GB2139698A - Micro-computer controlled digital ignition system - Google Patents

Abstract

The switching on and off of the current in the primary winding of the ignition coil of an internal combustion engine is controlled so that over a given range of engine speeds the unballasted coil switching is programmed to provide a five millisecond coil on time, thus preventing the coil overheating and saving energy. At higher speeds, this time reduces. The system includes a micro-computer (10), an opto-electronic trigger (12) and a coil switching system (14). The micro-computer includes a clock pulse generator (16), a PROM (18), two counters (20, 22) and a central processor unit 24. The PROM (18) has logarithmic look-up tables stored therein, in order that the system can determine the precise crankshaft positions at which the coil is to switch on and off at all engine speeds above a given value. Other look-up tables relating to manifold pressure and fuel rating can also be provided. <IMAGE>

Description

SPECIFICATION Micro-computer controlled digital ignition system The present invention relates to a micro-com puter controlled digital ignition system for an internal combination engine.

An opto-electronic system in which the coil is switched off to provide a spark at the correct crank angle position in advance of top dead-centre (TDC) is disclosed in our Patent Specification No. 1,410,782. Such a system utilizes two opto-electronic triggers to gener ate first and second series of voltage pulses in synchronism with the engine, the second series being high frequency pulses (every 1 or 2 degrees crank) compared with the first series of pulses. The system also utilizes a counter to count down a given number of the second series of pulses from a given crankshaft position as determined by the first series of pulses, the given number of pulses being presettable one cycle in advance according to the speed and/or load on the engine.The spark for ignition is initiated when said count down has been completed by comparison of two signals at the same logic levei.

A further development of this basic "double trigger" system is disclosed in our Patent Specification No. 1,420,814, in which we provide for the extinguishing of the spark at the spark plugs at a predetermined crank-shaft angle irrespective of the crank-shaft angle at which the spark has been initiated. In other words, the coil is turned on again at a fixed crank-shaft position (approximately a few degrees after TDC) in order to extinguish the spark, if it has not self extinguished, irrespective of the speed of the engine.

Yet a further development of this basic "double trigger" system is disclosed in our Patent Specification no. 1,483,611 in which we sense the commencement of the second stage of combustion, compare it with a fixed predetermined position, and adjust the position at which the coil turns off to initiate a spark, such that the sensed position of the second stage of combustion is coincident with a fixed predetermined position.

A still further development utilizing two counters and a read only memory (ROM) is disclosed in our Patent Specification No.

1,598,282. The first counter counts the number of second voltage pulses during a predet- ermined time interval (e.g. 2m.s.) from a given point in relation to the engine crankshaft position. The ROM which is operative from the first counter includes a program of the desired advance or retard of the ignition timing, and operates the second counter to count the second voltage pulses up to the number stored in the ROM in order to initiate engine ignition at the optimum crank-shaft position.

From studies of engine performance it seems desirable to additionally vary the crankshaft position of the turn on of the ignition coil, particularly at relatively low R.P.M. as well as the turn off of the coil to initiate the spark, in order to provide a coil on period which does not exceed a given time. With this method it is possible to use an unballasted coil, which not only saves energy but results in a cooler running of the coil.

According to a first aspect of the present invention there is provided a method of controlling the switching on and off of the current to the primary winding of an ignition coil of an internal combustion engine, which method includes the steps of (a) generating a square wave voltage signal in synchronism with the cyclic operation of the cylinders of the engine; (b) using the period or a predetermined part of the period of the waveform to gate a high frequency square wave voltage signal generated internally within a micro-computer.

(c) counting the number of pulses of the high frequency square wave voltage signal during the gated period in order to determine the engine speed; (d) using a function of this count to address a programmable read only memory (PROM) within the micro-computer; (e) calculating the crank-angle positions at which the coil is to be switched on and off from information previously stored in the PROM which information relates firstly to switching positions in relation to engine speed detected during the previous cycle and secondly maintaining for relatively low engine R.P.M. a predetermined coil on time; and (f) triggering the switching on and off of the primary current to the ignition coil in accordance with crank-angle positions which have been determined.

