JPH0733865B2 - Power unit control method - Google Patents

Description

Detailed Description of the Invention a. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control of a power unit including an engine having variable valve operating characteristics and a transmission that transmits the output of the engine.

The switching of the valve operating characteristics means switching at least one of the opening / closing timing of the intake valve or the exhaust valve, the opening period, and the valve lift amount, and at least one of a plurality of intake valves or exhaust valves in one cylinder. It also includes making the valve open period substantially zero and switching it to the closed state.

(Prior Art) An engine in which the valve operating characteristics of the intake valve and / or the exhaust valve can be switched between a low speed valve operating characteristic suitable for a low rotation range and a high speed valve operating characteristic suitable for a high rotation range. However, it is disclosed in Japanese Patent Publication No. 49-33289, in which the low-speed valve operating characteristic is set in the region where the engine speed is below a predetermined value and the intake negative pressure is below a predetermined value (vacuum side). Switching, and in other areas switches to high speed valve operating characteristics.

FIG. 6 shows an example of the output characteristic of such an engine. The characteristic shown by the line L when the low speed valve operating characteristic is used, and the characteristic shown by the line H when the high speed valve operating characteristic is used. Both characteristics intersect in terms of engine speed N _1, in the region of the engine speed Ne <N _1, the torque of the high-speed valve operating characteristic towards the torque (line A ₁₎ of the low-speed valve operating characteristic (line
B ₁ ), and in the region of Ne> N ₁ , the torque of the high speed valve actuation characteristic (line A ₂ ) is larger than the torque of the low speed valve actuation characteristic (line B ₂ ). In this case, the characteristic with the larger torque (characteristics shown by lines A ₁ and A ₂ ) is called the high-torque valve operation characteristic, and the characteristic with the smaller torque (characteristics shown by lines B ₁ and B ₂ ) is low. It is called the torque side valve operating characteristic. Note that normally, the valve operating characteristics on the high torque side are often used.

Further, as a transmission, for example, there is one disclosed in Japanese Patent Publication No. 61-24581, in which a plurality of power transmission elements are arranged between an input shaft and an output shaft.
Each power transmission element is mainly composed of a drive gear arranged on the input shaft, a driven gear arranged on the output shaft or the counter shaft, and a friction element such as a hydraulic clutch for selectively operating these gear trains. Thus, a combination of them constitutes one shift speed. Then, the control oil pressure for engaging the friction elements is set according to the engine output torque so that slippage does not occur on the friction surface, and as shown in FIG. 9a, the engine operates with the valve operating characteristics on the high torque side. If it is, the hydraulic pressure P _LH is set to have a margin with respect to the output torque HTVT at that time.

(Problems to be Solved by the Invention) However, when the valve operating characteristic switching mechanism for switching the valve operating characteristic of the engine as described above becomes defective, the valve operating characteristic is changed to the high speed valve operating characteristic (first Lines B ₁ and A _{2 in} Fig. 6 or low speed valve operation characteristics (same as above)
It will be fixed to A ₁ , B ₂ ). In this case,
When the engine is running on line B ₁ or B ₂ ,
When the hydraulic pressure P _{LH in} Fig. 9a is supplied, it means that the hydraulic pressure P _LH is higher than necessary in comparison with the hydraulic pressure P _LL corresponding to the output torque LTVT at that time. Cause Further, the friction element has an excessively large capacity with respect to the output torque LTVT, and the engagement thereof is rapidly performed to cause a shift shock.

Furthermore, regarding the shift map that sets the shift timing of the automatic transmission, the engine output torque characteristic when fixed to either valve operating characteristic as described above is:
Since the valve operating characteristic switching mechanism operates normally and is different from the torque characteristic when operating with the valve operating characteristic on the high torque side in the entire rotation range, for example, the shift map corresponding to the fixed valve operating characteristic is used as it is. In that case, there is a problem that the desired gear shift characteristic cannot be obtained.

On the other hand, the valve operating characteristic switching mechanism malfunctions and 1
If the valve operation characteristics are not fixed and the switching is irregular, or if the locked state cannot be detected,
If the control hydraulic pressure of the transmission is not sufficient, slippage will occur in the friction element when operating with the valve operating characteristics on the high torque side,
May wear.

In view of the above problems, the present invention provides a power unit control method capable of appropriately setting a control hydraulic pressure of a transmission, a shift map, and the like when the valve operating characteristic switching mechanism becomes abnormal. Is intended.

