Apex seals for rotary piston engines

Apex seal for rotary piston engines made of an iron-based material containing in weight 2.5 to 4.0% of C, 0.5 to 2.8% of Si, less than 1.0% of Mn, 0.25 to 2.0% of Ni, 0.25 to 2.0% of Mo, 0.25 to 2.0% of Cu, 0.15 to 0.4% of B, 0.15 to 1.5% of Cr, 0.1 - 0.4% of V and the balance of substantially iron. The material is of a chilled structure which is produced when the material is casted without any specific cooling. The apex seal provides remarkably improved wear and impact resistant properties.

The present invention relates to apex seals for rotary piston engines and 
more particularly to apex seals of such materials that have both 
wear-resistant and shock resistant properties. 
Conventional rotary piston engines comprise a casing which includes a rotor 
housing having an inner wall of trochoidal configuration and a pair of 
side housings gas-tightly secured to the opposite sides of the rotor 
housing to define a rotor cavity of trochoidal configuration. In the rotor 
housing, there is rotatably disposed a rotor of polygonal or usually a 
triangular configuration. The rotor includes a plurality of apex portions 
which are adapted to slidably engage the inner wall of the rotor housing 
so as to divide the rotor cavity into a plurality of working chambers 
having volumes which vary in response to the rotation of the rotor. 
In order to provide a gas-tight seal across the adjacent working chambers, 
the rotor is provided at each apex portion with a so-called apex seal. For 
the purpose, each apex portion of the rotor has an axially extending seal 
groove in which an elongated seal is received. The seal is resiliently 
biased against the inner wall of the rotor for sliding engagement 
therewith and moves along the inner wall of the rotor at a substantial 
speed as the rotary engine is operated. Thus, the apex seal must be of a 
highly wear-resistant material which is also durable to heat applied by 
engine combustion gas and produced in the seal due to the friction between 
the apex seal and the rotor housing wall. 
Further, the apex seal is also subjected to cyclic impact loads in use. 
Since the apex seal is used for gas-tightly separating two adjacent 
working chambers, when combustion takes place in one of the working 
chambers, the apex seal is forced under the pressure of combustion gas 
toward one of side walls of the seal groove in the rotor. As the rotor 
further rotates, combustion takes place in the other working chamber so 
that the apex seal is then forced toward the other direction under the 
pressure of combustion gas in said other working chamber. Thus, the apex 
seal is cyclically bumped against the side walls of the apex seal groove 
and therefore it is subjected to cyclical lateral impact loads of 
substantial value. Further, the movement of the apex seal is such that it 
is not always in close sliding contact with the inner wall of the rotor 
housing but cyclically moved away from the inner wall under the inertia 
force and again brought into bumping engagement with the inner wall under 
the biasing force and the centrifugal force on the apex seal. This 
movement of the apex seal causes cyclic impact loads thereon. 
Therefore, it will be understood that the apex seal should desirably be 
made of an impact and wear resistant material, however, it has been 
practically difficult to obtain a material which is satisfactory in 
respect of both impact and wear resistant properties since an improvement 
in the impact resistant property has generally caused poor wear resistant 
property. For example, proposals have been made of providing apex seals by 
a sintered alloy containing substantial amount of carbides such as TiC or 
by a cast iron containing high percentages of boron. Such apex seal 
materials have been found as possessing a satisfactory hardness as well as 
a superior wear resistant property, however, it does not have an adequate 
impact resistance. Therefore, in the known apex seals of the type that are 
made of the aforementioned materials, impact resistance has been provided 
by increasing the dimension, that is, the height and the thickness of the 
seal. However, such increase in the dimension of the apex seal is not 
recommendable from the view point of engine performance because any 
increase in the mass of the apex seal causes an increase in the sliding 
drag. 
There has also been proposed by Japanese patent application Sho 50-12142 
which has been disclosed for public inspection and at the same time 
published in Japanese Patent Gazette on July 30, 1976 under the disclosure 
number of Sho 51-87117 to provide an apex seal by a totally chilled cast 
iron. According to the proposal, the apex seal is made of a totally 
chilled iron-based material containing in weight 2.5 to 2.8% of total 
carbon, 1.5 to 3.0% of silicon, 0.5 to 1.0% of manganese, less than 0.30% 
of phosphorus, less than 0.10% of sulphur, 0.3 to 1.0% of chromium, 0.4 to 
1.0% of molybdenum, 0.4 to 1.0% of nickel or copper, 0.02 to 0.10% of 
boron and the balance of iron. However, this type of apex seal has been 
dissatisfactory in respect of wear-resistant property. 
