Balancer structure for three-cylinder engines

A balancer structure for a three-cylinder engine for eliminating the vibration in the engine especially the vibration caused by an inertia couple about an axis perpendicular to the crankshaft of the engine. A countershaft is rotated at the same speed as the crankshaft but in opposite direction. Two counterweights are secured to the crankshaft corresponding to the first and third cylinders of both ends for balancing of reciprocating masses and rotating masses. A counterweight is secured to the crankshaft for balancing of rotating masses. At least two balancers are secured to the countershaft at both ends thereof.

BACKGROUND OF THE INVENTION 
The present invention relates to a balancer structure for three-cylinder 
engines, and more particularly to a device provided with a countershaft 
rotated at the same speed but in the opposite direction of the crankshaft 
of the engine so as to balance the primary couple of inertia forces of the 
crankshaft about an intermediate position in the axial direction. 
There are two inertia forces of reciprocating masses and rotating masses, 
causing vibrations in the engine. The inertia forces of rotating masses 
may be balanced by providing a counterweight on the crankshaft in the 
opposite direction to a crank arm. The inertia forces of reciprocating 
masses may be balanced by the counterweight by a half of the inertia 
forces and the remainder may be balanced by the countershaft which is 
rotated in the opposite direction from the crankshaft and at the same 
speed. 
However, in the three-cylinder engine, the inertia forces of the first 
cylinder and the third cylinder act on the crankshaft symmetrically about 
an intermediate point corresponding to the second cylinder which is 
disposed between the first and third cylinders. Thus, an inertia couple 
acts about the intermediate point on the crankshaft. The couple of inertia 
causes a considerable vibration in the engine. Even if the inertia forces 
of rotating masses and reciprocating masses are balanced and further if 
the couple of inertia about the X-axis is balanced, the couple of inertia 
about an axis perpendicular to the crankshaft is inevitably generated. In 
order to balance such a couple of inertia, Japanese patent application 
laid open 55-6035 provides a balancer device of counterweights having a 
separated structure. Japanese patent publication 54-2333 discloses a 
countershaft which generates a couple of inertia equal to the couple of 
inertia of the crankshaft but opposite to the direction thereof. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a balancer device which 
can balance the couple of inertia about an axis perpendicular to the 
crankshaft of an engine in addition to the inertia forces of reciprocating 
masses and rotating masses. 
According to the present invention there is provided a balancer structure 
for a three-cylinder engine having three cylinders, a crankshaft and a 
countershaft rotated at the same speed as the crankshaft but in opposite 
direction, comprising: counterweights securely mounted on the crankshaft 
corresponding to the first and third cylinders disposed in both ends of 
the engine; a counterweight being disposed on the crankshaft corresponding 
to the second cylinder disposed at an intermediate position; two balancers 
securely mounted on the countershaft at both ends thereof. 
The present invention will be more apparent from the following description 
made with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Explaining a balancing system for one cylinder with reference to FIG. 1, a 
crankshaft 1 has three crank arms 2 angularly equidistant by 120.degree. 
with respect to each other. A connecting rod 4 is connected to each crank 
arm 2 by a crankpin 3 and to a piston 5. A counterweight 6 is secured to 
the crankshaft 1 along a line extending from the crank arm and at the 
opposite side of the arm for the balancing of the entire inertia forces of 
rotating masses and a half of inertia forces of reciprocating masses. A 
countershaft 7 is rotatably mounted in parallel with the crankshaft 1 and 
is adapted to be rotated at the same speed as but opposite direction 
relative to the crankshaft. A balancer 8 is secured to the countershaft 7 
for the balancing of the remainder of the inertia forces of the 
reciprocating masses. The balancer 8 is so disposed that turning angle 
.theta. of the balancer from the bottom on the Z-axis of the countershaft 
7 is equal to crank angle .theta. from the top dead center. 
Presenting the inertial mass of reciprocating parts mp and, for the 
convenience of explanation, the equivalent inertial mass at the crankpin 3 
of rotating parts mc, the mass of the counterweight 6 necessary for 
eliminating the vibration of the engine unit of FIG. 1 is mp/2+mc, because 
the mass of the counterweight 6 for balancing one half of the 
reciprocating inertial mass mp is mp/2 and the mass for balancing the 
entire rotating mass mc is mc. On the other hand, the mass of the balancer 
8 necessary for balancing the remainder of the reciprocating mass is mp/2. 
