Fuel injection split method

Abstract

A method for a direct injection engine for dividing a fuel injection corresponding to one engine cycle into minimum sub-injections, comprising the steps of: - determining a maximum required number of sub-injections (N) by dividing a fuel mass (M) to be injected during one engine cycle by a minimum injectable mass (Mmin) and rounding down to the nearest integer, - lower-bounding said maximum required number (N) by a maximum allowed number of sub-injections (Nmax), - verifying whether an injection duration (D) for such maximum required number of sub-injections (N) is less than a duration (Dmax) of a possible injection window, and if not, decrementing the maximum required number (N), iterating until the verification is positive.

Description

Technical Field

[0001] The present invention relates to the field of engine control, and more particularly to engine control in the case of direct injection gasoline engines. Background Art

[0002] As is known, in internal combustion engines, the fuel is injected directly into the cylinders. This can be achieved, for example, by means of a chamber (or in English "rail") containing the pressurized fuel and an injector that selectively closes the pipe connecting the chamber to the cylinders. When the injector opens, the fuel is sprayed into the cylinders to produce a liquefied gas that is able to mix as uniformly as possible with the air present in the cylinder.

[0003] There is the problem that the sprayed fuel may reach the opposite wall of the cylinder. This is harmful because the fuel is then deposited on the wall and tends to remain in liquid form, mainly when the engine is cold. The difficulty of vaporizing the fuel is then increased, which has a negative impact on its subsequent combustion. In addition, such deposits can interfere with the smooth and uniform mixing with the air and lead to harmful consequences in terms of performance and / or pollution. Summary of the invention

[0004] The risk of wall deposits, which is desired to be avoided, increases with the duration of the continuous injection. Therefore, in order to solve this problem, the invention proposes to divide an injection, which is usually carried out in the form of a single continuous spray / opening of the injector, into multiple sub-injections of individual shorter duration.

[0005] To this end, the subject of the invention is a method for a direct injection engine for dividing a fuel injection corresponding to one engine cycle into minimum sub-injections, the method comprising the following steps:

[0006] - the maximum required number of sub-injections is determined by dividing the fuel mass to be injected during one engine cycle by the minimum injectable mass and rounding down to the nearest integer,

[0007] - the maximum required number is under-limited by the maximum permissible number of sub-injections,

[0008] - verifying that the injection duration for such a maximum required number of sub-injections is less than the duration of the possible injection window, and

[0009] If not, decrement the maximum required number and iterate until the verification is positive.

[0010] Specific features or embodiments that may be used alone or in combination are:

[0011] - Lower bounded by: N := Min(N, Nmax), where Min is the minimum function, N is the maximum required number of sub-injections, and Nmax is the maximum allowed number of sub-injections,

[0012] - the injection duration is calculated according to the following formula: D = N * d + (N-1) * S, where D is the injection duration, N is the maximum required number of sub-injections, d is the sub-injection duration, and S is the minimum hydraulic separation duration to be observed between two sub-injections,

[0013] - Calculate the sub-injection duration according to the following formula: d = M / (N * Q), where d is the sub-injection duration, M is the fuel mass to be injected during one engine cycle, N is the maximum required number of sub-injections, and Q is the dynamic flow rate of the injector,

[0014] - applying the determined sub-injection sequence by starting the first sub-injection at the optimal injection start time,

[0015] - This method is more particularly applicable when the engine is cold (froid). BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be better understood by reading the following description given by way of example only and with reference to the accompanying drawings, in which:

[0017] Figure 1 shows a first injection timing diagram according to the prior art,

[0018] Figure 2 A second injection timing diagram according to the present invention is shown. DETAILED DESCRIPTION

[0019] refer to Figure 1 In each cycle, that is to say every two revolutions of the crankshaft, between two top dead centers PMH or TDC (from the English “Top Dead Center”: point mort haut, top dead center), a fuel injection is carried out in the cylinders of the internal combustion engine and a fuel mass M is injected.

