JP3308968B2 - Air filter for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an air filter for removing dust contained in air when the air is introduced into an internal combustion engine such as an engine.

[0002]

2. Description of the Related Art Conventionally, an internal combustion engine such as an engine which needs to supply an enormous amount of air has been included in air to prevent damage to cylinders and the like due to dust contained in air taken in from the atmosphere. An air filter is used for the purpose of removing dust.

[0003] Air filters of various shapes are used depending on the type of engine, displacement, and the like. However, in recent years, automobile engines have been shown in FIG. A cylindrical air filter 1A as shown in FIG.

In the conventional cylindrical air filter 1A shown in FIG. 7, a sheet-shaped filter medium 40 is formed by a mountain fold 41 and a valley fold 42.
Are alternately formed in a waveform, and the filter medium 40 formed in a tubular shape such that the ridge line of the mountain-folded portion 41 of the waveform forms a loop. in this way,
By forming the filter medium 40 in a waveform, a high-performance air filter 1A that is small and has a large surface area of the filter medium 40 is realized.

The conventional air filter 1A includes a nonwoven fabric,
A sheet-like filter medium 40 made of a material such as paper or glass fiber is used. Since the filter media made of these materials are formed by intertwining fibers with each other in a complicated manner, the shape of the gaps between the fibers serving as air passages at the time of filtration is complicated.
Dust contained in air taken in from the atmosphere is entangled in the gaps between the fibers of the filter medium which are intricately intricate.

[0006]

Since the filter medium removes dust by entanglement with the fiber when air is passed through, the dust entangled with the fiber by filtration is accumulated in the filter medium. Eventually clogs.
In particular, an air filter used for an engine of a work vehicle such as a concrete mixer truck used in a construction site where dust is flying tends to be clogged.

[0007] If the filter medium is clogged, it becomes difficult for the filter to pass air, so that the supply of air to the engine is hindered, incomplete combustion occurs, and consequently fuel consumption deteriorates, and harmful substances in exhaust gas are reduced. Environmental problems such as an increase in

For this reason, the air filter must be replaced before clogging occurs depending on the use condition of the engine.

As described above, the conventional air filter 1A uses a non-woven fabric, paper, glass fiber, or the like as the material of the filter medium 40, and has a structure in which the fibers are intricately entangled. The dust entangled in the filter medium by the filter reaches deep inside the fiber.

Therefore, the air filter after use is
Even if cleaning is attempted by blowing high-pressure air or the like, most of the dust remains inside the fibers of the filter medium 40 and clogging cannot be eliminated. Also,
When water is used for cleaning, the material of the conventional air filter 1A, particularly paper, may be deteriorated.

Therefore, conventional air filters using these filter media cannot be reused, and after use, most of them are disposed of by means such as incineration.
In addition, since conventional filter media use an organic chlorine adhesive or the like for adhesion, toxic substances such as dioxin are generated when incinerated, which causes environmental problems.

In view of the above problems, an object of the present invention is to provide an air filter for an internal combustion engine that has high durability, is reusable, and is easy to clean.

[0013]

This and other objects are achieved by the present invention which is defined below as (1) to (14).

(1) A cylindrical outer casing having a large number of openings, a cylindrical inner casing provided at intervals inside the outer casing and having a large number of openings,
An air filter for an internal combustion engine having a first filter medium and a second filter medium located between the outer casing and the inner casing, wherein the first filter medium has a mountain fold portion and a valley fold portion located alternately. It is composed of a stainless steel mesh bent in a corrugated shape, the stainless steel mesh has pores having a diameter of 10 to 100 μm, and a diameter of 20 to 200 μm
Are formed so that the side surfaces of either a vertical line or a horizontal line extending substantially in parallel are in contact with each other, and the second filter medium is formed of the outer casing and the first casing. An air filter for an internal combustion engine, which is located between the filter media.

(2) The air filter for an internal combustion engine according to the above (1), wherein the metal mesh forming the first filter medium is formed by a tatami weave. (3) The above-mentioned (1) or (2), wherein the first filter medium has pores having a diameter of 40 to 75 μm.
3. The air filter for an internal combustion engine according to item 1.

