Homebuilt Gas Turbine FAQ

Back To my Gas Turbine Engine Project This document originaly published at: http://aardvark.co.nz/pjet/faq.htm It may be linked to or redistributed freely and without restriction. THE HOMEBUILT GAS-TURBINE ENGINE FAQ Foreword on safety 1. Why build a gas-turbine engine? 2. What tools/equipment are needed? 3. What materials will I need? 4. Choosing a turbocharger 5. Where do I get some plans? 6. What Is The Combustor? 7. Choice of fuel 8. Oil Systems 9. Ignition 10. Instrumentation 11. Funding/Sponsorship 12. Where can I find out more about gas turbine engines? Please note that this is a "work in progress" and as such it may be incomplete in some areas and pleading for input from YOU in others. A Foreword On Safety: Anything involving explosive gases, fluids, flames and/or rapidly moving pieces of metal is dangerous -- and gas turbine engines are no exception. The dangers associated with a gas-turbine engine are manifold and include: * fire risks Not only does a gas-turbine engine use an intense and very hot flame to generate its power, but parts of the engine also get extremely hot and can cause severe burns if not treated with respect. Also be aware that even fuels with very low volatility (such as kerosene/paraffin) will burst into flames and produce a significant flash/fireball if they come into direct contact with the hot parts of your engine. Don't even think about starting or running a gas turbine engine without a suitable fire extinguisher within easy reach. This generally means a foam, dry-powder or CO2 extinguisher -- the garden hose will be useless against a gas or liquid fuel fire. However, having said that, do make sure that you have a bucket of cold water available just in case you sustain burns of any kind. Burns should be treated by immediately immersing the affected body in cool (not icy) water. * ballistic risks The turbines inside an operating gas-turbine engine will be spinning at anywhere from 30,000 to 150,000 RPMs -- which means that the edge of the turbine wheels may well be reaching speeds in excess of 1,000 MPH (1,600KPH). This is faster than a bullet fired from a handgun so, should a critical failure occur, there is a very real risk to life and limb if you are in the way when it happens. A good rule of thumb is that you should never stand in line with the turbine disks or the end of the turbine shaft (at the exhaust or intake). While there have been only a few incidents reported where a turbine has suffered catastrophic failure and, to the best of my knowledge, nobody has been injured in such an event, the potential for severe injury is significant and should not be treated lightly. This is particularly true of larger turbocharger units with bigger turbine wheels. * hearing protection Make no mistake, these engines, even the smallest of them, are extremely loud and if you do not wear a good set of ear muffs (and optionally plugs) you *will* damage your hearing. If you find your ears ringing after any exposure to loud noise then damage has occurred -- don't risk it! * construction safety When building a gas-turbine engine you will inevitably be using powertools and other items capable of causing severe injury. Treat all tools with respect and always use an isolating transformer or residual current device where appropriate. When welding, always use protective eye and handware! Remember -- building and running these engines can be a lot of fun -- but not enough to lose an eye, a finger or a life for. * choice of operating environment Be aware that your engine will create a very loud noise and this means that if you're running it in a built-up area you are bound to attract the attention of neighbors and/or passers-by. Give the various dangers mentioned above, be sure to allow for the fact that many people will be extremely curious about what you're doing and want to take a close look. Chances are they won't realize just how hot those bits of metal might be -- nor will they realize how much at risk their hearing is. If you don't get on well with your neighbors you might also want to consider that they could be far less impressed with the results of your hard work than you are. However, don't be tempted to run your engine in an enclosed space -- there's a very real risk of setting fire to objects some distance away from the engine and a risk of explosion or asphyxiation from the gases produced. I doubt your household insurance will cover "destruction by homebuilt gas-turbine engine operation." You might also need to check your local city bylaws and fire ordinances just to make sure you're not going to get an expensive citation when you fire things up. If you're in a residential area -- get your neighbor's permission before you start up and perhaps wait until they're mowing the lawns or making lots of noise of their own. 1.Why build a gas-turbine engine? This is the question that DIY gas turbine engine builders are most often asked... "Why