Method for Operating a Hydraulic Actuator

Abstract

A method for operating a hydraulic actuator for a prime mover or a drive device of a vehicle includes determining that the hydraulic actuator is to be actuated via an actuating system and detecting an operating variable influenced by a filling state of the hydraulic actuator indicative of pressure conditions in the hydraulic actuator, with the operating variable being separate from actuator operating variables of the hydraulic actuator and the actuating system. The method further includes pressurizing the actuator during a fast fill phase to one or more pressures over a fast fill time period in response to determining that the hydraulic actuator is to be actuated. The fast fill time period and the one or more pressures are adjusted as a function of the operating variable under certain operating conditions. Additionally, the method includes pressurizing the actuator during a pressure build-up phase after the fast fill phase.

Claims

1-24: (canceled)

25. A method for operating a hydraulic actuator (K0) for a prime mover or a drive device of a vehicle, the method comprising: determining that the hydraulic actuator (K0) is to be actuated via an actuating system controllable to adjust a fill level of a liquid operating medium in the hydraulic actuator (K0), the actuating system having a pressure-adjusting device controllable by an electronic control unit; detecting an operating variable (g, s), the operating variable (g, s) being influenced by a filling state of the hydraulic actuator (K0) indicative of pressure conditions in the hydraulic actuator (K0), the operating variable (g, s) being separate from an actuator operating variable (p) of the hydraulic actuator (K0) and an actuator operating variable (p) of the actuating system; pressurizing the hydraulic actuator (K0) during a fast fill phase to one or more pressures over a fast fill time period in response to determining that the hydraulic actuator (K0) is to be actuated, with the fast fill time period and the one or more pressures being adjusted as a function of the operating variable (g, s) under certain operating conditions; and pressurizing the hydraulic actuator (K0) during a pressure build-up phase after the fast fill phase.

26. The method of claim 25, wherein the actuator operating variable (p) of the hydraulic actuator (K0) is measured neither directly nor indirectly in the actuating system or in the hydraulic actuator (K0).

27. The method of claim 25, wherein the hydraulic actuator is a friction-locking shift element (K0).

28. The method of claim 27, wherein the friction-locking shift element (K0) is a multi-disk clutch or a multi-disk brake.

29. The method of claim 28, wherein the friction-locking shift element (K0) couples an internal combustion engine (VKM) to a drive train of the vehicle.

30. The method of claim 25, wherein the hydraulic actuator (K0) is a friction-locking shift element (K1, K2) within a drive device (DK).

31. The method of claim 25, wherein the hydraulic actuator (K0) is a hydraulic accumulator.

32. The method of claim 25, wherein the hydraulic actuator (K0) is a hydraulic cylinder with a piston.

33. The method of claim 32, wherein the hydraulic actuator (K0) is a device for actuating shifting rails.

34. The method of claim 27, wherein the vehicle includes at least one prime mover (EM, VKM), the operating variable (s) being an operating variable of the at least one prime mover (EM, VKM).

35. The method of claim 34, wherein the operating variable (s) is a torque of the at least one prime mover (EM, VKM), the pressure conditions in the friction-locking shift element (K0) being inferred based on a temporal profile of the torque of the at least one prime mover (EM, VKM).

36. The method of claim 34, wherein the operating variable (s) is a rotational speed of the at least one prime mover (EM, VKM), the pressure conditions in the friction-locking shift element (K0) being inferred based on a temporal profile of the rotational speed of the at least one prime mover (EM, VKM).

37. The method of claim 34, wherein the at least one prime mover comprises an electric motor (EM), the operating variable (s) being a current consumption profile over time (t) of the electric motor (EM), the pressure conditions in the friction-locking shift element (K0) and the filling state in the friction-locking shift element (K0) being inferred based on the current consumption profile over time (t) of the electric motor (EM).

38. The method of claim 25, wherein a fast fill pressure (p) during the fast fill phase is changed depending on a profile (s(t)) of the operating variable (s) over time (t).

39. The method of claim 25, wherein a fast fill pressure (p) is changed depending on a gradient (g(t)) of a profile of the operating variable (s) over time (t).

40. The method of claim 25, wherein a fast fill pressure (p) is changed depending on a temporal profile of the operating variable as well as a gradient (g(t)) of the temporal profile over time (t).