According to a second aspect of the present invention there is provided apparatus for controlling the switching on and off of the current to the primary winding of an ignition coil of an internal combustion engine, which apparatus includes (a) means for generating a square wave voltage signal in synchronism with the cyclic operation of the cylinders of the engine; (b) means for gating a high frequency square wave voltage signal generated internally within a microcomputer using the period or a predetermined part of the period of said engine related square waveform; (c) means for counting the number of pulses of the high frequency square wave voltage signal during the gated period in order to determine the engine speed; (d) a programmable read only memory (PROM) within the micro-computer which is addressed using a function of the count of the counting means;; (e) means for calculating the crank-angle position at which the coil is to be switched on and off from information previously stored in the PROM, which information relates firstly to switching positions in relation to engine speed detected during the previous cycle, and secondly maintaining for relatively low engine R.P.M. a predetermined coil on time; and (f) means for triggering the switching on and off of the primary current of the ignition coil in accordance with crank-angle positions which have been determined.

Preferably, the PROM has stored therein addressed look-up tables which correspond to particular counts of the counter, so that the desired ignition advance can be determined for all engine speeds.

Preferably, the look-up tables are arranged logarithmically, there being more look-up points at the lower end of the speed range than at the higher end.

Additionally, look-up tables which includemanifold pressure and/or fuel rating may be included within the PROM. Alternatively, two variables may be incorporated into the same look-up table.

For relatively low engine speeds, the coil is switched on at a given crank-shaft position which will allow a time of preferably 5ms to elapse before it is switched off.. This allows time for the primary current to build up to a maximum for an unballasted coil, thus saving energy as well as allowing the coil to run cooler.

For high engine speeds the coil is switched on and off at predetermined calculated crankshaft positions. In order to achieve this, two counters are associated with the PROM.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein Figure 1 is a block diagrammatic representation of one preferred form of micro-computer controlled digital ignition system for controlling the switching on and off of the current to the primary winding of an ignition coil of an eight cylinder internal combustion engine; Figure 2 is a set of waveforms illustrating the gating of the output of a fixed internal frequency from the microcomputer to provide an input to the first counter of the system shown in Figure 1; Figure 3 is a graph illustrating a look-up table of engine crank-shaft degrees before TDC plotted to a base of engine R.P.M;; Figure 4 is a timing diagram to illustrate the various sequences of events in the microcomputer controlled digital ignition system shown in Figure 1: Figure 5 is a series of coil on waveforms to illustrate the crank-shaft positions at which the coil switches on and off over a wide range of engine R.P.M; and Figure 6 is a series of coil on waveforms of a modified system showing the idling and cranking waveforms, in which the system operates entirely on a computerized basis even at cranking speeds.

Referring first to Figure 1, the micro-computer controlled digital ignition system includes a micro-computer 10; an opto-electronic trigger 12; and a coil switching system 14, whose output controls the switching on and off of the primary current in the ignition coil of the internal combustion engine.

The micro-computer 10 is based upon a known type of system such as INTEL MCS-51 series of micro-computers and micro-processors. The combination of MCS and a numerical suffix is a Registered Trade Mark. The basic elements shown in Figure 1 include: a clock pulse generator 1 6 which generates square-wave pulses at a frequency of 660 KHz; a PROM 18; first and second counters 20 and 22; and a central processor unit 24, hereinafter referred to as a CPU.

