DE1405792B2 - Control device for an electromagnetically excitable motor vehicle clutch - Google Patents

Description

The invention relates to a control device for an electromagnetically excitable clutch of Motor vehicles, the magnet winding of which to transmit one with the internal combustion engine speed increasing torque electrical energy in pulses by means of a pulse length determining, is supplied with a monostable multivibrator equipped with an input and output transistor, one with its primary winding to the output transistor and with its secondary winding im positive feedback meaning to the transformer connected to the input transistor and contains synchronous brought into its unstable operating state with the crankshaft revolutions of the internal combustion engine is supplied with power to the magnet winding and after a pulse length resulting from the inductance of the transformer automatically returns to its stable starting position simultaneous blocking of the energy supply for the magnet winding returns.

Such control devices are the subject of the earlier patent 1 405 788. They offer the advantage that after a minimum speed selected above the idling speed of the internal combustion engine has been exceeded the mean value of the magnetic excitation over time increases rapidly with increasing speed, but nevertheless causes a soft clutch insert.

However, it has been found necessary to adjust the idle speed when starting at low temperatures and consequently also to increase the speed at which the clutch disengages automatically. This increase should be effective until a specified minimum value of the operating temperature is reached and then switched off automatically.

To solve this problem, it is provided in a control device of the type described at the beginning, that according to the invention with the primary winding of the transformer a temperature-dependent resistor is connected in series, which is in heat-conducting connection with the internal combustion engine.

The drawings show an exemplary embodiment which is explained in more detail in the following description the control device according to the invention. It shows

Fig. 1 shows a four-cylinder four-stroke internal combustion engine with a magnetic particle clutch and a electronic control device in a schematic representation,

F i g. 2 shows a longitudinal section through the magnetic particle coupling according to Fig. 1,

F i g. 3 a circuit diagram of the control device provided for actuating the magnetic particle clutch,

F i g. 4 to 6 are diagrams for explaining the operation of the control device.

The internal combustion engine 10 intended to operate a motor vehicle (not shown) is operating together with a high-voltage ignition system, the distributor of which is indicated at 11. To the internal combustion engine the housing 12 is flanged to a magnetic particle clutch, which is supported by a In Fig. 3 shown in more detail electronic control device is automatically engaged when the speed of the internal combustion engine to a predetermined, lying above the idle speed Minimum value is increased beyond. A conventional one is attached to the housing of the magnetic particle clutch Gearbox 15 flanged, the individual gears of which are optionally engaged by a shift lever 16 can be.

Like F i g. 2 can be seen better, the magnetic particle clutch in its housing 12 contains a composed of two shells 21 and 22, firmly connected to the crankshaft 23 of the internal combustion engine, at the same time the flywheel of the internal combustion engine forming the cage and one on an output shaft 24 seated runner 25, the several adjacent grooves on its outer surface 26 has. Located between the outer surface of the rotor and the cylindrical inner wall 28 of the cage In reality there is an annular gap about 1 to 2 mm wide. Into the cavity between the shells 21 and 22 of the cage and the rotor 25 is filled with magnetizable powder 30, which when running Internal combustion engine is thrown into that annular gap as a result of centrifugal force. In each other facing end faces of the shells 21 and 22 is each one widening towards the axis of rotation Pierced annular groove 31 into which a magnet winding 35 is inserted. The ends of the magnet winding 35 are each guided to one of two slip rings 36 and 37, which are isolated from each other on the outer Front face of the shell 22 sit. On the inside of the housing 12 are also two against each other insulated abrasive brushes 38 and 39 attached. Each of these two grinding brushes works with one of the two slip rings together. Lines lead from the grinding brushes to one on the outside of the housing 12 seated junction box 40, through which the magnetic winding 35 of the magnetic particle coupling to the electronic Control device 13 is connected. The electronic control device supplies the Torque transfer required electrical energy in the form of roughly rectangular current pulses, one of which in FIG. 1 is indicated at 41. The input side is used to generate these current pulses the electronic control device 13 with the distributor 11 of the high-voltage ignition system a cable 42 is connected, via which a short control pulse indicated at 43 with each ignition process the control device shown in more detail in FIG. 3 is supplied.

The electronic control device is made up of a device provided for operating the high-voltage ignition system, Battery, not shown, is supplied with power via a negative line 50 and a positive line 51.

