In the world of internal combustion engines, technology has declared war on octane. Thanks to sophisticated engine control units and knock detection, ever fewer modern cars require high-octane fuel. Aircraft engines, naturally, have been left behind, stranded on their own little island of octane neediness. And so the perverse problem of finding a replacement for leaded 100-octane avgas.
But what if there’s just enough octane in the automotive gas pool to make it work in aircraft engines that had a little detonation control? That’s the idea behind Airplains’ resurrection of ADI or anti-detonation injection, a simple, proven technology that employs a light spray of methanol and water into the induction pipes to quench potential detonation, making it possible to burn lower-octane fuels in high-compression engines.
With a replacement avgas far from resolved, Airplains is betting that ADI will find some traction among owners who might be attracted to the mogas option if they own engines that require 100-octane but who would prefer to pay less for fuel. (Who wouldn’t?) In fact, even though it remains an underused option, most engines in the U.S. general aviation fleet can burn lower-octane mogas with nothing but an STC. But those that can’t burn mogas consume the majority of avgas sold and Airplains thinks some of those owners will like the idea of saving $12 to $20 an hour on fuel costs, even if they have to invest $12,000 to do it.
ADI is well-proven technology that simply fell out of favor for lack of need. In the heyday of high-performance piston engines during World War II, fighter aircraft were commonly equipped with ADI, despite the availability of higher-octane fuels than are produced today. ADI was typically used intermittently in tactical situations where a pilot needed every bit of horsepower the engine could produce for brief bursts. The injection could be controlled manually or automatically by a switch that activated at high throttle settings. ADI also found application on some civil airliners, including Lockheed’s famed Constellation, the DC-4 and the Martin 404. When piston fighters and airliners faded, so did ADI, although it actually found some use in jets, too, including the B-52.
Airplains’ iteration of ADI is a revisitation of technology originally developed by Todd Petersen in the 1980s as part of his larger program to market STCs for mogas. Market acceptance at the time, says Petersen, was anemic because the system wasn’t cheap and the price difference between avgas and mogas was only about 50 cents. “In those days, there was no hurry to get rid of 100 octane. Nobody thought it was just around the corner. We thought it was further down the road,” Petersen says. Leaded avgas’s demise is still not just around the corner, but it’s generally agreed we can see the end of it within five years or so.
Insufficient octane causes the flame front in engine cylinders to propagate too rapidly, leading to what is essentially an explosion with rapid pressure spikes and locally severe heating on the tops of pistons and in valves. ADI addresses this in two ways. First, it lowers the temperature and increases the density of the incoming induction charge, removing or at least lessening the conditions for detonation to begin. Second, the ADI blend provides enough cooling of combustion chambers and valve surfaces to suppress the rapid flame front advance that would otherwise lead to detonation.
Why a methanol/water blend? Would water or methanol alone suffice? Either would, but Petersen, in mining research done for ADI systems 70 years ago, found that a 60/40 ratio of methanol and water works best. (There’s also a dose of water-soluble oil in the fluid to reduce the corrosive effects of methanol. Owners can mix the stuff themselves.)
The methanol adds heat to the combustion charge, which isn’t produced by water alone, and it also provides freeze protection down to minus 40 degrees C. Straight methanol, on the other hand, has a low boiling point and could get vapor locked inside a hot engine compartment.
A report Petersen did describing his research revealed that during World War II, the Royal Aircraft Establishment referred to ADI as a “dual-fuel system.” Low octane was found satisfactory for cruise, but ADI resolved the octane deficiency for high-output takeoff or high cylinder-head temperatures. And that’s exactly how the Airplains Inpulse system works.
Airplains’ Mike Kelley and Rafael Soldan described the Inpulse system as simple or, to use Kelley’s descriptor, “dumb.” There’s not much to it. The basic plumbing consists of a five-gallon tank in the rear of the aircraft—at least in the Cessna 180 we flew—plus ¼-inch lines running forward to the engine bay, where they’re routed into a pair of nozzles. The nozzles are threaded into a custom plate that’s mounted in the induction system either upstream or downstream of the throttle plate, variable with aircraft and engine. A solenoid valve toggles the fluid flow—it’s either on or off, and it’s not on much.
The system is low pressure, driven by a pair of pumps—one primary and one backup—at about 6 PSI, As shown in the photo above, the pumps are tiny and live on mounts at the base of the tank structure. The tank itself is double-walled aluminum as a safety measure against the ADI fluid’s flammability and it has a filler nozzle outside the aircraft behind the baggage compartment.
