Page 728 - PPL-engelsk 2025

Aircraft general knowledge

8.8.3.1 Engine operation

1. Intake. The gas mixture is sucked in through the open intake valve.

2. Compression. The gas mixture is compressed. Both valves are closed. The

mixture is compressed. When the piston is at the top, voltage is supplied
from the two magnets to the two spark plugs located on top of each valve.

3. Power stroke. The mixture is ignited. The heat causes the air to expand, and

the pressure pushes the piston downwards.

4. Exhaust. When the mixture has burned, the piston moves up and the

exhaust valve opens. The combustion products are pushed out through the
open exhaust valve.

Note the position of the valves during the process. Also note that one sequence
requires two revolutions of the engine.

We have only looked at the working process in 1 cylinder, fortunately there are

usually at least 4 cylinders in an engine. This means that for every time the

propeller has turned ½ turn, there is a working stroke, and the more cylinders
the smoother the working process you get.

The figure is missing spark plugs. There are 2 spark plugs per cylinder in an
aircraft engine. This provides better safety, and it also provides better

combustion when the mixture is ignited in 2 different places.

Typically, the speed will drop 50 - 100 rpm when a cylinder only runs on one

magneto.

See animation here: http://www.entmontage.de/technik_e.htm

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 728