It has also been proposed by the U.S. Pat. No. 3,658,451 to provide a layer 
of chilled structure only in the sliding surface of an apex seal. In this 
type of apex seal, the wear-resistant property is provided by the layer of 
the chilled structure while the impact resistant property is provided by 
the basic cast iron material. The proposed apex seal is advantageous in 
that both of the wear resistance and the impact resistance can be provided 
without increasing the dimension of the seal. However, this type of apex 
seal has been found disadvangeous in that it comprises two layers of 
different coefficient of thermal expansion so that it is deformed under 
the heat produced in the engine operation and cannot maintain a line 
contact with the inner wall of the rotor housing throughout its length. 
Such deformation of the apex seal causes pressure leakage from one working 
chamber to another. Further, local non-uniform contact of the apex seal 
with the inner wall of the rotor housing may cause scratches therein 
possibly damaging the chromium plate layer on the rotor housing inner 
wall. 
It is therefore an object of the present invention to provide a material 
for apex seals of rotary piston engines which possesses both 
wear-resistant and impact resistant properties. 
Another object of the present invention is to provide apex seals for rotary 
piston engines which are satisfactory in respect of sealing property and 
do not have any adverse effect on the engine performance. 
A further object of the present invention is to provide apex seals for 
rotary piston engines in which the aforementioned disadvantages of prior 
art have been overcome. 
According to the present invention, the above and other objects can be 
accomplished by an apex seal for rotary piston engines which is made of an 
iron-based material containing in weight 2.5 to 4.0% of C, 0.5 to 2.8% of 
Si, less than 1.0% of Mn, 0.25 to 2.0% of Ni, 0.25 to 2.0% of Mo, 0.25 to 
2.0% of Cu, 0.15 to 0.4% of B, 0.15 to 1.5% of Cr, 0.1-0.4% of V and the 
balance of substantially iron, said material being of chilled structure 
throughout the apex seal. Said material may contain impurities such as 
phosphorus and sulphur. The chilled structure is provided simply through 
casting of the material without any specific cooling means. 
According to the concept of the present invention, the wear-resistant 
property is provided by the chilled structure and also by wear-resistant 
carbides of such elements, for example, B, Cr and V, that are essential in 
forming the chilled structure. Since boron has a remarkably adverse effect 
on the impact resistance, the amount of boron is maintained as small as 
possible within a limit in which a desired wear resistant property is 
obtained. 
Although there is a decrease in the impact resistance due to the boron 
content, such decrease is compensated for by addition of Ni, Cu, Mo and V. 
The boron content has been known as having a significant effect on the 
promotion of producing chilled structures but having a tendency of making 
the structure brittle. The V content has, when added by a suitable amount, 
an effect of producing fine chilled structures which contribute to improve 
the ductility. 
Thus, according to the present invention, an uniform chilled structure can 
be obtained throughout the body of the apex seal simply by casting the 
material without using any cooling means. It has been found that the 
chilled structure in accordance with the present invention has a wear 
resistance which is fifty percent higher than that obtained by a 
conventional apex seal made of an acicular cast iron having a chilled 
sliding surface. Although the apex seal has a chilled structure throughout 
the body thereof, it has also been found that the impact resistance 
thereof is fifty percent higher than that obtained by a conventional apex 
seal because the apex seal of the present invention has a fine structure 
and added with Ni, Cu and Mo. It will thus be understood that the apex 
seal of the present invention is made of a material which possesses both 
the wear resistant and the impact resistant properties. According to the 
present invention, therefore, it is not necessary to increase the 
dimension of the apex seal for the purpose of providing an adequate impact 
resistance. Further, it is possible to eliminate any thermal deformation. 
As the result, the apex seal in accordance with the present invention can 
provide an improved sealing property which contributes to an increase in 
the engine output as well as an improvement in fuel consumption. The fact 
that the apex seal has a uniform structure throughout the body thereof is 
effective to eliminate any thermal deformation. Moreover, since the apex 
seal has a chilled structure throughout the body thereof, the coefficient 
of thermal expansion is very small so that the thermal expansion of the 
apex seal can be maintained very low. This fact also contributes to the 
improvement of the sealing property. 
