Control of the dwell time of ignition coils in spark-ignition internal combustion engine

Abstract

A method for controlling an ignition unit for a spark plug of an internal combustion engine is provided. The ignition unit is driven by a spark demand pulse that is defined by start time point and an end time point. A voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse is determined, and then an adjustment amount based on the determined voltage level is determined. The end time point of the determined demand pulse is adjusted by the determined adjustment amount. A benefit of the invention is that a spark demand pulse is generated that is more appropriate for the prevailing battery voltage at the time the demand pulse is generated. Therefore, the process provides a more reliable approach for controlling the generation of spark at a spark plug as controlled by an ignition unit under conditions where the battery voltage may be compromised, for example for an aged battery and / or during cold engine conditions. There is also provided an engine system having a control unit configured to implement the method, a control unit comprising a processor to carry out the method, and a computer program product embodying the method.

Description

[0001] PH24078-2

[0002] IMPROVEMENTS RELATING TO CONTROL OF IGNITION COILS IN SPARK-IGNITION INTERNAL COMBUSTION ENGINES

[0003] Technical Field

[0004] This disclosure relates to improvements in control of ignition coils for spark-ignition internal combustion engines (ICEs). Such ICEs may be powered by a variety of fuels that are liquid or gaseous at room temperature, such as gasoline or hydrogen.

[0005] Background

[0006] Fuel injectors are integral components of a modern internal combustion engine, responsible for delivering fuel into the engine cylinders in a controlled and efficient manner. Fuel injectors work closely with ignition coils to ensure proper engine operation.

[0007] An ignition coil or ‘unit’ is responsible for converting the vehicle battery voltage (typically 12V in an ICE vehicle) into the high voltage needed to create a spark at the spark plug. The spark generated by the spark plug ignites the air-fuel mixture in the engine cylinder.

[0008] Ignition units generally consist of two windings or coils: a primary coil and a secondary coil. Energizing the primary coil generates a magnetic field. When the current to the primary coil is interrupted, the magnetic field collapses, which induces a high voltage in the secondary coil that is then delivered to the spark plug thereby generating a spark.

[0009] The dwell time (or charge time) refers to the duration for which the primary coil is energized. During the dwell time, the primary coil stores energy by virtue of the magnetic field. The longer the dwell time, the more energy the coil can store, resulting in a more powerful spark. However, excessively long dwell times can cause premature aging of the ignition units. Proper control of the dwell time is crucial to ensure that the ignition system performs efficiently and the engine operates smoothly.

[0010] Low battery voltage and low temperatures can have an adverse effect on the performance of ignition units. In the case of low battery voltage, typically caused by an aging or undercharged battery, the voltage available to charge the ignition unit during the dwell time may be insufficient, and this is particularly the case during engine cranking when the battery is being heavily loaded by the starter motor. This can lead to an incomplete charging process, where the ignition coil is unable to generate the necessary high voltage to produce a spark. This in turn can cause a misfire, leading to engine performance issues, poor fuel efficiency, and higher emissions. Low temperatures can exacerbate these problems, as they reduce the ability of the battery to supply sufficient voltage. At colder temperatures, the viscosity of the engine oil increases, requiring more energy for starting and additional strain on the electrical system. As a consequence, the available voltage for the ignition system may be lower than required, resulting in a reduced dwell time and potentially weak or inconsistent spark production.

[0011] It is with these issues in mind that the embodiments of the invention have been devised.

[0012] Summary of the Invention

[0013] Against this background, according to an aspect of the invention, there is provided a method for controlling an ignition unit for a spark plug of an ICE, wherein the ignition unit is associated with a respective engine cylinder in which combustion is to occur, wherein the ignition unit is driven by a spark demand pulse, the method comprising: determining a spark demand pulse for driving the ignition unit, wherein the spark demand pulse is defined by a start time point and an end time point, determining a voltage level of an associated engine battery at a time proximate to the start time point ofthe spark demand pulse, determining an adjustment amount based on the determined voltage level, and adjusting the end time point of the determined demand pulse by the determined adjustment amount.