41. The method of claim 25, wherein the certain operating conditions comprises when a transmission of the vehicle is in an operating state in which air accumulations are expected.

42. The method of claim 41, wherein air accumulations are expected when one or both of the vehicle transmission was shut off before a certain minimum time and an oil temperature of the vehicle transmission is below a certain value.

43. A drive device (DK) for a vehicle, the drive device (DK) comprising the hydraulic actuator (K0) operated by the method of claim 25.

44. The drive device of claim 43, wherein one or both of the hydraulic actuator (K0) and the actuating system does not have a sensor for detecting the actuator operating variable (p).

45. The drive device of claim 44, wherein the drive device is one of a multi-ratio automatic planetary transmission, a dual-clutch transmission (DK), a continuously variable automatic transmission, an axle drive, or a wheel drive.

46. A vehicle, comprising the hydraulic actuator (K0) operated by the method of claim 25 and at least one prime mover (EM, VKM).

47. The vehicle of claim 46, including a drive device having the hydraulic actuator (K0) operated by the method of claim 25, wherein the drive device is an automatic transmission, a dual-clutch transmission, an axle drive, or a wheel drive.

48. The vehicle of claim 46, wherein one or both of the hydraulic actuator (K0) and the actuating system does not have a sensor for detecting the actuator operating variable (p).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Exemplary embodiments of a method according to the prior art as well as of methods according to the invention are described in greater detail in the following, wherein:

[0041] FIGS. 1-4 show example pressure profiles over time of exemplary embodiments according to the prior art,

[0042] FIG. 5 shows example pressure profiles over time of a first exemplary embodiment of a method according to the invention,

[0043] FIG. 6 shows a schematic of a hybrid drive train suitable for use with the disclosed method,

[0044] FIGS. 7a-c show example time-dependency diagrams of different operating variables of the method according to the invention according to FIG. 5, and

[0045] FIG. 8 shows example pressure profiles over time of one further exemplary embodiment according to the invention.

DETAILED DESCRIPTION

[0046] Reference will now be made to embodiments of the invention, one or more examples of which are shown in the drawings. Each embodiment is provided by way of explanation of the invention, and not as a limitation of the invention. For example, features illustrated or described as part of one embodiment can be combined with another embodiment to yield still another embodiment. It is intended that the present invention include these and other modifications and variations to the embodiments described herein.

[0047] FIG. 1 shows a diagram in which the profiles of a pressure p over time t in response to a method for operating an automatic transmission according to the prior art are illustrated. The pressure p is the pressure in a friction-locking shift element, where the pressure p is to be raised or increased upon actuation of the shift element by a hydraulic control unit until the shift element halves of the friction-locking shift element are pressed against each other with a sufficient force such that the shift element halves are coupled to each other in a rotationally fixed manner and the shift element transmits a torque without slip at a point in time tC at the latest. FIG. 1 shows the profiles of a target pressure pS1 and of an actual pressure pI1 generated therefrom, which actually sets in in the shift element. An actual pressure is also referred to as a pressure sequence.

[0048] The target pressure pS1 is an idealized profile, which corresponds, for example, to an electric current, which is output by an electronic control unit to a valve device, such as, for example, an electronic pressure actuator. Therefore, the pressure in the shift element and also its temporal profile are adjustable by this electric current. An electronic pressure actuator is an actuator that converts an electric current into a hydraulic pressure. The part of the hydraulic system that encompasses the valve device up to the shift element is also referred to as an actuating system or actuating path. The drive device or the vehicle includes such electronic pressure actuators.

[0049] Against this background, the represented target pressure profile pS1 is to be understood such that the target pressure profile pS1 is actually an electric current that activates an electronic pressure actuator and adjusts a hydraulic pressure according to an ideally proportional relationship. The steep, pressure peak-like increase of the target pressure profile pS1 at a point in time t0 to a pressure value p11 is not to be understood such that the actual pressure profile pI1 should increase to this value, but rather that the pressure value p11 corresponds to an excessive current value, by which the electronic pressure actuator is transferred out of the rest condition into the operating condition.