B. Configuration of the Invention (Means for Solving the Problems) In order to achieve the above object, in the present invention, the valve operating characteristic switching mechanism becomes inoperable and the valve operating characteristic has two valve operating characteristics (high-speed valve operating characteristic). And low speed valve operation characteristics), the engine speed and the specific speed (the engine speed at which the output torque of the engine operated based on each valve operation characteristic is the same) are fixed. By comparison, it is determined whether the engine is operating based on the operating characteristics on the high torque side or the operating characteristics on the low torque side, and the transmission is controlled according to the determined operating characteristics. . If the valve operating characteristic switching mechanism malfunctions and the valve operating characteristics are no longer fixed to any of the characteristics, the operating characteristics on the high torque side regardless of the actual engine speed relative to the specified engine speed It is desirable to control the transmission according to the above.

(Operation) According to the control method described above, when the valve operating characteristic switching mechanism becomes inoperable and is fixed to one valve operating characteristic, the valve operating characteristic on the high torque side of the fixed valve operating characteristic is fixed. Alternatively, because the transmission is controlled according to the valve operating characteristics on the low torque side, friction elements can be prevented from slipping and wear can be prevented, and an appropriate shift map can be set to maintain a good operating condition at all times. It leads to doing. Also, if the valve operating characteristic switching mechanism malfunctions and one valve operating characteristic is not fixed, or if it is not possible to detect that it is fixed, the high torque side valve operating characteristic characteristic will be set in the entire rotation range. Since the transmission is controlled correspondingly, it is possible to prevent the friction element from slipping.

(Examples) Hereinafter, preferred examples of the present invention will be described with reference to the drawings.

FIG. 1 shows a power unit controlled by the method according to the present invention, which includes an engine E having a variable valve timing / lift mechanism VT and an automatic transmission AT controlled by a hydraulic control valve CV. Composed of. Variable valve timing / lift mechanism
The VT is a mechanism for switching the opening / closing timing, the opening period and the lift amount of the intake valve of the engine E between a low speed valve operation characteristic suitable for a low rotation range and a high speed valve operation characteristic suitable for a high rotation range. As described later, the predetermined hydraulic pressure is supplied / discharged by the ON / OFF operation of the solenoid valve 91 based on ON / OFF of the switching signal VTS from the control unit CU. Also, hydraulic control valve CV
Is a valve that controls the operation of the lockup clutch in the automatic transmission AT and the friction elements of the speed change clutch. Therefore, the control unit CU has a throttle sensor
Various signals such as a throttle opening signal from 93, an engine rotation signal from the engine rotation sensor 94, and a transmission output rotation signal (vehicle speed signal) from the transmission rotation sensor 95 are input.

First, the variable valve timing / lift mechanism VT will be described with reference to FIGS. 2 and 3. A pair of intake valves 1a, 1b are provided for each mechanism of the engine E, and the pair of intake valves 1a, 1b are provided on a camshaft 2 driven at a rotation ratio of 1/2 in synchronization with the rotation of the engine. A first low speed cam 3, a second low speed cam 3'and a high speed cam 5 which are integrally provided, and first, second and third rocker arms pivotally supported by a rocker shaft 6 parallel to the cam shaft 2. It is opened and closed by the action of 7,8,9.

The camshaft 2 is rotatably arranged above the engine body, and the first low speed cam 3 is integrally provided on the camshaft 2 at a position corresponding to one intake valve 1a. 3'is integrally provided on the camshaft 2 at a position corresponding to the other intake valve 1b. Further, the high speed cam 5 is integrally provided on the cam shaft 2 at a position corresponding to between the intake valves 1a and 1b. Moreover, the first and second low speed cams 3 and 3'have high positions 3a and 3a 'corresponding to low speed operation of the engine. The high-speed cam 5 has a high-order portion 5a corresponding to high-speed operation of the engine.

First to third rocker arms 7 to 9 are pivotally supported on the rocker shaft 6, and the first and second rocker arms 7 and 8 are extended to positions above the intake valves 1a and 1b. Also,
A cam slipper 10 slidingly contacting the low speed cam 3 is provided above the first rocker arm 7, and a cam slipper 11 capable of contacting the second low speed cam 4 is provided above the second rocker arm 8. In addition, each intake valve 1a, 1b has a valve spring 16,
The valve 17 is urged in the valve closing direction, that is, upward.

The third rocker arm 9 includes the first and second rocker arms 7,
It is pivotally supported by the rock shaft 6 between the 8th. The third rocker arm 9 is formed from the rocker shaft 6 to both intake valves 1a and 1b.
A cam slipper that extends slightly to the side and is in sliding contact with the high speed cam 5 is provided on the upper side.

