jag och en till från jobbet funderar på om vi skall testa att bygga en simpel jetturbin utav ett turboaggregat, har du några enkla tips för att vi skall lyckas?
hur räknar man ut förbränningskammarens mått/volym?
vad är lämpligast drivmedel, diesel? krävs förvärmning?
hur bör utblåset vara format?
Här får du lite att bita i, säg till när du vill gå djupare i teorierna så letar jag fram mer.
Rules Of Thumb - By John (racketmotorman) Wallis
Finding a suitable turbo
Find a turbo your budget can easily cope with, as you'll be spending a fair bit more on ancillaries. Try not to use too small a turbo as their turbine efficiencies can be so low as to make the engine a poor performer at the least or a non goer at the worst, as well small turbo's can have missmatched comp-turbine flow characteristics for our use, but which are OK for a turbo, try using a turbo from at least a 3 litre engine or bigger, a compressor wheelinlet dia ( NOT housing) 1.75-2" in dia or bigger.
If possible find one designed for high pressure "boost", something off a large generator or industrial stationary engine thats setup for continuous heavy boost, if you can view the entire compressor wheel with its housing removed, a compressor wheel that has its inlet dia around 2/3rds of overall diameteris a good possibility, eg 3" inlet 4.5" overall (outlet) dia, modern ones will probably have a bit better efficiency and hopefully a little more safety built into it, it should spin freely with no axial play (good thrust bearing), but there will be radial wobble if you grab the end of the shaft at either compressor or turbine, as long as neither compressor or turbine scrape against the housings without oil in the "brass bush" bearings, they should be right when there is oil pressure to them, check for bent or damaged vanes, reject the turbo if any signs of damage, as you will be running it harder than on any engines it might have come off.
Its turbine housing should be the "open" variety with no centre divide where the hot gases go in, the divide causes a loss in efficiency because of all that extra friction, and you'll end up with less thrust from your engine if you use a
divided housing.
To find max RPM of your chosen turbo, that corresponds to the groups accepted maximum compressor tip speed of 1450 feet per second, it can be worked out by measuring the outlet diameter of the compressor in inches multiply by "pi"(3.14) to get circumference in inches , divide that by 12 to get the circumference in feet, divide that into 1450, to get RPM/sec then multiply by 60 for RPM. eg. our compressor is 4.5 inches overall so 4.5 " X 3.14 =14.13" div by 12 = 1.1775 ft circumferance 1450 ft/sec div by 1.1775 = 1231.4 rps X 60 =73,884 RPM max.
Lubrication
The turbo will require good quality filtered oil at a pressure at least as high as it saw when on its original engine, pressure is measured from a pickup point in a tee piece at the turbo's oil inlet so as to get the correct reading.
Metal piping MUST be used near the turbine housing as it gets VERY hot and the radiant heat will degrade the strength of any other material, hot oil spraying on a "red hot" turbine housing and you've got big troubles.
The drain back to tank must have a continuous down hill slope and dump into the tank above the oil level, its inside diameter must be at least as large as the outlet hole from the centre housing of the turbo, the central housings drain hole can be positioned up to 30 degrees either side of the vertically down position.
Use 50 psi as a minimum pressure when running at high compressor discharge pressures ( P2's), sometimes a method of reducing pressure when starting is required if starter power is insufficient to get to self sustain speeds with full cold oil pressure, oil pressures of a 100psi can be run on large turbos to help the thrust bearing survive when running high P2's.
Various auto oil pumps with attached filters as well as power steering pumps have been used to provide oil supply, there are some 12V pumps available that are compatable with oil. Pumps need to have the capacity to not only suppy the pressure with hot oil but at least half a gallon per minute for a small turbo and at least one gallon /min for a larger one, the oil not only lubricates but cools the turbo as well. The oil need only be a type suitable for the engine the turbo came off, multigrade auto oils suitable for turbocharged engines should be good enough, just don't let it get too hot,somewhere around 65 C going into the turbo, so have a decent capacity oil tank or install an oil cooler for extended runs.
Combustor
If you want to construct a workable combustor that makes allowances for less than perfect air and fuel presentation prior to combustion taking place, these dimensions "should" provide some rough guidance.
