The Timing Point
(Others have written books about this topic, so I am also bound to concentrate on the most important items!)

The timing point considerably depends on the number of revolutions and the engine load.
As the engine load of a 2CV is not taken into account (because of the money) I will not get into it.
It is enough to know that it is like that.

We remember the way a 4-stroke-engine works. During the second phase the piston moves upwardly and the mixture in the combustion chamber is thereby compressed until the spark-plug ignites it and the burning mixture pushes the piston downward again. ( start of the third phase).
The ignition has to take place at the very right moment.

Why does the timing point depend on the number of revolutions?

When the mixture is ignited, it does not explode, as often wrongly thought, but it is burning. That is why it takes some time until the whole mixture is burning and the maximum pressure builds up. That is the reason why the mixture is ignited shortly before the piston has reached the top dead center (TDC) . So the maximum pressure is right at hand when the piston has just passed the TDC. The time from ignition to maximum pressure is nearly constant, that means it does not depend on the number of revolutions. (That is not at all true, but it is good enough for our observations) The piston covers a longer distance when the number of revolutions is higher than in the same time when the number is lower. Because of that with a rising number of revolutions the mixture has to be ignited earlier so that the maximum pressure is at the right moment. Therefore you need an adjustment of the timing point which depends on the number of revolutions. This work is taken over by the centrifugal governor.

How does the adjustment of the timing point work?

Centrifugal governor: Unfortunately I do not have an original illustration of a 2 CV but with this one you can easier explain the principle. In the case of a 2CV everything looks a bit different, the way it works is the same. The centrifugal governor is situated behind the casing of the breaker. The complete unit is fixed at the top of the camshaft.

With a rising number of revolutions the centrifugal force, which acts on the centrifugal weight increases. The weights are connected with the camshaft, which controls the breaker. With an increasing number of revolutions the weights are pushed on the outside and the camshaft turns a few degrees on the base plate and therefore the breaker opens earlier.

Timing point too early or too late:

Too late:
If the ignition is too late the motor will lose power and will need a vast amount of fuel. Mechanical damages are only possible with an ignition which has been adjusted hopelessly too late, but this is very unlikely.

Too early:
If the burning mixture reaches its maximum pressure before the piston gets to the top dead center (DTC), that means the ignition is too early, the piston will be pushed downward although it should have gone upward another bit. The result is an enormous pressure in the combustion chamber.

This causes a heavy mechanical strain for the motor and leads in practically no time (within seconds) to a piston failure, because the barrel and the piston get extremely hot. Anyway, the motor is then ready for the scrap- heap. That is way you have to be very careful with experiments concerning the timing point.


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June 12. 1999