BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to an engine fuel property estimation apparatus that injects fuel at the time of engine fuel cut and that estimates the ignition quality of the fuel from the engine torque produced by combustion of the injected fuel.

2. Description of Related Art

[0002] A diesel engine burns injected fuel by igniting it through compression.

Diesel engines use light oil as fuel. Commercially available light oils are different in their components, and vary in fuel properties such as the ignition quality. The ignition quality of fuel greatly affects the situation of occurrence of misfire, the engine output, etc. Therefore, in order to improve the output performance, the fuel economy performance and the emission performance of a diesel engine, it is necessary to check the ignition quality of the fuel that is currently used and to adjust the manners of execution of engine controls regarding the timing and amount of fuel injection, etc. according to a result of the checking of the ignition quality of the fuel.

[0003] The ignition quality of light oil as a fuel of diesel engines is evaluated by the cetane number. The cetane number of a specimen light oil is expressed by the volume percentage of the amount of cetane in a mixture of cetane and cc-methyl naphthalene which exhibits the same ignition quality as the specimen light oil.

[0004] Japanese Patent Application Publication No. 2010-024870 (JP 2010-024870 A) discloses an apparatus that performs a single injection of fuel when the engine rotation speed is decreasing in a state of no load and no fuel injection and that estimates the cetane number of the currently used fuel on the basis of the magnitude of the engine torque produced by the combustion of the injected fuel and the injection timing of the single injection. [0005] By the way, the fuel injection for estimating the cetane number is small in amount. Therefore, combustion of the fuel thus injected becomes less easy to occur if the in-cylinder flow (swirl and tumble flows) is excessively strong. Furthermore, the time from the injection of fuel to the combustion of the fuel varies depending on the temperature of the gas taken into the cylinder and the temperature of the cylinder wall. If the time from the injection to the combustion becomes long, the time from when the combustion starts until when the in-cylinder pressure or the in-cylinder temperature due to descent of the piston occurs becomes short, and the amount of fuel left unburned increases. As a result, the engine torque produced by the combustion of fuel becomes small.

[0006] Thus, depending on the states, such as the in-cylinder flow, the in-cylinder intake gas temperature, the cylinder wall temperature, etc. occurring when the fuel injection for estimating the cetane number of the fuel, or the like, the magnitudes of the engine torque produced by the combustion of the injected fuel vary even if the cetane number of fuel or the amount of fuel injection remains the same. Therefore, the estimation of the cetane number based on the magnitude of the engine torque produced by combustion achieves only a limited degree of accuracy.

SUMMARY OF THE INVENTION

[0007] The invention provides an engine fuel property estimation apparatus capable of estimating the ignition quality of fuel with improved accuracy.

[0008] A first aspect of the invention provides an engine fuel property estimation apparatus that injects fuel of an engine at a time of fuel cut of the engine and that estimates ignition quality of the fuel from magnitude of engine torque produced by combustion of the fuel injected. When engine load before the fuel cut starts is a first value, the estimation apparatus causes an upper limit value of engine rotation speed for implementing a fuel injection for estimating the ignition quality of the fuel to be smaller than when the engine load before the fuel cut starts is a second value that is lower than the first value.

[0009] The cylinder wall temperature affects the ignition delay time from injection of the fuel to ignition of the fuel. Then, if the ignition delay time changes, the magnitude of the engine torque produced by combustion of the fuel also changes.

[0010] During low-load operation of the engine, heat produced by the engine can be appropriately cooled or removed, so that the cylinder wall temperature is relatively stable. In contrast, during high-operation load operation of the engine, the cooling cannot match the production of heat by the engine, so that the cylinder wall temperature greatly increases. The amount of increase in the cylinder wall temperature at this time greatly varies depending on the duration of high-load operation, and the like. Therefore, in the case where the ignition quality of the fuel is estimated on the basis of the magnitude of the engine torque produced by combustion of the fuel at the time of the fuel cut following the high-load engine operation when the cylinder wall temperature greatly varies, the engine torque produced by combustion of the fuel varies due to the variations of the cylinder wall temperature, so that the accuracy in estimating the ignition quality deteriorates.

[0011] In the above-described construction, however, when the engine load prior to the fuel cut is high, the upper limit value of the engine rotation speed for implementing the fuel injection for the estimation of the ignition quality is lessened. If the upper limit value of the engine rotation speed is lessened, the time from the start of the fuel cut to the implementation of the fuel injection for the estimation of the ignition quality becomes long, so that the cylinder wall temperature risen due to the high-load engine operation can be sufficiently reduced prior to the implementation of the fuel injection for the estimation. Therefore, variations of the cylinder wall temperature at the time of implementation of the fuel injection for the ignition quality estimation can be restrained and therefore variations of the ignition delay time can be restrained. Hence, according to the foregoing construction, it is possible to restrain the deteriorate of the accuracy of the estimation caused by variations of the cylinder wall temperature during high-load engine operation and to estimate the ignition quality of the fuel with improved accuracy.

[0012] A second aspect of the invention provides an engine fuel property estimation apparatus that injects fuel of an engine at a time of fuel cut of the engine and that estimates ignition quality of the fuel from magnitude of engine torque produced by combustion of the fuel injected. When vehicle speed at a time of estimating the ignition quality of the fuel is a first speed, the estimation apparatus causes an upper limit value of engine rotation speed for implementing a fuel injection for estimating the ignition quality of the fuel to be smaller than when the vehicle speed at the time of estimation of the ignition quality is a second speed that is lower than the first speed.

