The principle of how model turbine engines work or any turbine engine is very simple. They suck in lots of air, compress it, add fuel to the compressed air, and then ignite the compressed air/fuel mixture. Seeing that the air is so highly compressed, more fuel per volume of air ratios are possible - this is why turbine engines are so powerful.
Igniting this already highly compressed air fuel mixture causes it to expand very quickly and seeing that there is highly compressed air at the front of the engine, this hot expanding air takes the path of least resistance – out the back end of the engine.
As the hot expanding air exits the back of the engine, it is forced by the turbine blades. This causes the turbine to spin and since the turbine blades are connected directly to the compressor blades by means of a shaft – the compressor spins and the whole cycle starts over. In other words the entire process is self sustaining.
The more fuel that is added, the hotter and greater the expansion of gas will be, causing the turbine to turn even quicker, thus sucking in and compressing even more air. The end result is lots of hot compressed expanding air exiting the back of the engine – THRUST!
You might be scratching your head right now thinking... "I understand all that, but how do you get this self sustained process going. If you just add fuel into the combustion chamber and ignite it, the hot expanding air will exit both the front and back of the engine?" That is a great question and you are absolutely right. The engine has to be spinning to create high enough pressure at the front of the engine so the hot expanding air can only exit out the back and past the turbine.
For model jet engines, this "pre" spinning is accomplished by either blowing compressed air into the front of the engine by say a leaf blower, spinning the engine with an external high speed electric starter, or by installing a permanent high speed electric motor to the front of the engine.
As you can see from this cut-away view of a Wren turbine engine above, most of today's commercially available model turbine engines are of the centrifugal-flow type. These engines use a single large centrifugal compressor blade to "throw" accelerating air outwards into the the convergence (compression) zone of the engine. This design is light and provides good compression efficiency from a single compressor blade. Simple & light weight, with very few moving parts compared to an axial-flow compressor with multiple smaller compressor blades to achieve compression efficiency.
Here is good good write up on the
de Havilland Goblin
full size centrifugal-flow turbo jet engine. It is a good read if you want to understand more about centrifugal-flow turbines and see how similar the scaled down model variety are (there is a good cut-away photo). Other full size centrifugal-flow turbo jet engines include Rolls-Royce's Derwent and General Electric's Allison J33 which powered popular jet fighters of the day such as the Lockheed P80's, Sabb 21R, and the Fiat G80 to name a few.
The picture below shows a Wren electric start turbo jet engine. The pod sticking out the front of the turbine encases the high speed electric motor to get the turbine spinning. Also notice this engine has screening around the intake. This is to protect the model turbine engine from FOD (foreign object debris).
Once the model turbine engine is spinning, only then can fuel be added to the combustion chamber and ignited. Now the next thing to realize is that in order for the fuel to ignite, it has to enter the combustion chamber in a gaseous state, not liquid. This isn’t a problem once the combustion chamber is hot – the liquid jet fuel (kerosene) will vaporize as it flows through the combustion tubes and is introduced to the high temperature air. It is however a problem when starting a cold engine.
To solve this cold start issue two methods are now used. The most common method at this time is to use propane or a propane/isobutane mixture as the starting fuel source. This "starting gas" is already in vapor form (at atmospheric pressure) so a glow plug can ignite the air fuel mixture, this gets the engine started and warmed up.
Once warmed up the propane/isobutane is turned off by the electric gas valve and the kerosene is introduced as the primary fuel source by turning on the second valve.
The picture above/right shows the two electrically controlled starting gas and main fuel valves used in an auto start model turbine engine.
The other method of starting a model turbine engine is by using what is known as a "KEROSTART" system. This type of starting system does away with the starting gas and uses the main fuel source (jet A or kerosene) to start the engine. A small ceramic pre-heater/ignitor is used to partially vaporize the liquid kerosene and then uses a high voltage electronic sparker to ignite it. The advantage to kerostart is you don't need to have starting gas and it sounds more realistic (no "propane pop" on startup).
The draw backs are it usually adds a few hundred bucks to the turbine and in cold weather can be problematic to get started but they are improving. The ceramic heating element can also fail and it costs much more to replace than a $10.00 glow plug. Both systems work well so it depends on what you deem more important (saving a few bucks and dealing with starting gas, or having a more realistic sounding startup and spending a few more bucks).
Here is video that shows an autostart kerostart sequence... They usually start much quicker than this (10 to 15 seconds), but I'm sure since it's a brand new build, there are some fuel ramp parameters to be tweaked or the lines needed to be primed.
Now when model turbine engines were first introduced about 10 years ago, all this switching of fuel sources and getting the engine spinning to a self sustaining speed had to be done manually. Once started, only then would the
FADEC or ECU
handle the fairly simple task of controlling the fuel pump to adjust the speed of the engine and monitor engine temperature.
Today's model turbine engine ECU's control everything from starting to shut down, making model turbines much more reliable and safe. This type of complete on board starting is called "auto start". It costs more than a manual start system and adds more weight to the RC turbine helicopter or airplane, but makes starting a turbine engine very easy and safe - very few if any model turbine engines theses days use manual start systems.
Now the next hurdle is getting all this model jet engine power to spin the main and tail rotor blades on our RC heli –
our next topic