Air is around 21% Oxygen. The bulk of air is non-flammable Nitrogen. Wouldn't surrounding the jet fuel with only O2 create more efficient and clean combustion?
$\begingroup$
$\endgroup$
Add a comment
|
4 Answers
$\begingroup$
$\endgroup$
7
Vehicles which carry their oxidizer with them are called rockets. It is a good thing that air is not pure $O_2$, because the nitrogen, being employed as a process gas, helps to get useful work from the combustion process. By being heated and accelerated in the combustor, it does the main work of driving the turbine and turning heat into mechanical work. Engines would be even cleaner if less oxygen is around, because already at 21% $O_2$ they run at fuel-to-air ratios well below the stoichiometric optimum.
If you consider the amount of air flowing through the core of a modern turbofan, you will see that the amount of oxygen which needs to be available will dwarf the amount of fuel, and most of it would not burn but would replace nitrogen in its role as the process gas. It could not be carried with the airplane nor isolated from surrounding air if any meaningful payload and range should remain.
For some background on the combustion process, please see this answer.
answered Dec 7, 2014 at 17:57
-
1$\begingroup$ Would high bypass turbofans or turbojets be more happy to breathe pure oxygen? $\endgroup$ Dec 8, 2014 at 14:52
-
$\begingroup$ For the same reason, some drivers in some races where the rules allow for have nitrogen boosters for their cars. $\endgroup$ Dec 8, 2014 at 20:04
-
8$\begingroup$ @ChristianHujer: Isn't this nitrous oxide, a compound of nitrogen and oxygen? It helps to boost performance when injected into the intake air of piston engines, because it contains more oxygen per gram than air and is a metastable compound, so its dissociation at 600°C sets free some more energy. Piston engines are different animals than jets; they profit from having oxygen-enriched air. $\endgroup$ Dec 8, 2014 at 20:59
-
2$\begingroup$ @shortstheory: No, they would run too hot. Please read the answer to this question for more details. $\endgroup$ Dec 8, 2014 at 21:02
-
$\begingroup$ @PeterKämpf Oh, you're right, I'm wrong. Thanks for pointing it out, and I just learned something new. $\endgroup$ Dec 9, 2014 at 2:38
$\begingroup$
$\endgroup$
3
Then the weight of the oxidizer would weight down the aircraft. For each kilogram of pure octane (one of the components of jet fuel) you would need around 1.7 kilogram of pure O2. In other words it would double the weight of the fuel you have to bring.
Also bringing your own oxidizer is what happens with rockets, it does provide higher thrust and can work where no oxygen is available in the environment like in space.
answered Dec 7, 2014 at 17:57
-
1$\begingroup$ I am not sure saying rockets have "higher thrust" make sense. The specific thrust is certainly lower (oxidizer now counts) and the total thrust depends on the engine. The other advantage of rockets, besides being able to operate in space, is that they can operate at hypersonic speeds while turbojets can't and scramjets are still mostly experimental. $\endgroup$ Dec 7, 2014 at 22:34
-
1$\begingroup$ It would nearly triple the amount of fuel: 1 kg of fuel plus 1.7 kg of O2 = 2.7kg, almost triple the original 1kg! $\endgroup$ Dec 8, 2014 at 10:00
-
$\begingroup$ Isn't O2 kind of a pain to carry, too? Wouldn't it either occupy a large volume or require either very high pressure or cryogenic storage? Rockets tend to opt for the latter solution, as far as I know. $\endgroup$– reirabDec 8, 2014 at 17:57
$\begingroup$
$\endgroup$
6
No.
As Ratchet freak has already mentioned, carrying oxygen would weigh an aircraft down.
As Peter has mentioned, a jet engine pumps air and burns some of the oxygen, not the full 21% available. Often it burns less than half of the available oxygen.
The reason for this is that the "adiabatic" (that means, assuming no heat transfer) flame temperature for complete combustion of all the oxygen in air is about 2000C. Add to that the fact that the compressor can preheat the incoming air up to 500C in some cases, and you would be looking at a temperature at the exit of the combustion chamber of 2500C.
A jet engine that could use all the oxygen in the incoming air would be more efficient, in accordance with the principles of the Carnot cycle, but unless built of "unobtanium" it would melt and fall apart.
A jet engine needs a turbine to do two things. The first is to drive its own compressor. The second is to drive a load (either a propellor or fan.) The turbine+prop/fan is needed because the exit velocity of the exhaust gases would be too high to be used efficiently for generating thrust. Hence putting a turbine in the exhaust path is unavoidable
A rocket has no turbine in the exhaust path, and as a result can run at the extremely high temperatures generated by burning oxygen. It also has extremely high exhaust exit velocities, which are hopelessly inefficient at the kind of vehicle velocities required for general aviation, but are excellent for space travel, where the more energy you can impart to each ton of propellant, the less propellant you have to carry. Also most rockets to date have been single use, unlike jet engines.
