Second revision (May, 2004)
Copyright Notice:
This book is copyright 2004 to Bruce Simpson and all commercial rights are reserved. Anyone discovering a violation of these terms is requested to contact the author through the webpage at http://aardvark.co.nz/contact/
Foreword
Modern jet engines, like the ones found on large passenger aircraft or military fighters are incredibly complex and expensive to make.
Built from thousands of individual parts, many of which are made from exotic alloys like titanium and Inconel, these engines are a masterpiece of modern engineering.
But what if I was to tell you that there is at least one type of jet engine that has been around for almost 100 years, can be built out of plain old steel using simple tools, and in some cases has no moving parts at all?
Well it’s true and I am, of course, talking about the pulsejet.
In this book I’ll do my best to explain how these engines work, how to build them, how to improve on the basic designs and how they can be used to power all manner of vehicles from model airplanes to gokarts.
In an attempt to make the information contained in this book accessible to the widest range of people, I’ve taken a few liberties in explaining some of the more complex concepts. I’m sure there will be physicists, engineers and mathematicians who will throw up their arms in disgust when they look at how I’ve presented some of this information.
My justification for this is that the majority of readers are probably not equipped nor interested in wading through pages of complex mathematical formulas in order to understand some aspect of a pulsejet’s operation. In such cases, I believe, it’s better to replace all this complexity with a simple analogy or basic calculation that hopefully anyone can follow. Another are of contention is the very explanation of how a pulsejet works.
Although there is some consensus on the basic mechanisms behind the operation of a pulsejet, much of the detail is a topic of hot debate and disagreement. Wherever possible I’ve tried to present all sides to an argument but obviously I favor my own opinions which are based on years of empirical observation and experimentation.
A Note About The Author
Who is Bruce Simpson?
Well I’m a middle-aged guy who has always had a strong interest in technology and things that go bang. From an early age you’d find me out in the garage playing with my chemistry set, building all manner of weird and wonderful devices from old discarded radios, or just reading books about science.
Since the age of about seven, I’ve also been an avid builder of model airplanes, mostly of my own design. Over the years I’ve created all manner of odd-ball flying creations including flying wings, flying saucers, flying lawnmowers, flying carpets, and many others. It was only natural therefore that eventually my
fascination with things that go bang, chemistry, physics and aerodynamics would collide and produce a strong interest in jet engine technology.
It was also inevitable that, rather than focus on currently fashionable small gas turbine technology, I’d instead concentrate my efforts on the almost forgotten pulsejet.
Over the past couple of years I’ve built dozens of different pulsejet designs, mainly to test my own ideas. As a result of this experimentation I’ve developed a reputation for being at the leading edge of this almost forgotten technology and have come up with a number of innovations such as the blast ring and a novel fuel-injection system that significantly extends the valve life of small pulsejet engines.
For about a year I was actively involved in the commercial manufacture of several of my pulsejet designs but unfortunately I rapidly found myself extremely embarrassed at being unable to keep up with the unexpected demand. As a result, I have sold the manufacturing rights for these engines and am again focused on pure research and development in this area, working on several projects including some commissioned by clients in the aerospace and defense industries.
I am also performing some design work on a new generation of ultra-low-cost high-speed pulsejet-powered UAVs designed for reconnaissance and other applications.
Contents:
How do pulsejets work? 5
The world’s simplest pulsejet 8
Pulsejets for models 10
How to design a pulsejet 12
Comparing intake valving systems 20
Making reed valves last 23
Fuel systems 29
Constructional techniques 37
Powering things with pulsejets 43
Schmidt’s contributions 46
Ignition systems 48
How to start a pulsejet 51
Valveless pulsejets 54
The design of valveless pulsejets 59
Improving pulsejet performance 66
Accidents and failures 69
A simple guide to anodizing 76
Making reed valves with electrochemical etching 82
Newton’s third law 88
The Reynolds effect 92
The Bernoulli effect 91
The Coanda effect 93
Plans 95
Wacky ideas 97
Afterburning augmentors 99
A Little History and a Few Important People 103
How Do Pulsejets Work?
The honest answer to this question is that nobody’s really 100 percent sure of all the mechanisms that drive a typical pulsejet engine.
While most of the basic principles of operation are understood fairly well, there are many small details that are still the subject of debate amongst engineers and experimenters to this very day. It is safe to say however, that the primary effect behind the function of a pulsejet is the fact that gases are compressible and tend to act like a spring.
This “springiness” is crucial to the way a pulsejet draws in a fresh mixture of air and fuel then expels the hot burning gasses that are generated when that fuel is ignited.
The Kadenacy Effect
The effect of this springiness has been labeled the Kadenacy effect and here’s how it works: Take a regular 12 inch rule and lay it over the edge of a table so that just two or three inches are held firmly against that table.
Pull the free end down an inch or so and release it. Did you see what happened?
The ruler, acting like a simple spring, quickly flicked back, away from your hand – but it didn’t stop once it became fully straightened – it continued to move and actually bent the other way very briefly.
This flexing back and forth probably continued for a second or so with the magnitude of each swing being slightly smaller than the previous one.
Now something very similar happens when we take a sealed container and fill it with a compressed gas such as air.
If we suddenly release that pressure by popping the cork, the compressed air will rush out but (and
here’s the surprising bit) – even once the pressure inside falls to match the pressure outside, the air will continue to flow out.
It’s pretty easy to see that this will cause the pressure inside the container to fall below the pressure outside – and then the gas will flow back inwards. This cycle of increasing and decreasing pressure will repeat a number of times, decreasing in magnitude each time.
For a practical demonstration, find an empty bottle that has an opening about the size of your finger or thumb.
Wet your finger or thumb and slide it into the neck of the bottle, allowing the air to escape as you do.
Now remove your finger quickly.
Hear that sound? That’s the air rapidly bouncing in and out – just like the springy ruler vibrated when you let the free end go.
See how easy this jet engine stuff is?
Now that we’ve seen how Kadeancy works it’s time to explain how it becomes the driving force behind a pulsejet engine.
The Pulsejet Engine Operating Cycle
Let’s assume that a mixture of finely atomized fuel and air has just been ignited inside our pulsejet.
The rapidly burning fuel generates gases such as carbon dioxide, carbon monoxide and water-vapor. These gases take up far more room than the air and fuel alone did so pressure build up inside the engine.
The heat generated by the combustion causes those gases to expand so the pressure is increased even more.
Our pulsejet has become a container filled with pressurized gases – just like the one described previously.
Those gases now rush out the opening at the end of the engine’s tailpipe and in doing so, they create thrust which pushes the engine (and whatever it’s attached to) in the other direction. It was Benjamin Newton who first described this effect when he said “for each and every action there is an equal and opposite reaction.”
A fraction of a second after the air/fuel is ignited and the hot gases have started flowing out the tailpipe, the pressure inside the engine has dropped to match that of the outside air.
However, thanks to the Kadenacy effect, the gases keep flowing down the tailpipe and a partial vacuum is created inside the engine.
At this point, two very important things start happening.
Firstly, the valves at the front of the engine open. They’re pushed open because the air outside the engine is at a higher pressure than that inside the engine. This pressure difference pushes on the valve and causes them to move aside, thus allowing fresh air and fuel to enter.