[0014] A benefit of the invention is that a spark demand pulse is generated that is more appropriate for the prevailing battery voltage at the time the demand pulse is generated. Therefore, the process provides a more reliable approach for controlling the generation of spark at a spark plug as controlled by an ignition unit under conditions where the battery voltage may be compromised, for example for an aged battery and / or during cold engine conditions. In the context of a hydrogen-fuelled ICE, the improved reliability of spark generation will help ensure that no unburned gases are present in the exhaust system or the cylinders.

[0015] It is envisaged that the most benefit may be achieved by performing the methodology whilst the engine is in the cranking mode of operation — that is, when the engine is not producing power but is being driven by a starter motor in order to initiate self-sustaining operation. Therefore, references to “spark demand pulse” and “dwell period” may be understood to be those features occurring during a cranking mode of operation, for example a “cranking dwell period” or a “cranking spark demand pulse”.

[0016] The examples of the invention may also be considered to reside in an engine system comprising at least one piston movable within an associated engine cylinder, and a spark plug associated with the cylinder, the spark plug coupled to an ignition unit, wherein the ignition unit is driven by a spark demand pulse, the engine system having a control unit configured to carry out the steps as defined above.

[0017] In some examples, the step of determining the spark demand pulse may further comprise: determining an energy demand value for a spark event of the ignition unit, prior to determining the spark demand pulse in dependence on the battery voltage level and, optionally, on the determined energy demand value. The energy demand value may be determined by reference to a relational data map associating the battery voltage level and at least one of engine temperature, air temperature, or engine load. The energy demand value may be the coil current that is required to initiate a spark at the spark plug, whilst factoring in one or more operational parameters of the engine, such as engine load, engine temperature, air temperature, or air pressure.

[0018] In some examples, the step of determining the spark demand pulse may include determining a filtered or averaged battery voltage level and determining the spark demand pulse in dependence also on the filtered or averaged battery voltage level.

[0019] In some examples, the step of determining a battery voltage level at a time proximate to the start time point of the spark demand pulse comprises measuring the battery voltage at one or more sample time points proximate the start time point of the spark demand pulse. Measurement of the battery voltage level may take place as a single measurement / sample or a plurality of measurements / samples. In the case of a plurality of measurements / samples, a suitable averaging may be applied, which may be a simple average or a weighted average, e.g., to bias towards the later measurements / samples.

[0020] In measuring the engine battery voltage proximate to the start of the dwell period, the at least one sample time point may be measured in a time window extending less than 2ms in advance and less than 2ms behind the start time point of the spark demand pulse. Expressed alternatively, the window may have a configurable width but for example between 5 crank angle degrees before and 5 crank angle degrees after the start time point of the spark demand pulse.

[0021] A plurality of battery voltage measurements may be taken at a plurality of sample time points, and an aggregate voltage level may be used to determine the voltage level. The samples may be taken in a window of between e.g. 1 to 5ms after the start time point of the spark demand pulse.

[0022] The control unit may take the form of an engine control unit (ECU) of the engine. The examples of the invention may also be expressed as a control unit comprising a processor or processing means configured to perform the method as defined in any of the above-defined features, or a computer program product comprising instructions which, when the program product is executed by a computer, cause the computer to carry out the method as in the above-defined features, or a computer-readable storage medium or a data carrier comprising instructions, for example in the form of a computer program product, which when executed by a computer, cause the computer to carry out the method of any of the above-defined features.

[0023] Further optional and advantageous features are referenced in the detailed description and the appended claims.

[0024] Brief Description of the Drawings

[0025] Examples of the invention will now be described with reference to the following figures:

[0026] Figure 1 is a schematic view of a simplified engine system in which the examples of the invention may be incorporated, where it should be noted the engine system shown provides a view of a single cylinder of what may be a multi-cylinder engine;

[0027] Figure 2 is a set of timing plots showing the conventional timing of a spark demand pulse or ‘dwell period’ (a), a coil current characteristic (b) associated with the spark demand pulse, and the spark generation (c) that is generated by the spark demand pulse;

[0028] Figure 3 is a flow chart depicting an example of a methodology for determining the duration and timing of a spark demand pulse in accordance with the invention;

[0029] Figure 4 is a timing plot illustrating a measured battery voltage profile;

[0030] Figure 5 is a flowchart illustrating a more detailed representation of Figure 3;

[0031] Figure 6 is a flowchart illustrating another example of a more detailed representation of Figure 3.

