Turbo Diesel Registry
Issue link: http://read.uberflip.com/i/594528
118 www.turbodieselregister.com TDR 90 DIESELS IN THE SKY Back in the 1920s it was unclear what kind of engines would power future aircraft. Gasoline-burning, air cooled radial piston engines were light, but their poor cooling forced operators to run rich. The weight of fuel that must be carried is subtracted from the payload, so this was a serious problem. Liquid-cooled gasoline engines were heavier and they didn't have to run rich. Liquid cooling itself brought problems – the US Navy reckoned that half the engine failures naval aircraft suffered in the early 1920s were cooling-system-related. The idea of supercharging to increase an engine's power existed, but putting it into practice was not easy. Supercharging pushed poor fuels into detonation, and even though improved stainless steels for exhaust valve use existed, valves ran hot and often broke at the "neck" where the slender stem met the flare of the valve's head. Besler's steam airplane showed a way to use low-quality fuels to power flight, but despite well publicized demonstrations, there were no takers. One unique feature of Besler's engine was that it was reversible! This made short-field landings a snap. The Diesel engine was the remaining possibility. The Diesel engine was the remaining possibility. Highly efficient in terms of fuel use, Diesels were mainly heavily-constructed industrial or marine engines. It would take a big dose of design-for-lightness for Diesels to take to the air. With their high compression ratios, Diesels already took much more energy out of their combustion gas than any spark-ignition engine could. That reduced exhaust gas temperature to the point that exhaust valve durability was a relatively minor issue. In 1927 the US Navy announced it would buy no more liquid-cooled aircraft engines (which caused Pratt & Whitney to spontaneously come into being). Prestige car maker Packard had a line of liquid- cooled engines that now looked like becoming surplus. (PT boats would be powered by Packard 2500 V-12s that had never taken to the air.) They now designed a 9-cylinder radial Diesel, making 225 horsepower from an encouragingly light 510 pounds. Another taker was a Dallas supplier of oil-field equipment, Guiberson. Their Diesel radial 9 was likewise able to reach a specific weight of 2 pounds per horsepower, a good achievement in 1932. Aviation writer Bill Gunston wrote that, "…the A-980 took the Waco 960 miles to Detroit on 96 gallons of furnace oil at 7 cents a gallon." Pretty attractive. Guiberson built tank engines in WW II and were never able to make a business of Diesel aircraft engines. Yet the attraction was there; with a specific fuel consumption of .382 pound/hp-hr, it used only 3/4 as much fuel as the gasoline-burning, spark-ignition engine. The age of airships encouraged construction of the giant Daimler- Benz 602 Diesel engine at over two tons, powering both the Hindenburg and Graf Zeppelin in sets of four. The maker, whose engines actually made it to mass production, was Junkers with their two-stroke, 6-cylinder, 12-piston, dual-crankshaft Jumo 205, which powered many Ju-52 transport planes. Although heavy for its power and demanding in terms of manufacturing precision, it achieved a cruise specific fuel consumption of .35 pound/hp-hr, or roughly 30% better than spark-ignition engines of the time. Other German makers prototyped larger Diesels of very high performance, for which there was at the time no application. The Soviets, with the added incentive of having no cheap alternatives by which to produce high octane number spark ignition fuels necessary for very high power, also produced some promising two-stroke Diesels for use in large aircraft. Well-known maker Allison had contracted to build a big two-stroke Diesel for airship use in 1927. It tested satisfactorily, but the Navy dropped it because of the difficulty of recovering water ballast from its exhaust (water recovery would partially offset the weight of fuel burned, requiring release of less helium as the fuel load decreased). And then the several tragic losses of large airships redirected aviation toward heavier-than-air craft. Then two developments completely changed the aircraft engine field. Then two developments completely changed the aircraft engine field. One was the wide adoption of Thomas Midgley's 1922 discovery that tetraethyl lead, added in small quantities to gasoline, greatly increased its resistance to detonation. The other was the perfecting of internally-cooled exhaust valves. This idea had originated with mercury as the internal coolant; by filling a hollow valve stem about 60% with the liquid metal, the motions of the valve would cause the metal to rapidly move heat from the failure-prone ‟neck" of the valve to its stem, and thence into the valve guide. Later, S.D. Heron experimented with mixtures of salts which improved heat transfer by their ability when molten to ‟wet" the interior of the valve stem. The final step was adoption of sodium as the coolant. A solid at room temperature, sodium melts to a liquid at just below boiling water temperature. Millions of such valves were produced. Thought Provoking Discussions with Automotive/Motorcycle Journalist Kevin Cameron