did you build it and what are you going to do with it?" The answers are many and varied, however I suspect most of us have built such an engine -- "because we can." Of course there's also a certain amount of prestige associated with not only having, but also having built, one's own jet engine. Most builders, at some stage or another, at least give passing thought to putting their engines to work powering a gokart, bike or some other vehicle, however few ever seem to get around to that. And of course... there's something obscenely satisfying about any piece of machinery that can make so much noise and heat. 2.What tools/equipment are needed? This varies widely -- depending on how sophisticated you want to make your engine. Some turbine builders have gone to extraordinary lengths to add such things as electronic engine management systems, strain-gauges to measure thrust, a dashboard full of dials to display every possible operating parameter, etc. Others have done little more than wire a few bathroom plumbing fixtures to an old and tired turbocharger then feed it with some fuel. Both approaches seem to produce results. However, if you're seriously interested in building one of these engines then you're probably going to need some fairly standard tools, including * a drill (electric preferred) * a vice and workbench * a hacksaw * screwdrivers, wrenches, etc * a range of files and, optionally (because you can always get some of the work done at an engineering workshop if you don't have these): * a welder (MIG, TIG or oxy for preference) * a pipe bender Not really a tool, but still an essential part of your inventory, will be a good strong leaf-blower which is the best/easiest way to start these engines. Of course if you have other gear such as a mill, drill-press, etc I'm sure you'll find ample opportunity to use it if you really want to. 3. What materials will I need? You'll need some lengths of pipe -- stainless steel is best but plain steel will suffice. If you're on a budget you can probably get away just using whatever is handy -- but remember those safety warnings!. "Alclad" exhaust tubing is a pretty good option -- it's an economically priced steel with a coating that protects it from rusting and corrosion and it's plenty strong for turbine engine use. It's much easier to work on your engine if you make a test trolley/rig into which it can be mounted. Some builders have welded these up out of 1" square-section steel tubing which is easy to work with and very economical. Once again however, you can use whatever is to hand and there's no reason why you can't just bolt a frame together out of steel, aluminum or whatever you find most convenient or available. In most cases you'll also need some flat-sheet steel of 1/8"-3/16" thickness for the top of the combustion chamber. A small amount of 1/4" steel sheet/bar will probably also be required to make up a flange to mate the combustion chamber to the turbocharger exhaust inlet. For such things as connecting the combustion chamber to the turbocharger compressor outlet you'll need some reinforced rubber hose of the right diameter. You can buy special turbocharger hose which is often a silicone type or you may find some reinforced rubberized tubing. Oil and fuel lines can be either steel, copper or reinforced rubberized hose (if the latter then make sure you're using a hose rated for oil, propane or fuel use as appropriate). Of course you'll also need miscellaneous nuts, bolts, cable-ties, etc -- and I highly recommend putting wheels on your test frame so that it can be easily wheeled out into a clear area for starting purposes. 4. Choosing a turbocharger There are so many different turbochargers of various types, sizes construction and design that it's impossible to give a list of all suitable types so here are a few notes to make the selection process easier. * Size: Unless you're very lucky you're probably going to end up with a turbo unit off a car. These are quite compact, fairly light and work well -- providing they're in reasonable condition. These smaller turbo units are also likely to be a good place to start because they're probably going to be cheaper -- and a lot safer than a bigger unit if something goes wrong. If the gods are smiling, you might just stumble on something bigger -- perhaps the turbo off a large diesel engine. Be aware that any damaged turbo can be little more than a grenade waiting to explode -- and the bigger the turbo, the bigger the potential bang. Even if you do find one of the much sought after giant-sized turbos -- you might want to consider keeping it under the bench until you've had the chance to play and learn with something a little smaller and safer first. * Condition The condition of your turbo will often be dependent (but not always) on the price you're paying -- but there are some things to check for when selecting a used turbo from the auto junkyard: Firstly make certain that the turbines