WO1993014387A1

WO1993014387A1 - Determining top dead centre

Info

Publication number: WO1993014387A1
Application number: PCT/GB1993/000044
Authority: WO
Inventors: David John Howard Peacock
Original assignee: David John Howard Peacock
Priority date: 1992-01-11
Filing date: 1993-01-11
Publication date: 1993-07-22
Also published as: GB2277995A; GB9200542D0; GB9413844D0; GB2277995B

Abstract

A method of accurately determining the top dead centre position of a piston (11) and a cylinder (12) of a reciprocating type of internal combustion engine (10) when inoperative which has a crankshaft (13) driven in use by the piston (11) via a piston rod (14), the method comprising the steps of connecting to a space (28) in the cylinder (12), a pressure sensitive switch (30, 31) of the kind which is adapted to change its operating state in response to sensing a pressure above or below a threshold pressure to open or close an electrical circuit, the method further comprising connecting the pressure sensitive switch (30, 31) in an electrical circuit which includes an indicating means (45, 46; 45a^_) to indicate when the operating state of the pressure sensitive switch (30, 31) changes, turning the crankshaft (13) manually until the indicating means (45, 46; 45a^_) indicates no change of operating state of the pressure sensitive switch (30, 31) for a corresponding movement of the crankshaft (13).

Description

Title: Determining top dead centre

Description of Invention

This invention relates to a method of accurately determining the top dead centre of a piston in a cylinder of a reciprocating type of internal combustion engine in which a crankshaft of the engine is manually turned whilst the engine is inoperative.

Methods of determining top dead centre are known which sense pressure changes in a volume of gas due to movement of the piston within the cylinder, and determine top dead centre as the position in which an incremental movement clockwise or anti clockwise of the crankshaft, does not result in any detected change of pressure.

For example, GB Patent specification No. 1233475 discloses the use of a manometer to detect such pressure changes, the pressure changes causing a piston within the manometer visibly to move in response.

However, such arrangements do not enable sufficiently accurate determination to be made of the top dead centre position of the piston particularly in a modem high performance engine.

According to the invention, I provide a method of accurately determining the top dead centre position of a piston in a cylinder of a reciprocating type of internal combustion engine when inoperative, which has a crankshaft driven in use by the piston via a piston rod, the method comprising the steps of connecting to a space in the cylinder, a pressure sensitive switch of a kind which is adapted to change its operating state in response to sensing a pressure above or below a threshold pressure to open or close an electrical circuit, the method further comprising connecting the pressure sensitive switch in an electrical circuit which includes an indicating means to indicate when the operating state of the pressure sensitive switch changes, turning the crankshaft manually until the indicating means indicates no change of operating state of the pressure sensitive switch for a corresponding movement of the crankshaft.

Thus utilising a simple pressure sensitive switch with a sufficiently high or low threshold pressure, a more accurate determination of top dead centre can be made than with hithertofore known methods.

In the prior art methods, the reasons why the methods are inherently inaccurate, are first that by the nature of the apparatus used , the pressure as the piston is moved, has to have a mechanical effect on a movable part of the pressure change indicator. The mass of the movable part and frictional effects, in known arrangements, causes inaccuracies.

Secondly, accuracy of the observation of the movement of the movable part is dependent upon operator skill.

Third, such apparatus require movement of a relatively large volume of air compared with the volume of the space above the piston when the piston is at its top dead centre position, and the additional air volume of the pressure change indicator tends to act as a buffer so that the piston may be moved significantly from or through the top dead centre position without effecting a sufficient pressure change in the large air volume to cause the movable member visually to move.

All of these disadvantages are overcome in the present invention which can utilise a proprietary kind of pressure sensitive switch to open or close an electrical circuit in response to pressure changes. There is virtually no mechamcal resistance for the pressure to overcome, and the method can be carried out by a relatively unskilled operator. Further, because such a pressure switch is inherently more sensitive than any purely mechanical arrangement can be, even if the volume of air upon which the piston acts is increased significantly when the pressure sensitive switch is connected to the space above the piston, a more accurate determination of top dead centre can be made.

The invention will now be described with reference to the accompanying drawings, in which: FIGURE 1 is an illustrative view of part of an apparatus for performing the method in accordance with the invention,

FIGURE 2 is an illustrative view of a further part of the apparatus for performing the invention,

FIGURE 3 is an illustrative view of a circuit for use with the apparatus of figures 1 and 2, and,

FIGURE 4 is an illustrative view of a simplified circuit for use in an alternative apparatus for performing the invention.

Referring first to figure 1, parts of an internal combustion engine 10 are shown, which is of the reciprocating type having at least one piston 11 slidable in a respective cylinder 12 and driving a rotatable crankshaft 13 via a piston rod 14 secured to the piston 11 and to a radially extending lobe formation 15 of the crankshaft 13.

