A jet engine works by pulling in air, squeezing it, mixing it with fuel, igniting it, and blasting the hot exhaust out the back to produce thrust. Every jet engine, from a massive commercial turbofan to a hobbyist’s garage build, follows this same four-step cycle. Understanding these steps is the foundation for building one at any scale.
The Four Steps That Create Thrust
Jet engines operate on what engineers call the Brayton cycle, a continuous loop of four events happening simultaneously in different sections of the engine.
Intake: Air enters the front of the engine. In flight, the inlet slows the incoming airstream, and that deceleration converts the air’s speed into pressure, giving the engine a head start on compression before the air even reaches the moving parts.
Compression: Rows of spinning blades, working like a powerful electric fan, squeeze the air to many times its original pressure. Each row of blades produces a small jump in pressure, and stacking dozens of rows together builds enormous compression ratios.
Combustion: The compressed air enters a combustion chamber where a small amount of fuel is sprayed in and ignited. This happens at constant pressure, and the temperature spike is extreme. Modern commercial engines reach 1,700°C (3,092°F) inside the combustion chamber, thanks to advanced heat-resistant coatings on the chamber walls. Without those coatings, the nickel-based superalloys used in engine construction would lose their strength at around 1,150°C and begin approaching their melting point.
Exhaust (turbine and nozzle): The hot gas rushes through a turbine, which works like a windmill. Instead of needing power to spin, the turbine extracts energy from the rushing gas. That extracted energy spins the compressor at the front, since the two are connected by a central shaft. After passing through the turbine, there’s still enough energy left in the gas to accelerate through a narrowing nozzle at the rear, producing the thrust that pushes the engine forward.
Core Components and What Each One Does
Every turbojet has five essential sections, and building a functioning engine at any scale means replicating each one.
- Inlet: Guides air smoothly into the compressor. Its shape matters because turbulent or uneven airflow reduces efficiency and can cause the compressor to stall.
- Compressor: Rows of precisely angled blades that act like airfoils, each adding a small pressure increase. Axial compressors (the type in full-size engines) stack many stages in a line. Centrifugal compressors (common in smaller engines and turbochargers) use a single spinning disc to fling air outward, achieving compression in one step.
- Combustion chamber: A carefully designed can or annular tube where fuel mixes with compressed air and burns continuously. The chamber must keep the flame stable across a wide range of fuel flow rates, from idle to full power, while ensuring the walls don’t overheat.
- Turbine: Extracts just enough energy from the exhaust gas to keep the compressor spinning. The turbine blades endure the highest temperatures and stresses of any component in the engine.
- Nozzle: Accelerates the remaining exhaust gas to high velocity. The difference between the speed of air entering the engine and the speed leaving the nozzle determines how much thrust is produced.
Why Turbine Blades Are So Hard to Make
The turbine section sits directly downstream of the combustion chamber, bathed in gas hot enough to melt the metal the blades are made from. This is the single greatest engineering challenge in building a jet engine, and it’s the reason full-scale engines cost millions of dollars.
Production turbine blades are made through investment casting. The process starts with a precise wax replica of the blade, including the tiny internal cooling passages that will eventually channel cooler air through the finished part. This wax pattern is coated in layers of ceramic to form a shell mold. The wax is melted out, and molten nickel superalloy is poured into the ceramic shell.
The most advanced version of this process produces single-crystal blades, where the entire blade is one continuous metal crystal with no grain boundaries. Grain boundaries are weak points where cracks can start, so eliminating them dramatically increases the blade’s ability to survive extreme heat and stress. This is achieved by cooling the casting very slowly from one end, coaxing the metal to solidify as a single crystal. It’s a process that took decades to develop and remains one of the most demanding manufacturing techniques in any industry.
