Fuel property determination system

A fuel property determination system for an internal combustion engine is arranged to determine a fuel property indicative that fuel in use is heavy or light, on the basis of a degree of change of a revolution speed during a period from an expansion stroke of a first fuel injection cylinder at engine start to an expansion stroke of a final fuel injection cylinder in a first round as to all cylinders of the engine.

BACKGROUND OF THE INVENTION

The present invention relates a system for determining a fuel property (heavy/light property) of fuel used in an internal combustion engine.

Japanese Published Patent Application No. 9-151777 discloses a method of determining a fuel property (heavy/light property) of fuel in use by detecting a revolution speed difference per each predetermined cycle such as a ½ engine revolution after a cranking speed reaches a predetermined cranking speed such as a cranking speed of 300 rpm, and by determining whether or not the number of the elapsed cycles is greater than or equal to a predetermined value when the sum of the revolution speed differences becomes greater than or equal to a predetermined value.

SUMMARY OF THE INVENTION

However, in case that the fuel property is determined after a predetermined time elapses from a first engine combustion, the feedback of the fuel property determination delays and there is a possibility that the determination accuracy degrades.

It is therefore an object of the present invention to provide a fuel property determination system for an internal combustion engine which system is capable of accurately determining a fuel property in quick response to engine start.

An aspect of the present invention resides in a fuel property determination system which is for an internal combustion engine and which comprises a control unit configured to determine a fuel property indicative that fuel in use is heavy or light, on the basis of a degree of change of a revolution speed during a period from an expansion stroke of a first fuel injection cylinder at engine start to an expansion stroke of a final fuel injection cylinder in a first round as to all cylinders of the engine.

Another aspect of the present invention resides in a method of determining a fuel property of fuel in use for an internal combustion engine. The method comprises an operation of determining a fuel property indicative that fuel in use is heavy or light, on the basis of a degree of change of a revolution speed during a period from an expansion stroke of a first fuel injection cylinder at engine start to an expansion stroke of a final fuel injection cylinder in a first round as to all cylinders of the engine.

A further aspect of the present invention resides in a fuel property determination system which is for an internal combustion engine and which comprises a control unit configured to detect a degree of change of a revolution speed during a period comprising a predetermined stroke of a cylinder being involved with a first fuel injection thereto, and to determine a fuel property being indicative of a specific gravity of fuel in use on the basis of the degree of change of the revolution speed.

DETAILED DESCRIPTION OF THE INVENTION

There are discussed preferred embodiments of a fuel property determination system according to the present invention with reference to the drawings.

Referring toFIGS. 1 through 6, there is discussed a first embodiment of the fuel property determination system according to the present invention.

As shown inFIG. 1, the fuel property determination system according to the present invention is applied to an engine system. An internal combustion engine1of the engine system comprises a plurality of combustion chambers3each of which is defined by each cylinder and each piston2. A spark plug4, an intake valve5and an exhaust valve6are provided at upper portions of combustion chamber3so as to surround combustion chamber3. An intake passage7and an exhaust passage8are also connected with combustion chamber3.

A throttle valve9is provided upstream of an intake manifold of intake passage7. A fuel injector10of an electromagnetic type is installed at each branch portion of the intake manifold by each cylinder so as to inject fuel toward intake valve5.

An engine control unit (ECU)11controls the operation of each fuel injector10. ECU11is connected with a cam angle sensor12, a crank angle sensor13, an airflow meter14, and a water temperature sensor15to receive signals therefrom. Cam angle sensor12detects a cam angle indicative signal employed for a cylinder determination. Crank angle sensor13outputs a crank angle signal synchronized with the engine revolution and is capable of determining an engine speed Ne of engine1. Airflow meter14is disposed upstream of throttle valve9in intake passage7and detects an intake air quantity Qa. Water temperature sensor15detects a temperature Tw of engine cooling water.

