- 19 min read

DIY Turbine Jet Engine

Building a Brayton cycle jet engine with a VW Jetta turbocharger

Background

Back in 2014, I was in highschool working on my senior project with a bandmate and close friend. Our plan was to build a fully functional turbojet engine from a turbocharger and some custom manufactured parts. The idea came from youtube, which is full of these homemade jet engines built in garages. It looked straight forward enough and the appeal was obvious. A blog I read on someone else's build put it perfectly: if you have to ask "why build a jet engine," it's probably not for you. We wrote our senior project proposal to build our jet engine with the automotives teacher as our mentor and surprisingly the school signed off.

POV: You're a high school advisor and these two

tell you they want to build a jet engine

Jet Engines 101

Before building a jet engine, it’s generally best to understand how they work. A jet engine can be broken into three basic parts:

the compressor
the combustion chamber
the turbine

The compressor brings in atmospheric air and squeezes it, increasing its pressure and density. The compressor feeds into the combustor, where the high pressure air is mixed with fuel and burned. This flame is usually lit with a sparkplug as the engine powers on but is self-sustaining afterwards. The exploded fuel and air exit the combustor as a torrent of hot gas and blows through a turbine wheel. The turbine converts some of the gas flow's kinetic energy into rotational motion, like how windmills are spun by the wind. To close the loop, the turbine is connected to the compressor by a common shaft, so that spinning the turbine also spins the compressor, allowing the engine to sustain itself as long as fuel is supplied.

Turbochargers

https://www.carsguide.com.au/oversteer/tech-through-time-turbocharging-59162

Next let's look at turbochargers and how they get us 2/3rds of a jet engine. The point of a turbocharger is to boost your car engine's cylinder pressure, which means squeezing more air into your pistons with each stroke. More air means you can burn more fuel and more fuel = more power per stroke = more power from your engine. To do this, your turbocharger uses a compressor - not unlike a jet engine.

Also like a jet engine, the compressor on your turbocharger is powered by a turbine, or the "hot side." The notable difference is that the pressurized gasses that spin this turbine come from the exhaust of your car engine. This is a clever way to get useful

work out of the energy that would otherwise be shot out your cars tailpipe. (After all, you paid for all that energy so you want to make the most of it)

So a turbocharger gives us both a compressor and a turbine. To make a jet engine, all we need to do is add a combustion chamber between the outlet of the compressor and the inlet of the turbine. Most DIY designs employ a can style combustor mounted to the inlet of the turbine, with a tube to carry air from the compressor. Air is sucked in and pressurized by the compressor, mixed with fuel and ignited in the combustion chamber, and spins the turbine as it rushes out, powering the compressor and continuing the cycle.

Now that we have the basic concepts we can start building!

The pneumonic for most thermodynamic cycles is suck, squeeze, bang, blow

2014 - The Build Begins

Sourcing a Turbo

Our first problem was finding a turbocharger. Josh and I spent several weekends going to repairshops soliciting free/cheap turbos for a good cause with no luck. Our breakthrough came from his dad, who gave us a turbo he replaced off his VW Jetta. I was concerned that it might be too small. The smaller a jet engine is, the more significant bearing friction and other energy losses will be, making it harder to start, but it was what we had.

We took measurements of the turbo to come up with dimensions for the combustor. There is a good starter video on the design process here, which shows how to size your combustor, flame tube and holes. It’s worth noting that these numbers are not exact and are derived from experimentation, so there is margin to fudge them a bit.

Combustion Chamber

The combustor design I came up with has four main components: an outer shell, the flame tube, and two end caps. The outer shell is just the casing for the combustor. The flametube is a smaller, concentric cylinder with many holes. The flametube helps the fuel and air mix and sustains combustion. The two endcaps hold and center the flametube within the shell. The endcap on the exhaust side of the combustion chamber has a large diameter hole through it that connects the flame tube to the turbine side of the turbocharger.

Picture a metryoshka doll but with more fire

I designed the combustor on Autodesk Inventor. I was learning machining from the robotics club, so I designed something that I could largely manufacture myself (turned end caps and rolled sheet metal), and that could be easily taken apart for troubleshooting.