(4) The air filter for an internal combustion engine according to any one of the above (1) to (3), wherein the one square inch surface of the stainless steel mesh has 4500 to 540000 holes. . (5) The first filter medium is formed by stacking a plurality of metal meshes in a thickness direction, and the metal mesh on the outer casing side has a hole having a larger diameter than the metal mesh on the inner casing side. The air filter for an internal combustion engine according to any one of the above (1) to (4), characterized in that:

(6) The second filter medium is provided in a space formed by at least two adjacent mountain folds of the first filter medium and the valley fold located between the mountain folds. The air filter for an internal combustion engine according to any one of the above (1) to (5). (7) The diameter of the pores of the second filter medium is larger than the diameter of the pores of the metal mesh used for the first filter medium (1) to (6). An air filter for an internal combustion engine according to any one of the above. (8) The air filter for an internal combustion engine according to (7), wherein the second filter medium is a metal mesh.

(9) An air filter for an internal combustion engine for removing dust contained in air supplied to the internal combustion engine, having pores having a diameter of 10 to 100 μm as a filter medium,
Internal combustion characterized by using a stainless steel mesh formed of a wire having a diameter of 20 to 200 μm and formed so that the side surfaces of one of a vertical wire and a horizontal wire extending substantially in parallel are in contact with each other. Engine air filter.

(10) The above (9), wherein the stainless steel mesh is formed by a tatami weave.
3. The air filter for an internal combustion engine according to item 1. (11) The stainless mesh has a diameter of 40 to 75.
The air filter for an internal combustion engine according to the above (9) or (10), which has a pore of μm. (12) The air filter for an internal combustion engine according to any one of the above (9) to (11), wherein a surface of one square inch of the stainless steel mesh has 4500 to 540000 holes.

(13) The internal combustion engine according to any of (9) to (12), wherein the stainless steel mesh is formed in a substantially cylindrical shape so that the ridge line of the mountain-folded portion forms a loop. Engine air filter. (14) The air for an internal combustion engine according to any of (9) to (13), wherein the stainless steel mesh is formed in a substantially plate shape so that a ridge line of the mountain-folded portion forms a straight line. filter.

Other objects, functions and effects of the present invention will become more apparent from the following description of embodiments with reference to the drawings.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an air filter for an internal combustion engine according to the present invention will be described in detail with reference to the drawings.

FIGS. 1 and 2 show an air filter 1 according to the present invention.
FIG. 1 is an explanatory view thereof, and FIG. 2 is a transverse sectional view thereof.

As shown in FIGS. 1 and 2, an air filter 1 according to the present invention comprises an outer casing 10 and an inner casing 11 in which a metal mesh is formed in a cylindrical shape, and upper end surfaces of the outer casing 10 and the inner casing 11, An upper cover 20 and a lower cover 22 covering a lower end surface, a first filter medium 30 located between the outer casing 10 and the inner casing 11, and a first filter medium 30 located between the first filter medium 30 and the outer casing 10. And the second filter medium 33.

Outer casing 10 and inner casing 1
Reference numeral 1 denotes a metal mesh formed in a cylindrical shape having different diameters as shown in FIG. 1, and an inner casing 11 is disposed substantially concentrically inside an outer casing 10 as shown in FIG. . These outer casings 10
For the inner casing 11, a sheet material such as a punched metal sheet can be used instead of the metal mesh used in the present embodiment. Thus, it is preferable to use a metal mesh.

The wire diameter and the hole diameter of the wire constituting the metal mesh are determined by the wire diameter of the wire constituting the metal mesh used in the first filter medium 30 and the second filter medium 33 which will be described in detail later. It is preferable to use one having a diameter larger than the diameter or the diameter of the hole.

As shown in FIG. 1, the upper cover 20 and the lower cover 22 cover the upper and lower surfaces of the outer casing 10 and the inner casing 11, respectively. Vent 2 for taking out air that has passed through second filter medium 33 and first filter medium 30
1 is provided. The lower cover 22 covers the entire lower end surface of the outer casing 10, and is not provided with the vent 21 as in the upper cover 20.

The upper cover 20 and the lower cover 22 are provided so that the air flows from the outside of the outer casing 10 toward the inside of the inner casing 11 when the air filter 1 is used.
The inner casing 11 has a function of ensuring airtightness of both end surfaces of the inner casing 11 and the inner casing 11.