The opto-electronic trigger 1 2 is one of our own designs, such as that disclosed in detail in our Patent Specification No.1,497,346 or No.2,030,765. It generates the waveform shown in waveform A of Figure 2. The present micro-computer controlled digital ignition system is designed to operate with any internal combustion engine using high voltage spark ignition. In the particular-example illustrated the engine is an 8-cylinder Rolls Royce engine. Accordingly the trigger 1 2 generates a waveform having an optimized fixed duty cycle which wavefform is synchronized with the cyclic operation of the cylinders in the engine.As shown in waveform A, the trigger output falls from logic 1 to logic 0 at the static timing positions of the cylinders 8" before top dead centre (TDC) and rises from logic 0 to logic 1 at 22 after TDC for each respective cylinder. The high to low logic transition of the waveform A is used by the micro-computer 10 as its reference position in all calculations. In an alternative form, successive transitions from low to high may be used by altering the mechanical position of the opto-electronic light chopper.

The output from the clock generator 1 6 is shown in waveform B of Figure 2. It is a square-wave pulse having a duty cycle of 50%, and a frequency of 660 KHz. The output from the clock pulse generator 1 6 and an output from the CPU 24 are applied to respective inputs of the first counter 20 which effectively operates to count the 660 KHz clock pulses for substantially the whole period of the waveform A as shown in waveform C.

This is because the CPU 24 enables the counter 20 for virtually the whole waveform A period---only requiring a negligible amount of time to store its value and reset it to zero after the reference point on waveform A. Thus as shown there is a short gap L at which the above events occur. These gated pulses are applied to the input of the first counter 20, the total number of pulses in the gated period being inversely proportional to engine R.P.M.

The PROM 18 is provided with a plurality of address locations, so that for every count of the first counter 20 there is a corresponding address location within the PROM 1 8. Each address location within the PROM contains a look-up value in engine crank-shaft degrees.

Thus for example the total range of the counts may be from 1,320 for an engine running at maximum speed up to 65,536 representing the slowest engine speed in the look-up table which corresponds to a speed of 1 52 R.P.M.

This range of counts is thus divided into a number of address locations each of which will give the required angle of advance for switching the coil "off" to initiate the spark for ignition.

The division of the range of a plurality of sections is done with reference to engine speed. A typical example of a look-up table is shown in Figure 3, by way of illustration, where stepped curve D represents ignition advance in crank-shaft degrees before TDC plotted to a base of engine speed. The look-up table is arranged logarithmically, there being many more look-up points provided at low and cruise speeds than there are at high speed. This is in accordance with engine requirements which call for far more timing changes at the lower end of the speed range than the upper end. Although not shown, the actual look-up table will contain approximately 40 steps.

During each cycle the first counter 20 counts the 660 KHz pulses coming from the clock generator 1 6 and let us suppose for example that the engine speed is such that for each gated period a total number of 30,000 pulses enter the first counter 20. This corresponds to a given engine speed which for example may be that which is associated with the 20th address to provide an advance requirement of 30 . As there are 30,000 counts per 90 crank-shaft angle, between successive reference points, it will be appreciated that 10,000 counts correspond to 30 crank-shaft angle.Therefore a dynamic advance requirement of 30 from the reference point requires a count of 20,000 corresponding to 60 crank-shaft after the previous reference point so that when the first counter has counted 20,000 pulses in the next cycle the counter 20 outputs a pulse to the CPU 24 which outputs a pulse to the coil switching system 14, which in turn causes the ignition coil to be switched off in order to provide the spark for ignition. The first counter 20 continues to count until the end of the gating period and the total number of 30,000 counted during the gating period represents the engine speed which is then used for calculating the address requirement of the next cycle.

In order to calculate the point at which the coil has to be switched on, the first counter 20 can be used to provide the coil switch on position provided that it is not in advance of the point at which the CPU 24 has finished its calculations. The micro-computer 10 having worked out the position to switch the coil off in terms of crank-shaft degrees can then calculate the position to switch the coil on in order to give a 5 millisecond period during which the coil is on.

A problem arises in the case where the coil on position is in front of the position at which the CPU 24 has finished its calculations.