[0013] When the vehicle speed at the time of the estimation of the ignition quality of the fuel is high, the engine is highly likely to have previously been operated under high load. As described above, the deterioration of the accuracy of the estimation due to variations of the cylinder wall temperature can be restrained by lessening the upper limit value of the engine rotation speed for implementing the fuel injection for estimating the ignition quality of the fuel. In conjunction with that respect, in this aspect, at the time of high vehicle speed when it is considered that the engine is highly likely to have been operated under high load, the upper limit value of the engine rotation speed for implementing the fuel injection for estimating the ignition quality of the fuel is lessened. Therefore, the deterioration of the estimation accuracy due to variations of the cylinder wall temperature is restrained, so that the ignition quality of the fuel can be estimated with improved accuracy.

[0014] A third aspect of the invention provides an engine fuel property estimation apparatus that injects fuel of an engine at a time of fuel cut of the engine and that estimates ignition quality of the fuel from magnitude of engine torque produced by combustion of the fuel injected. When estimation of the ignition quality of the fuel is performed while the vehicle is traveling, the estimation apparatus causes an upper limit value of engine rotation speed for implementing a fuel injection for estimating the ignition quality of the fuel to be smaller than when the estimation is performed during deceleration following racing of the engine.

[0015] Immediately after the fuel cut starts, the in-cylinder flow is strong, so that injection of a small amount of fuel does not easily achieve combustion. Therefore, in order to suitably perform the estimation of the ignition quality of the fuel, it is necessary to secure a sufficient time from the start of the fuel cut to the fuel injection for the estimation.

[0016] The racing, that is, the gunning of the engine, will greatly increase the engine rotation speed. At the time of fuel cut during deceleration after the racing of the engine, a sufficient time from the start of the fuel cut to the fuel injection for estimation of the ignition quality of the fuel can be secured even if the upper limit value of the engine rotation speed for implementing the fuel injection for the estimation is increased to a certain extent.

[0017] On another hand, at the time of fuel cut while the vehicle is traveling, it is often the case that the engine rotation speed before the fuel cut starts is not as high as in the case of the racing of the engine. Therefore, in the case where estimation of the ignition quality of the fuel is performed at the time of fuel cut while the vehicle is traveling, it is preferable to lessen the upper limit value of the engine rotation speed for implementing the fuel injection for the estimation and, after the in-cylinder flow becomes sufficiently weak, perform the fuel injection for the estimation.

[0018] In conjunction with that respect, in the foregoing construction, the upper limit value of the engine rotation speed for implementing the fuel injection for estimation of the ignition quality of the fuel is made large at the time of fuel cut during deceleration following the racing of the engine, and is made small at the time of fuel cut while the vehicle is traveling. Therefore, estimation of the ignition quality of the fuel is performed only in a situation where the estimation is performed in a good manner, so that the ignition quality of the fuel is estimated with improved accuracy.

[0019] At the time of fuel cut during deceleration following the racing of the engine, the influence of external disturbance is smaller than at the time of fuel cut while the vehicle is traveling, so that the ignition quality of the fuel can be estimated with improved accuracy. Therefore, if there is a need to estimate the ignition quality of the fuel with high accuracy, it is appropriate to estimate the ignition quality at the time of fuel cut during deceleration following the racing of the engine.

[0020] The cylinder wall temperature when the fuel injection for estimation of the ignition quality is implemented differs greatly between when the racing of the engine has been performed in a sufficiently warmed-up state and the other occasions. Then, variations of the cylinder wall temperature bring about variations of the fuel ignition delay time, having an unfavorable effect on the accuracy of estimation of the ignition quality of the fuel.

[0021] Therefore, in the third aspect, the estimation apparatus may implement fuel injection a predetermined number of times before performing the fuel injection for estimating the ignition quality of the fuel, only when the estimation of the ignition quality of the fuel is performed during deceleration following the racing of the engine. If fuel injection is performed a certain number of times prior to the fuel injection for the estimation, the cylinder wall surface can be heated by heat produced by combustion of the previously injected fuel even when the racing of the engine is performed in a state where the engine is not sufficiently warmed up. Therefore, implementation of such a preliminary fuel injection will restrain variations of the cylinder wall temperature at the time of the fuel injection for the estimation. Hence, according to the foregoing construction, the estimation of the ignition quality of the fuel at the time of fuel cut during deceleration following the racing of the engine can be performed with improved accuracy.

[0022] Furthermore, the engine fuel property estimation apparatus constructed as described above may be applied to the engine which includes a fuel pressure sensor that detects fuel pressure within an injector, and which finds amount of fuel actually injected from change in the fuel pressure detected by the fuel pressure sensor, and feeds back the detected amount of the fuel actually injected to a drive control of the injector.

[0023] In the engine described above, the accuracy in controlling the amount of fuel injection can be improved by the above-described feedback. In the above-described engine, if fuel injection is performed several times prior to the fuel injection for estimation of the ignition quality, the accuracy in controlling fuel injection at the time of the fuel injection for the estimation can be heightened by feeding back results of detection of changes in the fuel < pressure caused by the preceding fuel injection. Therefore, according to the foregoing construction, the estimation of the ignition quality at the time of fuel cut during deceleration following the racing of the engine can be performed with improved accuracy.

[0024] A fourth aspect of the invention provides an engine fuel property estimation apparatus that injects fuel of an engine at a time of fuel cut of the engine and that estimates ignition quality of the fuel from magnitude of engine torque produced by combustion of the fuel injected. When amount of intake air before the fuel cut starts is a first amount, the estimation apparatus causes an upper limit value of engine rotation speed for implementing a fuel injection for estimating the ignition quality of the fuel to be smaller than when the amount of intake air before the fuel cut starts is a second amount that is smaller than the first amount.

