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Title:
FUEL CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE
Document Type and Number:
WIPO Patent Application WO/2012/028944
Kind Code:
A1
Abstract:
A fuel control apparatus for an internal combustion engine (100) that is able to mix a first fuel with a second fuel that is higher in compression ignitability than the first fuel and then to use the mixture of the first fuel and the second fuel, includes a combustion pressure acquisition unit (62) that acquires a pressure in a combustion chamber (12) in which the first fuel and the second fuel are combusted; and a fuel control unit (60) that changes a percentage of the first fuel in fuel used on the basis of a maximum pressure at the time of combustion in the combustion chamber (12).

Inventors:
MASUBUCHI, Masahiko (1 Toyota-cho,Toyota-shi, Aichi-ken, 471-8571, JP)
Application Number:
IB2011/002017
Publication Date:
March 08, 2012
Filing Date:
September 01, 2011
Export Citation:
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Assignee:
TOYOTA JIDOSHA KABUSHIKI KAISHA (1 Toyota-cho, Toyota-shi, Aichi-ken, 471-8571, JP)
MASUBUCHI, Masahiko (1 Toyota-cho,Toyota-shi, Aichi-ken, 471-8571, JP)
International Classes:
F02D35/02; F02D19/10; F02D41/00
Domestic Patent References:
WO2005031134A1
Foreign References:
JP2006046075A
US20060032477A1
JPH06159182A
JPS62119445U
Download PDF:
Claims:
CLAIMS

1. A fuel control apparatus for an internal combustion engine that is able to mix a first fuel with a second fuel that is higher in compression ignitability than the first fuel and then to use the mixture of the first fuel and the second fuel, comprising:

a combustion pressure acquisition unit that acquires a pressure in a combustion chamber in which the first fuel and the second fuel are combusted; and

a fuel control unit that changes a percentage of the first fuel in fuel used on the basis of a maximum pressure at the time of combustion in the combustion chamber.

2. The fuel control apparatus according to claim 1, wherein

when the maximum pressure is higher than a predetermined threshold, the fuel control unit increases the percentage of the first fuel in the fuel used. 3. The fuel control apparatus according to claim 1 or 2, further comprising: a heat release rate pattern acquisition unit that acquires a heat release rate pattern at the time of combustion on the basis of a pressure in the combustion chamber; and a heat release rate pattern comparing unit that compares the heat release rate pattern with a predetermined ideal heat release rate pattern, wherein

the fuel control unit corrects the percentage of the first fuel in the fuel used on the basis of results of comparison made by the heat release rate pattern comparing unit.

4. The fuel control apparatus according to claim 3, wherein

the fuel control unit includes:

an injection timing correction amount calculation unit that calculates a correction amount of an injection timing of the second fuel, which is required to correct a deviation between the heat release rate pattern and the ideal heat release rate pattern, on the basis of the results of comparison; and

a correction amount converting unit that converts the correction amount of the injection timing into a correction amount of the percentage of the first fuel in the fuel used.

5. The fuel control apparatus according to claim 3 or 4, wherein

the heat release rate pattern comparing unit compares a rising timing of a heat release rate and an inclination angle after rising between the heat release rate pattern and the ideal heat release rate pattern.

6. The fuel control apparatus according to any one of claims 1 to 5, wherein the combustion pressure acquisition unit includes a combustion pressure sensor that is provided in the combustion chamber.

7. The fuel control apparatus according to any one of claims 1 to 6, wherein the first fuel is natural gas, and the second fuel is light oil.

8. A fuel control method for an internal combustion engine that is able to mix a first fuel with a second fuel that is higher in compression ignitability than the first fuel and then to combust the mixture of the first fuel and the second fuel in a combustion chamber, comprising:

acquiring a pressure in the combustion chamber in which the first fuel and the second fuel are combusted; and

changing a percentage of the first fuel in fuel used on the basis of a maximum pressure at the time of combustion in the combustion chamber. 9. The fuel control method according to claim 8, wherein

when the maximum pressure is higher than a predetermined threshold, the percentage of the first fuel in the fuel used is increased.