[0050] If a friction-locking shift element is to be transferred from a disengaged state, in which only a drag torque is transmittable between the shift element halves, into an engaged state, then, after the described activation of the electronic pressure actuator at the point in time t0, the target pressure pS1 is controlled to a pressure value p12 at a point in time t1, and so the volume of the friction-locking shift element, which is still pressureless, is filled as quickly as possible with the operating medium, usually hydraulic or transmission oil. This is the beginning of a so-called fast fill phase. After the point in time t1, the pressure in the shift element is raised to a pressure value p13 in a flat increase up to a point in time t11 such that the shift element halves are preloaded against one another counter to elastic forces, wherein the contact force upon the shift element halves is still too low, however, to enable a torque transmission that could be utilized for a vehicle operation. After a complete filling and preload have been achieved, the fast filling is ended by reducing the pressure in a very steep manner to a pressure value p14, which is reached at a point in time t12. At this point, a ramp-like pressure build-up now begins, which is concluded once a pressure value pC is reached at a point in time tC, where the shift element is now completely engaged and the torque is transmittable from the input side to the output side without slip. The period of time from t0 to t12 is also referred to as the fast fill time or the fast fill phase.

[0051] The target pressure pS1 is configured for an ideal filling of the actuating system. The deviation between the target pressure profile pS1 and the actual pressure profile p11 shown in the diagram in FIG. 1 sets in, for example, when the hydraulic system of the automatic transmission had been shut off and air had entered the hydraulic system due to an at least partial draining. This problem was not taken into account in the definition of the target pressure profile pS1, because this was based on a bleed actuating system. In the case of a restart after the operationally warm shutoff of the automatic transmission, it would be necessary to fill not only the elasticities or the air gap of the shift element, but also the air volumes that arose in the actuating system, by a fast fill. The deviation between the target pressure profile and the actual pressure profile is also referred to as a drag error and is that much more pronounced, the greater the amount of air it was that entered the hydraulic system. In the extreme case, this occurs possibly from very low temperatures below the freezing point, for example, upon the shutoff of a transmission at the maximum operating temperature, followed by cooling down overnight.

[0052] FIG. 1 shows a considerable deviation of the actual pressure pI1 from the target pressure profile pS1. The actual pressure profile pI1 slowly increases at the point in time t1 and reaches the pressure value p14 at the point in time t12 such that the actual pressure profile pI1 and the target pressure profile pS1 nearly coincide at this point. As the temporal profile continues, however, the flow filling the shift element according to the target pressure profile pS1 does not suffice for sufficiently quickly filling the volumes that have arisen due to the air pockets, and so the actual pressure profile pI1 rises only slowly and in a flat manner and, only after the point in time tC, increases to the pressure level predefined by the pressure value pC as the target value. According to the target pressure profile pS1, this should be achieved at the point in time tC. Due to this drag error, the pressure build-up in the shift element is prolonged in an impermissible and undesired way, which disadvantageously affects the spontaneity and the achieved accuracy of the actuation.

[0053] The additional volume that has arisen due to the air pockets is theoretically compensated for by an appropriate extension of the fast fill time. The diagram in FIG. 2 shows an extension of the fast fill time of this type. The end of the fast fill time has been shifted, with a modified target pressure profile pS2, from the point in time t12 by an extension time Δt1 to a point in time t22. The effect is reflected in a boost of an actual pressure profile pI2 that increases considerably faster than the actual pressure profile p11, which is also represented in the diagram in FIG. 2, and, thereby, also reaches the pressure value pC sooner. According to the target pressure profile pS2, the pressure value pC should be set at a point in time tC2. Therefore, the air in the actuating system has been at least proportionally taken into account.

[0054] The diagram in FIG. 3 shows a further extension of the fast fill time, by an extension time Δt2, as compared to the initial value t12 of a bleed actuating system. The extension of the fast fill time from initial time t12 up to a point in time t32 brings about a further boost of the actual pressure profile to an actual pressure profile pI3, which now increases, with a relatively low drag error as compared to a target pressure profile pS3, with the same slope in parallel thereto at the point in time t32 and reaches the pressure value pC slightly later than at a sought point in time tC3 due only to the principle-related, permissible hydraulic delay time. Therefore, the air in the actuating system has taken into account in a precisely suitable manner.

