My prediction is that MEPs will come back. Small turbines at mid altitudes are just incredibly inefficient compared to modern diesel engines. Those airplanes burn a lot of fuel that does not contribute to thrust but would with diesel engines. There is no inherent technical limitation that prevents us from building very efficient and highly reliable large diesel engines. Junkers did it before WW2.
When we get those large diesels, MEPs will resurface. Even a long written off low tech PT6 costs a fortune. It will probably require diesel electric concepts to make a change, just like modern diesel trains are all diesel electric. Put the engine somewhere in the fuselage and don’t waste 80% of the 2nd engine’s thrust just dragging the inefficient wing cowlings through the air.
I’m sure it’s coming, just taking an awful lot of time.
There seems to be a consensus that turbine engines need altitude for efficiency, but isn’t every aeroplane more fuel-efficient at higher altitudes? Is there a fundamental reason the need for altitude is more outspoken on turbines?
NB funny that you should mention this just while I posted my musings about the UB-14. Both ideas combined…?
NB the JuMo diesels were wonderful pieces of technology, but a 2-tonne twin doesn’t need that kind of power. A 2-cylinder 2-stroke diesel based on the same principle of opposing pistons is to be available soon from one company “Superior” but I am not holding my breath – this engine had been the Gemni before, and I think it was the DAIR before that… Good ideas do take very long times to come to market fruition, indeed.
PPSS mind you, the subject is “… the AVGAS twin” so no contradiction with the projected diesel.
There seems to be a consensus that turbine engines need altitude for efficiency, but isn’t every aeroplane more fuel-efficient at higher altitudes? Is there a fundamental reason the need for altitude is more outspoken on turbines?
Yes and yes. Mass flow through a turbine is pretty much constant and in order to get the same mass flow at altitude (with thinner air) you need to go faster. Also the task of a turbine’s entry stages is to compress the air and when going fast, the ram effect helps you which again improves efficiency just by flying faster. Similar for the exhaust portion, accelerating a big chunk of air by a smaller delta is more efficient than accelerating a smaller chunk of air by a larger speed delta. That is a very simplistic view, the whole maths with Carnot cycle, thermodynamic efficiency etc. is well explained here:
but isn’t every aeroplane more fuel-efficient at higher altitudes?
Piston engines, not really. The MAP of normally aspirated engines is decreasing all the time in the climb, so there’s a trade-off between the reduced engine power and the increased TAS with altitude. Apparently my normally aspirated IO-520 is “most efficient” at about 8,000 ft.
I try to flight-plan FL70 to FL100 and then negotiate the final cruising level (up to FL150, for me) based on the winds.
Piston engines, not really. The MAP of normally aspirated engines is decreasing all the time in the climb, so there’s a trade-off between the reduced engine power and the increased TAS with altitude. Apparently my normally aspirated IO-520 is “most efficient” at about 8,000 ft.
NA piston engines which aren’t really adequate for airplanes… 
Turbocharged pistons get more efficient with altitude because they need to produce less power (i.e. consume less fuel) for a given airspeed. The limit often is the turbocharger, at the critical altitude it can no longer compensate the effect of loss in ambient pressure. Where that is depends on the aircraft. With turbo normalized gasoline engines it is usually at the certification limit.
Diesel engines have the problem that they are heavily turbocharged already at MSL (100" MP is standard) which makes it very hard to keep power at altitude compared to gasoline engines that can get a good power to weight without turbocharging and can use the turbocharger mainly at altitude to compensate. Even with big turbos that extract all energy from the exhaust, a diesel will never perform at FL200 like a gasoline engine does. This means additional charging is required — traditionally through a crankshaft driven supercharger (Jumo again) — or in the future with an electric charger.
BTW: turbine engines have in principle no altitude limit, they get more efficient the higher you go without limits. What stops them (apart from the cabin/airframe) is usually supersonic physics for which normal turbines are not designed. If you design them that way, they will make you cruise most efficiently at the upper end of the atmosphere.
A 2-cylinder 2-stroke diesel based on the same principle of opposing pistons is to be available soon from one company “Superior” but I am not holding my breath – this engine had been the Gemni before, and I think it was the DAIR before that.
Maybe wishful thinking, but Superior manufacturing and selling the Gemini could be it. It is a 3 cylinder engine (6 pistons). It’s 100 – 130 HP depending on turbo or not. It should fit like a glove in any of the VLAs around DA20 or Aquila. It is right there with ULPower’s 260 and 350 and Jabiru 3300.
Twin engines are statistically amusing regarding redundancy (considering the aircraft can fly with only one engine). The MTBF of a piston is perhaps 50,000 hours which equals to a failure rate of 20 per million hours. The chance of both engines failing within the same hour is 0.0004 per million hours, thus the MTBF is 2500 million hours for this redundant system (quite a lot).
But, the chance of any one engine failing at any given time is double because of two engines. So the MTBF_serial is 25,000 hours or failure rate of 40 per million hours. You would have double chance of experiencing an engine failure with two engines, which is common sense. With one engine having failed, you only have one left, so the MTBF the remaining flight is 50,000 hours, same as a single engine aircraft.
The safety aspect of having two engines is tightly connected to how long you can expect to fly with only one engine, and the MTBF of the single remaining engine. A turbine has a MTBF of maybe 10 times a piston, maybe more? A SET therefore has a failure rate of 2 per million hours. If the flight is short, then the redundancy of the twin piston means a lot, but the longer the flight is, the more important will the actual reliability of each engine become. A twin turbine is 100 times more reliable than a twin piston for the same flight, and this difference increases the longer the flight is.
Even though a twin piston is much more reliable than a single piston, the comparison with a single turbine becomes more difficult. If safety is the issue, then a BRS (and good insurance) is better than a twin piston I would think.
A SET therefore has a failure rate of 2 per million hours.
Yes, maybe, but: All these MTBF figures reflect failure due to internal component malfunction. Most engine failure however are due to external influences like fuel contamination, blockage of the fuel supply, blockage of the air supply by ice or ingestion of birds or other objects or incorrect handling by the pilot (the usual carb-heat thing or wrong use of the mixture control). This is where the redundancy provided by a second engine with independent fuel and air supplies really changes the statistics.
Anyway, my personal experience differs from the numbers you quote by several orders of magnitude. In 2000hrs of MEP flying I have had three engine failures which means one every 700 hours. All due to internal defects of the engines. And a few weeks ago a pilot well known around here (ex German airforce F-104 pilot, now airline pilot, instructor and LBA examiner) died in an EFATO accident in a Bölkow Bo 207. About five years ago, the same pilot survived the crash landing of one of our flightschool Pa28s when the engine failed during an IR checkride. Statistically almost impossible with the numbers above…
This is where the redundancy provided by a second engine with independent fuel and air supplies really changes the statistics.
Yep, the failure rate goes up by over X 2 ’cause now, there are twice as many systems that could fail !
Piston engines, not really. The MAP of normally aspirated engines is decreasing all the time in the climb, so there’s a trade-off between the reduced engine power and the increased TAS with altitude. Apparently my normally aspirated IO-520 is “most efficient” at about 8,000 ft.
Understanding the intent of this, a clarification would be to say that the engine efficiency tends to be highest at the lowest altitude, and the airframe efficiency increases with altitude. The ‘trick’ is that for a while as you climb you can regain engine efficiency by increasing the throttle opening… until you run out of throttle opening to maintain the necessary power. From then on you need increase rpm to maintain power, which increases friction loss and thereby decreases efficiency. The happy medium at around 8000 ft results from that being the altitude where full throttle and normal rpm gives the desired % cruise power.