[0032] Detailed Description

[0033] In general, the examples of the invention provide strategies for controlling an ignition unit of a spark-ignition ICE. The strategies may provide benefits in terms of improving the reliability with which the ignition unit can energise a spark plug to generate a spark, particularly in conditions where battery voltage is relatively low, for example through an aged battery or reduced ambient temperatures. This can be particularly useful in gas-fuelled engines such as hydrogen ICEs as it helps to ensure that unburned hydrogen is not emitted in the exhaust.

[0034] Figure 1 depicts a simplified engine system 2 in schematic form that incorporates typical engine components relevant to the examples of the invention and into which the examples of the invention may be incorporated. It should be noted that a typical engine system would include many more components than shown in Figure 1 as would be well understood by a person skilled in the art. However, a full discussion of a complete engine system is not required here so has been omitted for brevity.

[0035] The engine system 2 comprises a cylinder 4. A piston 6 is received slidably within the cylinder 4 in the usual manner. The piston 6 is connected to one end of a connecting rod 8 at a gudgeon pin 9, and the other end of the connecting rod 8 is connected to a rotatable crank 10 at a crank pin 12, which are located at a lower end of the cylinder 4

[0036] Although not shown, the cylinder 4 is provided in an engine block 14. While a single cylinder 4 is shown in Figure 1 , it should be noted that this is for simplicity of description and a practical implementation would include a multi-cylinder engine.

[0037] A cylinder head 16 is provided at the upper end of the cylinder 4. Together, the cylinder head 16 and the piston 6 define a combustion chamber 18 in which combustion occurs when the piston 6 is at or near a top dead centre (TDC) position.

[0038] The cylinder head 16 provides an inlet duct 20 and an outlet duct 22. An inlet valve 24 is positioned to control the flow of a fuel / air mixture into the combustion chamber 18 from the inlet duct 20. The flow of air / fuel mixture along the inlet duct 20 is controlled by a throttle valve 26. The throttle valve 26 is controllable through any conventional means as would be well understood by a skilled person. Likewise, the delivery of the air / fuel mixture into the inlet duct 20 is provided by a suitable fuel delivery system (not shown) that would be well understood by the skilled person.

[0039] An outlet valve 28 is configured to control the flow of exhaust gas from the combustion chamber 18 into the outlet duct 22.

[0040] Movement of the inlet valve 24 is controlled by an inlet cam member 30, and movement of the outlet valve 28 is controlled by an outlet cam member 32. The cam members 30, 32 act on the respective valves 24, 28 in a manner that is known in the art. A spark plug 34 is positioned within the cylinder head 16 such that a tip end of the spark plug 34 protrudes into the combustion chamber 18. In this way, the spark plug 34 is operable to ignite an air / fuel charge that is delivered to the combustion chamber 18.

[0041] As is known, the piston 6 moves between a TDC position and a bottom dead centre (BDC) position within the cylinder 4 as it is driven by expanding gases in the combustion chamber 18 and, in doing so, rotates the crank 10. The TDC position is the position of the piston 6 when it defines a minimum volume of the combustion chamber 18 (i.e., the piston’s closest position to the spark plug 34). Conversely, the BDC position is the position of the piston 6 when it defines a maximum volume of the combustion chamber 18 (i.e., the piston’s furthest position away from the spark plug 34).

[0042] A cycle of the piston such that the piston 6 travels from a BDC position to the TDC position and back to the BDC position is half of a full cycle in a four-stroke ICE.

[0043] The spark plug 34 is coupled to an ignition unit 40. The ignition unit 40 incorporates the functionality of an ignition coil and associated control circuity required for operation. The ignition unit 40 is coupled to a vehicle battery 42 for the provision of a low voltage supply (typically a nominal voltage of approximately 12V). An ignition unit may sometimes simply be referred to as an ignition coil.

