Air filter for an internal combustion engine

An air filter for an internal combustion engine has a housing with an interior chamber. The housing has housing walls at least one of which is in the form of a filter element. One of the housing walls has a suction opening for connecting the air filter to a suction channel of the combustion engine, wherein combustion air flows through the filter element into the interior chamber and from the interior chamber through the suction opening into the combustion engine. Air flow resistance elements are arranged in the interior chamber upstream of the suction opening. The air flow resistance elements are arranged in a first and a second row. The first and second rows are spaced apart from one another in the direction of flow of the combustion air. The air flow resistance elements of the first row of the second row are spaced laterally from one another to form gaps therebetween. The air flow resistance elements of the first row are positioned so as to be substantially aligned with the gaps of the second row.

BACKGROUND OF THE INVENTION 
The present invention relates To an air filter for an internal combustion 
engine, especially for a two-stroke engine for hand-held working tools 
such as motor chainsaws, a cutter or trimmer etc. The air filter is 
comprised of a housing with housing walls formed by filter elements 
through which the combustion air is sucked into the interior chamber of 
the housing. A suction opening is provided in one of the housing walls for 
connecting the air filter to the suction channel of the combustion engine. 
In internal combustion engines, especially in two-stroke combustion 
engines, gas pulsations occur within the suction channel which, due to the 
design and operation of the combustion engine, cannot be avoided. Due to 
these pulsations, a portion of the combustion air enriched with fuel, and 
in the case of two-stroke combustion engines also with oil, flows counter 
to the suction direction back into the air filter. The fuel and/or the oil 
are deposited on the filter surfaces so that the wet filter surface is 
soiled and will subsequently be clogged. Thus, frequent filter changes are 
required. 
In order to minimize the escape of suction air flow into the air filter, it 
is suggested according to U.S. Pat. No. 4,600,418 to embody the surface of 
the air filter facing the suction channel as a rebound wall with a 
curvature such that the pulsating gas flow is deflected in the direction 
toward the suction channel in the direction of flow of suction air. With 
such a measure it is indeed possible to reduce the danger of wetting and 
soiling of the air filter. However, the resulting service life of such air 
filters cannot be increased to the extent desired. 
It is therefore an object of the present invention to provide an air filter 
of the aforementioned kind with which a wetting of the filter element by 
backwashing of the fuel/air mixture is prevented. 
SUMMARY OF THE INVENTION 
An air filter for an internal combustion engine according to the present 
invention is primarily characterized by: 
A housing having an interior chamber; 
The housing comprising housing wall at least one of which is in the form of 
a filter element; 
One of the housing walls having a suction opening for connecting the air 
filter to a suction channel of the combustion engine, wherein combustion 
air flows through the filter element into the interior chamber and from 
the interior chamber through the suction opening into the combustion 
engine; 
Air flow resistance elements arranged in the interior chamber upstream of 
the suction opening, the air flow resistance elements arranged in a first 
and a second row; 
The first and second rows spaced apart from another in the direction of 
flow of the combustion air; 
The air flow resistance elements of the first rows spaced laterally from 
one another to form gaps therebetween and the air flow resistance elements 
of the second row are spaced laterally from one another to form gaps 
therebetween; and 
The air flow resistance elements of the first row positioned so as to be 
substantially aligned with the gaps of the second row. 
Advantageously, the space between the first and second rows has a volume 
substantially equal to a pulsation volume of the internal combustion 
engine. 
The air flow resistance elements are preferably positioned substantially 
parallel to the central axis of the suction opening. 
The first and the second rows are positioned substantially in an annular 
arrangement about the central axis of the suction opening. The first and 
the second rows are arranged substantially concentrically relative to one 
another. 
Expediently, the air flow resistance elements have the shape of ring 
segments. 
Preferably, the air flow resistance elements are substantially U-shaped 
with legs pointing toward the suction opening. Preferably, the air flow 
resistance element of the first row, aligned with one of the gaps between 
two adjacent ones of the air flow resistance elements of the second row, 
is positioned such that each one of the legs of the air flow resistance 
element of the first row and the leg of the two air flow resistance 
elements of the second row adjacent to the gap are respectively positioned 
on a radial line extending from the suction opening. Preferably, the free 
ends of the legs are tapered. 
In a preferred embodiment of the present invention the air flow resistance 
elements project into the interior chamber. 
Advantageously, the air flow resistance elements extend from the first 
housing wall at which said suction opening is provided to the oppositely 
arranged housing wall. 