For the control of fuel injection of fuel injector10, ECU11calculates a basic fuel injection quantity Tp=K·Qa/Ne on the basis of intake air quantity Qa and engine speed Ne, determines a final fuel injection quantity Ti=Tp·COEF where COEF is a correction coefficient by properly correcting basis fuel injection quantity Tp, and outputs a drive pulse signal corresponding to final fuel injection quantity Ti at a timing in synchronization with the engine revolution to fuel injector10of each cylinder, wherein correction coefficient COEF includes an increase quantity correction coefficient (hereinafter referred to as start increase quantity correction quantity) KAS for increasing a fuel quantity during an engine start and thereafter, as shown by the following expression (1).
COEF=1+KAS+ . . .   (1)

Start increase quantity correction coefficient KAS is calculated from the following expression (2).
KAS=MTKAS×TMKAS  (2)
where MTKAS is a table value (water temperature increase rate) according to engine cooling water temperature Tw, and therefore MTKAS takes a large value when engine cooling water temperature Tw is low and decreases as engine cooling water temperature Tw rises.

This table value is changed according to the fuel property, that is, according to whether fuel in use is heavy fuel or light fuel.FIG. 2is a schematic graph showing tables of water temperature increase rate (MTKAS). A difference of fuel vaporization rate between heavy and light fuels is large when cooling water temperature Tw is low, and is decreased as cooling water temperature Tw rises. Therefore, increase quantity rate MTKAS is set according to the kind of fuel as shown inFIG. 2.

TMKAS is a table value (time correction coefficient) determined according to the elapsed time from the engine start and is decreased as the time elapses from the engine start.

In the first embodiment according to the present invention, ECU11executes a control program for achieving the fuel property determining process of the fuel property determining system. Hereinafter, there will be discussed a flowchart of a fuel property determination routine shown inFIG. 3. In the first embodiment, engine1is a four-cylinder engine.

At step S10ECU11determines a first fuel injection cylinder of four-cylinder engine1. More specifically, for the by-cylinder fuel injection control, ECU11determines the cylinder of executing the fuel injection. Bases on this cylinder determination, ECU11determines which stoke of intake, compression, expansion and exhaust strokes is being executed at each of the four cylinders.

Since the fuel injection is executed on the basis of the cylinder determination result, ECU11determines the first fuel injection cylinder at which the fuel injection is firstly executed and the expansion stroke is firstly executed. Normally, the fuel injection is executed during the exhaust stroke of each corresponding cylinder. However, in order to further rapidly start engine1, at the first fuel injection cylinder the fuel injection is executed during the intake stroke. Therefore, the fuel injection at the first fuel injection cylinder and the fuel injection at the second fuel injection cylinder are simultaneously executed.

When the first fuel injection cylinder, at which the fuel injection is firstly executed and the expansion stroke is firstly executed, is determined, ECU11sets a value Nc indicative of the number of cylinders from the first fuel injection cylinder at 1 (Nc=1), and the routine proceeds to step S20.

At step S20ECU11detects an angular speed ω1(deg/s) at a compression-stroke top dead center (TDC) of the first fuel injection cylinder (Nc=1). More specifically, ECU11detects angular speed ω at TDC and set the detected angular speed ω as TDC angular speed ω1.

At step S30ECU11detects a maximum angular speed ω2during the expansion stroke of the first fuel injection cylinder (Nc=1). More specifically, ECU11executes a subroutine shown inFIG. 4which is executed after angular speed ω1at TDC is detected.

At step S31of the subroutine shown inFIG. 4, ECU11initializes an angular speed ωmax at compression-stroke TDC (ωmax=0). At step S32ECU11detects angular speed ω at sampling intervals of 10° crank angle. At step S33ECU11compares the detected angular speed ω with maximum angular speed ωmax. When ω>ωmax at step S33, the subroutine proceeds to step S34wherein ECU11updates maximum angular speed ωmax by setting the detected angular speed ω at maximum angular speed ωmax (ωmax=ω). When ω≦ωmax at step S33, the subroutine jumps to step S35. At step S35subsequent to the execution of step S34or the negative determination at step S33, ECU11determines whether or not the crank angle of the first fuel injection cylinder reaches near a bottom dead center (BDC) at which the expansion stroke is terminated. When the determination at step S35is negative, that is, when the crank angle does not reach near BDC, the subroutine returns to step S32to repeating execute the sampling of the crank angle. When the determination at step35is affirmative, that is, when the crank angle of the first fuel injection cylinder reaches near BDC, the subroutine proceeds to step S36wherein ECU11sets maximum angular speed ωmax at this moment as an expansion stroke maximum angular speed ω2. Thereafter, the program returns to the main routine.