[0044] The ignition unit 40 is responsible for supplying the spark plug 34 with the high-voltage electrical charge necessary for spark generation. As is well known, the ignition unit 40 incorporates a transformer that steps up the voltage from a vehicle battery 42 to the tens of thousands of volts required to create a spark. Although the following components are not shown in Figure 1 , it will be understood that the ignition unit 40 consists of two windings: a primary winding that is coupled to battery voltage via an associated control circuit, and a secondary winding that is connected to the spark plug 34. When current flows through the primary winding, a magnetic field is created. When the current is interrupted, the magnetic field collapses, inducing a high-voltage current in the secondary winding. This high-voltage pulse is communicated to the spark plug 34 to produce the spark at its tip, known as the spark gap.

[0045] The timing of the spark, referred to as spark timing, is crucial for engine performance. The spark must occur at the correct moment in the cylinder cycle to enable optimal combustion. In most engines, this timing is adjusted to occur just before the piston 6 reaches the top of its compression stroke (at TDC). This is known as spark advance. At TDC, the piston 6 is at its highest point in the cylinder 4. Ideally, the spark should occur slightly before TDC, allowing time for the air / fuel mixture to ignite and begin expanding as the piston 6 starts its downward stroke. If the spark occurs too early (too much spark advance), the mixture may ignite before the piston 6 has reached the optimal compression, which can lead to engine knock or damage. Conversely, if the spark occurs too late (too much spark retard), the fuel may not burn efficiently, leading to a loss of power and increased emissions.

[0046] Modern engines typically use a timing control system (based on vehicle sensor data) to adjust the timing dynamically, depending on factors such as engine load, speed, and temperature. This ensures that the spark occurs at the most optimal point in each cycle, enhancing performance, fuel efficiency, and reducing harmful emissions.

[0047] In summary, the spark plug 34 and ignition unit 40 work together to initiate combustion in the cylinder 4, with the ignition unit 40 providing the necessary high-voltage current to the spark plug 34. The precise timing of the spark, closely tied to the position of the piston 6 in a particular cylinder cycle, is important for engine efficiency and performance.

[0048] The ignition unit 40 is also coupled to a control unit 44. The control unit 44 in this example of the invention provides the controlling input to the ignition unit 40 to cause it to charge the spark plug 34 to generate a spark. The control unit 44 may be a dedicated control unit or may represent functionality of the main ECU of the vehicle. The control unit 44 is adapted to receive data input 46 to sense operational parameters of the engine to provide suitable control output signals to the ignition unit 40 as discussed above, for example providing a drive signal 48. The control unit 44 also receives a timing signal 47 from a crank angle sensor 49. In this way, the control unit 44 has a precise measurement of the position of the pistons within the respective cylinders so appropriate drive signals 48 can be determined.

[0049] The control unit 44 includes a memory component 50. The memory component 50 stores data such as self-learnt control parameters and operating history data, as will be described in more detail later, which can be retrieved by the control unit 44 even after a power down cycle.

[0050] The control unit 44 may be operable to perform various engine monitoring and control objectives to manage the performance of the vehicle or engine system into which it is installed. It should be appreciated that the control unit 44 may be any suitable control environment provided by the engine system 2. The control unit 44 may be the ECU of the engine system 2 or another control unit that is configured to carry out other performance and monitoring tasks within the engine system 2 of the broader vehicle. In particular, the control unit 44 may be a control environment provided specifically for the purposes of performing the method as discussed in this disclosure. Irrespective of the functionality of the control unit 44, it will be appreciated that the control unit 44 has the necessary memory 50, processing environment 52, and communications interface 54 to be integrated into the engine system 2 and the broader system of an associated vehicle.

[0051] Having described the general overview of the engine system 2, the discussion will now focus on a methodology that may be embodied as suitable algorithms and software processes implemented on the control unit 44 for controlling the ignition unit 40 and, thus, the spark generation at the spark plug 34.