[0055] If the extension time and, thereby, the fast fill time is selected to be too long and a fast fill pressure p43 is reduced too late, as shown with reference to the example in FIG. 4, an actual pressure profile pI4 results, which is just as undesirable as a selection of the extension time in FIGS. 1 and 2 that is too short. Since the fast fill pressure, which moves between the pressure values p42 and p43, is applied for a very long time, according to a target pressure profile pS4, due to a very long extension time Δt3 from t12 up to a point in time t42, the shift element is completely filled such that, at a point in time t42, the actual pressure profile pI4 steeply increases in the form of an overshot to a pressure value p45. This increase is halted at the point in time t42, since the fast fill pressure is just now reduced from the pressure value p43 to a pressure value p44 and, thereby, the fast fill phase is terminated. The actual pressure profile pI4 therefore reaches a local maximum at the point in time t42 and is far above the desired pressure value p44. Due to this excess pressure, the power transmission capacity of the form-locking shift element also increases such that a torque shock occurs, which is to be avoided in the interest of component load and actuation comfort. After the point in time t42, the actual pressure profile pI4 swings around the target pressure profile pS4 and reaches the pressure value pC almost simultaneously therewith. Therefore, the air in the actuating system has been taken into account by too great an extent, which, in the extreme case, results in strong, undesired responses, which are to be avoided in any case.

[0056] The consequences of not achieving the appropriate extension time become apparent from the representations of the actual pressure profiles at different fast fill times or extension times. In practical applications, the setting of an appropriate extension time is highly problematic, since the amount of air in the actuating system is not known. Due to the serious disadvantages of an actual pressure profile pI4 in combination with an extension time that is too long, the tendency in practical applications is to select an extension time that is too short, as in FIG. 2. Although this results in a drag error, there is no need to worry about the serious disadvantages of an actual pressure profile pI4 with the excess pressure and the subsequent oscillations in FIG. 4. Overall, however, any selection of an extension time without knowledge of the air volumes in the actuating system yields an unsatisfactory result. The extension times Δt1, Δt2, and Δt3 were established under certain assumptions, which are based on test results or calculations, wherein, due to the different influencing variables, such as the standstill time of the automatic transmission, temperatures, component tolerances, shutoff state of moving parts, and volumes of the actuating system, it tends to be a coincidence whether an appropriate extension time such as the extension time Δt2 is achieved. For the sake of surety, an extension time that tends to be too short, such as Δt1, is always selected and a drag error is accepted.

[0057] As mentioned at the outset, this disadvantage is to be eliminated by arranging a pressure sensor in the actuating system and in the shift element. As a result, it is possible to observe the pressure build-up in the shift element and, thereby, select the extension time that is appropriate to the situation. The disadvantage, however, is the high costs and a high outlay of parts associated with additional pressure sensors.

[0058] The diagram in FIG. 5 shows the pressure profiles of a method according to the invention, which gets by without additional pressure sensors. Here, the effect of the actuating system is observed in the form of the response of other operating variables in the automatic transmission or of the vehicle, and the extension time is adjusted accordingly by reducing the fast fill pressure only once a criterion of the response has been established. Thus, in the method according to FIG. 5, a target pressure profile pS5 initially increases, as in the case of a completely filled actuating system, from a pressure value p52 to a maximum fast fill pressure value p53, which is reached at a point in time t11 and is held constant as the temporal profile increases up to a point in time t51. An actual pressure profile pI5 gradually increases at the point in time t11 and also remains nearly constant in the phase of the constant target pressure profile pS5 while the piston chamber of the shift element is filled. An increase takes place similarly to the increase of the piston restoring force, for example, from a return spring. Shortly before the point in time t51, the actuating system has been filled to such an extent that the actual pressure profile pI5 increases. At the point in time t51, the actual pressure profile pI5 exceeds a certain pressure value and the actual pressure profile pI5 changes its gradient.

[0059] If a pressure sensor were now arranged in the actuating system, this pressure increase could act as a criterion at the point in time t51 for triggering an abrupt reduction of the fast fill pressure or of the target pressure profile pS5, wherein, however, an essential problem addressed by the invention is that of avoiding the outlay of a pressure sensor. Without a pressure sensor in the actuating system and, therefore, without a measurement of the actual pressure pI5, its increase cannot be noticed. Since the increase of the actual pressure profile pI5 affects other operating variables outside the hydraulic actuator in the drive device or of the vehicle, however, which are already detected in an electronic control unit in order to fulfill the functions of the drive device, this is utilized according to the invention to infer the actual pressure profile pI5, as shown in FIGS. 7a-c.