Josh and I manufactured the combustor ourselves. We punched out the flame tube holes on a hydraulic press and rolled the metal sheets to roughly form the flame tube and shell. We turned the end caps from 316SS round stock, which required us to source carbide steel bits to use in the highschool shop. Even with the harder bits, turning the endcaps was gruesome. The cuts had to be very shallow and slow to prevent heat buildup which could work harden the stock. The chips it produced were extremely hot and would fly over your head and down your shirt. After cutting the pucks, we got access to a much nicer lathe with proper cooling fluid to bore out the large center hole in the exhaust side endcap.

We got help from a mentor to weld the combustor pieces closed and weld a base plate to the turbocharger for the combustor mount. The fuel injector was a ½” copper tube, crimped on the end with radial holes drilled in it. For ignition, we used a hobby plane spark plug near the fuel nozzle. With the combustion chamber manufactured and attached to the turbo via the threaded rods, the engine was ready to test.

Final assembly showing the threaded rods that secure the combustor and the copper fuel tube

2015 Testing

We began testing the evening before senior project presentations. We used compressed air to spool up the compressor and attempted to get the combustor to light. It was a little sketch piping in propane and trying to light the engine ourselves.

Unfortunately, we couldn’t get any fuel/air mixtures to light. The problem was the spark plug we were using was very small and likely not in the direct mix of propane. We tried adding another sparkplug between the combustor and the turbine but this also failed to light. The few times we were able to light a mixture were when no air was flowing and the combustion chamber was allowed to fully fill with propane. We ended up exploding the engine a few times by doing this, which shredded the AC tubing we were using between the compressor and inlet to the combustor. Without even coming close to running the engine we called it there.

We presented the next day on a jet engine that didn’t work, but was kind of cool to look at. The jet engine got put in storage and we headed off to college.

Our test setup the night before the final presentation. The

thin metal AC tubing was destroyed shortly after.

September 2020 - This time for real

After college, I felt a strong urge to pick up where I left off and hack the engine until it worked. I realized from taking thermo that it was actually much more realistic to make the jet engine work than I previously thought - even with such a small turbo and after all the inefficiencies of the system, with the amount of thermal energy I’d be dumping into the combustion chamber there should be plenty enough power to self sustain.

When I pulled the engine out of storage, I was impressed at how well it was built - especially for two high schoolers on a tiny budget and crunched for time. However, the engine needed some redesign and new systems we didn't take seriously the first time around. I wanted to rebuild the fuel nozzle, which was a 1/2" brass tube crimped on one end with some holes drilled in it. I also needed to redo the spark plug ignitor as it proved unable to light in 2015. Also, this time I planned to build a new test stand with oil and electrical systems to support the engine. Oil pressure is critical for the turbocharger's survival as the bearings can quickly wear and disintegrate at high RPMs and temperatures if run dry. We didn't put much thought or effort into the supporting systems back in high school and that contributed to our lack of success 5 years ago.

Finally, I decided to come at this project with a new strategy, which was to ensure success with thorough testing of each subsystem as I went. This means individually testing the ignition system, combustor, oil system, and electrical system so that I knew all the pieces worked before I put them together the first time.

New fuel nozzle

The first update was the fuel nozzle. I replaced the crimped tube with a pressure washer nozzle (a smaller orifice makes fuel mixture easier to control). I installed a bulkhead passthrough through the 1/2" hole for the original fuel tube with 1/4" NPT threaded holes for the fuel nozzle and fuel plumbing. I plugged the old spark plug hole next to the nozzle with a machine screw as it was no longer needed.

Oil System and Test Frame

In 2015, Josh and I tried to run the engine supported by a table vice and with a limited supply of oil from a single use pressurized tank. However, to properly sustain and support the engine, I needed a test stand with a closed-loop oil system and heat exchanger.

After a bit of digging (here's a good blog and informational video), I worked out a basic bill of materials for the oil system. The main components are:

12V oil pump
5L oil reservoir
Oil cooler
0-80psi regulator
Oil Filter
AN fittings on all pressurized lines

I chose to stick to a 12V power system to make everything standard, and because it’s easy enough to find 12V components from eBay autoshops.