"they ain't perfect but they're better than nuthin' "
)
To get the dimension of the flametube, measure the diameter of the compressor wheels inducer (bent fins plus hub at the front), where the air gets into the compressor wheel, multiply that diameter by 3 times for small turbo's and twice for large ones , and use that as the diameter of your flametube , for length of flametube, 6 times the compressor wheels inducer diameter for small or big turbo's.
For example a small turbo of 2 inch dia inducer, the flametube would be,2inch X 3 = 6 inch dia flametube, 2 inch X6 =12 inch long flametube,and for a big turbo with a 3.5 inch dia inducer the flametube would be,3.5 X 2 = 7 inch dia flametube, 3.5 X 6 = 21 inch long.
To get the dimensions of the flametube hole area , first measure the area of the compressor wheels inducer (the bent fins at the front), once you have the area, divide it into the various quantities to represent the three zones in the combustion process, namely primary, secondary and tertiary, the primary zone will require 30% of the compressor wheel inlet area for its holes, the secondary zone, 20% of the comps inlet area ,and the tertiary or main dilution zone the remaining 50%.
For example, if the compressor wheels inlet area is 10 square inches, 3 sq ins will be the total area of the holes around the fuel delivery end of flametube for about 1/4- 1/3 rd of its length, 2 sq ins would represent the area of holes
positioned about half way down the flametube and 5 sq ins would be the area of the main dilution holes at 3/4 length.
Use smaller holes in the primary zone, bigger in the secondary and reasonably large holes in the main dilution area, eg 1/4 - 3/8 - 5/8 inch dia if for a larger size turbo.
The outer combustor housing only needs to have 1/2 - 3/4 inch air gap between itself and the flametube for air to get to the flametube holes, a little extra clearance won't hurt though.
Smaller length and diameter combustors might work with propane, but kero doesn't burn as fast or is as forgiving of mixture strength variations as propane.
For a small turbo, your BBQ gas bottle will suffice for even full power runs, with a bigger turbo the BBQ bottle will be OK for starting and possibly lower power experimenting, but for higher fuel usage runs, the bottle's temperature
will plummet along with the fuel flow from it as the pressure inside drops, inverting the bottle and running liquid gas from the bottle can work but with added dangers, BEWARE.
Kerosine - Jet fuel, smells right and is possibly the best for bigger turbos, but will require more complicated fuel delivery than the BBQ bottle, for which a simple multi opening delivery tube to distribute the propane equally and
radially around inside the flametube will do. Spray nozzles suitable for kero are available from specialist suppliers of spraying equipment. Fuel pressure when using kero can be up to 1,000psi with some systems or around 250psi with others, all depends on how fine one wants to atomise the fuel and the nozzles used, vapourizing systems will use even lower fuel delivery pressures.
Small gear type pumps are used to get the pressures required, driven by mains power, 12V or even IC engines. The simplest fuel system is a variable speed pump to supply the fuel to the engine, then comes a constant speed pump delivering fuel that is "dumped" back to the tank thru a valve on the line to the spray nozzle, closing off the dump line forces more fuel to the spray nozzle increasing temperatures in the combustor and accelerating the engine rpm to a
higher power setting ( more thrust), even more complex systems with pressure differential valves can be used to get a "smoother" more linear delivery of fuel.
Ignition
The humble auto sparkplug is well up to the job of igniting propane and even atomized kero most of the time,the plug gap can be opened right up or the earth can be removed entirely, as long as the spark can jump the gap when tested prior to installation things shouldn't change much when its inside the combustor during a start, the air pressure inside the combustor is only just above ambient, unlike the situation inside the piston engine where the pressure at ignition can be very high, reducing the ability of the spark to jump the gap.
There are circuit diagrams on site for different ignition systems. Glow plugs might also be used in place of sparkplugs, and could be a better bet for kero ignition. Propane ignited by a spark plug as a "pilot light" or "torch igniter" for the subsequent reliable ignition of the main fuel, kero, is also used. Ignition points, either spark plug or glow plug need to be positioned so that start fuel and air come in contact with them. Propane is heavier than air so it will fall to the lower parts of the flametube if it is only "dribbled" in at start, so plugs may need to be situated there.
