Silencer

The silencer according to the invention contains two chambers (10,11) separated by a partition wall (14) and includes also a guide plate (25) guiding cooling air from a fan (27) through a channel (26) in which recesses (23) from one chamber terminate. By means of an ejecting function, then being formed, exhaust gases are evacuated into the channel and mixed with the cooling air to a lower temperature.

The present invention relates to a silencer for small engines in portable 
engine-driven tools, e.g. clearing saws. 
When using engines in the open air there is a risk of igniting inflammable 
material close to the gas outlet of the engine, if the exhaust gases are 
permitted to contain glowing particles which might cause fire. In order to 
reduce the fire risk stipulations have been issued stating the 
requirements for approval of such engines for use in woods and fields. 
Thus, a silencer should contain a spark trap or the like and have an 
arrangement for lowering the gas temperature. The simplest form of a spark 
trap is a net in a passage opening between different chambers of the 
silencer while, on the other hand, the arrangement for a temperature 
reduction can be much more complicated. Such a complicated arrangement is 
known, for instance, from the German publication DE-OS No. 29 29 965. 
By the present invention an arrangement of silencer is presented where, by 
means of an ejector function, the exhaust gases are ejected from a 
silencer chamber through a gap and mixed up with cooling air from the 
engine to a lower temperature. Besides the gap mentioned a guide shield is 
used which, on the one hand, catches the cooling air and on the other hand 
forms an outlet for the gas mixture at a lower temperature. A silencer 
designed according to the invention shall in this way have the features 
indicated more in detail in claim 1.

FIG. 1 shows a complete silencer in a preferred embodiment, suitable to be 
used in a chain saw. The silencer has two chambers 10, 11 which are 
principally located in a pair of metal shells 12 and 13, respectively, put 
together into a unit. Between the chambers there is a partition wall 14 
through which the chambers communicate via a passage opening 15. The 
silencer is mounted by means of three through screws in the holes 16, 17, 
18 onto an engine body 19 (FIG. 4). The exhaust gases then enter into the 
chamber 10 through a gate 20 between the two upper holes 16, 17. The metal 
shells are kept together by the screws which provide, when being 
tightened, a compact connection between the silencer and the exhaust gate 
of the engine. 
The joint between the metal shells has a special shaping according to FIG. 
2. On the outer edge of the shell 12 a flange 21 has been carried out 
which surrounds the whole shell. The flange overlaps the lower edge of the 
shell 13 providing a flange 22 turned inwards. In this flange there are 
some recesses 23 made along a long side of the shell 13. Through these 
recesses chamber 11 is connected to the environment so that the exhaust 
gases thereby can leave the chamber. Thanks to parallelism between the 
flanges 21, 22 the gas is guided a way which leads along the outside of 
the shell 13 (along the outlet arrow 24). 
The indicated way along the arrow 24 has also been created by means of a 
guide plate 25 located outside the corner of the shell 13 and forming in 
that way a channel 26 with limit surfaces in the guide plate and the 
outside of the shell. As appears in FIG. 4, the engine has a cooling fan 
27 which runs an air current from an inlet opening 28 through an engine 
room 29, limited by a casing 30, to an outlet opening 31 close to the 
silencer. Prior to the outlet through the opening the air passes the 
silencer and a part of the air current takes its way through the channel 
26 (see arrow 32, FIG. 2). The air through the channel provides an 
ejecting function on the exhaust outlet along the arrow 24 and decreases 
the resistance in the exhaust channel. Thanks to the bending of the 
channel round the corner of the silencer a turbulence is created in it 
which acts for a good mixture of gas and air. 
The spark trap mentioned in the introduction consists in the example 
described of a wire net 33 in the passage opening 15. Thanks to the net 
the glowing particles called sparks will remain in the chamber 10. Instead 
of the recesses 23 there will probably also be a possibility of carrying 
out inward bends in the flange 22 through which the exhaust gases may 
escape.