The subroutine for detecting expansion stroke maximum angular speed ω2may be arranged to detect an angular speed near an intermediate position of the expansion stroke as a maximum angular speed neighborhood value, or to detect an angular speed near BDC during the expansion stroke in addition to the detection of the maximum angular speed during the expansion stroke.

At step S40ECU11calculates an angular acceleration Δω=ω2−ω1from compression-stroke TDC angular speed ω1and expansion stroke maximum angular speed ω2. Further, ECU11may calculate the angular acceleration by employing the expression of Δω=(ω2−ω1)/dt where dt is a period from the detection of ω1to the detection of ω2.

At step S50ECU11executes the first combustion determination. More specifically, ECU11determines whether or not the first combustion is executed, on the basis of a comparison between angular acceleration Δω indicative of a changing degree of a revolution speed of each cylinder and a predetermined threshold ΔωS such as 40,000 deg/s2. When Δω≧ΔωS, ECU11determines that the first combustion was executed. When Δω<ΔωS, the program proceeds to step S60.

At step S60ECU11determines whether or not Nc=4 is satisfied, that is, ECU11determines whether or not the fourth cylinder was checked. When the determination at step S60is negative, the program proceeds to step S70wherein Nc is incremented by 1 (Nc=Nc+1).

Thereafter, the processing of steps S20through S40is executed to determine the first combustion of the next cylinder corresponding to Nc. More specifically, at step S50ECU11executes the first combustion of the next cylinder on the basis of angular acceleration Δω obtained from compression TDC angular speed ω1and expansion stroke maximum angular speed Δω2.

When the negative determination indicative that there is no combustion in any cylinder is made at step S50even though the first combustion determination is repeated from Nc=1 to Nc=4, that is, when no combustion has been executed within a first round wherein the combustion checks as to all cylinders were executed, ECU11determines that it is impossible to determine the property (heavy or light property) of fuel. Therefore, the present routine is terminated.

When the affirmative determination is made at step S50within the first round from Nc=1 to Nc=4, the program proceeds to step S80.

At step S80ECU11determines whether or not Δω≧ΔωL, by comparing angular acceleration Δω and a predetermined threshold ΔωS such as 100,000 deg/s2. When the determination at step S80is negative (Δω<ΔωL), the program proceeds to step S90.

At step S90ECU11determines whether or not Nc=4 is satisfied, that is, ECU11determines whether or not the fourth cylinder is checked. When the determination at step S90is negative, the program proceeds to step S100wherein Nc is incremented by 1 (Nc=Nc+1).

At step S110subsequent to the execution of step S100, ECU11detects compression-stroke TDC angular speed ω1(deg/s). At step S120ECU detects expansion stroke maximum angular speed ω2during the expansion stroke. At step S130ECU11calculates angular acceleration Δω=ω2−ω1from compression-stroke TDC angular speed ω1and expansion stroke maximum angular speed ω2. Thereafter, the program returns to step S80. That is, the processing of step S90through S130is repeated until the affirmative determination is made at step S80.

When the affirmative determination is made at step S80(Δω≧ΔωL), the program proceeds to step S140wherein ECU11determines that the fuel in use is light fuel. Thereafter, the present program terminated.

In contrast to this, when the affirmative determination is made at step S90(Nc=4) subsequent to the negative determination at step S80, the program proceeds to step S150wherein ECU11determines that the fuel in use is heavy fuel. Thereafter, the present program is terminated.

FIG. 5Ais an example showing changes of angular speeds ω (deg/s) relative to a crank angle from the start of the second time expansion stroke to the end of the sixth time expansion stroke. In the graph ofFIG. 5A, an X-axis represents a crank angle and a Y-axis represents angular speed ω.

FIG. 5Bshows angular acceleration Δω of each cylinder, which acceleration is derived from the change of angular speed ω inFIG. 5A. In a graph ofFIG. 5B, an X-axis represents the number of times the expansion stroke was executed, which corresponds to the X-axis ofFIG. 5a,and a Y-axis represents angular acceleration Δω.

InFIGS. 5A and 5B, continuous lines denote heavy fuel, and dotted lines denote light fuel.