[0025] When the amount of intake air prior to the start of the fuel cut is large, the engine is highly likely to have previously been operated under high load. As described above, the deterioration of the estimation accuracy due to variations of the cylinder wall temperature can be restrained by lessening the upper limit value of the engine rotation speed for implementing the fuel injection for estimation of the ignition quality of the fuel. In conjunction with that respect, in this aspect, when the amount of intake air prior to the start of the fuel cut is large, that is, when the engine is considered highly likely to have been operated under high load, the upper limit value of the engine rotation speed for implementing the fuel injection for the estimation is lessened. Therefore, it is possible to restrain the deterioration of the accuracy of the estimation caused by variations of the cylinder wall temperature during high-load engine operation and to estimate the ignition quality of the fuel with improved accuracy.

[0026] A fifth aspect of the invention provides an engine fuel property estimation apparatus that injects fuel of an engine at a time of fuel cut of the engine and that estimates ignition quality of the fuel from magnitude of engine torque produced by combustion of the fuel injected. When amount of fuel injection before the fuel cut starts is a first amount, the estimation apparatus causes an upper limit value of engine rotation speed for implementing a fuel injection for estimating the ignition quality of the fuel to be smaller than when the amount of fuel injection before the fuel cut starts is a second amount that is smaller than the first amount.

[0027] When the amount of fuel injection before fuel cut starts is relatively large, the engine is highly likely to have previously been operated under high load. As described above, the deterioration of the accuracy of the estimation due to variations of the cylinder wall temperature can be restrained by lessening the upper limit value of the engine rotation speed for implementing the fuel injection for estimating the ignition quality of the fuel. In conjunction with that respect, in this aspect, when the amount of fuel injection prior to start of fuel cut in a situation where the engine is considered highly likely to have been operated under high load is large, the upper limit value of the engine rotation speed implementing the fuel injection for estimating the ignition quality of the fuel is lessened. Therefore, the deterioration of the estimation accuracy due to variations of the cylinder wall temperature is restrained, so that the ignition quality of the fuel can be estimated with improved accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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 general diagram schematically showing an overall construction of an engine control apparatus in accordance with an embodiment of the invention;

FIG. 2 is a sectional view showing a structure of a side portion of an injector provided in a diesel engine to which the embodiment is applied;

FIG. 3 is a graph showing an example of a time waveform of a fuel injection rate; FIG. 4 is a flowchart of a cetane number estimation routine that is employed in the embodiment;

FIG. 5 is a graph showing a relation between the upper-limit rotation speed and the vehicle speed in the embodiment;

FIG. 6 is a graph showing a relation between the upper-limit intake air temperature and the vehicle speed in the embodiment;

FIG. 7 is a flowchart showing a processing procedure of a cetane number determination routine employed in the embodiment; and

FIG. 8 is a time chart showing transition of the engine rotation speed and transition of the rotation speed difference before and after execution of the fuel injection for detection of the cetane number.

DETAILED DESCRIPTION OF EMBODIMENTS

[0029] Hereinafter, an embodiment in which an engine fuel property estimation apparatus in accordance with the invention is embodied will be described in detail with reference to FIGS. 1 to 8. The estimation apparatus of this embodiment is applied to a diesel engine that is to be mounted in a vehicle.

[0030] As shown in FIG. 1 , a fuel tank 10 of a diesel engine to which the estimation apparatus of this embodiment is applied is provided with a fuel gauge 1 1 that measures the amount of fuel remaining in the fuel tank 10. Furthermore, a fuel supply passageway 12 through which fuel to be supplied to the engine passes is connected to the fuel tank 10. An intermediate portion of the fuel supply passageway 12 is provided with a high-pressure fuel pump 13 that pumps up fuel from the fuel tank 10, pressurizes it and discharges pressurized fuel. A downstream end of the fuel supply passageway 12 is connected to a common rail 14 that holds ^" pressurized fuel.

[0031] Injectors 16 for the cylinders of the diesel engine are connected to the common rail 14. Each injector 16 is provided with a fuel pressure sensor 17 that detects fuel pressure in the injector 16. Furthermore, the injectors 16 are connected to a return passageway 18 for returning surplus amounts of fuel to the fuel tank 10.

[0032] The thus-constructed diesel engine is controlled by an electronic control unit 19. The electronic control unit 19 includes a microcomputer that performs various computation processes related to the engine control. Furthermore, the electronic control unit 19 is provided with an input circuit that accepts input of signals from various sensors that detect situations of operation of the diesel engine. The fuel gauge 11 and the fuel pressure sensors 17 are connected to the input circuit. The other sensors connected to the input circuit include an intake pressure sensor 20, a rotation speed sensor 21 , a coolant temperature sensor 22 and an intake temperature sensor 25 that detect the intake pressure, the rotation speed, the coolant temperature and the intake air temperature, respectively, of the diesel engine as well as a vehicle speed sensor 24 that detects the vehicle speed, an accelerator pedal sensor 23 that detects the amount of depression of an accelerator pedal, etc.

[0033] Furthermore, the electronic control unit 19 is provided with drive circuits for actuators that drive various portions of the diesel engine. The drive circuits include circuits that drive the injectors 16 of the cylinders.

[0034] With reference to FIG. 2, further details of the construction of each of the injectors 16 provided for the individual cylinders of the diesel engine will be described. This diesel engine employs electrically-driven type injectors as the injectors 16.