[0020] Such an injection takes place in a possible injection window, which begins at the start of injection time SOI (from English “Start of Injection”) and ends at the latest at the end of injection time EOI (from English “End of Injection”). The duration Dmax of this injection window is therefore equal to EOI - SOI.

[0021] In a known manner, in a combustion cycle, the position or range is usually indicated relative to the top dead center TDC. The two top dead centers are 720° CRK apart. In a combustion cycle, the position or range can be indicated by an angle or an angle range. Alternatively and equivalently, in a combustion cycle, the position or range can be indicated by time or a time range or duration. The equivalence here is a bijection, requiring knowledge of the engine speed RM, which gives the angle / time conversion factor.

[0022] According to any method outside the framework of the present invention, all injection parameters SOI, EOI, Dmax and M are determined by the engine control in each cycle.

[0023] The object of the present invention is to reduce the continuous injection time, i.e. the time during which the injector is open. To this end, the method of the present invention proposes to divide a relatively long injection into several sub-injections, such as Figure 2 shown.

[0024] This segmentation should satisfy certain assumptions and comply with certain constraints.

[0025] A mandatory assumption is that the amount / mass M of fuel injected during a cycle remains constant. Therefore, the sum of the basic masses of all the sub-injections performed during a cycle should be equal to M.

[0026] Constraints associated with injector control, power consumption and / or heating of the control device are that the maximum number of sub-injections required per cycle is limited by a maximum permissible number Nmax which is considered a constant value or is determined by the actuation control device as a function of the engine speed and the battery voltage.

[0027] The constraints associated with the injector operating principle are that there is a minimum "rest" duration or minimum hydraulic separation duration S to be observed between two sub-injections, during which the injector should remain closed. Furthermore, due to its all-or-nothing control, the injector has a mass resolution or minimum injectable mass Mmin, below which it is not possible to drop. This minimum mass Mmin is a function of the fuel pressure in the rail and is known to the engine control, which is able to determine it at any time.

[0028] A final constraint associated with the engine cycle is that the entire injection or all sub-injections preferably take place during the injection window, that is to say between the SOI and the EOI.

[0029] The invention therefore proposes replacing longer injections with multiple sub-injections while respecting the above-mentioned constraints.

[0030] To this end, the method is adapted to determine a maximum required number of sub-injections N. It has been found that the problem of wall deposition can be improved by making the sub-injections as short as possible. In order for the sub-injections to be as short as possible, the maximum required number of sub-injections N is preferably as large as possible.

[0031] To simplify the calculations, a simplifying assumption is made - which is optimal, but not mandatory - of equal mass distribution between the sub-injections. Therefore, all sub-injections in a cycle inject the same mass / amount M / N of fuel. That is, by assuming a constant dynamic flow Q (taking into account the short duration of a cycle), all sub-injections in a cycle are carried out according to exactly the same duration d.

[0032] Thus, according to one feature, the maximum required sub-injection number N is first determined by dividing the fuel mass M to be injected during the engine cycle by the minimum injectable mass Mmin. Since the maximum required sub-injection number N is necessarily an integer, the ratio of M / Mmin is rounded down to the nearest integer. Thus, N = rounded (M / Mmin). This approach enables the incorporation of constraints associated with the minimum injected mass.

[0033] This maximum required number N is then compared with the maximum allowed number of sub-injections Nmax and, where appropriate, reduced to the maximum allowed number of sub-injections Nmax. A lower bound is thus performed. This is achieved, for example, by assigning N a value corresponding to the minimum value between the previously determined maximum required number N and the maximum allowed number Nmax. This makes it possible to incorporate the constraints associated with the maximum allowed number of sub-injections Nmax.

[0034] It is also advantageous to verify whether a maximum injection duration constraint is observed. To this end, the injection duration D is calculated as a function of the previously determined maximum required number of sub-injections N and compared with the duration Dmax of the injection window.