In this example shown inFIGS. 5A and 5B, the first fuel injection cylinder is a cylinder wherein the number of times of the expansion strokes is three. Even when any fuel (heavy or light fuels) is used, ECU11determines the first combustion determination by executing the determination at the first fuel injection cylinder (the number of expansion strokes is three) on the basis of angular acceleration Δω=(ω2−ω1)/dt obtained from compression-stroke TDC angular speed ω1and expansion stroke maximum angular speed ω2. When light fuel is used, Δω≧ΔωL is satisfied in simultaneously with the first combustion determination, and therefore ECU11determines that the fuel in use is light fuel. When heavy fuel is used, Δω≧ΔωL is not satisfied within the first round, that is, within a period that the number of expansion strokes is three to six, and therefore ECU11determines that the fuel in use is heavy fuel.

FIG. 6is a flowchart showing a fuel injection quantity control fuel property setting routine for executing a heavy fuel setting or light fuel setting for the fuel injection quantity control, using the fuel property determination according to the present invention. This routine starts in response to the turning on of an engine switch.

At step S101ECU11executes a heavy fuel setting as an initial setting. That is, ECU11firstly uses a heavy fuel table shown inFIG. 2where both of heavy and light fuel tables are shown. If the light fuel setting is firstly executed under a condition that heavy fuel is actually used, the engine starting performance degrades. Therefore, the heavy fuel setting is firstly executed.

At step S102ECU11determines whether or not the heavy fuel determination is terminated. Until the affirmative determination is made at step S102, step S102is repeated. When the affirmative determination is made at step S102, the program proceeds to step S103.

At step S103ECU11determines whether the fuel in use is heavy fuel, light fuel or indefinite. When ECU11determines that the fuel in use is heavy fuel, this routine is terminated since it is not necessary to change the initial setting as to the fuel property. When ECU11determines that the fuel in use is light fuel, the routine proceeds to step S104wherein ECU11changes the fuel property setting so as to use the light fuel table shown inFIG. 2. By this changing of the fuel property to the light fuel setting, the fuel consumption of engine1is improved.

When the fuel property is indefinite, that is, when the program shown inFIG. 3was terminated after the affirmative determination at step S6was made, the initial fuel property set at the heavy fuel property is maintained to mainly satisfy the engine starting performance and the engine combustion stability during the engine operation.

With the thus arranged first embodiment according to the present invention, taking account of a fact that the fuel injected during the first round from the first fuel injection cylinder for starting the engine is mainly attached to a wall of an intake port and remained as wall fuel since the intake port was dry, the mass of wall fuel increases as the fuel in use becomes heavier in property. Therefore, a fuel quantity supplied into each cylinder varies according to the fuel property of heavy or light. This variation due to the fuel property (heavy or light) generates the variation (large difference) of the degrees of changes of revolution speeds between heavy fuel and light fuel. Accordingly, on the basis of the degree of change of revolution speed (engine speed) during a period from an expansion stroke of the first fuel injection cylinder to the expansion stroke of the final (fourth) fuel injection cylinder in the first round, ECU11determines the fuel property (heavy or light) of fuel in use. This enables an accurate determination as to the fuel property (heavy or light) of the fuel in use within an extremely short time from an engine start until the termination of the first round of the fourth fuel injection cylinder.

Further, with the thus arranged first embodiment according to the present invention, by calculating the degree of change of the engine revolution speed such as an angular acceleration, on the basis of the difference (ω2−ω1) between an expansion stroke start (TDC neighborhood) angular speed ω1and an expansion stroke maximum angular speed (or neighborhood value thereof) ω2, it becomes possible to accurately detect the degree of change of revolution speed. Further, by using an angular speed at a point near the intermediate position during the expansion stroke or an angular speed at a position near BDC during the expansion stroke as a neighborhood value of the maximum angular acceleration during the expansion stroke, it becomes possible to easily detect the degree of change of the angular speed. Particularly, by using the angular speed at a point near the intermediate position during the expansion stroke, it becomes possible to further accurately detect the degree of change of the revolution speed since the difference between the absolute value of the expansion stroke start angular speed ω1and the absolute value of the angular speed ω2. Further, by using the angular speed at a position near the BDC during the expansion stroke, it becomes possible to stably detect the work load during the expansion stroke.

Furthermore, with the first embodiment according to the present invention, by determining the fuel property (heavy or light) through the comparison of the degree of the change of revolution speed with a predetermined threshold Δω1, the determination of the fuel property is easily executed. Further the determination of the fuel property is further accurately executed by repeating the comparison of the degree of change of the revolution speed of each cylinder with the threshold.

