Gas engine system

Abstract

A gas engine system controller: calculates a delay calculation value of a knocking occurrence ratio; determines a primary target ignition timing; sets the primary target ignition timing as a current ignition timing if the occurrence ratio difference is positive and an ignition timing does not exceed a converted value of a first advance rate; determines whether a rapid advance condition is satisfied if the occurrence ratio difference is positive and the ignition timing difference exceeds the converted value of the first advance rate; sets a secondary target ignition timing as the current ignition timing if the rapid advance condition is not satisfied, the secondary target ignition timing obtained by adding the converted value of the first advance rate to the previous ignition timing; and determines the current ignition timing so as to achieve a second advance rate greater than the first advance rate if the rapid advance condition is satisfied.

Claims

1. A gas engine system comprising: a gas engine including at least one cylinder where combustion of an air-fuel mixture of a fuel gas and air is performed; an ignitor provided in the at least one cylinder; a pressure sensor configured to detect a pressure in the at least one cylinder; and a controller controlling the ignitor, the controller being configured to: calculate a delay calculation value of a knocking occurrence ratio based on the detected pressure in the cylinder, which is detected by the pressure sensor; determine a primary target ignition timing based on an occurrence ratio difference calculated by subtracting the delay calculation value of the knocking occurrence ratio from a target occurrence ratio; advance an ignition timing in response to the occurrence ratio difference being calculated as positive, and retard the ignition timing in response to the occurrence ratio difference being calculated as negative; set the primary target ignition timing as a current ignition timing in response to (i) the occurrence ratio difference being calculated as positive and (ii) an ignition timing difference, which is calculated by subtracting a previous ignition timing from the primary target ignition timing, not exceeding a converted value of a first advance rate; when the occurrence ratio difference is positive and the ignition timing difference exceeds the converted value of the first advance rate, determine whether a rapid advance condition is satisfied; set a secondary target ignition timing as the current ignition timing when the rapid advance condition is not satisfied, the secondary target ignition timing being calculated by adding the converted value of the first advance rate to the previous ignition timing; determine the current ignition timing by advancing the ignition timing by a second advance rate greater than the first advance rate when the rapid advance condition is satisfied; and control the ignitor to ignite the air-fuel mixture in the at least one cylinder at the current ignition timing.

2. The gas engine system according to claim 1, wherein: the rapid advance condition is at least one of the following conditions (a) to (c): the condition (a) is a number of times of allowable knocking within a predetermined period previous to a current time is less than a predetermined number of times; the condition (b) is an absolute value of a difference between a target rotational speed and an actual rotational speed is less than a predetermined rotational speed; and the condition (c) is an absolute value of a time rate of change of a fuel gas injection period is less than a predetermined rate of change.

3. The gas engine system according to claim 1, wherein if when the rapid advance condition is satisfied, the controller is configured to determine the current ignition timing by adding (i) an additional amount calculated based on a preset addition rate to (ii) the secondary target ignition timing obtained by adding the converted value of the first advance rate to the previous ignition timing.

4. The gas engine system according to claim 1, wherein: the controller is configured to determine whether a reset condition is satisfied, and when the reset condition is satisfied, the controller is configured to correct the current ignition timing by subtracting a predetermined amount from the current ignition timing.

5. The gas engine system according to claim 4, wherein the reset condition is at least one of the following conditions (d) and (e): the condition (d) is an absolute value of a difference between a target rotational speed and an actual rotational speed is greater than a predetermined rotational speed; and the condition (e) is an absolute value of a time rate of change of a fuel gas injection period is greater than a predetermined rate of change.

6. The gas engine system according to claim 2, wherein when the rapid advance condition is satisfied, the controller is configured to determine the current ignition timing by adding (i) an additional amount calculated based on a preset addition rate to (ii) the secondary target ignition timing obtained by adding the converted value of the first advance rate to the previous ignition timing.

7. The gas engine system according to claim 2, wherein: the controller is configured to determine whether a reset condition is satisfied, and when the reset condition is satisfied, the controller is configured to correct the current ignition timing by subtracting a predetermined amount from the current ignition timing.

8. The gas engine system according to claim 3, wherein: the controller is configured to determine whether a reset condition is satisfied, and when the reset condition is satisfied, the controller is configured to correct the current ignition timing by subtracting a predetermined amount from the current ignition timing.

