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When will a diesel be available for my aircraft? |
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News, facts, and comments on the coming revolution for piston-engine aircraft.News of November 16, 2005Diesel Saga 2: How taxi diesels in Europe demonstrated a unique advantage (1950-1980).Yes, you all know that a diesel is more fuel efficient. However, this was only one of the three reasons why taxi drivers were first to promote diesels. The two others are: more longevity combined with lower maintenance costs (otherwise taxis wouldn’t touch it); and constant specific fuel consumption. And now if you really want to understand why diesel technology will rescue General Aviation, you have to bear with me for a little bit of theory. A diesel engine falls in the very general family of combustion engines. Steam engines, gasoline engines, gas engines (using natural gas or generated gas), diesel engines, turbojets and turboprops, and ramjets, are all combustion engines. All try to convert the heat from combustion, measured in Btu s or Joules, into some kind of mechanical energy measured in Horse-Power or Watts. And they only succeed to convert some of it: their efficiency is generally much less than 50%. (Do not complain too much however; remember that you need that heat to keep warm in winter and to clear your windshield.) All these engines burn a fuel, use the heat to incite motion and pressure in a gas, and recover that pressure in a mechanical device. An external combustion engine or steam engine has an efficiency of 4 to 8% when using straight steam and up to 17% when using dry hyperheated steam. An ignition engine, using an electric spark to trigger the combustion of a compressed fuel/air mixture, achieves 24 to 28%. A diesel engine achieves well over 30% and up to 50% for very large ship engines. A turbojet working in ideal conditions (40,000 ft high, stabilized cruise), gets closer to 60%. A ramjet at supersonic speeds can in theory reach 80%. (If you don’t know what a ramjet is, contact me through our Forum). The difference in efficiency is a consequence of the Carnot Principle (yes, Carnot was French, but he was right) which says that efficiency increases with the difference of temperature between the colder temperature and the higher temperature to which the gasified fuel is exposed. Which incidentally is why your engine runs better in winter… With internal combustion, your temperature can be the one at which your fuel burns, which is much higher than steam temperature; and it will be highest if materials exposed to combustion can stand highest temperatures which is much easier with turbines, and even easier in a ramjet where all parts are static (no moving parts). A diesel ignites its fuel by compressing air at a much higher temperature than a gasoline engine, high enough for combustion to be spontaneous: No spark plug, no magnetos, higher efficiency. This is why diesel is always more fuel efficient. But wait, it gets more complicated… The beauty of the reciprocating piston engine compared to turbine or other rotary system is that, at each revolution, the piston slows down, stops at maximal compression, and then moves again. In other words, it is kind enough to wait while the combustion takes place. This is why any piston engine can deliver power at variable rpms, typically between 500 rpm and 5 to 7,000 rpm. Here both diesel and gasoline engines are at an advantage, which is why you never saw a turbine automobile… Incidentally this was why the US Navy was reluctant, until the fifties, to adopt jet aircraft for aircraft carriers: At low speeds, a turbojet cannot deliver the force a piston engine delivers instantly with full throttle and full prop. And what will happen when the pilot misses its landing and needs to put full throttle to go around? Eventually the solution was found, part of it being longer runways on bigger carriers… Here visit http://hypertextbook.com/physics/thermal/engines But now comes the hidden advantage of diesel: Not only it can deliver power at different rpms, but on top of that, because the fuel is directly injected at exactly the right temperature without waiting for the propagation of a flame within the combustion chamber, (propagation depending itself on whether each of your plugs are clean and in good condition) you always use the exact amount of fuel you need whatever the rpm. So, efficiency remains high at all rpm, even when your taxi is fighting the traffic at 5 miles an hour on Fifth Avenue. European taxi drivers were the first to discover that their Mercedes 170 or 180 machines delivered a high mileage even in city driving, which is their main occupation! And diesel taxis started gaining market share, to the great benefit of the Daimler-Benz company. You can experience the same thing with a 172 Thielert, a Diamond diesel or a 182 SMA: Reduce your speed to maneuvering speed or to best glide speed and you will observe fuel flows going down to 2.5 to 3,5 GPH instead of 6 or 8! However we needed another actor to enhance the demonstration: the French IRS. This will be my next story… posted at 4:38 PM |
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