How pulsejet engines work (and a shameless plug for my book)

[CBMXX]

thank you

[MegaPelon2]

me and my freinds are wondering can a capped small jet pull engine will work..because we want to know if something small will work

[xjet]

@MegaPelon2 Small engines are much harder to get running than bigger engines. I don't know what kind of capped engine you're thinking of so I can't really comment further.

[pyasahaina]

well that was very helping but how are we going to inject the fuel inside?do we need some kind of pressure to inject in the fuel? how will the rest of the assembly will attached to it i am curious please help me............

[xjet]

@pyasahaina You can either inject fuel directly into the chamber or you can use an atomizing jet in the intake venturi that will draw fuel by suction.

[sbeer6er]

how do the pulse jets work with no moving parts?

[xjet]

@sbeer6er I'll do another video explaining how valveless pulsejets (with no moving parts) work.

[zvone151]

Hello, I need help. How gas,... com in, can you do a tutorial how to make this jet?

[xjet]

@zvone151 I will be publishing a full tutorial at some time in the fugure but there are plenty of helpful tips in my e-book.

[mercjoe72]

Great explanation. But how do valveless pulsejets work ? how do those possible work with the comustion chamber totally opened ?

[xjet]

@mercjoe72 I'll be doing another video explaining the operation of valveless engines soon.

[mercjoe72]

Great explanation. But how do valveless pulsejets work ? how do those possible work with the comustion chamber totally opened ?

[Wilzaza]

Hey its Willzilla from parkflyers, I have a whole year where I can make anything reasonable in metal work, along with free metal (and woodwork too). I have an old weedeater with a working engine and was going to make a Go-ped, but how fun would it be to make a awesome looking pulse jet scooter or cart?

Just wondering if it would cost heaps to run and not be worth it. Also just making a pulse jet engine would be bloody awesome. I also want to make something that makes hydrogen...

[treborly]

Why does it get so much hotter and make a lot more noise then your standard internal combustion engine?

[xjet]

It gets hotter because most of the fuel is actually turned into heat and noise rather than useful work.

The efficiency of a pulsejet engine (as a form of thrust) is very low but its efficiency as a conversion of fuel to heat is very high. That's why pulsejet burners are sometimes used in furnaces -- they're more efficient than regular (non-pulsating) burners.

[deepergroovation]

I understand the theory, how it keeps on running, etc. Although I cant get my head around the fuel source. Sure, it sucks some gas back in which hasn't escaped through the exhaust...But that can only burn for so long. Is there an external fuel source which gets fed in? Does it get fed through the valves? How do you make the mixture right?

Great video though. Will be purchasing this book when I've got some spare cash one payday :)

[xjet]

The fuel is introduced either at the front of the engine before the air passes through the valves (using a crude carburetor arrangement) or by direct injection into the combustion chamber.

[oldhoon01]

are you still making the pj15v for sale or any others for that matter i am highly interested in buying one. and how do contact you?

[justinatlanta]

finally somebody made me understand that

lol

[GTFORDMAN]

very well explained but you need to wear a mic for your next video (very little sound)

[grrumpff]

im a bit confused... it feels like there should be a spark plug or something similar...? :S

[xjet]

There is a spark plug -- but just to get the engine started. Once it's running the engine no longer needs the spark

[grrumpff]

thats awesome :)

[moomoomoomuu]

how the valve work????

[mac163]

In the engine that you are showing in the video what would be the total endurance/ comsumtion rate in minutes in say a 10 kilo propane tank? and how much thrust does it produce? thank you.

[xjet]

The engine shown will use about 220g (about 8oz) of fuel per minute.

[eumegaf]

Subscribed!

[Kinirosoma]

Ill make sure to check out your books and your other videos! Nice work!

[StonyRC]

Dude, a REALLY good explanation - forget that this is a "plug" or informercial. You're the first person that's ever really been able to explain to me how the damn thing works. Gonna have to stump up the cash for the e-book now. Excellent post.

[AntoBoesi]

great! even a 5 year old kid would understand you!

[heytisazn]

So when the flaps open, does it suck in both air AND fuel?

And if it does, where is the fuel before going through the flaps?

[xjet]

In most model-sized engines the fuel is mixed with air ahead of the valves but in larger ones it's more common to inject the fuel directly into the combustion chamber after the valves.

[electrician725]

Neat Stuff! At what speeds would the incoming air negate the need for the valves? Supersonic?

I suppose that is where a pulse jet overlaps with a Ramjet...

[kaplis117]

god video thanks

[stealhty1]

i have LONG EZ 46 RC could you tell me how far the location of the CG is since i believe mine is to far forward and take long to take off and dificul to land ,which of course meka me crash 3 times and counting

[LegendRepsaj]

Great video, made it a lot clearer. Me and my friend are planning to also build a pulsejet powered plane, but I still have a few questions.

