IGNITION CIRCUIT FOR AN INTERNAL COMBUSTION ENGINE

Description

[0001] The invention relates to an ignition circuit for an internal combustion engine with a spark plug arranged in a combustion chamber of the internal combustion engine, as known from US 5,447,131 A or US 7,536,983 B2. The combustion chamber is delimited by a piston which moves back and forth between a top dead center and a bottom dead center and drives a crankshaft via a connecting rod. The internal combustion engine has an intake duct in which a control element is provided for metering combustion air, on the one hand for when the internal combustion engine is idling and, on the other hand, for when the internal combustion engine is accelerating. When idling, a first amount of combustion air is supplied. When accelerating, a second amount of combustion air is supplied. The second amount of combustion air supplied is greater than the first amount of combustion air supplied.

[0002] By means of an electronic control circuit, an ignition spark is triggered at the spark plug at an ignition point depending on the crankshaft angle of the crankshaft in order to combust a mixture compressed in the combustion chamber. The ignition point can be varied by the ignition circuit depending on the operating state (idling, acceleration) of the internal combustion engine, with at least one ignition point for the idle state and at least one ignition point for the acceleration state being provided to the control circuit.

[0003] When the internal combustion engine is idling, the ignition timing is often set "late." Furthermore, the combustion of the mixture in the combustion chamber is irregular and often inefficient from cycle to cycle. Idling is controlled to minimize engine speed fluctuations.

[0004] During acceleration, the ignition timing is adjusted to "advance" in particular in order to achieve efficient combustion of the compressed mixture and thus good acceleration and rapid increase in engine speed.

[0005] The engine speed is increased by changing the amount of combustion air supplied and increasing the fuel supply, for which purpose at least the control element provided in the intake duct for metering combustion air is adjusted. For acceleration, the control circuit will expediently switch the ignition timing from, for example, "retarded" to "advanced." For this purpose, it is known to monitor the increase in engine speed so that the control circuit sets an "advanced" ignition timing as soon as an increase in engine speed beyond a predetermined value occurs due to the opening of the throttle valve and can be detected by the ignition circuit.

[0006] However, the "late" ignition timing set during idle and the associated inefficient combustion have the disadvantage that when the operating state changes from idle to acceleration, the engine's speed increases only slowly. Only when the engine's speed exceeds the specified limit does the control circuit switch to an "advanced" ignition timing for acceleration. The engine accelerates slowly and with a delay.

[0007] The invention is based on the object of designing an ignition circuit for an internal combustion engine such that the internal combustion engine accelerates powerfully and quickly from idle. Furthermore, a method for switching the ignition timing to the acceleration case is to be specified.

[0008] An ignition circuit according to the invention for an internal combustion engine is designed according to the features of claim 1 and solves the stated problem.

[0009] A method for early switching of the ignition timing to the acceleration case is specified in claim 8.

[0010] When the internal combustion engine is idling, the current speed is monitored over a predefined crankshaft angle range. The predefined crankshaft angle range is selected such that the compression stroke of the internal combustion engine lies at least partially within the predefined crankshaft angle range. Within the predefined crankshaft angle range, any drop in crankshaft speed that occurs is recorded as a value. This can be achieved, for example, by recording the current crankshaft speed at the beginning of the crankshaft angle range and the current crankshaft speed at the end of the crankshaft angle range, and the control circuit is designed, in particular, to determine the absolute value of the difference between the two recorded values as the drop in speed.

[0011] The value of the determined speed drop is compared with a predetermined value of a speed drop, wherein if the predetermined value of the speed drop is exceeded, the control circuit preferably switches immediately to an ignition point for the acceleration case.

[0012] The rapid switchover to an ignition timing for acceleration, particularly to an "advanced" ignition timing, effected by the control circuit as a function of the drop in the crankshaft speed during the compression stroke of the internal combustion engine, results in significantly improved acceleration of the internal combustion engine to a higher speed. Compared to the prior art, in which the switchover to the ignition timing for acceleration only occurs after an increase in speed due to the opening of the throttle valve, according to the subject matter of the invention, the switchover of the ignition timing to the acceleration case, advantageously to an "advanced" ignition timing, is carried out even before an increase in the crankshaft speed of the internal combustion engine.

