GB2087477A - Ignition Systems for Internal Combustion Engines - Google Patents

Abstract

The ignition system employs a single-chip micro-computer 1 to determine the required ignition advance for the prevailing engine speed and to control ignition coil switch-on and switch-off to provide sparks at the appropriate times. The task of determining ignition advance is simplified by dividing the engine speed range into bands, using simple approximations to calculate ignition advance for bands in the low- speed range, and employing a look-up table to determine ignition advance for bands in the high-speed range, The simplified approximations are arrived at by fitting, to the ignition advance/speed characteristic for an engine, a series of straight lines. The system includes a Hall effect sensor 4, the output of which is connected to the computer to determine the engine speed. The computer also receives inputs from an inlet manifold pressure sensor and a knock sensor, the computer retarding the ignition if knocking is detected. <IMAGE>

Description

SPECIFICATION Improvements in and Relating to Ignition Systems The invention relates to ignition systems, particularly those suitable for internal combustion engines.

Ignition systems employed in combustion engines are arranged to generate sparks at times appropriate for initiating combustion. In general, an ignition system employs an inductive coil as the principal means of generating the sparks, the coil being charged and discharged in synchronism with the engine to yield high voltages which are converted to sparks by means of spark plugs.

Charging of the coil is effected by connecting it to an energy source, usually a battery by means of a switch controlled by a camming member geared to the engine output shaft. Discharge is effected by opening the switch, the spark being provided as the switch opens.

In a conventional ignition system, a spark is provided in advance of the top dead centre (TDC) position of each cylinder on a compression stroke, and the amount of advance is varied by adjusting the switch position relative to the camming member.

It will be appreciated that in a system as described above the duration of the charging cycle of the coil varies with engine speed. It is desirable for best results that the coil should be just fully charged when the spark is required, but in a conventional system as described the coil may be overcharged at low engine speeds and undercharged at high engine speeds because the switch is generally closed, and opens only at set crankshaft angles.

It is an object of the present invention to provider an ignition system arranged to achieve optimum or near optimum spark conditions independently of engine speed.

According to the invention an ignition system includes: (i) an ignition coil, (ii) switch means, connected in series with the coil, arranged to permit and to interrupt current flow in the coil, (iii) means arranged to predict spark times from input data provided as periodically occurring pulses on an input port, (iv) means arranged to determine the minimum time required for charging the coil fully, (v) means arranged to predict, from each spark time and the coil charging time, each start of-charge time for the coil, and (vi) control means arranged to so control the switch means that, in operation, current flow in the coil is permitted from each start-of charge time to each spark time and is interrupted at the spark time.

Advantageously, the means arranged to determine the minimum time required for charging the coil fully is arranged to take account of changes of coil parameters with temperature.

Preferably the switch means is a transistor, and advantageously, the control means is arranged to so control the conduction cycle of the transistor that the coil is discharged at a controlled rate.

Advantageously, the ignition coil and the conduction cycle are such that coil current interruption lasts until all of the coil energy is discharged.

Advantageously, the means arranged to predict spark times from periodically occurring pulses is arranged to calculate the spark times from the pulses and to react adaptively to the pulses by classifying them in bands before performing the calculations.

Preferably, the calculations are effected by means of shift and add sequences which differ from band to band.

Advantageously, the means arranged to predict spark times from periodically occurring pulses is so arranged as to perform calculations on bands of pulses having periods longer than the calculation time and to provide preset values for pulses having periods shorter than the calculation time.

Advantageously, the means arranged to predict spark times is a computer which includes a fixed store, an alterable store, data input and output ports, and an arithmetic logic unit arranged to operate on input data in accordance with the contents of the fixed store to provide output data, wherein the contents of the fixed store to provide output data, wherein the contents of a first part of the fixed store provide control of and data for the arithmetic logic unit to calculate the spark times while the contents of a second part of the fixed data store provide preset data representing spark times.

Preferably, the computer is a single-chip microcomputer.

Advantageously, an internalcombustion engine having an ignition system according to the invention includes a pick-up device coupled to the crankshaft and arranged to supply, to the means arranged to predict spark times, periodically occurring pulses corresponding to top dead positions of the engine.

Preferably, the pick-up device is a Hall-effect sensor incorporated in a distributor, and advantageously, the ignition system is arranged to provide an energising current for the Hall-effect sensor.

Advantageously the internal combustion engine includes a knock sensor connected to an input port of the computer which is arranged to increase spark time delay when an input signal from the knock sensor is detected.

Preferably, the internal combustion engine includes an inlet manifold pressure transducer connected to a further input port of the computer which is arranged to vary spark time delay according to the transducer signal.

An ignition system according to the invention, applied to an internal combustion engine, will now be described by way of example only and with reference to the accompanying drawings in which Fig. 1 is a general block diagram representation of the ignition system connected to a distributor rotor sensor, showing it as a combination of a microcomputer, voltage regulator, coil and coil current control circuit, Fig. 2 is a more detailed representation of the voltage regulator and the coil current control circuit of Fig. 1, Fig. 3 is a flow diagram representation of the steps performed by the microcomputer in reacting to input signais from the distributor rotor sensor, and Fig. 4 is a graph representing a general speedignition advance characteristic for an internal combustion engine at one value of inlet manifold pressure along with an approximate characteristic having slopes restricted to values of +fl.

