JPH0127244B2 - - Google Patents

Description

Detailed Description of the Invention Technical Field The present invention uses a valve drive device for a diesel engine, more specifically, a hydraulic drive force to open a mushroom-shaped valve for gas exchange in a combustion chamber of an engine.
The present invention relates to a valve driving device that closes a valve using the repulsive force of a valve spring or hydraulic driving force.

Prior Art It is well known that the work done by a diesel engine is expressed by the area surrounded by a closed cycle curve drawn with the cylinder volume on the horizontal axis and the cylinder internal pressure on the vertical axis, as shown in Figure 1. . In contrast to the theoretical cycle a shown by the solid line in the exhaust process in Figure 1, in the actual cycle the pressure gradually decreases toward the bottom dead center as shown by the chain line b, and the closed curve area is compared to the closed curve area of the theoretical cycle. It decreases by ΔWi.
This area is work loss, and it is effective to reduce ΔWi as much as possible in order to improve the thermal efficiency of a diesel engine. That is, in FIG. 2, which shows an enlarged cycle diagram during the exhaust process, it is most effective to increase the valve opening speed as indicated by the broken line c.

However, in general, the exhaust valve of a diesel engine is
As the cam rotates, it is mechanically driven in the valve opening direction against the valve spring through the roller, push rod, and swinging arm that roll on the cam, and is driven in the valve closing direction by the force of the valve spring. It has become a mechanism. Therefore,
The lift curve of the exhaust valve basically depends on the profile of the cam, but in order to prevent the series of drive transmission members from the cam to the exhaust valve from coming out of pressure contact and causing the valve to float. , the curvature of the cam profile must be selected such that the product of the mass and acceleration of the part driven by the cam, i.e. the mechanical coupling member from the roller to the exhaust valve, is smaller than the force exerted by the valve spring of the exhaust valve. The change in the valve head must be very gradual, and the valve lift curve will become as shown by the solid line d in FIG. 3, and it cannot be made as shown by the broken line e, where the valve opening speed is increased.

As mentioned above, as an exhaust valve drive mechanism that is relatively less constrained by the inertia of the driven member, the cam pushes the plunger to compress the hydraulic oil sealed in the sealed container, and the pressure pushes the piston to exhaust the valve. Pressure oil supplied from a cam that drives the valve, a plunger type hydraulic drive mechanism, or a separate hydraulic source is connected to the cylinder via an electromagnetic switching valve or a switching valve that switches in synchronization with the movement of the engine via a cam, etc. There is a hydraulic drive device which opens an exhaust valve by supplying gas and pushing a piston, and closes the valve by a valve spring.

However, in the case of these hydraulic drive devices, in order to increase the opening speed of the exhaust valve, an amount of pressure oil corresponding to the product of the pressure receiving area and the stroke of the piston, that is, the displacement amount, must be pumped into the cylinder in a short period of time. Therefore, there is a problem that the hydraulic supply device becomes large because the discharge flow rate of the plunger or hydraulic pump and the flow rate passing through the switching valve must be increased per unit time. In other words, in the case of the cam plunger system, the plunger diameter increases and the inertial mass increases, so due to the design of the cam profile, it is not possible to achieve a large acceleration. Can not.

In addition, in an exhaust valve drive device using a separate hydraulic power source,
Generally, as shown in FIG. 4, a hydraulic cylinder 1 is connected to a cylinder head 12 of an engine, and a driving piston 2 is in pressure contact with the tip of a valve stem 14 of an exhaust valve 13, which is biased in the valve closing direction by a valve spring 16. engaged. By supplying the pressure oil generated by the pressure oil supply device 28 to the pressure chamber 19 surrounded by the cylinder casing 3 and the piston 2 by the 3-port switching valve 6, the force of the valve spring 16 and the pressure of the exhaust valve 13 are reduced. The piston 2 is pushed forward against the force of the cylinder internal pressure P of the combustion chamber 18 acting on the lower surface, the exhaust valve 13 is opened as shown in FIG. It is then discharged out of the engine. After a certain period of time, the switching valve 6 is switched to connect the pressure chamber 19 of the hydraulic cylinder 1 to the conduit 7 and the return conduit 9.
The piston 2 is raised by the force of the valve spring 16 to open the exhaust valve 13.
will be closed.

