area of 0.48m^2 and so for a speed of 50 mph, approximately 28lbs is required, does this sound sensible or will i need to incluse rolling resistance? (by the way this is for air at 15 degrees C , mass =1.25kg/m^3)

the equation im using is F=(1/2)P(v^2)AC and im taking f as the resultant force so if i equal it with thrust in the opposite direction (F=0) then v^2 will be my terminal horizontal velocity (on a frictionless surface)

does any of this sound sensible or am i making an idiot of myself?

[tufty]

i suppose that because Eric's calculator gives information on the frequency i could get arround the pulsting thrust problems with some strong damping, the strength required coming from treating it as SHM?

If I read you right, you're worrying about the pulsation affecting the thrust you'll see? If so, don't.

i have just been doing some calculations for drag (i have ignored weel friction for now) but do not know the referance area from the frontal view of a bike, pulsejet and rider so i subbed in 65lbs of thrust gives a speed of 75mph from Robert Maddox's efforts, this gave me a referance area of 0.48m^2

Sounds about right for a normal bike & rider in an "upright" position, yeah. Frontal area will vary depending on bicycle and rider position, though - how low can you go?

the equation im using is F=(1/2)P(v^2)AC and im taking f as the resultant force so if i equal it with thrust in the opposite direction (F=0) then v^2 will be my terminal horizontal velocity (on a frictionless surface)

That looks correct, yes, although it's been a while since I've done drag calcs. Make sure you're doing all your calculations in the right units, of course.

If you graph Rayleigh's equation above, you'll notice that it's driven by the v^2 element; assuming you're going for something more than about 1m/s you can quite happily ignore rolling resistance unless you have a bike with flat tyres and all its bearings welded solid. You may need to give a little push with the pedals to get going, but that's a small price to pay.

Personally, I'd tend to optimise the thrust for the following:

1 - How safe I'd feel on a top-heavy bicycle carrying a quantity of fuel in proximity to a piece of red-hot metal at a given speed[1]
2 - How much fuel and weight I'd want to be carrying on a bicycle (less power == less fuel and less weight)

I would suggest that 10mph, maybe 20 at a push, on the flat, would be more than adequate[2]

Aiming for 20mph reduces your thrust requirements massively, which is going to make your life an awful lot easier. Less fuel, less worry about mounting brackets being able to handle the power and vibration, less worry about your engine collapsing (a big worry with bigger engines), marginally less hot metal hanging off your bike, etc.

Simon

[1] And this, bearing in mind that I'm happy riding motorcycles at *very* high speed.
[2] 20mph is about as fast as most people ride bikes over a decent distance anyway. Sure, people might expect you to make a jet-powered bike that goes really frickin' fast, but making a jet powered bike is enough of a claim to fame, even if it's barely capable of propelling itself. After all, do *they* have a jet powered bicycle? Nope, didn't think so.

[rocketbat]

thanks for the help,
dont worry about units, everything is Newtons, Kilos and Meters

i have allready calculated my rolling resistance and have churned out that a 14Kg (30lbs) engine would be ideal for my particular bike, i was hoping a little more than 20mph as i would be taking many safety precautions, long flat straight quiet roads, full face helmet, some leather and some decent motorbike gloves!

i have also designed a heat sheild that will cool the engine significantly at speed (and slower speeds too as air will be drawn inside the gap between sheild and engine quite rapidly) and if i have time during manufacture, i will be aspirating cold water into the valve grid to vaporise inside the engine (is this a silly idea?)

should i still be worried? i am just over a week away from manufacture so a large design change will be quite a setback.

[tufty]

Well, a couple of things come to mind.

First off, a shroud is not necessarily a good idea, as it may (depending on implementation) interfere with the acoustics of the engine.

Secondly, if you are using the bike on the public road, you will almost certainly be breaking the law. Not that I have any problem with that personally, but as you said it was a school project...

Water into the valve grid is not a silly idea at all, it may well increase valve life, and has the potential of increasing power significantly. Eric Beck is our resident water injection expert, and can probably give you an idea as to how much boost you might expect.

If you find a "vanilla" engine is underperforming, there's always augmentation to consider.

[Eric]

What fuel are you planning on running it on, if you run it on pure methanol sprayed in the intake, it will give you extremely cold dense fuel air charges without any need to cool the valve system.

If you then add water injection in the chamber you could then get even more of a boost, but water injection can be tricky and in order to get significant boosts you need to do a lot of trial and error. If not, theres the very good chance that you will make things much worse.

With this being a school project I'd try to keep it simple, you are going to run into all kinds of problems you didnt expect, and the simpler the engine, the more likely you will be able to finish everything on time and have it working well.

[rocketbat]

ahh, can finaly get back on the internet.
i would not concider water injection untill my work was finished marked and given back, as a collage projet it will be kept simple . . . then brutaly modified and re produced later!

something that i have been pondering is how to hold the valve head in the talpipe/cc, it is a little cast alluminium argus style valve, i was wondering if anybody has any preferance, i was thinking simply about 8 bolts radialy going through holes in the steel and snugly fitting into holes in the valve but not with threads in the alluminium but the steel has the nuts spot welded to the outside to reduce wear on the alluminium from repetative removal and replacement of reeds,
the aluminium would have to be tight fitting though to not shake itself to bits although it will be done on a CNC router initialy so i need not worry about the fit.

one last main question before i begin manufacture, what kind of valve overlap am i to aim for?
the current design is each plate has 8 rectangular holes covered by four reeds (*4 to create 32 holes) i currently have 2mm around edges and 2mm running down the center, i could easily alter any of these figures.

[tufty]

the aluminium would have to be tight fitting though to not shake itself to bits although it will be done on a CNC router initialy so i need not worry about the fit.

Consider the coefficients of expansion of aluminium vs steel (or stainless) - once it heats up even a loose interference fit is going to be tighter than a tight thing. The same doesn't go for the screws, though, they will heat slower and less than the surrounding metal, will be loose, and they are gonna vibrate like a bastard - they'll happily "mill" the holes in your head. I'd probably go for a threaded insert in the CC / tailpipe, and a threaded section on the valvehead, myself, simply screw the head onto the tailpipe. That's how the tigerjet and atomjet work, at least.

one last main question before i begin manufacture, what kind of valve overlap am i to aim for?
the current design is each plate has 8 rectangular holes covered by four reeds (*4 to create 32 holes) i currently have 2mm around edges and 2mm running down the center, i could easily alter any of these figures.

Depends on the size and layout of your holes, I would say, but as little overlap as you can get away with whilst being able to register the valves properly - less size == less mass == faster / better valve action. Looking at the atomjet plans, the holes are 7.5mm dia, and the petals 9mm - that's .75mm each side.

Simon

[rocketbat]

thanks,
il probably use 1 mm all round the reeds in that case, heres a slide of my work that shows some 3d modeling of my valve head if it helps.