With reference to Fig. 1, an ignition system suitable for an internal combustion engine includes a microcomputer 1 having an output port 9 which is connected as the control input port of a coil current control circuit 2. An ignition coil 3 has a primary winding connected between a battery supply and the coil current control circuit 2, and a secondary winding one side of which is connected to the battery side of the primary winding. The coil current control circuit 2 is arranged to control the flow of current from the battery supply to the coil 3.

A voltage regulator 5, connected to the battery supply, provides regulated voltage supplies to the microcomputer 1, the coil current control 2, and a distributor rotor sensor 4 which has an output port connected to a data input port 6 of the microcomputer 1. The microcomputer 1 may include two further data input ports 7 and 8 connected to receive, respectively, engine "knock" indication and inlet manifold pressure.

"Knock" is, of course, engine shock caused by over-advanced ignition and low quality fuel.

The distributor rotor sensor 4 is a Hall-effect sensor which receives a current supply from the voltage regulator 5 and provides pulses on its output port at the times of the top dead centre positions of the engine crankshaft. Use of a Halleffect pick-up provides easy installation with minimal setting-up.

The system operates as follows:- The Hall-effect sensor detects top dead centre markers which are part of a magnetic circuit in the distributor of the engine. A comparator is provided in the system for reducing noise influencing the Hall-effect pick-up.

The times between the TDC markers are measured by the microcomputer which determines from these measurements, the engine speed and the required ignition advance angle.

After the computer has determined the required ignition advance angle and converted it to a time delay from the preceding TDC position, it subtracts from this delay the coil charge time to determine the coil start-of-charge time measured from the preceding top dead centre marker. The microcomputer then counts out the time from the previous top dead position, switches on the coil current control circuit 2 to achieve full charge as the spark is due, and switches the coil current control circuit off when the spark is due. The spark is produced in known manner when the coil current is interrupted.

The computer used in the system shown in Fig.

1 may be a relatively simple single-chip microcomputer such as the Texas Instruments TMS 1 000C CMOS 4-bit microcomputer. The ignition system as a "dedicated" system since the speed-ignition advance characteristic of the engine dictates the information stored in the fixed store of the computer. The information is represented by combinations of states of nonvolatile storage elements which make up the fixed store.

The extent of the fixed store, that is, the number of storage elements, required to effect adequate control of the system is minimised as follows: The speed range of the engine is divided into eight speed sub-ranges identified as speed ranges 1 to 8 where speed range 8 lies at the low end of the engine range. The engine speed is measured, classified as being within a particular speed subrange, and an ignition delay period determined from the preceding TDC position to give the necessary ignition advance value. This sequence of steps is shown in Fig. 3 which represents the overall arrangement of the control sequence for the system.

The speed sub-ranges are shown in Fig. 4 in relation to a general speed-ignition characteristic for an internal combustion engine at a single inlet manifold pressure (not identified). It will be appreciated that a full speed-ignition characteristic is a solid figure and that Fig. 4 is one slice of such a figure taken at one inlet manifold pressure.

in Fig. 4 the solid piece-wise linear characteristic A represents the ignition advance required at various speeds and at one inlet manifold pressure, while the dotted characteristic B represents an approximate characteristic the slopes of which are restricted to values of Tn where n is an integer. In Fig. 4 n lies between 5 and 8 for the values given. The speed sub-ranges are Sub-range 8- 300 to 1100 RPM.

Sub-range 7-1100 to 1600 RPM Sub-range 6-1600 to 1900 RPM Sub-range 5-1900 to 2800 RPM Sub-range 4 2800 to 4800 RPM Sub-range 3--4800 to 5000 RPM Sub-range 2-5000 to 5400 RPM Sub-range 1-5400 to 6000 RPM It may be understood from Fig. 4 that each ignition advance value on the dotted characteristic B is given by the product of speed and a constant K1 less a constant K2, where K1 and K2 change from one piecewise linear section to the next.

The ignition advance at any speed as given by the dotted characteristic B is therefore obtainable by identifying the speed range it belongs to in order to determine the values of K1 and K2, multiplying the speed value by K1 and subtracting K2.

Since K1 is permitted only values of Tn, multiplication is the same as a shift operation. The calculation performed by the microcomputer is therefore effected by a straightforward "shift n piaces and add" operation. Therefore, the required delay of the ignition point is provided by the microcomputer arranged as the combination of a shift register and a binary adder controlling a time delay circuit.

The microcomputer 1 is further adapted to its task by restricting calculation of the required spark advance to low speeds and employing parts of the fixed store as a look-up data store for high speeds. The fixed data store or read only memory (ROM) is utilised most efficiently because the calculation algorithm described above is short and the look-up store or map is kept small.

