This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 14th April 2007, 2 pm - 3 pm.
You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.
[Sound of a blackbird singing]
For some people the song of a blackbird is the most beautiful sound in the world. Other people's favourite sound might be Beethoven or Jimi Hendrix. But for me this is one of the most fantastic noises that I've ever heard...
[Sound of a pulsejet flyby]
It's the sound of a pulsejet engine. And if you lived in London from 1944 to 1945 you might not be quite so enthusiastic about hearing it. Pulsejets powered the 10,000 Nazi V1 missiles that rained down upon England during World War II.
I've got the engineering drawings of the V1 pulsejet engine sitting in front of me. It's pretty much the simplest machine imaginable. It's literally just an empty tube with one end blocked off by a bank of one-way reed valves.
[A pulsejet] works just like a two-stroke lawnmower motor -- but without the piston. Air is drawn in through the reed valves, fuel is injected, and then the fuel is exploded. But rather than the explosion pushing on a piston, it pushes hot gas out the back of the engine at high speed... thus producing thrust.
It couldn't be simpler, or cheaper to make. And it's long been the dream of aircraft engine manufacturers to use low cost pulse-jets on commercial aeroplanes.
So why don't they? Well, the answer is efficiency. Basically a pulsejet engine doesn't have any [efficiency]. The Nazi V1 missile used nearly 600 litres of fuel just to travel a few dozen kilometres across the English channel.
A pulsejet engine is inefficient because the fuel is combusted in a subsonic explosion. This means that the pulsejet operates with a very low compression ratio. An efficient diesel car engine might operate with a compression ratio of 20:1,
whereas the pistonless pulsejet engine can only achieve compression ratios of around 2:1.
But what if the explosion were like this...?
[Sound of a detonation explosion]
My ears are slightly ringing. That's the sort of supersonic detonation combustion that you can achieve if you get just the right conditions.
In this type of detonation explosion the compression ratio can reach 100:1, [which is] much higher than a normal jet engine. A pulsejet with this sort of compression ratio is called a pulse detonation engine. If such an engine could be successfully developed then it would be a breakthrough in aircraft efficiency and cost.
And that [could have] important implications for New Zealand. It would allow the air-transportation of goods and tourists (to and from) New Zealand at much lower cost, and using much less carbon dioxide-producing fuel. [In other words, reducing the energy consumption and greenhouse gas emissions for each 'air mile'.]
Dr John Hoke works for the United States Air Force Research Laboratory in Dayton, Ohio. He's the head researcher on their pulse detonation engine development programme. His research team have been testing their newly designed pulse detonation engine on a Rutan Long-EZ aircraft. I asked Dr Hoke how things have been going...
Dr John Hoke:
Well, we're doing basic and applied research here. We're able to detonate most practical fuels, [and] we've done high speed taxi tests with [the Rutan Long-EZ] aircraft with a pulse detonation engine attached. We've not flown that aircraft yet with a pulse detonation engine. We have every intent to do that, but at this point we're still doing research.
So you've successfully managed to achieve detonation combustion in your engine -- and therefore a much higher compression ratio than in a pulsejet. What sort of improvement in efficiency has
this translated into?
Dr John Hoke:
When you detonate a fuel-air mixture, you're going to get about three to four times improvement in efficiency over what the pulsejets are getting. You also have much higher exhaust gas velocities. So where the pulsejet typically operated at about Mach 0.6 or [Mach] 0.8, the pulse detonation engine is thought to be able to run very efficiently at Mach 2 to [Mach] 4.
Okay... Mach 2 to [Mach] 4 -- in other words between two and four times the speed of sound -- that's a much faster speed than passenger aircraft operate at today. So would the pulse detonation engine actually be a suitable replacement for the ordinary turbofan jet engines on commercial aircraft?
Dr John Hoke:
The turbofan [engine] is made for lower speed. You wouldn't put a pulse detonation engine on a commercial aircraft because typically they don't go Mach 2 to [Mach] 4. However, when you look at these things, the pulse detonation engine is a constant volume process, and the efficiency of that is inherently higher than a constant pressure process...
... which is the combustion process you'd have in a normal jet aircraft engine...