aren't damaged. Although you can't see the edge of the turbine wheels without pulling the unit apart, any turbo that has obvious damage to the visible vanes should be discarded. These wheels will be spinning at up to 150K RPMs so you want them to be perfect! A damaged turbine wheel will almost certainly cause a balance problem and that spells big trouble and the risk of catastrophic failure. What's more, any stress that deforms a turbine blade beyond its elastic limits (ie: caused a blade to bend) will have created a weakness in the metal -- and at 100,000 RPMs that means it could easily tear itself apart in a very dramatic and dangerous fashion. Now check that the turbo turns easily. Most smaller turbos use hydrodynamic bearings (ie: no ball bearings) so they won't spin ultra-freely and keep rotating if you give them a quick flick on the compressor nut but neither should there be any obviously tight-spots when rotated through a full turn. Check also that there's no noise -- this could be a turbine wheel scraping on the housing or some foreign body inside -- both spell big trouble. Check the bearings for wear by trying to push the compressor end of the turbo shaft from side to side by gripping the nut on the compressor wheel. You should get some movement -- maybe 1/16 of an inch (1.5mm) or so but not too much more. These bearings are designed to have some slop in them to allow the oil to flow through so, unless you have a unit that uses ballbearings, if there's no movement it could mean that the bearings are galled (ie: damaged). Also check the bearings for axial play -- ie: push and pull on the compressor nut so as to push/pull the shaft in/out of the turbo housing. There should be virtually no perceptible movement in this direction or the thrust bearing may be worn -- and that's the start of a mortal decay which will kill your engine in pretty quick time. Take a look at how carboned up the exhaust turbine wheel and inlet housing is. Excessive, thick carbon build-up may indicate that the turbine is well-worn because the engine on which it was mounted was blowing smoke -- likely due to high miles. Likewise check the compressor outlet pipe (the one that comes from the edge of the compressor -- not the one that goes in the middle). If this has a thick coat of sticky black oil then the engine on which it was mounted was also probably quite tired with worn rings or valve-guides -- another hint that the turbo might also be nearing the end of its useful life. Of course you'll have to weigh up the condition of the unit against the price. The last turbo I bought wasn't perfect -- but I managed to haggle over the price because the compressor intake was heavily coated in oil sludge and I used that fact to convince the shop that the turbo itself was likely to be almost stuffed. I got it for half the asking price in the end -- and it works just fine ;-) Another thing to note when buying a second-hand unit from the auto wreckers is to make sure you get all the oil/water fittings. These are the pipes and special bolts that connect the turbo to the oil (and sometimes water) system. If you don't get these bits then it will be a complex and time-consuming job to make the equivalents. Of course, if you try really hard -- and sound convincing, you might be able to get a discount on the price of a used turbocharger if you tell the people you buy it from what you plan to use it for. Tell them that if they're interested you'll bring them in a video or drop by and demonstrate it to them -- that often creates a lot of interest and they bend over backwards to help out. Ask if they have a sticker or something you can put on the engine in return for the help they're giving. Don't underestimate the effect that such an offer can have on the price you pay and the service you get. If they won't let you dismantle the turbo before you buy it, you should make sure you do so before you use it in your engine and carefully inspect the wheels for any kind of damage. Remember, it's an engine you're trying to build, not a bomb. If you get on well with your supplier they should be willing to replace any unit that turns out to be damaged. 5. Where do I get some plans? Unfortunately, due to the fact that just about everyone ends up with a different make and model of turbocharger for the heart of their engine, there's often little hope of duplicating another user's engine by way of a set of plans. However, you'll be pleased to know that the basic design of these engines is so simple that you don't really need a set of plans -- just a little commonsense and an understanding of what the various components are meant to do. 6. What Is The "Combustor"? Outside of the turbocharger itself, this is the most important part of the engine. It's where the fuel and air are mixed in a controlled manner so as to create a lot of heat and massive expansion of the incoming airflow. While it is possible to simply use an empty tube as a combustor -- with the air coming in one end, the fuel being burnt and the hot gasses passing out the other, this is neither very efficient nor desirable from a reliability perspective. The combustor usually consists of two tubes -- one inside the other. The outside tube is the combustion chamber and the airflow from the turbo's compressor is fed into this chamber. Inside however, is another tube called the "flame tube." It is inside this inner tube that the fuel and air are mixed and burnt. This two-tube system offers several advantages which include: a) the flame is protected from being blown out/about by the incoming air. b) the flame tube is designed so that only some of the incoming air is used to burn the fuel -- the rest of the air is actually mixed with the hot air from the flame which reduces the temperature of the exhaust gasses that get fed into our turbocharger. This is important so that the turbo wheel doesn't overheat and melt. c) the flame-tube/combustor combination is designed so that the fuel is burnt in a well defined area. This is achieved by way of some carefully placed and spaced holes in the flame tube that force the fuel to be burnt in a tightly confined swirling mass -- rather than just being blown along so as to impinge on (and overheat) the exhaust turbine. d) because the flame-tube runs up the middle of the combustor, the outside temperature of the combustion chamber is much lower than would otherwise be the case. The outside of the combustion chamber is effectively insulated from the flame by a layer of cooler air entering from the turbo compressor. Building a good, effective, reliable combustor is, in theory, quite simple. However, in practice it is often a trial-and-error process that requires several attempts before one gets it "just right." Symptoms of a poor combustor design include: a) hard starting b) flameouts c) excessive exhaust gas temperature (EGT) d) surging and/or fluttering e) no power 7. Choice of fuel The good thing about gasturbine engines is that they'll run on just about anything that burns. This means gaseous and liquid fuels -- and in some cases even solid fuels such as wood and coal providing a gasifier is used. * LPG/Propane The easiest, and often most convenient fuel is LPG or propane. This fuel has the advantage that it requires no fuel pump, no special jetting and is fairly cheap and readily available. It's also very clean burning -- leaving no carbon deposits or lingering smells. The downsides of LPG/propane are that it can be hard to get adequate pressure out of a gas cylinder during the colder months and there's not quite as much energy in a given quantity of LPG as there is in most liquid fuel alternatives. * Liquid fuels Many engine builders use either kerosene or diesel. These fuels however, require the use of a high-pressure (100 PSI or more) pump and special atomizing nozzles -- both of which can be obtained from home-heating outlets. Liquid fuel packs a lot more energy than gaseous fuels but do create more smoke, smell and mess. Engines powered by liquid fuels also risk a phenomena called "runaway" -- which occurs if fuel accidentally puddles inside the combustion chamber and causes the engine to run faster than full throttle once started. At least one turbine builder I've heard from has had his one of the turbine-wheels on his turbocharger explode when this occurred (remember the safety issues!!). Other liquid fuels such as gasoline, alcohol, etc can also be used -- however the highly volatile nature of these fuels means that they do represent a significantly higher risk of fire or explosion if something goes wrong. Alcohol is particularly nasty in this regard because it burns with an invisible, odorless flame that can cause significant damage or injury before it is detected. 8. Oil Systems As mentioned earlier, most turbochargers (especially smaller ones for auto use) use hydrodynamic bearings. This is simply a fancy name for a regular bearing that has no balls or rollers but relies on a thin film of oil (at around 40-50PSI pressure) to keep the sliding surfaces apart. Obviously, when operating at speeds of 100K RPMs or more, it is essential that the oil-feed to these bearings be very reliable. Loss of oil pressure for even a second or two will spell instant damage (or destruction) for your turbo. It is also important that you use an oil which is capable of handling the immense demands that one of these engines place upon it. There are several factors which make fully synthetic oils the only option: a) temperature Because we're running these turbochargers at temperatures which are often significantly higher than they would normally encounter in their intended applications, the oil becomes extremely hot as it passes through. Regular mineral oils not only loose much of their viscosity under such extreme temperatures but can also break-down and cause clogging of the fine oil paths inside the bearings -- with dire consequences.

