Haven't you always wanted a fire-breathing jet engine to power your car/motorcycle/Vespa/skateboard? Of course. Here's a handy, step-by-step guide. Have fun, and don't burn the house down! —Ed.
You don't have to be Jay Leno to own a jet powered motorcycle — we'll show you how to make your own jet engine to power your own wacky vehicles. This is an ongoing project, and plenty of additional info will be available on our website soon. The full build will be available at Bad Brothers Racing; additional info can also be found at Gary's Jet Journal.
Warning! Building your own jet engine can be dangerous. We suggest that you take all appropriate safety precautions when dealing with machinery and use extreme care while operating jet engines. Due to explosive fuels and dangerous moving parts, serious injury or death can occur while operating a jet turbine in close quarters. Extreme amounts of potential and kinetic energy are stored in operating engines. Always use caution and good judgment while operating engines and machinery and wear appropriate eye and hearing protection. Neither Bad Brothers Racing or Gary's Jet Journal accept any liability for your use or misuse of the information contained herein.
I started the build process of my engine with a design in the CAD program Solid Works. I find it much easier to work this way, and creating parts using CNC machining processes turns out a much nicer end result. The main thing I like about using the 3D process is the ability to see how the parts will fit together before fabrication, so that I can make changes before spending hours on a part. This step is really not neccesary, as anyone with decent drawing skills can sketch out the design on the back of an envelope rather quickly. When trying to fit the entire engine into my final project — a jet bike — it will certainly help.
That said, not everyone has the experience or background necessary to use computer-aided design tools. If you are attempting to build a jet engine or turbine-based project and don't know where to start, user groups like Yahoo Groups are the best place to begin. The years of combined experience available there will prove invaluable, and the contributors to those groups can help you get what you need. (For reference, I'm a regular on the Yahoo Groups DIY Gas Turbines forum.)
Use care when selecting your turbocharger! You need a large turbo with a single (non-divided) turbine inlet. The bigger the turbo, the more thrust your finished engine will produce. I like the turbos found on large diesel engines and earth-moving equipment. One of these units will yield enough thrust output to move most small vehicles — small motorcycles, go karts, etc. — pretty well. If possible, buy a rebuilt unit to maximize efficiency. Ebay is the way to go here.
Generally speaking, it's not so much the size of the turbo as it is the size of the inducer that matters. The inducer is the visible area of compressor blades that can be seen when looking at the turbo's compressor with its covers (housings) on. The turbo you see here, a Cummins ST-50 taken off an eighteen-wheel truck, is quite large — almost 5 inches in diameter — while the visible blades of the inducer are only 3 inches in diameter. This will easily create enough thrust to drive a small bike or kart.
It's time for the basics: Here's a quick summary of how jet engines work, and how to determine the size of combustion chamber — the power-producing bit — that your engine will need.
The combustion chamber allows compressed air coming from the turbo's compressor — the fan-shaped part inside the turbo — to be mixed with fuel and burned. The hot gases then escape through the rear of the combustion chamber and spin the turbine shaft which then powers the compressor attached to the other end to bring in more air and keep the process going. Additional energy left in the hot gases as they pass the turbine creates thrust. It sounds simple, but it's actually a bit complicated to build and get right.
The combustion chamber is made from a large piece of tubular steel with caps on both ends. Inside of this chamber is a flametube. This flametube is little more than a small piece of tubing, drilled full of holes, that runs the length of the combustion chamber. The holes allow the compressed air to pass through in predetermined ratios. This serves three purposes: 1) Mixing air and fuel for combustion, which also begins here; 2) Providing air for the completion of combustion; and 3) supplying cooling air to lower the charge temperature before the airstream comes into contact with the turbine blades.
To calculate the flametube dimensions, you double the diameter of the your turbocharger's inducer. This will give you the diameter of the flametube. Multiply the diameter of the inducer of the turbo by six to find the length of the flametube. (Again, the inducer is the compressor-blade area that can be seen from the front of the turbo with the housings on. While a compressor wheel in a turbo may be 5 or 6 inches in diameter, the inducer will be considerably smaller.)