Description:
FUEL CONTROL APPARATUS FOR INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a fuel control apparatus for an internal combustion engine that is able to use multiple types of fuel.

2. Description of Related Art

[0002] There is known an internal combustion engine that is able to use multiple types of fuel (for example, fuel gas and fuel oil) mixed with each other. Japanese Utility Model Application Publication No. 62- 119445 (JP-U-62-1 19445) describes an internal combustion engine that improves combustion efficiency in such a manner that the fuel injection timing is corrected on the basis of the mixture ratio of multiple fuels to maximize a maximum pressure Pmax at the time of combustion at an equivalent heat input.

[0003] In the above method in which a maximum pressure value (Pmax) at the time of combustion is maximized at an equivalent heat input, the gradient of the heat release rate at the time of combustion becomes steep, so a heat loss may increase to deteriorate thermal efficiency.

SUMMARY OF THE INVENTION

[0004] The invention provides a fuel control apparatus and fuel control method that suppress deterioration in thermal efficiency due to a heat loss in an internal combustion engine that is able to use multiple types of fuel.

[0005] A first aspect of the invention relates to a fuel control apparatus for an internal combustion engine. The fuel control apparatus is used for an internal combustion engine that is able to mix a first fuel with a second fuel that is higher in compression ignitability than the first fuel and then to use the mixture of the first fuel and the second fuel. The fuel control apparatus includes: a combustion pressure acquisition unit that acquires a pressure in a combustion chamber in which the first fuel and the second fuel are combusted; and a fuel control unit that changes a percentage of the first fuel in fuel used on the basis of a maximum pressure at the time of combustion in the combustion chamber.

[0006] When the maximum pressure is higher than a predetermined threshold, the fuel control unit may increase the percentage of the first fuel in the fuel used. [0007] The fuel control apparatus may further include: a heat release rate pattern acquisition unit that acquires a heat release rate pattern at the time of combustion on the basis of a pressure in the combustion chamber; and a heat release rate pattern comparing unit that compares the heat release rate pattern with a predetermined ideal heat release rate pattern, wherein the fuel control unit may correct the percentage of the first fuel in the fuel used on the basis of results of comparison made by the heat release rate pattern comparing unit.

[0008] The fuel control unit may include: an injection timing correction amount calculation unit that calculates a correction amount of an injection timing of the second fuel, which is required to correct a deviation between the heat release rate pattern and the ideal heat release rate pattern, on the basis of the results of comparison; and a correction amount converting unit that converts the correction amount of the injection timing into a correction amount of the percentage of the first fuel in the fuel used.

[0009] The heat release rate pattern comparing unit may compare a rising timing of a heat release rate and an inclination angle after rising between the heat release rate pattern and the ideal heat release rate pattern.

[0010] The combustion pressure acquisition unit may include a combustion pressure sensor that is provided in the combustion chamber.

[0011] The first fuel may be natural gas, and the second fuel may be light oil.

[0012] A second aspect of the invention relates to a fuel control method for an internal combustion engine. The fuel control method is used for an internal combustion engine that is able to mix a first fuel with a second fuel that is higher in compression ignitability than the first fuel and then to combust the mixture of the first fuel and the second fuel in a combustion chamber. The fuel control method includes: acquiring a pressure in the combustion chamber in which the first fuel and the second fuel are combusted; and changing a percentage of the first fuel in fuel used on the basis of a maximum pressure at the time of combustion in the combustion chamber.

[0013] When the maximum pressure is higher than a predetermined threshold, the percentage of the first fuel in the fuel used may be increased.