It's worth noting that in most turbochargers, the oil is considered part of the cooling system for the turbo-core (the bearings and shaft) assembly and even if your turbocharger has a water-jacket for additional cooling, the oil will still do most of the cooling work.

b) shearing forces Although I've been unable to obtain any definitive response from Mobil, I have been told that synthetic oils are much better able to stand up to the immense shear forces that occur inside a hydrodynamic bearing at 100K RPMs plus. Anecdotal reports suggest that these shear forces may cause conventional mineral oils to break down very quickly and loose much of their lubricating qualities. c) viscosity Conventional mineral oils tend to be thicker when cold and thinner when hot -- and neither of these are desirable qualities for DIY gas-turbine use. An oil that is thick when cold will make starting much harder (particularly in cooler climates), while an oil that becomes to thin when hot doesn't provide the protection required at high power/revs. Synthetic oils definitely have the best performance in this regard and maintain a more constant viscosity variation over a wider temperature range than conventional mineral oils. Mobil 1 5W50 or 0W40 oils have proven to be capable of handling the punishment that these engines dish out but other synthetics will probably also be fine. Just avoid the $4/gallon budget mineral oils they sell at the supermarkets if you value your engine! Also stay clear of those oil additives which are touted to provide extra protection. Things such as Slick 50 will do nothing but harm to your engine -- just stick to a good quality synthetic oil instead. Oil Pumps: Whatever oil pump you use, it (and the motor driving it) should be able to provide a sustained pressure of at least 50 PSI. Unfortunately, most modern auto engines have their oil pumps built into a machined recess in the engine block so are not easily converted for external use. Older engine designs such as the VW beetle, ford Escort and a few others have oil pumps suitable for mounting externally. These pumps can be driven by a 12VDC motor equipped with a suitable gearing or pulley system. Given that it's often desirable (or even necessary) to use slightly less oil pressure to get an engine started, a variable speed control on the oil-pump motor can be a big asset. The oil pressure can then be adjusted to suit the speed/power of the engine. Some have achieved this by using a variable-speed electric drill to drive the pump. Filters: It is absolutely essential that you have an oil filter in your system. The oil paths inside the turbo's bearings are so tiny that even the smallest fleck of rust or dirt could block them and produce major damage in the blink of an eye. A regular auto filter is fine and it doesn't hurt to use the biggest you can find. Oil Coolers: There's no avoiding the fact that as the oil passes through the turbo bearings it gets hot -- very, very hot. For best performance the oil should operate at a temperature somewhere between 50 and 100 degrees C (120-212 deg F). Unfortunately, when using a turbocharger as the heart of a gas-turbine engine, it's very hard to keep the oil cool enough without some kind of oil cooler. It should be remembered that in most turbochargers, the oil is used not only to lubricate the bearings but also to provide much-needed cooling. Even those turbochargers that also have a water-jacket will still heat the oil to very high temperatures. An oil cooler is less important if you have a large-capacity oil tank (a gallon or so) and run your engine only for short (2-3 minutes) periods. If you want to engage in sustained operation, an oil cooler is *essential*. 9. Ignition To start your engine you will need a source of ignition and, in most cases, a regular auto spark plug will do the trick quite nicely when mated up with an auto ignition coil and a simple electronic circuit. A suitable circuit to create a good spark can be built for just a few dollars by those with some experience with electronics -- and if anyone wants a ready-built unit for a nominal cost they can contact me. The ignition source is only required to start the combustion process and can be turned off once the engine is running. 