The first filter medium 30 is formed by bending a stainless steel mesh into a corrugated shape so that the mountain fold 31 and the valley fold 32 are positioned at substantially constant intervals, as shown in FIGS. 1 and 2. And wound into a substantially cylindrical shape. The first filter medium is disposed between the outer casing 10 and the inner casing 11, and the ridge line of the mountain-folded portion 31 is arranged along the inner surface of the outer casing 10. The ridge line of the valley fold 32 is arranged along the outer surface of the inner casing 11.

It should be noted that detailed specifications such as a wire diameter and a hole diameter of a wire constituting the stainless steel mesh used for the first filter medium 30 will be described later in detail.

The second filter medium 33 is provided in the outer casing 1.
0 and the first filter medium 30, and more specifically, as shown in FIGS. 1 and 2, two adjacent mountain folds 31 and the valley fold located between them. 32
And in the space defined by

The second filter medium 33 is formed by folding or rolling a metal mesh such as aluminum or stainless steel or a demister so as to fit in the space formed by the above-mentioned mountain fold 31 and valley fold 32. It is formed by rolling into a shape.

The wire diameter and pore diameter of the wire constituting the metal mesh used as the second filter medium 33 are determined by the wire diameter of the wire constituting the stainless mesh used for the first filter medium 30. Larger than the diameter of the pores and the diameter of the wire or the diameter of the pores constituting the metal mesh used for the outer casing 10 by using a wire having a large diameter contained in the air. Since dust can be removed in advance before reaching the first filter medium 30, it is possible to perform efficient filtration,
The working process at the time of cleaning is simplified.

In the present embodiment, the material of the second filter medium 33 is stainless steel or aluminum, which has high durability against oxidation and corrosion due to ease of cleaning and maintenance.
It is preferable to use a metal mesh such as titanium or a demister. However, the present invention is not limited to this embodiment, and it is also possible to use a nonwoven fabric or a filter medium made of a material used for a conventional air filter 1A such as glass fiber. .

Hereinafter, the operation of the air filter of the present embodiment will be described in detail. The air filter 1 of the present embodiment is configured such that an intake pipe of an engine or the like is connected to a vent 21 provided in the upper cover 20, similarly to the air filter 1A conventionally used. Air is taken in from the outside of the outer casing 10 of the air filter 1 into the inside of the inner casing 11 by suction from the air filter 1 or the like.

The air taken in from the outside of the outer casing 10 is supplied to the outer casing 10, the second filter medium 33, the first filter medium 30, and the inner casing 11
Pass through each hole in this order. Then, the air that has passed through the holes of the metal mesh and reached the inside of the inner casing 11 is sent out to an internal combustion engine such as an engine from a vent 21 provided in the upper cover 20. At this time, dust and the like contained in the atmosphere are adsorbed and removed by the outer casing 10, the second filter medium 33, and the first filter medium 30.

The air filter 1 of the present embodiment, like the conventional air filter 1A, causes clogging due to the adsorption of dust to the filter medium. Therefore, it is necessary to clean the filter medium every predetermined traveling distance. When cleaning the air filter 1 of the present embodiment, it is preferable to wash the air filter 1 by spraying water or a cleaning liquid from the inner casing 11 to the outer casing 10 of the air filter 1. By doing so, when the air filter 1 is used, dust adsorbed on the filter medium in the process of moving with the air from the outer casing 10 side toward the inner casing 11 side during use of the air filter 1, It moves together with the cleaning liquid from the 11 side to the outer casing 10 side and is removed.

Hereinafter, specific embodiments of the first filter medium 30 will be described.
That is, the specification of the first filter medium 30, such as the wire diameter and the hole diameter, will be described in detail.

1. Method of Creating First Filter Material 30 As the first filter material 30, a stainless mesh formed by forming a stainless wire by tatami weave such as plain tatami or twill tatami is used.

Here, the plain tatami weave means that the vertical line and the horizontal line are maintained at a fixed interval, and are laid one by one with each other so as to extend substantially parallel to either the vertical line or the horizontal line of the plain weave. This is a weaving method in which the side surfaces of two outgoing wires are arranged side by side. In addition, the twill tatami weave is such that a vertical line and a horizontal line are kept at a fixed interval, and two or more cross each other so as to extend substantially in parallel with either the vertical line or the horizontal line of the twill weave. This is a weaving method in which the side surfaces of the book wires are arranged side by side.