At the same time, I started building the new frame to support all the new systems. Originally, I wanted a metal test stand to use as a common ground for the electrical system, except I was fresh out of college with no job. After the ~$200 I spent on eBay auto parts, I didn’t want to splurge on anything that wasn't absolutely necessary to run the engine so I chose to stick with wood.

I designed the frame as parts arrived for the oil system, using quick clamps to mock up the layout. I followed these general guidelines:

Oil reservoir should be under the turbocharger. The return line must be able to flow unrestricted with gravity as the turbo cannot have any back pressure in the oil outlet to function properly.
Oil pump should be at the lowest point in the system to make sure it is always primed.
Oil filter before the oil pump to prevent debris from entering the pump.
Oil pump feeds straight to oil regulator. This is the highest pressure section of line.
Oil regulator return line must also flow unrestricted to the reservoir collector.

Early mockups of the test stand adding the engine, oil reservoir and oil cooler

October 2020/more building

With the test stand underway, there was still a lot of scrappy manufacturing to get everything mounted and plumbed. I needed to build a custom oil flange since some of the jet engine modifications to the turbocharger got in the way of the stock flange. I didn't have a milling machine, so I made a new flange with a file and a drill. Even with the custom flange and some filed down bolts, it was a tight fit. I think I barely squeezed out 3 threads of engagement and spent just as much time installing it with tweezers and a flashlight in my mouth as I spent making it.

Modified Retaining Plate (10/12/20)

The bulkhead passthrough for the new fuel nozzle was bigger than the original fuel line, so the sandwich plate I'd been using to hold the combustor needed a larger hole. Lauren drilled a new one with a hole saw. With this modification, the combustion chamber could be fully assembled with the new fuel nozzle and was ready for testing!

Testing

First Combustor Test (10/18/20)

At this point, the combustion chamber was ready for standalone tests. Josh joined me to try to run the combustor we built 5 years before. The combustor was mounted to a bare bones test stand, exhaust side up, with a glow plug held by vice locks at the exhaust. We piped the fuel nozzle directly to a standard bbq propane regulator and used a portable car battery to power the glow plug.

The test plan was to light the combustor using a shop vac to simulate air flow from the turbo compressor. This tests the new glow plug as an ignition source and what kind of gas flow the combustor can generate, on top of giving us a feel for how it runs and screening any unpredicted issues.

Right: the test stand for today. Left: Josh and I ponder how to start the combustor as Lauren observes in what can only be described as a modern day renaissance painting.

IPA cotton ball starter

Unfortunately, the ignition system we envisioned didn't work. The glowplug at the throat of the combustor was still too far from the propane, which is denser than air and pools up in the bottom of the chamber. The propane could be agitated by blowing into the air inlet with the shop vac, but this was very sketch. The propane would pop unpredictably in a fireball and snuff itself out. We realized we'd need to redesign the ignition system but decided to try to test the rest of the combustor, so we needed a new way to light the flame.

We considered sticking a barbecue lighter down the throat, but that would put your hand exactly where the flame would be so nobody wanted to. Eventually, we came up with a way to keep testing moving which consisted of dousing cotton balls in IPA, lighting them on fire and using kitchen tongs to drop them in the combustion chamber to act as a pilot flame. This got us ignition and we were finally able to test the combustor with airflow - sortof.

The next problem was that even with the propane valve all the way open, the flame would snuff out when we tried to blow air into the inlet with our shop vac. We learned there that bbq propane regulators regulate to about 0.5psi, so even with the valve all the way open we were not getting nearly enough fuel flow to match the air we were blowing in and the flame would die out. We were able to partially test at a much lower throttle by holding the blower hose some distance from the combustor inlet to control how much air was blowing in. In the video, there’s an audible low rumble even at this reduced throttle.

The first flame in the combustor since 2015 (and the first stable burn ever)

A view inside the heart of the combustor. Note the cotton balls that were used as a pilot light.