At the same time, those hot burning gases that were travelling down the tailpipe stop for an instant then start travelling back towards the front of the engine – driven by the air outside which is at a higher pressure than that inside.
You can see that at this point, we have fresh air and fuel coming in the front and still-burning gases coming back down the tailpipe.
Can you guess what happens when the two meet?
That’s right – the whole cycle starts all over again when the flames and hot gases from the tailpipe ignite the highly flammable mixture of air and fuel that has been sucked in through the valves in the front.
And of course, as soon as that fuel ignites, the pressures generated push the valves at the front of the engine closed, leaving the hot gases only one way to go – out the back.
When a pulsejet is running, this whole process is repeated many times per second and it is this repeated blasting of hot gases out the tailpipe that gives the engine its characteristic noisy bark. Wasn’t that simple?
The World’s Simplest Pulsejet
Here’s a chance to make your own ultra-simple pulsejet using nothing more than a hammer, a screwdriver, and a few readily available materials.
This simple design was first dreamt up by one of the grandfathers of pulsejet engines, a Dutchman by the name of Francois Henri Reynst back in the first part of last century. Although this pulsejet won’t hurt your ears or produce massive amounts of thrust, it’s still a good idea to include a few warnings at this point.
Safety
Pulsejets use explosive mixtures of air and fuel to create power. They also produce lots of burning hot gases when running.
For these reasons, you should never attempt to run a pulsejet (not even this very simple one) indoors or near anything that could catch fire.
Also be aware that because a pulsejet generates pressurized gases, there’s always a small risk that part of the engine could fly off and strike anyone standing nearby. This is particularly true if you’re using a glass jar in the following experiment. The glass can (and will eventually) crack and break due to the heat and pressures involved.
Here are some basic rules that will help keep you safe: 1. Always wear eye protection
2. Keep a safe distance from a pulsejet when it is running. 3. Keep a garden hose and/or bucket of water nearby at all times 4. Use hearing protection
Now on with the fun. Materials
In order to build our simple demonstration pulsejet you’ll need to find the following items: 1. A small jar with a screw-top metal lid (75mm or 3” diameter)
2. A screwdriver or nail.
3. Some methanol or model airplane fuel Here’s how we go about building our engine.
With the nail or screwdriver, make a hole in the center of the removable metal lid. You can then enlarge this hole to around a half-inch (13 millimeters) in diameter.
Now pour some methanol or model airplane fuel into the bottom of the jar to a depth of about a quarter inch (5 millimeters).
Replace the lid and swirl the liquid around in the bottom of the jar a few times.
Remember that you need to run this little engine well away from anything that could catch fire and one suggestion I have is to dig a small hole in the ground so that the jar can be inserted, leaving the lid slightly above ground-level. This will protect you and reduce the fire risk if the jar should crack.
Now bring a lighted match or flame near the hole in the jar’s lid – keeping your face and hands well away from the area directly above that hole – because a large flame may come shooting out with a whooshing noise that can give you a bit of a fright.
It is highly recommended that you wear eye-protection and at least a long-sleeved cotton shirt to protect yourself when performing this experiment.
If you’re lucky, your simple little “jam jar” engine should start puffing away – producing a series of little pulses of hot gas and perhaps a gentle purring noise.
IMPORTANT: do not let this simple jam-jar engine run for more than 5-6 seconds at a time or the glass will crack, possibly spilling burning fuel. You can stop it by covering the hole in the lid with a small piece of wood or even a suitably sized coin.
How Does It Work?
Now the more observant reader will have noticed that this pulsejet has no valves – so how does it work?
The answer is simple – when you ignited the air/fuel mixture that was originally in the jar it burnt, expelling that large jet of flame and making that whooshing noise.
Because the hot gases rush out of the jar with great speed, the pressure inside the jar quickly drops below normal atmospheric pressure and a weak vacuum is created – just as described in the previous chapter.
When this happens, a fresh gulp of air is sucked in through the hole in the lid and that air then mixes with highly flamable methanol vapor that is rising from the pool of fuel still sitting in the bottom of the jar. The jar once again contains a combustible mixture of air and fuel – but how does it ignite? After all, we had to use a match to ignite it the first time didn’t we?
Well, also inside the jar are some remnants of the hot gases generated from the last combustion cycle. Eventually the hot gases and the air/fuel mixture run into each other – whereupon ignition occurs, pushing its hot gases out the hole –starting the whole cycle all over again.
How’s that? We’re not even a quarter way through this book and you’ve already built your own pulsejet engine!
Pulsejets for Models
Forty or fifty years ago there were a number of manufacturers producing small pulsejet engines designed specifically for use on model aircraft.
Most of these engines are very similar in design and construction, consisting of a lightweight stainless-steel body and tailpipe, with a machined aluminum head and thin spring-steel valves. Quite a few of these engines were made in Eastern-bloc countries and at least one was made by the OS model engine company in Japan.
However, perhaps the most recognized model pulsejet of all time is the Dynajet. The Dynajet
Thousands of avid model airplane enthusiasts have owned, seen or lusted after this icon of the pulsejet era.
The Dynajet was so popular, and so many were sold, that it rapidly became the benchmark against which all other small pulsejets were compared.
Its simple design and lightweight construction made it perfect for use on model airplanes and well suited to the U-control speed models of the era.
Even today the Dynajet is a popular item on auction website such as eBay, often producing bids of several hundred dollars or more.
This picture is of an early model Dynajet that had the spark plug located right at the rear of the combustion
chamber and didn’t have an anodized head. In later models the sparkplug was moved forward and the aluminum head was anodized a rich red color which resulted in the engine being known as the Dynajet Red-head.
The body of these engines is made from two pressed stainless steel shells that are welded along an upright seam. This technique results in a nice smooth contour between the combustion chamber and the tailpipe, which probably helps the performance somewhat.
This picture shows the front of a later-model Dynajet with a more deeply finned valve-head section. You can clearly see the effects of the red anodizing on the aluminum and the more forward location of the sparkplug.
The company that used to make these engines, Dynafog, is still in business but now focuses on industrial fogging machines which
Another company, Bailey Machine Service, is still making a clone of the Dynajet engine which it sells for about US$250 although the supply is said to be somewhat erratic.
Many of the other pulsejet designs you’ll see from this era are very similar to the Dynajet in size, dimensions and performance, although there are still plenty of weird and wonderful variations on the basic theme.
The OS pulsejet
This engine was manufactured in the 1950s and 1960s by the Japanese company OS. There were apparently
two slightly different versions of this engine,
one being a little larger and more powerful than the other but they were both obviously very similar to the Dynajet in both form and function.
However, unlike the Dynajet which used a single machined piece of aluminum for its valve-head and venturi, the OS unit consists of two separate parts which screw together.
There was also a myriad of other pulsejet designs manufactured about the same time and sold under a raft of different names such as TigerJet, etc.
An even wider range of designs and ideas were published as plans for a generation of enthusiasts who, in a post-war era, were eager to build one of these magical “jet engines” for themselves.
As a result, many magazines such as Popular Science and Popular Mechanics were littered with advertisements for such plans.