Thus, the engine of FIG. 1 is balanced by the counterweight 6 and the 
balancer 8 having the above described respective masses. Therefore, the 
total mass of the counterweight 6 of a three-cylinder engine is 3 
((mp/2)+mc) and the total mass of the balancer is (3/2) mp. 
Explaining the balancing of the reciprocating inertial mass of the 
three-cylinder engine with reference to FIG. 2, each of the first cylinder 
to the third cylinder is designated by a numeral with suffix (a to c). In 
FIG. 2, the piston 5b of the second cylinder is at the top dead center, 
the piston 5a of the first cylinder is at 240.degree. crank angle and the 
piston 5c of the third cylinder is at 120.degree. crank angle. Vibration 
forces FP1 to FP3 of all cylinders at crank angle .theta. are as follows, 
where r is the radius from the center of the crankshaft to the crankpin 
and .omega. is the angular velocity of the crankshaft. 
EQU FP1=mpr.omega..sup.2 cos (.theta.+240.degree.) 
EQU FP2=mpr.omega..sup.2 cos .theta. 
EQU FP3=mpr.omega..sup.2 cos (.theta.+120.degree.) 
The total inertia force is 
EQU FP1+FP2+FP3=0 
Therefore the vibration forces are balanced. 
The couple of inertia of the crankshaft is expressed as 
EQU FP1.S+FP2(S+L)+FP3(S+2L). 
where S is a distance of a point P on the X-axis from the first cylinder, L 
is a pitch between adjacent cylinders. The above formula is substituted as 
follows. 
EQU FP1(S)+FP2(S+L)+FP3(S+2L)=.sqroot.3mpr.omega..sup.2 L sin .theta.(1) 
Thus, the couple of inertia about the Y-axis is produced in the crankshaft 
by reciprocating masses in the Z-axis direction. 
Explaining the half balancing of the inertia forces of the reciprocating 
masses by counterweights 6a, 6b and 6c with reference to FIG. 3, pistons 
5a-5c are in the same positions as FIG. 2 and each of counterweights 6a, 
6b and 6c is positioned at an angular position advanced 180.degree. from 
the corresponding crank arm 2a-2c. 
Forces Frec1 to Frec3 caused by the mass of each counterweight in the 
Z-axis direction at a crank angle .theta. are as follows. 
EQU Frec1=(mp/2)r.omega..sup.2 cos (.theta.+240.degree.-180.degree.) 
EQU Frec2=(mp/2)r.omega..sup.2 cos (.theta.+180.degree.) 
EQU Frec3=(mp/2)r.omega..sup.2 cos (.theta.+120.degree.+180.degree.) 
Therefore, inertia forces in the Z-axis direction are 
EQU Frec1+Frec2+Frec3=0 
Thus, the inertia forces are balanced. 
The couple of inertia caused by the inertia forces in the Z-axis about the 
Y-axis is expressed as 
EQU Frec1(S)+Frec2(S+L)+Frec3(S+2L)=(.sqroot.3/2)mpr.omega..sup.2 L sin 
.theta.(2a) 
Accordingly, couple of inertia about the Y-axis is also produced by masses 
of the counterweights 6a-6c. 
In addition, each of inertia forces of the counterweights 6a-6c has also a 
component in the Y-axis direction. The couple of inertia about the Z-axis 
is 
EQU -(.sqroot.3/2)mpr.omega..sup.2 L cos .theta. (2b) 
Thus, the counterweights 6a-6c produce the couple of inertia about the 
Y-axis and the couple of inertia about the Z-axis. The composite couple of 
inertia is presented as 
EQU (3/2)mpr.omega..sup.2 L sin .theta.-(.sqroot.3/2)mpr.omega..sup.2 L cos 
.theta.=(.sqroot.3/2)mpr.omega.L(sin .theta.=cos .theta.) (3) 
It is to be noted that it is possible to remove a counterweight for the 
second cylinder and the counterweight is distributed to the first and 
third cylinders. Explaining this with reference to FIG. 4, masses of 
counterweights 6a' and 6c' of first and third cylinders are (.sqroot.3/2) 
(mp/2). The counterweight 6a' of the first cylinder is positioned in 
advance 180.degree.+30.degree. from the crank arm 2a and the counterweight 
6c' of the third cylinder is positioned in advance 180.degree.-30.degree. 
from the crank arm 2c. That is counterweights 6a' and 6c' are 180.degree. 
opposed and make a right angle with the crank arm 2b. 