As shown in FIG. 3, first to third rocker arms 7,8,9
Are in sliding contact with each other, and the connecting means 21 capable of switching between a state in which the relative angular displacements thereof are possible and a state in which the respective rocker arms 7 to 9 are integrally connected are the first and second rocker arms 7. , 8,9.

The connecting means 21 connects the first and third rocker arms 7 and 9 and the position that connects the first and second rocker arms 9 and 8 and the third piston 2 and the second rocker arms 9 and 8 which are movable between the positions where the connection is released. The second piston 23 movable between the positions for releasing the connection and the first and second pistons 22, 23
24 for restricting the movement of the stopper 24 and the stopper 24 for moving the first and second pistons 22, 23 to the connection release position side.
And a spring 25 for urging.

The movement of the first and second pistons 22 and 23 is performed by the hydraulic pressure supplied into the hydraulic chamber 29 through the oil passages 31, 32 and 30 according to the operation of the solenoid valve 91.

Incidentally, such a variable valve timing / lift mechanism is disclosed in detail in, for example, Japanese Patent Laid-Open No. 62-121811.

Next, the variable valve timing /
The operation of the lift mechanism VT will be described.

When the engine E is operating at low speed, the solenoid valve 91 is OF
F, the communication between the oil passage 31 and the hydraulic source (not shown) is cut off as shown in FIG. 3, and the hydraulic chamber of the connection switching means 21 is disconnected.
The hydraulic pressure is not supplied to 29, and the stopper 24 is pressed toward the third rocker arm 9 side by the spring 25. Therefore, each rocker arm 7, 8, 9 can be independently displaced.

When the connection switching means 21 is in the connection released state, the driving operation of the cam shaft 2 causes the first rocker arm 7 to swing in response to the sliding contact with the first low speed cam 3, and the second rocker arm 8 to move. It swings in accordance with the sliding contact with the second low speed cam 3 '. Therefore, both intake valves 1a, 1b are opened and closed by the first and second low speed cams 3, 3 '. At this time,
The third rocker arm 9 swings due to the sliding contact with the high speed cam 5, but the swinging motion has no effect on the operation of both intake valves 1a, 1b.

Thus, during low speed operation of the engine E, one intake valve 1a has a timing and a lift amount corresponding to the shape of the first low speed cam 3 as shown by the broken line 3 and the alternate long and short dash line 3'in FIG. 5A. Opening and closing, the other intake valve
1b opens and closes at a timing and lift amount according to the shape of the second low speed cam 3 '. Therefore, an air-fuel mixture inflow speed suitable for low speed operation can be obtained, fuel consumption can be reduced and kicking can be prevented, and optimum low speed operation can be performed.

In order to obtain an air-fuel mixture inflow speed suitable for low-speed operation,
For example, as shown in FIG. 5B, the high position portion 3a 'of the second low speed cam 3'may be lowered to make the opening time / amount of the intake valve 1b extremely small during low speed operation. Alternatively, the high-level portion 3a 'may be set to zero, and the intake valve 1b may not be opened at all during low speed operation to create a valve resting state.

When operating the engine E at high speed, the solenoid valve 91
Is ON, and as shown in FIG. 4, a solenoid valve 91 connects an oil pressure source (not shown) to the oil passage 31,
The hydraulic pressure is supplied to the hydraulic chamber 29 of the connection switching means 21. As a result, as shown in FIG.
The first and second pistons 22 and 23 move until they contact the 36, the first and third rocker arms 7 and 9 are connected by the first piston 22, and the third and second rocker arms 9 and 9 are connected by the second piston 23. 8 are connected.

In this way, in the state where the first to third rocker arms 7, 8 and 9 are connected to each other by the connection switching means 21, the swing amount of the third rocker arm 9 slidingly contacting the high speed cam 5 is the largest, The first and second rocker arms 7 and 8 swing together with the third rocker arm 9. Therefore, the engine E
During high speed operation, both intake valves 1a and 1b open and close at the timing and lift amount according to the shape of the high speed cam 5, as shown by the solid line 5 in FIG. 5A. In this case, the timing and the lift amount are larger than those during low speed operation, and intake air suitable for high speed operation can be obtained, so that the engine output can be improved.