[0013] According to the invention, a faster response of the internal combustion engine to a change in operating state is achieved, namely from idling to acceleration. Even with an increase in the amount of combustion air supplied by adjusting the control element in the intake port (e.g., a throttle valve), the control circuit detects the change in the operating state of the internal combustion engine in the first compression stroke due to a significant drop in engine speed. The significant drop in crankshaft speed is caused by the increased compression work of the piston due to the larger amount of combustion air supplied.The significant drop in speed is an indication of the changed operating state of the combustion engine, so that the control circuit can ideally set the ignition timing for the acceleration case already for the next compression stroke, i.e. can expediently shift the ignition timing to "early".

[0014] There are several ways to determine the acceleration case by evaluating the speed drop in a monitored crankshaft angle range.

[0015] This allows a currently detected speed drop to be directly compared with a predefined speed drop limit. If the detected speed drop in a working cycle exceeds the predefined speed drop limit, the ignition timing is switched to the acceleration case. It may be useful to calculate an average of the current speed drop from several working cycles, e.g., 2 to 5 working cycles, and compare it with the predefined limit.

[0016] A difference between a current speed drop in the monitored crankshaft angle range and the speed increase of a previous work cycle in the monitored crankshaft angle range can also be calculated. The calculated difference is compared with a predefined limit value Δn G . If the calculated difference is greater than the predefined limit value Δn G , the ignition timing for the acceleration case is switched over. It may be expedient to provide the speed drop of a previous work cycle as a moving average and to calculate the speed drop from several previous work cycles, e.g., from 2 to 5, in particular 3, work cycles.

[0017] Instead of a predefined constant limit value as a comparison value, it may be useful to use a changing limit value. For example, the changing limit value can be calculated from a moving average of the speed drop over more than two working cycles plus a speed jump of, for example,

[0018] 100 rpm. This makes the comparison less dependent on the closed throttle speed, which can vary from engine to engine or depending on the idle setting.

[0019] The detection of the speed drop and the comparison to determine a significant speed drop are preferably carried out in an extended speed range relevant for idling, from approximately 1,500 rpm to 3,500 rpm.

[0020] The crankshaft angle range can advantageously extend over a crankshaft angle of up to 300°. The predetermined crankshaft angle range expediently extends over a crankshaft angle of no more than 180°CA. In a particular embodiment of the invention, the predetermined crankshaft angle range extends in the direction of rotation of the crankshaft at least between the bottom dead center of the piston and at least the top dead center of the piston. In this crankshaft angle range, the speed drop caused by the compression work of the piston is most pronounced. In an alternative embodiment, the crankshaft angle range can extend up to the ignition point of the compressed mixture.

[0021] The internal combustion engine can be a two-stroke engine. In a particular embodiment of the invention, the internal combustion engine is designed as a four-stroke engine. The two-stroke engine has one compression stroke per crankshaft revolution. The working cycle of a two-stroke engine corresponds to one rotation of the crankshaft through a crankshaft angle of 360°. The four-stroke engine has one compression stroke every other crankshaft revolution. The working cycle of a four-stroke engine corresponds to one rotation of the crankshaft through a crankshaft angle of 720°.

[0022] In a further development of the invention, the control circuit is configured to detect a speed drop occurring during multiple operating cycles of the internal combustion engine. The control circuit is further configured to derive the value of the predetermined speed drop from an average of the detected speed drop values. The predetermined value of the speed drop, above which the switch to the acceleration case occurs, can thus be a floating value or a fixed value.

[0023] The subject matter of the invention also relates to a method for switching an ignition timing in a spark-ignition internal combustion engine with a spark plug arranged in a combustion chamber of the internal combustion engine and a piston that compresses a fuel / air mixture supplied to the combustion chamber on its path from bottom dead center to top dead center. The supplied combustion air is metered via an intake duct with a control element depending on the operating state of the internal combustion engine. According to the invention, when the internal combustion engine is idling during a compression stroke, an occurring speed drop is detected, and the detected speed drop is compared with a predetermined speed drop. If the detected speed drop exceeds the limit of the predetermined speed drop, the ignition timing is switched to an ignition timing for the acceleration case, e.g., to an "advanced" ignition timing for the acceleration case.