[0014] With the fuel control apparatus and the fuel control method according to the aspects of the invention, it is possible to suppress deterioration of thermal efficiency due to a heat loss in an internal combustion engine that is able to use multiple types of fuel. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a view that shows the overall configuration of an internal combustion engine according to a first embodiment;

FIG. 2 is a view that shows the detailed configuration of a combustion chamber; FIG. 3 is a graph that shows the correlation between the percentage of CNG in fuel and a maximum combustion pressure;

FIG. 4 is a flowchart that shows control for correcting fuel injection according to the first embodiment;

FIG. 5A to FIG. 5F are graphs that show the correlations between a fuel injection timing and a relevant parameter;

FIG. 6 is a graph that shows the correlation between a fuel injection timing and a heat release rate pattern;

FIG. 7 is a graph that shows a deviation between an ideal heat release rate pattern and an actually measured heat release rate pattern;

FIG. 8 is a flowchart that shows control for correcting fuel injection according to a second embodiment;

FIG. 9A and FIG. 9B are graphs that respectively show correction amounts of the fuel injection timing; and

FIG. 10 is a graph that shows the correlation between a correction amount of the fuel injection timing and a correction amount of the percentage of CNG in fuel.

DETAILED DESCRIPTION OF EMBODIMENTS

[0016] FIG. 1 is a view that shows the overall configuration of an internal combustion engine according to a first embodiment. The internal combustion engine 100 is a dual-fuel internal combustion engine that is able to mix compressed natural gas (CNG) as a primary fuel with light oil as a secondary fuel and then to combust the mixture of the CNG and the light oil. The internal combustion engine 100, for example, includes an in-line four-cylinder engine block 10. A light oil injector 20 is provided in each combustion chamber 12 of the engine block 10. Light oil fuel is supplied from a light oil fuel tank 32 to the light oil injectors 20 via a high-pressure pump 33 and a common rail 34.

[0017] CNG injectors 22 are provided in an intake port 42 that communicates with the combustion chambers 12. CNG fuel is supplied from a CNG fuel tank 37 to the CNG injectors 22 via a regulator 38 and a CNG delivery 39.

[0018] The intake port 42, a throttle valve 44 for adjusting the flow rate, an intercooler 46, a turbocharger 48 and an air cleaner 49 are provided in an intake passage 40 of the engine block 10 in order from the downstream side. An exhaust port 52, the turbocharger 48 and a start converter 54 that incorporates catalyst for purifying exhaust gas are provided in an exhaust passage 50 of the engine block 10 in order from the upstream side.

[0019] In addition, the internal combustion engine 100 includes an electronic control unit (ECU) 60 that serves as a control unit. The ECU 60 acquires the operating conditions (operating load and engine rotational speed) of the internal combustion engine 100 on the basis of signals output from sensors, or the like (not shown). The signals indicate the opening degree of the throttle valve 44 and an engine rotational speed. In addition, the ECU 60 acquires a pressure at the time of combustion (combustion pressure) on the basis of a signal output from each combustion pressure sensor 62 provided in each of the combustion chambers 12. The ECU 60 executes fuel injection control over the light oil injectors 20 and the CNG injectors 22 on the basis of the acquired combustion pressures.

[0020] FIG. 2 is a view that shows the detailed configuration around each combustion chamber 12. Each combustion chamber 12 is defined by a cylinder 14, a piston 15 and a cylinder head 16. The light oil injector 20 is provided at the upper side in each combustion chamber 12. The intake side of each combustion chamber 12 communicates with the intake port 42 via an intake valve 17. An intake port upstream portion 42a is a space shared by all the combustion chambers 12. Intake port downstream portions 42b are passages formed separately for the respective combustion chambers 12 of the engine block 10. The exhaust side of each combustion chamber 12 communicates with the exhaust port 52 via an exhaust valve 18.

