JPH07111126B2 - Camshaft drive structure for OHC engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a structure for driving two cam shafts simultaneously by using one endless cogged belt, and more particularly to longitudinal vibration of the cogged belt, that is, a cogged belt. To a camshaft drive structure of the above type which has been modified to control the tension and compression vibrations along its length.

<Prior Art> Conventionally, a chain has been used to drive the camshaft of an OCH engine, but in recent years, a lightweight cogged belt has come to be used. Although the cog-belt is lightweight, its expansion and contraction is extremely small, and its longitudinal vibration is generally not a problem. However, the rotational torque of the camshaft of the engine periodically fluctuates because the valve opening / closing load fluctuates due to the cam drive, and torsional vibrations are generated on the camshaft. This torsional vibration is transmitted from the cam shaft pulley to the cogged belt and acts as an exciting force to the cogged belt. At the same time, if an excessive load is applied to the meshing portion of the cogged belt with the pulley, the root portions of the teeth may be deformed, and the cogged belt may be partially displaced in the longitudinal direction (longitudinal direction of the belt). Even though the expansion and contraction is small, since the cogged belt is an elastic body, minute expansion and contraction deformation due to fluctuations in the load acting on the belt is unavoidable. Vibration occurs. The vertical vibration of this cogged belt
When resonating with the load fluctuation cycle of the pair of camshaft pulleys in the OHC engine, the tooth deformation load at the meshing portion of the cogged belt with the pulley increases due to the amplitude opposite to the rotation direction of the pulleys. In particular, increase the maximum rotation speed in an engine in which the span length of the cogged belt between the camshaft pulleys tends to increase, such as a V-type engine, or in a multi-cylinder engine in which the torsional vibration excitation force of the camshaft tends to increase. In such a case, the longitudinal vibration of the cogged belt becomes excessive and the root strength of the cogged belt that meshes with the pulley may become a problem.

<Problems to be Solved by the Invention> Since such a movement of the camshaft drive structure is particularly coupled with the movement of the valve mechanism, it is extremely complicated and it is extremely difficult to predict its behavior. Is. Generally, by increasing the natural frequency of the entire system by making the second moment of inertia of the pulley as small as possible,
It is thought that it would be good if problems do not occur in the region below the maximum rotation speed of the engine, but according to the experiment conducted by the inventor, the OHC type of the type in which multiple camshafts are driven by a common cogged belt In the camshaft drive structure of the engine, such a general prediction does not hold true, and if the second moment of inertia of the camshaft pulley located on the tension side around the crankshaft pulley of the cogged belt is minimized, It was found that the belt load tends to increase below the normal speed range, and rather, it is better to increase the moment of inertia of inertia of the tension camshaft pulley somewhat.

Based on the drawbacks of the prior art and the knowledge of the inventor,
A main object of the present invention is to provide a camshaft drive structure for an OHC type engine that can effectively suppress longitudinal vibration of a cogged belt.

[Constitution of the Invention] <Means for Solving the Problems> According to the present invention, such an object is an OHC type in which one endless cogged belt is suspended around two cam shaft pulleys and a crank shaft pulley. In the camshaft drive structure of the engine,
Second moment of inertia (I ₂ ) of the camshaft pulley located on the loose side around the crankshaft pulley of the cogged belt
And the second moment of inertia (I ₁ ) of the camshaft pulley located on the tension side are set as I ₁ > I ₂ and the above I ₁ , I
OHC characterized by setting the ratio of ₂ within a predetermined range
This is accomplished by providing a camshaft drive structure for a rotary engine.

<Operation> Decide the moment of inertia of inertia of the camshaft pulley located on the loose side around the crankshaft pulley of the cogged belt to a sufficiently small value, and set the moment of inertia of inertia of the camshaft pulley located on the tension side from the same side as the slack side. It was found by experiments that the load applied to the cogged belt decreases with a gradual increase, and the load applied to the cogged belt begins to increase only when the second moment of inertia of the tension camshaft pulley exceeds a certain value. That is, the tension load of the cogged belt around the crankshaft pulley, which is the maximum value of the load acting on the cogged belt, is determined by the difference between the tension side load and the slack side load, but the slack side camshaft pulley and the tension side cam Generation of longitudinal vibration of the cogged belt by reducing both the inertia moment of inertia with the shaft pulley to the limit and setting the inertia moment of inertia of one cam shaft pulley to a value that minimizes the load on such a cogged belt. Is suppressed and the deformation of the tooth root portion is alleviated. Therefore, the durability of the cogged belt can be greatly improved.

<Embodiment> Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a front view showing a V-type multi-cylinder engine to which the present invention is applied with a part thereof being cut away. The engine 1 has a cylinder row composed of two banks 2a and 2b arranged in a V shape, and has a crankshaft 3 at a position corresponding to the intersection of these banks. The axial end of the engine on the side shown in FIG. 1 is covered with three-part covers 4a to 4c, and one end of the crank shaft 3 protruding from one of the covers 4c is cranked. The shaft pulley 5 is fixed. This pulley 5 has at least two pulley grooves, and a V-belt 8 suspended in these pulley grooves causes a power steering hydraulic pump 6 disposed on the upper surface of the cylinder block between both banks 2a and 2b. The pulleys 6a and 7a of the AC generator 7 are rotationally driven.

