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Rotax Aircraft Engine

Rotax Aircraft Engines, Design

Rotax four stroke aircraft engines have proven themselves as reliable pieces of machinery. They can be bought as experimental (UL), sport (S) or certified (A and F) engine and there are four main models (fuel injected too: i models) to date.

The original engine is the 80 hp 912, from which the 100 hp 912S was developed and Rotax also has a 115 hp turbocharged 914 engine. In 2015 BRP-Rotax announced the fuel injected 141 hp turbocharged 915iS.

Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the French market), the 912iS is green but without the carburetors and the 914 has red covers and a turbo. The 915 is a turbocharged version of the 912iS with blue covers. These are the most obvious differences.

Technical data on the Rotax engines can be found in these pdf files: Rotax 912 UL/A/F, Rotax 912 ULS/ULSFR/S and the Rotax 914 UL/F.

These engines are very popular in the experimental aviation community especially in two seat Ultralight (ULM) and Light Sport Aircraft (LSA) models and are becoming commonplace in the general aviation community too.

Some are also building a twin Rotax engined powered aircraft: Both Tecnam and Dyn'Aero are still using the 912S model for that. But I can see them using the fuel injected 912iS any time soon now!

Rotax four stroke

Rotax Engine Top View

The three four stroke models all share the same basic design you will find on most aero engines: four cylinders in boxer / horizontally opposed configuration, fuel carburetion or injection system with engine controller, turbo or naturally aspirated, dual ignition, intake and exhaust.

Rotax 916?

Sep 2021: Rumor has it that Rotax is working on a 160 hp version of their successful line of aircraft engines. This should be one to keep an eye out for! Maybe they started to work together with Edge Performance, I am guessing.

Rotax 915iS Engine

Turbocharged Rotax 915iS

During AirVenture 2015 at Oshkosh BRP Rotax announced the new fuel injected turbocharged 915iS 141 hp engine.

According to Rotax: "Based on the proven concept of the Rotax 912 / 914 engine series, the Rotax 915iS engine offers more power, the best power-to-weight ratio in its class, full takeoff power up to at least 15,000 feet (4,570 m) and a service ceiling of 23,000 feet (7,010 m)." This new 915iS has its roots in the 912iS with the same displacement (1358 cm3) and but with a turbo and intercooler (the 914 has no intercooler and a smaller turbo).

The engine has been available for OEM as of second half of 2017. This should be a great engine for the high performance experimental and light aircraft which needs just that amount of extra power to perform even better in high and hot conditions.

As of 2018 Rotax supplies sodium filled replacement exhaust valves for the 914 turbo engines, these were already included in the 915 engine. Sodium filled valve stems have a better heat conducting capability than normal valves, an upgrade which is worthwhile. You may mix standard and sodium valves in the same engine as their dimensions are the same. More at the Youtube channel of flyRotax.

Dec 2021: Rotax announces a 24 V option of their popular 915iS engine. It is capable of 800 watts of power to supply the most modern cockpit with enough power to run all equipment.

Rotax 912iS Engine

Fuel Injection, 912iS

2012 saw the introduction of the fuel injected 912iS with the same amount of power as the normal 912S, 100 hp. The automotive style ECU makes it possible to time each fuel charge per cylinder and it has dual injectors per cylinder. Rotax claims 20% - 30% reduction in fuel consumption and 2000 hours TBO (source: avweb, EAA).

To obtain lower fuel consumption Rotax designed the engine controller with two modes: power (takeoff and climb) and economy (cruise) controlled by the throttle setting (below 96,9% its economy, lean mixture and above 97,3% its in power mode, rich mixture).

A number of options can be installed: vacuum pump, alternator (the 912iS has an internal dual alternator 13 A for the ECU and 30 A for the aircraft), controllable propeller (not all options can be installed at the same time).


These have the designation C behind the type: for example iS and iSC for the injected model. Non-certified carburetor engines use the designation UL.

More Torque

Rotax has improved the torque curve with the 912iS Sport model. This modification in the ECU increases the torque between 4800 and 5000 RPM from under 120 Nm up to 130 Nm, a whopping 9 % increase! Power also increased a little bit. This model is better suited to high altitude operations.

For those of you wanting to read more information on this remarkable fuel efficient engine, follow the next link to the 912iS operators manual.

Soft Start module

Rotax has a soft start module for all engines after a certain serial number. The standard ignition modules fire at 3° BTDC (on the compression stroke) and then when the engine runs, it switches to 26° BTDC. The new soft start modules fire at 4° after TDC and then after about a couple of seconds swaps to 26° BTDC. This eases the stress on the engine and facilitates easier starts. You know when you have one of those modules as the engine will suddenly run smooth about 5 seconds after it has started. More info here at Rotax News Network.

Carburetor & Fuel Injected

The original Rotax has two automatic altitude compensating Bing carburetors with altitude adjustment (no mixture control for the pilot), dual electronic ignition, gearbox with 1 : 2.43 ratio (the 912 and early 914 have 1 : 2.27 ratio's), liquid cooled cylinder heads with a radiator, air cooled cylinders, external oil sump with radiator, mechanical fuel pump near the gearbox (the 914 has two electrical fuel pumps).

The new iS engines have a fuel injected system combined with a FADEC. These engines depend heavily on their redundant electric fuel pumps. Fuel consumption is field proven remarkably lower compared to the carburetor versions.

Complex engine?

When looking at a Rotax installation the first thing that everyone notices is the amount of wires, coolant and oil lines, its compact size (small, compared with an O-235) and apparent light weight. To hear a Rotax run for the first time has the same effect, it is not what you are used to and when one flies over, the sound is quite different too: you will notice the noise of the gearbox.

Other aero engines roar when they takeoff at full throttle, these engines whisper! Which helps with the noise requirements and or abatement procedures for your airport.

No direct drive

Rotax RPM Indicator

Where the LycoSaurus engines are direct drive and run the propeller at the same RPM as the engine. The Rotax engine is normally run (at cruise) between 4900 and 5300 RPM (engine RPM, divide that by the gear ratio to get propeller RPM) where they have their maximum torque / power combination. Maximum takeoff RPM is 5800 for 5 minutes. Maximum continuous is 5500 RPM and idle is around 1400 RPM.


Rotax FLYdatRotax FLYdat

There are a number of engine RPM indicators available on the market today which have the right color/RPM bands for a Rotax four stroke. These are very handy and it is a lot better than having to remember all the numbers like you need to do with a FLYdat. One look is just enough with one of these, see the picture.

Liquid cooled

Rotax was one of the first manufacturers who designed their engines with a liquid cooling system. Some say its too complex, but in my experience there hasn't been much trouble with it. The system consists of a radiator, some hoses, a coolant bottle with pressure cap and a cam shaft driven pump to cool the cylinder heads.

Thermostatic control

The radiator usually sits below the propeller in the free air stream. During the winter season you might need to block some of the airflow through the radiator to make sure the engine reaches the correct operating temperature. By using a thermostat in the coolant pipes together with a thermostat (82-90 °C) in the oil system the engine will remain perfectly on the correct operating temperature without the hassle of using duct tape to block the radiators. Read more in the Service Letter 912-001 / 914-009.

Written by EAI.

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