[0021] At the time of combustion, light oil is injected by the light oil injector 20 into the combustion chamber 12, and CNG is injected from the CNG injector 22 to the intake port downstream portion 42b via a metal pipe 27. By so doing, light oil and CNG are supplied into the combustion chamber to form a pre-mixture. CNG has a low compression ignitability, so CNG cannot be caused to combust solely; however, CNG is mixed with light oil having a high compression ignitability to thereby make it possible to combust CNG using light oil as a kindling charcoal. The ratio of CNG and light oil contained in fuel used may be appropriately changed in such a manner that the ECU 60 controls a fuel injection amount and a fuel injection timing from each light oil injector 20 and each CNG injector 22.

[0022] Here, in order to improve fuel consumption efficiency, there is an idea that a maximum combustion pressure (Pmax) in each combustion chamber 12 is maximized at an equivalent heat input. However, when the maximum combustion pressure Pmax is increased, the gradient of the heat release rate becomes steep, so a heat loss may increase to deteriorate thermal efficiency There are conceivably various factors that vary the maximum combustion pressure Pmax of injected fuel, such as a variation in the percentage of methane contained in CNG, variations due to devices and a malfunction of an injection system.

[0023] In the present embodiment, the fact that the internal combustion engine 100 uses two-type fuels having different ignitabilities is utilized to suppress the deterioration in thermal efficiency due to a heat loss. Hereinafter, this point will be described in detail.

[0024] FIG. 3 is a graph that shows the correlation between the amount of increase in CNG percentage in equal energy in fuel used and the amount of reduction in maximum combustion pressure Pmax. CNG contains methane that has a low flame propagation speed and that exhibits slow combustion, so the compression ignitability of CNG is lower than that of light oil. Thus, as the percentage of CNG in fuel used increases, the amount of reduction in maximum combustion pressure Pmax increases. In other words, when the maximum combustion pressure Pmax excessively increases, the percentage of CNG in fuel used is increased to thereby make it possible to decrease the maximum combustion pressure Pmax and moderate the gradient of the heat release rate.

[0025] FIG. 4 is a flowchart that shows control for correcting fuel injection according to the first embodiment. Initially, the ECU 60 that serves as a fuel control unit acquires a maximum combustion pressure Pmax (step S 10). The maximum combustion pressure Pmax may be acquired by each combustion pressure sensor 62. Subsequently, the ECU 60 compares the acquired maximum combustion pressure Pmax with a predetermined threshold Pth (step S 12). When the threshold Pth is higher than or equal to the maximum combustion pressure Pmax, the ECU 60 sets the correction amount of CNG percentage to "0" (step S 14). When the threshold Pth is lower than the maximum combustion pressure Pmax, the ECU 60 calculates the correction amount of CNG percentage required to suppress combustion (step S I 6). The ECU 60 stores the correction amount of CNG percentage, calculated in step S 14 or S 16 (step S I 8). The ECU 60 changes the CNG percentage in fuel used on the basis of the correction amount of CNG percentage, stored in step S 18, and then carries out fuel injection on the basis of the changed CNG percentage (step S20).

[0026] With the fuel control apparatus for the internal combustion engine 100 according to the first embodiment, the ECU 60 that serves as the fuel control unit changes the percentage of CNG (low ignitable fuel) in fuel used on the basis of a maximum combustion pressure Pmax at the time of combustion to adjust combustion speed. By so doing, feedback based on a combustion pressure is performed on the fuel injection system in real time to thereby make it possible to suppress a heat loss and deterioration in thermal efficiency due to the heat loss. In addition, by controlling fuel injection so that the combustion speed does not excessively increase, it is possible to suppress noise at the time of combustion and reduce emissions of NOx, or the like.

[0027] A second embodiment is an example in which fuel injection control is executed on the basis of a heat release rate pattern at the time of combustion. The configuration of an internal combustion engine according to the second embodiment is the same as that of the first embodiment (FIG. 1), so the detailed description is omitted.