[0024] After increasing the amount of combustion air supplied to increase the engine's speed, the user's acceleration request can be detected based on the resulting drop in engine speed during the piston's first compression stroke, and the ignition timing can be adjusted accordingly. Before the increased amount of combustion air supplied—and especially with an increased amount of fuel supplied—leads to an increase in the engine's speed, the acceleration request is already detected, and the ignition timing is switched to the acceleration case.

[0025] The speed drop is conveniently recorded over a crankshaft angle range of up to 300°. The crankshaft angle range is selected to cover at least the piston's bottom dead center and at least the piston's top dead center.

[0026] It may be appropriate to calculate the value of the specified speed drop as the threshold for switching from an average of the sum of previous speed drops. The specified value of the speed drop can thus be a floating value or a fixed value.

[0027] Further features of the invention emerge from the further claims, the description, and the drawing, which illustrates an exemplary embodiment of the invention. The features disclosed in the claims, the description, and the drawings can be combined with one another as desired. The exemplary embodiment illustrated in the drawing is described below. They show: Fig. 1 shows a schematic section through a working device with an internal combustion engine and an ignition circuit according to the invention, Fig. 2 shows a simplified representation of the speed curve of the crankshaft over the crankshaft angle with the throttle valve closed, Fig. 3 shows a simplified representation of the speed curve of the crankshaft over the crankshaft angle with the throttle valve open, Fig. 4 shows a simplified representation of the speed curve of the crankshaft over successive cycles of the internal combustion engine with a transition from a closed throttle valve (idling) to an open throttle valve (acceleration).

[0028] In Fig. 1 A working device 1 with an internal combustion engine 2 is shown. In the exemplary embodiment, the working device 1 is a motor-driven chainsaw with a saw chain 3' rotating on a guide bar 3. The motor-driven chainsaw is selected as an example from a variety of possible working devices such as blowers, brush cutters, hedge trimmers, or similar hand-held, particularly portable, working devices.

[0029] The illustrated internal combustion engine 2 is a two-stroke engine; alternatively, a four-stroke engine, in particular a mixture-lubricated four-stroke engine, can also be used to drive the implement 1. The following invention can be used with both two-stroke and four-stroke engines.

[0030] The internal combustion engine 2 has a cylinder 5 with a combustion chamber 4, which is delimited by a piston 6. The piston 6 moves back and forth between a top dead center TDC and a bottom dead center BDC. The fuel / air mixture sucked in via an intake port 7 flows into the combustion chamber 4 and is compressed in a compression stroke by the piston 6 moving in the direction 50 towards the top dead center TDC. The compressed mixture is ignited by the spark of a spark plug 21 in the combustion chamber 4, causing the piston 4 to move downwards at an accelerated rate towards the bottom dead center BDC. The piston 4 moving back and forth between the top dead center TDC and the bottom dead center BDC drives a crankshaft 9 of the internal combustion engine 2 in the direction of rotation 31 via a connecting rod 8. The crankshaft 9 drives a tool of the working device 1, in the present embodiment the saw chain 3' rotating on the guide rail 3.

[0031] In one power stroke (cycle), piston 6 moves from bottom dead center (BDC) to top dead center (TDC) and back to bottom dead center (BDC). The crankshaft rotates 360°CA.

[0032] The speed of the internal combustion engine 2 is determined by the supplied quantity and richness of the fuel / air mixture, which in the exemplary embodiment is metered via a carburetor 10. Alternatively, the supplied fuel can also be supplied via a fuel valve or an injection valve. A flap control can be provided to meter the necessary combustion air. In the exemplary embodiment shown, at least one throttle valve 11 is provided in the carburetor 10, which in Fig. 1 is shown in dashed lines in a closed position.

[0033] In the idle position of the throttle valve 11, the intake port 7 is closed by the throttle valve 11. The combustion air required for stable idling flows into the intake port 7 through a small opening in the throttle valve 11 or a bypass and is drawn into the combustion chamber 4 together with fuel as a fuel / air mixture. In the dash-dotted position of the throttle valve 11', the intake port 7 is fully open, allowing a large amount of combustion air for the fuel / air mixture to be drawn into the combustion chamber 4.

[0034] The position of the throttle valve 11, 11' can be changed by an adjusting element 13 via a lever arrangement 12. In the exemplary embodiment, the adjusting element 13 is pivotably mounted in the handle 14 of the working device 1. The adjusting element 13 can be designed as a throttle lever of the internal combustion engine 2.