The primary winding 52 of an otherwise not shown ignition coil of the high-voltage ignition system is arranged in series with the interrupter arm 53 of an ignition interrupter accommodated in the housing of the distributor 11 between the negative line 50 and the positive line 51. The interrupter arm works together with a fixed contact 54 connected to the negative line 50 and is lifted twice from the fixed contact 54 by a four-humped cam 55, which is also accommodated in the distributor housing, with each revolution of the internal combustion engine. Whenever the interrupter arm 53 interrupts the ignition current flowing through the primary winding 52, a voltage U is generated at the primary winding, which is fed to a multivibrator 56, described in more detail below, via a resistor 60, a capacitor 61, a line 62 and a rectifier 63. There are two in series across the capacitor 61

connected, between the positive lead 51 and the negative lead 50 resistors 64 of 10000 ohms and 65 of 5000 ohms connected. A capacitor 66 of approximately 0.02 μΡ leads from the resistor 60 to the positive line 51 and, in conjunction with the resistor 60, the capacitor 61 and the rectifier 63, serves to deliver a narrow control pulse 43 for the multivibrator 56, which occurs with each interruption opening.

In detail, the multivibrator 56 contains an input transistor 70 and an output transistor 71 as well as a transformer acting as a timing element, the primary winding 72 of which, comprising approximately 1000 turns, is connected to the collector of the output transistor 71. Its secondary winding 74, which sits on the common transformer core 73 and has about 8000 turns, is connected with one of its winding ends directly to the base of the input transistor connected to the rectifier 63 and is parallel to a resistor 75 of 2000 ohms, to which the other winding- ■ - r end is connected via a rectifier 76. The resistor 75 is connected together with the rectifier 76 to a resistor group of three resistors, namely a fixed resistor 78 of 7000 ohms connected in series with a temperature-dependent resistor 77 and a resistor 79 of 2000 ohms lying parallel to these two. This resistor group forms part of a voltage divider, since it is connected to the negative line 50 via an adjustable resistor 80 of approximately 10 kilohms and a fixed resistor 81 of 3 kilohms.

Between the input transistor 70 and the output transistor 71 of the multivibrator, a switching transistor 82 is provided, which is connected with its base on the one hand via a resistor 83 of 3000 ohms to the positive line and on the other hand via a protective resistor 84 of 8000 ohms and a rectifier 85 that is permeable in this direction the movable switching arm 86 of a switching contact is connected, the fixed contact 87 of which is connected to the negative line. This switching contact ¹ Jj reaches its closed position as soon as the one shown in FIG. 1 hand button of the shift lever 16 indicated at 115 for engaging or disengaging a gear of the gear! drive 15 is detected. It automatically returns to the open position shown as soon as the shift lever is released.

Between the rectifier 85 and the resistor 84 branches off a connection line to a 'switch arm 90 ■ one with a fixed counter contact 91 connected to the negative line 50; Switch contact, which is in its closed position at speeds below about 15 km / h, but at higher speeds, for example, by fleeing, not shown! weights is held in its open position.

For the mode of operation of the multivibrator 56 described in more detail below, it is initially assumed that the speed-dependent switching contact 90/91 is closed and the base of the switching transistor 82 is therefore connected to a potential of about 10 V established by the resistor 83 forming a voltage divider with the protective resistor 84 is when the operating voltage between the positive line 51 and the negative line 50 is 12 volts. The switching transistor 82 connected with its emitter to the collector of the input transistor 70 is therefore in the same operating state as the input transistor 70 as long as the switching contact 90/91 is closed.

When the internal combustion engine is at a standstill and - while it is running - immediately before the next ignition process, the input transistor 70 is in its current-conducting state, since its base is connected to one opposite the positive line 51 and

ίο the lower emitter connected to this Potential lies.

As soon as the cam 55 of the Zündunterbrechers after starting the internal combustion engine the interrupt arm 53 lifts from its contact 54 in the in Fig. 4 at t _Q indicated time and thereby / interrupts the current flowing since then through the primary winding 52 of the ignition coil ignition current arises due to the inductance of the Primary winding a voltage U by which the base of the input transistor is made strongly positive compared to the positive line and the input transistor is therefore blocked immediately. Then the switching transistor 82 is also de-energized, and the output transistor 71 immediately goes into its fully current-conducting operating state. As a result of the inductance of the primary winding 72 of the transformer, the collector current J _{c of} the output transistor 71 cannot immediately rise to the full final value.

