Rotax was founded in Dresden, Germany in 1920, but operations were moved to Austria in 1943.
The firm achieved success after WWII by building small engines for a variety of uses including snowmobiles from 1962.
Success with the latter led to their acquisition by Bombardier to produce engines for Bombardier Ski-doos.
Rotax has also achieved great success in engines for ultralight and light aircraft. This effort began in 1982 with two-stroke cycle single- and inline two-cylinder engines, production of which continue.
In the mid-1980s, they began developing larger four-stroke cycle engines for lightplanes.
Today, Rotax is the dominant international supplier of engines for ultralight and light aircraft manufacturers around the world.. They have reported the delivery of more than 100,000 aircraft engines.
The light sport airplane category for any length of time, that the vast majority of makes/models sold in the U.S. fly with Rotax four-stroke aircraft engines. While many readers are well-versed in the Lycoming and Continental engines that power larger and faster models, many may not know as much about Rotax engines.
“They came on the scene out of nowhere and very quickly were dominating the ultralight industry,” “Their engineers were putting in the work to produce different configurations to accommodate pusher-type and tractor-type airframes, very well-configured exhaust systems, and all different types of installations. It was pretty clear to me early on that these guys were going to be the leaders they are today.”
Rotax is the undisputed leader powering the light sport industry today, with an estimated 80 percent market share in new LSA models sold in the U.S. Other estimates put their market share as high as 90 percent.
The Rotax 916 on the Carbon Cub UL (Jon Bliss photo).
That successful run with their reliable four-stroke engine line started in 1989 with their 80 horsepower 912UL engine, which is still available to purchase. A 100 hp version, the 912ULS, is also available. As the company’s market share grew, so did their product line, and now, OEMs and experimental builders can choose not only the carbureted UL and ULS versions of the 912, they can pick the fuel-injected 100 hp 912iS Sport, the 115 hp 914UL, or the company’s largest powerplant, the 141 hp turbocharged 915iS.
That 141 hp turbo Rotax is now being used in the Sling TSi, with excellent results,. “The TSi is a very good four-place airplane, and Sling is seeing nearly a 1,000-pound useful load in that model by using the 141 hp Rotax engine,”
How Rotax engines came to be the gold standard with LSA manufacturers really comes down to engineering?
“Right from the very beginning, Rotax has been focused on making lightweight engines with great power-to-weight ratios,”
“They’ve designed engines that can cruise at 5,000 to 5,500 rpm all day long and go to TBO like that. From the metallurgy to the lubricants they use, it is all designed to allow the engine to run at high rpm.”
“You look at a 100 to 125 horsepower Lycomings or Continentals, and the Rotax engines are making that kind of power on about half the displacement, allowing for much lighter and more compact engines,”
“Many legacy aircraft engines do not have tight control over mixture cylinder to cylinder, so they must run rich of peak. The Rotax engines are very smart, with dual-path [engine control units] controlling the fuel injection. This allows for fully automated precise control over the fuel mixture which improves fuel efficiency, reliability and longevity providing super-efficient lean of peak operation.”
The Rotax 912ULS four-stroke engine is one of the most popular LSA engines ever produced. [Courtesy: Phil Lockwood]
While Rotax has factories in China and Mexico that can build the engines for some of their other applications, Lockwood said all their aircraft engines are only manufactured in their Linz, Austria, plant.
“The build quality of the Rotax engines coming out of Austria is very high. If you ever have a chance to go to the main factory in Austria, do so, because it is very impressive,”
As to what is in development for the Rotax aircraft engine line,“they seem to be quite committed to the aircraft engine market. And now they have quite a long history and are continuing to develop new engines. So exactly what is coming? I don't know. But it appears they're moving forward with higher horsepower engines.”
Rotax has scored a big hit with the 916.it’s also expensive, but that extra increment from 141 to 160 hp has opened the door to new performance in several designs that were good on the 915 but are significantly better with the newer engine. Some of those airplanes would be even better with more power.