[0052] Figure 2 illustrates a conventional spark demand pulse (SDP). A SDP may be understood as being the voltage pulse that is applied to the ignition unit 40 (and, more specifically, the primary coil thereof) to charge the primary coil prior to rapid termination of the voltage pulse. The SDP goes high for a predetermined time period, which is around 3ms-6ms in circumstances where there is a good battery voltage available at moderate temperature conditions. The rising edge T1 of the SDP represents the point at which battery voltage is applied to the ignition unit 40, and the falling edge T2 represents the point at which the applied voltage is terminated, thereby triggering a spark at the spark plug 34.

[0053] The duration of the SPD is the temporal separation between T1 and T2. During this time period, often referred to as the ‘dwell period’, the coil current increases to a predetermined level and then terminates abruptly at T2, following which a spark is generated by the collapsing coil current. Notably, the coil current level indicates the energy that accumulates in the ignition coil, which must be at a sufficient level to generate a spark.

[0054] The timing of the spark generation process is determined in the usual way by the control unit 44 in a manner that would be familiar to a person skilled in the art, as the spark timing is linked to the operation of the engine based on engine speed, engine position (as determined by the crank angle sensor 49), torque demand, and other operational engine parameters.

[0055] In ordinary circumstances, when battery voltage is at nominal levels and at moderate temperatures, the energy that is accumulated in the ignition unit 40 over the dwell period is sufficient to generate a spark at the spark plug 34. It is customary to calculate the dwell period based on an average of the battery voltage (or a filtered battery voltage value, using a simple configurable low pass filter with a time constant of typically between 10 to 100ms) for a given period in advance of the spark, typically greater than 100ms in advance of spark timing. This measure partially compensates for a reduced battery voltage level since the calculations can be made to increase the dwell period if the battery voltage is deemed too low. However, this process can be fallible in some circumstances, especially during engine cranking when a starter motor provides a large current load to the battery without an alternator providing compensation, causing the battery voltage to reduce significantly.

[0056] In accordance with an example of the invention, a control method 100 for the ignition unit 40, as shown in Figure 3, executes in respect of each cylinder of the engine. The control method 100 takes place during engine cranking when voltage levels are more variable, as the ignition unit 40 is fed with a voltage level that is determined by the vehicle battery 42. In contrast, during normal operation, the nominal 12V voltage level (running voltage usually between 13- 14.5V) is held constant due to operation of the alternator.

[0057] The methodology discussed below is executed for each cylinder event in a succession of cylinder events during which a spark is generated by the spark plug 34 to trigger combustion within the cylinder as it approaches TDC position during a compression stroke. Therefore, in a four-stroke engine running at approximately l OOOrpm, the method may have an execution interval of approximately 120ms. In a 4-cylinder four-stroke engine, the logic may run every 30ms (or equivalent crank angle timing in the rotational crank angle domain), as it needs to perform the same calculation for each cylinder. It should be appreciated that these values are merely exemplary. In practice, during cranking the crank speed will be lower than OOrpm so the execution interval may be increased compared to what is described here, for example, around 600ms per cylinder execution cycle when the engine is running at a cranking speed of 200rpm.

[0058] In general, the method 100 achieves the objective of determining the characteristics of a SDP.

[0059] As a first step, at step 102, the process determines the characteristics of a dwell period. The characteristics may be simply the duration of the SDP, but may also include the start time point and the end time point of the SDP. These characteristics are sufficient to determine when the SDP goes high and low. The time separation between the start time point and the end time point are defined by a pulse duration value. The characteristics may be expressed in milliseconds or in units of crank angle degrees.

[0060] The SDP may be determined by a conventional process. For example, a filtered value of battery voltage can be used as an input into calibration tables that also factor in engine load and ambient temperature (as examples of input variables) in order to determine the duration of the spark pulse. Still within step 102, once the characteristics of the SDP have been determined, the SDP is scheduled by the calculation of a start time point and an end time point for the SDP. Following this, the battery voltage is sampled proximate to the start time point at step 104. The sampling of the battery voltage may be done before, simultaneous with, or after the time the SDP is initiated. A single or plurality of measurements of the battery voltage may be taken. A suitable averaging process may be applied if a plurality of battery voltage measurements is taken.

