Reader Paul Nash (Vancouver, Canada) writes:
“While searching for some information about fuel use in aircraft I came across a research report that was eerily similar to your article on electric cars. It looks at all aspects of airline fuel efficiency from pre war days to today, and found that on a per passenger mile basis, the most efficient modern aircraft, the Airbus A380, has just managed to match that which was achieved by the piston engined Lockheed Constellation series in the 1950’s. As with cars, increases in speed have taken away any fuel advantage from jet engines, and it has taken 50 years to get back down to the same MJ per passenger-mile as before the jets. Given the advances in engines, materials and aerodynamics, one can only imagine what a modern version of the turboprop Constellation, flying at the same speed, would be like in terms of efficiency.”
Thanks, Paul. I have nothing more to add. “Fuel efficiency of commercial aircraft”, Dutch National Aerospace Laboratory, November 2005. From the summary:
“This report assesses how the fuel efficiency of commercial aircraft has developed since their introduction in the 1930s. Existing estimates, such as the oft-cited 70% improvement from the IPCC Special Report on Aviation and the Global Atmosphere, ignore the record of the pre-jet era. Based on bottom-up (micro) and top-down (macro) analyses of aircraft fuel efficiency, it can be concluded that the last piston-powered aircraft were as fuel-efficient as the current average jet.”
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I think we’ve got to the point where we write a conclusion.
Piston aircraft are more efficient than jets (orignal report that started this)
Diesel piston aircraft engines are even more efficient (but the last big one flew in the 1950’s)
Prop aircraft are not that much slower on short flights (because of the time spent in ascent and descent) and use much less fuel
In this day of high fuel prices and reducing resources piston/prop would use the least fuel for short/non-time critical flights
And NOBODY WANTS THE OLD TECHNOLOGY - pretty much the whole theme of this website really !
I don’t think a piston engine is more efficient than a turbine. Gas and steam turbines are used in power plants instead of piston engines as turbines are more efficient. Turbines can run with diesel fuel if necessary. Turbines for power plants can even run with crude oil. New aircraft piston engines will be able to run with jetfuel as avgas is leaded.
To keep fuel consumption low the plane simply has to fly at a lower speed. With geared fans further fuel savings can be realized.
Modern turbo-shafts with improved multiblade propellers have significant advantages over reciprocating engines: higher power to weight ratio, fewer moving parts and a longer service life. They run on what is basically kerosene rather than high octane aviation gasoline (a possible safety advantage) present a smaller frontal area reducing drag, and because of their lower weight reduce wing loading resulting in reduced wing structural requirements. Oh and they are easier to maintain.
Turboprops still fly at lower altitudes than jets. This prevents them from flying above bad weather. So they have more problems from the weather. On short distances this is not a differentiating factor, since jets are also climbing or descending most of the way. But it does affect flight scheduling and passenger comfort.
A much larger effect over the past decades has been improvements in scheduling and pricing that keep aircraft close to full. The fuel consumption per available seat mile improvements have been slow. The fuel consumption per actual passenger seat mile has been almost halved. Most of this improvement comes from aircraft being flown nearly full. That puts them near the designed optimal weight.
I still see plenty of turboprops flying short distances for commercial traffic.
Paul Nash, I stand corrected about the Constellation, ( and I see its much older than I thought) but the substance of what I said remains: piston engines, even with their extremely mature development in ground vehicles, are simply too heavy to use in aircraft. Witness the wholesale conversion of the fleet from piston to jet in the 1960’s. Conversely turbines don’t work well in ground vehicles. Turbo-props are jets, just with an external fan.
Its a bit moot- no one in this age of peak-everything is going to design, build, and certify a piston engine for commercial aviation.
However this article quotes DOT studies
as saying per seat per mile mileage for airlines is better than that for cars ( cars with one or two passengers - grain of salt required). While I couldn’t be less of a Big Oil reactionary, its quite remarkable that the fuel use is that good. Its a testament to some fine engineering, and some uncomfortable planes.
