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Feeding your Rotax 2-Stroke Aircraft Engine

Fuel | Engine Lubrication | Gearbox Lubrication | Cooling Liquid | Spark Plugs

Fuel

Octane rating

We recommend using a "premium" type automotive fuel with an octane rating of 91, a minimum of impurities and little or no alcohols (maximum 5%).

Some may have noticed that the Rotax documentation specifies an octane rating of 90, based on the RON standards used in Europe. This is the equivalent of an 87 rating under the Canadian AKI standards. A rating of 87 is indeed considered "regular" fuel, but we still recommend a "premium" fuel for two reasons:

  • First, when fuel is premixed with 2-stroke oil, the octane rating is reduced by about 2 points. An 87 octane fuel would therefore become 85 octane.
  • Second, fuel evaporates and loses its octane rating when it lays in your aircraft's fuel tank or in a plastic jug. A "premium", 91 octane fuel will see its octane rating reduced to unusable levels after as little as three weeks. Fuel with a lower octane rating would obviously have an even shorter usable life.

Too low an octane rating will create detonation and pre-ignition which can damage the piston crown and even melt a hole through it.

Where to buy

It is recommended to buy gas at the busier gas stations of the major oil companies, since their tanks are renewed often allowing the fuel to stay fresh and clean.

Alcohols

Certain fuels contain alcohols such as ethanol. These ingredients should be avoided since they absorb water which then creates corrosion inside a 2-stroke engine. They also have the effect of reducing the oil's lubricating properties. Rotax recommends avoiding fuels containing more than 5% alcohol.

Aviation Fuels

It is possible but not recommended to use 100LL AVGAS, since the the lead content will increase deposits in the combustion chamber and on crankshaft ball bearings, inducing premature wear. Its higher octane rating does not bring any significant advantage to the engine's operation.

To be avoided:

  • "Regular" fuel except if used with oil injection and burned entirely on the day of purchase;
  • "Premium" fuel which is more than 3 weeks old
  • Alcohol content of more than 5%
  • Diesel. You are not crossing the Atlantic in a Diamond TwinStar!

Engine Lubrication

2-stroke oil specifications

Rotax recommends using a "super" two-stroke oil which corresponds to ASTM/CEC standards and/or API-TC classification. It is also essential to choose an oil which is designed for an air cooled engine even if you own a liquid cooled engine.

2-stroke oil type

For most Canadian users a mineral or semi synthetic oil is recommended.

Synthetic oil should only be used by those who operate their engine nearly every day. Even when shut down, air is constantly circulating through a 2-stroke engine; it is never sealed like a 4-stroke engine. Even though it has excellent lubricating properties, a synthetic oil does not effectively protect a stopped 2-stroke engine against corrosion: it tends to attract moisture and will run off the parts rather than leave a protective coating.

Mixing

If you own an oil-injected engine, you simply need to keep your oil tank topped up frequently. Otherwise, it is necessary to premix your oil and fuel. The ratio is 50 to 1, or 2%. This means you would mix 400mL of oil in 20L of fuel, 500mL for 25L, and so on. Using more oil than recommended would not help your engine in any way: it will accelerate the formation of carbon deposits which will eventually break loose and accelerate wear.

Rotary valve lubrication

The oil used in the rotary valve lubrication circuit of liquid cooled engines (462, 532, 582, 618) should be the same 2-stroke oil used for primary engine lubrication.

To be avoided:

  • Oils whose label do not bear the above mentioned required specifications
  • Oils primarily designed for outboard 2-stroke engines
  • Mixing ratios other than 50:1

Gearbox Lubrication

Rotax recommends API-GL5 or GL6, SAE 140 EP or 85W-140 EP gear oil for gearbox lubrication. A synthetic type of oil can be recommended for this application thanks to the almost-sealed environment of the gearbox casing.

To be avoided:

  • Low quality lubricants which will deteriorate long before the recommended replacement interval (every 100 hours)

Cooling Liquid

For its liquid cooled Aircraft Engines, Rotax recommends a mix of 50% antifreeze concentrate without sulphates and phosphates, with anticorrosion additives designed for aluminium, and 50% distilled or demineralised water.

It is possible to use a higher proportion of water if you have overheating problems, but it is important to consider the effect on freezing point. The maximum ratios specified by the antifreeze manufacturer should not be exceeded since deposits may form inside the cooling circuit.

To be avoided:

  • Low phosphate and low sulphate antifreeze
  • Water which is not distilled or demineralised
  • Excessive mixing ratios

Spark Plugs

The recommended spark plugs are the NGK B8ES or BR8ES. The "R" denotes a resistance which helps suppress radio interference. The use of spark plugs with a solid tip, rather than the screwed-on tip, is mandatory. The latter can unscrew itself in flight and dislodge the spark plug connector cap, creating an ignition failure.

Spark plug gap

  • Allowable range: 0.4-0.5mm / .016-.020"
  • Optimal: 0.45mm / .018"
  • The gap can be reduced to its allowable minimum to help starting in very cold conditions

To be avoided:

  • Other spark plug models and other manufacturers' equivalents
  • Screwed-on tips
  • Unverified spark plug gaps