Using a Turbocharger for a Jet Engine
KKK-K26 Turbocharger
A turbocharger is used on internal combustion engines to increase the amount of air and consequently the amount of fuel that can be introduced into the engines cylinders and as a result increases the amount of power that can be produced for a given engine size.
Cross-Section Through a Typical Turbocharger
The turbocharger's compressor provides the pressurised air for the engines cylinders. The compressor wheel is driven by a turbine wheel via an interconnecting shaft. The turbine wheel is driven by the exhaust gases produced by the engine. The whole compressor/shaft/turbine rotating assembly is exactly the same setup as in a typical turbojet.
Flow Diagram for a Turbocharger in Normal Use
So, fortunately for us, a turbocharger already has two of the three major the elements that we need to build a turbojet, i.e. a compressor section and turbine section. The only difference between the turbocharger and a real commercial turbojet are the designs of the compressor and turbine wheels. In a commercial turbojet the wheels are designed to work 'axially' which means that the gases flow through the wheels along their axes of rotation.
Commercial Engine with Axial Wheels and Gas Flow
In a turbocharger, the wheels are designed to work 'radially' that is, the gases exit the compressor and enter the turbine in a radial direction, i.e. at right angles to their axis of rotation, which is the reason for the 'snail shell'-like shape to the housings. The reason for this is efficiency, radial compressors and turbines work more efficiently below a certain size, above this size axial compressors and turbines are used, but this is not an issue for us apart from one of design compactness.
Turbocharger with Radial Wheels and Gas Flow
The third element that we need to build our jet engine, requires us to build some form of suitable combustion chamber. A turbocharger, when bolted to an engine is almost behaving like a jet engine already, it provides compressed air to the engine's combustion chamber where fuel is burned, the resulting gases then being forced out of the chamber by the piston which spins the turbine wheel and hence driving the compressor. When we introduce a jet engine style combustion chamber we effectively replace the engine and it's cylinders for the burning of our fuel, turning the discrete 'suck, squeeze, bang, blow' cycle into a continuous one as in a real turbojet. The combustion chamber will essentially be a large can into which the fuel is sprayed and burned. The air from the turbocharger's compressor is fed in, fuel is added, burned and the resulting hot expanding gases exit the combustion chamber through a pipe connected to the inlet of the turbochargers turbine thereby completing the loop. Combustion chambers have been constructed using a variety of basic materials, built up from tubular steel or from modified fire extinguishers using mild or sometimes stainless steel for durability.
Because of the inherent design of the turbocharger ( radial inflow wheels as opposed to the more normal axial flow wheels ) and the fact that on most DIY jet engines we are using it 'as is', the combustion chamber needs to be constructed 'outside' of the turbocharger as a separate unit. This leads to the construction of a jet engine that is bulkier, heavier and far less efficient, thrust for thrust, than their more streamlined commercial brethren ( both full-size and model jets ) but is the price we have to pay in order to reduce complexity and cost to achieve a real working jet.
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Getting Started
Ok, so we know that we can use a turbocharger to build a jet engine, but how do we go about choosing the right turbo? What sort of thrust levels can we expect? How do I go about designing my combustion chamber? What other bits and pieces do we need to make it work such oil and fuel systems? Because of the numerous types of turbocharger out there and the varying levels of access to parts and materials that builders encounter, but moreover the fact that 'There Is More Than One Way To Do It..', there is no one definitive set of plans to go by. Instead, what is presented below are links to a set of guidelines ( 'Rules of Thumb' ) in Adobe Acrobat Reader format (.pdf) that have been an invaluable aid to myself and others to help get to grips with these problems and come up with working solutions. They were originally drawn up by Australian John Wallis a veteran DIY gas turbine builder and long time member of the DIYGasturbines Yahoo group. The 'Rules of Thumb' are reproduced here with his permission. You can see examples of John's (Racketmotorman) projects on the DIYGasturbines group and on Nick Haddocks website ( see Links below ).
The diagram below shows a typical layout of a DIY gas turbine and gives the names of the various parts ( click on diagram for a more detailed version ).
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Naming Conventions
| Abbreviations - Refer to these if you are unsure of certain terms |
| Rule of Thumb N.o. 1 - Choosing a Turbo |
| Rule of Thumb N.o. 2 - Oil Requirements |
| Rule of Thumb N.o. 3 - Combustion Chambers |
| Rule of Thumb N.o. 4 - Fuel Requirements |
| Rule of Thumb N.o. 5 - Ignition |
| Rule of Thumb N.o. 6 - Starters |
| Rule of Thumb N.o. 7 - Jet Pipes and Nozzles |
| Rule of Thumb N.o. 8 - Compressor Flows |
| Rule of Thumb N.o. 9 - Thrust |
| Rule of Thumb N.o. 10 - Fuel Consumption |
| Rule of Thumb N.o. 11 - Freepower Turbines |
| Rule of Thumb N.o. 12 - Afterburners |
| Rule of Thumb N.o. 13 - Evaporators |
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