[0028] FIG. 5A to FIG. 5F are graphs that show the correlations between a fuel injection timing and a relevant parameter. The abscissa axis of each graph represents the injection timing (crank angle [°CA]) of light oil fuel having a high compression ignitability, and a scale of 0 indicates a top dead center (TDC). The ordinate axes of the graphs respectively represent a net thermal efficiency in FIG. 5A, NOx emissions in FIG. 5B, HC emissions in FIG. 5C, smoke emissions in FIG. 5D, the magnitude of noise in FIG. 5E and a torque variation in FIG. 5F.

[0029] Here, the values of the respective parameters are desirable around a G point to which the light oil injection timing is advanced a certain amount from the TDC (see the area surrounded by the circle in each graph). Specifically, the net thermal efficiency is large (FIG. 5A), and the NOx emissions, the HC emissions, the smoke emissions, noise and the torque variation are small (FIG. 5B to FIG. 5F). Thus, the injection timing of light oil fuel is desirably set around the G point in the graphs.

[0030] FIG. 6 is a graph that shows the correlation between a fuel injection timing and a heat release rate pattern. The abscissa axis of the graph represents a crank angle, the ordinate axis represents a heat release rate, and the curves in the graph represents heat release rate patterns with variously changed injection timings of light oil fuel. The curve A has the latest injection timing of light oil, and the injection timing becomes earlier (is advanced) in order of the curves B, C, D, E, F, G (the same as G in FIG. 5) and H. In order from the curve A to the curve D, as the fuel injection timing is advanced, the rising timing of the heat release rate is advanced and the slope at the rising also becomes gradually steep. On the contrary, in order from the curve E to the curve H, as the fuel injection timing is advanced, the rising timing of the heat release rate is retarded and the slope at the time of rising also gradually becomes gentle. The ECU 60 stores the curve G in which the parameter relevant to fuel injection becomes a desirable value as an ideal heat release rate pattern.

[0031] FIG. 7 is a graph that shows a deviation between an ideal heat release rate pattern and an actually measured heat release rate pattern. The dotted line indicates the same ideal heat release rate pattern as that of the graph G in FIG. 6. The solid line indicates a heat release rate pattern acquired by each combustion pressure sensor 62. The actually measured heat release rate pattern may be calculated on the basis of a signal output from each combustion pressure sensor 62. As shown in the graph, the rising position (a) and slope at the time of rising (b) of the heat release rate pattern deviate between the ideal heat release rate pattern and the actually measured heat release rate pattern. The ECU 60 executes fuel injection control so that the heat release rate pattern at the time of combustion approaches the ideal heat release rate pattern. Hereinafter, this point will be described in detail.

[0032] FIG. 8 is a flowchart that shows control for correcting fuel injection according to the second embodiment. Initially, the ECU 60 acquires a heat release rate pattern during current operation on the basis of a signal output from each combustion pressure sensor 62 (step S30). Subsequently, the ECU 60 loads the ideal heat release rate pattern that becomes a reference (step S32). After that, the ECU 60 compares the rising positions (crank angles [°CA]) of the heat release rates in the two heat release rate patterns (step S34). Specifically, a deviation amount (AQ) between the rising positions in the two heat release rate patterns is acquired, and is compared with a predetermined threshold (Qth).

[0033] When the actually measured heat release rate pattern is advanced from the ideal heat release rate pattern by an amount larger than the threshold (Qth), the ECU 60 stores the advance-side deviation amount AQ (step S36). When the actually measured heat release rate pattern is retarded from the ideal heat release rate pattern by an amount larger than the threshold (Qth), the ECU 60 stores the retardation-side deviation amount AQ (step S38). When the deviation amount between the actually measured heat release rate pattern and the ideal heat release rate pattern is smaller than the threshold (Qth), the ECU 60 stores 0 as the deviation amount (step S40). The ECU 60 stores the deviation amount stored in any one of steps S36, S38 and S40 as a correction amount Q of the rising position (step S42).

[0034] Subsequently, the ECU 60 compares the rising inclination angles (inclination angles [J/(°CA) 2 ]) of the heat release rates in the two heat release rate patterns (step S44). Specifically, a deviation amount (AR) in inclination angle between the two heat release rate patterns is acquired, and is compared with a predetermined threshold (Rth).

