Valve operation control device for internal combustion engines

Abstract

A valve disabling operation control device for use in an internal combustion engine having inlet valve means, exhaust valve means, at least one of which comprises a plurality of valves. Disabling means selectively holds the at least one of the inlet valve means and the exhaust valve means in a first state wherein part of the valves thereof are inoperative, and in a second state wherein all the valves thereof are operative for alternate closing and opening actions. Transmission state detecting means detects whether power transmission means of the engine is in a connected state wherein power transmission between the engine output shaft and a load on the engine is allowed, and is a disconnected state wherein the power transmission is inhibited. Control means is responsive to an output from the detecting means to cause the disabling means to hold the at least one of the inlet valve means and the exhaust valve means in the first state when the power transmission means in the disconnected state, irrespective of the rotational speed of the engine.

Description

operation control or disabling device constructed as above will now be described. When the input signals applied to the three input terminals of the AND circuit 43 are all at a high level, that is, when the clutch is in an engaged state and simultaneously the engine rotational speed is higher than the particular speed N1, while at the same time the transmission is in a connected or power-transmitting state, the AND circuit 43 generates a high level output and applies same to the control terminal of the switching circuit 52 to bring same into the closed state, whereby the solenoid 33 of the solenoid controlled valve 32 is energized so that the spool 38 is displaced rightward as viewed in FIG. 8 to allow pressurized engine oil pumped from the oil pump 26 to be fed to the oil feeding passages 16a, 16a'. As a consequence, each of the back pressure chambers 18a, 18a' of the inlet and exhaust operating means A, A' is supplied with pressurized oil so that the piston 17 in each chamber is moved so as to drivingly connect the always operating rocker arm 15-1 with the disabling rocker arm 15-2 to thereby allow the engine to operate in the all valve operative state.

On the other hand, when the voltage applied to either one of the input terminals of the AND circuit 43 is at a low level, the AND circuit 43 generates a low level output and applies same to the control terminal of the switching circuit 52. Accordingly, the switching circuit 52 becomes open to cause deenergization of the solenoid 33 of the solenoid control valve 32 so that the spool 38 is moved leftward as viewed in FIG. 8, to thereby interrupt the supply of pressurized engine oil pumped from the oil pump 26 to the oil feeding passages 16a, 16a'. Consequently, the piston 17 is moved toward the always operating rocker arm 15-1 by the force of the spring 20 to release the always operating rocker arm 15-1 and the disabling rocker arm 15-2 from their driving coupling to allow the engine to operate in the partial valve inoperative state.

If a gear shifting operation is effected while the engine is operated in the all valve operative state, that is, while the AND circuit 43 has all its input terminals supplied with high level voltages, the transmission temporarily assumes its neutral position, and accordingly the neutral position switch 48 temporarily assumes an on position, so that the output from the AND circuit 43 temporarily goes low. If each time such temporary change in the control circuit take places in the high engine rotational speed region, the valve disabling device effects a changeover from the all valve operative state to the partial valve inoperative state, there can occur a sudden drop in the engine output, causing a mechanical shock upon the valve disabling device and component parts of the engine, which results in shortened lives of these components and even in degraded driveability of the vehicle. Therefore, according to the invention, the control circuit 34 of the valve disabling device is provided with the changeover temporary inhibit circuit 44 for temporarily holding the output voltage level from the AND circuit 43 unchanged, to thereby eliminate the above-mentioned disadvantage.

FIG. 10 shows a variation of the control circuit of the valve disabling device. According to this modified control circuit 34', contacts 61 of a relay and the solenoid 33 of the solenoid controlled valve 32 are serially connected between the anode of the battery 53 and the ground, while in parallel with the contacts 61 and the solenoid 33, switches 70, 71 and 72 and a coil 62 of the relay 60 are serially connected between the anode of the battery 53 and the ground. The switch 70 may be formed e.g. by contacts of a normally open relay, and is adapted to be closed when the engine rotational speed is above the particular speed N1. The switches 71, 72 may be formed by the clutch switch 50 and the neutral switch 48 in FIG. 9, and these switches 71, 72 are adapted to be closed when the clutch assumes an engaged position, and when the transmission assumes a position other than the neutral position, respectively.