In addition, 4 and 5 in the figure are oil passages, 8 is a pressure conduit,
Reference numeral 10 denotes a conduit, and the passage 5 and the conduit 10 are used to facilitate movement of the drive piston 2 and to drain leaked oil. Further, h ₁ is the stroke of the piston 2, and thus the opening stroke of the exhaust valve 13.

In the above-mentioned exhaust valve oil drive device using a separate hydraulic power source, the displacement speed V of the drive piston 2 is
Determined by the supply flow rate Q of pressure oil per unit time into the interior of 9 and the pressure receiving area A of the drive piston 2, V=
It is expressed as Q/A.

Further, the hydraulic driving force F of the drive piston 2 is determined by the pressure P ₁ of the supplied pressure oil and the pressure receiving area A of the drive piston 2, and is expressed by F=P ₁ ×A.

On the other hand, pressure oil supply equipment consisting of hydraulic pumps, tanks, strainers, accumulators, pressure regulating valves, conduits, various valves, etc. usually supplies pressure oil at a constant flow rate per unit time, and A pressure regulating valve is used to adjust the pressure to a constant value, and an accumulator is installed to absorb pressure fluctuations within the pipeline system.

The pressure oil supply capacity R of such pressure oil supply equipment is the product of supply pressure P ₁ and supply amount Q per unit time R = P ₁
It is represented by ×Q.

Therefore, the displacement speed V of the drive piston 2, that is, the opening speed of the exhaust valve, is expressed as V=R/P ₁ A=R/F, and the greater the pressure oil supply capacity R of the hydraulic source, the more the exhaust valve is driven. Therefore, the smaller the force F required to do this, the faster the valve opening speed becomes.

The force due to the cylinder internal pressure acting on the exhaust valve 13 is the product of the cylinder internal pressure P in the combustion chamber 18 at that time multiplied by the area of the valve 15 when the valve is opened, but after the valve is opened, the force due to the cylinder internal pressure in the combustion chamber 18 and the exhaust passage 17 is The pressure on the upper and lower surfaces of the valve shaft is equalized by communicating with the valve stem 1.
It is the product of the cross-sectional area of 4 and the cylinder internal pressure. Moreover, as the exhaust gas is discharged, the cylinder internal pressure itself rapidly decreases, so the force exerted by the gas becomes much smaller than when the valve starts opening.

In this way, the force required to open the exhaust valve is due to the force due to the cylinder internal pressure when the valve starts opening, and after the valve is opened, it is due to the valve spring force that is proportional to the opening stroke of the exhaust valve. It's decided. The latter is usually much smaller than the former, and is less than 1/3 even in the case of a conventional cam drive, which has a large inertial mass of the drive part, and is even smaller in the case of a hydraulic drive, which can reduce the inertial mass of the drive part. . On the other hand, the cylinder internal pressure when the exhaust valve is open tends to be higher due to the use of turbo compound engines, which effectively utilize the energy of exhaust gas to improve engine performance. Large driving force is required.

Therefore, the pressure-receiving area of the drive piston 2 is set to be sufficient to open the exhaust valve 13 against the force due to the cylinder internal pressure. However, as mentioned above, after the valve is opened, the driving force required to advance the opening stroke of the exhaust valve becomes very small, so as shown in Fig. 4, the pressure receiving area of the drive piston 2 becomes equal to the total stroke _h1 of the drive piston. In a conventional hydraulically driven exhaust valve drive device, which is constant over time, the pressure in the pressure chamber 19
P ₁ drops rapidly. On the other hand, the supply capacity of the pressure oil supply equipment is set to keep the pressure within the pipeline system constant, and a pressure chamber 19 that is part of the pipeline system is used.
It acts to immediately restore the pressure drop at.
Therefore, large pressure pulsations are generated within the pipe system, and if the supply capacity is not sufficient, it takes time to recover the pressure, resulting in a decrease in the overall pressure within the pipe system.

Engines usually consist of multiple combustion cylinders, and in the case of hydraulic drive, the pressure oil supply equipment is shared by each cylinder, so the piping system is connected to the exhaust valve drive device of each cylinder, and the exhaust gas is generated in each cylinder. This pressure pulsation and pressure drop not only destabilizes the hydraulic drive in other cylinders, but also accumulates in the piping system and impairs the function of the exhaust valve hydraulic control system of the entire engine.