Rotax takes the world view that its products have to work around the globe, which means they can’t depend on 100-octane fuel. In the next slot up, which I’ll call the 200-220 hp range, we basically have traditional aircraft engines that require big octane to make power. You could, of course, run an even bigger Lyconsaurus at a lower compression ratio but the power-to-weight figures get even uglier. So a Rotax product at this power level that could use, say, the equivalent of a high-octane auto gas (as the 916 does) would be a huge win and provide owners with the option of using auto gas, the currently available 94-octane unleaded avgas that’s sparsely available or something else. Point being that any future engine should be not designed to require 100LL.
It’s worth looking at where the Rotax line currently fits to help see a potential future. Of course, the 900-series engines are small four-stroke, flat-four engines with a gear reduction drive and liquid-cooled heads. The gear drive allows these small engines, which displace just under 83 cubic inches, to rev higher and make more horsepower. Our traditional engines, unburdened by gearboxes are, in effect, burdened by the inability to rev much higher than 2700-2900 rpm with any usefully sized propeller. As anyone who bases at an airport with a Cessna 185 present, Big Continental spinning 2850 rpm for takeoff with a large propeller is a gloriously noisy thing. But it’s not something you’d aim for with modern sensibilities or worldwide acceptance in mind.
By gearing the 900-series engines, Rotax allowed them to make more power than their displacement suggests but also to turn their propellers more slowly, which is good for noise and, to some extent, aerodynamics. The downside is weight and complexity, but the relative simplicity of the 900-series engines and their small size more than makes up for it. More advantages to the 900-series designs include modern combustion chambers—especially compared to the ancient chambers found in parallel-valve Continental and Lycoming engines, a good reason these engines need high-octane fuel to make decent power—and liquid cooling. In short, you can run a modern design harder—more compression ratio, more turbo boost—before destructive detonation begins to impact power and the liquid cooling further adds to the detonation margin.
Continental’s IO-520: Big, slow turning.
Rotax needs to build an engine in the 200-220 hp range. The 900 series it has reached its limits. The 912 iS makes 100 hp without a turbo on just under 83 cubic inches, which is 1.2 hp per cubic inch—itself more than double the power density of a Lycoming. It needs to make 91 pound-feet of torque at 5800 rpm to do so, and that requires a BMEP (brake mean effective pressure) in the combustion chamber of 165 psi to get there. While the horsepower-per-displacement number seems impressive, that’s actually a modest BMEP, or about what a 180-hp Lycoming runs. Clearly the 912 iS isn’t working all that hard.
But the 916 is. With 160 hp on tap for takeoff (limited to 5 minutes, then it’s 137 continuous), it’s working really hard. First of all, power-per-cube is a really impressive 1.93 hp/cubic inch. For that power, the engine has to make 145 foot-pounds of torque at 5800 rpm, a big jump from the 912’s figures, and that requires a BMEP of 263 psi. That is a huge number, one that really stresses the engine’s ability to resist detonation, which it surely only does through strict electronic control.
There are good reasons to have six smaller instead of four larger cylinders, mainly for smoothness. But the manufacturing cost difference in cylinder assemblies of different dimensions is virtually zero. So the fact that the six-banger has one-third more of them only adds to weight and cost.
It’s worth noting that Rotax is not the only player in this space. The ULPower 520T engine is impressive in its own right. It’s a 321 cubic inch six-cylinder engine with, like the Rotax, two pushrod-operated valves per cylinder. The 520T uses a modest amount of boost, just 38 inches for takeoff, and makes 220 hp at 2700 rpm. To do so, it’s making a phenomenal 428 pound-feet of torque from an almost-square bore/stroke ratio of 1.056:1. Heck, even a Lycoming is more “over square” than that, with a bore/stroke ratio of 1.717:1.
The ULPower’s BMEP is a modest 201 psi, above the naturally aspirated Rotax 912’s but well short of the turbo Rotaxes. No matter how nice the fins and effective air-cooling scheme is, it’s really hard to beat the efficiency of liquid cooling. The offset is the weight and complexity of a cooling system, of course, but make much more power from any given displacement.
Rotax is working on an engine in the next horsepower class up. Maybe two classes up. They could design the next engine with a more conventional up/down intake and exhaust system that would allow them to make both four- and six-cylinder versions with the same cylinders. Rotax will take a careful approach. The 900 series has been around for more than three decades, carefully developed and curated in that time to double the power output.
