The Turbojet!

turbocharges you gotta love them [Music] [Music] in the first three videos in this series we covered the construction of a hybrid turbojet engine based on electric ducted fans as the compressor unit the first video was sort of a proof of concept in the next two videos produced a pretty successful engine we made about 37 pounds of thrust and burnt a prodigious amount of fuel today what I'm going to do is discuss the construction of a an automotive turbocharger based turbojet engine if you go into YouTube there are dozens of videos about this subject and if you go on google there are forums that will go into a great deal of detail what I'm gonna do tonight is I'm gonna synthesize that information and we're gonna talk about the principles the design of the assembly and the operation of one of these turbocharger turbo jets there are three major components to a turbocharger there's the compressor there's the center section that houses the bearing and there is the turbine now in both cases there are turbine blades rotating in these two ends of the turbocharger this is considered the compressor turbine this is considered the turbine turbine bit of jargon but nevertheless the same process is happening in both parts just in Reverse the single most important number in sizing any turbocharger is what's called the inducer diameter on the input side of any turbocharger you have an aperture that allows the air to flow the inducer diameter is measured at the narrowest part in the throat of this input this is what allows the air to flow in you want to be a little careful when you go on eBay or some surplus sites they'll quote sometimes the outer diameter of this tube to make it sound like you got a bigger turbocharger however this is the controlling factor in the induction of air this number can vary anywhere from about a 20 millimeter diameter all the way up to monsters at 145 millimeters the inducer will determine the amount of thrust you can make because it controls the amount of air and as a rough guideline if you do everything right and I'm sure we probably you can expect to get about 10 pounds of thrust per square inch of cross-sectional area in your inducer or eight Newton's per square centimeter so the smallest turbochargers might make eight or nine pounds of thrust and the largest can top 200 pounds of thrust is an enormous range the other side of the turbocharger on the exhaust side is called the X deucer and it's the output from the turbine in a normal automotive application they want as little back pressure into their exhaust system as possible so the X deucer tends to be a little bit bigger than the inducer on the turbocharger but in our case we're going to want to convert some of that pressure into kinetic energy or velocity and so we're gonna want to put a nozzle on the end of the excuse ER to cause acceleration of the gas because there are a lot of variables the RPMs that you're running out the fuels you're burning it may be difficult to calculate that but as a rough guideline you're gonna probably start with an X deucer at about 1.2 times the size of your inducer and you're probably gonna want to start a finish with a nozzle at about 0.9 of the diameter of your inducer but to really nail it one trick is that when you put your exhaust tube on here either have a way to connect a throwaway component or make it extra long and when you couple this on here and run the turbocharger up to its maximum settings you can then take some sort of a clamp or vise and compress this side to form an ellipse or an oblong shape as you do so you're reducing the cross-sectional area and therefore increasing the velocity as you continue to do this stepwise you'll reach a plateau where you've maximized the thrust at that point you can take this piece of steel off or take the whole exhaust tube off take it over a piece of graph paper put your funny shape down there trace the shape whatever that ends up being in this case it's a rough circle allow for the wall thickness count your squares PI R squared determine the cross-sectional area and you'll know the diameter of a round tube that would give you that same area and you have a nice form for doing the taper or you could simply live leave it as an oblong it looks funky but it'll operate just as well now beside the X deucer and exhaust side you have another important number which is called the AR ratio and that represents the relationship between the flow path the area within the spirals where the compression and the expansion are happening and the distance from the axis of rotation out to the centroid of that flow path centroid is just a fancy way of saying the center of the area in a circle it's the center of the circle in an oblong it's the center point where the mass is flowing evenly about it these are close to a circle and basically what that means is that with a large AR ratio meaning area relative to radius with a large ratio you have a rather compact turbo with a rather short distance around here it will produce the least back pressure and for a given inducer diameter will have the highest flow through your turbocharger it's best for the very high end applications like say drag racing or offshore racing where at very high airflow rates this will produce less restriction on the other hand because the radius in that case is small you have a low moment arm over which that air is forcing the turbine around developing torque for compression so with a very high error AR ratio it'll be more difficult to start the turbocharger because you're gonna have to get up to very high air flows before you get to a self-sustaining compression ratio it also won't work very well at partial throttle settings ar ratios can vary between about 0.5 and 1.5 and it would be relatively conservative to pick maybe 0.7 to 1.1 if you're starting out and you want something that's going to compromise on both of those extremes the third number is called trim and it's related to the relationship or ratio between the diameter of the inlet part and the outlet part of the turbine to the actual overall diameter of the turbine a small trim means you have a small area relative to the diameter and therefore you have a much larger turbine that is accelerating the air as you might imagine with a small trim you're gonna have more compression more speed in your turbine but you're also gonna have a lot more resistance to flow so just like the AR ratio a small AR ratio and a small trim will tend to favor compression over airflow