Therefore, the pressure oil supply capacity R needs to be set so that the pressure in the pressure chamber 19 is maintained at the pressure _P1 of the pressure oil supplied from the hydraulic source, and the required drive of the exhaust valve after opening is required. It is necessary to increase the supply flow rate to compensate for the increase in the volume of the pressure chamber 19 due to the increase in the displacement speed of the drive piston 2 due to the decrease in force. Therefore, the pressure oil supply equipment is required to have an unnecessarily large supply capacity, which increases the size of the equipment and the manufacturing cost.

Furthermore, even if pressure oil supply equipment with a sufficiently large supply capacity is obtained, not only will it consume a lot of electricity to operate it, but the hydraulic driving force will be much larger than the required driving force of the exhaust valve, so the exhaust valve will not be able to operate. There is a disadvantage that the impact force upon completion of the full stroke movement is large and significantly shortens the life of the drive device and exhaust valve.
Furthermore, increasing the spring constant of the valve spring in order to reduce the impact not only increases the size of the valve spring, but also requires a large storage space in the cylinder head, which has many dimensional restrictions. This also results in a decrease in the opening speed of the exhaust valve 13.

Purpose The present invention solves the above-mentioned problems of the conventional hydraulically driven exhaust valve drive device, without causing a significant increase in the amount of pressure oil supplied, and without increasing the size of the pressure oil supply equipment. An object of the present invention is to provide a diesel engine valve hydraulic drive device that can increase the opening speed of a valve.

Structure The valve drive device of the present invention, which solves the above problems, is a large valve drive device that is slidably fitted into the cylinder bore of a hydraulic cylinder fixed to the cylinder head of an engine, and moved in the valve opening direction by the pressure of pressure oil. diameter piston;
a small-diameter piston that is bored concentrically with the large-diameter piston and slidably fits into a small-diameter cylinder bore in which a pressure chamber communicating with the pressure chamber of the cylinder bore is formed; The connecting rod engages with the valve that is urged in the closing direction by the valve closing means, and is pushed in the same direction when the large diameter piston moves in the valve opening direction by a pressing portion provided on the large diameter piston. The cylinder casing is provided with a stopper that limits the stroke of the large-diameter piston to a stop in the middle of the valve-opening stroke, and the small-diameter piston is capable of moving throughout the entire stroke of the valve. With this configuration, the pressure-receiving area of the pressure oil of the movable part of the drive piston 2 can be made large at the initial stage of valve opening when the gas force acting on the valve is large, and then made small in the valve-open state. Rapid opening of the valve is achieved without significantly increasing the supply flow rate of the pressure oil, and without increasing the size of the pressure oil supply equipment.

Hereinafter, the present invention will be explained in detail based on embodiments shown in the drawings.

FIG. 6 and FIG. 7 respectively show the state when the exhaust valve is closed and the state when the exhaust valve is fully opened in an embodiment in which the present invention is applied to an exhaust valve drive device. Valve spring 16, exhaust valve 1
3 is the same as the example of the conventional apparatus shown in FIGS. 4 and 5, and is therefore omitted from illustration.
Further, the connections of the switching valve 6, the pressure oil supply device 28, and the tank 11 are also the same as in the example of the conventional device described above.

However, in this embodiment, the drive piston consists of a large diameter piston 21 and a small diameter piston 22, the large diameter piston 21 is slidably fitted into the cylinder bore of the cylinder casing 20, and the small diameter piston 22 is fitted into the cylinder bore of the cylinder casing 20.
is slidably fitted into a small diameter cylinder bore concentrically formed in the large diameter piston 21, and a passage 2 is provided in the large diameter piston 21 so that pressure oil acts on the upper end surface of the small diameter cylinder bore.
7 is drilled. The lower end surface of the small diameter piston 22 is pressed into engagement with the upper end of the valve stem 14 of the exhaust valve, which is normally biased in the valve closing direction by the valve spring 16 of the exhaust valve. A stop 29 is arranged on the small diameter piston 22 and has a distance relative to a seat 30 fixed to the cylinder head 12 so that the piston does not displace beyond a predetermined stroke _h1 .