It is unlikely that many of those who purchased these plans ever managed to construct a working engine and at least a few of the designs were so fatally flawed that the publisher was obviously just trying to cash in on a craze.
How to design a pulsejet
One of the more interesting and more readable texts on pulsejet design and theory was written by a C.E. Tharratt while he was a staff scientist at the Chrysler Space Division in the late 1950s.
Titled “The Propulsive Duct”, this paper condensed much of the known pulsejet theory into a few simple formulas and constants.
Using these calculations Tharratt claimed that “we are in the surprising position of being
able to determine the dimensions of a duct capable of developing a given thrust literally on the back of an envelope and without knowing anything about its gas dynamics!”
Sounds great doesn’t it?
Unfortunately, while Tharrat’s formulas have stood the test of time and his understanding of the mechanisms behind the pulsejet (or “propulsive duct” as he called it) remains valid, when it comes to designing a powerful, reliable pulsejet engine, the devil is in the detail.
However, here is the simple formula that Tharratt proposed to be the core of pulsejet design (note: Tharratt’s papers and constants are presented in the imperial measurement system so that’s what I’ve used in this chapter. I’ll update with metric versions in the next release of this book):
V/L = 0.00316F Where:
V = engine volume (cu ft)
L = effective acoustic length of the engine (ft) F = thrust (lb)
The validity of this formula has been verified against a wide number of different and proven pulsejet designs including the Argus V1 and Dynajet.
Let’s take a look at what this formula actually means in terms of the way that the dimensions of a pulsejet affect its power output.
We can see that if we kept the volume of the engine (V) constant but increased its effective acoustic length (L) then the power would reduce. Note that in order to do this, the diameter (and cross-sectional area) of the engine would need to be reduced – so it would appear that there is a definite relationship between cross-sectional area and power.
Now, if we keep the length (L) constant but increase the volume (V), the power would
increase. To accomplish this however, we’d have to increase the diameter (and cross-sectional area) of the engine. This confirms that relationship between cross-sectional area and power output.
If we manipulate that simple formula a little more, we come up with a new formula containing a very interesting constant:
F = 2.2A Where:
F = thrust (lbs)
A = mean cross-sectional area (square inches)
Let’s just make an important point here – this 2.2lbs/sq-in constant is derived from a formula that includes the engine’s total volume as a factor. This is why it’s not just the cross-sectional area of the tailpipe that is important (as many would have you believe), but the mean (average) cross-sectional area of the entire engine along its total length.
However, it should be added that Tharratt didn’t expect engines built to his formulas to have a huge bulbous combustion area at the front so don’t expect that such a feature will significantly increase an engine’s performance over a straight pipe.
So now we can plug in our required power output of 10lbs thrust and get this: 10=2.2A
which simplifies to: A=10/2.2
A = 4.545 square inches
From this we can calculate the mean diameter of our engine as follows: D = 2√(4.545/π)
D = 2.4 inches
Now we need to decide on a length for the engine. Remember that making the engine longer or shorter won’t actually increase or decrease its power – only a change to the cross-sectional area will do that.
However, the length does have a bearing on the frequency at which our engine will operate. The only reference I can find from Tharratt in respect to the suggested length of a pulsejet engine is that it be at least eight times the mean diameter.
This length to diameter ratio is usually expressed as L/D and Tharratt notes that “with
geometrical ratios of L/D < 7 the development problems become particularly challenging”
and that in an engine with an L/D < 7 “combustion with chemical fuels is difficult to
It makes sense therefore, to use an L/D somewhat greater than 7 and it’s been my experience that a figure of about 14 is a good place to start for relatively small engines like this.
By way of comparison, the Dynajet has an L/D of 15 and the Argus V1 has an L/D of 9.6. As a rule of thumb, the smaller the engine, the higher the LD needs to be in order to get reliable operation and good power levels.
So now we can calculate the length of our engine as follows: L = 14D
L = 14 x 2.4 L = 33.6 inches
Okay, so now we know that to create a 10lb-thrust pulsejet engine we’ll need a piece of pipe that is 33.6 inches long and 2.4 inches in diameter -- but wait, there’s more!
Another key formula presented by Tharratt was one for calculating the valve area for an engine of a given size and power:
Valve area = 0.23 x mean cross-sectional area Or
Valve area = 0.1045F sq in
It is interesting to note that this 0.23 (or 23 percent) figure contrasts sharply with that proposed by other pulsejet “experts” of the era who suggest a figure of 0.4-0.5 is better. Let’s use Tharratt’s formula and constant to work out the size of the valve area we need for our 10lb-thrust engine.
Using the first formula we get:
Valve area = 0.23 x mean cross-sectional area Valve area = 0.23 x 4.545
Valve area = 1.045 sq in Using the second formual we get:
Valve area = 0.1045F Valve area = 0.1045 x 10
Valve area = 1.045 square inches
Yep, we get the same answer both times so we now know that the effective valved area for our l0lbs-thrust engine is just over 1 square inch.
Now Tharratt’s formulas assume that the intake is an open hole, with no losses due to the presence of valves. Unfortunately, the presence of spring-steel reed valves will significantly impact the flow of gas so we need to take those losses into account when designing our intake valving system.
The precise efficiency of a valving system depends on many factors and I suggest you read the chapter on intake valving for more information – but in the case of our little design, let’s use a simple petal valve and assume that it is just 50 percent efficient.
To get the actual area of the valve required to achieve an “effective” area of 1.045 square inches at 50 percent efficiency we simply divide by 0.5 to get a figure of 2.090.
So let’s see how our pulsejet is looking like so far:
Thrust 10lbs
Length (from valves to end of tailpipe) 33.6 inches
Mean diameter 2.4 inches
Total valve area (assuming 50% efficiency) 2.090 square inches So what about those valves then?
A petal valve system consists of a ring of holes over which a spring steel valve, consisting of a matching number of petals, is laid.
If we were to use 10 holes, spaced at 36 degree intervals, each hole would need to have an area of:
2.090/10 = 0.209 square inches which requires a diameter of 0.516 inches.
Alternatively, if we used a ring of 12 holes spaced at 30 degree intervals, each hole would need to have an area of:
2.090/12 = 0.174 square inches which requires a diameter of 0.470 inches
It’s been my experience that a half-inch hole is the upper limit for petal valve holes. Once you go beyond this size the pressure on the valves themselves cause them to be bent so that they begin to dish into the hole and this adversely affects their operation. For this reason, we’ll go with the 12-hole option.
You’ve probably already noticed that most small pulsejets have a much larger diameter section at the front, from where they funnel down to a narrower tailpipe. Many people mistakenly assume that this bulbous front is a combustion chamber, designed to contain the burning air/fuel mixture. While it may be true that much of the air/fuel is burnt inside this bulbous front section, the reason small engines are shaped this way is actually quite different and we’ll see why when we do the next set of calculations.
We’ve decided to use a petal valve system consisting of 12 holes, each of 0.47 inches in
diameter. If we assume that the holes will be placed in a ring around the edge of the pipe, and that we allow a certain amount of space between the holes so that there’s room for the spring-steel valves to rest, then we have a problem.