1. Did u weld the pipe yourself?

2. Is there a constant input of fuel, or is the fuel injected several times per second?

And, thanks again, you've convinced me of using a valved linear jet instead of valveless, u shaped one.

[xjet]

Yes, I weld up my own engines.

The fuel is injected constantly but because the pressure in the combustion chamber goes up and down with the burning of the fuel, the fuel flow tends to vary.

[JonnyTommyGuns]

It's amazing how technical yet simple it is.

[SPOOKSTR]

Great video mate.

[matthewmarrs123]

how would you make one of these out of scrap?

[xjet]

The answer to that would take far more space than is available here. I suggest you watch this video: v=YPrZ5J9nk-4 and you'll see how I did it on the TV series Scrapheap Challenge.

[matthewmarrs123]

oh ok thanks but i was just wondering because it seems that you can just take sheet metal and use a roller to make an exhaust outlet and then take another sheet metal and form a cone shape at the top it seems that easy?

[xjet]

I'm currently building an engine for a TV show and I've been videoing the process which I'll post to another of my YouTube channels in the near future. That'll show exactly what processes are involved and how it's done.

[Razorfish]

Pulse jets work better at slow speeds and will eventually stop working at some high speed. I generally understand this but more detail would be nice. Is this covered in your book? Not that I could make anything fast enough for that but it's interesting.

[xjet]

I haven't covered that aspect specifically but you are correct. Once they reach a sufficiently high speed the pressure of air coming in the front tends to hold the valves open and that affects operation. This can be overcome by placing the engine in a shroud with a diffuser at the front to slow the air down before it enters the engine.

[taofledermaus]

That's a really good explanation on how these work. I decided to make a rudimentary pulse jet out of a plastic 5 gallon carboy. I'm sure there are many design flaws with it, but I filmed it in slow motion hoping to show the workings in detail. You might get a kick out of it!

[xjet]

Yeah, I watched your video -- nice hi-frame-rate footage -- shame you can't capture combustion over several cycles.

[Goodman995]

were you on scrapheap challenge by any chance making a pulse jet? xD

[xjet]

Yep, that was me. "Expert" for The Destroyers

[Goodman995]

i saw! and you won! well done xD

[ComicalFerret]

i remember that one!

[mig3535]

man you are a smart guy, i need a grandpa like you

[hornyarab11]

lol ya no kidding

[0MoTheG]

could one put a muffler on one of these?

they seem to make more noise than anything else.

[xjet]

You could put a muffler on but, to be effective, it would have to be *huge* and that would make the whole setup rather impractical.

[0MoTheG]

why is there no nozzle cone on the end?

did you ever try to inject water at the end of the pipe?

[xjet]

A nozzle would upset the pumping action of the gases in the tailpipe and reduce performance.

[Arishtat]

Thanks, a great intro. I've seen a couple of videos mentioning a valveless pulse jet. How does that work?

[xjet]

Valveless engines rely on the inertia of air in the intake tube to provide a degree of containment which allows the combustion process to build up the pressure needed to create thrust. Because of this, they're usually not as powerful for a given size, especially when you're talking model-sized engines. However, the valvless engines on my dragster produced about 160lbs thrust each (video on my YT channel).

[Carliewilson88]

Good Video, so its kind of like a continual spud gun!!!

[xjet]

Yes, in a way you're right.

[Arvak777]

wow really great and informative video. nicely referenced too

[rohitlijhara]

great work sir which fuel should i use .....will petrol will be good enough

[xjet]

Yes, petrol is a good fuel. It is cheap, convenient, easy to find, ignites easily and will provide good power.

[SanchezAfricano]

I'm using isopropyl alcohol as my fuel, but it's not working too well. I suspect this is because the fuel is liquid, not gas. Would vaporizing the fuel make the engine more efficient?

[xjet]

Converting the liquid fuel to a vapor is essential. This is usually done by using an atomizing nozzle to break the flow of liquid fuel up into many tiny little droplets. Because these droplets have a much larger surface area for a given volume of fuel, the fuel evaporates into vapor far more quickly and thus you get an explosive air/fuel mixture being created.

[ratgreen]

but wont the heat of the combustion chamber vaporise the fuel anyway?

[xjet]

Not quickly enough to get good combustion.

[Elios0000]

whats the purity of the alcohol your using

rubbing alcohol has fillers that don't burn well

i use 99% for cleaning electronics but is a pain to track down

[xjet]

You could use either ethanol or methanol or a blend (denatured alcohol). I've used methanol before because it's fairly easy to find and affordable.

[SanchezAfricano]

You said anything combustible in air would work as a fuel. I've been trying to make an engine that works on isopropyl alcohol (it's all I have availabe) but it doesn't combust--it just burns. Does the fuel have to be vaporized in order to combust?

