Project που έκανα για την πλάκα μου προν 3 χρόνια. Ελπίζω να κινήσει κάποια στιγμή ένα kart και να μην ανατιναχτώ. Συγνώμη για τα αγγλικά, θα προσπαθήσω να γράψω εν συντομία τι γίνεται και πως λητουργεί.
How jet engines work:
A jet engine works on the principle of Sir Isaac Newton's third law of physics, i.e. for every action there is an equal and opposite re-action. The action of forcing gases out from the rear of the jet engine results in a re-active force in the opposite direction, and is commonly referred to as 'thrust'. This thrust is measured in Newtons (N) (or in older systems in pounds force (lbf ), kilograms force (kgf )).
In order to create thrust we need fuel, no matter if it is in gas or in liquid form, such as propane or kerosene. In order to burn fuel in a more efficient way and produce more gases for the thrust we use pressurized air, which is produced through forced inducing methods. In our case, this is going to be a turbocharger which was used to with a diesel engine. The expanding gases that are produced from the ignition of the fuel and the compressed air are used to create the engineΆs thrust.
Project description and goals:
The initial goal was to create a gas jet engine using a diesel truck turbocharger. This essentially entails the creation of a means to deliver and ignite fuel into a custom made combustion chamber, creating a high pressure oil delivery system for the turbo shaft and bearings, a thrust nozzle and all other associated equipment required to safely and reliably convert the turbo into a self sustaining turbine. We are still working on the reliability as the turbine sustains more than 80,000 RPM and more than 1100 C°. The lubrication cooling methods are an issue as the oil gets high temperatures and this can cause damage to the shaft of the turbine.
Turbo:
For this project we used the biggest turbo we could found. This turbo was initially used to induce air into a 12 liter V10 diesel engine of a truck. This turbocharger was taken out of the engine and was moved to our lab. When we obtained the turbo there was a film of oil and carbon sooth in the front, and while rust was noted on the back. After the turbo was dismantled the carbon and oil sooth and the rust were removed. This helped us to have a better view in order to inspect the condition of the shaft after opening the turbo housing. Then the shaft was balanced at 120,000 RPM.
Combustion Chamber, flame tube and fuel delivery:
A combustion chamber was totally constructed from the beginning by stainless steel (inox 304) tubes and foils, which were cut and then welded together. The combustion chamber was mounted on the turbocharger and then the compressor of the turbo was connected at the top of the combustion chamber. In Figure 1 bellow we can see the complete jet engine with its main parts attached together. The afterburner is not attached as we are studying the size of it and how it will be fitted.
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Figure 1: Complete view of the jet engine
Turbochargers work by harnessing the power of hot gases expanding out from the engine exhaust to move a turbine which in turn drives a compressor through a common shaft. The compressor pumps high pressure air into the engine which boosts its power output and makes more hot compressed gases available to the turbine. By attaching a combustion chamber to the turbine and feeding it with air from the turbo compressor the turbine is able to burn fuel and drive itself to extremely high RPM, being limited essentially only by how much fuel can be combusted in the chamber and how quickly the blades can spin before they catastrophically fail and fly apart.
Turbine combustion chambers are very intricate designs which involve a deep knowledge of thermodynamics, flame and combustion dynamics, heat transfer, materials science and fuel chemistry. Through computer modeling it is possible to optimize these factors so as to achieve very high performance in a package that is both lightweight and reliable. For our studying purposes, weight and reliability are secondary to price and ease of construction; as such, our challenge was to approximate the required parameters and adapt the desired design to fit existing materials.
The final combustion chamber design is welded from stainless steel (inox 304) pipes and flanges. The size of the combustion chamber was calculated from the beginning so as to be sufficient for the size of the turbine that would be used. Liquid propane gas is injected at the top of the combustion chamber through an injector (see Fig. 2) which was made for this purpose. The injector has a conical nozzle in order to increase the speed of the outgoing gas and prevent dangerous backfires. Finally, there are holes drilled so as the flame will not get extinguished until the right amount of the compressed air is driven into the combustion chamber.
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Figure 2: Fuel injector
At this stage, the most important consideration is the mixing of the compressed air with the fuel. Currently we use propane as fuel, so the compressed incoming air is easier to mix with the gaseous fuel. Furthermore, a swirl generator is attached inside the intake manifold in order to achieve better mixing of the air and the fuel. We can also notice the spark plugs (Fig. 3) that are placed inside the manifold, that can create a powerful spark and ignite the mixture.
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Figure 3a: Intake manifold
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Figure 3b: Inside the intake manifold
The flaming mixture is inserted at the center of the combustion chamber until it burns completely and is converted into expanded gases. The volume of those hot gases is bigger than the volume of the combustion chamber and thus forced outside to move the paddle-wheel of the turbine.
The structure of the combustion chamber is quite complicated. It was planned so that the fuel mixture is completely burned and transformed into gases. Otherwise, the unburned fuel mixture would escape the chamber and burn in the atmosphere without creating the appropriate work and pressure to move the turbine.
The cooling of the combustion chamber is very important, otherwise it would melt due to overheating. The combustion chamber consists of the main chamber, the flame tube and the gasket. The gasket is a flange which is mounded between the intake manifold and the main chamber. The fuel mixture comes through the center of the gasket and gets inside the flame tube. Though, cold air which is not burned with the fuel gets trough the ports that are opened on the gasket. The total surface of those ports is the 40% of the total surface of the central hole and the ports together. The cold air moves in the perimeter between the main chamber and the flame tube, keeping the temperature as low as possible.
The flame tube is drilled with holes of various sizes and distances between them. At the center of the flame tube there are seven rows of 12mm holes. The distance between them and their size is selected so as to maximize the flow. The firing mixture is inserted in the center of the tube and when it burns completely, expanding into high volume gases. Those expanded gases get trough the first set of holes and are mixed with the cold air between the main chamber and the flaming tube. Because of the momentum of the cold air the fumes are pushed to the lower part of the tube. The only way out is through the second set of bigger holes (20mm) of the flaming tube, as the bottom flange of the main chamber has no ports. Finally, the exhaust fumes pass trough the cone which is the exhaust manifold in our case. The velocity of the fumes is increased as their volume decreases. This helps the turbine to rev up easier and at a higher rate, since the fumes hit the paddle-wheel with higher momentum.
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Figure 4: Main chamber
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Figure 5: Flame tube
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Figure 6: Gasket
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Figure 7: Assembled combustion chamber without the intake manifold
Testing and development:
After all parts were gathered and assembled together we ignited the jet engine. The turbine gets up to speed quickly with a leaf blower held up to the intake. The ignition system is turned on and finally, using a fuel regulator we start the propane supply. The first time we did this the turbine spun up very quickly, reaching almost the half of its efficiency. After reguutf8g the fuel pressure we reached at a point on which the turbine was revving at a steady state, and with a short and conical shaped flame coming out of the exhaust.
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Figure 8: Starting up
Future research:
The future plans for this jet engine is to fabricate and fit an afterburner at the exhaust of the turbine. This will help us to increase more the thrust we extract. The size of the afterburner will be calculated so as to achieve better mixing and combustion of the compressed air with the fuel.
Finally, a temperature gauge will be fitted at the bottom of the combustion chamber to measure the temperature of the hot gasses and the lubricating system will be completed so as the shaft of the turbine will be not damaged.
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Figure 9a: Low boost
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Figure 9b: Full boost
