How Tech Made the Rotax a Monster

The Lycoming 320/360 twins so owned the midrange segment that almost no one tried scaling the great gray wall. Until three years ago, at least, when Rotax finally incremented itself into the 320’s power class. It arrived with a flourish.

Rotax is actually short for the company’s real name: BRP-Rotax. The BRP stands for Bombardier Recreational Products, makers of motorcycles, ATVs, snowmobiles, and marine equipment. That all of these vehicles require gasoline engines gives Rotax a powerful technical grounding, even if the aviation division is but a tiny sprite in the BRP world.

Rotax benefits from this engine-centric focus, true, but also tremendous buying power from suppliers. When Rotax calls, you answer. And what Rotax doesn’t want to buy, it can create in house. The combination means that pretty much anything you can put into the CAD-CAM software you can also build.

Specifically, Rotax’s approach to putting 160 hp to the prop flange is fundamentally different from Lycoming’s. The familiar great-gray aero engine uses displacement, all 319.8 cubic inches of it, to create a tidal wave of torque that permits it to run a paltry 2,700 rpm at the max. By eschewing a gearbox between the engine and the propeller, the Lycoming is simple and relatively light—around 265 pounds, depending on configuration. Because it’s air cooled, it doesn’t require an external radiator or coolant, though the necessary baffle system does add weight and wear items to the list.

Yes, the Lycoming is slow-turning and simple, using its displacement to counter the almost-crude (albeit simple) carburetor and magneto-fired, fixed-timing ignition system. Maybe we don’t all love these systems, but they are at least familiar and can be made reliable with proper maintenance.

Turning to the Rotax 916 iS, then. While it does share the basic architecture with the 912, introduced at 80 hp in 1992, it has been thoroughly developed over the years. (And bear in mind that 33 years is a fantastically long life for an engine in the so-called powersports industry, where it’s uncommon for a platform to carry on more than a decade.) 

And that basic architecture is this: a small, 4-cylinder engine of just 86 cubic inches with an integral prop-speed-reduction gear set. The 916 turns a maximum of 5,800 rpm for takeoff, which seems stratospheric by airplane standards but is actually fairly conservative when you consider a motorcycle engine of this displacement might rev to 10,000 rpm. A 2.54:1 reduction drive has the prop spinning just under 2,300 rpm for takeoff.

Each of the Rotax’s four tiny cylinder heads has two valves, just like the Lycoming, but is surrounded by a water jacket, thereby providing the most direct path to conducting heat out of the combustion chambers. (All the 900-series Rotax engines have air-cooled cylinders below the head because there’s not much advantage to fully jacketing the barrels and, besides, it allows Rotax to have separate heads and barrels, which is the modern way to do it.)

For the 916 iS to get 160 hp out of a mere 83 cubic inches takes more than high engine revs—it takes turbo boost. A lot of it. Typically, the 916 runs 50 inches of manifold pressure for takeoff. For reference, some of the most highly boosted aero engines top out around 40 inches. The Rotax benefits from an air-to-air intercooler to reduce the temperature of the air coming out of the turbo compressor, which adds back some power lost to that heating but also helps bolster detonation margins.

Something else helps keep the engine from grenading: electronics. Unlike your carbureted, magneto-fired Lycoming, the Rotax uses fully electronic ignition and fuel injection systems. The dual engine control units (ECUs) manage everything and are very carefully programmed to protect the engine at high power levels. By running a fairly high fuel flow (also a way to combat destructive detonation) and retarding ignition timing, the 916 can live through five minutes of very high power without trouble. Indeed, the initial TBO for the 916 is 2,000 hours.

After five minutes, the pilot is expected to pull back to a maximum-continuous power of 137 hp, where the ECU gets the fuel flow down and the ignition timing into the ideal range for best economy. It’s not unusual for the 916 to run at cruise  on around 9.7 gph; that’s comparable fuel consumption per horsepower to the most efficient aero engines—a fairly impressive feat.

Power management is where the pilot will most notice the differences between the Rotax and the Lycoming. You have to coax the Lycoming to start, especially when hot, and you’re in the loop to maintain the correct mixture setting for the phase of flight, not forgetting to lean during taxi, go to full rich (most of the time) for takeoff, and correctly leaning during cruise and descent.

With the 916, you follow the checklist for the engine start, engage both ECU channels (something Rotax calls “lanes”), flip on the electric fuel pumps, and engage the starter. Within a couple of blades, the Rotax falls into a chattery idle. The big change is that you need to raise rpm a bit to make sure the integral generator is engaged, and you’ll also need a minimum oil temperature before departure. In cold climates, it’s likely the 916-powered airplane will be waiting a bit longer to go than the O-320-powered neighbor.

In flight, the 916 asks no more of you than to come out of takeoff power at or before five minutes, watch the engine monitor for any anomalies as you fly, and keep fuel in the airplane. That’s it. You can cruise level, climb or descend, all without changing power settings and, well, the Rotax just doesn’t care. Liquid cooling means so-called shock cooling on descent just isn’t a thing.

On top of all this, the Rotax is light. Specs say the 916 is just under 190 pounds, including the turbo and exhaust system. By the time you’ve added the external cooling system and fluids, call it 210-215 pounds. 

That’s actually a bit of the 916’s problem. Because it’s so light, it’s not really feasible to retrofit it into airplanes that currently use the O-320 without significant mods. An example from the experimental world: A Van’s RV-9 was converted to a Rotax and it took a very long engine mount and nose so the lighter engine balanced the aircraft. Instead, we wait for an expansion of MOSAIC and new aircraft—trainers and travelers alike—to take best advantage of this new aero-engine tech.