The inertia forces of each cylinder by the counterweight in the Z-axis 
direction at a crank angle .theta. is as follows. 
EQU Frec1'=(.sqroot.3/2)(mp/2)r.omega..sup.2 cos 
(.theta.+240.degree.+180.degree.+30.degree.) 
EQU Frec3'=(.sqroot.3/2)(mp/2)r.omega..sup.2 cos 
(.theta.+120.degree.+180.degree.-30.degree.) 
The inertia forces in the A-axis direction are 
EQU Frec1'+Frec3'=0 
Thus, the inertia forces are balanced. 
The couple of inertia by the Z-axis forces about the Y-axis is 
EQU Frec1'.multidot.S+Frec3'(S+2L)=(.sqroot.3/2)mpr.omega..sup.2 L sin .theta. 
This formula is the same as the formula (2a). The couple of inertia about 
the Z-axis is also the same as the formula (2b). 
Thus, it will be understood that the inertia forces can be balanced by 
providing counterweights for all cylinders or for the first and third 
cylinders and that the couple of inertia in both cases are the same. 
The balancing of the couple of inertia about the Y-axis and Z-axis will be 
explained hereinafter. The composite couple of inertia of formulas (1) and 
(3) is 
EQU -.sqroot.3mpr.omega..sup.2 L sin .theta.+(.sqroot.3/2)mpr.omega..sup.2 L 
(sin .theta.-cos.theta.)=-(.sqroot.3/2)mpr.omega..sup.2 L (sin 
.theta.+cos.theta.) (4) 
A system for balancing such a couple of inertia by the countershaft will be 
described hereinafter with reference to FIG. 5. Balancers 8a, 8b and 8c 
balance a half of the inertia forces of reciprocating masses, and hence 
each mass is mp/2. As shown in the figure, the balancer 8b for the second 
cylinder is at the bottom when the piston 5b of the second cylinder is at 
the top dead center, the balancer 8a for the first cylinder is at a 
position advancing 240.degree.-180.degree. from the top in the 
counterclockwise direction, and the balancer 8c for the third cylinder is 
positioned advancing 120.degree.+180.degree.. 
Therefore, the inertia forces by the balancers in the Z-axis direction at 
an angle .theta. are as follows. 
EQU Frec1=(mp/2)r.omega..sup.2 cos (.theta.+240.degree.-180.degree.) 
EQU Frec2=(mp/2)r.omega..sup.2 cos (.theta.+180.degree.) 
EQU Frec3=(mp/2)r.omega..sup.2 cos (.theta.+120.degree.+180.degree.) 
Thus the inertia forces in the Z-axis direction are balanced. 
The couple of inertia about the Y-axis by the inertia forces in the Z-axis 
direction is 
EQU (.sqroot.3/2)mpr.omega..sup.2 L sin .theta. (2a') 
The inertia forces in the Y-axis direction are minus since the countershaft 
rotates in the counterdirection. However, the inertia forces are balanced. 
The couple of inertia about the Z-axis by the forces in the Y-axis 
direction is 
EQU (.sqroot.3/2)mpr.omega..sup.2 L cos .theta. (2b') 
The composite couple of inertia of formulas (2a') and (2b') is 
EQU (.sqroot.3/2)mpr.omega..sup.2 L (sin .theta.+cos .theta.) (4') 
If the formula (4') and the formula (4) are combined, 
EQU (.sqroot.3/2)mpr.omega..sup.2 L (sin.theta.+cos 
.theta.)-(.sqroot.3/2)mpr.omega..sup.2 L (sin .theta.+cos .theta.)=0 
Thus, couples of inertias about an axis perpendicular to the crankshaft may 
be balanced by balancers on the countershaft. 