[xjet]

Yes, liquid fuel itself doesn't burn -- only the vapor it generates when evaporation occurs is combustible. What you need is a stoichiometric ratio of fuel-vapor and air -- then you will get vigorous combustion. If you only get burning then you are not getting a good mixture of fuel vapor and air.

[COD5252]

good work sir,im also in the path of building a centrifugal turbojet without using turbocharger but its impellor as the aircraft engines do,i want ur advice since u r experienced only tell me its w0rth money and time or not

[RitzBlitz33]

Some great info. Thanks xjet. I'm considering building one of these for a landspeed-style rc.

[edd542]

Can this engine work in space?

[xjet]

No. Like all jet engines, it is "airbreathing" so will only operate within the confines of the earth's atmosphere.

[avroarrow135]

but what if you bring your own air like how NASA does with their rockets

[elliotmotoX]

How many RPM would it go?

The flaming gas going out then igniting the next mix.. How many times per minuter?

[xjet]

The cycle-rate of the engine is dependent on a number of factors but mainly the length of the tailpipe. Small engines cycle faster than big engines. A typical model-sized engine will cycle at between 200 and 350 cycles per second but large engines (such as the ones on my dragster) are much lower -- 60cps or so).

[tangnatalaga]

What I would like to know is if the valveless pulse jet that works by kadenacy effect be used on model jet.

[xjet]

Valveless engines in model-sizes are not particularly powerful and that makes them far less practical than valved engines. The other problem with small valveless engines is the difficulty associated with getting them to run on convenient (liquid) fuels. Virtually all the ones you see on YT are using propane gas. which is a bit impractical for most models.

[windsordrywallpro]

Absolutely brilliant!! I wish i would have had someone like you for a teacher or professor, you can really captivate someone into what you are saying.  Great bit of information

[alphaspida]

So its basically like a piston on an engine?

[xjet]

The movement of gas in the engine acts like a piston, yes.

[azzahighkite1]

he sounds exactly like Huey from heuys cooking

[smad333]

Is that you Bruce? You seem to have been missing for ages!

[xjet]

Not missing.. just really busy ;-)

[panamic855]

i wish i had him for a science teacher

[Sockmonkey666]

What ever happened with your x-jet design anyhow?

[xjet]

The XJet is alive and well, I am currently building new prototypes for testing and customer evaluation. I'll be posting new video and information in a few months.

[adrianrubi]

can you vary the thrust of a pulsejet while in flight?

[xjet]

Yes but there are a number of problems that make it far more difficult to do in practice than in theory.

[adrianrubi]

ok. so this should control the fuel metering in conjunction with metering the air into the combustion chamber then.

[xjet]

Maintaining an adequate air/fuel mixture under the effects of different airspeeds is problematic. You not only have the problem of varying static-head at the front of the engine but you also have the problem of the increased vacuum at the tailpipe as velocity increases. Both of these tend to lean-out the air/fuel mixture to the point where the engine stops. You can attempt to restrict the air entering the engine but this creates new problems.

[adrianrubi]

computer modelling of fluid flow is needed in the study of this combustion process then. it seems that a variable geometry engine is needed, one that adapts it's shape according to internal pressures.

[SarinoaIsMe]

wow really cool how do you do it while looking at the camera the whole time and ur voice is cool

[riflelord2]

what type was the v-1 engine?

[xjet]

The V1 was a valved pulsejet engine which works on exactly the principles I've described in this video.

[ShriveledTadpole]

your voice is cool

[xjet]

The dimensions are important which is why I have created a pulsejet calculator on my website at aardvark(dot)co(dot)nz(slash)p­jet

[Thefirstspectra]

Thank you very much for the infomertial, I will buy your book. You have given a good explination of how pulse jets work I mainly like the no-weld tube in tube design very cool. Good work

[centraflow]

The pulse jet was invented by the Germans in the secound world war and they used kerosine and they mixed it with a scound tank of water which reduced their fuel consumption by using super heated steam. I have a copy of these original plans which were with some old papers of my father when I was going through them after he died. I am 58 and a scientist and water is a fuel or catalist when it is broken down depending on how you want to use it.

[Zachary1080p]

hi santa!

[Syd1940]

Unfortunately, in the UK, pulse jets are also the fastest way to lose flying sites (because of the noise!) and so most clubs ban them. The only place I've seen them is on speed control line models at the BMFA National Champs in the middle of RAF Barkston Heath. Lots of fun but very, very noisy.

I guess they're OK where there's lots of space far away from habitation. Very interesting, never the less. Thanks

[g0fvt]

Love your website, a friend built a small Brauner pulsejet a long time ago. Amazing things.

[horizonflyer9]

ive always wondered how these things work. i never thought these things were so simple! thanks!

[stratosjoebar]

it has similar theory with a 2 stroke engine

[DarthMart]

aren't valeless pulse engines more reliable and easier to make.?

