24 this being a guarantee that the bulb valve element 28 39 is retained in the chamber 20 the diameter of the bulb valve element 28 39 is adapted to the diameter of the chamber 20 in such a way that when the engine is not running there is a gap between the bulb valve element 28 39 and the wall of the chamber 20 see fig4 a the dimensions of said gap are exaggerated for clarity reasons the gap has the effect that the pressure increase that occurs in the combustion chamber when the engine is started either manually or via the starter motor elevates the bulb valve element 28 39 somewhat see fig4 a in order to admit gas to flow out through the opening 24 and bypass the bulb valve element 28 39 said gas being discharged to the atmosphere via the radial holes 26 when the engine has started there is instantly through the combustion a very rapid heating of the bulb valve element 28 39 due to the fact that the spigot 29 39 is directly exposed towards the combustion chamber 22 the bulb valve element 28 39 then expands and the radial expansion brings about that the periphery of the bulb valve element 28 39 will sealingly contact the wall of the chamber 20 see fig4 b in the disclosed embodiment the bulb valve element 28 39 is designed with two axially separated sealing surfaces this is only an example of different sealing arrangements that are possible there.