Rotax Factory Tour

Gone are the carburetors in favor of fuel injection and dual-channel ECUs on the new 912 iS.



If there was anything surprising about Rotax’s rollout of its new 912 iS electronic engine last March, it’s that there was no surprise. Well, at least no showstoppers. As the dominant powerplant in the European ultralight market for more than two decades and the go-to engine for Light Sport Aircraft, Rotax was overdue for the follow-on engine. It was clear from the company’s unveiling at its Gunskirchen, Austria, factory that this project wasn’t being hatched half baked. By the time Rotax pulled the covers off, it was already testing the engine assembly line and preparing to ship the first engines. In an expansive tent set up on the factory grounds just for the purpose, a dozen ultralights and Light Sport airplanes were already equipped with the 912 iS, though few had actually flown with it. But Rotax has been flying a prototype for at least three years.

Nor was there anything particularly surprising about Rotax’s design strategy on the engine, with the possible exception of using two injectors per cylinder, rather than just one. The 912 iS has some features that could be considered uniquely Rotax, such as a pair of internal alternators, but the company’s basic approach to the engine is predictable. After all, Continental pioneered its FADEC engine in 1999, and Lycoming is coming up on the short strokes for certifying its iE2 technology for its large-displacement engines. Rotax may be late to the party.

The author visited Pipistrel Aircraft in Slovenia to get a look at the 912 iS installation in the Virus LSA. Pipistrel puts the water radiator on top of the engine, but it can go anywhere airflow and cooling drags suit.

Basic 912

Rotax has made some 40,000 of the 912/914 series engines, and given generally positive market acceptance, they’ve gotten the engine mostly right. Even though a reduction gearbox (engines have to reduce their maximum rpm from 5800 down to digestible prop speeds) could be a soft spot, the Rotax engines don’t seem to suffer for it. Some operators complain about having to tune the dual Bing carburetors, but that may be less onerous than doing mag inspections and swapping cylinders on a Lycoming. Although approved for it, the 912 doesn’t like leaded avgas, but the 912s are also approved for ethanol-blended avgas up to E10.

So in conceiving the 912 iS, Rotax used the opportunity to make some tweaks in the core engine. The cylinder-head internals were reshaped to improve coolant flow to provide more margin for lean running. The crankcase was also improved with new oil outlets and improved oil return, with a new pump providing two oil inlet choices to fit different installations. There’s also a new, more powerful starter. (Rotax will continue to offer the standard 912/914, by the way.)

Externally, the two engines are quite different. Gone are the two Bing altitude-compensating carburetors, replaced by an integral airbox on top of the engine that houses a sensor array. Induction is via four tubes that flow down into the intake ports, controlled by a mechanical throttle valve on top of the engine. Injection is port type, not direct injection, because the existing combustion patterns are sufficiently efficient to not benefit much from direct fuel injection, according to the company.

The 912 iS has an automotive-type common rail fuel-injection system held at 3 bar (43 psi) by a pair of Rotax-provided electric pumps housed in a single enclosed unit. The versions we saw had the pumps mounted on the firewall in an 8.4 x 6.18 x 2.87-inch housing. Although one pump is sufficient to operate the system, the second is used for redundancy and can be turned on for takeoff and landing. In the event that one pump fails in flight, the pilot has to bring the second pump online manually. The system requires a fuel return line to the tank from which it’s drawing fuel, which can be a main tank or a header tank.

Like its predecessor, the 912 iS has two automotive-type spark plugs per cylinder, with two direct-fire coils on each side of the engine. The common rail injection manifold is behind the green cover.

Rotax’s Christian Mundigler said one reason Rotax was able to move forward with the 912 iS was that the company felt comfortable with the quality of automotive electronic injectors, of which there are two per cylinder. Why two? For redundancy and to allow some finessing of the fuel charge. But this system may also have been designed to be scalable for a larger engine in the future. In normal operations, the two injectors work more or less together, though one is responsible for slightly more fuel volume than the other, and the system is smart enough to divide the fuel delivery to minimize wear on the injectors. Like all electronic injectors, the pulses can be timed precisely, and that’s how Rotax sets the fuel scheduling.