The casing 20 is provided at its upper end with a passage 4 for supplying and releasing pressure oil to the pressure chamber 25 for the large-diameter piston 21 and the pressure chamber 26 for the small-diameter piston 22 via the passage 27. In order to regulate the stroke h ₂ , the large diameter piston 22 is provided with a seat surface 33 which is provided as a step and which the seat surface 32 comes into contact with.

The casing 20 has a gap between the casing 20 and the large-diameter piston 21, a small-diameter bore, and a small-diameter piston 22 to facilitate the movement of the large-diameter piston 21.
Passages 23 and 24 are provided for draining drain oil leaked through the gap and open into the tank 11 via the conduit 10.

Since this device is configured as described above, when the switching valve 6 is switched to the position where the pressure conduit 8 and the conduit 7 communicate with each other as shown in FIG.
The pressure oil generated in step 8 flows into the pressure chamber 25 of the hydraulic cylinder 1 through the passage 4. Pressure oil is flowing through passage 2 at the same time.
7 and acts on the upper end surface of the small diameter cylinder 22, but the driving force is not strong enough to overcome the gas force that acts on the bottom surface of the exhaust valve 13, so the small diameter piston 22 acts on the upper end surface of the small diameter cylinder 22. On the other hand, when the large diameter piston 21 is pushed down by the pressure oil flowing into the pressure chamber 25 against the gas force and the valve spring force without relative displacement, the internal seating surface 31 of the large diameter piston 21 is moved above the small diameter piston 22. By pressing the end face, the small diameter piston 22 moves downward together with the large diameter piston 21, and the exhaust valve 13 is opened. When the exhaust valve 13 opens, the combustion chamber 18 (same as in FIGS. 4 and 5, and not shown in FIGS. 6 and 7) communicates with the exhaust passage 17 and acts on the exhaust valve 13. gas power decreases rapidly. Therefore, the large diameter piston 21
Even after the seat surface 32 comes into contact with the seat surface 33 provided in the cylinder after displacing the stroke of h ₂ and stops, the pressure of the pressure oil flowing into the pressure chamber 26 in the upper part of the small diameter piston 22 is maintained at the exhaust valve 13. The stopper 29 descends by overcoming the gas force and valve spring force acting on the engine, and the stopper 29 comes into contact with the seat 30 provided on the cylinder head of the engine and stops.The valve reaches the fully open state shown in FIG. 7, and the pressure chamber is closed. A pressure of P ₁ is maintained at 26. Small diameter piston 2 relative to large diameter piston 21 in the lowered position
The stroke _h3 of 2 is set so that the sum with the above _h2 becomes the stroke _h1 of the predetermined exhaust valve. The size of the stroke h ₂ is selected to be the minimum necessary so that the pressure of the pressure oil in the pressure chamber 25, passage 27, and pressure chamber 26 does not decrease and the movement of the small diameter piston 22 becomes unstable. Further, after the large diameter piston 21 has completed the stroke h ₂ , large hydraulic pressure continues to act on the seating surfaces 32 and 33, so that the passage 23
It is preferable to provide a suitable throttle (not shown) in the seat 32 or 33, or to provide a groove (not shown) in the seat surface 32 or 33 in order to cushion the shock when the seat is seated and to prevent the oil film from running out on the contact surface.

After maintaining the valve open state for a predetermined period of time, when the switching valve 6 is displaced and the conduit 7 is communicated with the return conduit 9, the pressure chambers 25 and 26 are opened to the tank 11, so that the spring force of the valve spring 16 is reduced. The exhaust valve 13 and the small diameter piston 22 move upward, and the stroke h ₃
When the upper end surface of the small-diameter piston 22 comes into contact with the seat surface 31 of the large-diameter piston 21, the small-diameter piston 22 pushes up the large-diameter piston 21, moves by a stroke of _h2, and reaches the state shown in FIG. Then, the exhaust valve 13 is closed to block the combustion chamber 18 and the exhaust passage 17.

If it is necessary to limit the speed of the exhaust valve 13 during the valve-closing operation described above, a suitable throttle 3 is installed in the return conduit 9.
4 can be provided. Further, in order to prevent the exhaust valve 13 from overshooting, it is preferable to provide a stopper (not shown) on the valve stem 14.

