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Ignition of the charge in the diesel cycle

Aviation Mechanic Powerplant (AMP) practice question 6 of 13 on Reciprocating Engines. ACS element AM.III.A.K10 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONIn an engine operating on the diesel cycle, what ignites the fuel sprayed into the cylinder?

  1. AThe heat of compression in the cylinder
  2. BAn electric spark timed before top center
  3. CA glow plug in the cylinder head
Show the answer and explanation

Correct answer: A. The heat of compression in the cylinder

  • A — correct The diesel cycle depends on high compression pressures; the resulting high pressure and temperature cause the fuel to burn when it is sprayed in.
  • B — incorrect Spark ignition timed before top center belongs to the four-stroke Otto cycle, not to the diesel cycle described in the handbook.
  • C — incorrect No separate ignition device is described for the diesel cycle; ignition comes from the pressure and temperature already in the cylinder.

BACKGROUND

The diesel cycle depends on high compression pressures to ignite the charge in the cylinder. Air is drawn into the cylinder and compressed by the piston, and at maximum pressure fuel is sprayed in. The high pressure and temperature then present cause the fuel to burn, raising internal cylinder pressure and driving the piston down to turn the crankshaft. Water-cooled and air-cooled engines that operate on JET A fuel, which is kerosene, use a version of the diesel cycle, and both two-stroke and four-stroke diesel cycles are in use.

STUDY TIP

Contrast the diesel cycle with the Otto cycle point by point: air alone is compressed, fuel is injected at maximum pressure, and no spark is involved.

KEY POINTS

  • The diesel cycle ignites fuel with the heat of high compression, not a spark.
  • Fuel is sprayed into the cylinder at maximum compression pressure.
  • Aircraft diesels can run on JET A kerosene in two-stroke or four-stroke form.

SOURCES

  • FAA-H-8083-32B ch01 "Diesel Cycle"
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MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.