[0035] When the actually measured heat release rate pattern is smaller in gradient from the ideal heat release rate by an amount larger than the threshold (Rth), the ECU 60 calculates a gradient correction retardation amount RA that is a retardation amount required to correct the deviation in inclination angle, and stores the gradient correction retardation amount RA (step S46). When the actually measured heat release rate pattern is larger in gradient from the ideal heat release rate pattern by an amount larger than the threshold (Rth), the ECU 60 calculates a gradient correction advance amount RB that is an advance amount required to correct the deviation in inclination angle, and stores the gradient correction advance amount RB (step S48). When the deviation amount in inclination angle between the actually measured heat release rate pattern and the ideal heat release rate pattern is smaller than the threshold (Rth), the ECU 60 stores 0 as the gradient correction amount (step S50). The ECU 60 stores the gradient correction amount (the advance amount or retardation amount of the fuel injection timing) stored in any one of steps S46, S48 and S50 as a correction amount R in inclination angle (step S52).

[0036] Furthermore, the ECU 60 calculates a combination of the correction amount Q of the rising position in step S42 and the correction amount R of the inclination angle in step S52 as a total correction amount S (step S54). The total correction amount S is a correction amount used to correct the deviation in heat release rate pattern by advancing or retarding the fuel injection timing, and indicates an advance amount (or a retardation amount) with respect to a current fuel injection timing.

[0037] Next, the ECU 60 converts the total correction amount calculated in step S54 into a correction amount in CNG percentage (step S56). The ECU 60 changes the CNG percentage in fuel used on the basis of the correction amount in CNG percentage, calculated in step S56, and carries out fuel injection on the basis of the changed CNG percentage (step S58). This step is the same as step S20 in the first embodiment (FIG. 4).

[0038] FIG. 9A and FIG. 9B are graphs that respectively show correction amounts of the fuel injection timing in steps S42 and S52 in FIG. 8. FIG. 9A shows a correction amount for a deviation in the rising position of the heat release rate pattern. FIG. 9B shows a correction amount for a deviation in the inclination angle at the time of the rising. As shown in FIG. 9A, while the deviation in the rising position falls below the predetermined threshold (Qth), the value of the correction amount is 0. When the deviation of the rising timing in the actually measured heat release rate pattern increases toward a positive side with respect to the ideal heat release rate pattern, the correction amount increases toward an advance side. On the contrary, when the deviation of the actually measured heat release rate pattern increases toward a negative side with respect to the ideal heat release rate pattern, the correction amount increases toward a retardation side. This is because, when the curve G in the graph of FIG. 6 is regarded as a reference, the rising position is retarded as the light oil fuel injection timing becomes earlier than that of the curve G (H) and the rising position is advanced as the light oil fuel injection timing becomes later than that of the curve G (F, E).

[0039] Similarly, in FIG. 9B, while the deviation in inclination angle falls below the predetermined threshold (Rth), the value of the correction amount is 0. When the deviation of the inclination angle in the actually measured heat release rate pattern increases toward a positive side with respect to the ideal heat release rate pattern, the correction amount increases toward an advance side. On the contrary, when the deviation of the actually measured heat release rate pattern increases toward a negative side with respect to the ideal heat release rate pattern, the correction amount increases toward a retardation side. This is because, when the curve G in the graph of FIG. 6 is regarded as a reference, the rising inclination angle becomes gentle as the light oil fuel injection timing becomes earlier than that of the curve G (H) and the rising position becomes steep as the light oil fuel injection timing becomes later than that of the curve G (F, E).