When all the switches 70, 71, 72 are closed and electric current from the battery 53 is applied to the coil 62 of the relay 60 at the same time, the contacts 61 of the relay 60 is closed to cause energization of the solenoid 33, whereas when the clutch is disengaged, or the transmission assumes its neutral position to interrupt power transmission, or the engine rotational speed is less than the particular speed N1, that is, at least one of the three switches 70, 71, and 72 is open, the supply of electric current to the coil 62 is interrupted to cause the contacts 61 to be open, whereby the solenoid 33 is deenergized.

In the above described manner, the energization of the solenoid 33 of the solenoid controlled valve 32 is controlled for changeover operation of the valve disabling device.

FIGS. 11 through 13 illustrate a second embodiment of the valve disabling device according to the invention. The device according to this second embodiment is characterized by its responsiveness to the engine rotational speed, the state of engagement or disengagement of the power transmission means, the engine oil temperature, and the throttle valve opening, for changeover of the states of the inlet valves 11 and exhaust valves 12 (FIG. 2). The device according to the second embodiment is distinguished from the first embodiment of FIGS. 2 through 10 previously described, in respect of the structures of the solenoid valve control circuit and the changeover temporary inhibit circuit, and also in that the control circuit is also responsive to outputs from a temperature sensor 35 and a throttle valve opening sensor 46, but in other respects is substantially identical with the latter. Therefore, parts or elements corresponding to those of the previous embodiments are designated by identical reference characters, and description thereof is omitted.

As shown in FIGS. 11 and 12, the solenoid valve control circuit 134 is electrically connected not only with the aforementioned engine rotational speed sensor 40, the neutral position switch 48, and the clutch switch 50, but also with the engine oil temperature sensor 35 and the throttle valve opening sensor 46. To be specific, the engine oil temperature sensor 35 has its output connected to the input of a comparator 45 which in turn has its output connected to a fourth input terminal of an AND circuit 143 which has first to third input terminals connected to respective ones of the comparator 42 and the level shifter units 49, 51. The output of the comparator 45 is also connected to a first input terminal of a changeover temporary inhibit circuit 44', which in turn has a second input terminal to which is connected the engine rotational speed sensor 40 by way of the f-V converter 41 and the comparator 42.

The temperature sensor 35 is mounted, for instance, on the coupling member D, and converts the temperature of engine oil as operating fluid of the valve disabling device into a corresponding voltage VT and applies same to the comparator 45. The comparator 45 compares this voltage VT with a predetermined voltage V2 corresponding to a predetermined value of the engine oil temperature, and when the relationship VT≧V2 stands, that is, when the engine oil temperature is above the predetermined value or the engine oil is low in viscosity, it applies a high level voltage to the AND circuit 143, whereas when the relationship VT<V2 holds, that is, when the engine oil temperature is below the predetermined value or the engine oil is high in viscosity such that the valve disabling device suffers from a large response lag, it applies a low level voltage to the AND circuit 143.

This engine oil temperature sensor 35 need not be a type of directly detecting the engine oil temperature. For instance, it may be formed by a sensor adapted to detect the temperature of the engine per se such as a cylinder or a cylinder head or a sensor adapted to detect the valve opening of a choke valve, the position of a choke lever, etc., from which an output signal is supplied to the comparator 45 as a function of the oil temperature.

The throttle valve opening sensor 46 has its output connected to the input of a comparator 47, the output of which is connected to a fifth input terminal of the AND circuit 143. The throttle valve opening sensor 46 converts the valve opening of the throttle valve into a corresponding voltage Vθ and applies same to the comparator 47, which in turn compares the input voltage Vθ with a predetermined voltage V3 corresponding to a predetermined value of the throttle valve opening (e.g. 1/12 times as large as the maximum valve opening). When the relationship Vθ≧V3 stands, that is, when the throttle valve opening is larger than the predetermined value, the comparator 47 applies a high level voltage to the AND circuit 143, whereas when the relationship Vθ<V3 stands, that is, when the throttle valve opening is smaller than the predetermined value or the absolute quantity of intake air is small, the comparator 47 applies a low level voltage to the AND circuit 143.

