Field of the Invention

This invention relates to a method of controlling fuel injectors which can operate in at least two different modes. It has particular but not exclusive application to Variable Orifice Nozzle (VON) fuel injectors.

Background to the invention

Variable Orifice Nozzle fuel injectors are fuel injectors whose effective discharge orifice can be varied e.g. by dispensing fuel into a combustion space through a variable number of orifices. Thus the injectors can be operated and thus switched between two or more modes where the effective orifice area is variable thus injecting different quantities of fuel.

When considering operation in different modes there will be thus at least two different gain curves relating the length of the pulse typically sent to the fuel injector valve to control fuel and the effective delivery quantity of fuel and consequently the torque provided. When moving from one mode to another, it is preferable that the torque produced is substantially the same in order to reduce vibrations, and also to increase efficiency. In order to do this the ECU must know the relationship between the electrical demand time e.g. pulse time length for both modes of operation to give the same torque.

However a problem with this is that the control system is calibrated with gain curves which may be accurate when new, but gain curves will drift with time due to factors such as seat wear, actuator ageing, coking, lacquering, etc. The particular problem with VON actuators is that this drift may vary in different ways for each mode of actuation (each gain curve).

It is an object of the invention to overcome such problems and provide for a system of compensation which ensures a smooth transition between the modes. Statement if the Invention

In one aspect of the invention is provided a method of controlling variable orifice nozzle fuel injectors in a combustion engine, said injectors being operable in at least two modes comprising: determining that there is a need to switch from a first mode to a second mode, and wherein switching of the modes is performed in progressive fashion.

The transition of modes for each injector may be performed sequentially.

The transition of modes for each injector may be performed after a set time or a set number of engine cycles

The point at which transition occurs for each injector may be performed at different set points

The injectors may be controlled to switch mode dependent on a set value of demand torque, and wherein said set value is arranged to differ for different injectors. There may be variation in the calibration of said injectors in terms of the point at which switching of modes takes place.

In a further aspect is provided a method of controlling and/or calibrating variable orifice nozzle fuel injectors in a combustion engine, said injectors being operable in at least two modes, comprising:

i) switching one injector between a first mode and a second mode;

ii) estimating or measuring the consequential torque or change in torque;

iii) adjusting the operation of said injector with respect to one and/or the other of the two modes, dependent on the consequential change in torque.

The adjusting may comprise proportionally/correspondingly varying the quantity of fuel delivered for a particular mode, at any set point.

Varying of the quantity of fuel may be such that there is no corresponding change in torque when said injector is switched modes.

Steps i) and ii) may be performed separately to normal mode transition. Steps i) and ii) may be performed, at a time when the engine is running at a substantially constant torque and or substantially close to the mode transition point.

Brief Description of Drawing Figure 1 shows two different gain curves for a dual mode VON injector with corresponding aged gain curves.

Description of the Invention

Typically fuel injectors operate to inject fuel into a combustion space through a nozzle by co- operation of a needle with a nozzle. Operation of a fuel injector is typically via application of an electrical pulse to control e.g. a solenoid or piezoelectric valve arrangement. In a typical design of fuel injector the nozzle may comprise a plurality of individual orifices (e.g holes) formed in the nozzle, such that mode selection allows the needle displacement to provide injection through one or more nozzles selectively. VON injectors are capable of producing two different gain curves, for the two different modes, as shown in figure 1. On the x-axis is shown the duration of an electrical pulse sent to the fuel injector (time on) and the y-axis show the resultant amount of fuel injected, and the torque consequentially developed will be a function of the delivered fuel quantity.

As can be seen line A shows the gain curve (it is actually a straight line but traditionally these have been referred to as "curves") for one mode where a single orifice is open to allow discharge of fuel and line B shows the gain curve corresponding to the use of two orifices. As can be seen, the two orifice mode allows more fuel to be injected for the same duration of pulse signal sent to the injector. Gain curves A' and B' correspond to the gain curves for mode A and B with an aged injector, which generally results in a reduced amount of fuel injected, for the same pulse duration.

As can be seen from the non-aged injector gain curves, when switching from the first mode A to the second mode B or vice versa, shown by bidirectional arrow E, the length of the electrical pulse has to be appropriately decreased /increased to achieve the same volume of fuel injected and hence torque. During calibration, the gain curves may be determined and the appropriate increase /decrease pulse length sent to injector determined in order to achieve the required volume of fuel/torque.

However as the injector ages, the changes in the gain curves are such that the deterioration results in problems such that the similar control of the electrical pulses will not give the same results; arrow F illustrates such problems. As can be seen, initiating the same change in electrical pulse duration, as before will, for an aged injector, result in a step change G

(decrease or increase) in the quantity of fuel delivered. This is manifested as a step change in torque and is undesirable.