9. The gas engine system according to claim 6, wherein: the controller is configured to determine whether a reset condition is satisfied, and when the reset condition is satisfied, the controller is configured to correct the current ignition timing by subtracting a predetermined amount from the current ignition timing.

10. The gas engine system according to claim 7, wherein the reset condition is at least one of the following conditions (d) and (e): the condition (d) is an absolute value of a difference between a target rotational speed and an actual rotational speed is greater than a predetermined rotational speed; and the condition (e) is an absolute value of a time rate of change of a fuel gas injection period is greater than a predetermined rate of change.

11. The gas engine system according to claim 8, wherein the reset condition is at least one of the following conditions (d) and (e): the condition (d) is an absolute value of a difference between a target rotational speed and an actual rotational speed is greater than a predetermined rotational speed; and the condition (e) is an absolute value of a time rate of change of a fuel gas injection period is greater than a predetermined rate of change.

12. The gas engine system according to claim 9, wherein the reset condition is at least one of the following conditions (d) and (e): the condition (d) is an absolute value of a difference between a target rotational speed and an actual rotational speed is greater than a predetermined rotational speed; and the condition (e) is an absolute value of a time rate of change of a fuel gas injection period is greater than a predetermined rate of change.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a schematic configuration of a gas engine system according to Embodiment 1 of the present invention.

(2) FIG. 2 is a flowchart of control performed by a controller in Embodiment 1.

(3) FIG. 3 shows one example of the waveform of the pressure in a cylinder.

(4) FIG. 4 is a diagram for describing a knocking occurrence ratio and its moving average value

(5) FIG. 5 is a graph showing temporal changes in a primary target ignition timing and a current ignition timing in Embodiment 1.

(6) FIG. 6 is a flowchart of control performed by the controller in Embodiment 2 of the present invention.

(7) FIG. 7 is a flowchart of control performed by the controller in Embodiment 2.

(8) FIG. 8 is a graph showing temporal changes in the primary target ignition timing and the current ignition timing in Embodiment 2.

DESCRIPTION OF EMBODIMENTS

Embodiment 1

(9) FIG. 1 shows a gas engine system 10 according to Embodiment 1 of the present invention. The gas engine system 10 includes a gas engine 1 and a controller 3.

(10) The gas engine 1 may be a pure gas engine that combusts only a fuel gas (e.g., natural gas), or may be a dual fuel engine that combusts one of or both a fuel gas and a fuel oil. In the present embodiment, the gas engine 1 is a 4-stroke engine. However, as an alternative, the gas engine 1 may be a 2-stroke engine.

(11) The gas engine 1 includes a plurality of cylinders 21 (FIG. 1 shows only one cylinder 21). Since all the cylinders 21 have the same configuration, only one cylinder 21 is described below.

(12) In the cylinder 21, a piston 22 is disposed such that the piston 22 is movable in a reciprocating manner. The cylinder 21 and the piston 22 form a combustion chamber 2. The piston 22 is coupled to an unshown crank shaft by an unshown connecting rod.

(13) In a case where the gas engine 1 is a 4-stroke engine, in the cylinder 21, the piston 22 reciprocates twice, and thereby one cycle of the gas engine 1 (intake, compression, expansion, and exhaust) is performed. The phase angle (0 to 720 degrees) of the gas engine 1 during one cycle of the cylinder 21 is detected by a phase angle detector 62. For example, the rotation angle of the crank shaft (i.e., crank angle) or the position of the piston 22 can be used as the phase angle. As one example, the phase angle detector 62 is an electromagnetic pickup, a proximity switch, or a rotary encoder. An actual rotational speed N (unit: rpm) of the gas engine 1 is also detected from the phase angle detector 62.

(14) Air is supplied to the combustion chamber 2 through an intake passage 41, and exhaust gas is discharged from the combustion chamber 2 through an exhaust passage 42. The intake passage 41 is provided with a main fuel injection valve 51, which injects a fuel gas into the air supplied to the combustion chamber 2.

(15) The cylinder 21 is provided with an intake valve 23, which opens and closes an intake port that is an opening of the intake passage 41, the opening facing the combustion chamber 2; and an exhaust valve 24, which opens and closes an exhaust port that is an opening of the exhaust passage 42, the opening facing the combustion chamber 2. The cylinder 21 is also provided with a spark plug 55 (one example of an ignitor) for igniting an air-fuel mixture of the fuel gas and the air in the combustion chamber 2. That is, the air-fuel mixture is combusted in the cylinder 21.

