Building your first gas turbine engine, the basics.
By Gary Packham
Disclaimer The information on this site is intended to give you an insight as to how a DIY gas turbine operates and is built. Our engines are not toys and need to be treated with respect. When built and operated within the safe limits set out by the members of the DIY Gas Turbine group, a safe sense of achievement will be gained. As always, playing with fire and fuel is dangerous, a responsible adult who is familiar with the operation of a gas turbine should always be present and most importantly a fire extinguisher should be at hand.
The first days, exploring the principles. I had lots of ideas in the beginning as to how to make my engine work, It was going to be the best "GT" because I could see things in a way that others couldn't. I started building some of my ideas, but at the end of the day its the adults in the group who have built these things for years that have all the answers, take my advice and take there advise, they are there to help, you only have to ask, it will save you a lot of time and effort. once you have built an engine that works, then you will have experience and you can tweak your engine using your own ideas.
Basics. You will need a turbo, oil pressure system, Ignition system, a fuel supply (gas is good to start with), a means to spin the turbo up for starting (a leaf blower) and you need to build a Combustion Chamber (CC).
Engine. My first engine was based on a Garrett T04 which I obtained from an engine out of a 7.5 ton lorry in a scrap yard. I made the CC simple, but easy to remove the Flame Tube (FT). The FT is the one part of the engine that will make the most difference to how or if your engine will run. The size of the CC is dependent on the turbo you are using, but it is a one way street. You can use a small CC with a small turbo but not with a big one. On the other hand you can run both a small or a large turbo with a large CC, the trick is to make the FT suit the turbo but make it easily changeable so you can up grade to a bigger turbo with minimal work involved latter on. The CC is basically a can with an outlet to suit the turbo, this can has a lid that houses the fuel delivery, spark plug and a pipe to measure the pressure in the CC. when bolted to the can, this lid will seal (not with sealant) the FT inside. The FT has holes around its length and is open at each end. It must be sealed in such a way to allow for expansion and only allow air to pass to the inside of the FT through the holes in its sides. There are three areas to the flame tube, Primary, secondary and tertiary. The inducer on your turbo (this is the part of your turbo where the cold air enters in) has a set area (area = pi x (r x r)). This area will equal the total area of the holes you will have in the FT, but is divided into the 3 areas as follows. 30% for primary, 20% for secondary and 50% for tertiary or dilution. For basic engine design use Jesses jet specs designer program. This program will give you a basic FT layout determined from the inducer size you put in. It is best to use stainless steel for the FT but 3mm mild steel will work, it just adds lots more weight.
The Principle. You first spin up the turbo using a leaf blower or air impingement, then, light a fire in the CC, inside the FT. This fire heats and expands the air. The expanded air has to go some were so it goes out to the turbine wheel. The hot gas going through the turbine blades causes the turbine wheel to turn, more gas more turns. The turbine is on a common shaft to the compressor wheel, so when the turbine turns the comp wheel turns at the same speed. The turning comp wheel compresses air into the CC where it must go through the holes in the FT. By adding more fuel to the extra air you get a bigger burn and more expanding air. The more hot air you can make the faster the turbine will turn. A small turbo might spin at 150,000 rpm or more where as a large turbo might spin at less than 80,000 rpm at max throttle. Idle or self sustain speed might be 30,000 rpm for both turbo's.
Flame Tube. After working out your FT size and either purchasing or rolling your own tube and having the hole position layout to hand, firstly mark the hole positions on to the FT and then drill them. To drill stainless you need to keep it cool with a cooling solution and use a sharp drill. SS will work harden especially if it gets hot, try to drill the size hole you want straight off (no pilot). If you roll your own tube, mark and drill the holes first but if you use a tube you will find it easier to support the tube on a square of wood with two battens either side to stop the tube rolling off and drill with a pillar drill.
Turbo. Normally on a turbo you can turn the comp and turbine scrolls (housings) on the core assembly. The core assembly must sit so that the small oil hole is at the top and the large oil hole is at the bottom. Oil enters the small hole under pressure at a minimum of 20 psi but 40 - 60 psi is good more is better. The oil will drain from the turbo with gravity so your reservoir should be under or nearly under your turbo. The drain pipe must not level out at any point but only go downwards. If your turbo has a waste gate it should be permanently held closed or blanked off. You will make a flange to mount the exhaust outlet of the turbo to, it is worth remembering at this point, that you will need to mount the turbo into a frame work. The flange you make, if made a bit larger, makes a good mounting point, (my TV81 with AB attached mounts ONLY with this flange). Most turbo's have a divided turbine scroll. This is not a problem if you protect the divider with a strip of SS. This way when the hot gasses travel into the scroll the divider will be protected by the SS. Without this protection the divider will melt causing molten metal to travel through the turbine and could cause a total critical failure. A critical failure could be critical to you if any turbine parts brake through the casing, they could be moving at 1450 feet/sec In English that is 988 MPH which is faster than a bullet. For this reason never stand in the rotational plane of a spinning turbo also never stand in front of the inducer when at speed, if the shaft brakes or the nut comes un done the comp wheel will try to exit the turbo in a forwards direction much like a prop on a plane.
