Autogas for everyone: Part II

The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.

In Part I of this series, we reviewed the history of the autogas Supplemental Type Certificates (STCs) that allow us to use the fuel in hundreds of different legacy aircraft and engines. But what do aircraft owners do when their airplanes are not covered by these STCs, for instance later model Beech Bonanzas, Barons, Cessna 210s and 310s, Mooneys, and other aircraft with high compression-ratio engines? Many of these aircraft will be covered by the latest development from Petersen Aviation, the Anti-Detonation Injection (ADI) system for light aircraft. First, a brief history and description how it works, contributed by Todd Petersen.

Pilots of aircraft such as this Beech Baron 58 owned by writer Mac McClellan could save thousands annually using the Petersen/AirPlains ADI system and autogas.

ADI first saw widespread use during World War II. At that time it was used in American fighters and bombers as a means of developing significantly more power. The P-47 developed as much as a 30% increase in power with ADI, useful if someone was on your tail. In bombers it was used primarily for added power on takeoff. The extra power didn’t come from ADI by itself. Intercoolers were also necessary, and the fuel system went into auto lean at the same time the pilot hit ADI. It is also important to remember that we were using 115/145 octane avgas back then. Between the fuel, intercoolers, ADI and auto lean, significant extra power could be wrung out of an engine.

ADI is also useful for a different purpose. It can be used to maintain rated power while using a lower octane fuel, much lower than the 100 octane many of us think our engines need. This is possible because of the cooling effect of ADI fluid, which is a mixture of methanol and water, sometimes mixed 70/30 and sometimes 60/40. When it is injected into the inlet air of an engine it evaporates very quickly, cooling the charge and slowing the burn rate. Methanol packs a lot of octane, around 120, and this makes it possible to pack more fluid into the engine than would be possible with water alone, plus it keeps things from freezing at altitude.

In World War II, ADI was usually activated by flipping a small hinged detent out of the way so you could get more travel on the throttle. When the throttles were advanced into that range it threw the mixture into auto lean and turned on ADI. Today we turn it on with an electrical circuit that monitors manifold pressure and temperature. The I0-520s and I0-470s that are approved begin detonating at about 80% power, so the system comes on at 70% power.

We also discovered during our testing that detonation could occur at almost any power setting if the CHTs were high enough. Say for example you’re cruising along and you have a broken ring on one cylinder, or you develop an intake leak which gives you a lean condition in one cylinder. In not too long a time this can give high CHTs in that one cylinder and it can be enough to cause serious detonation. Consequently our ADI system is tied into CHT probes on each cylinder. On the I0-520 and I0-470 detonation starts at around red line so our system will turn itself on if the CHTs exceed 400°F. A warning light in the cockpit calls the pilot’s attention to it so you know to enrichen the mixture or make other adjustments to address the heat issue but ADI keeps the engine safe in the meantime.

An ADI system consists of a tank, lines, pumps, filters, and nozzles, along with the electronics to operate it. ADI fluid is not placed into the aircraft fuel tanks nor does it ever pass through any of the existing aircraft fuel system. A separate tank and set of lines is installed to allow ADI fluid to travel through the airplane to the engine. Fifty-five gallon drums of ADI fluid are available for $2.82 per gallon. If an aircraft owner mixes his own ADI fluid the cost is reduced to about $2.11 per gallon. Approximately one half gallon of fluid is used per engine per takeoff.

Petersen Aviation, Inc. of Minden, Neb., and Air Plains Services of Wellington, Kan., have joined forces to bring the ADI systems back onto the market and to seek FAA approval for more of them. With today’s uncertainty over the future of avgas the time is right to reintroduce these systems. Owners of Beech Barons and Cessna 210s have little to fear over the eventual demise of 100LL given that our ADI system offers an alternative that has already gained FAA approval. Regardless of what 100LL is replaced with, VLL (very low lead), unleaded avgas or autogas, as long as it is no less than 91 octane AKI it will work with ADI.

Pilots in other parts of the world may benefit even more than in the US. In West Africa 100LL is $16 a gallon. In Australia 100LL may dry up altogether now that the military no long requires it. Indeed, throughout South Asia and in Africa 100LL is prohibitively expensive when it’s available and in some places it just simply never is.

This series of STCs will be offered for sale by the end of the year through Air Plains Services at a cost of approximately $15,000 for a Baron, and $2,500 less for a Cessna 210. Additional detonation and flight testing is anticipated for engines and airframes not previously approved, such as late model Beech Debonairs/Bonanzas and Cessna 310s, among others, depending on demand.

What’s the bottom line? For aviation writer Mac McClellan, whose 1975 Be-58 Baron (pictured above) burns around 30 gph in cruise, the cost differential between avgas and autogas (on April 13 it was $1.48 according to AirNav) would result in nearly $9,000 in annual fuel savings for 200 hours of flying, about average for a well-traveled Baron owner like Mac. In less than two years, he would have amortized the ADI’s cost and be operating his Baron $44 per hour cheaper on autogas than when using avgas. And he’d be contributing exactly zero lead to the atmosphere.

For those who are concerned about the need for a small tank of ADI fluid at $2.82 a gallon, consider the plight of pilots flying aircraft with the Rotax 912 series engines. Rotax strongly recommends the use of a lead scavenger if the lack of ethanol-free autogas requires that avgas must be used. With lead scavengers costing well over $100 a gallon, the use of leaded avgas in the Rotax, an engine that not only powers many LSA aircraft but is now used in several light twins, results in a significant additional cost penalty compared to autogas.

For more information on the ADI system, contact Air Plains Services at 620-326-8581 or Petersen Aviation, Inc. at 308-832-2200. In Part III of this series, we’ll describe the growing list of new aircraft and engines that have been designed from the outset to operate on autogas.


  1. Kent Misegades says

    Barton, ethanol-free has disappeared from many gas stations, but there is plenty of it at fuel terminals. Ethanol may not be pumped through gasoline pipelines, which guarantees a supply of ethanol-free at terminals. Over 75% of all fuel terminals in the country have 91+ “clear” gasoline. It is blended at the terminal with ethanol before being shipped to gas stations. Airports however are not supplied by gas stations, but directly from these terminals. Marinas, large farms, and a number of other important markets exist for ethanol-free gas. It is really up to airports to support a small autogas tank, the reason we created the Aviation Fuel Club to help them find affordable fuel and fuel systems.

  2. Barton says

    Great idea, just one problem. It requires unadulterated auto gas. Unless the EPA/Federal Government can be induced to provide a source of fuel you’ve wasted your money installing an ADI system. No?

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