Under these conditions, the counter 20 cannot be used and it is necessary to provide the second counter 22 in order to determine the crank-shaft position to switch the coil on. This second counter 22 receives ungated 660 KHz pulses from the clock pulse generator 1 6. The counter is preloaded from the CPU 24 to count up a given number of pulses on being started at the reference point by the CPU 24, and when this preloaded count has been reached it outputs a pulse via the CPU 24 to the coil switching system 14 in order to switch the coil on at say for example 10 after TDC. It will be appreciated that the second counter 22 only comes into operation when the coil switch on position occurs during the period of the CPU's calculation. Under all other conditions it is inhibited.

The above system will now be elaborated in greater detail with reference to Figures 4 and 5.

The diagranmatic representation shown in Figure 4 will help to sunmarize the above essential points of the micro-computer controlled digital ignition system. Waveform E is again the output from the opto-electronic trigger 12, the waveform changing from a logic 1 to a logic 0 at 8" before TDC, and changing back to a logic 1 at 22 after TDC, with reference to cylinder 1 or at 68 before TDC with reference to cylinder 2. The arrowed lines F represent the start and stop of the count of the counter 20. As mentioned above, the total count within this period is indirectly proportional to engine R.P.M. and is used to determine engine speed and calculate the advance required for the next cylinder firing position from the look-up tables stored in the PROM 18.

The arrowed line G represents the period in which the second counter 22 may switch the coil on in the case where the CPU 24 is too busy to continually examine the contents of the counter 20.

The arrowed lines H show the sequence of operations taking place in the main program.

The short line is used for calculation by the micro-computer, whilst the long line which takes place during the logic trigger output is the period where the first counter 20 determines the switch on and off points of the coil, except as mentioned above where the coil switch on position if advanced more than the end point of the CPU's calculation.

Finally, double sided arrows J and K represent the total range of coil on and off positions respectively for engine speed between idling and maximum.

Referring now to Figure 5, the on period of the ignition coil is shown for nine different engine speeds. In each waveform, the rise of a logic 1 presents the coil on position whilst the fall to logic O represents the coil off position.

The slowest speed of engine cranking 50 R.P.M. ffor an eight cylinder Rolls Royce engine is shown in waveform Z, whilst normal engine cranking of 150 R.P.M. is shown in waveform Y. It will be noted that the coil ON and OFF positions correspond to the logic high and low positions of the trigger waveform E in Figure 4. Thus for cranking the digital ignition system is effectively inoperative and the coil is switched on and off at fixed crank-shaft positions as in the manner disclosed in our Patent No. 1,497,346.

At speeds above normal engine cranking speeds, e.g. at approximately 400 R.P.M. the digital ignition system becomes operative, when the micro-computer 10 has detected an engine speed above 400 R.P.M. waveform X shows the normal idling speed af this type of engine (approximately 600 R.P.M.) and waveforms W to R show respectively increasing engine speeds up to a maximum of 5,000 R.P.M. At speeds between 400 R.P.M. and 1,800 R.P.M., the duration for which the coil is switched on can be maintained at 5 ms, i.e.

the coil is switched on at a given crank-shaft position so as to give an on duration of 5 ms before it is switched off to initiate the spark for ignition.

At speeds above approximately 1,800 R.P.M., since the coil on position must not occur earlier than 80" before TDC, the on duration of the coil cannot be maintained at 5 ms.

Thus in order to obtain maximum energy from the coil at high engine R.P.M. it is run unballasted. At lower speeds, the unballasted coil is prevented from overheating by setting the on position of the coil to be a fixed period of time (e.g. 5ms) before the firing point, this period of time being just sufficient to allow the coil current to rise to the required value.

As a result the coil will actually run colder at low speeds than in a ballasted ignition system.

When cranking a low to high transition of the trigger waveform A in Figure 1 will switch on the coil as shown in waveform Z (Figure 5) 30" after the reference point. If no subsequent high to low transistion occurs, for example if the engine stalls, the CPU 24 will prevent the coil over heating by switching it off within a few seconds.