How Fuel Gets Into the Combustion Chamber
You can’t just squirt liquid fuel into a high-speed airstream and expect it to burn well. The fuel must be atomized, broken into an extremely fine mist of tiny droplets so it mixes thoroughly with air and ignites efficiently. Fuel injectors in jet engines use several strategies to achieve this. Some force fuel through tiny orifices at high pressure (simplex and duplex nozzles). Others use the fast-moving air itself to shear the fuel stream into fine droplets (airblast atomizers), which are particularly effective with heavier fuels.
The injectors must work across a huge range of conditions. At idle, only a trickle of fuel flows. At full power, the flow rate increases many times over. The atomization quality at every point in that range affects ignition reliability, combustion efficiency, and emissions.
Building a Jet Engine From a Turbocharger
The most accessible way to actually build a functioning jet engine is to convert an automotive turbocharger. A turbocharger already contains the two hardest components: a compressor wheel and a turbine wheel, connected by a shaft. What it lacks is a combustion chamber, fuel system, ignition source, oil supply, and exhaust nozzle.
The combustion chamber is the most time-consuming part of the build. It’s typically fabricated from steel tubing as a “through-flow” design: an inner tube perforated with many holes, surrounded by an outer shell. Compressed air from the turbo’s compressor enters the space between the tubes and flows inward through the holes, mixing with fuel inside the inner tube. Getting the right air-to-fuel mixture requires extensive experimentation with the number, size, and placement of these holes. Too many fuel nozzles inside the chamber and you get a rich, inefficient burn. Too few and the flame won’t sustain.
Propane gas is the most common fuel choice for these builds because it’s easy to ignite, readily available, and its flow rate is simple to control with an adjustable regulator. A glow plug mounted on the outside of the combustion chamber provides the initial ignition spark.
Oil and Starting Systems
The turbocharger’s shaft bearing needs a constant supply of pressurized oil, which in a car comes from the engine’s oil pump. In a standalone jet engine conversion, you need to build your own oil system. A common approach uses a motorcycle gear oil pump (sourced cheaply from salvage) driven by a small 12-volt electric motor. The system needs an oil tank with a sight glass to monitor level and a thermometer to watch temperature, plus supply and drain lines connected to the turbo’s bearing housing.
Starting is another challenge. The compressor needs to be spun up to a speed where the combustion cycle can sustain itself. Compressed air directed at the compressor wheel is one approach. Others use an electric starter motor. Either way, you need a car battery to power the oil pump motor and the starter.
Critical Modifications
Turbochargers have a waste gate, a valve that normally limits boost pressure by bypassing exhaust around the turbine. In a jet engine conversion, this needs to be held closed (typically with a heavy spring) so all the exhaust energy drives the turbine. The connection between the combustion chamber and the turbo’s compressor outlet gets extremely hot, so heat shielding or active cooling of silicone hoses is necessary to prevent them from melting or catching fire. Monitoring exhaust gas temperature and boost pressure with gauges is essential for knowing whether the engine is running safely.
What Separates a Hobby Build From a Real Engine
A turbocharger conversion can produce a satisfying roar and visible thrust, but it operates at a tiny fraction of the power, efficiency, and reliability of a production engine. The gap comes down to materials, precision, and certification.
Production jet engines must pass blade containment tests mandated by federal regulations. Engineers deliberately fail the most critical compressor and turbine blades at maximum speed, and the engine casing must contain all the fragments without catching fire or breaking free from its mounts. The engine must continue running for at least 15 seconds after the failure (or shut itself down safely). This single test requirement drives enormous engineering effort into casing thickness, material selection, and structural analysis.
Additive manufacturing (industrial 3D printing) is beginning to change how some engine components are made. Silicon carbide composites are being produced using 3D printing techniques for turbine applications, allowing geometries that would be impossible with traditional casting. But these technologies remain in the domain of major manufacturers with specialized equipment.
A hobbyist turbocharger jet engine is a legitimate demonstration of the same thermodynamic principles powering a 747, built with accessible tools and materials. It won’t fly anything, but it will teach you more about how jet propulsion works than any textbook can.