Conclusion agreed. There is no reason a reliable, efficient aero diesel could not be made today - if they worked then, they could work much better now. They would be cheaper than gas turbines for small planes (like the Q400), and use 2/3 the fuel, or better. They have much better part load efficiency, so less fuel is wasted when on the ground.
They are just not sexy (=hi tech), that’s all.
If they built a propeller driven aircraft of this size, in which the propellers were driven by electric motors, could hydrogen fuel cells provide enough electrical power? How about one of those BloomBox fuel cells, which can use natural gas? I think Boeing had a fuel cell powered aircraft recently.
I notice that the Boeing 777 only goes 560 mph, whereas the fastest propeller driven aircraft is the Tupolev Tu-114, at 540 mph. So not that much slower (these figures are from wikipedia).
Nick Hein Morgantown WV
Didn’t see it mentioned here, but in the early 1980’s during the first fuel crisis I was involved in a project at McDonnell-Douglas to design fuel-efficient airliners that used propellers. GE, Rolls and Allison developed prototype engines. They were called Un-ducted Fans (UDF) or similar names. McDD and Boeing developed prototype airframes in the 150 passenger size. By 1988 gas got cheap again, neither the engine or airframe mfgrs wanted to pony up the development cost and so it was abandoned. The effort did have some lasting effect however. Conceptually the engines were treated by engineers as ultra-high bypass ratio (BPR) jet engines, with an effective BPR of 150 compared to ratios of 6 typical for airliners of the time. The next generation of jet engines were bumped up as high in BPR as they could go while still retaining a cowl (which simplified things like ice-shedding and blade-loss failures). The Boeing 777 and 787 have BPR of 9 and 11 respectively which also allows them to use very small engines with very big fans to get very high thrust. Still the focus has been on increasing thrust and not fuel efficiency. The UHB MD80 derivatives burned 66% LESS fuel than the turbofan versions - this in an industry where an airline executive would sell his grandmother for 1%.
The Austrian Diamond Air company offers a four seater airplane with a diesel engine. It is extremely fuel efficient compared to any other similarly sized airplane and while they had a problem with some of their early planes due to the closing down of their engine supplier, the company recently brought their engine manufacturing in house and they now offer the DA 42 NG : http://www.diamondaircraft.com/aircraft/da42/specs_da42_ng.php
I am not a pilot but there should be pilots here who might want to comment on this particular plane. And, there’s no reason why, with proper funding, larger aircraft with diesel engines could not be designed and offered using today’s more advanced technology.
Thanks for a very nice post, but then, that is to be expected here!
-I’m way late to this discussion (just came across Lowtech for the first time and am browsing some of the fantastic pieces from the past)
@matthias (comment #6)
Its actually exactly the opposite. As far as power generation goes, Recip engines are - for a given size - far more efficient than gas turbines. The problem is that there are physical limitations on just how big recip engines can be built. Wartsila makes a number of gas engines in the 10-20MW range that have efficiencies above 47%. Jenbacher makes smaller (2-6MW engines) that are 40-45% efficient.
Gas turbines can be made much larger (think Solar turbines, or the GE turbines), but they rarely achieve electrical efficiency above 40%.
Here’s the thing, though. Gas turbines produce enormous amounts of excess heat that, when captured, can be used in a steam turbine - so you can get higher overall efficiencies (still, not much more than 47-48%).
Another reason that turbines are frequently used in power plants instead of engines? Their O&M costs are much lower (not as many moving parts).
All of this holds together quite nicely with the OP - the piston engines from half-a-decade-ago are just as efficient than the jet-turbines we’re using today.
Two points to make on this.
1 - The piston engines used in the Constellation burned leaded avgas. Not exactly an environmentally friendly alternative to jet fuel. Fuel requirements for aviation a a lot more complicated than for other applications due to the changes in pressure and temperature experienced during a flight so using a different high-octane fuel like ethanol isn’t an option. Sure jet fuel/diesel (both basically kerosene) could be used in a diesel piston engine to achieve similar (if not better) efficiency but that brings me to my second point:
2 - Reliability. Turbines have a HUGE reliability advantage over piston engines and that is a big issue in aviation.