With the above arrangement, when the engine oil temperature is lower than the predetermined value, the output voltage from the AND circuit 143 remains at a low level even if the engine rotational speed rises above the particular speed N1. As the engine oil temperature rises above the predetermined value with warming-up of the engine, the output voltage from the AND circuit 143 goes high. However, in the event that the engine oil temperature rises above the predetermined value when the engine rotational speed is very high, a large shock can take place upon changeover of the valve disabling device from the partial valve inoperative state to the all valve operative state, attributable to a large difference in the engine output between the two states of the valves in a high rotational speed region of the engine, badly affecting the driveability of the vehicle and the durability of the valve disabling device and engine component parts. According to the invention, the second changeover temporary inhibit 44' operates to eliminate such inconvenience, in such a manner that while the engine rotational speed is above the particular speed N1, if the oil temperature exceeds the predetermined value for the first time, a logical operation circuit 44'a of the circuit 44' detects such state from the output signals from the comparators 42, 45, and applies a control signal to the base of a transistor 54 whose collector is connected to the junction of the output of the AND circuit 143 with the control terminal of the switching circuit 52 and emitter is grounded, to cause same to conduct, thereby rendering the voltage applied to the control terminal of the switching circuit 52 low in level. By virtue of the provision of the temporary inhibit circuit 44', in the event that the engine has been warmed up in a high rotational speed region, the valve disabling device is held in the partial valve inoperative state even after the completion of the warming-up of the engine, to inhibit changeover from the partial valve inoperative state to the all valve operative state. The second changeover inhibit circuit 44' is released from its inhibiting operation to inhibit changeover from the partial valve inoperative state to the all valve operative state when the engine rotational speed has dropped below the particular speed N1 after the engine has been warmed up, and thereafter, the changeover to the all valve operative state is effected when the engine rotational speed again rises above the particular speed N1.

As shown in detail in FIG. 13, the logical operation circuit 44'a of the changeover temporary inhibit circuit 44' comprises two one shot multivibrators 44'b, 44'd, an RS flip flop 44'c, and an AND circuit 44'e. The one shot multivibrator 44'b is triggered by the leading edge of an output signal from the comparator 45 upon rising from a low level to a high level when the engine oil temperature rises up to the predetermined value with warming-up of the engine, to apply a set pulse to a set pulse input terminal S of the flip flop 44'c. On the other hand, the one shot multivibrator 44'd is triggered by the trailing edge of an output signal from the comparator 42 upon breaking from a high level to a low level when the engine rotational speed drops below the particular speed N1 from a higher engine speed region, to apply a reset pulse to a reset pulse input terminal of the flip flop 44'c. The flip flop 44'c has its Q-output terminal connected to a first input terminal of the AND circuit 44'e, which in turn has a second input terminal connected to the output of the comparator 42, and its output connected to the base of the transistor 54. In the logical operation circuit 44'a constructed as above, when warming-up of the engine has been completed while the engine is operating in a high rotational speed region, a high level output is generated from the AND circuit 44'e immediately upon the completion of the warming-up, and applied to the transistor 54 to cause same to conduct, thereby allowing the valve disabling device to continually maintain the partial valve inoperative state. Afterwards, when the engine rotational speed drops below the particular speed N1, the Q-output signal from the flip flop 44'c is inverted from a high level to a low level to release the circuit 44' from its changeover inhibiting operation.

With the arrangement of FIG. 12, when all the conditions are satisfied, that is, when the clutch of the engine is in an engaged state, the throttle valve opening is above the predetermined value, the engine rotational speed is above the particular speed N1, the engine oil temperature is above the predetermined value, and the neutral position switch is in an off state, the AND circuit 143 has all its input terminals supplied with high level voltages to generate a high level output and apply same to the control terminal of the switching circuit 52. On the other hand, if any one of the input terminals of the AND circuit 143 is supplied with a low level input, the AND circuit 143 generates a low level output and applied same to the control terminal of the switching circuit 52. Thereafter, similar operations will be performed, to those described with respect to the first embodiment, description of which is therefore omitted.