[0035] As shown in FIG. 2, each injector 16 has a housing 30 that has a hollow cylindrical shape. Within the housing 30 there is disposed a needle valve 31 for reciprocating movements in the up-down directions in FIG. 2. Furthermore, a spring 32 that always urges the needle valve 31 downward in FIG. 2 is disposed within a portion of the housing 30 which is upward relative to the needle valve 31 in FIG. 2. [0036] Furthermore, within the housing 30 there are formed two fuel chambers that are separated from each other by the needle valve 31 , more specifically, a nozzle chamber 33 located relatively downward in FIG. 2 with respect to the needle valve 31 and a pressure chamber 34 located relatively upward in FIG. 2 with respect to the needle valve 31.

[0037] The nozzle chamber 33 is provided with injection holes 35 that provide communication between the inside of the nozzle chamber 33 and the outside of the housing 30. The nozzle chamber 33 is connected with an introduction passageway 36 that is formed within the housing 30. The introduction passageway 36 is connected to the common rail 14 (FIG. 1) so that fuel is supplied into the nozzle chamber 33 through the introduction passageway 36.

[0038] On another hand, the pressure chamber 34 is connected to the nozzle chamber 33 through a communication passageway 37, and to the aforementioned return passageway 18 through a discharge passageway 38. Furthermore, within the pressure chamber 34 there is provided a valve body 40 that is driven by a pressure-electric actuator 39 that is formed by laminating pressure-electric elements, for example, piezoelectric elements. Thus, a construction is provided such that the pressure chamber 34 is selectively caused to communicate with one of the communication passageway 37 and the discharge passageway 38 by driving the valve body 40.

[0039] A fuel pressure sensor 17 as described above is provided integrally with an upper portion of the injector 16 in FIG. 2. The fuel pressure sensor 17 is constructed so as to detect the pressure of fuel within the introduction passageway 36.

[0040] Each of the thus-constructed injectors 16 operates as follows. The pressure-electric actuator 39, when not energized with drive voltage, assumes a contracted state in which the entire length of the pressure-electric actuator 39 is reduced, so as to position the valve body 40 at such a position that the pressure chamber 34 communicates with the communication passageway 37 and is shut off from the discharge passageway 38. At this time, the nozzle chamber 33 and the pressure chamber 34 communicate with each other, so that the pressures in the two chambers are substantially equal. Therefore, at this time, the needle valve 31 has been disposed downward in FIG. 2 by the spring force of the spring 32 so that the injection holes 35 are closed. Hence, at this time, fuel is not injected from the injector 16.

[0041] On another hand, when the pressure-electric actuator 39 is energized with drive voltage, the entire length thereof increases so as to position the valve body 40 at such a position that the pressure chamber 34 is shut off from the communication passageway 37 and communicates with the discharge passageway 38. At this time, fuel is discharged from the pressure chamber 34 and the pressure in the pressure chamber 34 declines, so that the pressure in the nozzle chamber 33 is greater than the pressure in the pressure chamber 34. Due to the pressure difference, at this time, the needle valve 31 is displaced upward in FIG. 2, that is, so as to move away from the position at which the needle valve 31 closes the injection holes 35. Therefore, at this time, the injector 16 injects fuel.

[0042] In the embodiment constructed as described above, the electronic control unit 19 performs a fuel injection control of the diesel engine. Concretely, the electronic control unit 19 calculates a target value of the fuel injection amount (target fuel injection amount) from the engine rotation speed, the amount of depression of the accelerator pedal and an estimated value of the cetane number (control cetane number) of the fuel in use. Furthermore, the electronic control unit 19 calculates target values of the fuel injection timing and of the fuel injection duration from the target fuel injection amount and the engine rotation speed. Then, according to these calculated target values, the electronic control unit 19 applies the drive voltage to the pressure-electric actuator 39 of each injector 16 so as to control the fuel injection.

[0043] Furthermore, in this embodiment, in conjunction with the above-described fuel injection control, the electronic control unit 19 implements a control of forming a time waveform of the fuel injection rate of each injector 16 (the amount of fuel injected per unit time) on the basis of the fuel pressure detected by the fuel pressure sensors 17 provided for the individual injectors 16. This control is performed in the following manner.

[0044] After the needle valve 31 of an injector 16 starts to lift from the injection holes 35 according to the drive voltage applied to the pressure-electric actuator 39, the fuel pressure in the nozzle chamber 33 decreases with the increasing lift of the needle valve 31. Then, the application of the drive voltage stops and the lift of the needle valve 31 decreases. With the decreasing lift of the valve, the fuel pressure in the nozzle chamber 33 gradually rises. Therefore, from the fuel pressure detected by the fuel pressure sensor 17 of the injector 16, it is possible to specifically determine the timing at which the needle valve 31 starts to lift (valve opening-drive start timing Tos), the timing at which the fuel injection rate becomes maximum (maximum injection rate attainment timing Toe), the timing at which the fuel injection rate starts to decrease (fuel injection rate decrease start timing Tcs) and the timing at which the lift of the needle valve 31 ends (minimum lift attainment timing Tee). Then, from these determined timings, a time waveform of the fuel injection rate as shown in FIG. 3 can be found. From that waveform, it is possible to check the actual situation of fuel injection, that is, the actual fuel injection amount, the actual fuel injection timing, etc., with very high accuracy. In this embodiment, the electronic control unit 19 finds the rate of change of the fuel pressure (the time derivative of the fuel pressure) within each injector 16, and finds the aforementioned timings on the basis of the value of the rate of change.

[0045] Furthermore, in this embodiment, the electronic control unit 19 estimates the cetane number of the fuel currently in use, that is, estimates the ignition quality of the fuel. In the fuel property estimation apparatus of this embodiment, two modes of estimation of the cetane number are prepared, that is, an ordinary mode and a high-precision mode.