Advantageously, the air filter of the present invention further comprises 
an insert positioned at the suction opening opposite the air flow 
resistance elements, whereby the insert is comprised of an absorbent 
material and serves as a reservoir. The absorbent material is preferably a 
foamed rubber or foamed plastic material. Advantageously, the body extends 
along the entire height of the airflow resistance elements. 
Expediently, the air filter further comprises a body made of absorbent 
material, the body resting on the insert and extending in a direction of 
the height of the air flow resistance elements. Preferably, the body is 
positioned within the area delimited by the first row of the air flow 
resistance elements. Preferably, the body is arranged substantially 
perpendicular to the central axis of the suction opening. 
In yet another embodiment of the present invention the housing is comprised 
of two half shells wherein the air flow resistance elements and one of the 
half shells are formed as a unitary part. Advantageously, the half shell 
that comprises the air flow resistance elements also comprises the suction 
opening. 
With the staggered arrangement of at least two rows of air flow resistance 
elements a direct flow path from the suction opening back into the 
interior chamber of the air filter is interrupted. The combustion air 
flowing through the interior chamber to the suction opening encounters a 
low-resistance flow path due to the design of the air flow resistance 
elements and their staggered arrangement with elements of one row being 
positioned so as to be aligned with gaps of the other row. However, a gas 
flow backwashing from the suction opening into the interior chamber of the 
air filter encounters a great flow resistance. This great flow resistance 
results in turbulence so that fuel and oil contained in the gas mixture 
will be deposited on the air flow resistance elements. Thus, portions of 
the backwashing gas flow which in the most unfavorable situation could 
enter the area of the interior chamber beyond the air flow resistance 
elements have a very low contents of fuel and/or oil. 
Preferably, it is suggested that the space between the rows of the air flow 
resistance elements has a volume that corresponds to the gas pulsation 
volume of the combustion engine connected to the air filter so that the 
backwashing gas flow is caught in the buffer volume between the rows of 
the air flow resistance elements. Preferably, the air flow resistance 
elements are substantially positioned parallel to the axis of the suction 
opening, i.e., they extend essentially perpendicular to a plane in which 
the suction opening is located. In a preferred embodiment of the invention 
the rows of air flow resistance elements are essentially arranged as rings 
(annular arrangement) about the axis of the suction opening. 
The air flow resistance elements in a preferred embodiment are U-shaped 
whereby the legs point toward the interior of the rings. In this 
arrangement, one leg of an air flow resistance element of one row and the 
leg of a neighboring air flow resistance element of the other row can be 
positioned on a common radial line. In order to favor entraining fuel and 
oil deposited on the air flow resistance elements by the suction stream of 
combustion air, the free ends of the legs are preferably tapered. 
In a preferred embodiment of the invention an insert of an absorbent 
material such as foamed rubber or foamed plastic is arranged within the 
area opposite the air flow resistance elements at the suction opening. 
This insert serves as a reservoir for collecting fuel and oil deposited on 
the air flow resistance elements. In order to aid their return into the 
suction air stream, it is suggested to provide a body made of an absorbent 
material such as foamed rubber etc. which extends substantially in the 
direction of the height of the air flow resistance element, preferably in 
the area of the inner row. This arrangement provides for a side surface of 
the body facing the suction openings so that combustion air exiting from 
the interior chamber through the suction opening passes along the side of 
the reservoir body and absorbs and returns fuel and oil.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will now be described in detail with the aid of 
several specific embodiments utilizing FIGS. 1 through 5. 
The air filter 1 of the inventive embodiment shown in FIG. 5 is comprised 
of two housing parts 2, 3, preferably in the form of half shells. The 
assembled housing half shells are connected with screws that are received 
in receiving openings 4 (FIG. 2) within the housing half shell 2 and 
aligned receiving openings 5 (FIG. 4) in the other housing half shell 3. 
One such screw 6 is represented in a dashed line in FIG. 2. At the edge of 
the filter housing a further throughbore 40 can be provided for arranging 
a further fastening screw (FIG. 5). 
The assembled housing half shells 2 and 3 delimit an interior chamber 7 
that defines the clean room of the air filter and which is provided with a 
suction opening 8 (FIG. 1, FIG. 5) which is provided in the housing wall 
10 of the half shell 2. The housing of the air filter 1 has a small height 
H forming a peripheral narrow edge. The oppositely arranged housing walls 
9, 10 of a great surface area are in plan view slightly trapezoidal (FIG. 