When a stainless steel mesh is manufactured by these tatami weaves, it is possible to easily manufacture a stainless steel mesh having pores smaller than a stainless steel mesh woven by a plain weave or a twill weave, and it has a high strength. It is suitable for use as the first filter medium 30 of an air filter for an internal combustion engine requiring high rigidity and filtration accuracy.

2. Performance evaluation of first filter medium 30 The following performance evaluation was performed on the air filter 1 of the above embodiment.

(1) Change in performance curve due to difference in filter hole diameter and wire diameter Diesel engine EK-100 (Hino Motors, Ltd.)
, And a displacement of 13,260 cc, 270 hp), an unused air filter is attached, the engine speed when the engine speed is increased under no load condition, and the outside of the air filter 1 and the vent 21 Was measured, and performance was evaluated from a performance curve obtained from the measured value.

In this performance evaluation, the specifications of the weaving, the wire diameter, the number of meshes (the number of holes included in the surface of a 1-inch square mesh), and the diameters of the holes are shown in the table of FIG. Performance curves obtained from the measured values of the filters A to G,
This was performed by comparing a performance curve obtained from a measured value of a conventional air filter using filter paper as a filter medium.

FIG. 4 shows the measurement results. In this graph, the performance curve of each filter obtained by plotting the measured value for each filter with the differential pressure on the vertical axis and the rotation speed on the horizontal axis is shown.

From the measurement results, the air filter 1 of the present invention using the stainless steel meshes B to G except the stainless steel mesh A as the first filter medium 30 is the same as the conventional air filter 1 or the conventional one. The change in the differential pressure was lower than that of the air filter 1A, and it was confirmed that the air filter 1A had high performance.

On the other hand, as is clear from the measurement results of the stainless steel mesh A, when the diameter of the pores of the stainless steel mesh is smaller than 10 μm, the pressure difference is higher than that of the conventional air filter 1A, and the performance is low. It was confirmed that it became.

Therefore, the diameter of the pores of the stainless steel mesh is set to 10 to 100, which is the diameter of the pores of the meshes B to G.
μm, more preferably 40 to 75 μm, which is the diameter of the pores of the meshes D to F, and the wire diameter of the wire constituting the stainless steel mesh is 20 to 200 μm. By setting the number of vacancies included per unit to 4500 to 540000 used in the above measurement,
It has been confirmed that an air filter having performance equal to or higher than that of the conventional air filter 1A is realized.

It should be noted that a stainless steel mesh having a pore diameter of less than 10 μm has a lower performance than the conventional air filter 1A and is liable to be clogged. Therefore, it is used as a filter material for an air filter for an internal combustion engine. For this purpose, the performance must be improved by increasing the surface area of the filter medium, for example, by increasing the number of folds of the filter medium. Further, a stainless steel mesh having a pore diameter larger than 100 μm is suitable for use as a filter medium for an air filter for an internal combustion engine since dust passing through the pores of the mesh will damage the cylinder of the internal combustion engine. Absent.

(2) Restoration of filter performance by washing Diesel engine EK-100 used in the above measurement
(Hino Motor Co., Ltd., displacement 13,260cc, 27
0 hp), an unused state, a state after use, and an air filter in a state where the air filter in a state after use has been cleaned, and an engine when the engine speed is increased in a no-load state as in the above measurement. Rotation speed and air filter 1
The pressure difference between the outside of the sample and the vent 21 was measured, and the performance was evaluated from the performance curve obtained from the measured value.

The performance evaluation was performed on the filters B and E in the table of FIG. FIG. 5 shows the measurement result of the filter B, and FIG. 6 shows the measurement result of the filter C. These graphs show the performance curves obtained by plotting the measured values for the filters in each state, taking the differential pressure on the vertical axis and the rotation speed on the horizontal axis, as in the above measurement.

