Two stroke engine failure, engine ping, low octane fuel causing piston detonation.

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Diagnosing Rotax 2 stroke aircraft engine piston failure from detonation, or pre-ignition.

Diagnosing Rotax 2 stroke aircraft engine piston failure from detonation, or pre-ignition.

Seized Pistons

2 2A
Seized Pistons - (Ref: 2 & 2A)

The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminum.

If you inspect an  engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50.

The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film.

Possible causes:

  • too low an octane fuel used or fuel with too much alcohol used
  • jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburetor, air leak into crankcase
  • spark plug heat range too hot
  • ignition timing too far advanced, possible failure of the c.d. amplifier box
  • restricted exhaust system: back pressure too high
  • overheated, loosely-installed spark plug

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