According to the present invention, the amount of carbon content has been 
determined taking into consideration the fact that with the carbon content 
less than 2.5% in weight there will not be adequate eduction of carbides 
so that a satisfactory wear resistance cannot be obtained, while with the 
carbon content exceeding 4.0% in weight the material will become brittle 
due to a formation of excessive eutectic alloy. 
With the Si content less than 0.5%, an adequate casting property cannot be 
maintained, while, with the Si content exceeding 2.8%, there will be an 
adverse effect on the production of the chilled structure due to an 
increase in the amount of educed graphite. Manganese is added for the 
purpose of desulphurizing but it should not exceed 1.0% because an 
excessive Mn content has a tendency of making the structure brittle. 
Nickel, molybdenium and copper are added for the purpose previously 
discussed. However, they have no adequate improvement on the impact 
resistance where their contents are less than 0.5% but there will be no 
further improvement even when they are added more than 2.0%. 
Boron is the most important element because it produces hard carbides or 
composite carbides and has an effect of making it possible to produce a 
chilled structure simply by casting the material without any means for 
cooling. Thus, boron is essential in maintaining the wear-resistant 
property. With the boron content less than 0.15%, an adequate effect 
cannot be obtained but, with the boron content exceeding 0.4%, there is a 
remarkable tendency of producing a brittle structure and such defects 
cannot be eliminated by the addition of Ni, Cu, Mo and V. 
The boron content must be maintained as small as possible because excessive 
boron content leads to the disadvantages as discussed above. Therefore, it 
is practically unrecommendable to provide an adequate wear-resistant 
property solely by the addition of boron. Thus, chromium is added for 
supplementarily providing the wear-resistant property. With the Cr content 
less than 0.15%, however, the effect is not sufficient but with the Cr 
content exceeding 1.5% there will be an adverse effect on the machining 
property. 
Vanadium is one of the most important elements in accordance with the 
present invention. It produces carbides and has an effect of supplementing 
the effect of boron in producing a chilled structure simply by casting the 
material without any specific cooling means. It further has an effect of 
producing a fine structure which serves to compensate for the adverse 
effect of boron which makes the chilled structure brittle. The carbides 
produced by the addition of V provide an improved wear-resistant property. 
With vanadium content less than 0.1%, an adequate effect cannot be 
obtained. However, where the V content exceeds 0.4%, there will be an 
adverse effect of producing a brittle structure due to an excessive amount 
of carbides.

Referring to the drawings, particularly to FIGS. 1(a) and (b), the rotary 
piston engine shown therein includes a casing C which comprises a rotor 
housing R having an inner wall 4 of trochoidal configuration and a pair of 
side housings S secured to the opposite sides of the rotor housing R. In 
the casing C, there is disposed a rotor 2 of triangular configuration. The 
rotor is formed at each apex portion with a groove 3 in which an apex seal 
1 is received. The apex seal 1 is biased radially outwardly by means of a 
spring 8 which acts on the apex seal 1 and a wedge-shaped end piece 7. 
Thus, the apex seal 1 is thus forced into contact with the inner wall 4 of 
the rotor housing R and separates working chambers 5 and 6. The present 
invention can be applied to the apex seal 1. 
EXAMPLES 
Apex seals were produced from the materials listed in Table I. The apex 
seals had a configuration as shown in FIG. 1(c) wherein the height h was 
8.5 mm, the length l 80 mm, the thickness t 2.3 mm and the radius of 
curvature of the sliding surface r 2.0 mm. The materials were at first 
molten in an electric furnace and poured into shell moulds where the 
materials were cooled down until they were solidified. Then, the moulded 
parts were maintained at the temperature of 300.degree. to 600.degree. C. 
to remove strains and machined to the above dimensions. 
The samples listed in Table I were subjected to the following tests and the 
results were compared with those obtained from a conventional apex seal 
made of an acicular cast iron having a sliding surface of a chilled 
structure. The dimensions of the conventional apex seal were the same as 
those of the samples except that the thickness t was 3 mm. 