[0046] The cetane number estimation in the ordinary mode is performed during fuel cut of the diesel engine while the vehicle is traveling. The cetane number of the fuel estimated in the ordinary mode is reflected in engine controls such as a fuel injection control, an EGR control, a super charge rate control, etc. For example, when it is estimated that the cetane number of the fuel currently in use is low, various operations are performed in order to restrain occurrence of misfire resulting from low ignition quality of the fuel, for example, increasing the number of performances of pilot fuel injection, increasing the pilot fuel injection amount, advancing the timing of the pilot fuel injection and/or the main fuel injection, decreasing the EGR (exhaust gas recirculation), increasing the supercharging ratio, etc. The ordinary mode is entered when the following conditions (i) to (iii) are all satisfied, (i) The fuel cut of the diesel engine to be implemented according to reductions of the vehicle speed and the engine rotation speed resulting from discontinuation of the accelerator operation (i.e., of the depression of the accelerator pedal) is being executed, (ii) The total amount of fuel injection following the previous refueling (refilling of the tank) is greater than or equal to a predetermined value a. The predetermined value a is set to a value that is greater than a total amount of fuel that can be charged into the fuel channels extending from the fuel tank 10 to the injectors 16. That is, satisfaction of the condition (ii) means that after the previous refueling, the fuel in the aforementioned fuel channels has been replaced by the new fuel supplied from the fuel tank 10. (iii) After the previous refueling, the estimated value of the cetane number of the fuel has not been determined.

[0047] On another hand, the cetane number estimation in the high-accuracy mode is performed at the time of fuel cut following the racing of the diesel engine. A result of the estimation of the cetane number in the high-accuracy mode is used to specifically determine a factor in frequent occurrences of misfire if misfire occurs so. That is, when frequent occurrences of misfire are detected, the cetane number of the fuel is estimated in the high-accuracy mode. If as a result it is recognized that a low-cetane number fuel is being used, a factor in the misfire can be specifically determined as follows; that is, it can be determined that the fuel current in use has a problem or, if not, there is a problem with the fuel system of the diesel engine or the like. The above-described high-accuracy mode is entered when the diesel engine is gunned (without gear engagement) to at least a predetermined engine rotation speed, by following the procedure described in a maintenance manual or the like.

[0048] The estimation of the cetane number in this embodiment is performed through processing a cetane number estimation routine shown in FIG. 4. This routine is repeatedly executed at every predetermined control cycle time by the electronic control unit 19 while the fuel cut of the diesel engine is being implemented during the ordinary mode or the high-accuracy mode.

[0049] When the processing of the routine starts, the engine rotation speed NE, the intake air temperature THA and the vehicle speed SPD at the present time are firstly read in step SI 00. Then, in step SI 01 , the upper-limit rotation speed NEmax and the upper-limit intake air temperature THAmax are calculated on the basis of the present vehicle speed SPD. The upper-limit rotation speed NEmax is an upper limit value of the engine rotation speed NE for performing the fuel injection for the purpose of estimation of the cetane number of the fuel, and the upper-limit intake air temperature THAmax is an upper limit value of the intake air temperature THA for performing the same fuel injection.

[0050] The vehicle speed SPD used herein is used as an index value of the engine load that precedes the start of the fuel cut. That is, when the fuel cut is being implemented at high vehicle speed, it is considered that the engine is highly likely to have been operated under high load prior to the start of the fuel cut. Hence, in this example, the upper-limit rotation speed NEmax and the upper-limit intake air temperature THAmax are found on the assumption that the higher the vehicle speed SPD, the higher the engine load prior to the start of the fuel cut. During the high-accuracy mode of cetane number estimation, the vehicle speed SPD is "0".

[0051] As shown in FIG. 5, the higher the vehicle speed SPD is, the smaller the value of the upper-limit rotation speed NEmax is made. However, when the vehicle speed SPD is "0", that is, during the high-accuracy mode of cetane number estimation, the value of the upper-limit rotation speed NEmax is made larger than during the ordinary mode of cetane number estimation.

[0052] Furthermore, as shown in FIG. 6, the value of the upper-limit intake air temperature THAmax is also made smaller as the vehicle speed SPD is higher. However, when the vehicle speed SPD is "0", that is, during the high-accuracy mode of cetane number estimation, the value of the upper-limit intake air temperature THAmax is made larger than during the ordinary mode.

[0053] Subsequently, in step SI 02, it is determined whether the present engine rotation speed NE is less than or equal to the upper-limit rotation speed NEmax and the present intake air temperature THA is less than or equal to the upper-limit intake air temperature THAmax. If the present engine rotation speed NE is greater than the upper-limit rotation speed NEmax or the present intake air temperature THA is greater than the upper-limit intake air temperature THAmax (NO), the present process immediately ends. On the other hand, if the present engine, rotation speed NE less than or equal to the upper-limit rotation speed NEmax and the present intake air temperature THA is less than or equal to the upper-limit intake air temperature THAmax (YES), the process proceeds to step S I 03.

[0054] After the process proceeds to step SI 03, it is checked in step S I 03 whether the present mode of cetane number estimation is the high-accuracy mode or the ordinary mode. If the present mode of estimation is the high-accuracy mode, a preliminary fuel injection is implemented a predetermined number of times in step SI 04, and then the process proceeds to step SI 05. On the other hand, if the present mode of estimation is the ordinary mode, the process proceeds to step S 105 without implementing the preliminary fuel injection. Then, in step S I 05, cetane number determination is implemented, and then the present process of the main routine ends.