The crankshaft 13 has at one end, a formation 16, which is shown to be hexagonal in the example, which is adapted to receive a spanner 17 or other tool, to enable the crankshaft 13 to be rotated manually whilst the engine is inoperative.

The crankshaft 13 carries a pulley wheel 18 upon which is received a drive belt 19 which is adapted to transmit drive from the crankshaft 13 to an alternator 20 as is well known in the art.

The internal combustion engine 10 may be of the diesel type having a glow plug to facilitate cold starting of the engine 10, or a spark ignition type of engine having a spark plug to initiate combustion of fuel during a working cycle of the engine.

In either case, a cylinder 12 will have a threaded opening 25 to receive the glow plug or spark plug as appropriate. In the present example, the opening 25 has inserted therein, one end 26 of an adaptor 24, the one end 26 being threaded and receivable in fluid tight manner in the opening 25 from the cylinder 12. The adaptor 24 is connected at its other end, as hereinafter described with reference to figure 2, to pressure sensitive switches, the adaptor 24 having an internal passageway 27 to permit of communication between a space 28 in the cylinder 12 above the piston 11 and the pressure sensitive switches.

It will be appreciated that as the crankshaft 13 is rotated manually to bring the piston 11 towards a top dead centre position, the piston 11 will act upon air in the space 28 above the piston. With a glow plug or spark plug inserted in an opening 25, upward movement of the piston 11 would cause compression of the air, or air fuel mixture in the space 28. With the one end 26 of the adaptor 24 inserted, upward movement of the piston 11 will act to move a volume of air from within the space^~28 into the passageway 27 of the adaptor 24, towards the pressure sensitive switches.

Downward movement of the piston 11 will cause a volume of air to be drawn from the switches into the space 28.

However, when the piston 11 is at a top dead centre position, an incremental movement of the crankshaft 13 clockwise or anti-clockwise, will fail to cause a corresponding movement of the piston 11 sufficient to move a sufficient a volume of air from space 28 into the adaptor passageway 27 or vice versa, to actuate the pressure sensitive switches.

Referring now to figure 2, the adaptor 24 comprises, at its other end 28, a T piece 29, the passageway 27 extending through the limbs of the T piece 29 to one or other of a pair of pressure sensitive switches indicated at 30 and 31.

Each of the switches 30,31, are of a proprietory type and are of substantially similar construction, each comprising therein, a diaphragm.

In the case of switch 30, any increase of pressure within the passageway 27 caused by upward movement of piston 11 will cause the diaphragm to be moved against a switch element to cause completion of an electrical circuit in which the pressure sensitive switch 30 is deposed.

In the case of pressure sensitive switch 31, any decrease in pressure in the passageway 27 caused by downward movement of piston 11 will cause the diaphragm to move a switch element to cause completion of an electrical circuit in which the pressure sensitive switch 31 is deposed.

Suitable types of pressure sensitive switch 30 and 31 are sold by Micro Pneumatic Logic Incorporated under the designations MPL 501. Such pressure sensitive switches comprise a fixed electrical contact, and the movable switch element carrying another contact, such that movement of the diaphragm as hereinafter explained, completes an electrical circuit between the contacts and hence between respective electrical terminals 35 and 36 in the case of pressure switch 30, and 37 and 38 in the case of pressure switch 31.

The pressure sensitive switches 30 and 31 may each have an adjustment screw 39,40, respectively, so that the threshold pressure or vacuum required to cause movement of the diaphragm sufficient to move the respective switch element to complete the electrical circuit, can be adjusted, within limits.

Preferably, the switches 30 and 31 are adjusted such that switch 30 is responsive to close the electrical circuit between contacts 35 and 36 in the event of a pressure in passageway 27 of the adaptor of between 0.0093 and 0.1395 cm of mercury and in the case of pressure switch 31, a negative pressure in passageway 27 in this range.

Thus the pressure sensitive switches 30 and 31 are each extremely sensitive to changes of pressure occurring in the passageway 27 as a result of movement of the piston 11 in its cylinder 12.

Referring now to figure 3, there is shown an example of a circuit for use with the apparatus described with reference to figures 1 and 2.

The electrical circuit including switches 30 and 31 further comprises a pair of indicating means 45 and 46. It can be seen that when the circuit through pressure sensitive switch 30 is completed, light emitting diode 45 will become lit. Also when the electrical circuit through pressure sensitive switch 31 is completed, light emitting diode 46 will become lit.

Hence as the piston 11 is moved in incremental steps towards its top dead centre position, for each incremental movement, light emitting diode 45 will light until the piston 11 reaches its top dead centre position. If the incremental movements are too large and the piston 11 is moved through its top dead centre position and hence away from its top dead centre position, pressure sensitive switch 31 will be closed resulting in light emitting diode 47 becoming lit. Hence the direction of manual rotation of the crankshaft can be reversed and the incremental rotational steps made smaller.