Dr John Hoke (cont):
... yes. And what's thought for commercial application would be to take this constant volume combustor, and stick it in the middle of one of your turbofans. And then you're talking about potentially a 5 to 27 per cent increase in efficiency of fuel economy.
So by sticking a pulse detonation engine inside a normal jet engine -- to replace the combustor -- you can get up to a 27 per cent increase in fuel efficiency. In aircraft terms, that's huge!
But what about the case where you actually want to operate a commercial airliner at,
say, two or three times the speed of sound, maybe as a replacement for Concorde? Would a straight pulse detonation engine -- the sort you're working on now -- have an application in this context?
Dr John Hoke:
Potentially, yes. The one thing people point out is the noise. High noise-levels can have an impact on structures and what-not, but to our experience the noise [of our pulse detonation engine] is not a whole lot different than an aircraft on afterburner. I've stood right next to the thing when it's running, and it's loud, but it's acceptable.
Okay, that's quite surprising. I've heard a pulsejet engine, and they are really loud. But you're saying a pulse detonation engine isn't actually that bad?
Dr John Hoke:
Well, you're definitely wearing hearing protection. The sound levels coming out the back of the pulse detonation engine are very directional -- so if you're standing down behind the engine you're gonna see some pretty loud noise levels, I think. At the exit of the pulse detonation engine you're talking about 190 to 210 decibels, and I believe your ears start to bleed around 160 [decibels]. But when you're travelling Mach 2 [or] Mach 3 the sound is behind you. I think you have more serious [noise] issues with the aircraft sonic boom.
So talking about a pulse detonation engine in the context of a potential high-speed application [such as] a Concorde replacement -- it's so mechanically simple compared to a conventional supersonic aircraft engine -- have you got any feel for how much that might reduce cost?
Dr John Hoke:
Our best guess is about one hundred times cheaper. It could potentially be huge.
A hundred times cheaper really would be huge...
So coming back to something you mentioned earlier about
running your pulse detonation engine on a variety of fuels -- I was wondering if you'd tried it on bioethanol or biodiesel?
Dr John Hoke:
We've almost detonated everything, I'd say. We've done ethanol, we've done gasoline, we've done propane, we've done ethylene... hydrogen is one of my favourite fuels.
We've done aviation gasoline, [and] the jet fuels work fine. The one thing was biodiesel -- we haven't done that. But I wouldn't foresee any big issues with that because I don't think the combustion properties differ too much from regular jet fuel.
So when do you see pulse detonation engines being commercialized, and in what initial applications?
Dr John Hoke:
Theoretically the pulse detonation cycle [or] constant volume combustion cycle -- however you want to put it -- I think has a lot potential. As far as making it practical we still have yet to see how that's gonna all pan out. The thermodynamics says it should be more efficient than the constant pressure combustion [which is intrinsic to] the pulsejet and the gas turbine engine. So I'm very hopeful there.
As far as where it would first be used: it would probably be a pure pulse detonation engine, and it will be probably used for a drone or a missile-type of application -- where... it's un-manned and you're looking for a cheap engine and you're looking for fast flight.
Commercially -- and this may take off really quickly -- it depends on the engine companies and their research budgets. If the hybrid-turbine engine (using our constant volume [pulse detonation engine] combustor)... starts to pan out, that's gonna be a huge cost-saver for the airlines, and that could go very, very quickly.
So Pulse Detonation Engines offer the possibility of significantly reduced fuel consumption for normal sub-sonic commercial aircraft -- and even the option of running on biofuels.
But also, perhaps, they may usher in a new generation of lower-cost and more fuel-efficient supersonic aircraft. All of which is good news for a country as dependent on air transport as New Zealand.
Further information on pulse detonation engines:
- Read more about Dr John Hoke's research work into pulse detonation engines on the ISSI website (ISSI are civilian contractors to the United States Air Force Research Laboratory). Some interesting photographs of the prototype pulse detonation engines can be seen in their image gallery.
- Read more about the working principles of the United States Air Force Research Laboratory's pulse detonation engines in the ISSI FAQ section.
- Read more about pulse detonation engines (in general) in Wikipedia.
- Read more about Dr Hoke's test-bed aircraft: the Rutan Long-EZ.
- Read about How Pulsejets Work on Bruce Simpson's excellent Pulsejet website.
- Read more about the V1 missile in Wikipedia.