Page 729 - PPL-engelsk 2025

Aircraft general knowledge

8.8.3.2 Premature ignition and detonation

Normal combustion of the gasoline/air mixture is not a spontaneous explosion,

but an even and calm combustion.

There are 2 spark plugs in each cylinder which ensure that the mixture is ignited
in 2 places. If you want to burn a stack of straw, it is best to ignite in several

places, then the combustion will be faster and more efficient. We can see this,

for example, when we test magnets and only ignite with a spark plug. This gets

RPM to fall.

After ignition, the mixture will burn evenly at a speed of about 10 m per second.

The combustion speed increases to 45 m per second and decreases when the
mixture is about to burn out.

It can happen that the not yet ignited part of the mixture, in front of the flame

front, can become so hot and compressed that it self-ignites. This will result in a

violent detonation. Pressure waves spread and exert a destructive force in the
engine's interior.

Here you can see how combustion
normally takes place.

Lead deposits and exhaust residues can glow and cause ignition of the mixture

before calculation. This creates destructive pressure waves (detonation) in the

cylinders.

A too lean gasoline/air mixture can cause knocking and detonation. A too rich

mixture results in deposits in the engine.

So, make sure the mixture is correct!

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Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 729