Furthermore, with the first embodiment according to the present invention, when the degree Δω of change of the revolution speed of either cylinder within the first round becomes higher than or equal to the threshold value ΔωS, it is determined that the fuel in use is light fuel. This enables the fuel property determination to be easily executed. Further, when the degree Δω of change of the revolution speed of either cylinder within the first round does not become higher than or equal to the threshold value ΔωS, it is determines that the fuel in use is heavy fuel. This enables the fuel property determination to be accurately executed.

When the first combustion determination was not made within the first round for all cylinders, it is determined that the property of fuel in use cannot be determined, that is, ECU11prohibits the fuel property determination based on the degree of change of the revolution speed. This arrangement prevents misjudgment.

Further, with the first embodiment according to the present invention, the first combustion determination is executed on the basis of the comparison between the degree of change of the revolution speed ΔωS. Therefore, the first combustion determination is executed using the parameters as same as those of the fuel property determination.

Referring toFIG. 7, there is discussed a second embodiment of the fuel property determination system according to the present invention.

The construction of the second embodiment is basically the same as that of the first embodiment. The is second embodiment employs a flowchart for the fuel property determination routine shown inFIG. 7. The flowchart ofFIG. 7is different from that ofFIG. 3in further comprising step S160subsequent to the affirmative determination at step S60.

Accordingly, when the negative determination indicative that there is no combustion in any cylinder is made at step S50even though the first combustion determination is repeated from Nc=1 to Nc=4, that is, when no combustion has been executed within the first round wherein the combustion check as to all cylinders were executed, ECU11determines that the fuel in use is heavy fuel.

In the second embodiment according to the present invention, when it is impossible to determine the property (heavy or light property) of fuel from the degree of change of the revolution speed by each cylinder, it is considered that the reason of generating no first combustion in the first round is that the fuel vaporization rate is low. Therefore, in the second embodiment, it is determined that the fuel in use is heavy fuel at step S160when the affirmative determination is made at step S60. However, even if the affirmative determination is made at step S60inFIG. 3of the first embodiment, the fuel property table for heavy fuel is used. Therefore, the actual control based on the flowchart ofFIG. 7in the second embodiment is the same as that executed in the first embodiment.

Although the first and second embodiments have been shown and described such that it is determined whether the fuel in use is heavy fuel or light fuel by comparing the degree of change of the revolution speed with the predetermined threshold ΔωL, the degree of the fuel property may be determined according to the level of the degree of change of the revolution speed.

Referring toFIG. 8, there is discussed a third embodiment of the fuel property determination system according to the present invention.

The construction of the third embodiment is basically the same as that of the first embodiment. The third embodiment employs a flowchart for the fuel property determination routine shown inFIG. 8. The flowchart ofFIG. 8is different from that ofFIG. 3in further comprising step S5before step S10.

Accordingly, at step S5ECU11firstly determines whether or not engine1is started under an engine heated condition, that is, whether or not the engine start is a hot restart. More specifically, ECU11obtains a cooling water temperature Tw at a time just before the engine start from cooling water temperature sensor15and determines whether cooling water temperature Tw is higher than or equal to a predetermined temperature Twh such as 70° C. When cooling water temperature Tw is higher than or equal to predetermined temperature Twh, ECU11determines that the present engine start is the hot restart. Therefore, ECU11determines that it is impossible to determine the fuel property, and the program is terminated without executing the fuel property determination. When the negative determination is made at step S5, the program proceeds to step S10.

Although the third embodiment has been shown and described such that cooling water temperature Tw is used as an engine temperature representative value, a fuel temperature or oil temperature may be used instead of cooling water temperature Tw. Otherwise, the determination as to the hot restart may be executed by measuring an engine stop period before the engine start (corresponding to a period from a previous engine stop to a present engine start) and by determining whether or not the engine stop period is shorter than or equal to a predetermined period.

With the thus arranged third embodiment according to the present invention, the advantages given by the first embodiment are also obtained. Further, in case of the hot restart of engine1, the fuel property determination is prohibited to prevent erroneous determination (misjudgment) since the difference of the degrees of changes of the revolution speeds due to the fuel property becomes small in case of the hot restart. Further, the determination as to the hot restart is easily executed on the basis of the cooling water temperature or engine stop period before the engine start.