and again you probably want to pick a moderate number somewhere around maybe 0.55 to 0.6 on on the spec sheets for the turbochargers to be somewhere in the middle range now the final issue is the bearing the center section that contains the bearing they can either be ball bearings or steel or ceramic if you want greater longevity and typically in the higher-end larger turbochargers they will use ball bearings they have higher load capabilities and they're more precision with all that additional rotating mass but in the least-expensive turbochargers and the smaller ones they use what are called journal bearings and basically if you have a rotating turbine like this there is a small brass or bronze insert with some flute flow holes that allow oil to pass from the housing tube into and through the bearing and actually float the bearing or the shaft inside of the tube this is called a hydrodynamic bearing and it's less expensive it's less precise but nevertheless it works very well for most applications especially in the small turbos in all cases you're going to need cooling oil whether it's ball or journal and the oil is important because in this case it floats in the other case with ball bearings of lubricates important issue about the oil though is that all bearings leak they can't be perfectly sealed and if the turbine is in good condition and the journal bearings are in good condition the amount of leakage is going to be small and it's simply going to be into the compressor or into the turbine and pedal its fuel so it'll burn just like your fuel will burn and as a consequence that's not going to hurt anything however if the pressure is inside of this chamber exceed the oil delivery pressure you can get retrograde flow into your bearings starve the bearings of oil and destroy them so it's important to make sure that the oil delivery pressure exceeds the pressure ratios that you're going to reach in the turbo you'll rarely see these creating at above 30 psi gauge and so you'll want an oil pressure that's somewhere above 40 psi gauge in order to make sure that you have a little bit of safety margin for the delivery of oil in this turbo though you'll see that there are four tubes three up here and then one sort of buried underneath it and that's because this is a water cooled bearing housing water cooling is available on both the small and the large turbos and in most of the cases that we're going to be using this for they're probably not necessary it's probably possible for you to simply plug that up and ignore it but the reason that they include water cooling and most of these turbochargers is because of the need to prevent a process called heat soap when you drive these engines and you park a car or a truck the exhaust has made the turbine side of the turbocharger extremely hot and because you've turned off the engine and you've turned off oil delivery what happens is the heat from the hot end will soak into the more thermally sensitive components in the turbocharger potentially destroying them coking the oil and destroying the bearings so what happens just like with aluminum engine blocks there will be a continuous flow of water through the electrical water pump and the electric radiator that will allow cool water to pass through both the engine block and the turbo housing to drain away some of that heat so that it doesn't damage these components in the high-end turbochargers however sometimes that water cooling is part of the design process to keep the center housing cool so you can take a chance if you have water cooling on the turbocharger you can elect to plug it or as I've done if it's included in here we incorporated it in here and so we have both water and oil cooling occurring now the most time-consuming part of this project is the part that you have to do which is the construction of the combustion chamber the combustion chamber if you go online you'll find a bunch of different designs but by and large the most common is a cylindrical containment vessel that is essentially a pressure vessel it's hooked up so that the compressed air out of the turbocharger is able to feed into fuel entry end of the combustion chamber and it's often mounted somewhat off-center on the combustion chamber to allow a vortex of rotating air to pass around the flame tube this provides a more homogeneous availability of air so that you don't end up getting a hot spots in your flame tube placing this off-center typically the rule of thumb is you want it about halfway between dead center and and where this side is actually parallel to the side we're about 30% of the way over but halfway is kind of what I seem most recommended and it's probably a very conservative number when you move this way out you start to get to very complex hole patterns for the welding and so that might be a little bit problematic the size of the combustion chamber is pretty much defined by the size of the burn tube the burn tube which fits inside the combustion chamber is surrounded by this annular gap for the air flow the length of the combustion chamber is determined by this length and you want to make sure that it's large enough in diameter that it has at least half the spacing as the diameter of your inducer so if you have a thirty millimeter in do so you want at least fifteen millimeters you could make it a Norma's but there for you but then you would add a lot of additional weight and expense there's no need to do that now one little thing I'd like to do is a little shout-out to agent Jay Z anybody who really loves turbochargers and turbo jets and jet engines of any kind it's a wonderful channel on YouTube and he's got about two hundred and fifty odd videos and I want to say thank you because he has a great tutorial on wire locking techniques and I went ahead and did that and I think I got a very good result just following his recommendations so just wanted to let him know and give him credit for the information that he provided us in any case I want to talk a little bit about the flame tube this is probably the single most complicated part of any turbocharger turbojet conversion there's a lot of variables that enter into this even the type of fuel you use and the holes are designed to apply air or get air into the tube and their spacing and their size are all based on chemistry one of the most important things to understand about the burning of the hydrocarbon fuels is that there is a magic number called the stoichiometric mix that is the mix where if you have