Lets assume we need a ¼ inch gap between the holes – that means the total circumference of a circle drawn through the center of each hole will be:
NumOfHoles x diameter + NumOfGaps x GapSize And when we plug in our numbers we get:
12 x 0.47 + 11 x 0.25 = 8.39 inches That represents a circle with a diameter of:
8.39/π = 2.67 inches
What’s more, that figure is the diameter of a circle that runs through the center of each hole so we need to add two times the radius of the holes to get the size of a circle that will run around the outer edge of the ring of holes.
2.67 + 0.47 = 3.14 inches
Clearly, the absolute minimum diameter of our valve system (3.14 inches) is larger than the calculated diameter of our engine’s pipe (2.4 inches)
This disparity grows even further when we build in a bit of extra space so that the valves don’t scrape against the side of the engine when they swing open and closed. It’s been my
experience that you should allow a space between the outer edge of the valve holes and the side of the engine which represents an area equal to the total area of our valve holes. Or in other words, we need to leave 2.090 square inches of space around our ring of holes. That can be calculated as follows:
The area of a single circle covering our ring of valve holes would be: πR2 or 3.1415 x 1.57 x 1.57 = 7.74 square inches
Add 2 .090 square inches to get the size of the larger circle and we get 9.830 square inches From this we can calculate the diameter needed to provide that extra 2.090 square inches of space around the edge of the valve-ring:
Diameter = 2√(9.830/π) Diameter = 3.53 inches
Now that we know the diameter of our “combustion chamber” we need to calculate how long this section of the engine should be.
Since Tharratt didn’t use petal valves, he didn’t need this bulbous front section so has no advice (that I can find) for calculating this dimension.
However, we can look to the work performed by Schmidt (the guy who designed the Argus V1 engine) and my own empirical research that indicates the following:
During the intake phase of a pulsejet’s operation it will draw in a fresh charge of air equal to 15%-20% of the total engine volume.
I see no reason why we shouldn’t design the engine so that this front section is just large enough to hold this fresh charge of air/fuel. In that case we need to do some more calculations to determine its length:
If our engine were just a straight pipe of 33.6 x 2.4 inches then it would have a volume of 152.7 cubic inches. Such an engine would suck in 152.7 x 0.2 = 30.54 cubic inches of air during each cycle – so our front section needs to have a volume of 30.54 cubic inches. We’ve already calculated the area of this section as being 9.83 square inches we can calculate the required length as follows:
30.54 / 9.83 = 3.11 inches.
Of course more alert readers will notice that this 30.54 cubic inches no longer represents 20 percent of the engine’s total volume. This is because by making this front section wider without reducing the overall length of the engine, we’ve actually increased its total volume by an additional 16.4 cubic inches.
The total volume of our engine is now nearer 169 cubic inches so the 30.54 cubic inch front section only represents 18 percent of the total volume – but this difference is so small as to be unimportant.
The only thing remaining now is to join the front section of the engine to the tailpipe using a cone with a diameter of 3.53 inches at one end and 2.4 inches at the other.
How long (ie: what angle) should this cone be?
Well if we look at the Dynajet we can see that it’s hardly a cone at all – more of an abrupt transition. By comparison, the Argus V1 engine uses a very long, shallow angled cone to join the two sections. So how do we decide which is best?
Let’s look at the effects that the angle of this cone might have on an engine’s operation. Coming up with a suitable angle for this cone requires balancing a number of factors. To examine them, let’s look at extreme examples:
1. If we simply used a flat plate to join the two sections of the engine then the hot exhaust gases would have a rather torturous path to follow. Some of those gases would have to travel around two 90 degree bends to get from the combustion chamber to the tailpipe and that would potentially reduce the speed at which they were able to exit from the engine. Remember, the speed at which the gases leave the engine affects the thrust – we want those gases leaving as fast as possible. For this reason, a flat plate is obviously not such a good idea.
However, this configuration is not quite as bad as you might think, after all, the Dynajet has a very steeply angled cone that must constrict the flow of exhaust gases right? The very fact that it is so hard for the combustion gases to get into the tailpipe means that
immediately after ignition, pressure will build up inside the combustion chamber as all those gases try to rush around a tight bend and down the tailpipe.. Those higher pressures can improve combustion efficiency and actually increase the speed of the gases in the tailpipe. This is called post-ignition confinement (PIC).
It’s also worth noting that in the case of a flat plate, the hot gases that return from the engine's tailpipe and ignite the fresh air-fuel charge may do so far more efficiently. This is due to an interesting effect that occurs in the way they form a narrow jet that reaches deep into the chamber rather than a larger diffuse front that ignites the fuel more slowly.
Remember that the faster the fuel burns, the more power our pulsejet will develop because it will have less time to expand as it burns – thus producing the higher internal pressures that will, in turn, result in higher tailpipe gas velocities.
This diagram shows how ignition differs based on the angle of the cone between
combustion chamber and tailpipe. Note that in the second diagram, the distance between the hot gases and the engine body is far less than in the first – this is important.
The speed at which the combustion flame-front travels through the fresh air/fuel mixture is relatively slow (just a few tens of feet per second) in a low-compression engine like the pulsejet. Because of this, the mixture in the second diagram will be burnt far more quickly than that in the first, since the flame-front will be wider with a much shorter distance to travel.
2. If we used a very long cone that had a shallow taper all the way to the end of the engine (ie: no tailpipe as such) --then it would obviously be much easier for the combustion gases to
flow out under pressure. However, we’d also be significantly reducing the ability of the engine to create a vacuum after combustion is completed because a much smaller
percentage of the exhaust mass would be travelling at maximum velocity inside the engine. Remember that the “force” exerted by the escaping gases is equal to their mass times the velocity to which they’re accelerated (F=MA). For a given size of engine engine, the mass will always be the same but the velocity to which those gases are accelerated will depend very much on the design of the tailpipe. We need plenty of velocity to get the force required to establish a strong Kadenacy effect. In fact, tests conducted by the NACA during the 1950s indicated that an engine designed with just a long convergent cone instead of a straight tailpipe was very difficult to get running at all.
Once again it seems that a compromise is in order so we’ll chose an angle of 30 degrees for the section between the combustion chamber and the tailpipe. This will provide some post-combustion confinement to increase the internal operating pressures while ensuring that the engine still has good internal mass-flow speeds to provide maximum Kadenacy effect. A 30-degree cone will be a fairly short section – just 1.84 inches long.
So here are the final dimensions of our pulsejet engine, wasn’t that simple?
It should be noted that this is a very basic engine and there are still a few tricks we can use to improve its performance – but more of this later.
Comparing Intake Valving Systems
One of the most critical components of a traditional pulsejet engine is the intake valving system.
The demands placed on the intake valves are amazing.
They have to open and close several hundred times a second while being exposed to the thermal stresses associated with being alternately blasted by searing hot combustion gases and cold incoming air. At the same time, these thin strips of spring steel must resist metal fatigue and fracture resulting from the high mechanical stresses imposed.
What’s more, they have to do all this while providing a 100 percent seal against combustion gases when closed, and allowing the smooth, unimpeded flow of fresh air when open. To make life even harder, the only power available to open them is the tiny difference in pressure between the outside air and the small vacuum created inside the engine by the kadenacy effect of escaping exhaust gases down the tailpipe. (just a few psi).
