POSSIBLE EXTRAS

Bruce Simpson's book describes some performance enhancing additions that can be added to a pulsejet to inprove performance such as a thrust augmentor; this has been claimed to boost thrust by up to 50%. A cooled air intake has also been mentioned; I will consider these with the design and could look at evaluating the actual gain of adding such systems onto a pulsejet.

COLIN FURZE

A youtube search has uncovered a guy called Collin Furze; a complete legend who has made a few pulsejet engines including a massive one. On his website www.colinfurze.com he gives the plans he used to build it-He uses a Lockwood-Hiller design which is just a scaled up version of Bruce Simpson's design. I've decided I am also going to use Bruce Simpson's design and scale it to meet the materials I can source.

LOCKWOOD HILLER-CHOSEN DESIGN-NEXT STEPS

From further research around the Lockwood engine, I have found that Bruce Simpson has also published a guide for people wanting to build their own Lockwood Pulsejet (6).. this should be useful for general guidance.

I will look into the extra parts I will need for this design on top of the sheet metal I have and begin sourcing and getting a design sorted.

DESIGN COMPARISONS

To compare designs I have created a set of specifications for evaluating the suitability of each design;

Feasibility-Is it realistically possible for me to manufacture/construct this design with my limited knowledge and restricted access to tools/machinery.
Accessibility-Is there enough information and knowledge available to allow me to design/build one.
Price-Is the price of the materials needed for the design within reason for this project.-Ideally no more than £100?
Popularity-Is the design popular amongst hobbyists? If it isn't there'll be a reason for it.
Time-Is it realistically possible for me to manufacture/construct this design in the time-scale.

I have picked these as they matter to me the most for the case of this project; i could've done thrust, efficiency, weight stc. depending on the purpose of this pulsejet.

I am going to score each of these categories out of 10 to help decide which is the superior design for this project.
The comparison is given below:

The Lockwood-Hiller has come out with the highest score. Although I would have liked to design my own pulsejet, I have decided that there is a lack of information regarding the designing of this type. I am therefore going to investigate existing designs modified for my own purposes.

WELDING/METAL ROLLING ATTEMPT

As all of the designs I am considering involve the same mild steel construction and rolled sheet metal techniques, I began practising MIG welding.

Using my dad's angle grinder with a thin metal cutting disk in, it was fairly easy to cut rectangles out of the sheet metal. Using the sheet metal roller lowering the top roller by a half turn at a time, it was easy and simple to roll a tube which could be removed from the roller by detaching the top roller. I then (attempted to) MIG welded the gap to form a solid tube.

From looking at how roll cones cones/frustums (as opposed to tubes), the technique is the same; the shape of the material that is being rolled determines the shape of the cone. I will need to design nets and find a way of making these into full-size templates. I should be able to cut out these nets using an angle grinder again.


I originally tried welding with small strips. However I found this often burnt through the metal leaving a hole. I then started welding with small beads 'stitching' the weld.

From inspecting my welds, first hand comments from dad (who can weld and has had such jobs in the past) and research into what makes a good weld, I found that mine had poor penetration and protruding beads that needed grinding down; these mean the joint is relatively weak and basically, not good enough...I would like to construct the whole pulsejet myself so this will need work.

SHEET METAL SOURCING

All of the designs use the same mild steel construction from rolls sheet metal so even though I haven't decided on design I decided it would be appropriate and beneficial to start looking for suppliers of sheet metal. I've found many websites selling mild steel sheets which i will need. I approached the DT department in my school to see if they had any suppliers that I could possibly look at for sourcing these materials and acquired some names of suppliers they use-my thinking being they will be reputable and might have cheaper prices with them supplying to schools. However in doing so discovered that they had a collection of sheet mild steel at a suitable thickness (1.2mm) which was usable for my design and therefore purchased a few sheets at a brilliant price-regardless of whether I use these for the final product, they will be useful to practise techniques on nonetheless.

MATERIALS/TECHNIQUES

From looking at various designs and recommendations in Bruce Simpson's book, I have found that all of the designs are made from either mild steel or stainless steel. As this will have an influence on how I make the pulse-jet, I will need to compare the two materials for suitability. I have decided to do this now because it will allow me to start practising my techniques such as metal rolling/welding which will need to be developed over time.

Mild Steel
Cheap
Easily welded
Rusts (oxidises easily)
Heavy

Stainless Steel
More aesthetically pleasing
Stronger/more durable
Lighter
More expensive
Has to be TIG welded (as opposed to MIG therefore more difficult)

I have decided to go with mild steel because although stainless performs better, price and workability are more important for this project.

My dad has a MIG welder that I can use to weld the pulsejet; I will need to experiment with the different feed settings and techniques for the type and thickness of the steel.
In terms of actually rolling the metal, my Dad has a set of slip rolls that I can learn to use also.