an air-fuel mixture and it is and it reacts you do not have an excess of either component all the oxygen is used no extra fuel exists it's the perfect mixture it's the point at which the highest flame temperature is reached because you're not adding any additional unused components that cool the flame it's also the easiest flame to maintain or maintain a flame front it's the most stable the problem with the stoichiometric mix is that as you add additional fuel or additional air and it becomes more difficult to burn you reach a point called the flammability limits which you've gotten so much additional material in there that it won't burn at all and as a consequence you don't want to burn close to those limits but if you were to burn the fuels at the stoichiometric mix which is what you might think you want to do you're going to end up with flame temperatures in almost all the hydrocarbons that are around 2,000 degrees centigrade which would melt the turbine therefore you have to introduce some additional material cool it off you could use extra fuel but because you've got to carry that and you have to pay for it you'd want to use air and so we're gonna need to do is introduce far more air than the fuel needs to burn but we also don't want to add that initially in one big blast because we're gonna have a very difficult time maintaining a flame front and that's why we have the flow tube designed in such a way with these holes these holes are called the primary secondary and tertiary holes when the fuel exits the nozzle with a spark to initiate the flame in this end of the tube there's a large number of relatively small aperture holes that produce a lot of turbulent mixing of the fuel but a relatively low airflow rate down through the tube to give the flame time to burn as the flame continues to burn as it moves down here it's probably a little overly rich when you reach the secondary holes then what happens is any additional fuel that may remain can get additional air added and is less sensitive to blow out or to flame out because it's already so darn hot in here but the additional air ensures that you've got a complete burn which then continues down here and eventually that hot gas which is completely reacted then has the tertiary here holes pour another 50% of the air through them in order to cool the mixture before it enters the turbine and the blades exposed to the hot gases the sizing and the spacings of the holes black art but typically the diameter of the primary holes is 1/2 the secondary and the diameter of the secondary holes is 1/2 of the tertiary holes and whatever number of units you use to describe those holes if you add up all the cross-sectional area of all these holes they should equal the cross-sectional area of your inducer now the numbers of holes typically three times as many holes in the primary section as the secondary and as the tertiary section and there's a little bit of art to this and so as a consequence rather than follow the rule I just gave you the numbers of holes that we put in this particular unit which are two mill two and a half millimeters five millimeters and ten millimeters are about 90% of the cross-sectional area of the inducer the idea being that if we are a little bit wrong I'd rather add additional holes and to try to plug them all so when you buy this to buy enough that you can make a couple of them because you might mess up if it turns out that when the burn pattern occurs inside you take this too out after some use you find that you're not getting any heat damage or any heat effect until you get down to these holes you probably don't have enough holes up here and you're not giving the flame a lot of time to react before it gets down here so you probably have to add additional holes here if it turns out that your flame is all up here and as soon as you get down to about here everything down here appears to be cool then the reaction was forced to occur and only half the length of the flame tube you've probably got too many holes here and you probably don't have a complete so you can add and subtract holes you can vary the size of the holes in order to try to get a heat affected zone that begins here and continues nearly down to the tertiary holes now if you want to get Oh something I forgot to mention the dimensions on the flame tube are determined by the inducer typically you want a flame tube diameter that varies between 1 and 1/2 times and three times the inducer diameter so in the largest turbochargers you can be closer to the one and a half fold of the inducer diameter and in the smallest you want to be closer to three in this particular model it's at 2.4 the length of the flame tube should be six times the diameter of the inducer so six fold if you follow those rules you're probably going to have a successful build but again you may want to make some variations on the holes when you're sizing the turbocharger though to determine how you're going to get it as efficient as possible the single most important number is compression ratio and to illustrate that I'm going to talk a little bit about thermodynamics okay this is a pneumatic cylinder and if I plug the output from this cylinder and I compress the piston into the cylinder I'm doing work on the air within the cylinder as I compress it I've done work because it's force times distance even though the force varies during the compression nevertheless if you add up all of the little integrals of the full of the force times the distance I've done one unit of work let's just say that I compress the air to half its original volume I've doubled the pressure now if what I do is I allow us to expand and do work on my muscles I put one unit of work in and I've got one unit of work out the force and the distance was the same if however I compress the cylinder to the same point and then I place it in a plane and I the air within the cylinder if I bring the temperature of the air up to twice its original temperature so instead of 27 degrees centigrade or 300 degrees Kelvin I've run it up to 300 degrees centigrade or 600 degrees Kelvin what I've done is I've doubled the pressure inside the cylinder and so then when I allow it to do work against my muscles and expand I get two units of work out because at every point over that same distance the force against the piston is twice as great so one unit in and when you heat it two units out now if I do the same thing but I compress it even further and bring the pressure up to four times its