So we are stuck with jet engines which are unable to use all the oxygen in the fuel, due to materials issues. One way of boosting the power of a jet engine (but not the efficiency) is to use an afterburner. This is effectively a second combustion chamber, behind the turbine, where more fuel can be burnt. This creates a huge amount more energy, but comparatively little extra thrust. Remember kinetic energy=0.5mv^2 while momentum=mv. You need to match the exhaust velocity to the vehicle velocity for efficient thrust generation, and an afterburner does not do this, as its purpose is not efficiency, just a quick and dirty power boost.
There are some designs on the drawing board for the very large gas turbines in power stations, where the gas from the turbine, having been cooled by expansion, is sent to a second combustion chamber and then through a second turbine, to enable a higher average heat-addition temperature and improve efficiency. However the benefits are small and as far as I know it has not been tried yet. I suspect it will always be too complex for aviation.
More viable than oxygen would be water injection. Water/steam is now being used for cooling of industrial gas turbines, which reduces the amount of air that must be pumped by the compressor, improving efficiency. However, you would still have to carry it. Another option (also borrowed from industrial gas turbines) would be to put a heat exchanger in the exhaust gas flow, in order to generate steam for a steam-turbine driven propellor or fan. However, I don't expect to see these tried until all the oil in the world has nearly run out!
-
2$\begingroup$ I find afterburners to be such a disappointment! $\endgroup$– MichaelDec 8, 2014 at 15:59
-
$\begingroup$ Are expansion turbines in power generation stations really only "experimental"? I would have expected that in fixed generation plants such things would be common. In an aircraft, the extra weight of expansion stages would likely increase the amount of mechanical energy required to overcome drag more than it would increase the amount of energy extracted per unit of fuel, but in a fixed plant that would not be an issue. $\endgroup$– supercatDec 8, 2014 at 16:49
-
$\begingroup$ @Supercat my experience (industrial, not aviation) is from the heat recovery steam generator side (and more in industrial sizes of tens of MW rather than 100's of MW, but I did get seconded to the utility part of my company for one project.) When you say "expansion turbines" I'm not sure if we've understood each other. Multi-shaft gas turbines are common but not universal. What I was saying is that to have compressor - combustion chamber - turbine - 2nd combustion chamber - 2nd turbine in the flue gas path appeared in a brochure (I believe by GE) and was novel to my colleagues in Utilities $\endgroup$ Dec 8, 2014 at 20:15
-
$\begingroup$ @supercat Adding more rows of turbine blades gives diminishing returns, as the last row back-pressures the preceding ones. Most industrial installations now have a steam generator on the turbine exhaust, to boost overall system efficiency. It extracts heat from turbine exhaust gases while putting very little back pressure on the turbine. A 2nd burner may be employed between the gas turbine and the steam generator. This further improves system efficiency when the steam is used for cogeneration, but not when it's used for electric generation, as you have to consider losses in the steam cycle. $\endgroup$ Dec 8, 2014 at 20:33
-
$\begingroup$ @steveverrill: I'm familiar with the fact that adding rows of blades yields diminishing returns; I would expect the point where they cease to be worthwhile to be different in a plane than in a fixed installation. I'd missed the distinction that there's a combustion chamber between turbine assemblies. $\endgroup$– supercatDec 8, 2014 at 20:39
$\begingroup$
$\endgroup$
Basically, reaction engines are more energy efficient the larger the amount of propellant in relation to fuel is. With a jet engine the propellant is air which happens to already contain more oxygen than is needed to burn the fuel. In theory this might change for a very fast aircraft requiring very fast exhaust velocity, but the engine design would have to change too much for it to be relevant to the question.
The general benefit from more efficient burning (barring some specific performance requirement) would also be better fuel efficiency. Getting better fuel efficiency at the expense of carrying additional oxygen is unlikely to be useful.
I actually toyed with a concept for an engine similar to your idea, but, as mentioned by others, an engine that carries its own oxidizer is basically a rocket, so my concept was really a rocket-powered-bladeless-fan with characteristics similar to jet engines not an actual jet engine. So even approaching it from the opposite direction the answer is "The design would have to change too much."
People have actually tested engines based on using conflagration or detonation instead of combustion for more efficient burning of fuel. See Pulse Detonation Engine for example. These still fall in the category of "The design would have to change too much." Also note that they have not replaced jet engines despite the concept having been known for decades. Wikipedia has lots of articles on rare or experimental engines. You might find something close to your idea there.