Rather, it grows with a time constant τ = - = -,

where L is the inductance of primary winding 72 and R is the total resistance in the emitter-collector circuit of the output transistor. The increasing collector current induces in the secondary winding 74 a voltage that decreases from a high initial value with increasing collector current, which _{causes a current / s} flowing through the resistor 75 and the rectifier 76, through which the base of the input transistor 70 is kept positive even when the control pulse 43 generated by the ignition coil and supplied via the rectifier 63 has already disappeared. As a result, the input transistor is kept in its blocking state until the collector current J _{c of} the output transistor has approached its final value and therefore the voltage drop U _s generated at resistor 75, shown in FIG. 4, has decreased to a value that is approximately the same how the bias voltage U _v generated at the resistor 79 is. The output transistor 71 then flips back into its blocking state _{at time i 10.} As long as the output transistor 71 is in its current-conducting state, the transistor 93 coupled to its emitter is also current-conducting and for this period also brings the power transistor 94 into its fully current-conducting working area. Then a magnetizing current can flow through a resistor 95 of about 10 ohms connected to the collector of the power transistor 94, the magnet winding 35 of the magnetic particle coupling and two rectifiers 97 and 98 connected in series with this, the duration of which corresponds to the breakover duration of the multivibrator.

The faster the internal combustion engine runs, the faster these square-wave current pulses follow, the in F i g. 1 are indicated schematically at 41, one on top of the other and increase the mean value over time the magnetic excitation effective in the iron parts of the magnetic powder clutch. Once this

Excitation exceeds a minimum value, the magnetic particle clutch can generate a torque of the To transmit the internal combustion engine to the gearbox, so that the vehicle, when a gear is engaged, when the engine speed increases further.

However, when starting up at low temperatures it has been found necessary to ensure that that the magnetic particle clutch only engages automatically at significantly higher speeds, because otherwise there is a risk that the internal combustion engine which is not yet at operating temperature will run irregularly or even stop. Some internal combustion engines are also equipped with an automatic device Increase the idle speed equipped, which remains effective as long as the internal combustion engine has not yet reached a certain minimum temperature of around 40 ° C.

In order to relocate the clutch to the high speed area when the internal combustion engine is cold, a series-parallel connection of three resistors is provided in the collector circuit of the output transistor 71 , namely a thermistor 100 which is in heat-conducting connection with the cooling water of the internal combustion engine and a fixed resistor 101 in series with it of about 20 ohms and a resistor 102 lying parallel to this of 150 ohms. When the internal combustion engine reaches its operating temperature, the resistance of the thermistor 100 is small, but when it is cold it is high. It is thereby achieved that the decisive factor for the inductance of the primary winding 72

Time constant τ = - _D - with increasing operating temperature

temperature is greater and therefore a greater period of time is required until the output transistor 71 made conductive by one of the control pulses 43 can return to its blocking state. The at 41 in F i g. 1 indicated rectangular current pulses are therefore longer, the warmer the internal combustion engine becomes.

The switching arrangement just described interacts with an arrangement which is also described in more detail below and which also influences the pulse duration. This includes a transistor 105, the emitter of which is connected to a potentiometer 106. Its collector is at the connection point of the adjustable resistor 80 and the fixed resistor 81. The base of the transistor 105 is connected via a resistor 107 of 30 kilohms and a capacitor 109 of 1 μΡ connected in parallel to this. The capacitor 109 is bridged by two rectifiers 110 and 111 connected in series. Their common connection point 112 is connected via a capacitor 113 of 1 liF to the collector of the transistor 93 lying between the output transistor 71 of the multivibrator 56 and the power transistor 94 . The negative electrode of the rectifier 111 lies together with an assignment of the capacitor 109 and one end of a resistor 108 on a line section P, which connects a fixed resistor 114 of approximately 200 ohms connected to the positive line 51 with three resistors, of which the one indicated by 116 as NTC thermistor is formed and is in thermally conductive connection with the internal combustion engine. The fixed resistor 117 connected in series with the thermistor 116 has about 300 ohms, the third resistor 118, which is parallel to these two resistors, is set to 40,000 ohms. The line section P is therefore at a potential which becomes lower the higher the operating temperature of the internal combustion engine rises.

