FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsHard
A technician is preparing to start a reciprocating engine in extremely cold weather and preheats the engine before attempting a start. Beyond oil viscosity concerns, why is preheating important for successfully starting the ignition and engine?
- AThick, cold oil increases cranking drag, which lowers magneto rotational speed during cranking and reduces the spark energy produced
- BCold weather has no effect on the ignition system, only on fuel vaporization in the carburetor
- CPreheating automatically recalibrates the magneto's internal E-gap timing for cold conditions
- DCold temperatures cause the magneto to produce excessive voltage, which can damage spark plugs if not preheated
Show answer & explanationAnswer & explanation
Correct answer: A. Thick, cold oil increases cranking drag, which lowers magneto rotational speed during cranking and reduces the spark energy produced
Magneto output voltage at low cranking speeds is already marginal for reliable ignition. Cold, thick engine oil greatly increases mechanical drag on the crankshaft, further slowing cranking RPM and, in turn, the magneto rotor speed, which reduces the spark energy available to fire the plugs and can prevent a successful start.
Why the other options are wrong
- B. Cold weather significantly affects ignition performance, not just fuel vaporization.
- C. Preheating does not adjust internal timing; it only warms components to reduce drag/viscosity.
- D. Cold does not cause excessive magneto voltage; it reduces effective cranking speed instead.
Cold Weather Starting and Ignition
Cold, viscous oil increases engine cranking drag, slowing magneto rotor speed and reducing spark energy, making preheating important for reliable ignition during cold-weather starts.
- Magneto output is speed-dependent, weakest at low RPM
- Cold oil increases mechanical drag on cranking
- Preheating reduces drag, restoring adequate cranking speed
- Preheating also aids fuel vaporization and reduces engine wear
Memory trick: Cold oil = molasses drag = slow crank = weak spark.