When Rotax surprised us last year with the rollout of its new 912 iS engine, we were skeptical of a claimed 20 percent improvement in fuel economy against the old standby 912 ULS. In a four-cylinder, high-RPM geared engine, that sounded like an overpromise. During our factory tour at Rotax, we politely told the engineers we would reserve judgment until the engine hits the field.
Eighteen months later, it has and just as we suspected, Rotax’s initial claims were off the mark. But in the wrong direction, if the company’s recent year-long test of the 912 iS is to be believed. Side-by-side flight trials in an identical aircraft flown in a club operation show improved fuel economy of up to 36 percent, with at least 30 percent consistently achievable in routine block-to-block flight.
A back-of-the-envelope calculation reveals that these numbers rival diesel efficiency and if mogas is factored into the equation, flying a 912 iS-equipped aircraft is substantially less expensive than the equivalent 912 ULS.
Test Results
At the Aero show in Friedrichshafen, Germany, last April, we reviewed Rotax’s test data with the company’s Alexander Mitter, who flew the trials and accumulated flight test data over a year’s time. Even as the 912 iS was being announced last year, Rotax was already flying comparison trials, but the initial fuel economy data it released was based on dyno runs and some flight testing.
To expand the data envelope, multiple aircraft were refitted with the 912 iS. Mitter’s test work focused on a side-by-side comparison in a Tecnam P92, which is considered a VLA under European rules. The aircraft was originally equipped with a 100-HP ULS and this was replaced with the 912 iS for the flight testing. No other modifications were done, according to Mitter, other than fuel plumbing. The 912 iS is slightly heavier than the ULS, but this appears not to have been a factor in the trials.
Mitter said the test protocol called for trials on days of similar temperatures and density altitudes and defined run periods. For both engines, the aircraft was climbed to a specific altitude, leveled and allowed to stabilize on one fuel tank. Then the fuel source was switched to a second tank and the consumption precisely measured on the ground after exactly an hour of flight. This was done at multiple altitudes. In addition, Mitter collected broader data that showed that the 912 iS is slightly more efficient in climb than is the ULS.
According to the Rotax test data, the 912 ULS burned an average of 4.65 GPH in the test profile, while the 912 ULS burned 3.25 GPH for the equivalent power output, an efficiency improvement of 30.1 percent.
Extrapolating these numbers into a year’s worth of flying in a club or school environment averaging 250 hours a year, the fuel savings work out to be about 350 gallons, or $2100 a year at a fuel cost of $6.
Over the eventual 2000-hour TBO of the 912 iS, the engine will use a whopping 2800 gallons less than the ULS, for a savings of $16,800. Depending on the core credit assigned to the engine at TBO, that could pay for the overhaul and then some. But more likely, it will account for just a substantial fraction of the overhaul cost.
We expect the 912 iS overhaul costs to be higher than the ULS, but we don’t yet know how much higher. Typically, ULS overhauls costs are about $13,500 to $15,000, but can be higher if valve or cylinder work is needed. The iS has more components, including the fuel injection system, ECUs and dual, high-pressure fuel pumps. We don’t yet know if these will be required replacement items.
Obviously, the cost savings for owners using mogas in the Rotax engines—and many do—will be less. According to www.airnav.com, mogas averages about $4.68 as of June 2013. Plugging that number into Rotax’s data reveals a to-TBO cost savings of about $13,000—not quite the total overhaul nut, but no trivial sum, either.
Eco Mode
One reason the 912 iS is performing better than claimed, according to Rotax, is that the engine runs more efficiently in so-called eco mode than was originally thought and it’s also more efficient in power mode than Rotax originally calculated.
The 912 iS ECU is programmed to run in eco mode at 77 percent power or less, at which point the engine will operate at what European engineers know as Lambda 1, or a stoichiometric air-to-fuel ratio of 14 to 1. Without providing much detail, Rotax said the 912 iS is also capable Lambda 1.3 or lean-of-peak operation. In power mode, it runs at about Lambda 0.8 or 0.9.
The 912 iS is approved for 91 AKI mogas, but in equipping the engine with future knock-sensing capability, Rotax seems to be anticipating approvals for lower-octane fuels. The iS, by the way, has dual-injector electronic port fuel injection in place of the ULS’s dual Bing carburetors. Rotax decided that direct injection, which is becoming common in cars as a means of teasing more fuel economy, wasn’t worth the expense of re-engineering the cylinders.
If Mitter’s test data is confirmed by real-world results, they may we’ll be proven right. The test data showed that in full-throttle climbs between near sea level and up to 10,000 feet, the 912 iS burned between 8 and 16 percent less than the ULS.
Similarly, in cruise, the iS did better at low altitudes than Rotax originally calculated, but at higher altitudes—8000 feet and above—it did much better, showing as much as a 36 percent improvement over the ULS fuel burns.
Gas vs. Diesel
Rotax’s tests validate a trend that’s been apparent in the automotive industry for two decades. Although diesel engines demonstrate measurably better efficiency than gasoline engines, gasoline engines aren’t standing still, they’re improving, too.
Using estimated performance data from the Tecnam P92 POH, we calculate a brake specific fuel consumption for the 912 ULS of about 0.44 pounds per horsepower/hour. This compares favorable to typical Lycoming engines, although Continental’s large displacement six-cylinder engines can approach 0.39 BSFC when running lean of peak.
By comparison, the 912 iS appears to be running at about 0.36 to 0.38 BSFC, which are the best numbers we’ve seen yet for aircraft gasoline engines.
Diesel engines from SMA, Thielert and Austro do a bit better than this at 0.35, but when mogas versus Jet-A or diesel prices are factored, the fuel operating costs for these engines are more comparable. Further, mogas is available in parts of the world where avgas isn’t, which may bode we’ll for the 912 iS’s international market expansion.