THERMOJET

This is a valveless Pulsejet design very similar to the Chinese design except has two slightly shorter intake tubes going into the combustion chamber as opposed to just one.
Pulsejetengines.com says this is one of the best Pulsejet to build for the first time with it having 'the best range of performance characteristics and super easy starting'. (8.2)

(8)

LOCKWOOD HILLER

It turns out that this was the type of design actually used on scrapheap challenge.

A Lockwood-Hiller engine is a valveless pulsejet and consists of a combustion chamber, intake tube and exhaust pipe. The defining characteristic of this design is the U-bend in the exhaust making the intake and exhaust face the same direction.

It's easy to find the patent filed for the Lockwood engine in archives (17) which describes the design and a blog following a guy who made his own lockwood-hiller and documented the process (16).

If I choose a Lockwood-hiller I can be confident there is enough information from people who have already built their own to help with the construction of my own; the design seems to be well rated among enthusiasts, most common and most documented. However, with this type of design, there is a lack of standardised comprehensive mathematical laws governing their function and although backed by basic theory, many successful designs seem to be generated through empiricism due to the shear number of unpredictable variables involved with their function. (1.10)

(13)

CHINESE/FOCUSED WAVE PULSEJET

From what I can understand the Chinese and Focused-Wave designs are basically the same; the intake and exhaust face the same direction with the intake being a small tube directly leaving the combustion chamber.

I have found mixed reviews on this design; Bruce Simpson says it's poorly rated (1.6) whilst pulsejetengines.com rate them highly.

(8)

VALVED DESIGNS

When it comes to valved designs, the shape can be calculated using using calculations in Tharratts paper.

Valved engines have the benefit of a good power to weight ratio and are compact.
However they are more difficult to build (valves to manufacture).
(8.1)

The main factor that is to be chosen is the type of valve design to be used from:

Petal Valve-A thin piece of shaped metal attached to a plate with holes in. When the plate experiences a negative pressure (from the chamber after combustion), the metal 'petals' flex upwards exposing the holes underneath and allowing fresh air in. (1.3)









(12.1)

V-Valve-This is a valve similar to the petal valve but the valves are in a 'V' shape to provide a straighter root for the incoming air to travel down. These are: More efficient than petal valve, easier/cheaper to repair (valves can be replaced individually), easy to scale up or down by changing the number of valves, more complex/expensive than petal valves.(1.8)

This type of valve is very similar to the type used on the V1 bomb. (14)


A v-valve is also very similar to the reed valve on a 2-stroke engine:

(15)

Rotary Valve-A spinning plate on the front of the jet with holes that line up in sync with the engine cycle: Holes are created when the Pulsejet needs a 'breath' of air and are blocked when the gasses are combusting. (1.7)

The Enthusiasts Guide has also given me an insight into the mathematical work that has been performed on pulsejets (1.9); a large amount of work was done by Tharrat in his paper 'The Propulsive duct'. This is easy to find on online (12.2) and describes relationships between measurements and power output etc. This allows one to calculate the measurements of a pulsejet through the desired characteristics.
Fredrik Westberg also gives calculations that can be used to design a valved pulsejet with details such as valve area etc. (5.1)

For the purpose of deciding a design, I am going to treat all valved designs as one to evaluate against other designs.

TYPES OF DESIGN

Through reading and various searches I have studied the two types of pulsejet and picked the following designs to research, evaluate and compare:

VALVELESS
The 'Chinese Pulse jet'/Focused wave Pulsejet
Thermojet
Lockwood-Hiller Pulsejet

A valveless Pulsejet works by having a combustion chamber with two tubes leading off it of unequal length and diameter; an exhaust (larger tube) and an intake (smaller tube). A fuel/air mixture is combusted in the chamber (so-called) creating two spurts of hot expanding gas that travel mainly down the exhaust but some down the intake-this creates thrust. This leaves a void in the combustion chamber and as the hot expanding gasses are travelling down the exhaust, they have momentum and continue to travel down the tube even when the pressure in the combustion chamber drops below atmospheric. This creates a partial vacuum in the chamber. Because the pressure of air is now greater outside the pulsejet, air is pushed into the combustion chamber through the shorter intake tube and mixed with fuel. The hot gasses still in the exhaust are partially drawn back and cause this mixture to reignite. This cycle repeats at around 250 times a second in small pulsejets. The intake tube of a Valveless engine is usually faced backwards so expanding gasses don't create negative thrust. (7.1) 

                                                                                                  (image-10)

VALVED
Valved designs are all very similar in general shape/geometry; the main differences are the type of valve used:
Petal Valved
V-valve
Rotary Valve

Valved designs run on the same principal but have mechanical valves at the front to control the expanding gasses; they close when the gasses are expanding so they only go down the exhaust, and open when the pressure drops letting fresh air into the engine, a new mixture being created, it igniting, and the cycle repeating. (1.4)

(11)

I will explore all of these designs/types before comparing them and making a decision on which I will base my own on.