Tuesday, April 1, 2008
Airplanes, Cars—What’s the Difference?
What you need to know, on the road and in the air
|What’s the difference between car (or motorcycle) engines and aircraft engines? It’s a perennial question with a series of stock answers, none of which is sufficient.|
What do we have to do, and what do we want to do?
It won’t be long before 100LL disappears. (We’ve been hearing it for 30 years, so it must be true.) Lead is poison; we don’t want to use it when we don’t have to. We still have to use it because it’s the only practical way to boost octane (required by high-compression gasoline engines), and because the majority of aircraft piston engines need that octane to keep running. Though 80/87 worked for a lot of peashooters, sales volume was too low to allow its continued production, so we went to “low-lead” in the 1970s. (Note that 100-octane “low-lead” avgas contains two grams of tetraethyl lead per U.S. gallon, half that of aviation 80/87 and 100/130, but some 18 times that of the automotive premium of the 1970s.)
Eric Tucker, who knows the Rotax engines inside-out, said that auto gas (which is specified for the Rotax engines) and automotive-style (motorcycle-style, actually) oils make a good match, because all the components—engine, fuel, gearbox and oil—are designed to work together. When legislators (who are usually lawyers, not engineers) mandated the removal of lead from fuels, they had no idea of the secondary effects they were setting in motion (or they paid no attention). In addition to the octane-boosting effects of lead, Tucker said, “The designer relied on lead in the fuel to help reduce valve seat and valve wear, but now the old fleet is stuck because it must have the lead, or face expensive changes. Auto-engine oil has wear inhibitors to reduce wear, but it has created wear issues of another kind, such as camshaft and [flat-tappet] lifter issues.”
Tucker offered some advice for leaded-gas users: “Lead, combined with moisture, makes a chemical acid which can burn the bearings and leave pitting in aluminum surfaces. Long periods of nonuse, typical for an aircraft, cause many issues with lead contamination, one of them being the chemical actions that take place. The best practice is to change the oil before storage (draining the acids with the old oil). When people don’t do this, they keep the overhaulers busy.”
In the interest of fuel efficiency, we’ll need to have more-efficient engines. That means not only fuel-saving improvements (electronic engine controls, fuel injection, tighter tolerances, better cooling, improved lubricants), but also smaller and lighter engines. Metallurgy will provide many of the breakthroughs—lighter pistons and rods allow lighter cranks and cases, for instance—but additional fuel-flow, combustion and exhaust-design improvements are still necessary.
We also need lighter propellers that can produce good thrust from smaller diameters (reducing gyroscopic, straight-line and angular momentum, reducing the amplitudes of harmonics and running quieter at any given rpm while affording sufficient ground clearance, which in turn lowers airframe weight). Smaller engines can reduce frontal area or form drag; liquid-cooled engines often afford design flexibility in internal drag reduction that’s only sometimes offset by component weight and system complexity. Smaller engines also help reduce airframe weight: Attach points and hardware, engine mounts, etc., can all be smaller and lighter.
What’s being done, now?
We see constant improvement in existing engine technology: Aftermarket and OEM-direct shops such as Unison and K&N offer largely bolt-on incremental improvements through their STCs. New lubricants and coolants, including semi- and full synthetics, improve performance and component life. Detail improvements (such as improved metallurgy cylinders, new valve and head designs, roller lifters, plastic plenums) reduce weight or wear. Further along the evolutionary scale, increased use of turbochargers helps produce more power from any given package; FADEC (full-authority digital engine control) systems outperform even the most experienced and attentive pilots. Additional movement is promised by geared engines, diesels and compound-turbo-supercharged technology.
Ian Walsh, VP and General Manager at Lycoming (also a Six Sigma black belt), noted that the obvious costs can be reduced, and not only through higher production numbers. “Automotive manufacturers have become best in class when it comes to lean and cost-out efforts,” says Walsh, “because they’ve pioneered lean implementation, Kaizen ‘continuous improvement,’ Six Sigma methodologies and supply-base rationalization and transformation. Aviation is learning the same techniques and process improvements to make innovation more affordable.”
The piston engine will be around for a long time in its present configuration, if for no other reasons than that the installed base is so great and the costs of replacement are so high. New-generation induction, engine control and flow management (both internal and external—combustion and cooling) improvements will become more economical and widespread. Diesel technology, accepted worldwide (and grudgingly in the States), will become more popular as avgas becomes ever more dear. Smaller engines, geared engines, lower-power engines—all are in the near future. In other words, we’ll see progress on all fronts…except in bureaucracy.
Walsh concluded, “We should be excited that aviation engines are becoming more technologically advanced, economical and will undoubtedly become more affordable as automotive thinking plays a more influential role. Competition, in the final analysis, is the best creator of all!”
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