The inducer of the turbos I like to use (ST-50 and VT-50 models) is 3 inches in diameter, so the flame tube dimensions would be 6 inches in diameter by eighteen inches in length. This is a recommended starting point; it can be fudged a little. I wanted a slightly smaller combustion chamber, so I decided to use a 5-inch diameter flametube with a 10-inch length. I chose the 5-inch diameter primarily because the tubing is easy to acquire — it's the same size as readily available diesel-truck exhaust pipe. The 10-inch length was chosen because my engine will eventually end up in the small frame of a motorcycle.
With the size of the flame tube calculated, you can find the size of the combustion chamber. Since the flametube will fit inside the combustion chamber, the chamber housing will have to be a larger diameter. A recomended starting point is to have a minimum 1 inch space around the flametube; the length should be the same as the flametube. I chose an 8-inch diameter chamber housing because it fits the need for the airspace and it is a commonly available size in steel tubing. With the 5-inch diameter flametube, I will have a 1.5 inch gap between the flametube and the combustion-chamber housing. Try to use steel tubing instead of pipe when possible.
Now that you have your engine's rough dimensions, you can put it together with the caps on the ends and the fuel injectors. All of these parts combine to form the complete combustion chamber.
The combustion chamber is a simple bolt-together piece. I use a method of constructing rings that will not only provide a surface to which the end caps can be bolted, but that also centers the flametube in the chamber.
The rings are fabricated to an outside diameter of 8 inches with an inside diameter of 5 and 1/32 inches. The extra space provided by the 1/32 inch will make inserting the flametube easier when construction is complete, and will serve as a buffer to allow for some expansion of the flametube as it gets hot.
The rings are made from 1/4-inch plate steel. I had mine laser cut from 3-D drawings I created in solid works. I find going this route much easier that trying to machine the parts. You can use a milling machine, water jet, or hand tools to make the rings. Any method which gives acceptable results will work. The 1/4-inch thickness will allow for the rings to be welded on with less chance of warpage, and will provide a stable mounting base for the end caps. It will also allow for the flametube to be constructed 3/16ths of an inch shorter than the overall combustion chamber length and allow for heat expansion in the axial plane.
Twelve bolt holes should be drilled around the ring in a circular pattern for the mounting of the end caps. By welding nuts on the back of these holes, bolts can be threaded right in. This is a requirement since the back side of the rings will be inaccessible for holding nuts with a wrench once mounted on the combustor. You could still replace a nut inside of the combustor if one were to strip out, making this a better method than tapping the holes in the rings for threads. Three tack welds placed on every other flat of the each nut should hold them tight enough to keep them in place.
Now that the end rings are ready, they can be welded onto the combustor housing. The housing must first be cut to the proper length and have the ends squared up so that everything will align properly.
Start by taking a large sheet of posterboard and wrapping it around the steel tube so that the ends are squared with each other and the posterboard is pulled tight. It should make a cylinder shape around the tube, and the ends of the posterboard will be nice and square. Slide the posterboard to one end of the tube so that the edge of the tube and posterboard cylinder ends are almost touching, making sure there is enough room to make a mark around the tube so you can grind down the metal flush with the mark. This will square one end of the tube. Most metal suppliers cut the tubing with a bandsaw, and the margin of error for their cuts is plus or minus 1/16th inch. If not corrected, this could make for a less-than-perfect cut and a wobbly end.
Next, measure from the squared-up end toward the other for the length you want the combustion chamber and flame tube to be. Since the end rings that will be welded on are 1/4 inch each, be sure to subtract 1/2 inch from your measurement first. (Since my combustor will be 10 inches in length, my measurement will be taken at 9.5 inches.) Scribe the tube using the posterboard to create a nice, even marking.
I find that using a cut-off wheel in an angle grinder does the job of cutting through the 1/8th-inch tubing very nicely. Make nice, even strokes with the wheel and rotate the tube as you go, cutting a little deeper with each pass. Don't worry about making the cut perfect — it's best to leave a little excess material and clean it up later. I like to use flap discs in the angle grinder for the final cleanup.
Once the cut is made and cleaned up, use the flap disc to bevel the outside edges of both ends of the tubing to get good weld penetration. The tube is then ready for welding.