While we were able to achieve a sweet whirling flame and low growl, this test showed there were a few fixes needed. The glow plug would need to be much closer to the fuel nozzle to be effective and we need a lot more propane pressure to match our air flow.

New Ignition System (10/24/20)

After what we learned from the combustor test, I worked through different ways to arrange the glow plug closer to the fuel nozzle. The solution I pursued was to pass the glow plug through one of the flame tube’s primary burner holes, close to the fuel nozzle. I wanted to be sure this layout would work before drilling a hole in the combustion chamber, so I made a new test stand which mocked up the combustion chamber outer shell with a creatine bottle. The glowplug fit through a burner hole without any modifications to the flametube.

The creatine bottle allowed me to test the flame tube as if it were inside the combustion chamber. The fuel was fed from the same propane bottle (still with the 0.5psi regulator). After heating the glowplug yellow-hot, I opened the propane supply and the combustor popped on with a woosh:

Really need to stop calling the fuel nozzle a "fuel injector"

Excited with the results, I worked on adding the new glowplug layout to the engine. I drilled a hole through the outer shell of the combustor that lined up with one of the primary burner holes in the flame tube. I took the glowplug to Ace to find a nut to hold it and used high temp weld putty to join the nut to the outer shell. The glowplug could be threaded into the nut and would locate nicely inside the flametube. I finished up by sanding a nice fillet around the cold weld.

High pressure regulator test (10/26/20)

The next day my new 0-30psi propane regulator came in the mail. I ran a test opening up the regulator through the unbuilt flame tube nozzle to see what kind of flame it could make. Without passing 2 psi the setup was roaring like an actual jet engine. It felt lucid! The flame itself was invisible but extremely loud. For the first time since starting the project 5 years ago it felt like I was so close.

Notice on the pavement the shadow of the column of hot gasses blowing out the end

2nd Combustor Test (w upgrades)

With the ignition and high pressure reg figured out, I built up the combustion chamber to reattempt the full combustor test. This test used the same setup as before, but with the variable high pressure regulator and a working ignition system (massive leap from IPA cotton balls). The test went like this:

Turn on battery and wait. The glowplug takes ~10s to get red hot
Open propane supply. The combustor popped almost instantly and began burning at atmospheric pressure.
Slowly increase airflow with a shop vac as a blower

Unlike before, the combustor sustained its flame with full airflow! Notice in the video how the flame was initially burning outside the throat of the combustion chamber, likely due to the lack of air flow making the mixture too rich inside the flame tube. However, once I induce airflow, the flame shoots back inside and the combustor begins roaring. I slowly moved the blower nozzle up to the air intake before clamping it in at full power.

The combustor goes full jet engine roar the instant it's fed air

self-titled

Wiring the stand

With the combustor design fully vetted, I turned my attention to the electrical system. For power distribution, I bought this terminal bus bar from amazon. The #4 screws easily accommodate ring wire terminals for easy connections, and it can easily handle the current draw for a cooling fan and oil pump. I wired everything up with toggle switches rated for my current draw. Based off this wiring guide, I bought 6 AWG wire for the power supply to power rail lines. Here's a basic schematic of my electrical system:

Power Supply and Electrical System Test (8/29)

I was able to power on the electrical system using my car charger to test everything. While the charger might be able to power the system for a quick test, I don’t want to rely on it for running the jet since continuous oil flow is needed several minutes after shutting down to cool the heat soak. I found a 600 watt server power supply on eBay capable of 49 amps at 12V to replace it for full tests (49 x 12 ~= 600). The connectors on the front were proprietary and it took some googling to find out how to wire up the I/O pins to enable the 12vdc output. Josh and I hacked and soldered some brass screws to the power rails as our terminals and made a small LED circuit to show when the power was on for safety. Although 12 volts isn’t enough to harm a person, if the power supply were shorted, the high amperage of the unit could spark a fire or melt whatever shorted the leads.

The power supply was the last step before testing. We finished soldering October 30 and ran a full systems check minus combustion. The oil reservoir was filled and pumping oil through the turbo. All the switches were wired and components working. The plan was to test the next day. Very shortly I would know if the several hundred dollars I’d dropped from my internship savings were wasted or not.