The “dumb” part, says Kelley, comes in the control of the system. It’s sensor spare, needing only to know when manifold pressure is higher than 25 inches or CHTs are above 400 degrees F. That sensing is done by a 2¼-inch panel-mount computer with a pair of switches. One turns it on and tests the system; the second allows overriding the computer’s commands to run the backup pump and flow ADI fluid continuously, independent of MAP and CHT. Total installed weight of the Inpulse, including fluid, is about 65 pounds. Not a huge hit against payload, but not trivial either.
The system flows 6 GPH of fluid through the two nozzles, which are pointed at each other in the throat of the induction plate. That sounds like a lot of fluid for a single engine to gulp and it is. But because ADI is mostly about addressing octane shortfall during takeoff, it doesn’t flow much fluid on a typical flight. A high-horsepower, normally aspirated aircraft usually climbs briskly and in under five minutes, it’s likely to be below 25 inches MAP, if the pilot hasn’t reduced power in a lower level off. So on a typical takeoff, you’d consume about a pint of fluid, although a takeoff heavily loaded on a hot day might require double that if CHTs remain high during the climb. Soldan told us that on a trip from Kansas to Georgia and back, he used about a quart of fluid.
Then why carry five gallons of it, which contributes mightily to system weight? It’s a tradeoff, explains Petersen. To guard against running out of fluid and getting stranded where you can’t find any, you need to carry enough to last for awhile. Carrying jugs of it in the baggage compartment probably isn’t practical, since you’re still hauling the weight without the protection of the double-walled tank.
The cost of the fluid, by the way, isn’t much of a factor. In bulk, methanol costs about $2.50 a gallon and the soluble oil, the same type used for machine cutting tools, is readily available. Stashing a small drum of methanol in the hangar and mixing it as needed would be practical.
To sample the system’s performance, we departed from Wellington, Kansas, in Airplains’ Cessna 180. Having Inpulse aboard doesn’t add much workload for the pilot, other than requir ing a pre-takeoff check to see if the system has fluid and is flowing it. That involves running the pumps manually and watching for indications that the induction is digesting the water.
Interestingly, the effect of the water is not instantaneous. With the 180’s IO-520 at run-up power, there’s not enough MAP to switch on the fluid, so you do it manually and watch for a slight decrease in indicated MAP. But the decrease is indeed slight and it’s surprisingly slow in coming—several seconds. Switching off the injection had the reverse effect; a slower-than-expected rise in manifold pressure.
Todd Petersen told us that his developmental research indicated that quenching detonation in a cylinder is more or less instantaneous when the fluid stabilizes the flame front but without knock sensing, you’re unlikely to notice it through any physical manifestations. Test data Soldan reviewed with us confirms this.
Because of the thermal mass of cylinder heads, CHT indicators are slow to respond after post-quench detonation. Once injection is applied, it takes fully 30 seconds to even measure a CHT effect and more than a minute before the temperatures cool significantly. Soldan said cooling is on the order of about 20 degrees and although that doesn’t sound like much, it’s sufficient because detonation tends to have a sharp temperature/pressure threshold and reducing CHT by that amount is quite likely to snuff it out.
Because we flew on a cold, late-winter day in an airplane without good CHT sensing, we’re accepting Airplains’ test data, backed up by Petersen’s experience and that of Inpulse user John Otte. In our view, there’s no argument that the system works; the challenge is economic, not technical.
Airplains’ Mike Kelley recognizes that Inpulse’s appeal is neither universal nor a cinch for those owners who could use it. How, he asks, are you going to convince a Baron owner who’s running a split fuel system—avgas for takeoff, mogas for cruise—without the bother of an STC to buy an expensive system that carves into payload? On the other hand, Airplains is getting nibbles from non-U.S. customers where mogas is both more available than in the U.S. and sold at a much higher price Delta. Typically, in the U.S., mogas is $1 to $1.50 less than avgas, but it can be more. Even at $1, that’s as much as $18 an hour in fuel savings for large displacement engines. If you flew but 50 hours a year, you’ll save $900 in fuel. Not bad, but against a $12,000 investment, that’s a long payback. But if the avgas/mogas price difference rises to $2, which it very well could once 100LL is gone and replaced by an unleaded fuel, and you flew 120 hours, the savings would be $3600 a year. In our view, that looks more attractive.
Still, Airplains will need a lot more engine and airframe STCs to gain much market penetration and these are expensive to develop individually. The company is hoping to make deals with interested owners and we can only hope that in the meantime, mogas finds wider distribution. Right now, only about 115 U.S. airports carry it.
To us, Inpulse looks like a long-term play that may find buyers after the 100LL crisis is resolved. Nothing will quite focus owner attention on mogas like $7 or $8 avgas. It’s already gaining some notice in offshore markets.