[0061] Measuring the battery voltage proximate to the start of the SDP means that the system determines a more accurate and contemporaneous evaluation of the battery voltage. Using the measured battery voltage, the required characteristics of the SDP can be recalculated at step 106. In some examples, a conventional calibration map approach can be used to recalculate the end time point of the SDP, using the ‘new’ battery voltage measurement. The end time point of the spark demand pulse can then be adjusted during the duration of the SDP at step 108.

[0062] The benefit of this approach is that the length of the SDP is increased based on the instantaneous measurement of the battery voltage, which ensures sufficient energy levels will occur within the ignition unit 40 in order to generate a spark reliably.

[0063] This process is illustrated graphically in the series of plots shown in Figure 4. Figure 4(a) depicts the battery voltage, which is shown reducing from a BDC position of the cylinder to a TDC position, during cranking when the alternator is not running to support the battery voltage. This may be, for example, from about 8V at a BDC position to about 6V at a TDC position.

[0064] The start time point of the SDP triggers at T1 (see Fig. 4(b)), at which point the coil current begins to increase (Fig. 4(c)). The dashed line indicates the current rise profile that would be present with a nominal battery voltage. However, since the battery voltage is much reduced, the solid line indicates the resulting coil current rise profile. As can be seen, the coil current takes longer to increase to a sufficient level when subject to a reduced battery voltage.

[0065] The measurement time of the battery voltage is shown at TO, by way of example. Here, Vbatt is measured before T1 , but it could also be measured simultaneously with or after T1 , as discussed above. Since the battery voltage is relatively low, the charge duration has been extended compared to what a nominal battery voltage would be. This is seen as the time period AT, which extends the falling edge of the SDP or the ‘end time point’, shown at T2.

[0066] The spark generation is shown at Figure 4(d).

[0067] The resultant dwell period (charge duration) is therefore extended compared to the case where the battery voltage is at a higher value. The benefit of this is that the process compensates for lower battery voltage levels that would otherwise compromise the ability of the ignition unit 40 to charge to a sufficient level to generate a spark.

[0068] Figure 5 illustrates one example of a more detailed method 200 to that described above with reference to Figure 3, although the general principle is the same.

[0069] Method 200 starts with determining the characteristics for an initial SDP. Step 202 involves calculating the required energy for the coil of ignition unit 40 to trigger a spark. Step 204 involves calculating the duration of the SDP based on a filtered battery voltage level.

[0070] Taking step 202 in more detail, it should be noted that the ignition unit 40 can be compared to an LR circuit, having a specified resistance R and inductance L. The energy required in the ignition unit 40 at the end of a charge period may be determined based on the electrical characteristics. This can be a stored value in the memory component 50 in a suitable calibration table, since it is generally an understood parameter for the required energy of the coil to generate a spark successfully, although the stored value may suitably be adjusted by appropriate compensation strategies.

[0071] The charging period results in energy being stored in the primary coil of the ignition unit 40 at the end of the charging period. The stored energy can be expressed as:

[0072] E = -2LI^d(1)

[0073] Therefore, it can be appreciated that the charging current at the end of the charging period determines the energy stored in the primary coil of the ignition unit 40. So, charging starts when current is at 0A and then increases to an end current level (lend) at the end of the dwell period (tend).

[0074] The end time point is dependent on the spark timing derived from general vehicle operations, since the end time point marks the time at which a spark is required to be generated at the spark plug 34.

[0075] Once the energy required to charge the ignition unit 40 has been determined at step 204, this value can be used to calculate the length of the SDP based on the voltage level. More specifically, conventionally it is known to use a filtered value of battery voltage to calculate the dwell period, and it is this process that is implemented in step 204. A suitable calibration table or ‘look up table’ is constructed to derive a multiplication factor or ‘k factor’ from values of voltage, engine load, engine temperature, and air temperature. This and other conventional means to calculate the duration of the SDP would be known to a skilled person. The k factor is used as a multiplier with the required end current level lend to calculate the SDP duration.