(16) In the present embodiment, the combustion chamber 2 includes: a main combustion chamber 2A, which communicates with the intake passage 41 and the exhaust passage 42; and an auxiliary combustion chamber 2B, which is divided from the main combustion chamber 2A by a dividing wall 25 with communication holes formed therein. The spark plug 55 is disposed in the auxiliary combustion chamber 2B. The fuel gas is injected into the auxiliary combustion chamber 2B by an auxiliary fuel injection valve 52. A rich air-fuel mixture is formed in the auxiliary combustion chamber 2B by the injection of the fuel gas from the auxiliary fuel injection valve 52, and the air-fuel mixture is ignited by the spark plug 55. As a result, a flame is generated in the auxiliary combustion chamber 2B, and the flame propagates into the main combustion chamber 2A through the communication holes formed in the dividing wall 25. Consequently, a lean air-fuel mixture in the main combustion chamber 2A is also ignited. The cylinder 21 is provided with a pressure sensor 61, which detects a pressure P in the cylinder 21 (the pressure in the main combustion chamber 2A).

(17) It should be noted that the ignitor is not limited to the spark plug 55, which ignites the air-fuel mixture in the auxiliary combustion chamber 2B. For example, a pilot fuel injection valve that directly injects a high-pressure pilot fuel (oil or fuel gas) into the main combustion chamber 2A to cause self-ignition of the pilot fuel can be adopted as the ignitor (in this case, the auxiliary combustion chamber 2B may be eliminated).

(18) The fuel injection valves 51 and 52 and the spark plug 55 are controlled by the controller 3 based on, for example, the phase angle detected by the phase angle detector 62. The controller 3 adjusts the ignition timing for each cycle, such that a delay calculation value of a knocking occurrence ratio Kr becomes a target occurrence ratio Kt (e.g., 7.5%). In the present embodiment, the ignition timing is an angle (crank angle) relative to the top dead center, and indicates how much before the top dead center the ignition should be performed.

(19) The knocking occurrence ratio Kr is the ratio of the number of cycles Ck in which allowable knocking has occurred to a predetermined number of cycles Cy (i.e., Kr=Ck/Cy). The predetermined number of cycles Cy may be, for example, preset to a particular number of cycles, such as 50 cycles, or may be, for example, the number of cycles measured within a preset period of time, such as 5 to 15 seconds. In the present embodiment, the delay calculation value of the knocking occurrence ratio Kr is a moving average value of the knocking occurrence ratio Kr. However, as an alternative, the delay calculation value may be a first-order lag calculation value.

(20) Hereinafter, control performed by the controller 3 is described in detail with reference to the flowchart of FIG. 2.

(21) Based on the pressure P in the cylinder 21, which is detected by the pressure sensor 61, the controller 3 determines, for each cycle, whether the combustion state is unallowable knocking (D), allowable knocking (C), normal (A), or misfire (B) (step S1).

(22) The combustion state is determined, for example, by the following method. The controller 3 passes the waveform, shown in FIG. 3, of the pressure in the cylinder 21 through a filter to extract high-frequency components within a predetermined period T, which is a period of time from when the piston 22 reaches the top dead center. Then, the controller 3 samples a plurality of high-frequency components from among the extracted high-frequency components, and calculates an average value PA of the sampled high-frequency components. If the average value PA is greater than or equal to a first threshold T1 (T1PA), the combustion state is determined to be unallowable knocking. If the average value PA is less than the first threshold T1, but greater than or equal to a second threshold T2 (T2PA<T1), the combustion state is determined to be allowable knocking. The controller 3 also calculates a difference P between the pressure P in the cylinder 21 before reaching the top dead center and the pressure P in the cylinder 21 after reaching the top dead center. If the difference P is less than a third threshold T3 (P<T3), the combustion state is determined to be misfire. If the difference P is greater than or equal to the third threshold T3, and the average value PA is less than the second threshold T2 (T3P, PA<T2), the combustion state is determined to be normal.

(23) As shown in FIG. 4, assuming that the current cycle is the nth cycle, the controller 3 stores therein the combustion state of each of the past Cy cycles including the current cycle. The controller 3 obtains the number of cycles Ck, in which allowable knocking (C) has occurred, among the past Cy cycles including the current cycle, and divides Ck by Cy, thereby calculating the knocking occurrence ratio Kr.sub.n in the current cycle (step S2). It should be noted that, as shown in FIG. 4, the calculated knocking occurrence ratios Kr over predetermined H cycles including the current cycle are stored in the controller 3.