As an additional feature, in order to prevent over-revving, the ignition system will also function as a revolution limiter and temporarily discontinue sparking at engine speeds in excess of a predetermined limit (e.g. 5,000 R.P.M.) although the digital advance system itself will be capable of functioning at higher speeds.

Whilst the above described micro-computer controlled digital ignition system has been described with reference to using look-up tables in relation to engine speeds, the system can be expanded to take into account load of the engine by using the manifold pressure.

This can be achieved either by using two separate look-up tables or by incorporating two variables into the same look-up table. For example, the above described system could use boht speed and manifold pressure as two separate co-ordinates for a single ignition advance requirement.

It will be further appreciated that there is room in the micro-computer for an extra three look-up tables of the size already incorporated so that different advance characteristics can be selected to suit the octane rating of the fuel/C.R. of engine.

Whilst the specific example described relates to an eight cylinder engine, it will be appreciated that the system is readily adaptable to engines having different numbers of cylinders.

Yet further, whilst the above system has been described only in connection with spark ignition for an internal combustion engine, the addition of fuel injection to the ignition sustem could bring greater benefits to the engine than the ignition system on its own. Nothing which has been stated above excludes the possibility of using the micro-computer to control fuel injection in addition to the switching on and off of the coil as outlined above.

In the above described micro-computer controlled digital ignition system engine generated signals are used for both the coil on and off (ignition) points during cranking and idling to give stable non-electronic, non-calculated operation during these conditions in which engine speeds fluctuate dramatically within each revolution. It will be appreciated that electronic measurement during the first few cycles of operation on starting an engine is inaccurate and causes unstable engine running.

At higher than cranking speeds, the system switches over at approximately 400R.P.M. for the eight cylinder Rolls Royce referred to in the above example, to a truly electronic computerized system measuring both speed and load and switching the coil on and off at crank-shaft positions which are determined according engine requirements as determined from the look-up tables.

The accuracy of time measurement by using the same optical switching transition between either successive switch on points of the elec tro-optical trigger or two successive switch off points, provide an accuracy for base timing of less than 0.5" crank-shaft throughout the life of the vehicle and complete precision of time measurement equal to the accuracy of the blades of the chopper disc or apertures in the chopper disc.

Referring now to the modified system in which the system operates entirely on a fully computerized basis even at cranking speeds, Figure 6 shows three waveforms which illustrate crank-shaft positions at which the coil switches on and off.. Waveform X' illustrates the situation at the idling speed of 600 R.P.M., in which the coil off position coincides with the reference point and the coil on position is 5 ms earlier, whilst waveforms Y' and Z' illustrate the two cranking speeds of 150 and 50 R.P.M. respectively. In this case the coil on positions are referenced at the reference point 8" before TDC and the coil off positions occur 5 ms later; It will be noted that at idling and cranking speeds the coil off and/or on positions are referenced at the actual reference point, whereas for all the other waveforms W to R, the reference point at which calculations are based on is the earlier one.

Whilst in the above embodiments we have chosen to use a single fixed reference point so that the gated period is equal to the period of the waveform, it will be appreciated that we may use two reference points in the waveform so that the gated period is a predetermined part of the total period of the waveform.

Claims (20)

1. A method of controlling the switching on and off of the current to the primary winding of an ignition coil of an internal combustion engine, which method includes the steps of: (a) generating a square wave voltage signal in synchronism with the cyclic operation of the cylinders of the engine; (b) using the period or a predetermined part of the period of the waveform to gate a high frequency square wave voltage signal generated internally within a micro-computer; (c) counting the number of pulses of the high frequency square wave voltage signal during the gated period in order to determine the engine speed; (d) using a function of this count to address a programmable read only memory (PROM) within the micro-computer;; (e) calculating the crank-angle positions at which the coil is to be switched on and off from information previously stored in the PROM which information relates firstly to switching positions in relation to engine speed detected during the previous cycle and secondly maintaining for relatively low engine R.P.M. a predetermined coil on time; and (f) triggering the switching on and off of the primary current to the ignition coil in accordance with crank-angle positions which have been determined.