It’s no wonder therefore, that no aspect of pulsejet design and construction has caused more sleepless nights, scratched heads and frustration than the valving.
Lets examine the alternatives: Petal Valves
Small engines almost always use a petal-valve. These valves offer the following benefits: 1. simplicity. The valve can be etched or cut from a single piece of spring-steel.
2. Low cost. As a side effect of their simplicity, petal valves can also be very economical to manufacture – especially when you consider that the valve plate consists of a simple piece of aluminum with a ring of holes drilled in it.
Unfortunately, the petal valve also has a number of disadvantages:
1. poor aerodynamic performance. Since the air passing through a petal valve must negotiate two near-90 degree bends on its way into the engine, the efficiency of such a system is not particularly high.
2. low durability. Because the tips of the petals are directly exposed to the hot combustion gases, petal valves often suffer from premature tip cracking or fracture.
3. High maintenance. Since petal valves are usually made as a single piece, the failure of individual petal requires the replacement of the entire spring-steel valve.
Despite their drawbacks, petal valves are generally the best option for small pulsejet engines, although I wouldn’t recommend them for any engine larger than about 20lbs of thrust. The V or multi-V valve
Generally only seen on larger engines, these valves are generally more efficient than petal valves because they produce less deflection of the airflow when they’re in an open position.
There are two basic methods of constructing such a valve system – one involves the use of two or more flat metal plates with holes in them, joined at an angle (45 degrees is a good starting point).
The other method of forming a V valve is the one used in the Argus V1 where a cast or machined spacer with
multiple ribs is used to hold the valves in position and limit their movement as in the diagram below:
V valves provide the following benefits:
1. Higher efficiency than a simple petal valve. Since the incoming air has a far straighter pathway into the engine, more air is able to flow for a given size of valve opening when compared to a petal-valve.
2. Lower maintenance costs. Since the individual spring steel valves in a V-valve system can be replaced as/when they fail, maintaining the engine becomes a less expensive task and all valves can be used to the full extent of their lifespan.
3. Scalability. Unlike the petal-valve, a V-valve can be easily scaled to create the required valve area by simply increasing the length or number of V-valves in the array.
Of course there are downsides too:
1. Greater complexity. A V-valve generally requires more machining steps and a higher component count than a petal-valve setup.
2. Increased expense. As a side effect of this complexity, the production cost for a V-valve system is significantly higher than for a petal-valve. This is another reason why most cheap model engines don’t use V-valving.
Less commonly used valving systems
Petal and V-valves are not the only systems that have been used on pulsejets but they are by far the most common.
Perhaps the only other practical valving system for a pulsejet is: The Rotary Valve
These generally consist of either a spinning disk containing a hole that controls the flow of gas by covering and uncovering a matching hole in the front of the engine, or a spinning butterfly-type valve that alternately blocks and allows the flow of gas.
Rotary valves can be made very robust and thus have the potential to create very reliable, long-lived pulsejets. Unfortunately however, they are fraught with hidden complexities, not the least of which includes the issue of timing.
In a conventional pulsejet valving system, the valve timing is automatically controlled by the changing pressure inside the engine. When the internal pressure goes up (because the fuel has ignited) then the valves close. When the pressure falls (due to the Kadenacy effect) then the valves open). This results in a very simple and quite reliable system that automatically compensates for any fluctuations in the engine’s operating frequency or phase.
Rotary valves on the other hand, have no such intrinsic timing control and therefore require a very sophisticated system to control their rotational speed and phase relationship to the engine’s basic operating cycle. This immediately negates the pulsejet’s two single most endearing qualities – simplicity and low cost.
Research done in the USA during the 1940s cited engines using the rotary valve as offering “very long useful operating periods” along with “good thrust and specific fuel
consumption” but also mentioned the complexity associated with driving such a valve in a
synchronous manner.
Never the less, rotary valves are being considered as a viable option for the new generation of pulse detonation engines (PDEs) currently under development. Since these PDEs already require a significant number of ancillary control systems anyway, the overhead of the rotary valve adds little to the cost or complexity of these engines.
Making Reed Valves Last Longer
The thin spring-steel valves normally used to control the flow of air into a pulsejet and stop the hot combustion gasses from escaping out the front are the most highly stressed part of a conventional pulsejet engine.
The life of the reed valves in most pulsejet designs is measured in minutes rather than hours and they must be considered a “consumable” part of any conventional pulsejet engine. It's not hard to understand why these fragile little pieces of metal don't last long. They're slammed back and forth between the intake and retainer plates with great force, several hundred times per second. What's more, they're usually exposed to extremely hot combustion gasses
There are three factors that contribute to reed-valve failure: • heat-damage
• impact damage • fatigue due to flexing
A well designed valve system attempts to minimize all these factors so as to provide maximum valve life but (woudn’t you know it) there are always compromises involved.
Let’s look at the simplest and easiest to solve issue first: Fatigue due to flexing
This picture shows the effect of valve failure due to metal fatigue brought about by the flexing motion of a petal valve. Note that one of the petals has completely broken near the root. Close inspection of this valve showed that stress cracks were starting to appear at the root of the other petals. In this case, failure was due to a poorly designed valve-retainer which had an uneven radius of curvature.
Obviously, reed valves must flex in order to operate properly. The key to avoiding premature failure however, is to limit this flexing to a bare minimum and try to keep the flex radius as large as possible.
To this end, the conventional petal-valve arrangement with a curved valve-retainer is quite good – providing the curvature of the retainer is of a constant radius.
If the valve retainer doesn’t have an even, large radius curve to it, most of the valve flexing will be concentrated over a small area near the root of the petal. In a fairly short space of time, the stresses caused by this flexing will cause the spring steel to crack and fracture.
This diagram shows the right and wrong way to design the valve retainer for a petal valve system. Note that when a small radius is used near the base of the valve retainer, most of the reed valve remains unbent so all the stress is concentrated at the root.
There are several ways to make a valve retainer that has a large, even radius but because it’s so much easier to make a straight-sided retainer, people often make the mistake of creating what amounts to a shallow cone instead– with predictably bad results.
Impact Damage
Most small pulsejets run at somewhere between 180 and 250 cycles per second. This means that the valves must open and close as often as 15,000 times per minute.
Each time the valves close, they slam into the valve-plate at quite high speed and therefore with significant force. All the energy that is contained in these fast-moving valves has to go somewhere – and some of it is absorbed by the valve material itself.
This constant hammering eventually causes minute cracks to form at the tips of the valves after which they begin to fray and small fragments will eventually flake off. If you’re running your pulsejet at night, these small fragments can be seen as impressive sparks flying out the tailpipe.
This picture shows a badly frayed petal valve that has certainly reached the end of its useful life. It is a very good idea to replace valves long before they get to this state because the sharp, ragged ends will soon damage the comparatively soft material of the aluminum valve-plate against which they impact.
There are a few techniques that can be used to reduce impact damage to reed valves. • Reduce the amount of valve travel. If the valves can open too far then they will reach a
much greater velocity when they’re closing and this will increase the forces applied to them as they impact the valve seat. Of course limiting the valve-opening will also tend to reduce an engine’s power as it means that less air can be drawn in during the intake phase. • Use a softer material for the valve-plate. Most small pulsejets already use an aluminum
alloy for the valve plate so there’s not a lot of room for improvement here. However, the NACA did conduct tests on engines which had a think coating of neoprene on the valve-plates. This was said to almost double valve-life by reducing the impact shock
Note that although it might seem like a good idea to simply increase the amount that a valve overlaps the hole it covers so that the air trapped between the valve and the plate acts as a cushion to soften the impact, this is actually a bad idea.