[0060] On the basis of the criterion that is to tag an end of the fast fill phase, the target value of the fast fill pressure is abruptly reduced, at the point in time t51, from the pressure value p53 to a pressure value p54, which is reached at a point in time t52. This now yields an extension time Δt4 from t12 to t52 for the fast fill phase as compared to the point in time t11. Due to the pressure drop, the actual pressure profile pI5 does not continue to increase, but rather dwells at the pressure value p55 for a certain length of time and follows, with a slight time delay, the target pressure profile pS5 in the ramped increasing pressure build-up phase. The pressure value pC, as the target value, is likewise reached by the actual pressure profile pI5 having a drag error with a slight time delay as compared to a desired point in time tC5 and with the same slope as the target pressure profile pS5.

[0061] Regardless of the air volumes present in the actuating system, the correct extension time is now always set and the air volume is now always appropriately taken into account due to the control of the fast fill pressure on the basis of the criterion.

[0062] If the expected response does not take place by the end of the regular fast fill time, the filling activation is maintained until an expected response is observed. The filling activation is suitably concluded at the point in time at which the expected response is observed. Here, “suitably” means that the operating sequence is either abruptly terminated or a conclusion or transition process is carried out, or a further time is activated as an overrun.

[0063] This example is based on an automatic transmission that is a dual-clutch transmission having a hybrid design. The diagrammatic configuration thereof is represented in FIG. 6. An internal combustion engine VKM is coupleable to an electric motor EM by a clutch K0. A dual clutch DK is rotationally fixed to the electric motor EM as a drive device, wherein the dual clutch DK includes two clutches K1 and K2, which are selectively engageable during the driving operation. The diagrammatic dual-clutch transmission is merely one example of the application of the method, which is usable in the same manner in any other type of transmission that includes hydraulic actuators to be filled with an operating medium for the actuation thereof.

[0064] The prime mover is an internal combustion engine or an electric motor or a hybrid drive including an internal combustion engine and an electric machine.

[0065] If, in the case of the dual-clutch transmission in FIG. 6, when an internal combustion engine VKM is stopped and the clutches K0, K1, and K2 are disengaged or slipping, the electric motor EM is now driven and the clutch K0 is filled and pressurized in the fast fill phase for the actuation thereof, a slight torque builds up, which, in order to maintain the current rotational speed, becomes evident in the current consumption of the electric motor. The current consumption of the electric motor EM is a measure of its actual torque.

[0066] In the present system, therefore, use is made of the fact that the electric motor EM must counteract the internal combustion engine VKM drag torque (when the internal combustion engine VKM is not running) coupled in via the clutch K0 in order to maintain the rotational speed, for example, when the vehicle is at a standstill with an active idling speed controller. In situations in which the clutch K0 is to be actuated based on the driving strategy (for example, starting the internal combustion engine VKM at a standstill), an observation of the response is made on the basis of suitable signals (for example, torques or rotational speeds in the drive train. Here, the signal value as well as the change of the signal value (gradient) are observed and evaluated.

[0067] The current consumption is detected in the electronic control unit as a signal s for the torque, which is represented over time in the diagram in FIG. 7c as a signal curve s(t) over time t. The diagram in FIG. 7b shows an associated gradient profile g(t), i.e., the derivative of the signal profile s(t) over time t. FIG. 7a shows, in a diagram, the actual pressure profile pI5 from FIG. 5 as the operating variable.

[0068] Further operating variables, the signal or the derivative of which are usable as a criterion, are all signals, in principle, that permit an inference of the actual torque applied at the shift element. These operating variables are, for example, a torque balance at the electric motor EM, the rotational speed profile of the electric motor EM, the rotational speed profile of the internal combustion engine VKM, the torque of the internal combustion engine VKM, and signals from a torque sensor, a system pressure sensor, a flow rate sensor, an acceleration sensor, or a position sensor.

[0069] At the point in time t51, a response to the increase of the actual pressure profile pI5 in FIG. 7a is evident in the signal profile s(t) as well as in the gradient profile g(t), which means an increase of torque in the electric motor EM. The level of the gradient value g as well as of the signal value s as well as both variables act as a criterion here, upon the achievement of which the fast fill pressure is reduced by reducing the target pressure profile pS5 at the point in time t51 from the pressure value p53 to the pressure value p54, which is to be reached at the point in time t52. In this way, the gradient profile g(t) provides information about the dynamics, for example, of a change in torque at a driving machine. The signal value s(t) itself is also meaningful when this is, itself, already a measure of the actuation.