Fuel. Propane will be your primary fuel even if you intend running on diesel but for your first engine just propane will be fine. Don't use a regulator at the bottle, you will need more gas than that. A straight through pipe is good with a needle valve to use as a throttle and a stop cock as a shut off should you have a problem. The gas jet can be a blanked off steel pipe or screw in cap with about 6 or 7 1mm holes drilled in it. These small holes will stop a flame from passing back up the pipe. You should make the jet removable from the CC for changing latter on, it should also sit near to the spark plug so that the gas can be easily lit.
Ignition. There are many ideas for an ignition system. A simple system is all you need, I use two auto relays and a capacitor. This gives me a good spark from my motorcycle coil at a rate of 4 sparks/second. You can increase or decrease the spark rate by increasing or decreasing the value of the capacitor.
Oil. I use a standard turbo engine oil. These oils are designed to work with engines that have turbo's, The oil will get very hot when you run the engine and you may need an oil cooler depending on how long you run it for. When you first start the engine the oil could be cold enough to stop you spinning the turbo fast enough for a start. There are two ways around this, the first you should have regardless and this is a bypass system. You take oil from your tank and through an oil cooler (if you have one) and into your pump (I use a Jabsco 12v domestic water pump like you would find on a boat or in a caravan) from here the oil goes through an oil filter to a "T" piece. The "T" piece goes one side to the turbo and the other side back to the tank. On the turbo side, all you need is a pressure gauge to tell you the oil pressure at the turbo. the oil is fed into the top of the turbo and will drain out the bottom. The tank side of the "T" will need a stop cock so you can stop the flow of oil from going back to the tank. By opening or closing the stop cock you will decrease or increase the pressure to the turbo. This allows you to lower the pressure for starting allowing faster spinning of the turbo. Once the engine is at sustain speed REMEMBER to bring the oil pressure up to 40 - 60 psi. The other alternative is to pre heat the oil. This method allows you to start with a higher oil pressure to start with and so is a safer procedure.
Starting. There is a knack to starting a home built gas turbine engine. All our engines are different because they are made in different ways. You can use forced air (leaf blower) or air impingement (air line) to spin up your turbine. You should have your oil pump and ignition on. You may have to reduce your oil pressure if you have trouble getting to self sustain speed but remember to bring it up again as soon as is possible. Self sustain speed is again different for each engine, but it is the point at which the engine will run without your chosen starting method but also at a suitable running temperature. Our engines run high TOT's (Turbine outlet temperature's) when started until the compressor wheel is turning fast enough to push an amount of air through the engine that is suitable for both combustion and cooling. The faster you run then engine the cooler it will run, up to a point where the compressor starts to lack in efficiency, then the TOT's will start to rise. The safe Turbine outlet temperature is 600'c. Once you have spun up your turbo to a decent speed start to open the fuel. You should here a pop as the propane ignites, the speed of the turbo will start to increase, so gradually continue to open the fuel. Avoid opening the fuel to fast, this will cause the flame to push forward into the turbine and exceed the safe max temps allowed, this is visible as a flame exiting the turbine and will not increase the speed of the turbo. If this happens just back of the fuel slightly until the turbo starts to pick up speed again. You should also avoid going to slowly as this also will give you high temp's. With practice you will manage to get it just right. Constantly check the oil pressure as an oil supply frailer could be fatal. A sudden increase in speed of the turbo for no apparent reason could be an indication of oil failure. You will soon learn the sound of your engine and will instinctively know if something is wrong, this also helps when tweaking the engine.
This should give you a running engine and allow you to gather data. Once you have learnt the way your engine starts and you are getting consistent readings you can start making small changes to improve the performance. I have built a few engines and my latest one is a Garrett TV81 which has a 79mm inducer. I use it with an after burner to power a sit on tractor, it has been named "Turbitrac" On its first test at one of the Bovingdon meetings it managed a healthy 35 mph in 3/10ths of an uphill mile. a longer runway would produce a higher top speed.