[0035] The spark plug 21 is controlled by an ignition circuit 20 to emit an ignition spark depending on the crankshaft angle °KW of the crankshaft 9. The ignition timing ZZP can fundamentally depend on the operating state of the internal combustion engine 2 and the crankshaft angle °KW of the crankshaft 9, i.e., the actual angular position of the crankshaft 9 and thus the stroke position of the piston 6.

[0036] To detect the crankshaft angle °CA of the crankshaft 9, a sensor 15 designed as a speed sensor can be provided, for example. The sensor 15 is arranged on the crankshaft 9 and senses its angular position. The combination of the sensor 15 with the crankshaft 9 can be designed such that the sensor 15 emits an output signal each time the crankshaft rotates by 1°CA. Over a crankshaft revolution of 360°CA, the sensor 15 then emits exactly 360 output signals. The output signals of the sensor 15 are fed via a signal line 16 to an input circuit 17 of the ignition circuit 20. In the input circuit 17, the speed of the crankshaft 9 and the rotational position of the crankshaft 9, namely the crankshaft angle °CA of the crankshaft 9, are determined on the one hand based on the received output signals of the sensor 15. This information is fed via an output 18 to a control circuit 19 of the ignition circuit 20.Based on the speed signal and the angle signal, the ignition timing ZZP of the spark plug 8 is controlled by the control circuit 19.

[0037] In the exemplary embodiment shown, the control circuit 19 is advantageously provided with a first ignition point ZZP L for the idling case of the internal combustion engine 2 and a second ignition point ZZP B for an acceleration case or full load case of the internal combustion engine 2. The ignition points ZZP L and ZZP B are advantageously provided to the control circuit 19 from a memory 22. An ignition point ZZP L for idling can be from 15 °CA before TDC to 0 °CA before TDC. An ignition point ZZP B for an acceleration case or full load case can be from 20 °CA before TDC to 40 °CA before TDC. It can be expedient to provide several ignition points for the idling case and several ignition points for the acceleration case. In an advantageous embodiment, ignition characteristics are stored in the memory 22, which provide ignition points ZZP adapted as a function of the speed of the crankshaft 9.

[0038] The speed curve 40 in the idle case of the combustion engine 2 is in Fig. 2 shown. The idle speed is, for example, between 2,000 and 3,000 rpm. In the compression stroke of the internal combustion engine 2, ie when the piston 6 moves from the bottom dead center UT in the direction of 50 to the top dead center OT and in the process compresses the fuel / air mixture in the combustion chamber 4, this generally leads to a speed drop Δn due to the compression work carried out, which in Fig. 2 is shown as Δn zu. Such a speed drop Δn zu can, for example, reach the value of 125 rpm. The ignition is shown schematically in the dashed curve 45 drawn below the speed curve 40. In the idling case, a later ignition point is provided. Around the area of top dead center TDC, the control circuit 19 triggers an ignition spark at the spark plug 21, which leads to the ignition of the compressed fuel / air mixture. As can be seen from the speed curve 40, the speed N of the internal combustion engine 2 then increases again, only to fall again in a subsequent compression stroke due to the compression work performed.

[0039] In Fig. 3 1 shows the speed curve 40 during acceleration of the internal combustion engine 2. When the internal combustion engine is at full load, the speed is between 7,000 rpm and 11,000 rpm. The maximum speed is between 8,000 rpm and 15,000 rpm. If the throttle valve 11 is pivoted into the open position of the throttle valve 11' during idling, for example by pressing down the actuating element 13 in the handle 14 of the working device 1, the intake duct 7 is fully open. When the intake duct 7 is fully open, a large amount of combustion air can flow into the combustion chamber 4, to which a correspondingly large amount of fuel is supplied to form an ignitable fuel / air mixture. The amount of combustion air flowing into the combustion chamber 4 when the throttle valve 11' is in the open position is greater than the amount of combustion air flowing into the combustion chamber 4 when the internal combustion engine 2 is idling.The amount of combustion air flowing in when the throttle valve 11' is in the open position is in particular a multiple of the amount of combustion air flowing to the combustion engine 2 when idling.