From these measurement results, it was confirmed that the performance of the air filter 1 of the present invention was almost restored to the state before use by washing after use. In addition, since the restoration of the performance due to the cleaning was confirmed in both the filter B and the filter E, the stainless filter was cleaned after use regardless of the diameter of the pores, the diameter of the wire rod, and the number of meshes. As a result, it was confirmed that the performance returned to almost the state before use.

Incidentally, [mmH ₂ O], which is a unit of the differential pressure used in this measurement, can be converted into [Pa] in SI units by the following equation. 1 [mmH ₂ O] ≒ 0.0098 [Pa]

In the above measurement, the pore diameter was 5 to 100
The twill tatami weave and the plain tatami weave stainless steel meshes A to G were used. As is clear from the measurement results shown in the graph of FIG. That is, the diameter of the hole is 10 to 1
When a stainless steel mesh of 00 μm was used, it was confirmed that the performance was equal to or higher than that of the filter medium 40 of the conventional air filter 1A.

Therefore, from the above measurement results, in order to realize an air filter having the same performance as the conventional air filter 1A using the filter paper as the filter medium 40 by the air filter 1 of the present invention, the filtration accuracy of the stainless mesh, that is, The diameter of pores of stainless mesh is 10 to 10
It has been confirmed that the thickness is preferably 0 μm, more preferably 40 to 75 μm, and the stainless steel mesh used is preferably made of a wire having a wire diameter of 20 to 200 μm. Even when a stainless steel mesh A having a pore diameter of 5 to 6 μm is used as the filter medium 30, the number of folds of the filter medium 30 is increased and the surface area of the filter medium 30 is increased, so that the conventional air filter 1 </ b> A It is considered that an air filter having similar performance can be realized.

In this embodiment, the stainless steel mesh is used as the metal mesh forming the first filter medium 30. However, the present invention is not limited to this embodiment, and a metal material having high durability against oxidation and corrosion, for example, aluminum It is also possible to use a metal mesh made of titanium or titanium.

In the present embodiment, the first filter medium 30 is formed of one stainless steel mesh. However, the present invention is not limited to this embodiment, and the first filter medium 30 may be formed of a plurality of metal meshes in the thickness direction. It is also possible to form by superimposing. In this case, the metal mesh on the outer casing 10 side has a larger diameter hole than the metal mesh on the inner casing 11 side of the superposed metal meshes, thereby facilitating operations such as cleaning. become.

As described above, the air filter 1 of the present invention has been described in detail based on the embodiment. However, the air filter of the present invention does not necessarily need to be provided with the outer casing 10 and the inner casing 11 as in the above embodiment. As in the case of the conventional air filter 1A shown in FIG. 7, these casings are omitted, and the first filter medium is simply formed into a substantially cylindrical shape so that the ridge line of the mountain-folded portion forms a loop. An air filter having the upper cover 20 and the lower cover 22 attached thereto as in the embodiment can also be used.

The air filter of the present invention does not necessarily have to be of a substantially cylindrical type as in the above-described embodiment. If necessary, the upper end face and the lower end face of the cylindrical air filter as in the above-described embodiment may be used. The truncated cone shape having different diameters, a polygonal cylindrical shape having a square or triangular upper end surface and a lower end surface, and a metal mesh bent in a corrugated shape as in the above-described embodiment. It is also possible to adopt a flat plate shape formed in a substantially plate shape inside the frame so that the ridge line of the portion forms a straight line.

Finally, it is needless to say that the present invention is not limited to the above-described embodiment, and various changes and improvements can be made within the scope described in the claims.

[0061]

As described above, according to the present invention,
The dust is efficiently removed by using the two types of filter media, the second filter media and the first filter media, and the dust is not entangled deep in the fibers of the filter media unlike conventional filter media. The filter can be reused by washing, and a highly durable air filter for an internal combustion engine is realized.

Further, the air filter of the present invention has a filter material of 1%.
Since a stainless steel mesh having pores of 0 to 100 μm is used, the performance is the same as that of a conventional filter, and the structure is less prone to clogging.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing the overall structure of an air filter of the present invention.

FIG. 2 is a cross-sectional view of the air filter of the present invention.

FIG. 3 is a table showing specifications of stainless steel meshes A to G and filter paper used for a filter medium of an air filter used in an experiment.

FIG. 4 is a graph showing a change in differential pressure with respect to a change in engine speed when an air filter using stainless steel meshes A to G and filter paper as a filter medium is used.