1, FIG. 4) whereby the housing walls 9 and 10 are in the form of a grate 
with the exception of the area 12 in which the suction opening 8 is 
provided. The grate serves as a support 11 for a filter element (mesh) 41. 
Preferably, the housing parts 2, 3 of the air filter 1 are made of plastic 
whereby the filter element (mesh) 41 can be embodied as an unitary part 
together with the support 11. 
The wall section 12 in which the suction opening 8 is provided is of a 
semi-circular shape whereby a corresponding continuous wall section 12a is 
provided at the opposite housing half shell 3. 
In the plan view according to FIG. 1 it can be seen that the suction 
opening 8 is preferably surrounded by two rows 15, 25 of air flow 
resistance elements 20, 21 which relative to the axis 18 of the suction 
opening 8 are positioned at a radial distance a from one another. It is 
also possible to provide three or more rows of such air flow resistance 
elements in the shown staggered arrangements. The rows 15, 25 are 
essentially arranged in the form of a ring about the axis 18 of the 
suction opening 8 whereby the rows 15, 25 are preferably concentric to one 
another. The distance b of the inner row 15 to the edge of the suction 
opening 8 is selected to be greater than the radial distance a between the 
inner row 15 and the outer row 25. 
As shown in FIG. 5, the air flow resistance elements 20, 21 project from 
the housing wall 10, extend toward the other housing wall 9, and rest at 
the wall section 12a. The housing edge of the air filter in the area of 
the suction opening 8, the inner row 15, and the wall section 12a of the 
housing half shell 3 form a suction ante room 13 which is connected via 
the suction opening 8 directly to the suction channel of a combustion 
engine, especially of a two-stroke combustion engine. The space delimited 
by the inner row 15 and the outer row 25 as well as by the wall section 12 
of the housing half shell 2 and the wall section 12a of the housing half 
shell 3 forms a buffer volume 14 for backwashing gas flow, as will be 
described in the following. 
As can be seen especially in FIG. 1, each air flow resistance element 20, 
21 is preferably shaped as a ring segment whereby neighboring air flow 
resistance elements 20, 21 of each row 15, 25 are spaced from one another 
by a lateral distance L. Preferably, the distance L of neighboring air 
flow resistance elements 20 in the outer row 25 is identical to the 
distance L of neighboring air flow resistance elements 21 of the inner row 
15. The air flow resistance elements 21 of the inner row 15 are positioned 
such, relative to a radial line of the axis 18 of the suction opening 8, 
that they essentially cover the gaps 16 formed by spacing the air flow 
resistance elements 20 at the distance L. Preferably, the air flow 
resistance element 20, 21 are slightly wider than the gaps 16 to be 
covered in the flow direction of the combustion air. Due to the concentric 
arrangement of the rows 15, 25 relative to one another and the arrangement 
of the air flow resistance elements 20, 21 on the respective circular arc, 
the center point of which is preferably the axis 18 of the suction opening 
8, the circular arc section of a gap 16 is thus smaller than the circular 
arc section of the air flow resistance element 20, 21 covering this gap 
16. Preferably, the air flow resistance elements 20, 21 of one row 15, 25 
are embodied identical to one another and are positioned adjacent to one 
another with an equidistant spacing L. 
In the shown embodiment the air flow resistance elements 20, 21 are 
substantially U-shaped whereby the legs 22 are pointing toward the suction 
opening 8, i.e., they are pointing towards the inner area of the ring or 
circular arc. Preferably, the corresponding legs 22 of an air flow 
resistance element 20 of the outer row 25 are arranged such relative to a 
neighboring leg 23 of an air flow resistance element 21 of the inner row 
15 that the legs 22 and 23 are positioned on a common radial line 24 to 
the axis 18 of the suction opening 8. It may be expedient to provide the 
free ends of the legs so as to be tapered. 