Diameter D ₂ of small diameter piston 22 and valve spring 16
The spring constant is determined by the fact that the exhaust valve 13 moves together with the small-diameter piston 22 during the opening movement of the small-diameter piston 22, and that the stopper 29 moves against the seat 30 in relation to the flow rate of pressure oil supplied to the pressure chamber 26. Conditions such as the impact force at the time of contact not being excessive and the spring force of the valve spring 16 being large enough to almost complete the closing operation of the exhaust valve 13 in a sufficiently short time are taken into consideration. The selection will be made based on the following.

In the embodiment configured as described above, the exhaust valve 13
The amount of pressure oil supplied to open the valve is h ₂ · (D ₁ /2) ² π + h ₃ (D 2 /2) ₂ where the diameter of the large diameter piston is D ₁ and the diameter of the small diameter piston is _D ^2. π, and Fig. 4,
When compared with the supply amount h ₁ (D ₁ /2) ² π in the case of the conventional configuration shown in FIG. 5, the ratio _r becomes as follows.

r= _h2・^D2 / ₁ + _h3・^D2 / ₂ / _h1・^D2 / ₁ =1− _h3 / _h1 {
1-(D ₂ /D ₁ ) ² } For example, if h ₃ /h ₁ = 9/10 and D ₂ /D ₁ = 1/2, then r = 0.325, which reduces the amount of pressure oil to approximately 1/3. can also be reduced.

Due to the significant reduction in the required supply amount of pressure oil, the supply flow rate of the hydraulic source per unit time, flow rate through the switching valve, and pressure/flow compensation, which are problems when supplying pressure oil in a very short time. It becomes possible to achieve rapid opening of the exhaust valve by hydraulic drive without significantly increasing the pressure accumulation capacity of the accumulator.

In the above embodiment, a mechanical coil spring is used as the valve spring that closes the valve, but the valve spring is not necessarily limited to a mechanical spring. A gas spring (air spring) is used as a valve spring, in which a compressed gas is filled in the cylinder, and when the piston moves inside the cylinder and the volume of the gas changes, pressure is generated that is inversely proportional to the volume based on Boyle-Schard's law, which acts as a spring. It is also possible to do so.

Furthermore, it is also possible to perform the valve closing action not by a valve spring but by hydraulic drive. The embodiment will be explained based on FIG. The valve shown in FIG. 8 is shown in the closed state, and the structure of the hydraulic cylinder 1 for opening the valve is the same as that of the embodiment described in FIGS. 6 and 7, so the same members are given the same reference numerals. A further explanation will be omitted.

In this example, the valve spring 1 in the previous example
6, a hydraulic cylinder chamber 40 is provided in the cylinder head 12 of the engine surrounding the valve stem 14, and a portion 41 of the valve stem 14 has a larger diameter than the other portion. It fits into the inner diameter of the piston and acts as a piston. The upper end of the hydraulic cylinder chamber is connected via a conduit 42 to a conduit 10 that opens into the tank 11. On the other hand, a conduit 43 is connected to the lower end of the hydraulic cylinder chamber. Unlike the previous embodiment, the hydraulic switching valve 6' is a 4-port switching valve, and in the switching position shown in FIG.
The conduit 43 is connected to the pressure oil supply device 28, and the conduit 7 connected to the pressure chambers 25, 26 of the valve-opening hydraulic cylinder 1 is connected to the conduit 9 that opens to the tank 11.

When opening, switching the switching valve 6' to the other position connects the conduit 43 to the conduit 9 which opens into the tank 11, and the conduit 7 to the hydraulic source 28.

Therefore, in the valve open state, as in the previous embodiment,
Initially, the large diameter piston 27 and the small diameter piston 22 move together to open the valve with a large force, and after the valve has opened to a certain extent, the small diameter piston alone opens the valve to a predetermined opening degree. When the valve is opened, as the valve stem 14 moves downward, the piston part 41 provided on the valve stem 14 moves, and the oil in the pressure chamber below that of the cylinder chamber 40 is discharged into the tank 11 through the conduit 43. , the cylinder chamber above the piston part 41 contains conduits 10 and 4.
Since the valve-closing hydraulic cylinder 40 is exposed to the atmosphere at the tank 11 via the valve 2, the valve-closing hydraulic cylinder 40 does not provide any resistance to the valve-opening operation.

