I don’t know that space – are helicopter turbines seeing much development?
The latest incarnations of Allison 250 series, now in the hands of Rolls-Royce, are called RR300 (300 shp) and RR500 (450 shp).
Peter,
Bizzarely, weight is not a major MPG factor in light GA, because we fly so far beyond the Vbg speed.
Actually, you guys got me to calculate a bit. Especcially the Carson Speed and Vbg caught my attention. I looked it up for my plane and found that Vbg is 105 mph and therefore Carson speed is 138 mph. Interestingly, these being IAS it fits pretty well with the speed envelope that results from the settings in the POH. Maybe that is one reason why we do get such a good performance/consumption figure.
Weight however is a massive point for light GA, as most airplanes are notoriously short on payload.
to state that basically a 2-stroke is inherently less efficient than a 4-stroke, but makes up for that in small engines with less weight and less complexity. Hence their success in scooters and lawn mowers. The Jumo’s were indeed marvels of engineering and efficiency, but that took a good deal of complexity too.
Are you mixing 2-stroke gasoline engines and 2-stroke compression ignition (aka diesel) engines here? A 2-stroke gasoline engine has the problem of incomplete combustion due to poor cylinder scavenging. Nothing like that in a 2-stroke diesel. They are the most efficient combustion engines out there, take the SFC of the big ship diesels. As to complexity, they really aren’t more complex. Even though they were only used for a few years, they did show very satisfying reliability in the 1930s. For one, this concept finally allows to greatly reduce vibration. 4-stroke engines have a cylinder contribute for only 1/4th of the time, the rest of the time it is driven by the other cylinders and with the much more violent combustion in a diesel, this makes for an interesting challenge.
The reason we don’t have this technology today is because shortly after it was about to see critical mass, the turbine developed by Hans von Oheim and Frank Whittle was ready. The Messerschmitt 262 of 1942 was so impressive that the whole world jumped onto the turbine. Fuel consumption was not an important consideration back then as the main goal was to get more HP out of the engine. For decades fuel played no role at all (think of the Concorde).
Back in the 1930s, the Jumos were able to deliver sea level power up to FL400 and were cruising up to FL500 at a SFC that has never been attained again, not with piston engines and even less so by turbines. I believe that 2-stroke diesels could replace turbines entirely for aircraft up to 20 passengers or more. It will happen eventually because fuel will continue to get more expensive and turbines are just incredibly wasteful. We haven’t seen any serious R&D since the 1950s but eventually we’ll have to do just that. If fuel makes up for 50% of the operating costs, then there is an incentive to rethink engines.
Achimha, I too believe very much in the potential of the diesel two-stroke – the WW2 Jumo’s produced some 600-900 HP so they could easily power a Twin Otter or so, though I suspect them to be a good deal heavier than the turbines, likely requiring modifications to the wing structure which would add more weight, thus further reducing payload.
But I always understood the scavenging problem is essential to the two-stroke concept, after all the cylinder, valves, piston do not know what fuel is being burned, neither how it is ignited. The Jumo’s avoided this problem by having two pistons in each cylinder, and of course one did the charging, the other the scavenging. For further improvement, the pistons were not exactly in phase (some 19 degrees offset, I seem to remember).
But I had already wondered how the problem is solved in the marine diesels, or, to remain closer to aviation, in the Wilksch WAM engines (of which we have regrettably little news). Shall be glad to learn more!
The mixture is pushed in (via a turbocharger) at the bottom through ports and pushes out the exhaust through a valve. Not as clean and complete as a dedicated salvage cycle but can be tuned quite well in slow running machines. Exhaust quality will be aehm suboptimal but SFC great.
Having opposed pistons in a single chamber makes sense for weight. The hp/kg ratio is rather good, much better than what today’s piston engines have. The Jumos had only seen a few years of development and back then metallurgy and fabrication was nowhere what is possible today. Aluminum can be used, better steels, etc. The hp/kg will no be as good as a turbine but with fuel becoming the most important consideration, it can still trump turbines. One would no longer have to climb to FL260+ for the typical European short hop. It could really have a major impact.
I think that we will see that development happen in the drone world. The Command in Chief of the Free World™ likes to have plenty of them flying at all times everywhere. Some of the drones use the Thielert engines, the bigger ones jet engines. However, the longer it can stay airborne, the better so I am sure the military will switch to 2-stroke diesels of some sort. Normally these drones don’t loiter where they have a landing strip nearby so every hour of additional endurance can save billions or greatly increase the utility value. Pipeline/railway observation is most often done with turbines in low altitude operations. I live right next to the main NATO fuel pipeline and see it every other day. An incredible waste of resources and a sizeable market. It will happen, just not in GA because GA is kind of irrelevant and more importantly it is a shrinking market and every business man knows that shrinking markets are to be avoided at all cost.
When I was a kid (no comments please 
) I read a great story – English original which I actually have – from the 1920s or early 1930s about a mad count who had a grudge against society and built a subterranean boring machine which he used to terrorise his enemies (basically everybody, but especially the British).
The engine was a diesel with two opposing pistons.
On one occasion he raided an airfield and sucked out a load of avgas from a fuel truck he captured, so he was ok with avgas, maybe 130 octane back then?
The Sterling Motor, right?
Kid: :-) :-)
@Alexis: mind your spelling when the language-hyper-critical Flemishman is around! That engine (far underestimated today, in my view, if one considers the amounts of energy we are deliberately throwing away with airco’s and more such counternatural contraptions) was invented by Mr. Robert Stirling.
@Peter: your story might well have been the inspiration to the “Terranef”, appearing in a comic book of my boyhood – the ’’Suske en Wiske" album “De knokkersburcht”.
And now for serious, and on-topic:
@achim: thanks for the explanation, but it doesn’t really satisfy. What you neatly show looks like only “minimalising damage”, again doing the scavenge as far away from the load as possible; and reducing engine speed to gain time. But in aviation we want high revs, to get the maximum power out of a minimal engine weight. As the Rotax 912 illustrates, reduction gears are worth their weight today even in small engines, as they were in yesterday’s larger Jumo, Merlin, and the multi-row US’an radials.
Yes, Jan – sorry, STIRLING, of course.
Don’t worry, I hate bad spelling (although i make so many typos, typing all day …)
Actually, we don’t want high revs, not more than ca 2800 rpm because we want to do without a reduction gear. Also the Rotax 912 is not the way it is because some aviation genius has come up with the best possible solution in a clean slate design. It has a long history like most engines, coming from non aero applications. There is another fundamental difference between 2-stroke diesel and gasoline engines: lubrication. Gasoline engines need to burn a mix of oil and gasoline for lubrication while diesel has builtin lubrication properties.
What really counts is the SFC and power/weight ratio. Both were rather favorable in the 1930s designs and would only be better today. If scavenging was such an issue, then SFC would suffer. As long as there are no environmental standards for aero engine exhaust, it is the way to go. Getting to clean exhaust would be very difficult, for the same reason the Wankel (rotary) engine died a few years ago.
The real beauty is the unique smoothness of the ignition, apart from power/weight the second major advantage of a turbine. A good 2-stroke diesel gets there as well as:
What I find to be incredible is how in the 1930s they managed to build Roots blowers that allowed the Jumo engines to keep their rated power up to FL400 (!!!) and cruise up to FL500. Compare that to the latest SMA diesel which quickly loses power starting around FL100. Of course the Jumos were supercharged in addition to turbocharging.