[0076] Once the characteristics of the SDP have been determined, the process moves to step 206 at which point the start time point of the SDP is scheduled. As the skilled person will understand, the start time point is a defined point set by the required spark timing. The spark timing, i.e. the start time point and optionally the end time point, may occur before the calculation of the SDP duration. Given the determined start time point, and the SDP duration, the end time point of the spark demand pulse can also be determined.

[0077] The method then enters a pause period at which it loops through check point 208 to monitor for the start of the SDP. It should be appreciated at this point that the loop achieves the dynamic adjustment of the end time point of the SDP as it monitors for the start of the SDP before sampling the battery voltage that triggers the adjustment of the end time point once the dwell is due to start. Other implementations may be acceptable, such as a software or hardware interrupt.

[0078] One the SDP starts (see T1 on Figure 4), the monitoring step 208 terminates and the process flows to step 210. At step 210, the battery voltage is measured contemporaneously with the start of the SDP. The precise point of measurement is not crucial, but there needs to be sufficient timing within the duration of the SDP to take the voltage measurement and for the algorithm to perform the subsequent calculations to adjust the end point time of the SDP as discussed above. It is envisaged that battery voltage is sampled shortly after the start time point T1. Although it could be a ‘time triggered’ sample, meaning that the sample is taken contemporaneously with the start time point, it is envisaged that the sample may be taken within 3ms, or preferably within 1 ms of the start time point.

[0079] In the illustrated example, it is envisaged that a single voltage sample is taken. However, it may also be acceptable to take a plurality of battery voltage samples and to derive an average value. The average value may be calculated as a straight average or a weighted average that biases towards later voltage samples.

[0080] Once the battery voltage value proximate the start time point has been calculated, the dwell period is adjusted, as shown at step 212. This process can be achieved by re-calculating the dwell period in the manner set out above with reference to step 204. This has the effect of postponing the end time point of the dwell period, allowing a longer time period for coil current to build up within the ignition unit 40. Another example in accordance with the invention is shown in Figure 6. Steps 302 to 306 provide the same process as described for steps 202 to 206 above.

[0081] Step 308 represents a monitoring loop during which the method 300 monitors for the start of the dwell period.

[0082] Steps 310 to 314 represent an energy calculation loop during which the current building in the ignition unit 40 is evaluated in real time. By the term ‘real time’ it is meant that the calculations are being performed while the dwell period is in operation. That is, the control unit 44 is measuring values and performing calculations at the same time the measurements are being made, rather than storing the data and performing the calculation at a later time.

[0083] By way of further explanation of the process covered at steps 310 to 314, the following principles can be considered.

[0084] The voltage across the ignition coil can be expressed as:

[0085] Vs = VR+VL (2)

[0086] Equation (2) can be expressed in terms of the obtained electrical characteristics of the ignition coil (L and R) as:

[0087] Vs (t) = l(t)R + Ldl(t) / dt (3)

[0088] So, the equation for charging of the ignition unit 40 can be expressed as: dl(t) / dt = Vs(t) / L - l(t)R / L (4)

[0089] The charging period results in energy being stored in the primary coil of the ignition unit 40 at the end of the charging period. The stored energy can be expressed as:

[0090] E = -2LI^d(5)

[0091] Therefore, it can be appreciated that the charging current at the end of the charging period determines the energy stored in the primary coil of the ignition unit 40. So, charging starts when current is at 0A and then increases to an end current level (lend) at the end of the dwell period (tend). The required charging current at the end of the dwell period has been determined at step 302, as discussed above. Equation (4) can be expressed as:

[0092] Thus, equation (6) provides an iterative approach to calculating the coil current at each time step based on a measured voltage level. Thus, at step 310, a measurement of the battery voltage (Vs) is taken at a time step n; at step 312, the coil current at time step n+1 is determined based on the measured voltage and the previous coil current (starting at zero for the first sample, e.g. at the start time point of the spark demand pulse); and at step 314, the calculated coil current is compared with the required coil current for a successful spark, as determined previously at step 302.