Pulleys 10 and 20 for the cogged belt are provided on the ends of the cam shafts 9 and 19 inside the covers 4a and 4b and on the outer periphery of the crank shaft 3, respectively.
・ 11 are fixed. A cogged belt 14 is suspended between these three pulleys, and the cogged belt 14 between the pulleys 10 and 20 of both camshafts 9 and 19 is suspended on the back surface thereof by the pulley 12 of the cooling water pump. The cogged belt 14 between the pulley 10 of the cam shaft 9 of one bank 2a and the pulley 11 on the crankshaft side is suspended on the idler pulley 13 of the belt tensioner on the back surface thereof.

FIG. 2 is a vibration model of the camshaft drive structure in the embodiment of FIG. Here, the rotation direction of the crankshaft pulley 11 is clockwise as indicated by the arrow A, and the inertia moment of inertia of the camshaft pulley 20 located on the belt tension side around the crankshaft pulley is I ₁ , the radius. Is r ₁ , the angular displacement is θ _1, and the camshaft pulley 10 is also located on the loose side.
Inertia second moment and I _2, radius r _2, the angular displacement theta ₂ of, k ₁ the blade constant of cogged belt portion between the cam shaft pulley 20 of the pulley 11 and the tight side of the crank shaft, tension The spring constant of the cogged belt between the camshaft pulley 20 on the side of the camshaft and the camshaft pulley 10 on the side of the slack is k ₂ , and the spring of the cogzobelt between the pulley 11 on the crankshaft and the camshaft pulley 10 on the side of the slack When the constant is k ₃ , the movement of both camshaft pulleys 10 and 20 is expressed by the following equation.

I _{1 1} = k ₁ r ₁ ² θ ₁ + k ₂ r ₁ (−r ₁ θ ₁ + r ₂ θ ₂ ) + A ₁ sin ωt I _{2 2} = −k ₃ r ₂ ² θ ₂ + k ₂ r ₂ (−r ₂ θ ₂ + r ₁ θ ₁ ) + A ₂ sin (ωt + 2/3 ・ π) (1) At this time, the pulley 12 and idler pulley of the cooling water pump
Reference numeral 13 contacts the back surface of the cogged belt and can be ignored when analyzing the longitudinal vibration of the cogged belt. Further, A ₁ sin ωt and A ₂ sin (ωt + 2/3 · π) represent the excitation force of the valve mechanism for both camshaft pulleys 10/20.

Next, the above equation (1) is solved by numerical calculation and substituted into the following equation, f ₁ = k ₁ r ₁ θ ₁ f ₂ = k ₂ (−r ₁ θ ₁ + r ₂ θ ₂ ) f ₃ = -K ₃ r ₂ θ ₂ (2) When the tensions f ₁ , f ₂ and f ₃ in each span of the cogged belt 14 are calculated, the load F _T of the cogged belt on the crankshaft pulley 11 is finally as follows. It is represented by the formula.

F _T = f ₁ −f ₃ (3) According to the numerical analysis performed by the inventor, the inertia moment of inertia I ₁ of the camshaft pulley 20 on the tension side is fixed and the inertia of the camshaft pulley 10 on the slack side is fixed. When the second moment I ₂ is gradually increased from the minimum possible value, the maximum value F _{Tmax of the} belt load increases in both the low speed region and the high speed region of the engine rotation speed Ne as shown in FIG. It was found to do. Actually, when the maximum value F _Tmax of the load of the cogged belt 14 is plotted against the inertia moment of inertia I _{2 of} the slack side camshaft pulley 10, as shown in FIG. 4, the inertia moment of inertia is maximized. It has been found that smaller values give better results. As a result, the secondary moments of inertia I ₁ and I ₂ of both camshaft pulleys 10 and 20 are reduced as much as possible and their natural frequencies are increased, to the high-speed range outside the normal engine speed range. The resonance points of both camshaft pulleys 10 and 20 shift,
It can be seen that it is effective in reducing the maximum value F _Tmax of the belt load.