[0070] FIG. 8 shows an optimized embodiment of the method with an observation of a response outside the actuating system. Here, in a target pressure profile pS6, a pressure value p62 is output as the fast fill pressure after the first pressure peak and is held constant up to a point in time t61, which corresponds to the point in time t12 of the target pressure profile pS1. This takes place in order not to activate the final fast fill pressure level too quickly at the beginning of the filling while the actuating system has not yet responded. As a result, the flow rate, which largely results due to the actuated pressure differential between a pressure value p62 or p63 and ambient pressure p0, without the expected back pressure initially for filling a system acted upon by air, is not higher than desired, which could otherwise result in an excessively steep pressure build-up with pressure values that are too high.

[0071] At the point in time t61, an extension time Δt5 is appended as compared to the target pressure profile pS1, within which the fast fill pressure is reduced from the pressure value p62 to a pressure value p63, which is reached at a point in time t62, in order to more slowly fill the shift element at the end of the fast fill phase such that the response is less pronounced up to the conclusion of the activation. It is useful, however, to maintain a minimum level. At the point in time t62, the fast fill pressure is held constant at the pressure value p63 until the response of the operating variable, which acts as a criterion, takes place at a point in time t63, whereupon the fast fill pressure in the target pressure profile pS6 is abruptly reduced to a pressure value p64, which is to be reached at a point in time t64. The actual pressure profile pI6, which increased toward the end of the fast fill phase, i.e., before the point in time t64, since the volume to be filled was full, now remains constant, due to the decrease of the target pressure profile pS6, at a pressure value p65, which deviates only slightly from the desired pressure value p64 and follows the target pressure profile pS6 with a drag error in parallel with the same slope up to the pressure value pC at a point in time tC8 as the target value.

[0072] In the case of highly actuation-sensitive systems, in order not to always have to wait, in principle, for a response, which then results in disruptive responses, the observation is activated only when air is probably in the system and the shutoff is then carried out in a timely manner. This is the case, for example, after long standstill times, during intense cooling of the drive device during the standstill, or during operating conditions with heavy oil foaming.

[0073] Modifications and variations can be made to the embodiments illustrated or described herein without departing from the scope and spirit of the invention as set forth in the appended claims. In the claims, reference characters corresponding to elements recited in the detailed description and the drawings may be recited. Such reference characters are enclosed within parentheses and are provided as an aid for reference to example embodiments described in the detailed description and the drawings. Such reference characters are provided for convenience only and have no effect on the scope of the claims. In particular, such reference characters are not intended to limit the claims to the particular example embodiments described in the detailed description and the drawings.

TABLE-US-00001 Reference characters DK dual clutch EM electric motor g gradient g(t) time profile of the gradient K0 clutch K1 first clutch of the dual clutch K2 second clutch of the dual clutch p pressure p0 pressure value p11 pressure value p12 pressure value p13 pressure value p14 pressure value p22 pressure value p23 pressure value p24 pressure value p32 pressure value p33 pressure value p34 pressure value p42 pressure value p43 pressure value p44 pressure value p45 pressure value p52 pressure value p53 pressure value p54 pressure value p55 pressure value p62 pressure value p63 pressure value p64 pressure value p65 pressure value pC pressure value pI1 actual pressure profile pI2 actual pressure profile pI3 actual pressure profile pI4 actual pressure profile pI5 actual pressure profile pI6 actual pressure profile pS1 target pressure profile pS2 target pressure profile pS3 target pressure profile pS4 target pressure profile pS5 target pressure profile pS6 target pressure profile s signal s(t) time profile of the signal t time t0 point in time t1 point in time t11 point in time t12 point in time t22 point in time t32 point in time t41 point in time t42 point in time t51 point in time t52 point in time t61 point in time t62 point in time t63 point in time t64 point in time tC point in time tC2 point in time tC3 point in time tC4 point in time tC5 point in time tC8 point in time Δt1 time difference Δt2 time difference Δt3 time difference Δt4 time difference Δt5 time difference VKM internal combustion engine