Making "custom" terminals. We also added an LED circuit to see when the terminals were live.

Halloween/Full Start Attempt

Starting the Engine (In Theory)

Starting up a turbocharger jet engine is the same as starting an engine on a commercial jet: spool the shaft with external power, light the combustor, and spin up until the engine can power itself. In aviation, the external power is either an on-board APU (Auxiliary Power Unit) or alternatively blown in from a ground support cart. Both provide high pressure air directly to the jet engine's turbines, which begin to spool the engine. Once there is enough airflow, the fuel is opened, the combustor is lit, and the engine's own thrust gradually begins to take over. Eventually, you can remove the external air source and the engine will self-sustain. In my case, I'm using a shop vac as my ground support cart.

By Airman 1st Class Benjamin Gonsier, U.S. Air Force - www.defense.gov, Public Domain, https://commons.wikimedia.org/w/index.php?curid=20667725

First Test!

Some friends came to visit us for Halloween. Between setting up parts for the test stand I was shaving my head for my ATLA costume.

I practiced lighting the combustor several times without airflow. This consists of warming the glow plug for ~15 seconds then opening the fuel reg. If the mixture is right you can hear a pop, followed by a low furnace-y sound as the propane burns. If the combustor failed to light, I would purge it out with the shop vac to reset the mixture to fresh air (a throwback to the lighter fluid potato cannon days).

After getting the timing down, I started attempting to spool up the jet engine. To do this, I would light the fuel and slowly introduce more air flow as I opened the throttle. Ideally, this would keep the mixture between the lower and upper explosive limits, otherwise the combustion chamber would flameout and I would have to start over.

When I finally got it, the engine spooled up and got loud very quickly. I kept adding power until the shop vac was bottomed out into the compressor inlet and I couldn’t add anymore air. The jet engine was extremely loud by this point, but every time I tried to pull the shop vac away it would wind back down again, meaning my engine was not self-sustaining yet. I tried this several more times and tried every propane throttle setting but still no luck.

It seemed like the engine was on the verge of taking over, so I tried the same process with a much larger shop vac. Heat the glow plug, wait, windmill the compressor, open the throttle a bit, listen for the pop, and walk up the fuel and air. Again the engine kept getting louder and higher pitched, but this time it wasn't levelling off. It was getting harder and harder to push the shop vac against the engine as the gap closed I don’t know if it was the static pressure pushing back or my own body reacting to the roaring engine. I pushed the shop vac on all the way and opened the throttle the last bit. This time, as I pulled away the shop vac, I had to overcome a noticeable suction to remove it and the jet engine kept going. 5 years after starting, my jet engine was running!

jet engine go brrrrr

It Dies

At last, we were able to spool up the turbo and have it fully self-sustain - but not for very long. Very quickly, the engine would start billowing smoke from the turbo core, most likely pressurized oil leaking out through the flange and burning on the hot metal. Additionally, the turbine blades would glow bright orange and shoot sparks out the back. The engine was alive, but in agony. After each run, I kept the oil circulating for ~5 minutes to help dissipate heat and would also flow air through with the shop vac.

The grandparents came out to see! I like to think my grandpa was impressed but he mostly looks confused at the pile of smoking loud junk his grandson has been obsessing over for the past several weeks.

During shut down on the 5th or so run, the engine finally blew up. As I was killing the throttle, the turbo could take no more and blew apart in the middle with a puff of smoke. On the bright side, that was a good place to call it for the night.

it died :/

Rebuild + Upgrades

I was excited to finally spool up the jet engine, but I also wanted to leave it in working condition. Luckily, the turbo split at a joint in the casing and could be rejoined. It seems that sometime ago I removed the bolted flange that retained the outlet housing to the core, so in its place I used several bolts and oversized washers to span the joint.