(24) In a case where the gas engine 1 is used in, for example, power generating equipment, i.e., in a case where the load on the gas engine 1 does not vary frequently, it is desirable that H be relatively great (e.g., 1500 to 2000 cycles). On the other hand, in a case where the gas engine 1 is used as, for example, a main engine of a ship, i.e., in a case where the load on the gas engine 1 varies frequently, it is desirable that H be relatively small (e.g., 100 to 500 cycles).

(25) Next, the controller 3 calculates a moving average value Kr (AVE) of the knocking occurrence ratio Kr (step S3). To be more specific, as shown in FIG. 4, assuming that the current cycle is the nth cycle, the controller averages the knocking occurrence ratios Kr over the past H cycles including the current cycle (Kr (AVE)=(Kr.sub.n+Kr.sub.n1+ . . . +Kr.sub.nH+1)/H).

(26) Next, the controller 3 calculates an occurrence ratio difference K (=KtKr (AVE)) by subtracting the moving average Kr (AVE) of the knocking occurrence ratio Kr from the target occurrence ratio Kt (step S4). If the occurrence ratio difference K is positive, the ignition timing should be advanced, and if the occurrence ratio difference K is negative, the ignition timing should be retarded. Thereafter, based on the occurrence ratio difference K, the controller 3 determines a primary target ignition timing ITf, such that the ignition timing is advanced if the occurrence ratio difference K is positive, and such that the ignition timing is retarded if the occurrence ratio difference K is negative. That is, if the occurrence ratio difference K is positive, the controller 3 determines the primary target ignition timing ITf to be greater than a previous ignition timing IT.sub.n1, and if the occurrence ratio difference K is positive, the controller 3 determines the primary target ignition timing ITf to be less than the previous ignition timing IT.sub.n1. More details of the method of determining the primary target ignition timing ITf are as disclosed in, for example, Patent Literature 1.

(27) Next, the controller 3 determines whether or not the speed of changing the ignition timing from the previous ignition timing IT.sub.n1 to the primary target ignition timing ITf is within an allowable range. If the aforementioned occurrence ratio difference K is positive, an ignition timing difference IT (=ITfIT.sub.n1) obtained by subtracting the previous ignition timing from the primary target ignition timing ITf is also positive, whereas if the occurrence ratio difference K is negative, the ignition timing difference IT is also negative.

(28) Specifically, the controller 3 stores therein a preset first advance rate (unit: deg/sec) and a preset retard rate (unit: deg/sec). For example, the first advance rate is 0.01 to 0.05 [deg/sec], and the retard rate is 0.2 to 0.05 [deg/sec]. Then, the controller 3 compares the ignition timing difference IT with a converted value (/K) of the first advance rate (step S6), and compares the ignition timing difference IT with a converted value (/K) of the retard rate (step S7). Here, K is a coefficient for converting the unit of the advance rate or the retard rate from deg/sec into deg/cycle. In the present embodiment, since the gas engine 1 is a 4-cycle engine, K=N/120 [cycle/sec]. It should be noted that it does not matter if step S6 proceeds or follows step S7.

(29) If the ignition timing difference IT is less than /K and greater than /K (YES in step S6 and YES in step S7), i.e., if the ignition timing difference IT does not exceed the converted value of the first advance rate and the converted value of the retard rate , the controller 3 sets the primary target ignition timing ITf as a secondary target ignition timing ITs (step S8). If the ignition timing difference IT is less than /K, i.e., if the ignition timing difference IT exceeds the converted value of the retard rate , the controller 3 determines the secondary target ignition timing ITs based on the retard rate . Specifically, the controller 3 adds the converted value of the retard rate to the previous ignition timing IT.sub.n1, and sets the resulting value as the secondary target ignition timing ITs (ITs=IT.sub.n1+/K).

(30) On the other hand, if the ignition timing difference IT is greater than /K, i.e., if the ignition timing difference IT exceeds the converted value of the first advance rate , the controller 3 determines whether or not a rapid advance condition is satisfied (step S10). In the present embodiment, the rapid advance condition contains all of the conditions (a) to (c) described below. However, it will suffice if the rapid advance condition is at least one of the conditions (a) to (c). If the rapid advance condition is at least one of the conditions (a) to (c), frequent allowable knocking and the occurrence of unallowable knocking can be prevented even if the ignition timing is advanced rapidly.