If the overlap area is too great, the air can’t get out of the way quickly enough and the tips of the valves are actually bent backwards by this trapped air. As a result, tip fraying is
dramatically increased because any cracks that form grow very quickly due to this additional stress.
Determining the ideal overlap area is something best done by trial and error. If the overlap is too great then you’ll get premature fraying and poor engine performance (due to late closing of the valves). If the overlap is too small then the valve plate will be damaged by the high pressure loadings and this will ultimately affect engine performance because the valves will no longer seal properly.
Heat Damage
Reed valves are usually made from hardened, tempered spring steel because it’s strong and will return to its original position after flexing.
The problem with spring steel is that the hotter it gets, the softer it gets. If it gets too hot then it will lose much of its strength and some of its springiness.
Unfortunately, the inside of a pulsejet engine is a very hot place and it is the pressure
generated by extremely hot (1,500 deg C) combustion gasses that actually cause the valves to be closed.
So why don’t the valves simply get red hot and go all floppy?
Well fortunately, the valves are only exposed to the hot combustion gasses for part of the operating cycle. During the intake cycle they’re cooled by the incoming charge of fresh air. In a petal valve setup, the valve retainer also provides a measure of protection from the heat of combustion by shielding most of the valve from direct exposure to the hot gases.
However, the tips of the reed valves will still get hot and, as a result, they will become softer than the rest of the valve. What’s more, the valve retainer will itself heat up once the engine is running and some of this heat will be transferred to the valves when they’re open.
A number of solutions have been proposed to the problem of valve heating but most of them will reduce an engine’s performance to some degree.
One of the simplest solutions is to place a flame-trap in the engine directly behind the valves. This flame trap consists of little more than a mesh of stainless steel or some similar heat-resistant metal.
It’s well known that a metal mesh with suitably sized holes will not allow a flame to pass through it – but it will allow air and other gases to do so. This was the principle behind the
old miner’s safety lamps. Before the days of battery-operated flashlights, miners still needed some form of illumination while working underground. A naked flame such as that from a candle or oil lamp would pose a very real danger as it risked igniting underground pockets of explosive gases such as methane.
By enclosing the flame in a metal mesh, the flame could not extend beyond that mesh so the “safety lamp” could provide light and still be used without risk of sparking an explosion. Now, the problem with a using a flame-trap mesh in a pulsejet is that in order to be effective, the size of the holes in the mesh must be quite small. As a result, the mesh represents a significant obstacle to the flow of the incoming fresh air charge. This means less air is drawn in during each intake cycle so less power is produced..
Never the less, a flame-trap mesh is one way of making a relatively low-powered engine that will run for far longer between valve-changes than a regular pulsejet.
Over the past two years I’ve given quite a bit of thought to the issue of extending valve life and have come up with two ideas of my own.
The first is the Blast Ring concept which works by providing a physical shield between the hot exhaust gases and the valve tips.
By blocking the direct path of the combustion flame to the valve-tips, the operating
temperature of the valves is significantly reduced. However, because the Blast Ring has a very large hole in the middle it doesn’t restrict the flow of the incoming charge of fresh air to the same degree as a flame-trap mesh.
This picture shows my PJ15 design running with a Blast Ring in place.
You’ll notice that the ring itself is glowing red-hot, an indication that it is indeed absorbing much of the heat that would otherwise be reaching the valves.
However, even this system imposes about a 15%-20% performance penalty on the power levels that would otherwise be obtained from an engine.
Another method for reducing the valve heating is to design the engine so that there’s a buffer of cold, dense air between the valves and the combustion gases. The easiest way to do this is to inject the fuel into the combustion chamber some distance from the front.
In such an engine, the air in front of the injection point will not contain any fuel thus will not actually take part in the combustion process. It will however, act as an insulating buffer between the hot combustion gases and the valves.
Unfortunately, as with the other methods mentioned so far, there’s a performance penalty associated with this method.
Since a pulsejet normally only draws in a fresh charge of air equal to about 15-20 percent of its total volume, creating a buffer zone which contains no fuel leaves less available for the
combustion process. That means less fuel can be added and, as a result, the engine produces less power.
There are very few free lunches in the world of pulsejet design.
More recently however, I have come up with what appears to be a system that imposes no performance penalty, yet significantly improves valve-life. This system works by creating a two-layer valve retainer that is cooled by the incoming fuel.
As you can see in this diagram, the fuel (purple) passes between the two thin dished valve-retainer plates before mixing with the
incoming air.
As you can see, this method allows virtually all the air in the combustion chamber to be mixed with fuel and therefore provides good power.
This provides multiple other benefits over the traditional system.
1. The fuel is pre-heated and/or vaporized before it mixes with the incoming air. This provides a much better (and more combustible) mixture than is normally achieved either by direct injection or atomization.
2. As it atomizes, the fuel absorbs a tremendous amount of heat from the two dished plates, this cooling them to a much lower temperature than they would otherwise run at.
3. The two disks, with the small gap between them, act as a far more efficient heat shield than does the normal one-piece valve retainer.
4. The small gap between the retainer disks tends to absorb some of the hot combustion gases that would otherwise reach the valves.
Here’s how I’ve implemented this design concept on my PJ15 engine. The two disks are formed from thin 0.020” (0.5mm) stainless steel which is spun to shape on a lathe.
I’ve actually observed a small power increase after changing from a conventional one-piece valve retainer to this new concept and valve-life has been almost doubled.
Unfortunately, building a valve system like this takes more time and more time and skill than the traditional one-piece valve-retainer – remember what I was saying about free lunches?
Fuel Systems
Next on the list of critical elements of a pulsejet must surely be the fuel system. Atomization
Smaller engines such as the Dynajet have traditionally used a very crude form of carburetor that using the incoming air to create a spray of rather coarsely atomized fuel droplets.
This atomizing process occurs right at the front of the engine when the incoming air is forced through a slight venturi.
An Italian by the name of Bernoulli discovered that the faster air flows, the lower its pressure becomes. This observation was promptly labeled (wait for it…) the Bernoulli Effect.
The atomizer on these small pulsejets uses a venturi to squeeze the incoming air through a narrowing in the intake. As it squeezes through, it has to speed up. As it speeds up – the pressure drops.
Now, if we stick a pipe carrying some fuel into the middle of this low-pressure area, the fuel is literally sucked out and turned into a fine spray of droplets.
What could be simpler?
Unfortunately, although this system does work, the magnitude of the low-pressure area created in the pulsejet’s venturi is quite small and this means that there’s not much energy available to suck that fuel through.