[0055] The cetane number determination in step SI 05 is performed through the process of a cetane number determination routine shown in FIG. 7. After the process of this routine starts, the timing of fuel injection for detecting the cetane number is set on the basis of the rotation speed, the coolant temperature and the intake air pressure in step S200. The engine rotation speed, the engine coolant temperature and the intake air pressure are used in calculating the fuel injection timing for the following reason.

[0056] The amount of fuel left unburned changes depending on the fuel injection timing in addition to the ignition quality of the fuel. If the fuel injection timing of a cylinder is relatively early, the time from when fuel is injected to when decline of the in-cylinder pressure or the in-cylinder temperature occurs due to descent of the piston becomes long. Therefore, the combustion continues for a longer duration, so that the amount of fuel left unburned becomes less. On another hand, if the fuel injection timing is late, the aforementioned time becomes short and therefore the duration of the combustion also becomes short, so that the amount of fuel left unburned becomes larger. The time from when fuel is injected to when decline of the in-cylinder pressure or of the in-cylinder temperature occurs becomes shorter as the engine rotation speed is higher. Therefore, in order to uniformalize the combustion condition, the timing of the fuel injection for detection of the cetane number of the fuel needs to be further advanced as the engine rotation speed is higher.

[0057] Furthermore, when the cylinder wall temperature is low, the maximum value of the in-cylinder temperature (peak temperature) in the engine compression stroke becomes low. When the intake air pressure is low, the maximum value of in-cylinder pressure (peak pressure) in the engine compression stroke becomes low. As the peak temperature or the peak pressure is lower, the duration of a high-temperature and high-pressure state in the cylinder is shorter and the duration of combustion is shorter. Therefore, in order to uniformalize the combustion condition, the timing of the fuel injection for detection of the cetane number needs to be further advanced as the cylinder wall temperature is lower or as the intake air pressure is lower.

[0058] Therefore, in this embodiment, in order to uniformalize the condition of combustion of the fuel injected for detection of the cetane number, the injection timing of the fuel is adjusted according to the then engine rotation speed, the then cylinder wall temperature and the then intake air pressure. Concretely, in this embodiment, the timing of the fuel injection for detection of the cetane number of the fuel is further advanced as the engine rotation speed is higher. Likewise, in this embodiment, the timing of the fuel injection for detection of the cetane number of the fuel is further advanced as the engine coolant temperature, which is an index value of the cylinder wall temperature, is lower. Furthermore, in this embodiment, the timing of the fuel injection for detecting the cetane number is further advanced as the intake air pressure is lower.

[0059] After the fuel injection timing is set in the above-described manner, a predetermined amount of fuel injection is carried out at the set timing in the subsequent step S201. Then, in step S202, the magnitude of the torque produced by that fuel injection is found.

[0060] The calculation of the produced torque in step S202 is performed in the following manner. The electronic control unit 19 acquires the engine rotation speed at every predetermined cycle time, and finds a difference between the acquired engine rotation speed and the engine rotation speed acquired the previous cycle time before (rotation speed difference ΔΝΕ).

[0061] An upper section of FIG. 8 shows transition of the engine rotation speed before and after execution of the fuel injection for detection of the cetane number, and a lower section of FIG. 8 shows transition of the rotation speed difference ΔΝΕ at that time. As the engine torque is produced due to execution of the fuel injection for detection of the cetane number of the fuel, the engine rotation speed increases or the rate of decrease in the engine rotation speed decreases, so that the rotation speed difference ΔΝΕ increases. The time integrated value of the increase in the rotation speed difference ΔΝΕ (which corresponds to the area of a hatched portion in FIG. 8) is larger as the produced torque is larger. Therefore, in this embodiment, the time integrated value of the increase in the rotation speed difference ΔΝΕ is calculated as an amount of change in rotation ΣΔΝΕ, and the value of the amount is used as an index value of the produced torque.

[0062] Subsequently, in step S203, the actual fuel injection timing and the actual amount of fuel injection are found from the time waveform of the rate of fuel injection in the fuel injection performed in step S201 , and errors of the actual values of the fuel injection timing and of the amount of fuel injection from their command values (the injection timing error and the injection amount error) are calculated. Then, on the basis of the injection timing error and the injection amount error, the amount of change in rotation ΣΔΝΕ is corrected. This correction is performed to reduce the influence that the injection timing error and the injection amount error have on the result of estimation of the cetane number of the fuel by performing correction of an amount that corresponds to the amount of change in engine torque that is caused by the injection timing error and the injection amount error. Concretely, as the injection timing error to the advanced side (the side to which the injection timing becomes further advanced) is larger, the produced torque is larger, so that the amount of change in rotation ΣΔΝΕ is more greatly reduced for correction. Furthermore, as the injection amount error to the side of increased amount is larger, the produced torque is larger, so that the amount of change in rotation ΣΔΝΕ is more greatly reduced for correction.

[0063] Subsequently, in step S204, an estimated cetane number of the fuel is calculated on the basis of the post-correction amount of change in rotation ΣΔΝΕ and the engine rotation speed occurring at the time of execution of the fuel injection. The microcomputer of the electronic control unit 19 pre-stores empirically predetermined relations of the cetane number of fuel with the amount of change in rotation ΣΔΝΕ and the engine rotation speed. The calculation in step S204 is performed on the basis of the pre-stored relations. After the estimated cetane number is calculated, the present processing of the routine ends.

