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The only reason leaded avgas still exists is to deliver high octane cheaply and the only reason for octane is to prevent detonation in high-power, high-compression engines. But octane isn't the only way to quench detonation, something engineers have known for years.
Injecting a water-methanol spray into the combustion chamber—so-called anti-detonation or anti-detonant injection (ADI)—was once a common technique for military aircraft when octane wasn't available or when aircraft designers wanted excess power in bursts, even when burning high-octane fuel. It was also used in civil transport applications.
If it worked 60 years ago, why not now? That's exactly what Air Plains is proposing in its resurrection of ADI STCs developed by Todd Petersen during the 1980s, when mogas as an alternative fuel was in vogue. But just as mogas fell out of favor, a victim of a shrinking cost difference between it and avgas and a flood of ethanol in the fuel stream, so too did Petersen's ADI fade. It never found wide market application. But like Petersen's hundreds of mogas approvals, the STCs remained alive.
Air Plains, which is a mod and engine conversion house in Wellington, Kansas, is updating Petersen's original work using state-of-the-art electronic controls developed by Electronics International. Air Plains has in mind systems that would allow high-horsepower engines to operate safely on either 9lUL or mogas of suitable octane.
The underlying idea behind ADI is to use a water/methanol mix to cool the combustion event and slow the propagation of the flame front, which tends to accelerate when the fuel/air mixture encounters the hot surfaces of cylinder walls, valves and pistons. This leads to instant, explosive ignition that we know as detonation. Even high-octane fuels will detonate if the cylinders get hot enough, but octane serves to yield a more orderly flame front and thus provides detonation margin.
Although water alone is an effective anti-detonation agent, early research revealed that a methanol/water blend (about 60/40) is more effective and the methanol actually adds a little energy to the combustion process. It also serves as anti-freeze, protecting the ADI reservoir from freezing down to about minus 40 degrees C.
Air Plains isn't the only company exploring ADI. At least one European manufacturer, Tecnam, will reportedly use ADI in a new twin called the P2012. Powered by Lycoming's new TEO-540-A1A, Tecnam says the airplane will be operable on mogas. Another Lycoming-powered aircraft, Grumman's pilot-optional Firebird drone, will also have ADI, presumably to operate in theaters where 100LL isn't available but mogas is.
Traditionally, ADI injects the fluid not directly into the cylinders but into the induction manifold downstream of the carburetor or fuel injection throttle valve. This simplifies and lightens the system and it's the approach that Air Plains is following with the revised ADI system it will soon have ready for the market.
"The main goal we're working on now is to have it available for today's technology. By that, I mean some of the electrical equipment used before is a little outdated," says Air Plains' Rafael Soldan.
Petersen's original STCs covered the IO-470 and IO-520 families and four airframes, the Cessna 188 and 210 and Beechcraft 55 and 58 Barons. Although the existing engine STCs approve the engines, Air Plains will need to seek additional airframe approvals to expand the market. So far, only non-turbocharged engines are approved, but Air Plains says it has turbocharged engines on the to-do list. Soldan told us the system could be made available for Experimental aircraft if there's sufficient interest.
The system consists of a baggage-compartment-mounted tank—5.5 gallons for singles and 11 gallons for twins—plus two pumps, a main and a backup pump. The injection point is through an add-on plate downstream of the throttle body.
The ADI is controlled by what Air Plains calls a TPCU or temperature and pressure monitoring unit. The system is set up to inject when either of two parameters are met: 25 inches or more of manifold pressure or CHTs of 400 degrees F or more.
"This system prevents detonation in a way that's super conservative. When it was tested originally, detonation occurred at way higher parameters than what we're using," Soldan said. Electronics International has been engaged to make the control unit, which is pictured here. The system is essentially automatic, switching on when the ADI parameters are met. It can be switched off or on manually by the pilot. The backup pump is operated from a separate electrical bus. There are also low fluid warning lights.
Fluid flow is either on or off, with no modulation and consumption is one gallon per 10 minutes of operation, or about 6 GPH. However, injection typically wouldn't be used in cruise flight or even high-altitude climb in normally aspirated engines. The fluid blend is 60 percent methanol, 39 percent water and 1 percent soluble oil to keep the system lubricated. Methanol bought in 55-gallon lots is about $2 a gallon and Air Plains figures the pre-mixed fluid cost at about $2.11 a gallon.
Weight of the system is estimated to be about 42 pounds for a single-engine airplane, including a full tank of fluid. As of early 2013, Air Plains said a single-engine ADI system would be priced at about $9000 and a twin around $11,000. For more, see AirPlains.com or call 800-752-8481.
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