[0093] If the comparison step 314 determines that the coil current has not yet reached or exceeded the required current, the process loops back to step 310. The process steps 310, 312, 314 therefore repeat in a loop until the determined coil current reaches or exceeds the required current. Upon this occurrence, the process exits the looping steps 310-314 and moves to step 316, at which point the process 300 terminates the SDP by triggering the end time point, as shown in Figures.

[0094] The initially calculated end time point at step 304, or at least the calculated SDP duration, is therefore extended in order to increase the accumulated energy in the ignition unit 40, which improves the reliability of spark generation during cranking. This effect is the same as the method 200 as described above with respect to Figure 5, in that the end time point of the spark demand pulse is extended in time, as shown in Figure 4.

[0095] In the above discussion, the sampling of the battery voltage may be configured appropriately to ensure enough battery samples can be taken within a typical SDP. For example, the sampling rate may be every 1 ms, and so take place over 3ms to 15ms (by way of example only), which would cover the typical length of spark demand pulses.

[0096] It will be appreciated that various adaptations may be made to the specific implementations described above without departing from the inventive concept as defined by the claims.

Claims

Claims1 . A method for controlling an ignition unit for a spark plug of an internal combustion engine, wherein the ignition coil is associated with a respective engine cylinder in which combustion is to occur, wherein the ignition coil is driven by a spark demand pulse, the method comprising: determining a spark demand pulse for driving the ignition unit, wherein the spark demand pulse is defined by a start time point and an end time point, determining a voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse, determining an adjustment amount based on the determined voltage level, adjusting the end time point of the determined demand pulse by the determined adjustment amount.

2. The method of Claim 1 , wherein the determining the spark demand pulse further comprises: determining an energy demand value for a spark event of the ignition unit, prior to determining the spark demand pulse in dependence on the battery voltage level and the determined energy demand value.

3. The method of Claim 2, wherein the energy demand value is determined by reference to a relational data map associating the battery voltage level and at least one of engine temperature, air temperature, or engine load.

4. The method of any of the preceding claims, wherein determining the spark demand pulse includes determining a filtered battery voltage level, and determining the spark demand pulse in dependence also on the filtered battery voltage level.

5. The method of any of the preceding claims, where determining a voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse, comprises measuring the battery voltage at one or more sample time points proximate the start time point of the spark demand pulse.

6. The method of Claims 5, wherein the or each sample time point is measured in a time window extending less than 2ms in advance and less than 2ms behind the start time point of the spark demand pulse.

7. The method of Claims 5 or 6, wherein a plurality of battery voltage measurements is taken at a plurality of sample time points, and an aggregate voltage level is used to determine the voltage level.

8. The method of Claims 5 or 6, where determining a voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse comprises measuring the battery voltage at a single sample time point proximate the start time point of the spark demand pulse.

9. The method of Claims 1 to 4, wherein the step of determining a voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse, comprises monitoring the battery voltage for a time period after the start time point of the spark demand pulse.

10. The method of Claim 9, wherein, in monitoring the battery voltage for a time period, the method further includes determining the energy in the ignition unit in real time over the monitored time period.11 . The method of Claim 10, wherein, in determining in real time the energy in the ignition unit over the monitored time period, the method includes setting the determined adjustment amount based on when the monitored energy in the ignition unit reaches or exceeds the determined energy demand value.

12. An engine system comprising at least one piston movable within an associated engine cylinder, and a spark plug associated with the cylinder, the spark plug coupled to an ignition unit, wherein the ignition unit is driven by a spark demand pulse, the engine system having a control unit configured to carry out the steps of:determining a spark demand pulse for driving the ignition unit, wherein the spark demand pulse is defined by start time point and an end time point, determining a voltage level of an associated engine battery at a time proximate to the start time point of the spark demand pulse, determining an adjustment amount based on the determined voltage level, adjusting the end time point of the determined demand pulse by the determined adjustment amount.

13. A control unit comprising a processor configured to perform the method of any one of Claims 1 to 11.

14. A computer program product comprising instructions which, when the program product is executed by a computer, cause the computer to carry out the method of any one of Claims 1 to 11.

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