This arrangement is used to use a preceding square-wave current pulse 41 to increase the duration of the subsequent current pulse when the next current pulse follows quickly at high speed, or to reduce it when the speed is low. As already explained above, the input transistor 70 enters its blocking state for each of the control pulses 43 supplied via the rectifier 63 and derived from the primary winding 52 of the ignition coil and only becomes conductive again when the induction in the secondary winding 74 of the transformer 72 to 74 The voltage has dropped so far that the voltage drop U _s occurring across the resistor 75 no longer outweighs the bias voltage U _v across the resistor 79. However, this bias voltage is changed by the collector current of transistor 105 in the time periods between a preceding and a subsequent control pulse. In detail, reference is made to the in FIG. 5 indicated the diagram.

In Fig. 5 it is assumed that at the time t _x the current flowing through the transistor 93 , which is indicated by the line 120 , ends with a steep trailing edge because the transistor 93 at this moment through the output transistor 71, which also flips back into its blocking state is blocked. Up to this point in time, the transistor 93 was fully conductive. The capacitor 113 could therefore, because on the one hand it via the very low-resistance fixed resistor 114 and the rectifier 111, on the other hand via a low-resistance resistor 119 in the emitter circuit of the transistor 93 and the current-conducting connection between the emitter of the transistor 93 and the base of the power transistor 94 with the common positive line 51 was connected, discharged except for a residual voltage that is unimportant for the mode of operation. At the time t _x of passing into its restricted transistor 93 brings the occupancy of the capacitor 113 connected to its collector connected to the potential of the negative conductor 50. Since the capacitor, however, can not be charged via the rectifier 111, its charging current must through the resistor 108 and the capacitor 109 and the rectifier 110 go, at which it finds practically no resistance. The connection point Q between the resistors 107 and 108 therefore immediately assumes a very low potential and brings the transistor 105 into a fully current-conducting state, while the capacitor 109 is initially still uncharged, but is now connected to voltage. From the time t _t onwards, the capacitor 113 can continuously be charged via the resistor 108 and the resistor 107 in series with the emitter-base path of the transistor 105 , so that the potential q of the connection point Q corresponds to the one shown in FIG. 5 is indicated by the line 121 course. As the potential at connection point Q increases, the current of transistor 105 passing through resistor 81 slowly decreases, so that bias voltage U _v at resistor 79 can slowly increase in the manner indicated by line 123. If now at time f, the input transistor 70

is blocked by the next following control pulse 43 and thereby makes the output transistor 71 conductive, a current that increases exponentially due to the inductance of the primary winding begins to flow through the primary winding 72. The magnetic field generated by the current induces a voltage in the secondary winding 74 of the transformer, which causes the voltage drop U _s indicated by a line 125 at the resistor 75 and keeps the input transistor 70 blocked until the voltage drop U _s has dropped to a value that is smaller than the then existing value of the slowly increasing bias voltage U _v . In Fig. 5 the lines 123 and 125 intersect at the point in time denoted by t _s. At this point in time the input transistor 70 becomes conductive again and blocks the output transistor 71 and thus also the subsequent transistor 93 and the power transistor 94. The square-wave current pulse is therefore ended again at the point in time ί _Ά. Its duration T ₁ extends from time t ₂ to time t _v while the pulse repetition time is indicated _{by Z 1.} At time / ₃ , the game described is repeated anew, with _{the next control pulse 43 blocking input transistor 70 at time i 4} for generating the subsequent square-wave current pulse, not shown in the drawing.

In the previous considerations it has been assumed that the control pulses 43 follow one another relatively quickly. If, on the other hand, the internal combustion engine is running more slowly, the control pulses also follow one another more slowly. At a very low speed, the next control pulse is therefore not generated until time t _{5, for example.} Then the voltage drop U _s running after the line 125 appears in the drawing strongly shifted to the right, as is indicated by the broken line 126 . However, since the bias voltage U _v has risen considerably as a result of the increasing discharge of the capacitor 109 , the voltage drop U _s falls below the value of the bias voltage U _v already at time i ₆ , which is indicated by the intersection of lines 123 and 126 . The rectangular current pulse generated in this way is therefore very short and then has the course indicated by broken lines. It begins at time t ₅ . Its associated pulse train time, which is determined by the time interval between two successive ignition processes, is indicated in FIG. 5 by Z i. From this diagram it can therefore be clearly seen that at low speed a very short pulse duration T ₂ is associated with a long pulse repetition time Z _2. At high speeds, on the other hand, the pulse repetition time Z _{1 is} significantly shorter, while the pulse duration T _{1 is} considerably greater, so that the desired strong extension of the pulse duration occurs with increasing speed. This enables the clutch torque to be so low at idle speed that the vehicle does not push and the full clutch torque is transferred even if the driving speed is not too high, so that a sufficiently hard clutch application can be achieved.