FIG. 6 shows an example in which the balancer for the second cylinder is 
omitted like FIG. 4. The mass of the balancer 8a' for the first cylinder 
and the mass of the balancer 8c' and for the third cylinder is (mp/2) 
(.sqroot.3/2) respectively. The balancer 8a' is advanced 30.degree. from 
the position of FIG. 5 and the balancer 8c' is retarded 30.degree.. By 
these conditions, inertia forces of reciprocating masses are balanced. 
The balancing of the forces by rotating parts will be explained 
hereinafter. The construction of the balancer device is the same as FIG. 
2. Forces Fc1, Fc2, Fc3 in the first to third cylinders at a crank angle 
.theta. are 
EQU Fc1=mcr.omega..sup.2 cos (.theta.+240.degree.) 
EQU Fc2=mcr.omega..sup.2 cos .theta. 
EQU Fc3=mcr.omega..sup.2 cos (.theta.+120.degree.) 
The couple of inertia about the Y-axis by the rotating masses is 
EQU -.sqroot.3mcr.omega..sup.2 L sin .theta. (5a) 
The couple of inertia about the Z-axis is 
EQU .sqroot.3mcr.omega..sup.2 L cos .theta. (5b) 
The composite couple of inertia is 
EQU -.sqroot.3mcr.omega..sup.2 L (sin .theta.-cos .theta.) (6) 
The balancing system with the counterweights 6a, 6b, 6c for the couple of 
inertia by the rotating masses are described hereinafter. The construction 
of the system is the same as FIG. 3. Forces Frot1, Frot2 and Frot3 by 
masses of counterweights 6a to 6c are 
EQU Frot1=mcr.omega..sup.2 cos (.theta.+240.degree.-180.degree.) 
EQU Frot2=mcr.omega..sup.2 cos (.theta.+180.degree.) 
EQU Frot3=mcr.omega..sup.2 cos (.theta.+120.degree.+180.degree.) 
The couple of inertia about the Y-axis is 
EQU .sqroot.3mcr.omega..sup.2 L sin .theta. (7a) 
The couple of inertia about the Z-axis is 
EQU .sqroot.3mcr.omega..sup.2 L cos .theta. (7b) 
The composite couple of inertia is 
EQU .sqroot.3mcr.omega..sup.2 L (sin .theta.-cos .theta.) (8) 
Thus, the composite couple of inertia of the formula (6) is also balanced 
by the composite couple of inertia of the formula (8). 
The forces of rotating masses may also be balanced by separating 
counterweights into the first and third cylinders as described with 
respect to FIG. 4. The mass of the counterweight is mc(.sqroot.3/2) and 
the phase of the counterweight is advanced or retarded by 30.degree.. 
The present invention is based on the above described principle. In 
accordance with the present invention, inertia forces and couple of 
inertia caused by rotating masses are balanced by counterweights mounted 
on the crankshaft and vibration caused by reciprocating masses is 
eliminated by counterweights on the crankshaft and balancers on the 
countershaft. 
Referring to FIG. 7, counterweights 6a-1, 6a-2, 6b-1, 6b-2, 6c-1, and 6c-2 
are provided for each cylinder opposite to the crankarm like FIG. 3 for 
balancing the rotating masses. Further, for the first cylinder, two 
counterweights 6a'-1 and 6a'-2 are provided and counterweights 6c'-1 and 
6c'-2 are provided for the third cylinder for balancing reciprocating 
masses. 
The countershaft 7 is provided with balancers 8a' and 8c' at positions 
corresponding to bearings 9a and 9d of both ends except the second 
cylinder. Since counterweights 6a'-1, 6a'-2, 6c'-1 and 6c'-2 for the 
reciprocating masses are separated into two positions, each counterweight 
is (mp/2) (.sqroot.3/2) and the phase of each counterweight is advanced or 
retarded by 30.degree. whereby they make right angles with the crank arm 
2b (see description with respect to FIG. 4). If the pitch between the 
cylinders is L, the couple of inertia is balanced by the following 
conditions. 