Physically, the two injectors are placed side by side on the top of the cylinders, and the rail itself hides behind a cover bearing the 912 iS logo. If one injector fails—unlikely, given the quality of high-volume automotive components—the engine ECU can detect the fault and instantaneously reschedule the good injector. The engine monitoring unit (EMU)—there will be several aftermarket choices here—will alarm an injector fault.

The 912 iS fuel system has two electric pumps contained in a single housing. One is primary, the other for backup and for takeoff and landing, if the builder wishes.

Like other systems of this ilk, the 912 iS has two electronic control units (ECUs), with the Rotax version provided by Rockwell Collins. Rotax constructed its dual-channel architecture with one ECU running “Lane A” and the second running “Lane B.” Lane A is the default operator, but the two channels compare data and feedback, and except for some minor sensor-input differences, Lane B is identical to Lane A. It can take over and run the engine at any time.

Both ECUs fit into a single module that measures 9.18 x 5.7 x 1.83 inches and should be mounted on the cold side of the firewall. At Gunskirchen, we saw 10 installations and noticed that LSA builders had placed the 912 iS’s components in various places, including the hot side of the firewall. But during a visit to Pipistrel in Ajdovščina, Slovenia, to get a closer look at its installation, all of the electricals were placed on the cold side of the firewall, behind the glareshield. This requires a port through the firewall to pass the thick-as-your-thumb harness that Rotax provides as part of the engine installation kit.


At Pipistrel, Tine Tomazic said the 912 iS consists of four major components: the engine itself, a fuse box or smart load center, the two pumps and the ECUs. For ignition, the iS has four coils mounted on top of the cylinders with short leads to the plugs. The engine has two automotive-type spark plugs per cylinder, and the system is set up so that one coil fires two cylinders on each side, providing full redundancy in the event of either a coil or plug failure.

Rotax plans to certify the 912 iS—that model will be the 912 iSc—so it had to satisfy regulators (and common sense) with some kind of redundancy for both the fuel and ignition system. To meet this requirement, Rotax stuck with the conceptual idea of the flywheel-mounted alternator used on the 912.

But for this iteration, Rotax uses a permanent magnet alternator design, called generators in the tech specs, consisting of rotating magnets on the flywheel and fixed coils. For redundancy, there are two alternators, one at 16 amps capacity, the other at 30 amps. To accommodate the higher power output, the alternator coils are cooled by oil immersion.

Rotax 912 iS.

Because both alternators are PMA designs, they don’t need external current for excitation but will generate sufficient current if the engine is spinning at a minimum rpm, which appears to be about 2000. For startup, the system relies on the starting battery, but once it’s firing and alternator A is making at least 6.5 volts, the smart fuse box switches from battery power to alternator power for the pumps and the ECUs. In other words, the engine will operate completely independently of ship’s power, just as with magnetos. Alternator A (16 amps) runs only the ECUs and the pumps, while alternator B runs everything else in the airplane and does the battery charging. If alternator A fails, the fuse box is smart enough to switch the engine over to the second alternator with no pilot action required, though it may shed the charging function. The fuse box can also cross-link for charging purposes. If the aircraft systems need more power, an optional 42-amp external alternator is available.

If both internal alternators quit, so will the engine. However, for tertiary redundancy, Rotax built in the ability to back up the system from the ship’s battery, which Pipistrel did with an emergency battery switch. It will run the engine fuel pumps and ECUs for 30 to 40 minutes. (The 912 iS requires about 15 amps to run it, so limp-home time will vary with battery capacity and condition.)

Pipistrel also did some significant development to integrate the engine into its airframe in a way that will appear seamless to a pilot used to the conventional Lycoming or Continental. Tomazic said that Rotax originally proposed engine cockpit controls that would consist of seven toggle switches: one each for the two ECUs, a master switch, an avionics switch, one for each of the two fuel pumps and the emergency battery switch. Rotax also proposed two momentary buttons for starting.