Using magnetic welding clamps, center the end rings on the ends of the tubing and make sure they are flush with the tube. Tack weld the rings in place and allow to cool. Once the tacks are set, use stitch welds of roughly 1 inch in length to close the weld bead around the rings. Make a stich weld, then alternate to the other side and do the same. Use a fashion similar to tightening the lug nuts on a car. Go slowly so as not to overheat the metal and warp the rings.
With the main combustor housing complete, you will need two end caps for the combustor assembly. One end cap will be for the fuel injector side, and the other will route the hot exhaust gases to the turbine.
Fabricate two plates with the same diameter of your combustion chamber, in our case, this measurement is 8 inches. Place 12 bolt holes around the perimeter to align with the bolt holes on the end rings so they can be attached later. (Twelve is just the number of bolts I use, you can use more or less on the rings and end caps.)
The injector cap need only have two holes in it. One will be for the fuel injector and the other will be for the spark plug. You can add more holes for more injectors if you like; this is a personal preference. I use five injectors, with one in the center and four in a circular pattern around it. The only requirement is that the injectors be placed so that they end up in the flametube when the parts are bolted together. For our design, this means that they must fit into the center of a 5-inch diameter circle in the middle of the end cap. I used 1/2-inch holes for mounting the injectors.
Next, and slightly offset from the center, you will add the hole for your spark plug. The hole should be drilled and tapped for a 14mm x 1.25mm thread. Again, the design in the pictures has two spark plugs — this is just a matter of preference for me in case one plug chooses to go out of service. Make sure that the plugs are also within the confines of the flametube.
In the photo of the injector cap, you can see the little tubes that stick out of the cap. These are for mounting the injectors. As I said, I will have five of them, but you can get by with one in the center for your first attempt. The tubes are made from 1/2-inch diameter tubing with a 3/8th-inch inside diameter. They get cut to 1.25 inches, after which a bevel is placed on the edges by chucking them in the drill press and rotating them while hitting them with an angle grinder. It is a neat little trick that turns out decent results. Both ends are threaded with a 1/8th inch NPT tapered pipe thread. I hold the tubes in a vise under the drill press and chuck up the pipe tap so that I can start the threads nice and straight in the tubes. After starting the threads, I finish them by hand turning the tap to the required depth. They are welded in place with 1/2 inch of the tube protruding from each side of the plate. The fuel supply lines will attach to one side and the injectors will screw into the other. I like to weld them to the inside of the plate to make the outside of the combustor have a clean appearance.
To make the exhaust cap, you will need to cut an opening for the hot gases to escape from. In my case, I sized it to the same dimensions as the entrance to the turbine scroll on the turbo. This is 2 inches by 3 inches on our turbo. A small plate, or turbine flange, is then made to bolt to the turbine housing. The turbine flange should have the same sized opening as the turbine inlet as well, plus four bolt holes to secure it to the turbo. The exhaust end cap and the turbine flange can be welded together by making a simple rectangular box section to go between the two. In the photo of the exhaust manifold below, you can see the turbine flange to the right and the exhaust cap face down on the ground. The transition bend had to be made for the application this engine will see in the jet bike, but it could have easily been made with just a simple straight-in rectangular section created from sheet steel. Weld the parts together, keeping your welds on the outside of the pieces only so that the air flow will not have any obstructions or turbulence created by inside beads.
You are now getting closer to having a finshed jet engine. It is time to bolt the parts together to see if everything fits as it should.
Start by bolting the turbine flange and end cap assembly (the exhaust manifold) to your turbo. The combustor housing then bolts to the exhaust assembly, and finally the injector cap bolts to the main combustor housing. If you have done everything right so far, it should look similar to the second picture below. If it doesn't, back up and see where you made your mistake.
It is important to note that the turbine and compressor sections of the turbo can be rotated against each other by loosening the clamps in the middle. Different turbos use many kinds of clamps, but it should be easy to see which bolts must be loosened to make the parts rotate.