10. Instrumentation As mentioned earlier, the level of instrumentation used by DIY gas-turbine builders varies immensely -- however there are at least three basic measurements you should be able to watch: * Oil pressure Since the turbocharger internals are so very dependent on a good strong supply of oil at 40-50 PSI, it is very important that you're able to monitor the pressure accurately. A regular auto-gauge works fine for this application and costs very little. * Exhaust Gas Temperature (EGT) The life (and very survival) of the exhaust turbine is very much dependent on ensuring that it doesn't overheat. The only way to ensure you don't overheat the turbine is to keep an eye on the temperature of the gases coming out of the exhaust. While you can invest in a purpose-built "pyrometer" (hi-temperature thermometer), you'll find that many relatively low-cost digital multimeters have a temperature facility and require only the addition of a suitable probe rated to at least 1,200 degrees C (1,900 degrees F). Knowing the EGT of your engine can provide many valuable clues as to exactly how it's performing. * Compressor pressure The amount of pressure being produced by the turbocharger compressor is a very good indicator of the amount of power your engine is producing and is a valuable aid to starting. Most engines will need to be producing a certain amount of compressor output before they will sustain (keep running) so having this gauge helps you to know when to remove your starter (the leaf blower). Just a regular compressed air gauge (0-35 or 40PSI) will do the job fine. On one of these engines you're unlikely (ill-advised) to use more than about 30 PSI of compressor boost or you risk over-speeding the turbine wheels. Other gauges that are 'nice to have' but not essential are: * tachometer It's nice to know how many RPMs your engine is doing -- even if just for the "shock factor." There aren't many "off the shelf" options for a tachometer that will handle this many RPMs but many builders have used digital multimeters with an in-built frequency counter to do the job. This method often requires the addition of a simple photo-transistor and amplifier circuit [reference to circuit diagram goes here] or if you'd like to use a re-calibrated analog meter from an auto-tachometer a circuit for building your own tach will be added to this guide shortly. * oil temperature As mentioned previously, the oil one of these engines gets very hot so it's nice to be able to keep a watchful eye on just how hot it is. * fuel pressure Particularly useful if you're using a liquid fuel, this is effectively your throttle setting. 11. Funding/Sponsorship Once you get your engine going it will likely attract a lot of attention -- both positive and negative. Avoid the negative by being considerate about when and where you make all the noise and disturbance that these engines can generate -- and take advantage of the positive interest that can be created. You might be very surprised at the levels of interest that will be expressed by the companies you've had to purchase parts from during the construction of your engine. When you're buying the bits to build your engine, tell the people you're dealing with what you're doing -- and promise to bring them a video or to demonstrate the engine to them when you're done. You may be very pleasantly surprised at the response you get when you follow up on that promise. Once they've seen that your engine runs you might also want to offer them a chance to provide some sponsorship by way of free products or a good discount on future purchases. It will help if you can organize some publicity by way of the local newspaper or TV station So far I've managed to pick up some free synthetic oil sponsorship from Amsoil and have been getting my turbochargers at a cut-rate by making a promotional video featuring my engine for the auto-parts store where I buy them. My engine and I also featured as a three-page article in the July 2000 New Zealand edition of the PC World magazine under the heading "Rocket Man" -- this all helps attract interest and sponsorship. Another builder, Mark Nye, was featured along with mention of his homebuilt gas turbines, in an article by the New York Times. It's available online at: http://www.nytimes.com/library/tech/00/06/biztech/technology/07poll3.html. 12. Where can I find out more about gas turbine engines? Check out the Links Page on this site. It has a list of other sites that contain a wealth of information on the theory and practice of gas turbine engines. The links page also has links to many websites created by other gas turbine engine builders. These are a tremendous resource for anyone contemplating building their own engine and I highly recommend you visit them all. Also check out the eGroups mailing list linked from the Links Page. As of 9 September 2000, this mailing list has some 67 members, including a number of people with a tremendous amount of knowledge who are more than willing to answer the questions of newbies or those having problems. A Work In Progress As with all good FAQs, this document is a work in progress and will be added to as new questions and suggestions come along.

Back To my Gas Turbine Engine Project