In the above operation, the first and second low speed cams
The opening / closing timing and lift amount of the intake valves 1a, 1b based on 3, 3'are referred to as a low speed valve operating characteristic, and the opening / closing timing and lift amount of the intake valves 1a, 1b based on the high speed cam 5 are referred to as a high speed valve operating characteristic. Both valve operating characteristics are used by dividing into a low speed operation area and a high speed operation area,
The relationship between the engine output torque and the engine speed at this time is as shown in FIG. As described above, the characteristic in the low speed valve operation characteristic operation is shown by the line L, and the characteristic in the high speed valve operation characteristic operation is shown by the line H in this figure.
The characteristics indicated by A ₁ and A ₂ are the high torque valve operation characteristics,
The characteristics shown by B ₁ and B ₂ are the low torque valve operating characteristics.

Next, the automatic transmission AT will be described with reference to FIG.

This automatic transmission AT includes a torque converter 40 and a transmission mechanism.
The torque converter 40 includes a pump 46a connected to the engine output shaft E _OP and an output shaft (transmission mechanism input shaft) 61.
46b connected to the turbine and a stator 46c fixedly held
Further, it has a lock-up clutch 47 capable of engaging and disengaging the pump 46a and the turbine 46b.

The speed change mechanism 50 is an input shaft integrated with the torque converter output shaft.
61, and has a counter shaft 62 and an output shaft 63 parallel to this. Between the input shaft 61 and the counter shaft 62, there are five sets of gear trains that mesh with each other, that is, first speed gear trains 51a, 51.
b, second speed gear trains 52a, 52b, third speed gear trains 53a, 53b, fourth speed gear trains 54a, 54b, and reverse gear trains 55a, 55b, 55c. Hydraulic actuating clutches 64 to 68 for selecting each gear train are provided as the drive gears or driven gears of each gear train. By selectively operating these hydraulic actuating clutches 64 to 68, either gear train is operated. The power transmission path is selectively switched, and gear shifting is performed.

Output gear trains 59a, 59b are arranged between the counter shaft 62 and the output shaft 63, and the power changed as described above is transmitted to the output shaft via the output gear trains 59a, 59b. .

One-way clutches 56 and 57 are attached to the first-speed driven gear 51b and the second-speed driven gear 52b, and an emblem clutch 69 for locking and holding these one-way clutches 56 and 57 is provided.

Lock-up clutch in the automatic transmission AT having the above configuration
The operation control of 47 and the operation control of each clutch 64 to 69 of the speed change mechanism 50 are performed by the control valve CV shown in FIG.

In the power unit configured as described above, the control of the automatic transmission AT when the variable valve timing / lift mechanism VT is abnormal will be described with reference to the control flow of FIG.

First, in step S1, the throttle sensor of FIG.
93, the rotation sensor 94 and the transmission rotation sensor 95,
The throttle opening θ _th , the engine speed Ne and the vehicle speed V are detected. Next, in step S2, it is determined whether or not the variable valve timing / lift mechanism VT is abnormal. The method is as follows, for example. When this engine is operating with the high torque valve actuation characteristic, when shifting from the low speed valve actuation characteristic side (on the line A _{1 in} FIG. 6) to the high speed valve actuation characteristic side (also on the line A ₂ ), normally, The connection switching means 21 changes from the connection release state as shown in FIG. 3 to the connection state as shown in FIG. 4 at the rotation speed N ₁ based on the switching signal VTS sent from the control unit CU to the solenoid valve 91. Therefore, if the predetermined hydraulic pressure does not rise in the oil passage 31 even if the switching signal VTS is turned on, it is determined that the variable valve timing / lift mechanism VT is abnormal.

If it is determined in step S2 that the variable valve timing / lift mechanism VT is abnormal,
Proceeding to S3, whether or not the variable valve timing / lift mechanism VT is fixed to either the high speed valve operation characteristic or the low speed valve operation characteristic, that is, the connection switching means 21 is in the completely connected state or the completely disconnected state. It is determined by detecting the position of the piston (21, 22) or the like.

In step S3, when the variable valve timing / lift mechanism VT determines that the high speed valve operating characteristic is fixed to any of the low speed valve operating characteristics, the process proceeds to step S4, and the fixed valve operating characteristic is high speed. Judge whether the valve operating characteristic is HVT or the low speed valve operating characteristic is LVT. If it is the high-speed valve operating characteristic HVT, proceed to step S5,
By comparing the engine speed Ne detected in step S1 with the engine speed N _{1 at the} point where the engine output torques match (switching point) in both valve operating characteristics, the low torque side valve operating characteristics (line B in FIG. 6) are compared. Determine whether the operation is in ₁ ) or the valve operating characteristics on the high torque side (also line A ₂ ).