Table I 
__________________________________________________________________________ 
SAM- 
PLE C Si Mn P S Ni Cr Mo V B Cu OTHERS 
__________________________________________________________________________ 
1 3.32 
2.23 
0.5 0.065 
0.03 
1.5 0.82 
1.38 
0.15 
0.31 
1.10 
Ti 0.3 
2 3.35 
2.10 
0.42 
0.065 
0.02 
1.35 
0.51 
1.53 
0.20 
0.25 
1.00 
Nb 0.5 
3 3.36 
1.02 
0.73 
0.063 
0.03 
2.4 1.0 2.0 0.10 
0.30 
0.97 
-- 
4 3.40 
2.30 
0.41 
0.063 
0.03 
1.03 
0.52 
1.35 
0.12 
0.20 
1.20 
-- 
5 3.42 
1.83 
0.52 
0.063 
0.02 
2.5 0.48 
1.50 
0.15 
0.38 
0.70 
-- 
6 3.45 
1.75 
0.50 
0.060 
0.03 
1.03 
0.50 
1.15 
0.17 
0.15 
0.83 
W 0.5 
7 3.48 
2.20 
0.49 
0.065 
0.02 
2.00 
0.49 
0.50 
0.12 
0.25 
1.30 
-- 
8 3.50 
2.16 
0.45 
0.063 
0.02 
1.80 
0.53 
1.51 
0.18 
0.20 
0.78 
Ti 0.5 
9 3.52 
2.30 
0.50 
0.065 
0.03 
1.00 
0.48 
1.35 
0.20 
0.20 
1.30 
-- 
10 3.57 
2.23 
0.41 
0.061 
0.03 
1.03 
0.50 
1.37 
0.11 
0.15 
0.63 
-- 
__________________________________________________________________________ 
Wear Test 
1. Test Procedure: 
A rotatable disc having a chromium plated surface was prepared and the 
specimens were maintained in sliding contact with the rotating disc. The 
amount of wear was measured in terms of decrease in the height of the apex 
seal. 
2. Test Conditions: 
Back-up Pressure; 4.5 Kg 
Sliding Speed; 5 m/sec. 
Time; 20 min. 
Lubrication; None 
3. Test Results: 
The Results are shown in Table II. 
Table II 
______________________________________ 
PRIOR 
SAMPLES 1 2 3 4 5 6 7 8 9 10 ART 
______________________________________ 
WEAR (.mu.) 
40 36 38 37 32 44 38 39 36 37 55 
______________________________________ 
From the test results, it will be noted that the apex seals in accordance 
with the present invention have superior wear resistant property as 
compared with the conventional apex seal. 
Impact Tests 
The samples as listed in the Table I were subjected to impact tests and the 
results were compared with those obtained from the conventional apex seal. 
1. Test Procedure: 
In order to simulate the servicing conditions of the apex seals in engines, 
the samples were subjected to 50 cycles of heating, each cycle comprising 
heating the sample to the temperature of 400.degree. C. for 10 minutes and 
then cooling it down in water. The samples were then subjected to impact 
tests in a manner as shown in FIG. 2, wherein the distance S was 50 mm. 
The test results were measured in terms of the impact load W at which the 
specimen was broken. The same tests were also conducted on samples which 
were not subjected to repeated heating cycles. The results are shown in 
Table III. 
Table III 
______________________________________ 
SAMPLES 1 2 3 4 5 6 7 
______________________________________ 
WITHOUT 
HEATING 15.9 16.7 16.7 17.9 15.9 22.7 18.7 
WITH 
HEATING 16.7 18.2 18.1 19.5 17.3 24.6 19.6 
______________________________________ 
______________________________________ 
SAMPLES 8 9 10 PRIOR ART 
______________________________________ 
WITHOUT 
HEATING 17.1 18.3 15.9 13.7 
WITH 
HEATING 18.0 19.8 17.0 12.7 
______________________________________ 
Values are in Kg.m/cm.sup.2 
______________________________________ 
From the table, it will be noted that the apex seals in accordance with the 
present invention have impact resistance which is significantly higher 
than that of the conventional apex seal. Although the test specimens of 
the apex seals in accordance with the present invention were 2.3 mm thick, 
it should be noted that similar results would be obtained with specimens 
of 3.0 mm thick. The test results shown in Table III apparently show that 
the materials of the apex seal in accordance with the present invention 
have superior impact resistance as compared with the conventional apex 
seal material. Thus, according to the present invention, it is unnecessary 
to have an apex seal of increased dimension since an adequate impact 
resistance can be provided even in an apex seal of smaller dimension. 
