News, facts, and comments on the coming revolution for piston-engine aircraft.
In 1998, one diesel engine flew on a converted airplane for the first time since 1945. Today, close to 4,000 singles and twins are flying. This is the beginning of a worldwide trend which will eventually allow a rebirth of the piston-engined aircraft, around new specs and new missions.
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News of January 24, 2010
Discussing respective advantages of 2-stroke and 4-stroke diesels.
One of our subscribers sends us a question we already heard in the past, so the question and our answer are published here for discussion. Feel free to send me your comments at email@example.com.
Q. Could you please expound on the differences of the harmonics produced between a 2 stroke and a 4 stroke diesel. We have experiences with both 2 and 4 stroke diesels in our truck fleet, and found that the 2 strokes last longer, with only a fraction of maintenance, along with fewer fatigue cracking in thin or weak surrounding metal areas, and longer lasting clutch assemblies, plus we learned that the worlds most efficient Diesels are found in large ships and happen to be 2 strokes.
A. In theory it is correct: A 2-stroke of same configuration and number of cylinders generates less vibration than a 4-stroke. This is why often in the automobile and vehicle world we have seen a 2 cylinder 2 stroke or a 3 cylinder 2 stroke giving comparable vibration patterns than their equivalent 4 cyl. and 6 cyl. 4 strokes, for less weight and cost. It is also correct to say that opposite cylinder engines and radial engines have a better vibration behavior than In-Line engines. What is unknown yet about diesel 2-strokes applied to aviation is the wear & tear behavior due to the fact that you have no valves, but instead some kind of static openings for gas intake and exhaust, which induces in theory both friction problems and heat stress problems. And these problems are complicated or simplified by lubrication technology used: A very simple 2-stroke engine burning by design a lot of oil mixed to gasoline (the old motorcycle and scooter design of my youth) is very low cost and lasts a long time. Of course modern 2-strokes have a separate lubrication system. Nevertheless 4-stroke was ultimately preferred for reasons of fuel efficiency and pollution.
Q. We are hoping for a 350 hp 2 stroke to become available for our PA-46 in the near future, and are currently looking at purchasing a Cirrus for LoPresti to convert into the SR2X Delta Hawk 2 stroke Diesel when that becomes available in the next year.
A. We expect to see progress for powers of 280-400HP of 3 different manufacturers offering different technologies: Thielert/Centurion, SMA, and Deltahawk. The others may come only later. So we will have the 3 solutions available. Which one is the best? See next question.
Q. Would you agree that 2 strokes are a better investment, and would make a better aircraft engine? I personally cannot see how the V8 Thielert could be made as light or as long lasting as a V configuration Delta Hawk or a flat Gemini, 2 stroke, and I think that there is a reason for Thielerts clutch assembly not lasting due to harmonics, (which is why I request your comment), but after saying all the above, if Thielert were to produce a 350 hp engine that would fit in a PA46 and still allow the nose gear to fully retract, then just because it's a Diesel, we would be first in line to purchase one.
A. You are right. This is why so many users of the first 1.7 liter Thielert kept their engine and waited patiently for repair kits: They had no choice because Avgas was simply not available anymore where they are.
Now let's discuss what's best: In-line or opposite/radial or other? Air-cooled or liquid-cooled? 2 stroke or 4 stroke? Well, imagine we go back to 1945 when you had, active in military operations, the Rolls Royce Merlin, a liquid-cooled in-line V12 geared design; (see our photos); the radial engine, offered by Wright and P&W, and the Jumo Opposite Piston 2 stroke diesel:
The discussion then was exactly the same! Note that the three fastest piston engine airplanes ever (say the Hawker Tempest/Fury with Napier H-24 or Centaurus radial, the North American Mustang with Merlin, and the Grumman Bearcat with R2800) are still competing with both radial versus in-line, (except the Napier has disappeared because of too high maintenance costs)! As for diesel, already at the time the Jumo was demonstrating a much lower specific fuel burn on the Ju-86P high altitude bomber and on the Dornier and Blohm & Voss patrol flying boats. The Germans, however, experienced that diesels were at a disadvantage in combat, and more efficient on a peaceful, long range cruise. Which is what we need today. That's another story.
Now here is another aspect: any engine design makes fantastic improvements as its production and applications develops. The first Merlin with 1,000HP had a TBO of 30 hours in 1939; and a TBO of 200 hours with 1,850HP in 1945! You cannot assess what all these engines will become until they are produced and flown in significant numbers, and backed by full time permanent design engineering teams.
As of today, my forecast is:
2-strokes position: They will invade the 100-120HP (LSA and flight academies) market; and DeltaHawk has enough time to position itself as supplier of a 2-stroke 160HP for Cessna 172s in the US and Canada, because Centurion/Thielert has got itself a bad name. Outside the US it is a different story because Centurion has established a service network and many 172 Thielerts keep flying. But DeltaHawk should have a weight and cost advantage in that power.
300HP plus: There is no competition yet between SMA and Centurion because of difference of power offered. Within a few years, we expect to see them competing with fully certified engines in the 280-450HP range. We do not see yet which 2-stroke manufacturer will be ready and certified to compete with them. By then, we expect for each of them some kind of strategic alliance to better address the US market.
posted at 4:54 AM
News of January 10, 2010
This paper gives a good forecast on the incidence of biofuel on JetA. It foresees $3.20/gallon in 2020 for a fuel containing 50% biofuel. Brazil to become a big supplier? I personally doubt that Brazil alone can fill the gap for other reasons: intelligent use of their cultivated land. But other countries may address that market too... Click on the link to read more. This may affect future aero diesel engines because we don't know yet what will be the Cetane rating of such a fuel. Cetane rating measures the speed at which a diesel fuel will burn once ignition has been triggered by compression.
posted at 12:05 AM
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Every month: news, facts, and comments on the coming revolution for piston-engines aircrafts between 130 and 400 HP: Retrofitting a diesel engine to run on Jetfuel or Kerosene, reduce Gallons/Hour by some 30%, eliminate ignition systems (magnetos, spark plugs) and their problems, eliminate mixture control, increase TBO to 2,400-3,000 hours, increase performance between 6,000 and 12,500 ft., and drastically reduce Operating Costs.
The letter is intended for piston engines aircraft owners, manufacturers, fleet operators and FBOs, re-manufacturers of engines for these aircrafts, manufacturers of engine components and ancillaries, and all professionals acting in decisions of engine exchange or refitting at TBO, in North and South America, Pacific Rim, African continent, and all parts of the world were Avgas, Mogas, Kerosene and Jetfuel are available.
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