What About Those Diesels? Part 1

The first practical engine was built in 1897, yet in aviation diesel technology is still in its infancy

Adding weight surely does that, but represents the least elegant solution. More-typical expediencies include incorporation of clutches or slipper washers in the drive system (sometimes in conjunction with a gearbox, which can also be tuned), experimentation with firing order, and more-sophisticated control of ignition and burn rates within the combustion chamber.

Additional work has been done with materials — some are quite good at changing the harmonics, but cost a bundle. (Anyone seen titanium engine cases lately?) Lower-cost manufacturing sometimes involves higher-cost materials or processes and smaller or less-complicated parts.

As engine designers work through this calculus, we sit on the sidelines and ask,

“When can I have one?”

Several attempts have been made, and some have met a degree of success.

There are a lot of factors behind this: necessity is one. For almost as long as I can remember, we have heard of the imminent demise of avgas. Like the preacher who predicted the end of the world six times already, the doomsayers shout even louder: “This time, we really mean it!” And this time, they’re probably right, at least about leaded avgas.

Unleaded gas, the readily available alternative, is commonplace in cars, but cars don’t need 100 percent power settings for every takeoff, or 75 percent power settings for most of their lives. Worse for mogas in aviation is the ever-changing blending of different auto fuels and the inclusion of hygroscopic (water-seeking) Ethanol in the mix. Auto gas, because of its inconsistency, changing formulations (usually dictated by unpredictable bureaucracies), uncertain storage and transportation, and lowered power content, does not offer a promising alternative to avgas. Additionally, the lack of lead in unleaded gasoline may pose problems for some engines’ internals.

Aviation diesel fuel, commonly known as Jet-A, is regulated, relatively consistent, and of high power content. (Jet fuel contains more Btus per pound — more available energy — than even the best gasoline.) An engine built to use Jet-A will deliver more-consistent results, crucial in aviation. Further, jet fuel, because of the enormous demand, will be around for decades.

Not long ago, Jet-A was considerably less-expensive than avgas in the U.S., and that difference fueled a lot of interest in the black-smokers; but the relative prices have recently closed or inverted. Relative fuel costs are no longer significant.

Diesel engines are more efficient than gasoline engines: they convert more of the available energy into useful energy; and they use a more powerful fuel. It stands to reason that diesels should be better at power production. As a byproduct of their thermal efficiency, they also emit less of many undesirable compounds into the air. Although we all are familiar with diesel smoke, “aroma,” and soot (as accumulates in exhaust pipes), these byproducts are considered less-harmful than many of the invisible pollutants produced by gasoline engines.

Still, it is the power to weight ratio of the full system that matters in aviation. Diesel advocates often talk about fuel burn in “gallons per hour,” knowing that diesel fuel is roughly 19 percent heavier than avgas. (This has implications for the airframer, too: what are the overall weight impacts on structure and payload? Are there possible CG effects in adding weight to fuel tanks?) Note, too, that there are significant differences in thermal expansion between gasoline and diesel. So, to get a more-valid comparison, think “pounds per hour” on fuel burn, but accommodate the weight difference and temperature/volume effects.

Where’s the progress to be made?

Diesel technology in aviation is still in its infancy, while gasoline aero-engines are well-developed. There is simply more progress yet to be made in innovating with aero-diesels. Past achievements in gasoline engine technology actually work against innovation and further development in gas engines. Since improvements in gas engines will likely be small, there is less incentive to pursue them. Diesels face the opposite prospect: innovations will be rewarded by their use on large numbers of engines. AMT


In the concluding episode, we’ll look at current and near-future aero-diesel technology.

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