FIG. 14 shows a variation of the control circuit 134 of the second embodiment appearing in FIGS. 11 through 13. The control circuit 134' of FIG. 14 has a similar construction to the control circuit 34' of FIG. 10, and as compared with the latter, it further includes switches 73 and 74 serially connected between the switch 72 and the relay coil 62 which both have their equivalents in the control circuit 34' of FIG. 10. The switch 73 may be formed by contacts of a normally open type relay driven by a driving circuit composed of a thermistor and a transistor, and is adapted to be closed when the engine oil temperature is above the predetermined value. The switch 74 may be formed by a mechanical switch operable in response to the throttle valve of the engine, and is adapted to be closed when the throttle valve opening is larger than the predetermined value. The operation of the control circuit 134' is basically similar to those of the control circuits 134, 34' of FIGS. 12, 10, as can be easily deduced from comparison between the figures, description of which is therefore omitted.

Although in the foregoing embodiments the changeover means of the valve disabling device is composed of a control means adapted to control the energization of the solenoid of a solenoid controlled valve, this is not limitative. The valve disabling device according to the invention may have another type changeover means insofar as it is operable in response to part or all of engine rotational speed, throttle valve opening, engine oil temperature, and state of engagement or disengagement of the power transmission means.

Although the operative state of the valve disabling device is changed in response to engine rotational speed and state of engagement or disengagement of the power transmission means in the first embodiment, and in response to throttle valve opening and engine oil temperature besides the above parameters in the second embodiment, respectively, these are not limitative, but other combinations of the four parameters can apply to changeover of the operative state of the valve disabling device, for instance, a combination of engine rotational speed and engine oil temperature, or a combination of engine rotational speed, engine oil temperature, and throttle valve opening.

Moreover, part or all of the predetermined or particular values of engine rotational speed, throttle valve opening, and engine oil temperature for determining whether to effect changeover of the operative state of the valve disabling device may be provided with hysteresis margins so as to make the changeover action more stable. For instance, the particular speed (e.g. 2000 rpm) N1 may be provided with a hysteresis margin of ±100 rpm such that when the engine rotational speed rises above 2100 rpm, a changeover occurs from the partial valve inoperative state to the all valve operative state, while when the engine rotational speed drops below 1900 rpm, a changeover occurs from the all valve operative state to the partial valve inoperative state. By providing a suitable hysteresis-imparting circuit in the control circuit 34, 134, it can be avoided that repeated changeover actions occur when the engine is operated at speeds in the vicinity of the particular speed N1 or 2000 rpm. The same results as this can be obtained by providing the predetermined value of engine oil temperature or throttle valve opening with a hysteresis margin.

Claims

1. A valve disabling operation control device for use in an internal combustion engine having inlet valve means, exhaust valve means, at least one of which comprises a plurality of valves, an output shaft, and power transmission means for selectively establishing and interrupting power transmission between said output shaft and a load on said engine, the device comprising:

disabling valve operating means for selectively holding said at least one of said inlet valve means and said exhaust valve means in a first state wherein part of said valves thereof are inoperative, and in a second state wherein all said valves thereof are operative for alternate closing and opening actions;

transmission state detecting means for detecting whether said power transmission means is in a connected state wherein said power transmission is allowed, or in a disconnected state wherein said power transmission is inhibited; and

control means responsive to an output from said detecting means to cause said disabling valve operating means to hold said at least one of said inlet valve means and said exhaust valve means in said first state when said power transmission means is in said disconnected state, irrespective of the rotational speed of said engine.

2. A valve disabling operation control device as claimed in claim 1, including operating oil supply means, said disabling valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, and temperature sensor means for sensing the temperature of said operating oil, and wherein said control means is responsive to an output from said engine speed sensor means and an output from said temperature sensor means for controlling the supply of said operating oil from said operating oil supply means to said disabling valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value and the temperature of said operating oil is higher than a predetermined value at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof or the temperature of said operating oil is lower than said predetermined value thereof, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state.