(31) Condition (a): the number of times allowable knocking has occurred within a predetermined period (e.g., 60 seconds) previous to the current time is less than a predetermined number of times (e.g., zero or several times).

(32) Condition (b): the absolute value |N| of the difference between a target rotational speed Nt and the actual rotational speed N is less than a predetermined rotational speed (e.g., 2 to 10 [rpm]).

(33) Condition (c): the absolute value |dD| of the time rate of change of the fuel gas injection period is less than a predetermined rate of change (e.g., 0.1 to 1 [deg/sec]).

(34) The condition (a) is a condition for determining that the combustion state is stable within a proper range, and the conditions (b) and (c) are conditions for determining that the load has become substantially constant. It should be noted that, instead of the conditions (b) and (c), for example, at least one of the conditions (M1) to (M4) described below can be adopted as the condition(s) for determining that the load has become substantially constant.

(35) Condition (M1): the absolute value of the time rate of change of a necessary fuel gas injection amount is less than a threshold.

(36) Condition (M2): the absolute value of the time rate of change of a torque detected by a torque meter provided on the output shaft of the gas engine 1 is less than a threshold.

(37) Condition (M3): in a case where a turbocharger including a compressor and a turbine is connected to the gas engine 1 the absolute value of the difference between an actual charge air pressure and a target charge air pressure is less than a threshold.

(38) If the rapid advance condition is not satisfied (NO in step S10), the controller 3 determines the secondary target ignition timing as based on the first advance rate (step S11). Specifically, the controller 3 adds the converted value of the first advance rate to the previous ignition timing IT.sub.n1, and sets the resulting value as the secondary target ignition timing ITs (ITs=IT.sub.n1+/K). On the other hand, if the rapid advance condition is satisfied (YES in step S10), the controller 3 determines the secondary target ignition timing ITs so as to achieve a second advance rate , which is greater than the first advance rate (step S12). The second advance rate is preset and stored in the controller 3. The second advance rate is, for example, 0.1 to 0.5 [deg/sec]. Specifically, the controller 3 adds a converted value of the second advance rate to the previous ignition timing IT.sub.n1, and sets the resulting value as the secondary target ignition timing ITs (ITs=IT.sub.n1+/K).

(39) When the secondary target ignition timing ITs is determined in any of steps S8, S9, S11, and S12, the controller 3 sets the secondary target ignition timing ITs as a current ignition timing IT.sub.n (step S14), and controls the spark plug 55 such that the spark plug 55 ignites the air-fuel mixture at the current ignition timing IT.sub.n (step S14). Thereafter, the controller 3 returns to step S1.

(40) As described above, in the gas engine system 10 of the present embodiment, if the rapid advance condition is satisfied, the ignition timing is advanced by the second advance rate , which is greater than the first advance rate . Accordingly, the ignition timing is advanced rapidly, and the time required for the optimization of the ignition timing can be reduced. For example, as shown in FIG. 5, in a case where the primary target ignition timing ITf is allowed to be great close to a maximum ignition timing ITm, the current ignition timing IT.sub.n can be brought close to the primary target ignition timing ITf at an early stage.

Embodiment 2

(41) Next, with reference to FIG. 6 and FIG. 7, a gas engine system according to Embodiment 2 of the present invention is described. It should be noted that the only difference between Embodiment 1 and Embodiment 2 lies in the control performed by the controller 3, and the configuration of the gas engine system according to Embodiment 2 is as shown in FIG. 1.

(42) The flowchart of the present embodiment is the same as the flowchart of FIG. 2 from step S1 to step S9. In the present embodiment, in the case of NO in step S6, the flow thereafter is different from the flow in the flowchart of FIG. 2.

(43) Also in the present embodiment, if the rapid advance condition is satisfied when the ignition timing difference IT (=ITfIT.sub.n1) exceeds the converted value (/K) of the first advance rate , the controller 3 determines the current ignition timing IT.sub.n so as to achieve the second advance rate , which is greater than the first advance rate . Specifically, if the ignition timing difference IT is greater than /K (No in step S6), the controller 3 first determines the secondary target ignition timing ITs based on the first advance rate (step S21), and then determines whether or not the rapid advance condition is satisfied (step S22).

