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 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.
Getting to know a Rotax with its different systems takes some time, here we explain them and what to look for.
The Rotax engine has a number of subsystems for operation as the cooling, fuel, lubrication, electronic ignition and propeller. For a better understanding of the engine we will describe each of them in a bit more detail.
The liquid cooling system cools the cylinder heads only and air is used to cool the cylinders through the fins and ram air. The coolant pump is driven by the cam shaft and pumps the liquid to the cylinder heads after which it is collected in the expansion tank on top of the engine. From there it flows to the radiator to be cooled by outside air and it flows back to the coolant pump.
The expansion tank is fitted with a pressure valve which opens when the pressure reaches 0.9 / 1.2 bar, this depends on the type of valve used (there is a SB on this item). When the coolant heats up it expands and with enough pressure it opens the valve and it can drain to the overflow bottle. There should always be some coolant in the bottle when the engine is cold.
There is no indication to measure the coolant temperature directly. Coolant temperature is measured indirectly by sensors in the cylinder heads.
This engine can run on AVgas and Mogas. Mogas is preferred and Rotax recommends that when using AVgas up to a maximum 30% of the time, the oil change interval must be shortened from 100 hours to 50 hours. Should you run primarily on AVgas then change oil every 25 hours. This is due to the high lead content of AVgas. Semi-synthetic oil must also be used when running on AVgas, as a full synthetic oil has difficulties cleaning the lead deposits inside the engine. Not complying with this will ruin your engine with no chance on any warranty claim.
Rotax mentions the use of Decalin Runup or Alcor TCP in SI-912-016 when using leaded fuels to prevent lead deposits in the engine. Have a look at the Decalin technical datasheet here.
Fuel should be fed from the tank through a coarse filter (in the tank), tank on/off and drain valve, fuel fine filter, then to an auxiliary electrical fuel pump and on to the engine mechanical fuel pump after which it is divided to both carburetors and fuel pressure gauge. A fuel return line should be included to reduce fuel bubbles in the fuel line to the carburetors. When using electric fuel pumps exclusively (ie. 914 engine) you need to read "Rotax Fuel Systems" here on site.
If fuel is carried in multiple tanks, a fuel tank switch should be installed and properly labelled to preclude any mis selections. The fuel fine filter should either be changed, cleaned or replaced accordingly to the aircraft maintenance program.
The engine is equipped with a dry sump lubrication system with a gear type oil pump and integrated pressure regulator and sensor for cockpit indications, an oil filter with oil temperature sensor is also provided. The Rotax oil filter contains an high pressure valve which opens if the filter gets blocked for any reason so that oil keeps flowing and can do its job.
The oil pump sucks oil through the oil radiator out of the external oil tank / sump and pressurizes it to pump it through the engine for lubrication. Oil is then collected in the crank case and is returned under pressure (by blowby gases) to the external oil tank / sump.
Keep in mind that if the oil tank / sump sits higher than the crankcase, oil will slowly return to the crankcase while the engine is not running. Hence the need to rotate the propeller by hand before start. This pre-lubricates the bearings and returns all oil to the external tank / sump.
Sometimes when you work or perform maintenance on the engine you need to disconnect the oil sump/ radiator or its any of its tubing. Air may enter the system during removal and installation of these items and MUST be removed as per Rotax instructions found in Service Instruction SI 912-018-914-020.
Dec 2018: Some Rotax 915iS engines showed oil seepage from the oil tank into the turbo sump due to insufficient sealing after a long standstill, see SB-915 i A-004/SB-915 i B-004 Replacement of Oil Pump Housing for more details on the affected engines. Kit part nr: 481550 is available for repair.
Equipped with a fully dual electronic DCDI (Dual Capacitor Discharge Ignition) system, each fed by a separate power supply, which is totally isolated from the aircraft electrical system. The indications for the RPM gauge is also fed from a separate stator coil. This system is practically maintenance free. Ignition sequence is 1-4-2-3.
This electronic ignition system should be fully shielded to protect it from injection of external high power RF signals. Certain installations have proven to be susceptible to RFI interference from radio stations.
This consists of a magnet on the flywheel and separate coils on the stator. This alternating voltage is then fed to an electronic regulator with a large capacitor for rectification. The voltage should be around 13.8 V with the engine running. It pays to install a volt and ammeter in the system for monitoring. See the heavy maintenance manual section 74 for a schematic.
The propeller is driven through a reduction gear with a 1:2.43 or a 1:2.27 ratio, this depends on the engine model. Certified engines (and non certified series 2/3) have an overload clutch installed to protect the crankshaft in case of a propeller ground strike. It also dampens the engine/propeller vibrations. This overload clutch can cause a backlash in the propeller during start, stop and power changes which is totally harmless. During engine warmup maintain RPM at 2000 - 2500 to keep the gearbox loaded, this reduces gear wear and prolongs its life.
Provision is made to install either a vacuum pump or a hydraulic governor for a constant speed propeller to the propeller shaft.