Referring toFIGS. 9 and 10, there is discussed a fourth embodiment of the fuel property determination system according to the present invention.

The construction of the fourth embodiment is basically the same as that of the first embodiment. The fourth embodiment employs a flowchart for the fuel property determination routine shown inFIG. 9. The flowchart ofFIG. 9is different from that ofFIG. 3in further comprising step S5before step S10and step S55subsequent to the affirmative determination at step S50.

Accordingly, at step S5ECU11firstly determines whether or not engine1is started under an engine heated condition, that is, whether or not the engine start is a hot restart. More specifically, ECU11obtains a cooling water temperature Tw at a time just before the engine start from cooling water temperature sensor15and determines whether cooling water temperature Tw is higher than or equal to a predetermined temperature Twh. When cooling water temperature Tw is higher than or equal to predetermined temperature Twh, ECU11determines that the present engine start is the hot restart. Therefore, ECU11determines it is impossible to determines the fuel property, and the program proceeds is terminated without executing the fuel property determination. When the negative determination is made at step S5, the program proceeds to step S10.

Although the fourth embodiment has been shown and described such that cooling water temperature Tw is used as an engine temperature representative value, a fuel temperature or oil temperature may be used instead of cooling water temperature Tw. Otherwise, the determination as to the hot restart may be executed by measuring an engine stop period before the engine start (corresponding to a period from a previous engine stop to a present engine start) and by determining whether or not the engine stop period is shorter than or equal to a predetermined period.

Further, at step S55subsequent to the affirmative determination at step S50, ECU11sets the threshold ΔωL according to engine cooling water temperature Tw. More specifically, ECU11determines threshold ΔωL from engine cooling water temperature Tw and with reference to a table shown inFIG. 10which shows a relationship between threshold ΔωL and engine cooling water temperature Tw. Threshold ΔωL has been determined such that threshold ΔωL linearly increases as cooling water temperature increases. InFIG. 10, an applicable range of engine cooling water temperature Tw ranges from −40° C. to 70° C., and an applicable range of threshold ΔωL ranges from 30,000 deg/s2to 100,000 deg/s2relative to the applicable range of engine cooling water temperature Tw. After the execution of step S55, the program proceeds to step S80.

With the thus arranged fourth embodiment according to the present invention, the advantages given by the first embodiment are also obtained. Further, the variation of the in-cylinder flowing fuel quantity due to the fuel property decreases as the engine temperature at the engine start increases and as the in-cylinder flowing fuel quantity increases. Although this variation generates the misjudgment as to the fuel property, it becomes possible to highly maintain the determination accuracy by changing the threshold ΔωL according to the engine temperature condition such as the cooling water temperature Tw.

Referring toFIGS. 11,12A and12B, there is discussed a fifth embodiment of the fuel property determination system according to the present invention.

The construction of the fifth embodiment is basically the same as that of the first embodiment. The fifth embodiment employs a flowchart for the fuel property determination routine shown inFIG. 11. The flowchart ofFIG. 11is different from that ofFIG. 3in further comprising steps S72, S74and S76between steps S50and S80.

At step S72subsequent to the affirmative determination at step S50, ECU11determines whether or not Nc=1, that is, whether or not a cylinder of executing the fuel injection is a first fuel injection cylinder of four-cylinder engine1. In other words, ECU11determines whether or not the cylinder to be determined in the fuel property is a first fuel injection cylinder. When the determination at step S72is affirmative (Nc=1), the program proceeds to step S74wherein ECU11sets a relatively large value ΔωL1such as 100,000 deg/s2at threshold ΔωL. When the determination at step S72is negative, that is, the fuel injection cylinder is a second cylinder or later cylinders, the fuel injection is an exhaust stroke fuel injection. Therefore, the program proceeds to step S76wherein ECU11sets a relatively small value ΔωL2such as 80,000 deg/s2at threshold ΔωL where ΔωL1>ΔωL2. After the execution of step S74or S76, the program proceeds to step S80.

With the thus arranged fifth embodiment according to the present invention, the advantages given by the first embodiment are also obtained. Further, as shown inFIGS. 5A and 5B, the first fuel injection cylinder is a cylinder wherein the number of times the expansion stroke is three. Even when any fuel (heavy or light fuels) is used, ECU11determines the first combustion determination by executing the determination at the first fuel injection cylinder (the number of expansion strokes is three) on the basis of angular acceleration Δω=(ω2−ω1)/dt obtained from compression TDC angular speed ω1and expansion stroke maximum angular speed ω2.