On the other hand, the second moment of inertia of the camshaft pulley 10 on the loosening side I ₂
Is set to the minimum value described above and the inertia moment of inertia I ₁ of the camshaft pulley 20 on the tension side is gradually increased from the same value as that on the slack side, as shown in FIG. Second moment of inertia I _{1 of} camshaft pulley 20 and that of slack side I ₂
If and are the same value, the maximum value F _Tmax of the belt load tends to increase as a whole, and as the secondary moment of inertia I ₁ of the tension side camshaft pulley 20 increases, the belt load in the high speed range increases. It was found that the maximum value F _Tmax increases, but the maximum value F _Tmax of the belt load in the low speed range decreases. The maximum value of the belt load of the cogged belt 14 with respect to the second moment of inertia I ₁ of the camshaft pulley 20 on the tension side
When F _Tmax is plotted, the results shown in Fig. 6 are obtained, and when the ratio of the inertia moments of inertia I ₁ and I ₂ of both camshaft pulleys 10 and 20 is within the specified range, the tension side It is possible to minimize the maximum value F _Tmax of the load on the cogged belt 14 by making the inertia moment of inertia I ₁ of the camshaft pulley 20 of ₆ above a little larger than the inertia moment of inertia I ₂ of the camshaft pulley 10 on the loose side. Was found. From these facts, if the inertia moments of inertia I ₁ and I _{2 of} both camshaft pulleys 10 and 20 are equal,
Since the rotational fluctuations of both camshaft pulleys 10 and 20 resonate, the maximum value F _Tmax of the load on the cogged belt 14 increases, and the secondary moments of inertia I ₁ and I ₂ of both camshaft pulleys 10 and 20 differ from each other. As a result, the resonance points of the camshaft pulleys 10 and 20 are also displaced from each other, and as a result, it is considered that the maximum value F _Tmax of the belt load around the crankshaft pulley 11 can be reduced. In addition, if the tension-side camshaft pulley 20 is equal to or lighter than the slack-side camshaft pulley 10, the oscillating force of the slack-side camshaft pulley 10 will cause the tension-side camshaft via the cogged belt 14. It is presumed that this is transmitted to the shaft pulley 20, and this is directly transmitted to the crankshaft pulley 11. Therefore, by setting the inertia moment of inertia I ₁ of the tension side cam shaft pulley 20 to be slightly larger than that I ₂ of the slack side cam shaft pulley 10, the slack side cam shaft pulley 10 starts up. Since it is effective in damping the vibration force, the maximum value of belt load F
It seems that the reduction of _Tmax is more effective. When the value of the inertia moment of inertia I ₁ of the tension side camshaft pulley 20 exceeds a certain area, the maximum belt load value F
There is a tendency for _Tmax to increase, but this is because when the inertia moment of inertia I ₁ of the camshaft pulley 20 on the tension side becomes excessive, the resonance frequency of this shifts to the low rotation speed side, which is the dominant effect. Is assumed to be large.

Generally, a cogged belt has an idler pulley 13 and a cooling water pump pulley 12 for setting its initial tension.
, Etc. are in contact with each other, but they are in contact with the back surface of the cogged belt 14, and it is considered that the influence on the tooth deformation load is extremely small. Therefore, its existence can be ignored as far as longitudinal vibration is concerned.

[Effects of the Invention] As described above, according to the present invention, the longitudinal vibration of the cogged belt can be effectively suppressed, the load on the cogged belt can be reduced, and the durability of the cogged belt can be improved. It is extremely large.

[Brief description of drawings]

FIG. 1 shows an OHC having a camshaft drive structure according to the present invention.
FIG. 3 is a front view showing a partially broken type V engine. FIG. 2 is a vibration model of the camshaft drive structure of FIG. FIG. 3 is a graph showing changes in the maximum load F _Tmax of the cogged belt with respect to the rotation speed of the engine with respect to changes in the inertia moment of inertia I ₂ of the slack side camshaft pulley. Fig. 4 shows the inertia moment of inertia of the camshaft pulley on the loose side.
7 is a graph showing changes in maximum load F _Tmax of the cogged belt with respect to changes in I ₂ . FIG. 5 is a graph showing changes in the maximum load F _Tmax of the cogged belt with respect to the rotation speed of the engine with respect to changes in the inertia moment of inertia I ₁ of the camshaft pulley on the tension side. Fig. 6 shows the secondary moment of inertia of the camshaft pulley on the tension side.
7 is a graph showing changes in the maximum load F _Tmax of the cogged belt with respect to changes in I ₁ . 1 ... Engine, 2 ... Cylinder block 2a, 2b ... Bank, 3 ... Crankshaft 4a, 4b, 4c ... Cover 5 ... Crankshaft pulley, 6 ... Hydraulic pump 7 ... AC generator, 6a, 7a …… Pulley 8 …… V belt, 9 …… Cam shaft 10, 11, 12 …… Pulley 13 …… Idler pulley, 14 …… Cogged belt 19 …… Cam shaft, 20 …… Pulley

Claims (1)

[Claims] 1. A camshaft drive structure for an OHC engine in which one endless cogged belt is suspended between two camshaft pulleys and a crankshaft pulley, wherein the cogged belt is loose around the crankshaft pulley. Second moment of inertia of camshaft pulley located on side (I ₂ )
And the second moment of inertia (I ₁ ) of the camshaft pulley located on the pasting side are set as I ₁ > I ₂ and the ratio of I ₁ and I ₂ is set within a predetermined range. OHC engine camshaft drive structure characterized by