2. The method according to claim 1, wherein the calculating step includes extracting from look-up tables stored in the PROM particular values of the coil on and off crankshaft positions which correspond to the particular number of pulses counted, so that the desired ignition advance can be determined at all engine speeds above and including idling speeds.

3. The method according to claim 2, wherein said look-up tables are arranged logarithmically, there being more look-up points at the lower end of the speed range than at the higher end.

4. The method according to claim 2 or 3, wherein two variables are incorporated into the same look-up table.

5. The method according toclaim 2, 3 or 4, wherein additional look-up tables are included within the PROM, said additional look-up tables corresponding to manifold pressure and/or fuel rating.

6. The method according to any one of the preceding claims, wherein between two different engine speeds the coil on time is fixed at a given duration in time.

7. The method according to claim 6, wherein said time duration is five milliseconds.

8. The method according to any one of the preceding claims 1 to 5, wherein for high engine speeds the coil is switched on and off at predetermined calculated crank-shaft positions.

9. Apparatus for controlling the switching on and off of the current to the primary winding of an ignition coil of an internal combustion engine, which apparatus includes: (a) means for generating a square wave voltage singal in synchronism with the cyclic operation of the cylinders of the engine; (b) means for gating a high frequency square wave voltage signal generated internally within a micro-computer using the period or a predetermined part of the period of said engine related square waveform; (c) means for counting the number of pulses of the high frequency square wave voltage signal during the gated period in order to determine the engine speed; (d) a progranmable read only memory (PROM) within the micro-computer which is addressed using a function of the count of the counting means;; (e) means for calculating the crank-angle positions at which the coil is to be switched on and off from information previously stored in the PROM, which information relates firstly to switching positions in relation to engine speed detected during the previous cycle, and secondly maintaining for relatively low engine R.P.M. a predetermined coil on time; and (f) means for triggering the switching on and off of the primary current of the ignition coil in accordance with crank-angle positions which have been determined.

10. Apparatus according to claim 9, wherein the PROM has stored therein addressed look-up tables which correspond to particular counts of the counter, so that the desired ignition advance can be determined for all engine speeds.

11. Apparatus according to claim 10, wherein the look-up tables are arranged logarithmically, there being more look-up points at the lower end of the speed range than at the higher end.

1 2. Apparatus according to claim 10 or 11, wherein two variables are incorporated into the same look-up table.

13. Apparatus according to claim 10 or 11, wherein the PROM additionally has stored therein look-up tables for manifold pressure and/or fuel rating.

14. Apparatus according to any one of the preceding claims 9 to 13, wherein for relatively low engine speeds the coil is switched on at a given crank-shaft position which will allow a predetermined time to elapse before it is switched off..

1 5. Apparatus according to claim 14, wherein said predetermined time is five milliseconds.

16. Apparatus according to any one of the preceding claims 9 to 13, wherein for high engine speeds the coil is switched on and off at predetermined calculated cranked shaft positions.

1 7. Apparatus according to claim 1 6, wherein said counting means comprises a pair of counters.

18. Apparatus according to any one of the preceding claimd 9 to 17, wherein for speeds below a predetermined speed which is less than normal engine idling speed, the system switches over to a non-timed system in which the coil on and off positions occur at fixed crank-shaft positions.

1 9. A method of controlling the switching on and off of the current to the primary winding of the ignition coil of an internal combustion engine, substantially as described with reference to and as illustrated in the accompanying drawings.

20. Apparatus for controlling the switching on and off of the current to the primary winding of the ignition coil of an internal combustion engine, constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.