Flying a Big-Bore Engine Without the Lead

Continental modifies an SR22 engine to use unleaded fuel.

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Cirrus SR22 Turbo

My test flight in Continental Motors' developmental Cirrus SR22 was among the most unusual and interesting of my career. That was in part because I was testing a major mod of an airplane in which I have a lot of flying experience, and in part because I was glimpsing into the future by flying a big-bore-powered airplane using unleaded fuel.

The airplane I flew was topped off with 94UL in Mobile, Alabama, that morning. By the time it made it out to Austin, Texas, we still had a couple of hours of unleaded fuel, enough to see how the TSIO-550 performed on 100-No-Lead before adding 100LL for the remainder of the flight.

The worry has always been that heavily breathing engines like the turbocharged 550 would be the acid test for unleaded fuel, but today the prospects are far less scary.

When it comes to aviation piston engines, lead is magical stuff. It keeps the temps down, internal parts lubricated and the octane nicely boosted. But current guidelines call for an end to 100LL by 2017, which is much closer than you might imagine.

Luckily, Continental believes that it can live with that deadline and is taking what it calls a "two-pronged" approach to the problem, coming up with modifications to its gas piston models while actively (albeit quietly) developing piston engines capable of burning jet-A -- yes, diesels.

Continental President Rhett Ross tells me that the company will be ready for the unleaded change. Continental's team believes that with some workable changes to the valve seats in some models and to the combustion and ignition systems in others, and with no changes to some models, its gas piston engines will be compatible with the new fuel. Of course, because 94UL is lower in octane than 100LL, its potential energy will be lower across the board, regardless of whose engine the future fuel is being used to power.

That said, the performance of the SR22 I flew on 94UL was impressive. Not only did it produce a good deal more power on takeoff and in climb on 94UL than the factory SR22 does on 100LL, but it also ran cooler, even at power settings in the high teens.

There are a lot of questions to be answered here, and one of them is how 94UL will do on a hot day at high altitude and a high power setting. The answer for us was "great." Had we climbed into the 20s (which we did not do), the book likely would have required some limitations on power settings to maintain the required detonation margins, says Continental's Keith Chatten.

According to Ross, the company has a plan to have its engines ready for 94UL when the time comes, and the TSIO-55 will be among the first to get approval.

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