Because the main control function of the microcomputer 1 is simplified to that of a multiple shift and subtract operation, there is time available at low speeds to form other functions, such as the detection of "knock." The microcomputer may be arranged to poll or be interrupted on a "knock" input port such as 7 in Fig. 1 and to react by retarding the ignition point to avoid "knock". The use of a knock sensing system with the microcomputer in this way will permit operation of an engine on poor quality fuel and/or improve fuel economy and engine power output. It will be understood that such knock detection feedback represents an ability to react to over-advanced ignition settings. A suitable knock detector would be an accelerometer attached to the cylinder block of the engine.

Reference is now make to Fig. 2 in which the coil current control 2 of Fig. 1 is shown in more detail as consisting of a driver circuit 10 and a switching transistor 11. The driver circuit 10 includes a comparator circuit which performs the function of removing noise from the Hall-effect sensor 4 and providing pulses in synchronism with engine TDC positions on the microcomputer input port 6.

The remainder of the driver circuit 10 provides drive current for the output transistor 11. The coil discharge is ideally a constant energy discharge in which the driver circuit 10 brings the transistor 11 out of saturation, gradually increasing its effective series resistance and therefore its power dissipation. Here, the collector-emitter voltage is increasing at constant current.

Where constant energy discharge is not required, the system is reduced by having the output transistor 11 driven by a buffer transistor controlled by the microcomputer 1.

The system may be further reduced by having the Hall-effect sensor driving the microcomputer directly.

The spark characteristic may be changed as required by using a driver circuit 10 with different output signal characteristics without any change to the microcomputer 1.

It is possible to effect constant energy discharge of the coil without exceeding the power rating of the transistor because the coil conduction part of the cycle is kept at its minimum so that transistor dissipation on the conduction part of the cycle is minimised. This permits an increase in switch-off dissipation.

The features of an ignition system as described could be summarized as follows:- (i) Reduction of maintenance (ii) Consistency from unit to unit.

(iii) Comparable cost with existing mechanical systems (iv) Avoids the use of a coil ballast resistor.

(v) Low component count.

(vi) Idle speed control helping to reduce fuel consumption.

(vii) The coil is charged to its optimum state.

(viii) The coil is discharged fully.

(ix) There is fuel saving, power output is increased, and knock is avoided.

(x) The system is tailored to the engine and a change in microcomputer map alone is needed for a different engine.

(xi) Constant energy discharge is possible.

(xii) Spurious discharges are avoided.

(xiii) The energy discharge characteristic may be altered independently of the microcomputer control memory.

ft will be understood that high performance engines which have relatively complex ignition advance/speed/inlet manifold pressure characteristics would require a microcomputer with more program memory space than the TMS 1000 used here, but that the principles applied above are still applicable to ensuring that the required memory space and operating speed are minimised.

Claims (13)

Claims

1. An ignition system including, (i) an ignition coil, (ii) switch means, connected in series with the coil, arranged to permit and to interrupt current flow in the coil, (iii) means arranged to predict spark times from input data provided as periodically occurring pulses on an input port, (iv) means arranged to determine the minimum time required for charging the coil fully, (v) means arranged to predict, from each spark time and the coil charging time, each start of-charge time for the coil, and (vi) control means arranged to so control the switch means that, in operation, current flow in the coil is permitted from each start-of charge time to each spark time and is interrupted at the spark time.

2. An ignition system as claimed in claim 1, wherein the means arranged to predict spark times from periodically occurring pulses is arranged to classify the periodically occurring pulses into bands and to predict spark times according to the band classification.

3. An ignition system as claimed in claim 2, wherein the means arranged to predict spark times is so arranged as to perform calculations on bands of pulses having periods longer than the calculation time and to provide preset values for pulses having periods shorter than the calculation time.

4. An ignition system as claimed in claim 3, wherein the spark times are calculated by means of shift-and-add sequences according to the band classification.

5. An ignition system as claimed in any one of claims 1 to 4, wherein the means arranged to predict spark times includes a fixed store, an alterable store, data input and output ports, and an arithmetic logic unit arranged to operate on input data in accordance with the contents of the fixed store to provide output data, and a first part of the fixed store provides control of and data for the arithmetic logic unit to calculate spark times while a second part of the fixed data store provides preset data representing spark times.

6. An ignition system as claimed in claim 5, wherein the means arranged to predict spark times is a single-chip computer.

7. An ignition system as claimed in any one of claims 1 to 6, arranged to achieve full charge of the coil as the spark is due.

8. An ignition system as claimed in claim 7, wherein the means arranged to determine the minimum time for achieving full charge of the coil is arranged to take account of changes of coil parameters with ambient conditions.

9. An ignition system substantially as herein described with reference to and as illustrated by the accompanying drawings.

10. An internal combustion engine including an ignition system as claimed in any one of claims 1 to 9, and a pick-up device coupled to the crankshaft arranged to supply pulses at set positions of the engine to the means arranged to predict spark times.

11. An internal combustion engine as claimed in claim 10, wherein the pick-up device includes a Hall-effect sensor.

12. An internal combustion engine as claimed in claim 10 or 11, and including a knock sensor connected to the ignition system which is arranged to delay the spark when a signal from the knock sensor is present.

13. An internal combustion engine as claimed in any one of claims 10 to 12, and including an inlet manifold pressure transducer connected to the ignition system which is arranged to change the spark time delay according to the transducer signal.