Now, let us consider the control of the control hydraulic pressure of each friction element that selects the gear stage as an example of the control of the transmission. When the variable valve timing / lift mechanism VT is operating normally, as shown in Fig. 9a, if the operation is based on the high torque side valve operating characteristic shown by the solid line HTVT, it is shown by the thick solid line P _LH . If the control oil pressure is set to the oil pressure and the operation is based on the low torque side valve operating characteristic indicated by the broken line LTVT, the control oil pressure is set to the oil pressure indicated by the thick broken line P _LL .

FIG. 9b shows the control oil pressure when the high-speed valve operating characteristic HVT is fixed as described above. Step
When Ne <N ₁ is determined in S5, the operation is at the low torque side valve operation characteristic LTVT. Therefore, step S6
Go to and set the control oil pressure P _LL corresponding to the low torque side valve operating characteristic LTVT. If it is determined that Ne> N ₁ , the operation is at the high torque side valve operation characteristic HTVT, so the process proceeds to step S7, and the high torque side valve operation characteristic HTVT is performed.
Set the control oil pressure P _LH corresponding to.

If it is determined in step S4 that the fixed valve operating characteristic is the low speed valve operating characteristic,
Proceed to step S8. In step S8, step S5
In the same manner as the above, based on the engine speed Ne detected in step S1, the operation at the high torque side valve operating characteristic (line A _{1 in} FIG. 6) or the low torque side valve operating characteristic (also line B ₂ ) is performed. Determine if you are driving. The control hydraulic pressure in this case shows
FIG. 9c is a diagram. If Ne <N ₁ in step S8, the operation is at the high torque side valve operation characteristic HTVT, so the process proceeds to step S6, and the control oil pressure P _LH corresponding to the high torque side valve operation characteristic HTVT is set. If Ne> N ₁ , the operation is at the high torque side valve operation characteristic HTVT, so the process proceeds to step S7, and the control oil pressure P _LL corresponding to the high torque side valve operation characteristic HTVT is set.

The control of the control hydraulic pressure has been described above as an example of the transmission control when the variable valve timing / lift mechanism is fixed to one of the valve operating characteristics.Next, the setting of the shift map will be described with reference to FIG. It will be explained with reference to FIG.

In FIG. 10, when shifting up from one shift speed to the next shift speed, the shift map in the operation with the high torque side valve operation characteristic HTVT is shown by a solid line, and in the operation with the low tokule side valve operation characteristic LTVT. The shift map is shown by a broken line. In step S6, when the transmission control corresponding to the low torque side valve operating characteristic is performed, the shift map indicated by the broken line is used for the shift control, and in step S7, the control corresponding to the high torque side valve operating characteristic is performed. When performing, the shift map shown by the solid line is used. This way
This is the same as using a shift map corresponding to a fixed valve operating characteristic, and a smooth shift is performed.

On the other hand, in step S3, if it is determined that the variable valve operating characteristic switching mechanism VT is not fixed to the high speed valve operating characteristic or the low speed valve operating characteristic, or if it is fixed, it cannot be detected. In this case, in order to prevent the friction element of the transmission from slipping and wearing, the transmission is controlled in accordance with the high torque side valve operating characteristic in order to sufficiently increase the control hydraulic pressure of the transmission. So proceed to step S7.

As described above, according to the above control method, when the variable valve timing / lift mechanism VT becomes inoperable and is fixed to the high speed valve operation characteristic or the low speed valve operation characteristic, the fixed valve operation characteristic is fixed. Since the transmission is controlled according to the high torque side valve operation characteristic or low torque side valve operation characteristic of the friction element, wear can be prevented without slipping the friction element, and an appropriate shift map can be set. It is possible to maintain a good operating condition. Also, the variable valve timing / lift mechanism VT has malfunctioned,
If it is not fixed to one valve operating characteristic, or if it is not possible to detect that it is fixed, the transmission is controlled corresponding to the high torque side characteristic of the high speed valve operating characteristic and the low speed valve operating characteristic. Wear can be prevented without causing slippage in the friction element.

