GB2325703A

GB2325703A - Internal combustion engine spark scheduling

Info

Publication number: GB2325703A
Application number: GB9711137A
Authority: GB
Inventors: Alan Hoy; Jon Dixon
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1997-05-30
Filing date: 1997-05-30
Publication date: 1998-12-02
Also published as: US6012427A; EP0881383B1; EP0881383A2; EP0881383A3; GB9711137D0; DE69812473D1; DE69812473T2

Abstract

The present invention relates to apparatus for scheduling the ignition sparks for a spark ignition internal combustion engine. An ignition system for a spark ignition engine, comprises means for generating a series of pulses 12 upon each revolution of a crankshaft 1 and means 8,118 for identifying pulses in the series relative to a top dead centre (TDC) position for the engine, one or more ignition coils and means to charge a coil and then to discharge the coil to generate an ignition spark at a desired spark angle 54 relative to the top dead centre position, characterised in that the ignition system comprises means to identify a first particular pulse 60 occurring after the charging starts and before the discharging begins, and means to calculate a first interval I D from the first particular pulse 60 so that the discharging happens at the desired spark angle 54.

Description

2325703 Internal Combustion Engine Spark Scheduling The present invention

relates to apparatus f or scheduling the ignition sparks for a spark ignition internal combustion engine.

combustion engines are mechanical ignition sparks for internal conventionally scheduled using either distributors or more reliable electronic ignition systems.

A c,oil must be charged during a dwell time and then discharged to ignite the fuel in a cylinder at a time corresponding to the correct spark angle relative to the engine top dead centre position. it is often desirable to alter either the dwell time or the spark angle is depending on the engine operating characteristics.

Electronic systems offer greater flexibility than mechanical systems in controlling the coil charging and discharging, but problems remain in scheduling the spark event. In particular, if the engine speed is changing rapidly, for example, during revving up or while braking, the actual spark angle may be shif ted appreciably f rom, the desired spark angle, because the electronic ignition calculates the correct point in time for the coil discharge on the assumption that engine speed is constant.

The scheduling of spark events is usually calculated with reference to a known orientation, or "angle", of the engine cylinders. For example the engine angle may be known at two positions, 1800 apart, from a toothed wheel and sensor arrangement on the engine crankshaft. Whilst such an arrangement gives good performance at relatively constant engine speeds, the spark angle may become shifted at varying conditions. For example, engine speed may change by up to 2 over a 900 engine angle, and if thit4M97-0408 30 my 1997 angle corresponds to the time taken. from the time at which the spark event is scheduled to the discharge time, then the discharge time will be out by about 71P. This is a significant error and will result in non- OPtiMal firing of the engine cylinders.

it is an object of the present invention to provide an ignition system which provides a more accurate control of the spark angle and, optionally, of the dwell time.

Accordingly, the present invention provides an ignition system for a spark ignition engine, comprising means for generating a series of pulses upon each revolution of a crankshaft and means for identifying pulses in the series relative to a top dead centre position for the engine, one or more ignition coils and means to charge a coil and then to discharge the coil to generate an ignition spark at a desired spark angle relative to the top dead centre position, characterised in that the ignition system comprises means to identify a f' irst particular pulse occurring after the charging starts and before the discharging begins, and means to calculate a first interval f rom the f irst particular pulse so that the discharging happens at the desired spark angle.

The desired spark angle may need to be varied, for example, increasing at higher engine speeds.

The pulses may be electrical or optical. In the case of features or teeth cast into the crankshaft I the sensor may be a variable reluctance sensor producing a generally sinusoidal series of electrical pulses, with at least one pulse being identifiable, for example having a different width or occurring after a missing pulse. Such pulses may then be digitised in a known manner for input into an GR91-0402 os? 1 -15 - 3 electronic ignition system, which may be based on a microprocessor which determines the f irst interval and, optionally the dwell time.

in general the greater the number of pulses during one revolution of the crankshaft, the greater the accuracy will be in scheduling the coil discharge relative to the desired spark angle. it is therefore advantageous if the series of pulses cises at least about nine pulses. However, using too great a number of pulses may provide a negligible benefit. Therefore the series of pulses should comprise no more than about 360 pulses.

Allso in general, the closer in the time the first particular pulse is prior to the spark event the greater the accuracy will be in scheduling the coil discharge relative to the desired spark angle. It may therefore be advantageous always to use as the first particular pulse the pulse immediately preceding the time corresponding to the desired spark angle. However, for a moderate number of pulses, for example of the order of thirty-six, there may be insuf f icient time at higher engine speeds f or a microprocessor to calculate the correct first interval and to initiate the discharge of the coil at the desired spark angle. In any event, there may be an insignificant benef it associated with the decreased interval at higher engine speeds. Therefore, it is particularly advantageous if the first particular pulse is selected from amongst a plurality of pulses by the means to identify the first particular pulse, so that the first interval remains albove a minimum interval as the engine speed increases.

In a similar manner to that described above for scheduling the spark event, the start of coil charging may also be scheduled, according to a desired dwell time. Depending G897-0409 30 may 199,7 on the time taken between the time at which an engine management system the time taken from the time at which the start of coil charging is scheduled to the actual time at which coil charging starts, engine speed variations may alter the actual dwell time.