(44) If the rapid advance condition is not satisfied (NO in step S22), the flow proceeds to step S31, in which the controller 3 sets the secondary target ignition timing ITs as the current ignition timing IT.sub.n. The flow proceeds to step S31 also from step S8 and step S9. On the other hand, if the rapid advance condition is satisfied (YES in step S22), the controller 3 calculates an additional amount A (unit: deg/cycle) based on a preset addition rate (unit: deg/sec) (step S23). Specifically, the controller 3 calculates the additional amount A by dividing the addition rate by a coefficient K (A=/K).

(45) Thereafter, the controller 3 compares ITs+A with a rapid advance limit value ITL (step S24). The rapid advance limit value ITL may be a fixed value, or may be a variable corresponding to the load when the rapid advance condition is satisfied. If ITs+A is greater than the rapid advance limit value ITL (NO in step S24), the controller 3 proceeds to step S31, in which the controller 3 sets the secondary target ignition timing ITs as the current ignition timing IT.sub.n. In other words, even if the rapid advance condition is satisfied, rapid advance of the ignition timing is not performed.

(46) On the other hand, if ITs+A is less than the rapid advance limit value ITL (YES in step S24), the controller 3 determines the current ignition timing IT.sub.n by adding the additional amount A to the secondary target ignition timing ITs (step S25). In other words, the controller 3 sets ITs+A as the current ignition timing IT.sub.n. That is, in the present embodiment, the second advance rate is the sum of the first advance rate and the addition rate .

(47) When the current ignition timing IT.sub.n is determined in step S31 or step S25, the controller 3 determines whether or not a reset condition is satisfied (step S32). For example, the reset condition is a condition for determining that the load has increased rapidly, and at least one of the conditions (d) and (e) described below can be adopted as the reset condition.

(48) Condition (d): the absolute value |N| of the difference between the target rotational speed Nt and the actual rotational speed N is greater than a predetermined rotational speed (the predetermined rotational speed of the condition (d) may be the same as or different from the predetermined rotational speed of the condition (b)).
Condition (e): the absolute value |dD| of the time rate of change of the fuel gas injection period is greater than a predetermined rate of change (the predetermined rate of change of the condition (e) may be the same as or different from the predetermined rate of change of the condition (c)).

(49) If the reset condition is not satisfied (NO in step S32), the controller 3 does not correct the current ignition timing IT.sub.n. On the other hand, if the reset condition is satisfied (YES in step S32), the controller 3 corrects the current ignition timing IT.sub.n by subtracting a predetermined amount from the current ignition timing IT.sub.n. The predetermined amount may be a fixed value, or may be a variable corresponding to the situation. For example, the predetermined amount may be greater than or equal to a cumulative value A of the additional amount A. In this case, as shown in FIG. 8, when the reset condition is satisfied after the ignition timing is advanced rapidly, the current ignition timing IT.sub.n can be shifted to be on a line that is based on the first advance rate , or shifted to fall below the line. Alternatively, the predetermined amount may be less than the cumulative value A.

(50) Thereafter, the controller 3 controls the spark plug 55 such that the spark plug 55 ignites the air-fuel mixture at the current ignition timing IT.sub.n (step S34), and returns to step S1.

(51) The present embodiment can provide the same advantageous effects as those provided by Embodiment 1. Further, in the present embodiment, since the additional amount A is added to the secondary target ignition timing ITs in step S25, the proportion of the advance amount derived from the first advance rate in the advance amount derived from the second advance rate can be stored as a history.

(52) Further, in the present embodiment, in a case where the reset condition is satisfied after the ignition timing is advanced rapidly, the ignition timing can be retarded instantaneously. For example, assume a case where the reset condition is the presence of rapid increase in the load. In this case, even if the load increases rapidly after the ignition timing is advanced rapidly, the occurrence of unallowable knocking can be prevented. In addition, if the predetermined amount subtracted from the current ignition timing IT.sub.n is greater than or equal to the cumulative value A of the additional amount A, the advantage that the occurrence of unallowable knocking is prevented can be obtained notably.

Other Embodiments

(53) The present invention is not limited to the above-described embodiment. Various modifications can be made without departing from the spirit of the present invention.

(54) For example, step S32 and step S34 of Embodiment 2 may be performed between step S13 and step S14 of Embodiment 1.

(55) In Embodiment 1, a step of comparing the secondary target ignition timing ITs with the ignition timing limit value ITL may be performed after step S12, and the flow may proceed to step S11 if ITs>ITL.

REFERENCE SIGNS LIST

(56) 10 gas engine system 1 gas engine 21 cylinder 3 controller 55 spark plug (ignitor) 61 pressure sensor