A Note About Atomization and Vaporization
Another problem with the simple atomizer is that the fuel droplets created tend to be very large and therefore do not vaporize particularly well. It should be remembered that liquid fuels themselves don’t actually burn – only the vapors that they emit will ignite. In order to obtain good vaporization, the goal should be to create the smallest possible droplets because this results in the largest surface area (from which vapor is emitted) for a given volume of liquid. Fortunately however, the inside of a pulsejet engine is a very hot place so, despite the fact that the simple atomizer does a poor job of converting liquid fuel into a nice fine spray, the high internal temperatures of the engine greatly assist the conversion of those large droplets of fuel into vapor.
[endnote]
The end result is that most of these small pulsejets are extremely sensitive to just where the fuel tank is placed relative to the atomizer assembly.
If you place the tank too low then the engine won’t have enough “suck” to pull the fuel up to the atomizer nozzle.
What’s worse, even if you do get the engine running nicely, moving the fuel tank up or down by even an inch or two can cause it to stop because the fuel flow is affected.
There are ways to reduce this sensitivity to fuel-head however and perhaps the simplest is to use a pressurized fuel tank.
By delivering the fuel under pressure, the effect of a changing fuel-level is dramatically reduced. The big problem is how do we generate this pressure?
One option is to simply pump some compressed gas into the fuel tank then seal it up. In order for this to work, the tank should only be filled with fuel to only about 25 percent of its capacity otherwise the pressure inside will drop significantly as the fuel is drawn off.
Alternatively, the compressed gas can be stored in a separate container and fed into the fuel tank through a regulator. This is how the fuel system for the Argus engine that powered the V1 flying bomb was configured and is illustrated in the diagram above.
Rather than rely on a large reservoir of compressed air inside the tank, it is possible to tap into the pressure produced by the combustion of the pulsejet itself. This diagram shows how some of that pressure can be directed into the tank to keep it pressurized. Note the small reed valve that stops the pressure from leaking back into the engine during the intake phase.
In practice, the reed valve should be placed in the pipe that leads from the engine to the tank rather than in the tank itself. Surprisingly, there’s little risk that the hot gases from inside the engine will ignite the fuel in the tank. This system can be used with both atomized and injected fuel systems.
Another simple way to achieve fuel pressurization is to use something like a small balloon for a fuel tank. This configuration is called a “bladder tank”. The elasticity of the balloon will automatically pressurize its contents – but be aware that some fuels will quickly break down the rubber from which normal balloons are made and if it goes “pop”, you’ll have a very real fire danger.
Some of those using pulsejets in model airplanes often use these bladder tanks to ensure good pressurization and reliable fuel feed under varying G-forces. It’s worth noting however, that the rubber tank is usually contained inside another leak-proof container such as a plastic soda bottle. This way, if the bladder bursts, the fuel remains contained
Most flyers of pulse-jet powered model airplanes also use a device called a Cline regulator to ensure not only that the fuel pressure remains constant but also to automatically shut off the fuel flow if the engine stops unexpectedly.
You should also be aware that any leak in a fully pressurized fuel system can result in large amounts of flammable liquid being dumped onto the
ground or in the general area of the engine. This is an obvious fire risk. What’s even worse is that if the engine stops for any reason, the flow of fuel will continue to flood into what is now a red-hot steel tube. That can result in a very impressive fireball that could also be very dangerous.
Injection
Virtually all engines over 20lbs of thrust use direct fuel injection rather than atomization. In such a system, the fuel is squirted directly into the engine’s combustion chamber under some form of pressure.
This makes the engine’s operation far more reliable and adds the additional benefit that by varying the amount of fuel being injected, the engine’s power can be varied. Yes, a throttleable pulsejet!
The Argus V1 engine used direct injection but, to the best of my knowledge, no attempt was made to provide any form of throttle control – not that it would have been of any use on a flying-bomb anyway.
The downside of fuel injection is that you need some method of pressurizing the fuel to force it into the engine in a fine spray.
The V1-flying bombs used the latter option and the fuel tank was pressurized using the same compressed-air source which drove the missile’s gyroscopes and other onboard systems. Most of my injected engines use propane as a fuel because this has the advantage of being self-pressurizing. Your common BBQ tank has around 100psi of pressure in it so you can use this for direct injection without the need for a supply of compressed air or a fuel pump.
Using such a system, the pulsejet remains a stand-alone engine that requires no extra bits and pieces to keep it running.
The simplest injection system for a petal-valved engine simply involves locating a cross-drilled injection nozzle directly behind the valve-retainer plate.
This nozzle is drilled so that the incoming fuel is sprayed out directly towards the side of the combustion
chamber. This ensures optimum mixing with the air and (in the case of liquid fuels) means that any droplets of fuel that aren’t vaporized by the incoming air will be instantly flashed into vapor when they hit the hot combustion chamber walls.
A more recent innovation I’ve come up with however involves placing an additional disk behind the valve retainer, separated by just a small space.
By injecting the fuel in the same radial pattern as with the previous system but between the two disks, the fuel is not only vaporized more effectively but also serves to cool down the valve retainer disk (and the valves). Building a system like this does however, require access to a lathe in order to turn up the key component which is this radial injector nozzle.
Using this double-disk setup I’ve been able to double the life of the reed valves used in a petal valve engine while also slightly increasing the engine’s performance and throttle range.
Timed Injection
One disadvantage of direct fuel injection is that simple systems such as the one used in the Argus V1 engine tend to spray fuel throughout the engine’s operating cycle.
Fuel will only burn efficiently (or at all) when mixed with exactly the right amount of air. This combustible mixture of air to fuel is referred to as the “stoichiometric ratio” and it varies depending on the type of fuel being used.
It makes little sense therefore, to waste fuel by injecting it when there is no incoming air to mix with it as that fuel will be unable to burn inside the engine thus contributes nothing to the thrust being generated.
Back in 1947, the guys at Princeton University came to this same conclusion and suggested that using timed fuel injection would be a way to improve the fuel-efficiency of pulsejet engines.
Now there are two ways in which timed fuel injection could be done: the simple way and the complex way.
Given that the simplicity of a pulsejet is its single greatest virtue, I’m all in favor of keeping a timed fuel injection system simple too.
I regularly get email from people who think it would be a good idea to use an electrically driven fuel injector like the ones used in modern car engines – but I disagree.
In order to make one of these injectors work you’d need a rather complex system that involved a battery to drive the injector, sensors to measure the pressure inside the combustion chamber for timing, and some electronics to tie the whole thing together.
This setup, although I’m sure it could be made to work, would be costly, complex and offer only minimal benefits over the system I use to obtain timed fuel injection.
Fortunately it is a simple job to synchronize the injection of fuel into the engine with the intake of a fresh air charge. This is because the pressure inside the engine falls to below 1
atmosphere (14.7psi at sea-level) during the intake phase and rises to as much as twice atmospheric (30psi+) during combustion and exhaust phases.
A valve placed over the fuel jet is sufficient to provide a degree of injection timing and the addition of this mechanism can provide a noticeable improvement in the fuel-efficiency of a large pulsejet. I’ve experimented with a number of different valved injectors ranging from a simple bolt drilled length-wise with a flap of spring-steel over the end like the one illustrated here…
To this carefully machined injector made from stainless steel and nickel-plated steel components I fitted to the 100lbs-thrust engine on my gokart. I noted a very definite improvement in the fuel-efficiency of this engine after fitting the timed injector system.
What Fuel is Best?