Reconfigure Inlet Scrolls

I wanted to improve how the engine ran, so I solicited some advice from the DIY Turbines group. Other builders recommended adjusting the turbine inlet scrolls to be only 30% open. The turbine inlet scrolls are normally connected to the wastegate valve, which passively controls how much exhaust bypasses the turbine wheel to regulate engine backpressure. I had assumed that in order to get full power, I should set scrolls full open. However, I learned that this can choke the turbine and actually results in less energy transfer into the rotation of the shaft. The reason for this is that the scrolls also control the angle of attack of the air, which dramatically affects how much energy is imparted into the turbine blades. I set the scrolls to 30% open (30% above tangent), which helps spin the exhaust against the turbine blades for more impulse. This increases the turbine's efficiency and means the engine requires less added heat to sustain.

Variable vane geometry turbo. The optimal setting is 30% from tangent. https://www.carsguide.com.au/oversteer/what-happened-to-variable-geometry-turbos-68205

Radial Spray Fuel Nozzle

During the rebuild, I opened up the combustor to have a look inside and noticed something strange. The propane flame tempered the metal, leaving visible color changes. What was surprising was that it seemed to indicate that the hottest part of the flame was occurring near the exhaust end of the flametube. This is not how the combustor is supposed to burn. In a properly designed combustor, the flame should burn at the bottom 1/3rd of the flametube at the primary burners. The secondary burner holes should add enough air to finish burning whatever fuel is left, and the tertiary holes are purely for cool air mixing before the hot gas enters the turbine wheel. In my combustor, the flame was burning at the tertiary burners, so it was blowing straight flames into the turbine wheel with no cool air mixing. This is extremely bad for the turbine wheel, and would explain the glowing orange blades and sparks.

Flame tube should not look like this

I double checked my combustor design, which was slightly oversize but not extremely out of line. The next suspect was the fuel nozzle, which was a jet nozzle off a power washer. I realized that the nozzle was spraying the fuel in a tight stream, which shoots it most of the way down the flame tube before it had time to mix and burn. My solution build a custom radial spray nozzle with a 1/4" NPT plug from ace and some small drill holes. The idea was that spraying fuel radially would result in thorough mixing and burning within the primary burner zone, allowing the secondary and tertiary holes to mix cool air as intended.

Running Cool

These modifications took a few days to complete, but the next time I tried starting up the jet engine the difference was night and day. It took some practice to relearn how to start the engine as the new fuel nozzle affected the way the throttle responded. Most notably, the new fuel plug had about 4x the flow area of the original jet washer, so the throttle was much more sensitive (the same pressure would cause ~4x the original propane flow). This made the mixture harder to control.

The engine was clearly running much cooler. There were no more sparks or smoke, and the turbine wheel didn't glow. I was also able to throttle the engine over a range of operating speeds by controlling the fuel pressure. The engine runs best at ~5psi boost, but can operate anywhere from around 3-11 psi. Check out the full demo:

This is what they mean by "cool your jets"

After all these years!

With the latest combustor and inlet scroll modifications, the jet engine I started over 5 years ago was complete and in good condition! The thought of building my own jet engine has been a dream for so many years that I nearly wrote off as too ambitious many times. I feel like this project was the cornerstone of my late teen years so it was a euphoric and gratifying moment when the jet spooled up the first time.

Thank you to everyone who helped me on this journey and everyone who shared in my excitement! In particular, thanks to the JATO group, Josh Griffin, Lauren Pearson, and the many inspirations and mentors that helped me when this project started in 2015, and to my family for putting up with all my messes and long-winded explanations.

Photo Dump (in no particular order)

What's Next?

Since tuning the engine to run cool, I haven't done too much with it. Most recently, I showed it off to some college friends and we used Lauren's car to power the electrical system (link). I also rearranged the test frame to shrink the engine so I can bring it with me to LA.

I will also release another project blog that focuses on the math, theory and design for builders. However, this writeup took several months to write so I'm in no rush to do it again.

As far as upgrades, I've got a jet pipe or afterburner on my mind. Both require more in depth instrumentation, thermodynamics calcs, and careful performance tuning, which sounds like a sweet future project.

Thanks for reading! If you liked this, check out my flute controlled robot or snare drum build.

Ad Astra!

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