3. A valve disabling operation control device as claimed in claim 1, including engine speed sensor means for sensing the rotational speed of said engine, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means and an output from said throttle valve opening sensor for controlling said disabling valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value and the valve opening of said throttle valve is larger than a predetermined value at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof or the valve opening of said throttle valve is smaller than said predetermined value thereof, said disabling valve operatingmeans holds said at least one of said inlet valve means and said exhaust valve means in said first state.

4. A valve disabling operation control device as claimed in claim 1, including engine speed sensor means for sensing the rotational speed of said engine, and wherein said control means is responsive to the output from said transmission state detecting means and an output from said engine speed sensor means for controlling said disabling valve operating means in a manner such that when said power transmission means is in said connected state and the rotational speed of said engine is higher than a predetermined value at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when said power transmission means is in said disconnected state or the rotational speed of said engine is lower than said predetermined value, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state.

5. A valve disabling operation control device as claimed in claim 1, including operating oil supply means, said disabling valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, temperature sensor means for sensing the temperature of said operating oil, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means, an output from said temperature sensor means, and an output from said throttle valve opening sensor means, for controlling the supply of said operating oil from said operating oil supply means to said disabling valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the temperature of said operating oil is higher than a predetermined value, and the valve opening of said throttle valve is larger than a predetermined value at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the temperature of said operating oil is lower than said predetermined value thereof, or the valve opening of said throttle valve is smaller than said predetermined value thereof, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state.

6. A valve disabling operation control device as claimed in claim 1, including engine speed sensor means for sensing the rotational speed of said engine, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means, an output from said throttle valve opening sensor, and the output from said transmission state detecting means for controlling said disabling valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the valve opening of said throttle valve is larger than a predetermined value, and said power transmission means is in said connected state at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the valve opening of said throttle valve is smaller than said predetermined value thereof, or said power transmission means is in said disconnected state, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state.

7. A valve disabling operation control device as claimed in claim 1, including operating oil supply means, said disabling valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, temperature sensor means for sensing the temperature of said operating oil, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to the output from said transmission state detecting means, an output from said engine speed sensor means, an output from said temperature sensor means, and an output from said throttle valve opening sensor means for controlling the supply of said operating oil from said operating oil supply means to said disabling valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the temperature of said operating oil is higher than a predetermined value, the valve opening of said throttle valve is larger than a predetermined value, and said power transmission means is in said connected state, at the same time, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the temperature of said operating oil is lower than said predetermined value thereof, the valve opening of said throttle valve is smaller than said predetermined value thereof, or said power transmission means is in said disconnected state, said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state.

8. A valve disabling operation control device as claimed in claim 1, wherein said power transmission means comprises a transmission disposed such that said power transmission is interrupted when said transmission assumes a neutral position, and a clutch disposed such that said power transmission is interrupted when said clutch is in a disengaged state, said transmission state detecting means comprising a neutral position switch for generating a signal indicative of whether said transmission assumes said neutral position thereof, and a clutch switch for generating a signal indicative of whether said clutch is in said disconnected state.

9. A valve disabling operation control device as claimed in any of claims 2, 5 or 7, wherein said predetermined value of said temperature of said operating oil is set at such a value that when the temperature of said operating oil is lower than said predetermined value thereof, a change over between said first state and said second state by said disabling valve operating means can take place at a value of the rotational speed of said engine substantially different from said predetermined value of the rotational speed of said engine.

10. A valve disabling operation control device as claimed in any of claims 3, 5, 6, or 7, wherein said predetermined value of the valve opening of said throttle valve is set at such a value that said engine can produce a larger output when said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state than when said disabling means holds said at least one of said inlet valve means and said exhaust valve means in said second state, irrespective of the rotational speed of said engine.

11. A valve disabling operation control device as claimed in any of claims 2-7, wherein said predetermined value of the rotational speed of said engine is set at such a value that when the rotational speed of said engine is lower than said predetermined value thereof, said engine can produce a larger output when said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state than when said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state.