C. EFFECTS OF THE INVENTION According to the above control method, when the valve operating characteristic switching mechanism becomes inoperable and is fixed to one valve operating characteristic, it is a switching point between the actual engine speed and the valve operating characteristic. It is possible to easily and clearly grasp the operating condition of the engine (operation using the operating characteristics on the high torque side or operating using the operating characteristics on the low torque side) by comparing it with the specific speed. it can. Since the transmission is controlled according to the high torque side valve operation characteristic or the low torque side valve operation characteristic in use, friction elements can be prevented from slipping and wear and other problems can be prevented. It is possible to always maintain a good operating condition such as setting. Further, if the valve operating characteristic switching mechanism malfunctions and one valve operating characteristic is not fixed, or if the fixed state cannot be detected, the valve operating characteristic may be changed randomly. Since the transmission is controlled in accordance with the valve operating characteristic characteristics on the high torque side after grasping the rotation state, it is possible to prevent the friction element from slipping, prevent wear, and improve its durability.

[Brief description of drawings]

FIG. 1 is a schematic view showing a power unit controlled by the method according to the present invention, FIG. 2 is a sectional view of a variable valve timing / lift mechanism used in an engine constituting the power unit, FIGS. 3 and 4 FIG. 5 is a cross-sectional view of this mechanism, FIGS. 5A and 5B are graphs showing the opening / closing operation characteristics of the intake valve, FIG. 6 is a graph showing the relationship between the output torque and the rotational speed of the engine, and FIG. FIG. 8 is a schematic diagram showing a power transmission system of an automatic transmission which constitutes a drive unit, FIG. 8 is a flowchart showing control according to the present invention, and FIGS. 9a to 9c show that the variable valve timing / lift mechanism is normally operated. When operating, when the variable valve timing lift mechanism is fixed to the high speed valve operating characteristic, and when the variable valve timing lift mechanism is fixed to the low speed valve operating characteristic. The automatic transmission conceptually showing graph control pressure in the case, FIG. 10 is a graph showing the shift map of the automatic transmission. 3,3 '…… Low speed cam, 5 …… High speed cam 6 …… Rocker shaft, 21 …… Connection switching means 22,23 …… Piston, 29 …… Hydraulic chamber 50 …… Transmission mechanism, 63 …… Shift Machine input shaft 91 …… Solenoid valve 93 …… Throttle sensor 94 …… Rotation sensor, 95 …… Transmission rotation sensor CU …… Control unit VT …… Variable valve timing / lift mechanism

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadashi Hanaoka 1-4-1, Chuo, Wako-shi, Saitama Honda R & D Co., Ltd. (72) Inventor Junichi Miyake 1-4-1, Wako-shi, Saitama No. Incorporated in Honda R & D Co., Ltd. (72) Inventor Masao Kubotera 1-4-1, Chuo, Wako, Saitama Incorporated in Honda R & D Co., Ltd. (56) Reference JP-A-62-149524 (JP, A) KO 49-33289 (JP, B1)

Claims (2)

[Claims] 1. An engine comprising at least one of an intake valve and an exhaust valve whose valve operating characteristics are switchable, and a transmission connected to an output shaft of the engine, wherein the engine switches the valve operating characteristics. A valve operating characteristic switching mechanism that is operated, and the engine is operated by operating the valve operating characteristic switching mechanism at a specific engine speed at which the output torque of the engine operated based on each of the valve operating characteristics matches. In a power unit capable of selecting an operation based on a combination of operating characteristics on the high torque side or an operation based on a combination of operating characteristics on a low torque side in each of the valve operating characteristics, the valve operating characteristic switching mechanism becomes inoperable and the valve When the operating characteristic is fixed to the characteristic of 1, the actual engine speed and the characteristic The engine speed is compared to determine whether the engine is operating based on the operating characteristics on the high torque side or operating on the operating characteristics on the low torque side, and the speed change according to the determined operating characteristics. A method for controlling a power unit, comprising controlling the machine. 2. An engine in which at least one of the intake valve and the exhaust valve has a valve operating characteristic that can be switched, and a transmission connected to an output shaft of the engine, the engine switching the valve operating characteristic. A valve operating characteristic switching mechanism that is operated, and the engine is operated by operating the valve operating characteristic switching mechanism at a specific engine speed at which the output torque of the engine operated based on each of the valve operating characteristics matches. In a power unit capable of selecting an operation based on a combination of operating characteristics on the high torque side in each of the valve operating characteristics or an operation based on a combination of operating characteristics on the low torque side, the valve operating characteristic switching mechanism malfunctions and the valve operates. If the operating characteristics are no longer fixed to any of the characteristics, the engine Regardless for the particular engine rotational speed of the rolling speed, the control method of the power unit and performs control of the transmission in accordance with the operating characteristics of the high torque side.