One thing jets have going for them is passenger comfort. My father-in-law flew several times in a military Super Constellation during the early 1960’s. He said his ears rang for days afterward and called it the loudest plane he’d ever flown in.
Several issues immediately arise:
What is the economic value of cruising speed? Jets cruise faster and higher — is the time not spent in the air worth the extra money / fuel? The compare with the older generation prop craft may not be relevant. The best of the best (current generation turbo props with the skewback blades) have quite good efficiency as long as speeds are kept lower than a jet.
There never was a production turboprop Constellation, although one was considered.
These airplanes are probably very similar to the Douglas series (DC6/7) in BTU/seat-mile as the engines were the same and the aerodynamics not that different.
The Wright Turbo-compound engines were quite efficient due to recover of power from exhaust gasses, but I doubt they were any better than modern turbines. They were vastly less durable and reliable. Speed is the real controlling factor….
Carlos in ATx
The Super Constellation was about as efficient a propeller driven aircraft as could be designed using piston engines. It could only hold 96 passengers on it’s largest version. Even the smallest of the modern turbofan powered airliners the Boeing 717/MD-90 carries more passengers at 60% faster speeds. The 737’s will carry 50% more passengers again at faster speeds. On a per passenger mile, the Super Conny might rival them but it is no way as efficient a transport because it would have to make 2 trips to carry an equivalent load of passengers. Never mind the bigger planes like the 747 which can haul 4x the passengers of the Super Conny. It would be nice to see the Super Conny equipped with a pair of modern turbofans that are of equivalent power to their old prop engines and then see how efficient it could be. It would have less drag and therefore could haul passengers faster for the same fuel consumption.
Also, the Conny used a fully ’teardrop’ fuselage. It’s much more aerodynamic than a tube with a constant radius. They aren’t used more because it means every curve is a compound one, and every bulkhead is a different size. It increases the cost of making an aircraft by a significant amount, and I don’t believe any pressurized airliner has been made with a non-cylindrical fuselage since.
Well the efficiency of a jet is only achieved at high altitude. High altitude allows you to fly over weather, and achieve a higher ground speed due to lower air density. Turboprops are more efficient when travel is shorter as you spend your time cruising rather than climbing. Piston engines in regular use just dont get that high as they need to have some type of forced induction to get enough air into the engine to operate. Forced induction increases cost and complexity.
Then you have reliability to consider. On the radial engine airliners you talk about, engine failure was a common occurance. It was not an if but a when. Most of the time it was in flight and could be dealt with without too much hoopla. But when it wasnt it was usually followed by a tragedy that back then was all to familiar to the public. Modern piston engines are robust and designed to provide a rated amount of power for the 2200 (can be as low as 1800 or as high as 2500) hours it gets to run before it is overhauled. It is very overbuilt for the power output as the designers want it to last without failure for the entire life, as a failure leads to an emergency every time. And a lot of times that emergency turns tragic.
Motor design focuses on reliability. Power and fuel economy are secondary. Normally aspirated 540 cubic inch motors are producing 230 hp. In a car 540 cubic inches gets you 500 plus hp. Far more efficient are the modern automotive engines.
The power to weight ratios needed are not to be taken lightly.
High performance petrol engines are not efficient and they loose power with rapidly with altitude.
See the specs here: the Twin Wasp was thirsty (295 g/(kW•h))and look what happens to its power at only 4 km altitude.
To most a 1930s diesel doesn’t sound like it would be a real candidate…
But look here:
By the end of WWII they had the answer, an opposed piston Junkers diesel of almost 30 liters displacement, running 3000 rpm, with exhaust driven turbocharger.
Consider a plane that used it:
“Luftwaffe tested the prototype Ju 86P with a longer wingspan, pressurized cabin, Jumo 207A1 turbocharged diesel engines, and a two-man crew… The British Westland Welkin and Soviet Yakovlev Yak-9PD were developed specifically to counter this threat….a modified Spitfire V shot one down over Egypt at some 14,500 m”
What you say to making a new one.