One of the great advantages of pulsejet engines is that they can, at least in theory, be made to run on almost any type of combustible liquid or gas.
Pulsejets aren’t limited to liquid or gas fuels however – on at least two occasions, coal dust has been used as a fuel. It is rumored that the Germans attempted to run the Argus V1 engine on coal dust when liquid fuel supplies became almost unobtainable near the end of WW2 and some of Reynst’s pulsed combustors were designed specifically to use this unusual fuel. Before you start worrying too much about what is the best fuel, it’s worth citing part of a report published by Princeton University in 1947 that summarized a large amount of the research done into pulsejet engines up to that time. It said “the pulsating jet engine of
contemporary design ran on almost any common fuel with negligible variations in performance.”
The only caveat the report included was that “principal [sic] differences were in the degree
of body heating and the rapidity of valve destruction.”
They found that even the use of exotic fuels such as nitropropane or nitromethane offered only a slight power increase at the expense of doubling an engine’s fuel consumption. It makes sense therefore to choose your fuel on the basis of whatever’s cheapest or most convenient to use.
For most of us however, the choice of fuels is fairly simple and boils down to one of these: Gasoline
This has the advantage that it’s relatively cheap, very easy to obtain, and is pretty clean burning. It’s also quite volatile so atomizes easily to promote easy starting.
Note that, contrary to what you might think, higher-octane gasoline is not going to produce any more power than regular gasoline. In fact (in theory) it may produce slightly less power. If you plan to use gasoline, just use whatever’s cheapest.
Propane (LPG)
Thanks to the popularity of gas-fired BBQs, propane has also become quite easy to obtain and suitable 20lb refillable tanks can be bought for well under $50.
In some countries, propane is even cheaper than gasoline and it burns very cleanly indeed – leaving no smell and very little residue at all. Despite the fact that it’s stored under pressure, it is actually quite a bit safer to use than gasoline because its vapors dissipate very quickly in the open air.
Since the boiling point of propane is well below normal room temperature, it either comes out of the tank as a gas (thus avoiding the need for vaporization) or, when drawn off as a liquid, instantly boils into a vapor. This makes a propane-powered pulsejet one of the easiest to start. Note that bigger engines will almost certainly demand to be fed with liquid propane because an average BBQ tank simply can’t provide gas at a sufficiently high rate to keep up.
If you’re planning to use a BBQ tank of propane as a fuel, you’ll have get rid of the regulator that is normally used to limit the flow of gas. This regulator reduces the pressure of the gas to just a few psi, far too low for a pulsejet’s needs.
To give you an idea of just how much gas is needed to run a pulsejet, my own 15-lbs-thrust engines (PJ15) will drink all the propane gas you can feed them – with the regulator removed. The Lockwood valveless engines will drink all the liquid propane you can feed them without any regulator in place.
If you try to use propane without removing the regulator then all you’ll get from a pulsejet is a few bangs and pops – it won’t run.
However, you will still need some form of control over the flow of gas into the engine and for this I recommend buying a cheap propane/air torch – of the type often used for soldering or brazing.
These torches are available from almost any hardware store and cost just $25-$30. Note that depending on the exact make/model of torch you buy, you may need to purchase an additional adapter fitting so that it can be screwed directly onto a 10lb or 20lb propane tank.
To use a torch like this as the gas-control valve for
your pulsejet, simply unscrew the burner fitting on the end and slide your propane-certified plastic fuel pipe over the end, securing it with a small hose-clip.
The gas-flow knob on the torch will now enable you to control the amount of gas that is delivered to your engine. If you invert your BBQ tank of propane, the torch will still serve as a very simple way to control the flow of liquid propane to larger engines. Very simple, very inexpensive, and very effective.
Another method of controlling the flow of propane to your engine is to simply use a device called a needle-valve. These valves are readily available from a number of sources and, just like the gas-torch, offer a very fine degree of control over fuel-flow.
Butane
It should be noted that although it is also often sold for use on small camp stoves, butane is not a good substitute for propane. It contains less energy and doesn’t produce as much pressure as propane at room temperature. In short – don’t waste your time or money trying to use butane as a fuel for pulsejet engines.
White Spirit/Coleman fluid
This is simply a very low octane unleaded form of gasoline which has no fancy anti-knock or combustion “improvemnet” additives included. It’s actually a better fuel than high-octane
gasoline for pulsejet use. Many of the early small pulsejet engines such as the Dynajet run best on this fuel.
Methanol
This is my second-favorite pulsejet fuel. It has the advantage that it will burn over a very wide range of rich/lean mixture settings – making an engine less sensitive to fuel head or starting conditions.
It also burns very cleanly with no smelly or oily residue and creating little more than water vapor and some C02 as combustion byproducts.
On the downside, methanol is more expensive than gasoline, your engine will burn more of it for a given amount of power, and it can be very dangerous if spilled because it burns with an almost invisible flame. Many people have been burnt because they’ve walked straight into a methanol fire without seeing it.
Despite the downsides, I prefer to use methanol for all my aspirated engines because it generates a little more power, allows the valves to run cooler, and doesn’t leave my hands stinking of gasoline.
Note that you shouldn’t use pre-mixed model airplane fuel instead of straight methanol. Model airplane fuel contains up to 20% oil that will leave significant deposits inside your pulsejet and also affects the vaporization of the mixture. It’s also a lot more expensive than plain old methanol so you’ll be wasting money.
Your local hot-rod or drag-racing club ought to be able to help you find a source of methanol but if all else fails, try one of the major oil companies like Mobil – they sell me 5-gallon drums of the stuff when I want it.
Another thing to watch when using methanol as a fuel is that it is very hydroscopic – which is to say that it tends to absorb moisture out of the air. If you leave a can of methanol uncapped then it may well absorb so much moisture that its combustibility is affected and this can result in hard-starting.
Also be aware that when you use methanol as a fuel, one of the combustion byproducts is water (albeit as water vapor). This means that the spring-steel reed valves used in an engine run on methanol are prone to rusting unless you oil them lightly before storing your engine after each run.
Constructional Techniques
Once you’ve calculated the dimensions for a pulsejet, how do you then go about building one? Of course It really helps if you’ve got access to a workshop or some basic metalworking tools such as a hacksaw, drill, welder, etc. but don’t be put off if your resources are a little more modest.
You’d be surprised how helpful your local welder or engineer can be when you explain that you’re building a jet engine and would be happy to demonstrate it to them when it’s done. It’s also amazing what you can do with a minimum of tools – if you’ve got enough patience. The Engine Body/Tailpipe
Most commercial pulsejets are made from thin stainless steel sheet that is rolled or otherwise formed into tubes and cones before being welded together.
This results in a durable engine that is light enough to be practical for such uses as powering model airplanes.
These cones and tubes are formed using a device known as a slip-roll which looks rather like an old washing-machine wringer and consists of three rollers that can be adjusted to both grip and curve the metal sheet as it’s wound through.
A hand-operated slip-roll like this one is limited to rolling stainless steel that is no more than 1mm thick – and even then it’s damned hard work if you’re rolling a piece the full 600mm long which is the maximum this set of rolls can handle.
For larger engines it really pays to find someone who has a set of motorized rolls that can handle the thicker material and longer lengths you’ll need to use.