On the other hand, in the state shown in FIG. 8 when the valve is closed, pressure oil is supplied from the oil pressure source 28 to the lower part of the piston part 41 of the cylinder chamber 40 through the switching valve 6' conduit 43, and the piston 41 is moved upward. to press. At this time, the pressure chambers 25 and 26 of the hydraulic cylinder 1 are connected to the tank 11.
Since the valve stem 14 is open to the outside, the valve stem 14 can easily move upward and close the valve.

Also in this embodiment, a throttle 34 is provided in the return pipe 9.
By providing this, it is possible to limit the speed of oil drainage.

Although a 4-port switching valve is used as the hydraulic switching valve in the example shown in Fig. 8, the same effect can be obtained by using two 3-port switching valves connected as shown in Fig. 9 instead. can.

In the above explanation, the exhaust valve has been described, but the valve drive device of the present invention can also be applied to the drive device of the air supply valve of a 4-cycle engine, and can be applied to both the exhaust valve and the air intake valve. By hydraulically driving the valve, a mechanical valve mechanism such as a rocker arm can be omitted.

Effects As described above, according to the present invention, the supply flow rate of the hydraulic source, the flow rate passing through the switching valve, the pressure of the hydraulic source, the accumulator pressure accumulation capacity for flow rate compensation, etc., are not significantly increased, and the pressure oil can be reduced. Since the valve can be opened stably and rapidly by hydraulic drive without increasing the size of the supply equipment, the thermal efficiency of the engine can be improved at a correspondingly lower cost.

[Brief explanation of drawings]

Figure 1 is an example of a cycle diagram of a diesel engine, Figure 2 is an enlarged view of the cycle diagram during the exhaust process, Figure 3 is an example of a valve lift diagram of an exhaust valve, and Figures 4 and 5 are FIGS. 6 and 7 are longitudinal cross-sectional views illustrating the structure and operation of an example of a conventional hydraulically driven exhaust valve drive device in a closed state and an open state, respectively, and FIGS. FIG. 8 is a vertical cross-sectional view showing the valve state in a valve closed state; FIG. 9 is a schematic diagram showing a modified example of the hydraulic switching valve. It is a diagram. 1... Drive unit (hydraulic cylinder), 6... Switching valve,
9...Return conduit, 12...Cylinder head, 13
...Exhaust valve (valve), 14...Valve rod (connecting rod), 16
...Valve spring, 20...Casing, 21...Large diameter piston, 22...Small diameter piston, 25, 26...
...Pressure chamber, 29...Stopper, 30...Seat, 31
...Internal seat surface (pressing part), h ₁ ...Exhaust valve stroke, h ₂
...Large diameter piston stroke.

Claims (1)

[Scope of Claims] 1. A driving device for a mushroom-shaped valve for gas exchange in a combustion chamber of a diesel engine, in which a valve is opened by a hydraulic cylinder and the valve is closed by a valve-closing means, in which the hydraulic cylinder is used at the initial stage of opening. The hydraulic cylinder is slidably fitted into the cylinder casing and the cylinder bore of the cylinder casing, and is moved in the valve opening direction by the pressure of the pressure oil. a large-diameter piston having a pressure-receiving area approximately equal to the large pressure-receiving area required at the initial stage of valve opening, and a pressure chamber that is bored concentrically with the large-diameter piston and communicates with the pressure chamber of the cylinder bore. and a small-diameter piston that is slidably fitted into a small-diameter cylinder bore in which a connecting rod is formed and has a pressure-receiving area that is approximately equal to the small pressure-receiving area that is required after the initial period of valve opening. engages with the valve biased in the valve closing direction by the valve closing means through the valve closing means;
When the large-diameter piston moves in the valve-opening direction, the large-diameter piston is pushed in the same direction by a pressing part provided on the large-diameter piston and is carried along, and the cylinder casing has a pressure section that controls the stroke of the large-diameter piston in the middle of the valve's opening stroke. 1. A valve driving device characterized in that a stopper is provided to restrict the valve to a stop, and the small-diameter piston is movable over the entire stroke of the valve. 2. The valve drive device for a diesel engine according to claim 1, wherein a throttle is disposed in a return conduit from a directional control valve that controls the hydraulic cylinder. 3. The valve driving device according to claim 1 or 2, wherein the valve closing means is a coil spring. 4. The valve driving device according to claim 1 or 2, wherein the valve closing means is an air spring. 5. The valve drive device according to claim 1 or 2, wherein the valve closing means is a hydraulic drive means.