Rotax four stroke aircraft engines have proven themselves as reliable engines. They can be bought as experimental (UL) or certified (A and F) engine and there are three main models to date. The original (click to see their images) 80 Hp 912, the 100 Hp 912S and the 115 Hp turbocharged 914 engine.

Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the french market) and the 914 has red covers and a turbo. 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. The documents will open in a new window for easy reading.

Rotax four stroke engine design

The three four stroke models are basically the same: four cylinders in boxer / horizontally opposed configuration, two altitude compensating (no mixture control!) Bing carburettors, 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). A number of options can be installed on the engine: vacuum pump, alternator, controlable propeller (some options can not be installed at the same time).

When looking at a Rotax engine installation the first thing that everyone notices is the amount of wires, coolant and oil lines, it's compact size (small, compared with an O-235) and lightweight. 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.

Rotax FLYdat Indicator

The Rotax engine is normally run between 4900 and 5100 RPM (engine RPM, divide by the gear ratio to get propeller RPM) where they have their maximum torque. Maximum take-off RPM is 5800 for 5 minutes. Maximum continuous is 5500 RPM and idle is around 1400 RPM. 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.

Operating a Rotax

Before cold starting this engine it is recommended to open the oil sump, leave the cap off and rotate the propeller by hand (ignition off, of course) until you hear a murmling sound coming from the sump. This way you know for sure all oil is in the sump and the engine is loose (especially helpful in wintertime). Now is also the time to check the oil level. You can do that after the flight too, but be very careful then. Hot oil can burn very nasty.

Cold starting a Rotax is not that difficult: use the fuel pump (if any), activate the choke fully, place the throttle in idle position (or else the choke will not work on these Bing carbs) and activate the starter, ten seconds maximum saves it from overheating, you will need to wait two minutes to cool the starter before you take another attempt to start the engine.

When it starts, close the choke slowly and take over with the throttle to keep RPM constant. If it does not start it is probably flooded: close the choke, open the throttle and start again. Close the throttle when the engine fires. If it still does not start you might have used not enough choke.

Running a Rotax

When the engine starts, keep it running below 2000 RPM. Warm the engine for the first two minutes up at 2000 RPM, this way the oil slowly warms up and is distributed in the engine. After two minutes increase RPM to 2500 and continue warm up. The engine is ready for taxi and run up checks when the oil reaches a minimum of 40 - 50°C. Oil operating temperature is between 75°C and 110°C. Make sure that the oil reaches this temperature to boil off any condensation that might have accumulated. This keeps the rust out of the engine.

Rotax CHT Temperature Chart

During flight oil temperature should be between 75°C and 110°C, cylinder head temperature should also be around 80-90°C and must not exceed 135°C depending on the type of coolant used, for details: move your mouse over the image to the right. Oil pressure is in the range of 2 - 5 bar (29 - 73 PSI) on average around 4 bar.

After landing and taxi back make sure to cool the engine for a couple of minutes before shutting down. Else the risk of boiling cylinder heads will be high. Cooling a 914 is even more important, reason: the turbo. The turbo can be red hot while operating and runs at a maximum of 160,000 RPM !! It needs a minimum of five minutes cooling time before the engine can be shut down. Turbos are expensive machinery so make sure to cool it properly and that the engine is within operating temperatures before take-off.

At altitude

The only Rotax engine capable of producing sufficient power at altitude is the turbocharged 914, keeping its continuous power of 100 Hp up to 16,000 ft (the 115 Hp is only available at 5800 RPM for a maximum of five minutes at a time). All other types loose power with altitude. It's a sort of automatic throttle reduction and it reduces at about 1" manifold pressure every 1000 ft of altitude gain.

This brings me to the next point: I have seen pilots reducing RPM just after takeoff between 300' and 500' AGL. After asking them why, they said is was to reduce stress on the engine. They probably did not realize that by reducing power it will take longer to get to altitude and the engine needs more total RPM to get to a certain altitude. Plus that the engine may run 5500 RPM forever and 5800 RPM for five minutes and this will normally get you higher than 300' AGL in those five minutes.

And, changing engine power at such a low altitude (when things are running smoothly) is usually not a good idea. Get some altitude first before any changes are introduced. This will give you options in case of trouble.

Maintaining

Rotax Oil Viscosity Chart

As the gearbox of the engine is also lubricated by the engine oil it needs therefore a type of oil suitable for gearboxes. Motorbike oil is fine for this purpose. Normal car or aircraft engine oil is not suitable and will ruin the gears eventually. Use API SG or higher with additives for gearboxes. As for viscosity we recommend 10W40, as this will do fine in most climates, see the chart.
Service Instruction SI-18-1997-R5 from Rotax explains it in more detail.

This engine can run on Avgas and Mogas. Mogas is preferred. Rotax recommends that when the engine uses Avgas for more than 30% of the time, oil change interval must be shortened from 100 hours to 50 hours. This is due to the high lead content of Avgas. Semi-synthetic oil must be used when running on Avgas, full synthetic oil has difficulties cleaning the lead deposits in the engine.

The Bing carburettors are attached to the engine by means of a rubber flange. This flange needs to be checked every 100 hours for cracks. These cracks are mainly on the inside and you will need to remove the carbs to see them, if any. If these cracks are allowed to progress an engine failure is possible.

The engine has water cooled cylinder heads with a pressurized system containing ethylene glycol or NPG+^TM. Early models used a 0.9 bar pressure cap but the latest models use a 1.2 bar cap. Optimum temperature can be read of the chart above, or click here for a larger version. More information on the new pressure cap and required coolant is in this Rotax Service Bulletin SB-912-043-914-029.

Waterless Coolant
Rotax GmbH recommends a waterless coolant for its engines, more can be read in this article from Phillip Lockwood and these installation instructions from Evans.

Electronic Ignition

The engine is equipped with an electronic ignition. Personal experience has led to the following recommendation: when flying near high powered radio transmitter stations do keep a safe distance from the antenna's. Normally you need to be 1000 ft higher than any obstacle within 600 meters, in this case stay at least 2 miles away.

Certain radio stations can emit high powered radio signals and flying close to them has the effect that these radio signals sometimes can be heard in your intercom. If this is the case, the electronic ignition can probably 'hear' them too.
The result of these unwanted signals in the electronics will be that the ignition is going to run irregular/erratic and could even fail at some point and the engine will start running rough, missing power strokes or even stop working altogether resulting in an off airport landing.