[0064] Next, operation of the embodiment will be described. If while the vehicle is traveling, the fuel cut of the diesel engine is implemented with all the conditions (i) to (iii) satisfied, the estimation of the cetane number of the fuel in the ordinary mode is performed. In this estimation process, the injection of a small amount of fuel for cetane number estimation is implemented at the time point at which the engine rotation speed NE decreases to or below the upper-limit rotation speed NEmax calculated on the basis of the vehicle speed. As described above, the value of the upper-limit rotation speed NEmax is made smaller as the vehicle speed SPD is higher, that is, as the engine load prior to the fuel cut is estimated to be higher. Therefore, when the vehicle speed SPD is high, the engine rotation speed needs to decrease to a lower rotation speed in order for the fuel injection for cetane number estimation to be implemented than when the vehicle speed SPD is low.

[0065] The cylinder wall temperature affects the ignition delay time from the injection of fuel to the ignition of the injected fuel. As the ignition delay time changes, the magnitude of engine torque produced by combustion of the fuel also changes. Therefore, if the cylinder wall temperature at the time of the fuel injection for cetane number estimation varies, it becomes impossible to accurately estimate the cetane number on the basis of the engine torque produced by combustion of the fuel.

[0066] On another hand, during low-load operation of the diesel engine, heat produced in the engine can be appropriately removed (or cooled), so that the cylinder wall temperature is relatively stable. In contrast, during high-load operation of the diesel engine, the cooling fails to match the heat production of the engine, so that the cylinder wall temperature greatly increases. The amount of increase in the cylinder wall temperature at this time greatly varies depending on the duration of high-load operation. Therefore, during high-load operation of the engine, the cylinder wall temperature varies greater than during low-load operation. For a certain amount of time following high-load operation of the engine, the in-cylinder gas flow is strong, so that the small amount of fuel injected for cetane number estimation sometimes fails to accomplish combustion.

[0067] In conjunction with that respect, in this embodiment, when the engine load prior to the fuel cut is high, the fuel injection for cetane number estimation is not performed until the engine rotation speed NE decreases to the lower rotation speed. In that case, the time from the start of the fuel cut to the implementation of the fuel injection becomes long so that the cylinder wall surface heated due to the high-load operation can be sufficiently cooled before the fuel injection for cetane number estimation is performed. Therefore, even in the case where the engine is operated under high load prior to fuel cut, the cylinder wall temperature at the time of the fuel injection for cetane number estimation does not greatly vary. Furthermore, since a sufficient amount of time is secured before the fuel injection, the in-cylinder flow made stronger by the high-load operation eases off. Therefore, even after high-load operation, the combustion of injected fuel is stable, so that the cetane number can be estimated with high accuracy.

[0068] If the fuel cut of the diesel engine is implemented after the racing of the engine, estimation of the cetane number in the high-accuracy mode is performed. During this mode, the upper-limit rotation speed NEmax is set greater than during the ordinary mode. Depending on the racing of the engine, the engine rotation speed NE greatly increases. At the time of fuel cut during deceleration following the racing of the engine, it is possible to implement the fuel injection for cetane number estimation at high engine rotation speed NE while securing a sufficient time from the start of the fuel cut to the implementation of the fuel injection. Then, if the upper-limit rotation speed NEmax is increased, the engine rotation speed range in which the estimation of the cetane number of the fuel is implemented is expanded.

[0069] Furthermore, in the high-accuracy mode, the fuel injection for cetane number estimation is preceded by a predetermined number of performances of preliminary fuel injection. Due to such preliminary fuel injection, heat produced by combustion of the injected fuel appropriately heats the cylinder wall surface even if the cylinder wall temperature immediately prior to the preliminary fuel injection is low. Therefore, variations in the cylinder wall temperature occurring when the fuel injection for cetane number estimation is performed are restrained. Therefore, due to variations of the cylinder wall temperature, the occurrence of a difference in the situation of production of the engine torque is restrained, and accurate estimation of the cetane number of the fuel becomes possible. [0070] In this embodiment, the amount of fuel actually injected is found from change in the fuel pressure, and the found amount of fuel is fed back to the drive control of the injectors 16. If the preliminary fuel injection is implemented immediately prior to the fuel injection for cetane number estimation, results of the preliminary fuel injection can be fed back, so that the accuracy of the control of the fuel injection for cetane number estimation will heighten. In turn, the accuracy of estimation of the cetane number of the fuel will improves.

[0071] According to the engine fuel property estimation apparatus of the embodiment described above, the following effects can be achieved. (1 ) In this embodiment, when the vehicle speed SPD is high and the engine load prior to the fuel cut is estimated to be high, the upper-limit rotation speed NEmax, which is the upper limit value of the engine rotation speed NE for implementing the fuel injection for cetane number estimation is made smaller than when the vehicle speed SPD is low and the engine load prior to the fuel cut is estimated to be low. Therefore, even at the time of fuel cut following high-load operation at which the cylinder wall temperature varies greatly, it is possible to restrain variation of the cylinder wall temperature at the time of the fuel injection for cetane number estimation and therefore variation of the ignition delay time, and therefore it is possible to estimate the cetane number of the fuel with improved accuracy.