EQU (mp/2)(.sqroot.3/2)2L=(.sqroot.3/2)mpL 
Therefore, presenting the composite mass of the counterweights 6a'-1 and 
6a'-2 "Mca'", and the composite mass of the counterweights 6c'-1 and 6c'-2 
"Mcc'", for the balancing of the inertia forces on the crankshaft 1, it is 
necessary to keep Mca'=Mcc'. 
Further, presenting the position of the composite center of gravity of the 
counterweights 6a'-1 and 6a'-2 in respect to the Y-axis "L+X'", and the 
position of the composite center of the gravity of the counterweights 
6c'-1 and 6c'-2 "L+Y'", it is necessary to satisfy the following equation 
EQU Mca'(L+X'+L+Y')=(.sqroot.3/2)mpL 
Hence, 
EQU Mca'=Mcc'=(.sqroot.3/2)mpL/(2L+X'+Y') (9a) 
As to the counterweights 6a-1, 6a-2, 6b-1, 6b-2, and 6c-1, 6c-2 for 
balancing the mass of the rotating parts, it is necessary to keep 
Mca=Mcb=Mcc, where Mca, Mcb and Mcc are composite mass respectively. If 
the position of the composite center of the gravity of counterweights 6a-1 
and 6a-2 for the composite center of the gravity of counterweights 6b-1 
and 6b-2 is L+X and the composite center of gravity of counterweights 6c-1 
and 6c-2 for the composite center of the gravity of counterweights 6b-1 
and 6b-2 is L+Y, 
EQU Mca(L+X)=Mcc(L+Y) 
Thus, X=Y is necessary. For the couple of inertia in the Y-axis direction, 
it is necessary to satisfy 
EQU ((Mca(L+X)+Mcc(L+Y)) cos 30.degree.=.sqroot.3mcL 
This is changed to the following general formula. 
EQU Mca=Mcb=Mcc=mcL/(L+X) (9b) 
Thus, the mass of the counterweight may be suitably determined in 
dependency on the composite center of the gravity. The mass decreases with 
the increase of X', Y', X and Y. If the center of gravity of the first 
cylinder coincides with that of the third cylinder and the center of 
gravity of the second cylinder is positioned at the center of gravity, 
X'=Y'=X=Y=0. Therefore, the mass of the counterweight of the first and 
third cylinder is (.sqroot.3/4) mp respectively. And the mass of the 
counterweight of the second cylinder is mc. It will be noted that although 
counterweights 6a-1 and 6a'-1 and others are separated by an angle of 
30.degree., in practice these counterweights are integrally combined. 
The masses of the balancers on the countershaft 7, as described above, 
correspond to the masses of the reciprocating parts of the engine and each 
mass is (mp/2) (.sqroot.3/2) and its phase is adjusted by 30.degree.. The 
massess are so arranged as to produce the following couple of inertia, 
EQU (mp/2)(.sqroot.3/2)2L=(.sqroot.3/2)mpL 
Therefore, considering the balancing of the inertia forces on the 
countershaft, it is necessary to keep Mba'=Mbc', where Mba' is the mass of 
the balancer 8a' and Mbc' is the mass of the balancer 8c'. If the 
positions of the centers of gravities of the balancers 8a' and 8c' are 
L+X" and L+Y", the following conditions are necessary: 
EQU Mba'(L+X"+L+Y")=(.sqroot.3/2)mpL 
As a result, 
EQU Mba'=Mbc'=(.sqroot.3/2)mpL/(2L+X"+Y") (10) 
Thus, the masses Mba' and Mbc' also decrease with the increase of X" and 
Y". Accordingly, the device may be made in a small size. 