When the piston 11 is substantially at its top dead centre position such that a (small) incremental movement of the crankshaft 13 in either direction is insufficient to cause pressure sensitive switches 30 and 31 to be closed, neither light emitting diode 45 or 46 will become lit. The electrical circuit comprises further light emitting diodes 47 and 48 which are each driven respectively by a transistor 49,50, from a logic controller 51. In the event that an incremental movement of the crankshaft 13 fails to light either light emitting diodes 45 or 46. respectively, light emitting diodes 47,48 will be lit.

The circuit includes a power supply 52 which may be a battery or transformer or the like, an on/off switch 53, and various resistances to bias the transistors 49 and 50 and the logic controller 51, and to provide the necessary voltage drops across light emitting diodes 45,46,47 and 48.

It will be appreciated that the circuit shown in figure 3 is only an example of a suitable circuit which may be used. If the light emitting diodes 47 and 48 are not required, the transistors 49 and 50, and logic controller 51 could be omitted.

The apparatus described above, may be modified.

For example, in place of two pressure sensitive switches 30 and 31 to detect both pressure increases and decreases in the passageway 27 communicating with the space 28 above the cylinder, if desired, only one pressure sensitive switch may be provided. For example, if a pressure sensitive switch such as switch 30 only were provided, a simplified electrical circuit such as shown in figure 4 may be provided. In this arrangement, for each incremental movement of the crankshaft 13 towards the top dead centre position of the piston 11 only, the corresponding pressure increase in the space 28 above the piston 11 is communicated via the passageway 27 to the switch 30 and this will cause completion of an electrical circuit and an otherwise unlit light emitting diode 45a. to be lit. When a very small incremental movement of the crankshaft 13 fails to cause light emitting diode 45a. to be lit, an operator thus knows that the piston 11 is at its top dead centre position.

Alternatively, one pressure sensitive switch only such as switch 31 may be provided, only sensitive to a decrease in pressure in the space 28 above the piston, such that only movement of the piston 11 away from its top dead centre position causes a circuit within the pressure sensitive switch to be completed. In each case, a light emitting diode will indicate to an operator by not lighting, where top dead centre occurs.

In place of switches 30,31 which each complete an electrical circuit when sensing a pressure above or below the threshold pressure set on the switch 30,31, if desired, an alternative type of pressure sensitive switch may be provided in which an electrical circuit therethrough is broken when a pressure above or below a threshold pressure is sensed. In such an arrangement, an alternative kind of electrical circuit to that described with reference to figures 3 or 4 would need to be provided such that an indicating means such as a light emitting diode becomes lit or extinguished upon an incremental movement of the crankshaft 13, to indicate to an operator top dead centre position when the electrical circuit through the respective switch or switches 30,31, is broken.

Further alternatively, one switch 30 or 31 may close an electrical circuit when sufficient pressure or vacuum is sensed, and the other switch opens an electrical circuit, with indicating devices being provided as desired.

In a typical example, the volume of the space 28 in the cylinder above the piston when the piston is at top dead centre position, for a diesel engine is about 20 millilitres or for a petrol engine is about 50 millilitres. Preferably, the gas volume of the passageway 27 including the T piece 29 and within the switches 30,31, is not significant compared with the volume of the space 28 by which I mean that it does not have a gas volume of more than 30% and preferably 15% of the volume of space 28 when the piston is at top dead centre. For example, in a typical application, the dead volume of gas within the or each switch 30,31, to one side of the diaphragm therein, is about 0.20 millilitres and the volume of passageway 27 within T piece 29 about 0.4 millilitres. Passageway 27 of the adaptor 24 has an overall length between T piece 29 and opening 25 of about 1 metre and an internal diameter of 1.5 millimetres giving a volume of about 1.7 millilitres. Hence the total volume of adapter 24 and switches 30,31, and piece 29 adds up to about 2.57 millilitres. For yet more accurate determination of top dead centre position than possible with the apparatus described, a significantly shorter and/or narrow diameter passageway 27 may be provided. For example, the pressure sensitive switches 30,31, may be connected to the space 28 so as not to increase the volume of gas upon which the piston otherwise would act, by more than 10%.

Where only one switch 30 or 31 is provided, again preferably the volume of switch 30 or 31 and the connection to the space 28 does not increase the gas volume of gas on which piston 11 would otherwise act by more than 30%, more preferably not more than 15%, or for more accurate determination, by more than 10%.

Referring again to figure 1, in an alternative arrangement, in addition to, or instead of visible indicating means comprising lights or diodes etc., the indicating means comprises a marking device M which, upon each incremental movement of the crankshaft 13 towards and/or away from its top dead centre position, marks a movable part of the engine 10 in this example the drive belt 19, at a reference position R. However, when the piston 11 is substantially at its top dead centre position such that the pressure switch or switches 30,31, do not respond to an incremental movement of the crankshaft 13, the marking device M is arranged not to provide any marking on the drive belt 19.