[0035] If and as soon as verification is obtained that the injection duration D is less than the duration Dmax of the injection window, the previously determined maximum required number of sub-injections N is valid.

[0036] Injection based on N sub-injections may be performed.

[0037] If, on the other hand, the calculated injection duration D is greater than the duration Dmax of the injection window, the maximum required sub-injection number N is reduced by decrementing (e.g., by one unit). The injection duration D is recalculated based on the new maximum required sub-injection number N. After decrementing N, the injection duration D should also be reduced. It is re-verified whether the injection duration D is less than the duration Dmax of the injection window. If this verification is positive, i.e. D is less than Dmax, the last obtained maximum required sub-injection number N is retained. Otherwise, the iteration continues: decrementing and re-verifying until a positive verification is obtained.

[0038] It can be noted that this iterative process will inevitably converge to an acceptable and operational solution. In fact, the case of N = 1 corresponding to the injection parameters determined by the engine control is at least possible.

[0039] The first constraint N < Nmax can be verified, and then the second constraint associated with the range Dmax of the injection window can be verified. Alternatively, the second constraint can be verified, N can be reduced until a range D of a sub-injection sequence is found that is contained within the range Dmax of the injection window, and then the second constraint can be applied if necessary to bound N with Nmax.

[0040] To describe a possible embodiment in detail, the following delimitation is performed according to the following formula: N := Min(N, Nmax), where N is the maximum required sub-injection number and Nmax is the maximum allowed sub-injection number. Therefore, the maximum required number N that remains is the previously obtained sub-injection number, or if the maximum allowed injection number Nmax is smaller, N is replaced by Nmax.

[0041] like Figure 2 As shown, an injection is divided into N sub-injections. Each sub-injection requires a duration d. Between each two consecutive sub-injections, an injector-specific minimum duration or minimum hydraulic separation duration S must be observed, which is a function of the fuel pressure in the rail and is therefore known. Therefore, according to one feature, the duration D of a segmented injection sequence comprising N sequentially ordered sub-injections according to the minimum duration can be calculated according to the following formula: D = N * d + (N-1) * S, where D is the injection duration, N is the maximum required number of sub-injections, d is the sub-injection duration, and S is the minimum hydraulic separation duration to be observed between two sub-injections.

[0042] According to one feature, the formula is time-dependent, with d, D, S and Dmax expressed in seconds (or ms). Alternatively, in an equivalent manner, this formula may be angle-dependent, with d, D, S and Dmax expressed in °CRK.

[0043] In absolute terms, the dynamic flow rate Q of an injector depends on the diameter of the injector, which is constant, and on the fuel pressure in the rail, which is known by the engine control system. It is assumed that the dynamic flow rate Q is known or can be determined by the engine control. It is assumed that the dynamic flow rate Q of an injector is constant during one cycle or at least during one injection. Therefore, the sub-injection duration d is proportional to the mass M / N to be injected during said sub-injection, the proportionality factor being the inverse of the dynamic flow rate 1 / Q. The duration d can be calculated according to the formula d = M / (N * Q), where d is the sub-injection duration, M is the mass of fuel to be injected during one engine cycle, N is the maximum required number of sub-injections, and Q is the dynamic flow rate of the injector.

[0044] The previous formula can be simplified by eliminating d: D = M / Q + (N-1) * S.

[0045] Once the maximum required number of sub-injections N has been determined and verified according to the aforementioned steps, the sub-injection sequence thus determined is applied. The sequence starts with the first sub-injection at the optimal start of injection time SOI. This optimal start of injection time is determined by the actuation control device, including the case where a single injection is applied. The sequence is then executed by performing N sub-injections in succession, with only the minimum hydraulic separation duration S between two sub-injections.

[0046] When the engine is still relatively cold, the above-mentioned one or more methods are particularly advantageous. Therefore, it is particularly decided to apply such methods in this case.