Variations in dwell time due to engine speed changes are not as serious as variations between desired and actual spark angle because the coil charging can be such that the nominal coil charge is more than sufficient to ignite a cylinder under all normal conditions. However, it may stilLbe desired to be able to control the dwell time and hence the coil charging more accurately in order to deliver a more optimal amount of energy to ignite the cylinder. Dwell time may also need to be varied, in order to provide optimal energy in a spark, according to various factors, such as battery voltage and coil temperature.

In a similar manner to that described above for scheduling the spark event, the ignition system may therefore comprise means to identify a second particular pulse occurring before the charging starts, and means to calculate a second interval from the second particular pulse so that the charging begins at the desired dwell time prior to the beginning of discharge.

Similarly again, the second particular pulse may be selected from amongst a plurality of pulses by the means to identify the second particular pulse, so that the second interval remains above a minimum interval as the engine speed increases.

If the ignition system comprises means to calculate a desired spark angle according to engine operating parameters, then the first, and optionally the second, =91-0401 39 Way 1PP7 intervals or particular pulses may be then be appropriated calculated or selected by the ignition system.

In a preferred embodiment of the invention, the means for generating the series of pulses upon each revolution Of a crankshaft comprises a toothed wheel turned by the crankshaft. A sensor may then sense each tooth passing the sensor and outputs the series of pulses, the wheel having at least one reference tooth which produces a reference pulse as the reference tooth passes the sensor.

In this way each pulse in the series of pulses may be identified relative to the engine top dead centre position.

The invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a toothed wheel, showing the series of pulses and the scheduling of the spark event and dwell time according to the invention; and Figure 2 is a schematic block diagram of an ignition system according to the invention having as its input the series of pulses of Figure 1.

Referring first to Figure 1, a crankshaft 1 has secured around its periphery a toothed wheel 2. The toothed wheel 2 has positions for thirtysix evenly spaced teeth 4 with gaps 6 of equal width to the teeth therebetween. Each tooth and gap therefore represents 50 of angle. one of the thirty-six teeth is missing, leaving a gap 8 three times longer than the other gaps 6. The gap 8 is therefore equivalent to 150 of crankshaft rotation.

GB97.0405 39 Nay 1997 - 6 is A variable reluctance sensor, or VRS, (not shown) produces a raw sinusoidal VRS output signal 10, which is inverted and digitized into a buf f ered VRS signal, or BVRS 12. Each of the thirty-five pulses in the BVRS signal therefore has a period equivalent to 100 of crankshaft rotation.

The toothed wheel rotates anticlockwise, as indicated by the arrow 14 and is illustrated for convenience with top dead centre (TDC) at the top of the drawing.

The start of the 150 gap 8 for the missing tooth is set to occur at 1000 before TDC. This is purely for convenience owing to the fact that a prior art toothed wheel, shown on Figure 1 as PIP (for Profile Ignition Pickup) provided two pulses per revolution on the crankshaft, with edges 18,20 respectively at 1000 before TDC and at 800 after TDC. The prior art system used one or the other of these PIP pulses to schedule both the start of charging and beginning of the spark discharge.

The preferred embodiment of the invention described hereinafter uses reference edges 118,120 of one or the other of the BVRS pulses at the same angles to schedule the start of charging of the coil.

Referring now to Pigure 2, which shows a preferred embodiment of an ignition system 21, the so-called 1136-1 toothed wheelfl 2 rotates past a VRS sensor 22 to produce the raw VRS signal 10, which is passed to a buf f ering input 24 of an engine management module 26. The module comprises a microprocessor unit 28 based on an 8065 chip manufactured by Intel Corporation. Using the input BVRS signal 12 the microprocessor unit calculates (as will be explained in more detail below) the correct time for the start of charging and the beginning of discharging, of one 0891-0400 30 may 1997 7 - is 2 0 or the other of a pair of coils 30,32 and supplies a digital output signal 34,36 to one or the other of a pair of coil drivers 38,40, which in turn produce drive signals 42,44 for the coils 30,32.

Each coll driver also produces an analogue output 46,48 passed to a comparator 50, which supplies a digital output 52 to the microprocessor 28 which goes high when either one or the other of the coils has been charged half way. The dwell time T. to charge the coil may vary depending on a number of factors, particularly the battery voltage and the coil resistance, which is a function of coil temperature. Therefore, the signal microprocessor 28 uses the digital signal 52 to calculate an expected or predetermined dwell time for charging the coil.

Referring now also to Figure 1, the microprocessor will aim to discharge the coil at a particular desired spark angle 54, here 200. The expected dwell time TD terminates in the discharge of the call and the spark event. The frequency with which the BVRS pulses reach the microprocessor 28 is a measure of the engine speed at a given instant, and f rom this, the microprocessor 28 can determine an expected time interval 12 starting f rom the detected edge 118 of the BVRS signal until the occurrence of the desired spark angle 54. Since the dwell time T. is predetermined, a time interval 1. from the edge 118 to the start of charging is also known, and the microprocessor 28 therefore begins to charge the coil at this time. In this example, this time interval begins at 580 before TDC.