[0040] The larger quantity of combustion air supplied to the combustion chamber 4 during acceleration leads to a larger cylinder charge, ie, there is a larger quantity of a fuel / air mixture in the combustion chamber, whereby the compression work of the piston 6 increases. This greater compression work of the piston 6 leads - as the speed curve 40 in Fig. 3 shows - to a significant drop in the speed N of the crankshaft 9. On its way from the bottom dead center UT in the direction of 50 to the top dead center OT, a speed drop Δn occurs, which in Fig. 3 is shown as Δn o. The speed drop Δn o occurring when the throttle valve 11 is open is significantly greater than the speed drop Δn zu occurring when the throttle valve 11 is closed when idling. The greater speed drop Δn zu occurring due to the greater compression work performed can have a value of 260 rpm, for example. If the large amount of fuel / air mixture in the combustion chamber 4 is ignited in the region of top dead center TDC when accelerating, there is a sharp increase in speed above the idling speed. The control circuit 19 controls the ignition timing ZZP of the spark plug 21 according to the advantageously predetermined ignition timing ZZP B for the acceleration case.

[0041] According to the invention, when the internal combustion engine 2 is idling, the profile of the rotational speed N of the crankshaft 9 in the compression stroke is monitored over a predetermined crankshaft angle range 30. Advantageously, the monitoring of the rotational speed for switching the ignition timing is provided in a rotational speed range relevant for idling, from approximately 1500 rpm to 3500 rpm.

[0042] When the throttle valve 11 is closed, the speed drop Δn zu occurring during the compression stroke in the predefined crankshaft angle range 30 is recorded as a value. The speed drop Δn zu is reported in the ignition circuit 20 as speed drop Δn to a monitoring circuit 23, and the value of the recorded speed drop Δn is fed to a comparator 24. The comparator 24 directly compares the value of the speed drop Δn recorded in the predefined crankshaft angle range 30 with a predefined limit value Δn G , which is advantageously stored in the comparator 24. The value of such a limit value Δn G can, for example, be a constant of 200 rpm.

[0043] If the throttle valve 11 is open, the piston 5 must perform a greater compression work due to the larger filling of the combustion chamber 4, which is why there is a greater drop in speed Δn o in the monitored crankshaft angle range 30 ( Fig. 3 ).

[0044] If the recorded value of the speed drop Δn, in the case of the open throttle valve 11, is the speed drop Δn o ( Fig. 3 ), equal to or greater than the specified limit value Δn G , the comparator 24 outputs a signal on the control line 25. If the control circuit 19 receives the signal from the comparator 24, the control circuit immediately switches to an ignition point ZZP B for the acceleration case. This is shown schematically in Fig. 4 reproduced.

[0045] The predetermined speed drop Δn G provided as a limit value is advantageously compared directly with the current speed drop Δn or Δn o or Δn to .

[0046] It may be advantageous to calculate a moving average of the speed drop Δn over several working cycles, e.g., over 2 to 5, especially 3, working cycles. The specified limit value of the speed drop Δn G is selected so that switching to the ignition timing for acceleration is avoided in the event of speed noise.

[0047] It may also be advantageous to compare a predetermined limit value for the speed drop Δn G with the difference between a determined current speed drop Δn n of a first working cycle and the speed drop Δn n-1 of a previous working cycle. In particular, the speed drop Δn n-1 of a previous working cycle can also be provided as a moving average. Thus, the speed drop Δn n-1 of a previous working cycle can be averaged from the mean value of, for example, 2 to 5 previous working cycles, in particular three working cycles. This allows individual outliers or noise to be smoothed out. The limit value for the speed drop Δn G can advantageously be selected closer to zero.

[0048] It may also be advantageous to calculate the limit value of the speed drop Δn G from a moving average of the speed drop Δn ni from previous work cycles, plus the addition of a speed jump of, for example, 100 rpm. This makes the function less dependent on the speed during idle, which can vary from engine to engine or depending on the idle setting.

[0049] The acceleration of the internal combustion engine 2 can be detected significantly earlier, before the acceleration can be detected by the increase in the speed of the crankshaft 9. The reaction of the internal combustion engine 2 to an opening of the throttle valve 11, 11' occurs significantly faster than with a control system that switches to an ignition timing ZZP for the acceleration case based on an increase in speed.