When assembled, the incoming air (see arrow 19) must change its course 
twice on its way to the suction opening 8 due to the construction of the 
air filter 1 whereby the combustion air flowing radially through the gaps 
16 of the outer row 25 into the buffer volume 14 flows about the inner air 
flow resistance elements 21 and through their gaps 16 into the suction 
ante chamber in order to be guided via the suction opening 8 to the 
combustion engine. The backwashing or back pulsation of combustion air 
enriched with fuel and oil, which occurs in combustion engines and 
especially in two-stroke combustion engines, is favorably affected by the 
inventive construction of the air filter 1. The U-shaped air flow 
resistance elements 21 facing the suction opening 8 represent a 
considerable flow resistance for a flow from the suction ante chamber 13 
into the clean room 7a of the air filter whereby due to the shape of the 
air flow resistance elements 21 great turbulence is produced that effects 
that the fuel and/or oil are deposited on the air flow resistance elements 
20, 21. Preferably, it is furthermore suggested that the buffer volume 14 
between the rows 15 and 21 of the air flow resistance elements 20 and 21 
is to be adjusted in volume to the pulsation volume of the combustion 
engine so that a backwashing gas flow ideally is trapped within the buffer 
volume 14. In a subsequent suction stroke the gas flow caught within the 
buffer volume 14 is then washed out by the flow of fresh combustion air 
flowing through the clean room 7a and ante chamber 13 into the suction 
opening 8 so that the buffer volume 14 is again freed of air laden with 
fuel and/or oil. The oil, respectively, the fuel that has been deposited 
on the air flow resistance elements 20 and 21 is entrained during the 
suction stroke by the combustion air flowing in the direction of arrow 19. 
For enhancing entrainment the legs 22 and 23 of the air flow resistance 
elements 20, 21 are preferably tapered. 
In FIG. 1 the inner row 15 is comprised of only three air flow resistance 
elements 21, while the outer row is comprised of five air flow resistances 
elements 20. In the area of the inner row 15 fastening screws 6 are guided 
through the receiving openings 4 so that the fastening screws 6 themselves 
form air flow resistance elements which cover a corresponding gap 16 of 
the outer row 25 of the air resistance elements 20. 
This staggered arrangement of at least two rows 15, 25 of air flow 
resistance elements effects a retention of the backwashing gas flow 
whereby simultaneously, due to the shape of the air resistance elements, a 
deposition of fuel and oil is favored. In the direction of flow 19 of the 
combustion air, on the other hand, the air flow resistance elements 20 are 
flow-dynamically designed so that for the combustion air flowing toward 
the suction opening 8 no great flow resistance is present. The suction 
resistance of the air filter is not increased. Due to the concentric 
arrangement of the air flow resistance elements 20, 21 and the gaps 16 of 
substantially the same width within the inner or the outer ring 15, 25, 
the air flow resistance elements 20 of the outer row 25 can be of the same 
size, relative to the segment angle, as the air flow resistance elements 
20 of the inner row 15. The segment angle 27 is preferably substantially 
30.degree.. 
In a further embodiment of the invention the wall section 12a of the second 
half shell 3 receives a body 30 made of liquid-absorbent materials such as 
foamed rubber or foamed plastic etc. as is shown in dashed lines in FIG. 4 
(see also FIG. 5a for its position within the filter). This body 30 
extends essentially perpendicular to the axis 18 of the suction opening 8. 
This absorbent body 30 ensures that droplets forming at the bottom area of 
the air flow resistance elements 20, 21 can be captured, stored and 
released in a subsequent suction stroke by the incoming combustion air, 
indicated by arrow 19. 
The shown air filter 1 is mostly used such that the gravitational force 
acts in the direction of arrow G. This, over an extended operational 
period could lead to the collection of liquid in form of a fuel/oil 
mixture at the lower edge 17 of the housing. In the area of the edge 17 a 
receiving chamber 26 is provided within the housing half shell 2 as well 
as within the housing half shell 3 for a reservoir body (insert) 31 
(indicated in dashed lines in FIG. 4). This insert 31 is preferably made 
of foamed rubber or plastic and is essentially in the shape of a 
parallelepiped that is pressed into the receiving chamber 26 and conforms 
to the shape of the chamber 26. The upper part with surface 31a extends in 
the direction of the height of the air flow resistance elements 20, 21. 
The reservoir insert 31 preferably extends over the entire height h of the 
air flow resistance elements 20, 21 from one housing wall 10 to the other 
housing wall 9. The surface 31a of the reservoir body 31 which is facing 
the suction opening 8 is positioned substantially at the level of the 
inner ring 15 so that it is ensured that this surface 31a over its entire 
length is subjected to the stream of outflowing combustion air leaving 
through the suction opening 8 so that the reservoir body 31 can release 
fuel and oil. 
FIG. 5a shows schematically the relative arrangement of body 30 and insert 
31 within the air filter 1 (the hatching of the two parts 30, 31 is only 
used to better illustrate the parts within the filter and is not to be 
understood to represent a metal). This shown embodiment represents a 
non-limiting example of the arrangement of the body and insert. Other 
arrangements will be apparent to a person skilled in the art. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.