When Δω>ΔωL1is satisfied simultaneously with the first combustion determination or when Δω>ΔωL2is satisfied at one of second, third and fourth cylinder (the number of expansion strokes is within a range from 4 to 6), ECU11determines that the fuel in use is light fuel. On the other hand, if the fuel in use is heavy fuel, Δω>ΔωL1is not satisfied simultaneously with the first combustion determination and Δω>ΔωL2is not satisfied at one of second, third and fourth cylinders (the number of expansion strokes is within a range from 4 to 6). Therefore, ECU11determines that the fuel in use is heavy fuel.

The reason for changing threshold ΔωL in the first fuel injection cylinder (intake stroke fuel injection cylinder) and the fuel injection cylinder thereafter is that since the first fuel injection cylinder is an intake stroke cylinder, the in cylinder flowing fuel quantity increases. However, since a time for vaporizing the fuel is short, the difference of the degree of change of the revolution speed due to the difference of the fuel property becomes small.

More specifically, as shown inFIG. 12A, in case of the intake stroke fuel injection, a variation range of the estimated degree Δω of change of the revolution speed in the situation using heavy fuel and a variation range of the estimated degree Δω of change of the revolution speed in the situation using light fuel are partly overlapped since the difference therebetween is small. Therefore it is difficult to accurately determine the fuel property of the fuel in use. Accordingly, the threshold of determining the fuel property is set at a relatively large value ΔωL1which is larger than the variation range of the estimated degree of change of the revolution speed.

That is, if it is erroneously determined that the fuel in use is light fuel even though the fuel in use is actually heavy fuel, the engine operation (drivability) degrades. Therefore in order to prevent such degradation of the engine operation (drivability), the threshold for determining the fuel property is set at the relatively large value ΔωL1. Generally, when it is determined that the fuel in use is heavy fuel even though the actually used fuel is light fuel, the drivability does not degrade although the fuel consumption degrades. That is, when it is difficult to determine the fuel property, the fuel property determination system according to the present invention is basically arranged to determine the fuel in use as heavy fuel. In this fifth embodiment, when the degree Δω of change of the revolution speed becomes higher than threshold ΔωL, it is directly determined that the fuel in use is light fuel. However, when the degree Δω does not become higher than threshold ΔωL, it is not directly determined that the fuel in use is heavy fuel. Therefore, by setting a threshold for the first fuel injection cylinder (intake stroke fuel injection cylinder) at a relatively large value, it becomes possible to prevent the misjudgment of the fuel property and to improve the determination accuracy.

On the other hand, as shown inFIG. 12B, in case of the exhaust stroke fuel injection, a variation range of the estimated degree Δω of change of the revolution speed using heavy fuel and a variation range of the estimated degree Δω of change of the revolution speed in the situation using light fuel are separately positioned since the difference therebetween is large. Therefore it becomes easy to accurately determine the fuel property of the fuel in use. Accordingly, the threshold of determining the fuel property is set at a relatively small value ΔωL2which is smaller that the value ΔωL1and which is an intermediate value between the variation range of the estimated degree of change of the revolution speed using heavy fuel and the variation range of the estimated degree Δω of change of the revolution speed using light fuel.

With the thus arranged fifth embodiment according to the present invention, the advantages given by the first embodiment are also obtained. Further, at the cylinder in which the fuel injection is executed during the intake stroke in the first round, such as the first fuel injection cylinder, the difference of the degree of change of the revolution speed due to the fuel property becomes small as compared with a case of the cylinder where the fuel injection is executed during other stroke except for the intake stroke. Therefore, there is a possibility that the erroneous determination as to the fuel property is made. However, by changing the threshold ΔωL according to whether the cylinder to be checked is a cylinder of executing the fuel injection during the intake stroke or cylinder of executing the fuel injection during other stroke (exhaust stroke) except for the intake stroke, the diagnosis accuracy as to the fuel property is improved. More specifically, by setting the threshold ΔωL1for the degree Δω of change of the revolution speed at the cylinder where the fuel injection is executed during the intake stroke, so as to be greater than the threshold ΔωL2for the degree Δω of change of the revolution speed at the cylinder where the fuel injection is executed during the other strokes except for the intake stroke, the diagnosis accuracy is improved.