[0047] According to another feature, the method / strategy according to the invention does not take precedence. Thus, if another injection strategy is applicable with a certain degree (even a lower degree) of urgency, this other strategy will prevail as long as the two strategies are incompatible.

[0048] To illustrate this method, reference is made to Figure 2 A numerical application example is given. Assume that the engine speed is 2000 rpm, the engine control determines that the mass M to be injected in the next cycle is 20 mg, the SOI is 270°CRK before top dead center PMH or TDC, and the maximum injection window width is 120°CRK. It is also assumed that the injector enables a dynamic flow Q of 5 mg / ms, the minimum hydraulic separation duration S observed is 1 ms, the minimum injectable mass Mmin = 2.5 mg, and the maximum allowed number of injections Nmax is 5 times.

[0049] The first maximum required number of injections N is determined by dividing M / Mmin, i.e. 20 / 2.5, i.e. 8. Taking the minimum of this result and Nmax, we get N = 5. From this, it follows that the mass of each sub-injection M / N = 4 mg and the duration d are M / (N *Q), i.e. d = 0.8. The calculation of the duration D of the injection sequence is given by D = N * d + (N-1) * S, i.e. D = 5 *0.8 + 4 * 1 = 8ms. At 2000 rpm, these 8ms correspond to 96°CRK. It is easy to verify that this range is smaller than the injection window range of 120°CRK. Therefore, according to Figure 2 According to the timing diagram in , one injection can be divided into five sub-injections.

[0050] The invention has been illustrated and described in detail in the drawings and the above description. The description should be considered illustrative and given in the name of example, and should not be considered to limit the invention to the description. Numerous implementation variations are possible.

[0051] Reference numerals list

[0052] d: duration of sub-injection,

[0053] D: injection duration,

[0054] Dmax: Maximum duration of the injection window,

[0055] N: maximum required number of sub-injections,

[0056] Nmax: maximum allowed number of sub-injections,

[0057] M: the mass to be ejected in the current cycle,

[0058] Mmin: minimum ejectable mass,

[0059] Q: Dynamic flow rate of the ejector,

[0060] S: minimum hydraulic separation duration to be observed between two sub-injections,

[0061] SOI: Injection started.

Claims

1. A method for a direct injection engine for dividing a fuel injection corresponding to one engine cycle into minimum sub-injections, characterized in that The method comprises the following steps: - the maximum required number of sub-injections (N) is determined by dividing the fuel mass (M) to be injected during one engine cycle by the minimum injectable mass (Mmin) and rounding down to the nearest integer, - the maximum required number of sub-injections (N) is under-bounded by the maximum allowed number of sub-injections (Nmax), - verifying whether the injection duration (D) for such maximum required number of sub-injections (N) is less than the duration of a possible injection window (Dmax), and if the calculated injection duration (D) is greater than the duration of the injection window (Dmax), decrementing the maximum required number of sub-injections (N), iterating until the verification is positive.

2. The method according to claim 1, wherein: The lower bound is performed according to the following formula: N=Min(N, Nmax), where Min is the minimum function, N is the maximum required sub-injection number, and Nmax is the maximum allowed sub-injection number.

3. The method according to claim 1 or 2, wherein: The injection duration (D) is calculated according to the following formula: D=N*d+(N-1)*S, where D is the injection duration, N is the maximum required number of sub-injections, d is the duration of one sub-injection, and S is the minimum hydraulic separation duration to be observed between two sub-injections.

4. The method according to claim 3, wherein: The sub-injection duration (d) is calculated according to the following formula: d = M / (N*Q), where d is the sub-injection duration, M is the fuel mass to be injected during this engine cycle, N is the maximum required number of sub-injections, and Q is the dynamic flow of the injector.

5. The method according to claim 1 or 2, wherein: The determined sub-injection sequence is applied by starting the first sub-injection at an optimal start of injection (SOI).

6. A method according to claim 1 or 2, more particularly applicable when the engine is cold.

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