Hence the crankshaft 13 can be rotated to bring the piston 11 to a top dead centre position subsequently without needing to utilise the output from the pressure sensitive switches 30,31, by rotating the crankshaft 13 to the position where the markings cease adjacent the reference position R.

In another arrangement, the marking device M may be arranged to mark the drive belt 19 or other movable part of the engine only when an incremental movement of the crankshaft 13 fails to result in a response from the or either pressure sensitive switch 30,31.

In yet another arrangement, the electrical circuit in which the pressure sensitive switch or switches is/are contained, includes a memory means, and a movable part of the engine 10, such as the drive belt 19 is provided with a transducer such as an inductive transducer t clamped to the belt 19, there being a sensor to sense the position of the transducer as the crankshaft 13 is manually rotated, and to provide a signal to the circuit.

Hence the top dead centre position of piston 11 as sensed by switch or switches 30,31, can be memorised and the piston 11 returned to its top dead centre position subsequently by relying upon a signal from the memory, and the circuit recognising the position of the transducer relative to the sensor.

Claims

1. A method of accurately determining the top dead centre position of a piston in a cylinder of a reciprocating type of internal combustion engine when inoperative which has a crankshaft driven in use by the piston via a piston rod, the method comprising the steps of connecting to a space in the cylinder, a pressure sensitive switch of the kind which is adapted to change its operating state in response to sensing a pressure above or below a threshold pressure to open or close an electrical circuit, the method further comprising connecting the pressure sensitive switch in an electrical circuit which includes an indicating means to indicate when the operating state of the pressure sensitive switch changes, turning the crankshaft manually until the indicating means indicates no change of operating state of the pressure sensitive switch for a corresponding movement of the crankshaft.

2. A method according to claim 1 in which the pressure sensitive switch is connected to the space in a manner such as not to increase the volume of the gas upon which the piston otherwise would act, by more than 30% or preferably 15%.

3. A method according to claim 1 or claim 2 in which the pressure sensitive switch is connected to the space in a manner such as not to increase the volume of the gas upon which the piston would otherwise act, by more than 10%.

4. A method according to any one of claims 1 to 3 wherein there is provided an adaptor, the method including inserting one end of the adaptor into an opening of the engine which is connected to the space of the cylinder, and securing the adaptor thereto in generally gas tight manner, securing the adaptor at its other end in generally gas tight manner to the pressure sensitive switch, the adaptor having a passageway between the opening of the cylinder and the pressure sensitive switch to permit of communication therebetween.

5. A method according to any one of claims 1 to 4 wherein the pressure sensitive switch comprises a diaphragm movable in response to a pressure above or below a threshold pressure, the diaphragm acting upon a switch element to complete or disconnect an electrical circuit.

6. A method according to any one of the preceding claims wherein the gas space in the cylinder is connected to each of a pair of pressure sensitive switches, one of the pressure sensitive switches being adapted to change its operating state in response to sensing a pressure above a higher threshold pressure to open or close an electrical circuit, and the other of the pair of switches being adapted to change its operating state in response to sensing a pressure below a lower threshold pressure, each pressure sensitive switch being in an electrical circuit having an indicating means which indicates when the operating state of the respective switch changes.

7. An apparatus for performing the method according to any one of the preceding claims comprising a pressure sensitive switch, means to connect the switch to a space in the cylinder, and electrical control means including an electrical circuit including the pressure sensitive switch, and an indicating means responsive to a change of operating state of the pressure sensitive switch.

8. A method according to claim 7 wherein the indicating means comprising a light which is lit or extinguished in response to the pressure sensitive switch changing its operating state as the piston is moved towards or away from top dead centre position by an incremental movement of the crankshaft, and remains unlit or lit respectively when the crankshaft is moved incrementally with the piston at its top dead centre position.

9. Apparatus according to claim 7 or claim 8 wherein the indicating means comprises a marking device to mark a movable part of the engine which moves as the crankshaft is rotated manually, either in response to the pressure sensitive switch changing its operating state, or in the event that the pressure sensitive switch does not change its operating state upon an incremental movement of the crankshaft.

10. Apparatus according to any one of claims 7 to 9 wherein the electrical control means includes a memory means and the apparatus includes a transducer associated with a part of the engine which moves as the crankshaft is incremently rotated to produce a signal to the control means indicative of the position of the movable part, the control means correlating the signal from the transducer and the output of the pressure sensitive switch for each incremental movement of the crankshaft and providing information to the memory means whereby the top dead centre position can be found subsequently by utilising the signal from the transducer and information from the memory means without utilising the output from the pressure sensitive switch.