According to an aspect of the invention, when it is determined (e.g. as a result of an increase in demand torque) that there needs to be a change of mode, aspects of the invention provide for the switching of modes to be done progressively. This can be implemented in a variety of ways according to different aspects of the invention. The transition for each injector/cylinder in turn may occur after a set or variable time or a set or variable number of engine cycles, depending on operating conditions. This ensures that if there is a discrepancy in the gain curves, then the individual steps in the torque as each injector is switched, is less than if they were all switched together

In a first example, during mode switching, the cylinders are switched one after another, e.g. in a progressive fashion. So for example in a 4 cylinder engine with one injector per cylinder, where there are two modes A and B and all the injectors are operating in mode A and there is a decision (e.g. based on torque demand) to switch to mode B, the first VON injector may be switched to mode B, and after a set period of time or number of engine cycles, the second VON injector is switched to mode B, and after a set period of time or number of engine cycles, the third injector is switched to mode B and so on. Thus, for example, in a four cylinder engine, the step torque is then only 25% of what it would have been otherwise. In other words after a decision to transition between modes, the injectors could transition progressively based on a time interval or after a certain number of engine cycles, thus reducing any large step change which may arise when all the injectors are switched at the same time.

The injector modes may be controlled (switched) progressively according to another embodiment as follows. The control may be such that each injector is switched according to (different) torque demand. So where the absolute or relative torque demand reaches a certain point (e.g. threshold) a particular injector is switched (e.g. from mode A to mode B) and when the torque demand reaches a different threshold value, another injector has its mode switched from A to B. So where in prior art system all injectors are switched at 50% for example, according to this aspect, the injectors are switched progressively one at a time, over a certain period. So at 48% torque no injectors are switched, but at 52% torque first one is switched at 56% another injector is switched, until all injectors were switched.

In another alternative strategy, the transition torque may be calibrated differently for each injector. This example is similar to the one described above, but rather than the ECU, for example, controlling when the injectors are switched, a single command is sent, e.g.

indicative of demand torque, to the injectors, and injectors would be adapted to switch mode dependent on this demand torque and an inherently set threshold torque. Again each injector would be switched at a different engine torque. For instance the first injector at 45% torque the second at 50% the third at 55% etc. This means that if engine torque demand were at 48% torque continuously one injector would be switched and the other 3 not switched.

Aspect 2

In one aspect of the invention, to overcome the aforementioned problems, the relative drift in the gain curves for the different modes for a particular injector is tracked. One way of doing this is to select one of the injectors and switching this from one mode (e.g. two orifices to another mode (one orifice), or vice versa. The torque produced on the corresponding cylinder which has the fuel injector mode switched is measured and compared when operating in mode A and in mode B. This may be done separately to the mode transition, typically at a time when the engine is running at a fairly consistent torque which is near to the transition point. The measurement of torque differences is often already done in the cylinder balancing strategy - which looks at the crankshaft acceleration cylinder to cylinder and thus corrects injector drift cylinder to cylinder. Alternatively, in the event that cylinder pressure sensors are fitted then a suitable feedback signal on the fuel delivered in each operating mode can readily be obtained by that method. The two gain curves on the one injector can now be compared with each other rather than between different cylinders.

Thus in an example, for a particular cylinder or injector, a switch of mode is made and the torque (or change in torque) for the two different modes is measured. If for example, the torque from mode A (one orifice/row operation) is 5% less than from mode B (two orifices/rows) operation, then mode A operation can be corrected/adjusted by increasing fuel flow, e.g. the electrical pulse duration, by 10%. (in the case of a 50:50 flow split). If the mode B (two row operation is 5% less than the mode A (one row operation) then likewise the two row (Mode B) operation of the injector can be corrected up by 10%. In general the following correction strategy may be applied:

Rowl_Correction = max(0, (Delivery_row2 - Delivery_rowl)/Delivery_row2 / (row2 % of total nominal injector flow))

Row2_Correction = max(0, (Delivery_rowl - Delivery_row2)/Delivery_rowl / (rowl % of total nominal injector flow))

In essence, for a particular torque, if switching from mode A to mode B reduces torque by a value of x%, then with respect to the correction factor, the fuel delivered in mode B is increased by an appropriate amount, which may be the value of x multiplied by a factor, said factor being a function of the ratio of effective orifice areas for the two modes.

When a certain fuel delivery is demanded, the gain curve gives a "time on" (TON) value for the injector. It is common to adjust this value before it is applied to the injector. Corrections may be applied for the zero crossing point on the gain curve, the gradient, points of inflection, wear, pressure wave modulation etc. The benefit of applying the corrections as described above is that only one nominal gain curve is required for all injectors, over their whole lifetime, in any operating mode. The real gain curve of each injector at any point in its lifetime does not change, but the corrections allow the real gain curve to be adjusted to fit the nominal gain curve. The advantage of the above methodology is that one can be much more certain about a correction of this nature than a correction applied between cylinders using for instance the cylinder balancing strategy. This new strategy thus limits the total negative drift of the injector to the minimum of the drift of either row of the injector. This assumes that injector gain curve drift is only ever a reduction in the fuel delivered for a particular electrical pulse length. This advantage is provided because one is able to look and compare the performance on an individual cylinder with all other engine parameters(friction, air path, etc) exactly the same, except the injector operating mode (i.e. the effect of switching between 1 and 2 rows). Further it allows detection of failure to inject from one or the other rows by comparison of the gain curves with each other and with a maximum drift value for normal operation. Torque measurements (or estimates thereof e.g. from pressure measurements) are thus according to one aspect used to calibrate changes to the delivered fuel quantity. By making the measurements at two different torques, adjustments are made to the corrections for both to the gradient of the gain curve and to the zero crossing point. Additional measurement torques might allow more complicated gain curve shapes to also be characterised.