It should further be noted that in the apex seal of prior art the impact 
resistance is decreased after heating, however, in the apex seals of the 
present invention, there is an increase in the impact resistance after 
heating. It is understood that in the conventional apex seal the material 
became brittle through the application of heat shock because the grains of 
the unchilled casting material are grown when heated. By the contrary, 
according to the present invention, the material is less sensitive because 
the apex seal has a chilled structure throughout the body thereof. 
The effect of boron content was thereafter investigated. For the purpose, 
the wear and impact tests were performed with samples having the same 
compositions as the sample 1 except that the boron content was changed. 
The results are shown in FIG. 3(a). 
It will be noted in FIG. 3(a), with the boron content less than 0.15%, the 
amount of wear exceeds 50 microns so that the wear resistant property is 
unsatisfactory. Further, with the boron content exceeding 0.4%, the impact 
resistance decreases below 13 kg.m/cm.sup.2 so that the apex seal can no 
longer have required strength. 
Further investigations have been made on samples of such compositions that 
are the same as those of the sample 9 except having various vanadium 
contents so as to find the effect of vanadium. The samples were subjected 
to the wear and impact tests and the results are shown in FIG. 3(b). 
In FIG. 3(b), it will be noted that where the vanadium content is less than 
0.1% the amount of wear exceeds 43 microns and at the same time there is a 
decrease in the impact resistance. With the vanadium content exceeding 
0.4%, there is no further improvement in the wear-resistant property in 
response to an increase in the vanadium content. Further, there is a 
noticeable decrease in the impact resistance. 
Thus, according to the present invention, it is possible to provide an apex 
seal which is satisfactory in respect of both the wear and impact 
resistant properties by using an iron based material containing 0.15 to 
0.4 percent in weight of boron and 0.1 to 0.4 percent in weight of 
vanadium. In accordance with the present invention, the dimension of the 
apex seal can even be decreased to provide desired properties and the seal 
is substantially free from any thermal distortion. 
In Tables II and III, it will be noted that in accordance with the present 
invention the values on the wear and impact resistance properties are 
fifty percent higher than those in the conventional apex seals. Therefore, 
it is even possible to decrease the dimensions of the apex seal in the 
present invention to provide the wear and impact resistant properties 
which are equivalent to those of the conventional apex seal having normal 
dimensions. More specifically, according to the present invention, the 
apex seal of 2.3 mm thick has the impact resistance comparable to that of 
the conventional apex seal of 3.0 mm thick. 
Thus, according to the present invention, it is possible to decrease the 
thickness of the apex seal so that the mass of the apex seal can be 
correspondingly decreased. This leads to a decrease in the sliding drag 
and an improvement in the engine performance. 
In the arrangement of the apex seal as shown in FIGS. 1(a) and (b), the 
reduction in the thickness of the apex seal provides further advantages. 
As shown in FIG. 1(a), there is a gap 9 at the end of the apex seal 1 and 
the volume of the gap 9 is decreased as the thickness of the apex seal 
decreases. Therefore, it is possible to decrease gas leakage through the 
gap 9 resulting in an increase in the brake mean effective pressure or 
output of the engine and also in an improvement in fuel consumption. In 
order to confirm the fact, the following test was performed. 
Engine Performance Test 
Test Procedure: 
Two rotary piston engines were provided for the test. Each engine was of 
two rotor type having a single working chamber displacement of 573 cc. One 
of the engines was equipped with apex seals of the present invention 
having the thickness of 2.3 mm. The other engine was equipped with 
conventional apex seals of 3.0 mm thick. The engines were operated with 
wide open throttle and the brake mean effective pressure and the brake 
output were measured under various engine speeds. The results are shown in 
FIG. 4. 
Fuel Consumption Test 
Test Procedure: 
The above two engines were used for the fuel consumption test. The engines 
were operated under 1500 rpm and the brake mean effective pressure and the 
fuel consumption were measured. The results are shown in FIG. 5. 
In FIGS. 4 and 5, it will be noted that the thin apex seals are effective 
to improve the engine performance in respect of the brake mean effective 
pressure, the output and the fuel consumption. Of course, the present 
invention is not limited to a specific thickness of the apex seal but it 
should be noted that according to the present invention there is a 
substantial room for decreasing the thickness of the apex seal to obtain 
an improved engine performance. 
The invention has thus been described with reference to specific examples, 
however, it should be noted that the invention is in no way limited to the 
details of such examples.