With the parts attached and the orientation of your turbo set, you will need to fabricate a pipe to connect the compressor outlet opening to the combustor housing. This pipe should be the same diameter as the compressor outlet, and will eventually be attached to the compressor with a rubber or silicon hose coupler. The other end will need to fit flush with the combustor and be welded into place once a hole has been cut into the side of the combustor housing. It does not matter where the hole is on the side of the combustor so long as the air has a nice smooth path to get in. This means no sharp corners, and keep the welds on the outside. For our combustor, I chose to use a piece of 3.5-inch diameter, mandrel-bent exhaust tubing. The image above shows a hand-fabricated pipe that enlarges and slows the air down before entering the combustor.
You should now have a nice clean path for the air to take all the way from the inlet of the compressor, down the pipe to the combustor, through the exhaust manifold, and past the turbine section. Everything should be pretty much airtight, and you should check all welding to make sure that it is solid. Blowing a leaf blower through the front of the engine should cause the air to flow through and turn the turbine blades.
Many builders consider this to be the hardest part. The flame tube is what lets the air into the center of the combustion chamber and keeps the flame held in place so that it must exit to the turbine side only, not the compressor side.
The picture above shows my flametube. From left to right, the hole patterns have special names and functions. The small holes to the left are the primary holes, the middle larger holes are the secondaries, and the largest to the right are the tertiary or dilution holes. (Note that there are also some additional small holes in this design to help create a curtain of air to keep the flametube walls cooler)
The primary holes supply the air for fuel and air mixing; this is where the burn process begins.
The secondary holes supply the air to complete the combustion process.
The tertiary/dilution holes provide the air for cooling of the gases before they leave the combustor; this helps keep the turbo's turbine blades from overheating.
The size and placement of the holes is a mathematical equation at best and a logistical nightmare at worst. To make the process of calculating the holes easy, I have provided a program here that will do the work for you. It is a Windows program, so if you are on a Mac or Linux box you will have to do the equations longhand. The program, dubbed Jet Spec Designer, can also be used to determine the thrust output of a particular turbo.
Before making any holes in the flametube, you will need to size it to fit into the combustor. As our combustor is 10 inches long from the outside of the ring ends on one side to the other, you will need to cut the flametube to that length (make sure you cut to fit your combustor length). Use the posterboard wrapped around the flametube to square up one end, then measure and cut the other. I would suggest making the flametube almost 3/16ths of an inch shorter to allow for expansion for the metal as it gets hot. It will still be able to be captured inside of the end rings, and will "float" inside of them.
Once you've cut it to length, get going on those holes. There will be a lot of them, and a "unibit" or stepped drill bit is very handy to have here. The flametube can be made of stainless or regular mild steel. Stainless will of course last longer and hold up to the heat better than mild steel.
Now that you have the flame tube drilled, open the combustor housing and insert it between the rings until it snugs down into the back against the exhaust cap. Replace the injector side cap and tighten the bolts. I like to use hex-head cap bolts just for the look, but the convenience is also nice as you dont have to fiddle with a regular wrench.
Fuel and Oil Pumps: Plumb It Right, Don't Die
Now you will need to get fuel to the system and oil to the bearings. This part is not as complicated as it may seem. For the fuel side, you will need a pump capable of high pressure and a flow of at least 20 gallons per hour. For the oil side, you will need a pump capable of at least 50 psi pressure with a flow of about 2-3 gallons per minute. Fortunately, the same type of pump can be used for both. My suggestion is the Shurflo pump, model number 8000-643-236. Other alternatives are power steering pumps, furnace pumps, and automotive fuel pumps. The best price I have found on the Shurflo is from here, currently $77 US. Do not skimp out and buy the other Shurflo pumps which look the same but are cheaper. The valves and seals in the pumps will not work with petroleum based products and I cannot guarantee that you will have much luck with them.
I have provided a diagram for the fuel system here; the oil system for the turbo will work the same way. If your pump does not have a bypass return directly on it (the Shurflo does not, but some furnace pumps do) then you can omit the pump bypass as it is only there to catch blowby from the pump itself.
The idea of the plumbing systems is to regulate pressure with a bypass valve setup. The pumps will always have a full flow with this method, and any unused fluid will be returned to its holding tank. By going this route, you will avoid back pressure on the pump and the pumps will last longer too. The system will work equally well for fuel and oil systems. For the oil system you will need to have a filter and an oil cooler, both of which would go in line after the pump, but before the bypass valve.