[0040] FIG. 10 is a graph that shows the correlation between a correction amount of the fuel injection timing and a correction amount of the CNG percentage in fuel. The abscissa axis represents a total correction amount of the fuel injection timing, calculated in step S54, and the ordinate axis represents a correction amount of the percentage of CNG fuel contained in fuel used. When the total correction amount of the fuel injection timing is 0, the correction amount of the CNG percentage is also 0. When the correction amount of the fuel injection timing is increased toward an advance side, the correction amount of the CNG percentage increases toward a positive side (the percentage of CNG contained in fuel increases). On the contrary, when the correction amount of the fuel injection timing is increased toward a retardation side, the correction amount of the CNG percentage increases toward a negative side (the percentage of CNG contained in fuel reduces).

[0041] As is described in the first embodiment, when the percentage of CNG in fuel used increases and then the combustion speed decreases, the rising timing in the heat release rate pattern is retarded and then the inclination angle becomes moderate (shifts from the pattern of the curve G to the pattern of the curve H). On the contrary, when the percentage of CNG in fuel used reduces and then the combustion speed increases, the rising timing in the heat release rate pattern is advanced, and the inclination angle becomes steep (shifts from the pattern of the curve G to the pattern of the curve F or E). In this way, the total correction amount of the fuel injection timing may be converted into the correction amount of the CNG percentage, and the CNG percentage in fuel used is changed to thereby make it possible to bring the heat release rate pattern close to an ideal shape.

[0042] With the fuel control apparatus for an internal combustion engine according to the second embodiment, the ECU 60 that serves as the fuel control unit changes the percentage of CNG (low ignitable fuel) in fuel used on the basis of a heat release rate pattern at the time of combustion. By so doing, the heat release rate pattern may be brought close to an ideal shape, so it is possible to improve thermal efficiency by suppressing a heat loss. In addition, by bringing the heat release rate pattern close to an ideal shape, as shown in FIG. 5B to FIG. 5F, emissions of a toxic substance may be suppressed, and noise and torque variations may be reduced.

[0043] Note that, after the total correction amount S is calculated in step S54, the fuel injection timing may be changed (advanced or retarded) instead without converting the total correction amount S into the correction amount of the CNG percentage and changing the CNG percentage. The fuel injection timing may be determined on the basis of the total correction amount S of the fuel injection timing. In this case as well, the heat release rate pattern may be brought close to an ideal shape as in the case of the second embodiment.

[0044] In the second embodiment, the ECU 60 functions as a heat release rate pattern comparing unit that acquires a heat release rate pattern at the time of combustion and a heat release rate pattern comparing unit that compares the acquired heat release rate pattern with an ideal heat release rate pattern. The ECU 60 that serves as the heat release rate pattern comparing unit specifically compares the rising timings of the heat release rates and compares the inclination angles after rising. Furthermore, the ECU 60 that serves as the fuel control unit also functions as a fuel injection timing correction amount calculation unit that calculates the correction amount of the light oil fuel injection timing required to correct a deviation between an actually measured heat release rate pattern and an ideal heat release rate pattern and a correction amount converting unit that converts the injection timing correction amount into the correction amount of the CNG percentage in fuel used.

[0045] As described above, in the first embodiment, the method of changing the percentage of fuel used on the basis of a maximum pressure Pmax at the time of combustion is described, and, in the second embodiment, the method of changing the percentage of fuel used on the basis of a heat release rate pattern at the time of combustion is described. During fuel injection control, only one of the methods may be carried out or the two methods may be used at the same time. The maximum pressure Pmax and the heat release rate pattern both may be acquired by each combustion pressure sensor 62; instead, the maximum pressure Pmax and the heat release rate pattern may be acquired by a component other than the combustion pressure sensor 62.

[0046] In addition, in the first and second embodiments, CNG is used as a first fuel, and light oil is used as a second oil; instead, fuels other than the above may be used. At this time, the second fuel is desirably higher in compression ignitability (cetane number) than the first fuel.

[0047] The embodiments of the invention are described in detail above; however, the aspect of the invention is not limited to the above specific embodiments. The aspect of the invention may be altered or modified in various forms within the scope of the invention recited in the appended claims.