Aircraft Piston Engines: Detonation and Pre-Ignition

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Pratt & Whitney R-4360 Wasp Major – A 28 Cylinder Aircraft Piston Engine. Photo Credit: D. Miller

A typical piston engine making use of the Otto Cycle provides a systematic approach to the conversion of fuel energy into mechanical energy. Consistent and precise movements of the piston heads, with the help of expanding gases, provide power for aircraft piston engines such as those commonly used in the Cessna – 172 and other similar planes.

During the flight of a propeller-driven plane, the engine provides the force which propels the aircraft. An aircraft’s piston engine basically generates mechanical power, which in turn produces a force of thrust via airplane propellers. Without the force of thrust, the aircraft cannot maintain flight, hence the significance of effective aircraft engines.

Internal Combustion vs. Detonation in Aircraft Piston Engines

Normal activity in the internal combustion engine entails a progressive burning of the fuel-air mixture within the cylinders.

  1. The gasses within the cylinders are ignited from the top.
  2. The flame produced as a consequence travels down synchronously.
  3. This combustive force, equally applied to the piston in a stable manner, pushes the piston down.
  4. The downward motion of the pistons in aircraft engines is then mechanically transferred to the propeller.

However, detonation occurs differently. A common synonym for the term “detonation” is explosion. As opposed to the progressive burning of the fuel-air mixture in aircraft piston engines, detonation involves a rather explosive burning of the mixture. Here is how it happens:

  1. A rise in temperature of the gasses in the compression stroke occurs.
  2. The gasses are ignited.
  3. A “spontaneous combustion” of the gasses takes place, as opposed to the progressive travel of the flame through the gas.
  4. An abrupt and explosive force (energized by the spontaneous combustion) is applied to the piston which practically shoves the piston down.
  5. This excessive pressure on the pistons damages the engine and its components.

Detonation may occur due to an increase in the temperature of the fuel-air mixture in aircraft piston engines. Increased internal temperature may occur for a variety of reasons, including an overheated engine, or the use of expired fuel.

Pre-Ignition in Aircraft Piston Engines

Another form of abnormal combustion in aircraft piston engines is pre-ignition. No explosive combustion occurs in this case. However, the cylinders in aircraft piston engines are susceptible to pre-ignition at all times.

In pre-ignition, the fuel-air mixture is ignited prior to the timed ignition initiated by the spark plugs. This affects the whole cycle of the piston, and is thus considered to be “destructive” for the cylinder heads.

Lead deposits within the cylinder or around the spark plugs are the most common cause of pre-ignition.  However, overheated spark plugs or other immediate components may also cause pre-ignition in aircraft piston engines.

To prevent abnormal combustion in aircraft piston engines, operating limitations for the engine must be followed along with the use of properly refined and high-quality fuel.

The radial piston engine of a Pt-17 Boeing Stearman. Photo Credit: anttler

Resources

Federal Aviation Administration, Flight standards Service. Airplane Flying Handbook. (2004).

Federal Aviation Administration, Flight standards Service. Pilot’s Handbook of Aeronautical Knowledge. (2008).

Trevor, T. Aeroplane General Knowledge and Aerodynamics. Aviation Theory Centre. (2004).

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