Oil Cooler: Plumb it Right, Still Don't Die
For an oil cooler, I suggest B&M transmission coolers. Oil filters can be the regular screw-on type by using a remote oil filter mount. Make sure that all lines running to the turbo are made of "hard line" such as copper tubing with compression fittings. Flexible line such as rubber can blow off and end in disaster. Oil or fuel hitting a hot turbine housing will burst into flame very quickly. Also of note is the pressure involved in these pump systems. Rubber hose will soften with heat, and the high pressures from the pumps will cause the lines to rupture and slip off of fittings. Be safe and use hard lines. It is just as inexpensive as flexible lines. You have been warned of the dangers; I accept no liability for your unwillingness to follow instructions!
When plumbing the oil lines to the turbo, make sure that your oil inlet is on the top of the turbo and that the drain is at the bottom. The inlet is usually the smaller of the two openings. If you are using a wate-cooled turbo, it is not neccessary to use the water jacket at all, and nothing need be hooked to these ports. It will only be useful if you would like to supply a flow of water for cooling the turbo upon shutdown.
Tanks, Injectors, and Oil
Tanks for fuel can be any size, and oil tanks should be capable of holding at least one gallon. Do not place the pick up lines near the return lines in tanks, or the aeration caused by the returning fluids will cause air bubbles to enter the pick up lines and the pumps will cavitate and lose pressure!
For fuel injectors, I recomend HAGO nozzles from McMaster Carr. Look on page 1939 of the online catalog for the stainless-steel water-misting nozzles. An engine of this size will need a flow of approximately 14 gallons per hour at full bore.
With regard to oil, I use Castrol fully synthetic 5W-20 right now. A fully synthetic oil with a low viscosity is a must. The synthetic will have a much higher flash point and be less likely to ignite, and the low viscosity will help the turbine to rotate easier on startup.
For more information about calculating fuel requirements and such, I suggest you join a user group such as the Yahoo Forums "DIYgasturbines" user group. There is a wealth of information there, and I am a regular member.
Sparking That Mother Up
Ahh, you will need a source of ignition! Since there are numerous ways to get a spark from a sparkplug, I won't go into this in-depth. I leave it to you to search the Internet for a nice high -oltage circuit to get a spark, or you can cheap out and wire an automotive flasher relay to a coil and get a rather slow, but usable, spark out of your plug.
For the power to all of the 12-volt systems, I like to use 12-volt, 7 or 12 amp-hour, sealed gel-cell batteries like the ones you find in burglar alarms. They are small, light, and well suited to the task, and they easily fit into a jet kart or other small vehicle.
Ok, so you've made it this far. All you need now is a stand on which to mount your engine. You can see the test stand I made in other pictures here and get an idea of how to make one for yourself. Do you have your leaf blower ready? Let's get it started!
Note: The engine in the video is not the author's engine. Video of that engine can be found here.
This is the fun part! The parts you will need are...
1) The engine
2) Ear muffs
3) Lots of fuel (diesel, kerosene, or Jet-A)
4) A leaf blower
This is where things get interesting.
First, set up the jet in a place where you can actually start it without making anyone mad with the loud noise. Next, you'll need to fuel it. I like to use Jet-A — aviation jet fuel, available at any small airport — because it works well and offers the right smell. Switch on your oil system and set the oil pressure to a minimum of 30 psi. Put on your ear defenders and spool up the turbine by blowing air through the engine with the leaf blower. Yes, you can use electric or air starting on these engines, but it is not the norm, and it is much easier to just use the leaf blower.
Next, turn on the ignition circuit and slowly apply the fuel by closing the bypass needle valve on the fuel system until you hear a "pop" when the combustor lights. Keep increasing the fuel flow, and you will start to hear the roar of your new jet engine. Gradually pull the leaf blower away and see if the engine speeds up on its own. If it does not, reapply the leaf blower and give it more fuel until it does.
That's it! Congratulations — you've built a jet engine! Don't burn the house down!
Russ Moore is a contributor for Instructables.com, a "web-based documentation platform where passionate people share what they do and how they do it, and learn from and collaborate with others." This story originally appeared on Instructables on April 17, 2006.