12. A valve disabling operation control device as claimed in claim 1, wherein said control means includes changeover temporary inhibiting means operable to cause said control means to respond to a change in the output from said transmission state detecting means corresponding to a changeover of said power transmission means from said connected state to said disconnected state, with a predetermined time delay set at a value larger than a period of time for which said transmission temporarily assumes said neutral position and also larger than a period of time for which said clutch is temporarily held in said disengaged state, when said disabling valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state.

13. A valve disabling operation control device as claimed in claim 2, wherein said control means includes a second changeover temporary inhibiting means operable to cause said disabling valve operating means to continually hold said at least one of said inlet valve means and said exhaust valve means in said first state even if the temperature of said operating oil changes from a value lower than said predetermined value thereof to a value higher than the latter, when the rotational speed of said engine is higher than said predetermined value thereof, and to cause said disabling valve operating means to hold said at least one of said inlet valve means and said exhaust valve means in said second state only after the rotational speed of said engine rises above said predetermined value thereof after dropping below the latter.

14. A valve disabling operation control device as claimed in claim 1, wherein said engine includes a rotatable camshaft formed with an inlet cam and an exhaust cam, a pair of inlet and exhaust rocker arm shafts, and a pair of inlet and exhaust rocker arms journalled by respective ones of said inlet and exhaust rocker arm shafts for causing alternate closing and opening actions of respective ones of said inlet valve means and said exhaust valve means, said disabling valve operating means comprising one of said rocker arms corresponding to said at least one of said inlet valve means and said exhaust valve means, said one of said rocker arms comprising a disabling rocker arm corresponding to part of said at least one of said inlet valve means and said exhaust valve means, and a normally operating rocker arm corresponding to the remainder of said at least one of said inlet valve means and said exhaust valve means and disposed in slidable contact with a corresponding one of said inlet cam and said exhaust cam in a manner selectively drivingly connectible with or disconnectible from said disabling rocker arm, selecting means for selectively causing driving connection or disconnection between said disabling rocker arm and said normally operating rocker arm, whereby when disconnected from said normally operating rocker arm, said disabling rocker arm holds said part of said at least one of said inlet valve means and said exhaust valve means in a closed position thereof.

15. A valve disabling operation control device as claimed in claim 14, wherein said disabling rocker arm and said normally operating rocker arm have continuous guide bores, said selecting means having a piston slidably disposed within said guide bores, said piston being disposed to selectively assume a first position wherein it holds said rocker arms drivingly connected together, and a second position wherein it holds said rocker arms disconnected from each other, said control means comprising operating oil supply means, oil feeding passage means connecting said operating oil supply means with a back pressure chamber defined by a portion of said guide bores and said piston, valve means selectively establishing or interrupting communication between said operating oil supply means and said back pressure chamber, whereby when said communication between said operating oil supply means and said back pressure chamber is established by said valve means, operating oil is supplied to said back pressure chamber to thereby cause said piston to assume said first position, while said communication between said operating oil supply means and said back pressure chamber is interrupted by said valve means, the supply of operating oil to said back pressure chamber is interrupted to cause said piston to assume said second position.

16. A valve disabling operation control device as claimed in claim 1, wherein said load on said engine comprises driving wheels of a vehicle in which said engine is installed.

17. A valve operation control device for use in an internal combustion engine having inlet valve means, exhaust valve means, at least one of which comprises a plurality of valves, an output shaft, and power transmission means for selectively establishing and interrupting power transmission between said output shaft and a load on said engine, the device comprising:

valve operating means for selectively holding said at least one of said inlet valve means and said exhaust valve means in a first state wherein the operation of said valves presents less than the total sectional area available for gas flow through said valves for engine operation and in a second state wherein the operation of said valves presents the total sectional area available for gas flow through said valves for engine operation;

transmission state detecting means for detecting whether said power transmission means is in a connected state wherein said power transmission is allowed, or in a disconnected state wherein said power transmission is inhibited; and