Once the fires alight, the ignition source can be deactivated for the remainder of the engine run.
Starting the engine
The garden leaf blower is a good unit for spinning up the engine at start, just "plug" it into the turbo inlet and force air thru the compressor, combustor and turbine, probably better on larger turbos as the smaller ones may be a bit restrictive and cause flow problems for the leafblower.
Smaller turbos might better be started with the exhaust from a domestic vacuum cleaner. Very high pressure air blown across the compressor blades is another start method.
Mechanical connection of a very high speed electric motor to the compressor nut
by means of a socket fitted to the electric motor to spool the engine up works as well, once up to speed just pull the starter away.
There could be "dynamics" problems concerning the shafts stability with anything other than the "leaf blower" type start method. Leaf blowers are also used to blow cooling air thru the engine at shutdown in conjunction with the oil supply continuing, to cool the turbo down and stop heat soakback into the bearings from causing coking problems, unlike in the turbos original use where cooldown is achieved by idling the engine for a few minutes when shutting down from high power and heat settings, we shut our GT engines down from relatively high temps and rpm, even at idle.
Jet pipe
Jet pipe construction is important in maximising as much of the pressure left after the turbine has taken its share from the pressure produced by the compressor. Idealy the jetpipe shouldn't detrimentally disturb the gas flow on its way from turbine to jet nozzle.
Gas pressure and velocity exit profiles from the turbine are a real mixed up mess and need some time to settle out, so a certain length of jet pipe is required for this to happen ,8-9 inches should be enough length, if its too long the extra frictional losses will reduce thrust, so "not too short and not too long but just right "
)
Jet pipes need to have their inside diameter the same diameter as the turbine housing at the turbine exducer, NO step outs or flairs , the same size from turbine wheel to jet nozzle, because every time there is a change in cross sectional area there are losses. Place TOT gauge and total pressure gauge pitot tube at the downstream end of the jetpipe just before the jet nozzle starts to restrict things, that way you will have the best and hopefully most accurate readings, for working out jet nozzle sizes , tuning etc.
Straightening vanes in the jet pipe to remove any swirl in the gases exiting the turbine are something that can be experimented with, but the need for them will depend on whether there is swirl produced by the turbine, if vanes are inserted in the jetpipe place them well downstream from the turbine, and only weld their extreme upstream end to the jetpipe, otherwise expansion warpage can be a problem.
Jet nozzles convert any left over pressure coming out of the turbine, ( after the pressure drop required to power the compressor has been absorbed by the turbine ), into jet velocity. For there to be any leftover pressure, the compressor and turbine have to be running efficiently and there has to be a certain temperature rise in the combustor. When running a DIY engine without a jet nozzle, that is with no leftover pressure downstream of the turbine, our combustion temperature rise is modest maybe only 3-400 deg C but when the jet nozzle is installed the combustion temperature rise will have to be around 600 deg C at the same max comp discharge pressure (P2), with a big increase in fuel burnt.
Jet nozzle sizing will depend on the "leftover" pressure as well as the gas temperature and the mass flow of air going thru the engine. Because we are limited by the max temp that the turbine can take and stay together ( we don't want bits flying off )our turbine inlet temperature ( T I T ) is limited to 1450 F -788degC which will drop to a 1200 F - 650C or less, turbine outlet temperature ( TOT), this temperature will continue to drop as the gases go thru the jet nozzle and the leftover pressure increases the gas velocity as its static pressure drops to ambient.
As long as the temperature measured ( TOT) in the jet pipe is below that 1200 F, the jet nozzle isn't too small, if the jet nozzle is too large for the mass flow ,TOT should be less than the 1200 .A jet nozzle about the same diameter as the compressor wheels inlet is somewhere to start from,there are too many variables to give a definite sizing,
Its better to start with an oversized jet nozzle with low TOT's and thrust, than one too small, with TOT's climbing too high, before max rpm are reached, with the possibility of doing damage to the turbine or having surge problems from the compressor because the flow has been restricted.
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