Aluminum block, wet liners, common rail injectors, better turbocharger, liquid cooled pistons in the manner of the straight 5 Mercedes, and making it as large or larger than the 207.
…And put it in a completely new airframe:
Narrow body jetliner fuselage aerodynamics and construction techniques, long unswept wings, fuel in rear fuselage instead of wings, passengers in forward compartment in rear facing seats, first class in rear compartment.
Use engine coolant for thermal deicing.
Piston engines are complex, high maintenance, and a waste for modern large commercial aircraft. The USAF got rid of them for good reason. Liquid cooling would be vulnerable to leaks and corrosion vs bleed air deicing. One pinhole and you are done, hence the preference for air-cooled radial engines for combat aircraft.
There are plenty of modern, large turboprop cargo/pax aircraft. They called “C-130s” and dandy for tactical airlift. No need for Constellation nostalgia.
There are more important concerns than fuel efficiency. We can always make more aviation fuel. Turbines which trim to temperature are very old news and more tolerant of a wide range of fuels than piston engines. Turbines are low-tech and a breeze to work on. Been there, done that.
J. M. Korhonen
Thanks for interesting article.
I’ve been researching the piston/turbine transition in aviation for my PhD thesis. I concur with several posters (who seem to have experience in aviation engines): reliability is a huge concern. Ease of maintenance is another - and it’s closely tied to reliability. There simply are far fewer moving parts in a high-efficiency turbine than there are in a high-efficiency piston engine, and hence, much fewer potential points of failure.
Turbines may never be exactly as efficient as piston engines, but engine failures no longer drop fully laden passenger aircraft out from the sky. That was a regular occurrence in the 1950s, even though there was much less air travel.
If we are willing to roll back safety standards to those prevailing during the age of piston engines, I don’t see why fuel scarcity needs to be a problem ever. Just to take one example, fast spectrum fission in sodium or lead cooled, unpressurized reactors alone could easily and relatively cheaply deliver more than enough energy for any conceivable need 9-10 billion people might have for liquid fuel synthesis from CO2, if equivalent safety standards and risks were to be accepted by the general public.
The turbocompound Wright 3350s were the most maintenance-intensive powerplants ever used in air carrier aviation, and indeed in aviation of any sort except for racing applications. By contrast, the early jets were models of simplicity, and although they were not fuel-efficient, the fuel was cheap kerosene and in any case all fuels were cheap. The jets needed much less maintenance, and also, a 707 or DC-8 could make two cross-continent or cross-Atlantic trips in a single day, where the Connie and DC-7 could manage one. The jet made a lot more money with much less effort.
The aviation diesel, however, is a textbook case of a superior technology that was mostly ignored. Before WWII, the primary objection was that it cost more and required a non-standard fuel with no benefit in speed. With the introduction of turbines, which offered a substantial increase in speed, the alternate fuel supply became more palatable (although it should be remarked that early jet engines would operate on gasoline as well as kerosene). However, the diesel offered improved fire safety and reliability (because no ignition system was needed.) A purpose built, modern, clean-sheet-of-paper piston diesel light aircraft engine could be very successful, and efforts along that line are being made, but the primary problem with light aircraft in general today is that corporate jets are more profitable.
The difference between a high-bypass jet engine and a turboprop is the number of propeller blades and being wrapped in a shroud.
If you look at the figure10 at p.22 of the “Fuel efficiency of commercial aircraft” (.pdf), and then interpolate the curve, then you would find out the limit is reached.
The study cited in the original post is Peeters et al. (2005).
Paul Peeters completed his PhD dissertation in November 2017 (10 years in the making). I contains an update of the study that started this thread, and much much more. 343 pages. It is open-access.
“Tourism’s impact on climate change and its mitigation challenges: How can tourism become ‘climatically sustainable’?”
Fascinating post and conversation in the comments. One other factor I didn’t see mentioned: contrails? I’ve seen people say contrails account for about half of all the climate impact of aircraft – albeit a much shorter-lasting impact than carbon emissions. What types of flying/types of plane could address this problem, and also address fuel efficiency at the same time?