Further, in case of the cylinder where the fuel injection is executed during the intake stroke, when the variation range of the estimated degree Δω of change of the revolution speed in the situation using heavy fuel and the variation range of the estimated degree Δω of change of the revolution speed in the situation using light fuel are partly overlapped, the threshold ΔωL is set at a value greater than the variation range of the estimated degree Δω of change of the revolution speed using heavy fuel. This accurately improves the determination accuracy of the fuel property determination.

Referring toFIG. 12, there is discussed a sixth embodiment of the fuel property determination system according to the present invention.

The construction of the sixth embodiment is basically the same as that of the first embodiment. The sixth embodiment employs a flowchart for the fuel property determination routine shown inFIG. 13. The flowchart ofFIG. 13is different from that ofFIG. 3in further comprising step S72subsequent to the affirmative determination at step S50.

At step S72subsequent to the affirmative determination at step S50, ECU11determines whether or not Nc=1, that is, whether or not a cylinder of executing the fuel injection is a first fuel injection cylinder of four-cylinder engine1. In other words, ECU11determines whether or not the cylinder to be determined in the fuel property is a first fuel injection cylinder. When the determination at step S72is affirmative (Nc=1), the program jumps to step S100wherein ECU11increments Nc by 1 (Nc=NC+1). When the determination at step S72is negative, that is, the fuel injection cylinder is a second cylinder or later cylinders, the fuel injection is an exhaust stroke fuel injection. Therefore, the program proceeds to step S80wherein ECU11determines whether or not Δω≧ΔωL by comparing angular acceleration Δω indicative of the degree of change of the revolution speed at each cylinder, which is calculated at step S40with predetermined threshold ΔωL which is larger than ΔωS and corresponds to the value ΔωL2shown inFIGS. 12A and 12B. When the negative determination is made at step S80, the program proceeds to step S90. When the affirmative determination is made at step S80, the program proceeds to step S140.

At step S90ECU11determines whether or not Nc=4 is satisfied, that is, ECU11determines whether or not the fourth cylinder is checked. When the determination at step S90is negative, the program proceeds to step S100wherein Nc is incremented by 1 (Nc=Nc+1).

At step S110subsequent to the execution of step S100, ECU11detects compression TDC angular speed ω1(deg/s). At step S120ECU detects expansion stroke maximum angular speed ω2during the expansion stroke. At step S130ECU11calculates angular acceleration Δω=ω2−ω1from compression TDC angular speed ω1and expansion stroke maximum angular speed ω2. Thereafter, the program returns to step S80. That is, the processing of step S90through S130is repeated until the affirmative determination is made at step S80.

When the affirmative determination is made at step S80(Δω≧ΔωL), the program proceeds to step S140wherein ECU11determines that the fuel in use is light fuel. Thereafter, the present program is terminated. In contrast to this, when the affirmative determination is made at step S90(Nc=4) subsequent to the negative determination at step S80, the program proceeds to step S150wherein ECU11determines that the fuel in use is heavy fuel. Thereafter, the present program is terminated.

With the thus arranged sixth embodiment according to the present invention, the advantages given by the fifth embodiment are also obtained. Further, since the sixth embodiment is arranged to prohibit the fuel property determination when the cylinder to be checked is a cylinder in which the fuel injection is executed during the intake stroke of the first round and to execute the fuel property determination when the cylinder to be checked is a cylinder in which the fuel injection is executed during the other stroke except for the intake stroke in the first round, it becomes possible to present the misjudgment due to the decrease of the difference between the degrees of changes of the respective revolution speeds in the respective situations using heavy fuel or light fuel. Further, since the sixth embodiment according to the present invention is arranged such that the fuel property determination is executed on the basis of the degree of change of the revolution speed at the cylinder in which the fuel injection is executed during the other stroke except for the intake stroke, such as the exhaust stroke, it becomes possible to improve the accuracy of the fuel property determination.

This application is based on Japanese Patent Application Nos. 2003-326990, 2003-326991 and 2003-326992 filed on Sep. 19, 2003 in Japan and No. 2003-329357 filed on Sep. 22, 2003 in Japan. The entire contents of these Japanese Patent Applications are incorporated herein by reference.

Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, in light of the above teaching. The scope of the invention is defined with reference to the following claims.