control means responsive to an output from said detecting means to cause said valve operating means to hold said at least one of said inlet valve means and said exhaust valve means in said first state when said power transmission means is in said disconnected state, irrespective of the rotational speed of said engine. 18. A valve operation control device as claimed in claim 17, including operating oil supply means, said valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, and temperature sensor means for sensing the temperature of said operating oil, and wherein said control means is responsive to an output from said engine speed sensor means and an output from said temperature sensor means for controlling the supply of said operating oil from said operating oil supply means to said valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value and the temperature of said operating oil is higher than a predetermined value at the same time, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof or the temperature of said operating oil is lower than said predetermined value thereof, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 19. A valve operation control device as claimed in claim 17, including engine speed sensor means for sensing the rotational speed of said engine, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means and an output from said throttle valve opening sensor for controlling said valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value and the valve opening of said throttle valve is larger than a predetermined value at the same time, said valve operating means holds at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof or the valve opening of said throttle valve is smaller than said predetermined value thereof, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 20. A valve operation control device as claimed in claim 17, including engine speed sensor means for sensing the rotational speed of said engine, and wherein said control means is responsive to the output from said transmission state detecting means and an output from said engine speed sensor means for controlling said valve operating means in a manner such that when said power transmission means is in said connected state and the rotational speed of said engine is higher than a predetermined value at the same time, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when said power transmission means is in said disconnected state of the rotational speed of said engine is lower than said predetermined value, said valve operation means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 21. A valve operation control device as claimed in claim 17, including operating oil supply means, said valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, temperature sensor means for sensing the temperature of said operating oil, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means, an output from said temperature sensor means, and an output from said throttle valve opening sensor means, for controlling the supply of said operating oil from said operating oil supply means to said valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the temperature of said operating oil is higher than a predetermined value, and the valve opening of said throttle valve is larger than a predetermined value at the same time, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the temperature of said operating oil is lower than said predetermined value thereof, of the valve opening of said throttle valve is smaller than said predetermined value thereof, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 22. A valve operation control device as claimed in claim 17, including engine speed sensor means for sensing the rotational speed of said engine, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to an output from said engine speed sensor means, an output from said throttle valve opening sensor, and the output from said transmission state detecting means for controlling said valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the valve opening of said throttle valve is larger than a predetermined value, and said power transmission means is in said connected state at the same time, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the valve opening of said throttle is smaller than said predetermined value thereof, or said power transmission means is in said disconnected state, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 23. A valve operation control device as claimed in claim 17, including operating oil supply means, said valve operating means being operable in response to the pressure of operating oil supplied from said operating oil supply means, engine speed sensor means for sensing the rotational speed of said engine, temperature sensor means for sensing the temperature of said operating oil, and throttle valve opening sensor means for sensing the valve opening of a throttle valve of said engine, and wherein said control means is responsive to the output from said transmission state detecting means, and output from said engine speed sensor means, an output from said temperature sensor means, and an output from said throttle valve opening sensor means for controlling the supply of said operating oil from said operating oil supply means to said valve operating means in a manner such that when the rotational speed of said engine is higher than a predetermined value, the temperature of said operating oil is higher than a predetermined value, the valve opening of said throttle valve is larger than a predetermined value, and said power transmission means is in said connected state, at the same time, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state, while when the rotational speed of said engine is lower than said predetermined value thereof, the temperature of said operating oil is lower than said predetermined value thereof, the valve opening of said throttle valve is smaller than said predetermined value thereof, or said power transmission means is in said disconnected state, said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state. 