In Fig. 6, a hyperbola 130 indicates how long the pulse duration T of the square-wave current pulses in the magnet winding must be at least if the clutch is to transmit a minimum torque required for rolling the vehicle. Since the current pulses follow one another more quickly with increasing speed η , the required time average value is already achieved with shorter current pulses than at lower speeds. It is therefore necessary to use the _{basic length of the current pulses indicated by T 01} , which applies to an idling speed n ₀₁ = 600 rpm, which is valid when the internal combustion engine is warm, when the internal combustion engine is cold

ίο and an idling speed increased to n ₀₂ = 1000 rpm to be greatly shortened to a length indicated _{by T 02.} At the same time, when the internal combustion engine is cold, the onset of the increase in the pulse duration with the speed must be shifted from the speed A ₁ to the higher speed A _2.

The pulse shortening required when the engine is cold is achieved not only by the thermistor 116 in the base circuit of the transistor 105 but also by the thermistor 100 in the collector circuit of the output transistor 71, which assumes a high resistance at low engine temperatures and then the time constant τ of the inductive timing element of the value τ = T ₁ shortened to τ = τ ₂ = 0.4 _{T 1} when the internal combustion engine is warm.

Like F i g. 4 shows, the voltage i / _s drops more rapidly at a low internal combustion engine temperature and reaches the value of the bias voltage at the resistor 79, which has been raised to U _V2 by the thermistor 116, much earlier.

In this way, by choosing the temperature profile of the resistor group comprising the resistors 101 and 102 and the thermistor 100 , the resistance group shown in FIG. 6 parallel to the "axis running curve is shifted against smaller or larger values T of the basic length of the current pulses, while with the resistor group located in the base circle of transistor 105 and comprising thermistor 116 , that operating point A can be set from which the in multivibrator 56 generated square-wave current pulses can be greatly lengthened as the speed increases above the idle value.

When changing gears, it is important that the clutch is released quickly and the torque transmission is interrupted quickly. For this purpose, the shift arm 86 coupled to the gearshift lever 16 applies the base of the input transistor 70 to the negative line 50 via a protective resistor 135 and a rectifier 136 which is permeable in this direction.

This makes the base so negative that the input transistor 70 is no longer through the Rectifier 63 incoming control pulses 43 can be blocked.

Then, as long as the switch contact 86, 87 is closed, the output transistor 71 remains in his Blocked state and can no longer use the power transistor 94 to deliver current pulses to the Induce magnet winding 35 of the clutch, because at the same time via the closed switching contact 86, 87 and the rectifier 85, which then comes into effect, the switching transistor 82 in current-conducting State is maintained.

Thus in this transition of the power transistor caused by the closing of the switching contact 86, 87 94 in the blocking state, the magnetic field present in the iron parts of the coupling completely is reduced, the winding end connected to rectifiers 97 and 98 is the

A resistor 138 is provided for the winding 35 and the positive line 51, which forms a voltage divider with the two rectifiers mentioned, while the other end of the winding is connected to the resistor 95 connected to the collector of the power transistor 94. This winding end also relates to a current-permeable in that direction rectifier 139 to the switching contact 86,87 in connection that connects the coil end to the negative line 50 upon detection of the shift lever, so that an opposite direction to the induced from the power transistor current surges J _k flowing degaussing J _{e can} adjust. This cancels the residual magnetic field in the magnetic winding of the clutch resulting from the last magnetizing current surges, so that this is completely released immediately after the current pulses caused by the multivibrator 56 have disappeared. As soon as the shift lever 16 is released again and the shift arm 86 returns to its open position, the multivibrator 56 can either cause the power transistor 94 to emit an uninterrupted current to re-engage the clutch, or in the manner described above, depending on the then prevailing driving speed cause it to deliver a square-wave current pulse 41 with each ignition process, the length of which increases with increasing speed in the direction shown in FIG. 6 increases manner shown.