This struck Pipistrel as a bit busy, so they built their own box to integrate these functions; the pilot is presented with a conventional four-position ignition switch labeled Lane A, Lane B, Both and Start. It uses small LEDs as annunciators to show when the two lanes are deactivated during a routine runup.

Fuel Scheduling

Rotax is ever mindful that its entire livelihood depends on the burning of hydrocarbons for fun and it seems to be feeling the pressure from the green lobby to produce low-emission engines. “We like what we’re doing here, and we want to keep doing it,” one executive said during our tour of Gunskirchen.

Accordingly, the 912 iS is being pitched as an “eco” engine with lower CO and CO2 emissions and up to 21%-better fuel economy over like engines. The 912 has a reputation for sipping fuel, but it would more accurately be called economical than efficient. Its brake-specific fuel consumption is about 0.44 pounds per horsepower hour—similar to what a typical Lycoming delivers. If Rotax really delivers the 21% claim against the legacy 912, the 912 iS will be running at 0.36 BSFC, an impressive number indeed for a gasoline engine and close to par with aerodiesels. (We asked if the legacy 912 was considered a “like” engine for the fuel economy comparison, but Rotax never clarified this.)

When Rotax announced the 912 iS, Stock Flight Systems announced an engine monitoring unit for the new engine. It was shipping last spring. Garmin and Dynon also have monitoring capability in the works.

Rotax’s VP of product development, Wolfgang Wukisiewitsch, said that the 912 iS will have two modes—power and “eco” mode, which you are free to take as economy or ecology. Above about 77% power, the engine will operate in power mode, but below that—as determined by a combination of throttle position and MAP sensing—it will automatically switch to eco mode. Unlike most modern cars, the 912 iS doesn’t use mass airflow sensing because, according to Wukisiewitsch, the induction runs are too short to provide the zonal airflow mass air sensing needs to be accurate. “Throttle position,” he said, “is accurate enough, and it’s more robust.” Each cylinder’s main feedback datapoint is exhaust-gas temp (EGT) sensing, but there are future hooks for cylinder-head temps (CHT) and knock sensing already installed, albeit not programmed for initial shipments.

In the U.S., we tend to think in terms of rich- or lean-of-peak operations, but Rotax prefers the technical engineering term, Lambda. In eco mode, the 912 iS will operate close to Lambda 1, which is a stoichiometric 14.7:1 air-to-fuel ratio. Rotax wasn’t specific on precise fuel scheduling, but allowed that Lambda 1.3 (leaner) might be possible in some operating environments. In power mode, the 912 iS will run closer to Lambda 0.8 or 0.9. The 912 iS is approved for 91 AKI mogas, but Rotax, in including future knock sensing, may be anticipating lower-octane fuels or may just want more detonation margin. Or both.


Rotax left delivery customer schedules up to its distributors, so we checked with Mark Paskevich at Rotech Research, the distributor for the certified versions of Rotax engines for the Americas and non-certified in Canada, to ask about availability. He said initial shipments to select OEM customers would begin as early as May 2012, with larger volume coming in June and July. Before selling to builders, Rotech wants to complete its own installation and develop a training program, Paskevich said.

“Installation of the engine is critical. It has to be right or you’re going to have problems,” Paskevich said. “So we are selling the engine with an installation course included in the price. A customer won’t get his ECU for the engine until he’s attended an engine installation training program. This way, we can keep control with a builder who’s stuffing one into an airplane that’s never had one before. What we don’t want is for this engine to get a bad reputation due to a bad installation.”

How much does all of this cost? The complete U.S. kit version—engine, air box, ECUs, fuse box, pumps, ring mount, exhaust and training system, but not the radiators and other externals—sells for about $25,000. Builders are also on their own for the engine monitoring unit (the CANaerospace unit is $3893.50). Others may be in the works. (See sidebar.)

Finally, if you’re thinking dumping those old Bings in favor of some lightweight electronic boxes will save a few pounds, it won’t. Rotax says the 912 iS will be up to 13 pounds heavier than the carbureted engine, depending on installation variables. But look at it this way: If the fuel economy pans out, you can carry 20% less fuel to fly the same distance. Plug the numbers into a spreadsheet to see if the tradeoff is worth it.


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