24. A valve operation control device as claimed in claim 17, wherein said power transmission means comprises a transmission disposed such that said power transmission is interrupted when said transmission assumes a neutral position, and a clutch disposed such that said power transmission is interrupted when said clutch is in a disengaged state, said transmission state detecting means comprising a neutral position switch for generating a signal indicative of whether said transmission assumes said neutral position thereof, and a clutch switch for generating a signal indicative of whether said clutch is in said disconnected state. 25. A valve operation control device as claimed in any of claims 18, 21 or 23, wherein said predetermined value of said temperature of said operating oil is set at such a value that when the temperature of said operating oil is lower than said predetermined value thereof, a change over between said first state and said second state by said valve operating means can take place at a value of the rotational speed of said engine substantially different from said predetermined value of the rotational speed of said engine. 26. A valve operation control device as claimed in any of claims 19, 21, 22 or 23, wherein said predetermined value of the valve opening of said throttle valve is set at such a value that said engine can produce a larger output when said valve operating means holds at least one of said inlet valve means and said exhaust valve means in said first state than when said disabling means holds said at least one of said inlet valve means and said exhaust valve means in said second state, irrespective of the rotational speed of said engine. 27. A valve operation control device as claimed in any of claims 18 through 23, wherein said predetermined value of the rotational speed of said engine is set at such a value that when the rotational speed of said engine is lower than said predetermined value thereof, said engine can produce a larger output when said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said first state that when said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state. 28. A valve operation control device as claimed in claim 17, wherein said control means includes changeover temporary inhibiting means operable to cause said control means to respond to a change in the output from said transmission state detecting means corresponding to a changeover of said power transmission means from said connected state to said disconnected state, with a predetermined time delay set at a value larger than a period of time for which said transmission temporarily assumes said neutral position and also larger than a period of time for which said clutch is temporarily held in said disengaged state, when said valve operating means holds said at least one of said inlet valve means and said exhaust valve means in said second state. 29. A valve operation control device as claimed in claim 18, wherein said control means includes a second changeover temporary inhibiting means operable to cause said valve operating means to continually hold said at least one of said inlet valve means and said exhaust valve means in said first state even if the temperature of said operating oil changes from a value lower than said predetermined value thereof to a value higher than the latter, when the rotational speed of said engine is higher than said predetermined value thereof, and to cause said valve operating means to hold said at least one of said inlet valve means and said exhaust valve means in said second state only after the rotational speed of said engine rises above said predetermined value thereof after dropping below the latter. 30. A valve operation control device as claimed in claim 17, wherein said engine includes a rotatable camshaft formed with an inlet cam and an exhaust cam, a pair of inlet and exhaust rocker arm shafts, and a pair of inlet and exhaust rocker arms journalled by respective ones of said inlet and exhaust rocker arm shafts for causing alternate closing and opening actions of respective ones of said inlet valve means and said exhaust valve means, said valve operating means comprising one of said rocker arms corresponding to said at least one of said inlet valve means and said exhaust valve means, said one of said rocker arms comprising a disabling rocker arm corresponding to part of said at least one of said inlet valve means and said exhaust valve means, and a normally operating rocker arm corresponding to the remainder of said at least one of said inlet valve means and said exhaust valve means and disposed in slidable contact with a corresponding one of said inlet cam and exhaust cam in a manner selectively drivingly connectible with or disconnectible from said disabling rocker arm, selecting means for selectively causing driving connection or disconnection between said disabling rocker arm and said normally operating rocker arm, whereby when disconnected from said normally operating rocker arm, said disabling rocker arm holds said part of said at least one of said inlet valve means and said exhaust valve means in a closed position thereof. 31. A valve operation control device as claimed in claim 30, wherein said disabling rocker arm and said normally operating rocker arm have continuous guide bores, said selecting means having a piston slidably disposed within said guide bores, said piston being disposed to selectively assume a first position wherein it holds said rocker arms drivingly connected together, and a second position wherein it holds said rocker arms disconnected from each other, said control means comprising operating oil supply means, oil feeding passage means connecting said operating oil supply means with a back pressure chamber defined by a portion of said guide bores and said piston, valve means selectively establishing or interrupting communication between said operating oil supply means and said back pressure chamber, whereby when said communication between said operating oil supply means and said back pressure chamber is established by said valve means, operating oil is supplied to said back pressure chamber to thereby cause said piston to assume said first position, while said communication between said operating oil supply means and said back pressure chamber is interrupted by said valve means, the supply of operating oil to said back pressure chamber is interrupted to cause said piston to assume said second position. 32. A valve operation control device as claimed in claim 17, wherein said load on said engine comprises driving wheels of a vehicle in which said engine is installed.