Since it is desirable when upshifting from a lower to a higher gear that the clutch engages as quickly as possible after releasing the shift lever 16 so that the motor vehicle can be accelerated quickly, but on the other hand, when downshifting to utilize the braking force of the internal combustion engine, the clutch is as soft and as soft as possible starts jerk-free, a switch with a switching arm 140 and a fixed mating contact 141 is placed in parallel with the resistor 95. The switching arm 140 is coupled to the accelerator pedal (not shown) of the motor vehicle in such a way that it moves into its closed position as soon as the accelerator pedal is depressed to accelerate the motor vehicle. Then he short-circuits the resistor 95 so that the clutch is at full battery voltage, while when the accelerator pedal is released, the resistor 95 comes into effect and limits the clutch current and thus the magnetic field in the clutch. In this way, a clutch behavior adapted to the respective driving conditions is achieved in a simple manner.

Claims (8)

Claims: 55 1. Control device for an electromagnetically excitable clutch of motor vehicles, the magnet winding of which is fed to the transmission of a torque increasing with the engine speed with electrical energy in pulses by means of a monostable multivibrator which determines the pulse length and is equipped with an input and output transistor, one of which is supplied with its primary winding to the output transistor and contains a transformer connected to the input transistor with its secondary winding in the positive feedback sense and is brought into its unstable operating state synchronously with the crankshaft revolutions of the internal combustion engine, while the magnet winding is supplied with current and automatically into its own after a period of time determined by the inductance of the transformer, which results in the pulse length stable starting position returns with simultaneous blocking of the energy supply for the magnet winding, characterized in that the primary winding (7 2) a temperature-dependent resistor (100) is connected in series, which is in a thermally conductive connection with the internal combustion engine (10). 2. Control device according to claim 1, characterized in that a transistor (105) is provided in the base circuit of the input transistor (70) belonging to the multivibrator, the base of which is connected to a voltage divider which has at least one temperature-dependent which is in thermally conductive connection with the internal combustion engine Contains resistor (116). 3. Control device according to claims 1 and 2, characterized in that in a connecting line between the base of the transistor (105) and the voltage divider (114, 116, 117, 118) a parallel circuit of a resistor (108), a capacitor (109 ) and two rectifiers (110 and 111) lying in series with one another, from the connection point (112) of the two rectifiers a capacitor (113) leads to a transistor (93) connected to the output of the multivibrator. 4. Control device according to claims 1 to 3 with a multi-stage gear connected to the electromagnetically excitable clutch, the gear shift lever of which is coupled to an electrical switch which is in its working position when it is detected, characterized in that the switch (86, 87) is connected on one side to a of the two supply lines of an operating current source for the magnet winding (35) of the clutch is connected and is connected to the base of the input transistor (70) of the multivibrator (56) connected at its emitter to the other supply line of the operating current source via a rectifier (136) , wherein the forward direction of the rectifier is in the same sense as the emitter-base path of the input transistor (70). 5. Control device according to one of claims 1 to 4, characterized in that the magnet winding (35) of the coupling is connected with one of its winding ends to a voltage divider which is above a current source used to supply power to the magnet winding, and that the other end of the winding is at an arbitrarily actuatable one, releasing the clutch effecting switch (86, 87) is connected to the magnet winding for the purpose of demagnetization in a current device opposite to the operating current to the voltage divider allowed to connect. 6. Control device according to claims 1 and 5, characterized in that the voltage divider at least one operated in the forward direction, such as a highly doped p-n-germanium semiconductor body containing rectifier (97, 98) and that when the closed Switch (86,87) the magnet winding (35) in one Shunt path to this rectifier is. 7. Control device according to claims 1, and 6 with a power transistor serving to power their magnet winding, characterized in that between the connection point of the magnet winding (35) and the Power transistor (94), a rectifier (139) leading to the switch (86, 87) is provided. 8. Control device according to claims and 7, characterized in that between the connection point of the magnet winding (35) and the rectifier (139) to the collector electrode of the power transistor (94), a resistor (95) is provided, to which a when stepping down of the gas pedal of the internal combustion engine in its closed position switch (140, 141) is parallel. 1 sheet of drawings