FIG. 5 is a graph showing a change in differential pressure with respect to a change in engine speed when an air filter using a stainless steel mesh B as a filter medium before, after and after washing is used.

FIG. 6 is a graph showing a change in differential pressure with respect to a change in engine speed when an air filter using a stainless steel mesh E as a filter medium before, after and after use is used.

FIG. 7 is an explanatory view showing the entire structure of a conventional air filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Air filter (this invention) 1A air filter (conventional) 10 Outer casing 11 Inner casing 20 Upper cover 21 Vent 22 Lower cover 30 First filter medium 31 Mountain fold part 32 Valley fold part 33 Second filter medium 40 Filter medium 41 Mountain fold part 42 Tanioribe

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) F02M 35/024 B01D 39/10-39/12 B01D 46/12-46/14

Claims (14)

(57) [Claims] 1. A cylindrical outer casing having a number of openings, a cylindrical inner casing having a plurality of openings provided at intervals inside the outer casing, and a cylindrical outer casing having a number of openings. An air filter for an internal combustion engine having a first filter medium and a second filter medium positioned therebetween, wherein the first filter medium is a stainless mesh that is bent into a waveform in which mountain-folded portions and valley-folded portions are alternately positioned. The stainless steel mesh has a diameter of 10
It has a hole of １００100 μm, is formed of a wire having a diameter of 20 to 200 μm, and is formed so that the side surfaces of either one of a vertical line or a horizontal line extending substantially in parallel are in contact with each other, The air filter for an internal combustion engine, wherein the second filter medium is located between the outer casing and the first filter medium. 2. The metal mesh forming the first filter medium,
2. The method according to claim 1, wherein the woven fabric is formed by a tatami weave.
3. The air filter for an internal combustion engine according to 1.). 3. The first filter medium has a diameter of 40 to 75 μm.
3. The method according to claim 1, further comprising:
3. The air filter for an internal combustion engine according to 1.). 4. The air filter for an internal combustion engine according to claim 1, wherein the one square inch surface of the stainless steel mesh has 4500 to 540000 holes. 5. The first filter medium is formed by laminating a plurality of metal meshes in a thickness direction, and the metal mesh on the outer casing side has a larger diameter than the metal mesh on the inner casing side. The air filter for an internal combustion engine according to any one of claims 1 to 4, comprising: 6. The method according to claim 6, wherein the second filter medium is at least the first filter medium.
The filter medium according to any one of claims 1 to 5, wherein the filter medium is provided in a space formed by two adjacent mountain folds of the filter medium and the valley fold located between the mountain folds. Air filter for internal combustion engine. 7. The diameter of the pores of the second filter medium is equal to that of the first filter medium.
The air filter for an internal combustion engine according to any one of claims 1 to 6, wherein a diameter of the air filter is larger than a diameter of pores of the metal mesh used for the filter medium. 8. The air filter for an internal combustion engine according to claim 7, wherein the second filter medium is a metal mesh. 9. An air filter for an internal combustion engine for removing dust contained in air supplied to the internal combustion engine, comprising:
The filter medium has pores with a diameter of 10 to 100 μm and a diameter of 2
An internal combustion engine characterized by using a stainless steel mesh formed of a wire having a diameter of 0 to 200 μm and formed so that the side surfaces of one of a vertical wire and a horizontal wire extending substantially in parallel are in contact with each other. Air filter. 10. The air filter for an internal combustion engine according to claim 9, wherein the stainless steel mesh is formed by a tatami weave. 11. The stainless steel mesh has a diameter of 40.
The air filter for an internal combustion engine according to claim 9 or 10, wherein the air filter has pores of up to 75 µm. 12. The air filter for an internal combustion engine according to claim 9, wherein a surface of one square inch of the stainless steel mesh has 450 to 540000 holes. 13. The air filter for an internal combustion engine according to claim 9, wherein the stainless steel mesh is formed in a substantially cylindrical shape so that a ridge line of the mountain-folded portion forms a loop. . 14. The air filter for an internal combustion engine according to claim 9, wherein the stainless steel mesh is formed in a substantially plate shape so that a ridge line of the mountain-folded portion forms a straight line. .