[0072] (2) In this embodiment, when the cetane number is estimated in the ordinary mode that is entered while the vehicle is traveling, the upper-limit rotation speed NEmax, which is the upper limit value of the engine rotation speed NE for implementing the fuel injection for estimation of the cetane number of the fuel is made smaller than when the cetane number is estimated in the high-accuracy mode that is entered during deceleration following the racing of the engine. Depending on the racing of the engine, the engine rotation speed NE greatly increases. At the time of the fuel cut during deceleration following the racing of the engine, it is possible to implement the fuel injection for cetane number estimation at high engine rotation speed while securing a sufficient time from the start of the fuel cut to the implementation of the fuel injection for estimation of the cetane number of the fuel. Therefore, estimation of the cetane number of the fuel is implemented only in a situation where the in-cylinder flow has sufficiently weakened and therefore the estimation can be appropriately performed, so that a suitable accuracy of the estimation of the cetane number of the fuel can be secured. Furthermore, during the high-accuracy mode, the engine rotation speed range in which the estimation is implemented is expanded, so that the estimation of the cetane number in the high-accuracy mode can be more reliably performed.

[0073] (3) In this embodiment, at the time of estimation of the cetane number of the fuel in the high-accuracy mode that is entered during deceleration following the racing of the engine, the fuel injection for the estimation is preceded by a predetermined number of implementations of preliminary fuel injection. Therefore, the variation in the cylinder wall temperature at the time of the fuel injection for estimation of the cetane number of the fuel can be restrained. Furthermore, since results of the detection of change in the fuel pressure in the injectors 16 at the time of immediately previously performed preliminary fuel injection are fed back to the drive control of the injectors 16, the fuel injection for estimation of the cetane number of the fuel can be performed with high accuracy. Therefore, according to the embodiment, the accuracy of the cetane number estimation in the high-accuracy mode can be improved.

[0074] (4) In the embodiment, when the vehicle speed SPD is high and the engine load prior to the fuel cut is estimated to be high, the upper limit value of the intake air temperature for implementing the fuel injection for cetane number estimation is made smaller. On the other hand, as the intake air temperature is higher, the variation of the in-cylinder gas temperature tends to be greater, and the variation of the ignition delay time becomes greater, so that the accuracy of the estimation of the cetane number more easily deteriorates. Therefore, it is possible to avoid implementation of estimation of the cetane number of the fuel in a situation where securement of an estimation accuracy is difficult, and to estimate the cetane number of the fuel with improved accuracy. [0075] The foregoing embodiment can also be carried out with the following modifications. In the diesel engine to which the embodiment is applied, the amount of fuel actually injected is found from change in the fuel pressure within an injector 16 detected by the fuel pressure sensor 17, and the found amount of fuel actually injected is fed back to the drive control of the injectors. The estimation of the cetane number (ignition quality of the fuel) in the foregoing embodiment can be applied in the same manner to a diesel engine that does not perform the above-described feedback.

[0076] In the foregoing embodiment, the preliminary fuel injection is implemented a predetermined number of times prior to the fuel injection for estimating the cetane number of the fuel in the high-accuracy mode which is performed during deceleration following the racing of the diesel engine. However, the preliminary fuel injection may be omitted.

[0077] Although in the foregoing embodiment, the estimation of the cetane number of the fuel in the high-accuracy mode is performed during deceleration following the racing of the engine, it is also pennissible to omit the estimation of the cetane number of the fuel in the high-accuracy mode and perform only the estimation of the cetane number in the ordinary mode which is performed at the time of fuel cut while the vehicle is traveling. In that case, too, if the upper-limit rotation speed NEmax at the time of high vehicle speed is made smaller than at the time of low vehicle speed, the variation in the ignition delay time can be restrained, and the cetane number of the fuel can be estimated with improved accuracy.

[0078] In the foregoing embodiment, the vehicle speed SPD is used as an index value of the engine load prior to start of the fuel cut, and the upper-limit rotation speed NEmax and the upper-limit intake air temperature THAmax are set on the basis of the vehicle speed SPD. As such an index value, it is also possible to use parameters other than the vehicle speed SPD, for example, the amount of intake air prior to the start of the fuel cut, the amount of fuel injection prior to the start of the fuel cut, etc. That is, when the amount of intake air or the amount of fuel injection prior to the start of the fuel cut is large, the engine load prior to the start of the fuel cut can be estimated to be high, and when the amount of intake air or the amount of fuel injection prior to the start of the fuel cut is small, the engine load prior to the start of the fuel cut can be estimated to be low. The amount of intake air and the amount of fuel injection prior to the start of the fuel cut and the like can be used as index values of the engine load prior to the start of the fuel cut similar to the vehicle speed SPD, effects that are substantially the same as the effects (1) and (4) can be obtained.

[0079] Although in the embodiment, the magnitude of the engine torque produced by combustion of the fuel is found from the amount of change in the engine rotation speed, the magnitude of the engine torque produced by combustion of the fuel may also be found from other parameters, such as the amount of increase in the in-cylinder pressure that is associated with the combustion, or the like.

[0080] Although in the foregoing embodiment, the timing of the fuel injection for detection of the cetane number of the fuel is adjusted according to the intake air pressure, the adjustment of the fuel injection timing may be omitted if the intake air pressure at the time of detection of the cetane number can be assumed to be substantially constant or if the changes in the produced torque that are caused by differences in the intake air pressure are sufficiently small.

[0081] Although in the embodiment, the timing of the fuel injection for detection of the cetane number of the fuel is adjusted according to the cylinder wall temperature, the adjustment of the fuel injection timing may be omitted if the cylinder wall temperature at the time of detection of the cetane number can be assumed to be substantially constant or if the changes in the produced torque that are caused by differences in the cylinder wall temperature are sufficiently small.

[0082] Although in the embodiment, the timing of the fuel injection for detection of the cetane number of the fuel is adjusted according to the engine rotation speed, the adjustment of the fuel injection timing may be omitted if the engine rotation speed at the time of detection of the cetane number can be assumed to be substantially constant or if the changes in the produced torque that are caused by differences engine rotation speed are sufficiently small.