original level and again put it inside the heat source and again double the temperature it's the same number of molecules of air so it takes the exact same amount of heat to bring it to that temperature but now the force again is twice what it was when I compressed it so over the same distance I have twice the amount of force doing work on my muscles as a consequence of pushing it in harder and investing more in the compression I get more of the energy of that same amount of heat back in energy or in back in use for work that's why when you invest more in the compression process you get more out in useful work and that's why diesel engines are more efficient than gas engines because they have a higher compression ratio maybe twenty to one instead of ten to one octane is a description of the of an a chain hydrocarbon but it's also an adjective that's used to describe the resistance of a fuel to pre detonation or ignition interestingly high octane gasoline actually contain less thermal energy than low octane gasoline but because they permit you to use an engine that has a higher compression ratio they'll actually give you higher efficiency in the conversion of that heat energy into useful work energy that is the reason why our original turbojet in the first series was so fuel inefficient because it had a compression ratio of only about one point three to one a typical RC turbojet may have a compression ratio of say three to one you can achieve about three to one in a turbocharger based turbojet so you're going to want to end up running the turbocharger very near its maximum setting to get the highest compression ratios however there's a limit to how hard you can run the turbine before it runs out of its tolerance limits and so if you want to get even higher compression ratios and higher efficiencies there is a way to do that and that's called series turbo charging effectively what you do is you take a second larger turbo that you connect up to this turbocharger in series so that the output from the compressor feeds the inducer of this compressor and then this compressor in turn compresses the air further into the combustion chamber so rather than getting a compression ratio of maybe two and a half to one you get a compression ratio of two and a half times two and a half to one or six and a quarter to one that much higher compression ratio adds to efficiency it also means that because you have a denser air burn in here you can still continue to use the relatively smaller combustion chamber in flame tube which saves a little bit on weight and compensates for the fact that you've added the weight of the second turbocharger in addition it's possible to bypass some of the air per stage and as our in trust stage I guess it would be to allow you to control the boost pressure over a broader range and give you more flexibility in the output at high efficiencies we'll get into that in more detail when we go to the next stage on this build and we're putting the second turbocharger in series with this one what I also want to cover is the support in order to keep the turbocharger running as I said we need cool oil input and oil output as well as the water flow that goes through these tubes if we go down below you'll see the systems that I've got up to set to do set up to do that the oil drains out of the system into this reservoir tank here at the bottom of that reservoir tank you'll see a valve or a small port that allows the oil to drain out of the reservoir tank and into the radiator where the oil passes through these fins past these fins and the fans cool the oil this then allows cooler oil to flow up here and into the inline fuel oil pump which supplies the pressurized oil here to the oil filter the oil filter then allows clean pressurized oil to flow up here and into the bypass assembly what this allows us to do is by opening and closing this valve we can control how much of the oil from the oil pump goes up into the turbocharger and how much is allowed back into the fluid loop to again reach the radiator and continue to circulate in a in a circle and as a result that allows us to have the additional flow beyond what the turbocharger may demand based on pressure to cool the in-line pump which requires a certain amount of minimum flow to keep from overheating finally you'll see this little module down here where the oil temperature and pressure gauges are located so we can monitor both of those on the device below this table in addition if you look in the back of the unit you'll see a water pump a radiator with a fan and a water reservoir and this simply circulates cool water to maintain the water cooling operation on the center section of the turbocharger finally below this table we have the Diagnostics located here the exhaust gas temperature reading that comes off next to the output nozzle near the edge juicer the boost pressure the oil temperature and oil pressure are all read on these gauges over here finally this is a frequency counter with these very small and very high speed turbochargers the turbine can rotate it as much as 330 thousand rpm and at that very high speed there is no way to measure the RPM with a contact method you need to use something optical and so what you do is you shine an LED or a laser in here and then with a photodiode you mount you measure the pulses that are coming off reflected off the blades that pulse frequency is then interpreted by the frequency counter into an RPM and that's important if you're going to be running the engine or the turbocharger near its limits to prevent it from over speeding now what I'm gonna do once we finish the video is I'm gonna go ahead assemble this into a complete unit then we're gonna go outside and in the next video I'm gonna run this thing up I'm gonna test it I'll also explain to you why we decided to build such a small turbocharger for this first build it's we've got some pretty interesting applications and we'll get into that in the next videos so at this point I want to say thank you very much for watching it's really appreciated if you subscribe because it does help out the channel a great deal in any case you have a wonderful new things tonight [Music]

Channel: Tech Ingredients

Views: 472,562

Rating: 4.9147563 out of 5

Keywords: Jet engine, Turbocharger, Combustion chamber, Combustion ratio, Flame tube, Turbine

Id: JzwfzgfJiJ4

Length: 27min 43sec (1663 seconds)

Published: Wed Mar 13 2019

Reddit Comments

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