If the engine speed is constant, then after the predetermined dwell time T. the engine will be at the correct spark angle. However, if the engine speed has varied between the detection of the edge Ila and the end QB97.040 30 May 1997 - a is of the expected interval I., then the engine will not be cat the correct spark angle after the predetermined dwell time T,,. in order to schedule the spark event at the desired spark event, the microprocessor therefore selects a particular pulse in the series of MS pulses occurring after the start of charging and before the end of the expected interval I,,. In order to allow enough time for the microprocessor to complete its real- time control of the coil discharge, it is here not desirable to select the pulse 56 immediately preceding the pulse during which the spark event will occur; instead the microprocessor selects the leading edge 58 of the penultimate pulse 60 prior to that pulse 56 as the basis for calculating a time interval, referred to hereinafter as a discharge interval I., ending in the discharge of the coil.

The discharge interval 1 D is calculated based on the measured engine speed as determined by the frequency of the BVRS pulses immediately preceding the penultimate pulse 56, and from the angle, here 2 0 0, calculated between the desired spark angle 54 and the penultimate pulse leading edge 58. The discharge of the coil is then scheduled at the end of the discharge interval 1..

The above description is the case at low engine speeds.

At moderate engine speeds, the time interval ic f rom the edge 118 to the start of charging will become small, and at maximum engine speed, the start of charging may need to begin at angles in excess of 2705. Therefore, depending on the frequency of the BVRS pulses, the microprocessor 28 selects one or the other of the BVRS pulse reference edges liS,120 from which to base its calculation of the expected interval 1. and the time interval 1. to start of charging.

G597 "0408 30 X&Y 1997 Similarly, in order to avoid the discharge interval 1 D f rom becoming too small, pulses progressively further f rom the pulse 56 at the spark angle are selected by the microprocessor 28 for the calculation of the discharge interval 1.. At a maximum engine speed of the order of 6500 rpm, the pulse from which the interval 1. is calculated may be up to 600 prior to the desired spark angle.

If the engine speed varies significantly between a reference edge 118,120 and the start of charging, then the dwell time T. may, for larger time intervals IC vary from its optimum value. This however, is not a particular problem as long as the nominal coil charge is sufficiently is beyond a minimum level. It would, however, be possible to ensure that the dwell time was more optimal by selecting one of the BVRS pulses prior to the start of coil charging, for example a penultimate prior pulse, in a similar manner to that described above for scheduling the coil discharge.

An ignition system as described above may be used to improve the control of the spark angle and, optionally, of the dwell time. The ignition system improves scheduling of the spark event, and optionally the amount of coil charging when the engine speed is varying significantly.

Ga97-0-901 30 may 2.9.7 -

Claims

Claims is 1. An ignition system f or a spark ignition engine, comprising

means f or generating a series of pulses upon each revolution of a crankshaft and means for identifying pulses in the series relative to a top dead centre position for the engine, one or more ignition coils and means to charge a coil and then to discharge the coil to generate an ignition spark at a desired spark angle relative to the top dead centre position, characterised in that the ignition system comprises means to identify a first particular pulse occurring after the charging starts and before the discharging begins, and means to calculate a first interval from the first particular pulse so that the discharging happens at the desired spark angle.
2. An ignition system as claimed Claim 1, in which the first particular pulse is selected from amongst a plurality of pulses by the means to identify the first particular pulse, so that the first interval remains above a minimum interval as the engine speed increases.
3. An ignition system as claimed in Claim 1 or Claim 2, in which the time between the start of charging and beginning of discharge corresponds to a desired dwell time, the ignition system comprising means to identify a second particular pulse occurring before the charging starts, and means to calculate a second interval from the second particular pulse so that the charging begins at the desired dwell time prior to the beginning of discharge.
4. An ignition system as claimed Claim 3, in which the second particular pulse is selected from amongst a plurality of pulses by the means to identify the second C097-0408 30 may 1997 11 particular pulse, SO that the second interval remains above a minimum interval as the engine speed increases.
5. An ignition system as claimed in any preceding claim, in which the series of pulses comprises at least nine pulses.

is
6. An ignition system as claimed in any preceding claim, in which the series of pulses comprises no more than 360 pulses.
7. An ignition system as claimed in any preceding claim, in which the igni tion system comprises means to calculate a desired spark angle according to engine operating parameters.
8. An ignition system as claimed in any preceding claim, in which the means for generating the series of pulses upon each revolution of a crankshaft comprises a toothed wheel turned by the crankshaft, a sensor which senses each tooth passing the sensor and which outputs the series of pulses, the wheel having at least one reference tooth which produces a reference pulse as the reference tooth passes the sensor.
9. An internal combustion engine comprising an ignition system as claimed in any preceding claim.
10. An ignition system substantially as herein described, with reference to the accompanying drawings.
11. An internal cmMustion engine substantially as herein described. with reference to the accompanying drawjngs.

G997.0405 30 Xay 1997