Claims

1. Ignition circuit for a combustion engine (2), having a spark plug (21) which is arranged in a combustion chamber (4) of the combustion engine (2), • wherein the combustion chamber (4) is delimited by a piston (6) which is moved to and fro between a top dead centre (OT) and a bottom dead centre (UT) and, via a connecting rod (8), drives a crankshaft (9), • and having an intake channel (7) with a control element (27) for apportioning combustion air for an idling situation and an acceleration situation of the combustion engine, • and having an electronic control circuit (19), which is designed to trigger an ignition spark at the spark plug (21) at an ignition time (ZZP), as a function of a crankshaft angle (°KW) of the crankshaft (9), • wherein the control circuit (19) is provided with at least one ignition time (ZZPL) for the idling situation and at least one ignition time (ZZPB) for the acceleration situation, • and the control circuit (19) is designed, in the idling situation, to set the specified ignition time (ZZPL) for the idling situation, characterized • in that the control circuit (19) is designed, in the idling situation of the combustion engine (2), to monitor the course of a rotational speed (N) of the crankshaft (9) over at least one specified crankshaft angle range (30), • wherein the compression stroke of the combustion engine (2) at least partially lies in the specified crankshaft angle range (30), • and, in the idling situation, to detect a drop (Δn), occurring within the specified crankshaft angle range (30), in the rotational speed (N) of the crankshaft (9) as a value, • and is designed to compare the value of the detected drop in rotational speed (Δn) with a specified value of a drop in rotational speed (ΔnG), • and, when the specified value (ΔnG) of the drop in rotational speed (Δn) is exceeded, to change over to the ignition time (ZZPB) for the acceleration situation.

2. Ignition circuit according to claim 1, characterized in that the control circuit (19) is active at least in a rotational speed range of 1500 1 / min to 3500 1 / min.

3. Ignition circuit according to claim 1 or 2, characterized in that the crankshaft angle range (30) extends over a crankshaft angle (°KW) of up to 300°.

4. Ignition circuit according to any of claims 1 to 3, characterized in that the specified crankshaft angle range (30) extends over a crankshaft angle (°KW) of not more than 180°KW.

5. Ignition circuit according to any of claims 1 to 4, characterized in that, in the rotation direction (31) of the crankshaft (9), the specified crankshaft angle range (30) extends at least between the bottom dead centre (UT) of the piston (6) and at least the top dead centre (OT) of the piston (6).

6. Ignition circuit according to any of claims 1 to 5, characterized in that the specified crankshaft angle range (30) extends at least up to the ignition time (ZZP).

7. Ignition circuit according to any of claims 1 to 6, characterized in that the combustion engine (2) is a four-stroke engine.

8. Ignition circuit according to any of claims 1 to 7, characterized in that the control circuit (19) is designed to detect a drop in rotational speed (Δn) occurring in a plurality of combustion cycles of the combustion engine (2) and is designed to derive the value of the specified drop in rotational speed (ΔnG) from a mean of the detected values of the drop in rotational speed.

9. Method for switching over an ignition time in a spark-ignition combustion engine (2) having a spark plug (21), which is arranged in a combustion chamber (4) of the combustion engine (2), and a piston (6) which compresses a fuel / air mixture, supplied to the combustion chamber (4), on its way from a bottom dead centre (UT) to a top dead centre (OT), wherein the supplied combustion air is apportioned with a control element (27) via an intake channel (7) depending on the operating state of the combustion engine, characterized in that, in the idling situation of the combustion engine (2), a drop in rotational speed (Δn) occurring in the compression stroke is detected, in that the detected drop in rotational speed (Δn) is compared with a specified drop in rotational speed (ΔnG), and in that, when the detected drop in rotational speed (Δn) exceeds the specified drop in rotational speed (ΔnG), a changeover is made to an ignition time for the acceleration situation.

10. Method according to claim 9, characterized in that the drop in rotational speed (Δn) is detected in a crankshaft angle range (30) of up to 300°.

11. Method according to claim 9 or 10, characterized in that the drop in rotational speed (Δn) is detected in a crankshaft angle range (30) of at least between the bottom dead centre of the piston (6) and at least the top dead centre of the piston (6).

12. Method according to claim 9, characterized in that the value of the specified drop in rotational speed (ΔnG) is formed from a mean of the sum of preceding drops in rotational speed (Δn).

13. Method according to any of claims 9 to 12, characterized in that the drop in rotational speed is detected at least in a rotational speed range of 1500 1 / min to 3500 1 / min.