As is understood from the foregoing, the masses of the counterweights 
6a'-1, 6a'-2, and 6c'-1, 6c'-2 are provided to satisfy the equation (9a) 
and positioned to make a right angle with the crank arm 2b of the second 
cylinder. Counterweights 6a-1, 6a-2, 6b-1, 6b-2, and 6c-1, 6c-2 of formula 
(9a) are opposite to respective crankpin. On the other hand, the balancers 
8a' and 8c' of the formula (10) are positioned, corresponding to the 
bearings 9a and 9d. The balancers 8a' and 8c' are so disposed that when 
the piston of the second cylinder is at the top dead center, the balancer 
8a' is at the same angular position as the counterweights 6c'-1 and 6c'-2 
and the balancer 8c' is at the same angular position as the counterweights 
6a'-1 and 6a'-2. Thus, the primary inertia forces, primary couple of 
inertia of the reciprocating and rotating parts of a three-cylinder engine 
and the primary couple of inertia of the crankshaft about axes 
perpendicular to the crankshaft are balanced by the counterweights on the 
crankshaft and balancers on the crankshaft. 
Since the balancers 8a ' and 8c' are disposed corresponding to the bearings 
at both ends so as not to engage with the counterweights, the countershaft 
may be disposed adjacent to the crankshaft. In addition, since the 
counterweights for the reciprocating masses are provided for the first and 
third cylinders with the exception of the second cylinder, the engine may 
be made of a small size compared with an engine provided with 
counterweights at every cylinders. 
FIG. 8 shows an example in which an engine 10 is transversely mounted on an 
automobile at a rear portion thereof for driving the rear wheels. An air 
cleaner 11, carburetor 12 and induction pipe 13 are horizontally disposed 
and connected with each other. Further, a compressor 14 for an air 
conditioner and ACG 15 are also attached. In accordance with the present 
invention, the countershaft 7 can be disposed adjacent to the crankshaft 1 
without interfering with upper devices such as the carburetor 12. 
FIG. 8 shows another example in which the engine 10 is transversely mounted 
on an automobile at a front portion for driving the front wheels. An 
exhaust pipe 16 and a catalytic converter 17 for an emission control 
system are disposed in a front side of the engine. In this example, the 
countershaft 7 can also be disposed adjacent to the crankshaft 1 without 
interfering with such the equipments. 
Referring to FIG. 10 showing the second embodiment of the present 
invention, each of balancers 8a' and 8c' of this embodiment is formed as a 
part of a journal for bearing the countershaft 7. The balancer 8a' is 
eccentrically formed in a cylindrical journal 20 which is coaxial and 
integral with the countershaft 7. The journal 20 is supported by a journal 
bearing 21 in a frame 22 which supports the crankshaft 1 by the bearing 
9a. The balancer 8c' is formed in the same manner as the balancer 8a' and 
supported in a frame 24. Other parts are the same as the first embodiment 
of FIG. 7. 
In accordance with this embodiment, extra portions for bearing the 
countershaft are not provided, whereby the device may be assembled into a 
small size. 
In the third embodiment of FIG. 11, balancer 8a' is divided into balancers 
8a'-1 and 8a'-2 and balancer 8c is divided into 8c'-1 and 8c'-2. In this 
embodiment, composite mass Mba' of the balancers 8a'-1 and 8a'-2 and 
composite mass Mbc' of the balancers 8c'-1 and 8c'-2 must be equal. Each 
of balancers 8a'-1 and 8c'-2 at both ends is formed as journal 20 as the 
second embodiment and supported by the journal bearings 21. 
In the fourth embodiment of FIG. 12, the balancer comprises three balancers 
8a, 8b and 8c according to the principle of FIG. 5. Also in this 
embodiment, mass Mba, Mbb and Mbc of balancers 8a, 8b and 8c must be equal 
each other. 
FIG. 13 shows the fifth embodiment of the present invention. In this 
embodiment, the balancer comprises three balancers 8a, 8b and 8c like the 
fourth embodiment. Each of balancers 8a and 8c at both ends is formed as 
journal 20 and the same parts as FIG. 11 are identified by the same 
numerals. 
In the sixth embodiment shown in FIG. 14, the central balancer of the fifth 
embodiment of FIG. 13 is divided into balancers 8b-1 and 8b-2. The center 
of gravity of the balancers 8b-1 and 8b-2 is positioned at a center of the 
second cylinder. The balancer 8b-1 corresponds to a bearing 9b for the 
crankshaft 1 and the balancer 8b-2 corresponds to a bearing 9c. Thus, the 
space in the engine may be effectively used.