[xjet]

Yes, but in smaller sizes like these, valveless engines don't produce nearly as much power. I'll do another video demonstrating and explaining the theory behind valveless engines.

[DarthMart]

yeah that'll be great, thanks

[1metiz]

I don't know if I agree with that. New developements - expanding tails/ divergants and such have realy pumped the power of those small valveless engines. Like the Raptor engine. Please correct me if I'm wrong here Bruno.

[xjet]

I have yet to see a pulsejet (including the Raptor valveless models) that offers the same power-weight/volume ratio as something like the valved P90 used by Team Helmond. 85 Newtons (20 lbs from an engine that's just 930mm (36") long and 90mm (3.5") in diameter. You might also want to compare the mean and peak exhaust gas velocities between a valveless (with divergent cone) and a valved engine. In smalls sizes, valved engines are far more practical/effective for fast model aircraft.

[1metiz]

I stand corrected. Although if you fix the fueling problem (i.e liquid), valveless pulse-jets are just as usable on small aircraft as valved engines albeit less powerfull. At high speed, there are also (or so I'm told) some interesting aero/thermo(?)dynamic advantages to be gained with valveless engines.

[xjet]

It's not surprising that there haven't been any valveless powered RC models, it's very hard to match the simplicity and performance of a regular valved engine. No fuel pump, no highly pressurized propane and a much easier form-factor to fit to most airframe designs. Valveless engines have their place but it's just not on relatively small model aircraft.

[1metiz]

No fuel pump? how does a valved engine get it's fuel then - surely it can't just suck up the fuel at such a high rate. One turn and you're dry. Also if the fueling is sorted, so a fuel pump a a atomizing injector, asuming some-one else did this developement for you (so you can copy it) I'd say a valveless is actually a better idea in some cases. they will run forever and if one happens to smash into the ground, you don't need to replace any complex/ expensive/ hard to find valves.

[xjet]

The valved engine in my Long-EZ has no fuel pump. It uses combustion pressure to push the fuel to the atomizer. After much experimentation with non-return valves and demand-regulators I have found that simple is best and removed all such accessories. The fuel system is now 100% reliable and unaffected by G-forces or the change in static-head caused by varying air-speed. Valves aren't that expensive (US$3-4 each) or hard to find (you can even make your own in a few minutes).

[MonkeyWithAWrench]

on the valves, how much overlap is acceptable? i have built 2 *running* valvless pulsjets (i just emailed you a few days ago on tuning the second,i got lucky, it was tuned properly with the untrimmed intake and exaust) and want to build a large valved pulsejet to run at the salt flats. but i need to figure out how to build the valves cheeply with little to no tooling. i was thinking of half inch wide .004" shim stock cut in 3" lenghts bent in the middle with a small cross bar to hold it in place

[xjet]

I've written a lot about valve-design on my other website. Google for: aardvark factors affecting valve life. That will pull up a page that answers a lot of what you need to know. Plain steel shim isn't a good valve-material, it's too soft and weak.  It'll deform almost immediately. You need spring steel and you won't easily be able to bend that (without it breaking).

[111alien111]

helo im intersted in you site...how evah id like 2 know how much speed in mph these things can give out . thankyou muchly

[xjet]

Pulsejet engines have the potential to propel aircraft or vehicles up to speeds of around 400mph. The best examples of this are the German V1 "buzz-bomb" that flew at around 385mph and some models aircraft that have been measured at over 400kph.

[locoloic]

how about for larger engines? For examble the one you did with the lazy-boy - would it be better to used a valved engine? granted it would be harder to make, but sureley there'd be significantly more power?

[xjet]

In larger sizes, valveless engines are a match for valved ones in terms of power and they are significantly more reliable due to the lack of moving parts. Once you go down in size however, valveless engines produce significantly less power and are harder to make run on convenient (liquid) fuels than valved engines. That's why most pulsejet-powered model planes use valved engines and why I used valveless on my dragster and the Lazy-Boy jet-powered recliner.

[locoloic]

Ok, thanks. One more question: if my bottleneck is fuelling - 1 tank of propane, inverted and unregulated - what's the optimal size for my engine? I made a LH with a 10cm dia meter U-pipe which could run at full throttle with the fuel turned up. Would it be best to make my new engine the same size, or make it larger? Same sized engine at 100% throttle, or 50% bigger engine, running at maybe 70% throttle (limited by fuel input)? thanks again.

[xjet]

I found that a Lockwood with a 100mm U-bend will almost take everything an inverted 9Kg propane cylinder can deliver so it is a good size if that's your fuel source. This is why I used two 100mm Lockwoods on my dragster rather than a single 150mm unit. There are other problems with valveless engines as you start o increase the size -- it becomes difficult to keep them from imploding without using very heavy and hard to work materials.