Abstract:
In order to determine the quality of fuel in an auto-igniting internal combustion engine, a defined fuel amount is injected at defined crank shaft angles during a deceleration fuel shut-off phase of the internal combustion engine. The thereby created crankshaft torque contribution is detected and an absolute or relative measure is determined for the fuel quality thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2008/063236 filed Oct. 2, 2008, which designates the United States of America, and claims priority to German Application No. 10 2007 054 650.7 filed Nov. 15, 2007, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates to a method for determining fuel quality in an auto-igniting internal combustion engine and to a corresponding device. 
     BACKGROUND 
     Fuel quality plays a significant role in the operation of an internal combustion engine. In auto-igniting internal combustion engines the quality of the fuel is critically important for the ignition of the fuel. For diesel fuels, therefore, the cetane number is usually specified, this number being a measure for how fast the fuel combusts in a diesel-powered internal combustion engine. In auto-igniting internal combustion engines the fuel is ignited by means of the compression heat. In addition to other parameters, such as compression level, crankshaft angle and fuel quantity, the cetane number in particular also influences the ignition quality following an injection. An auto-igniting internal combustion engine which is operated with fuel having a comparatively low cetane number is reluctant to start, runs more roughly and louder, and has poorer exhaust gas emission values. Fuels having a higher cetane number lead to faster ignition than fuels having a lower cetane number. 
     Since the cetane number therefore has an effect on the operating characteristics and in particular on the exhaust gas properties of an internal combustion engine, it is becoming an increasingly common practice to provide fuel quality sensors in the fuel supply tract of an internal combustion engine in order to measure the fuel quality. Even a deviation from a nominal fuel quality due to time-related and/or regional factors can then remain without undesirable consequences for the operation of the internal combustion engine. 
     SUMMARY 
     Since fuel quality sensors are, of course, expensive components and complicate the design of a fuel supply system, according to various embodiments a method for determining the fuel quality in an auto-igniting internal combustion engine as well as a corresponding device can be achieved. 
     According to an embodiment, in a method for determining the fuel quality in an auto-igniting internal combustion engine, a defined fuel quantity which differs by a specific amount from the fuel quantity required for the operating state is injected at defined crankshaft angles during a deceleration fuel cutoff phase of the internal combustion engine, the crankshaft torque contribution effected thereby is recorded, and an absolute or relative measure for the fuel quality is calculated therefrom. 
     According to a further embodiment, the method can be repeated and a statistical evaluation or an averaging over the recorded crankshaft torque contributions or determined measured values for the fuel quality can be performed. According to a further embodiment, the method can be repeated with at least one further crankshaft angle. According to a further embodiment, the crankshaft torque contributions determined for different crankshaft angles or measured values for the fuel quality can be linked in a linear regression in order to obtain an improved measure for the fuel quality. 
     According to another embodiment, a device for determining the fuel quality in an auto-igniting internal combustion engine, may comprise a control unit for influencing the operation of the internal combustion engine, which control unit controls the internal combustion engine for the purpose of performing one of the above described methods and performs the necessary calculations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below by way of example with reference to the figures of the drawings, in which: 
         FIG. 1  shows a block diagram of a first variant of a method for determining the fuel quality of an auto-igniting internal combustion engine, 
         FIG. 2  shows a partial block diagram relating to a modified method for determining the fuel quality, and 
         FIG. 3  shows a partial block diagram of another modified method for determining the fuel quality. 
     
    
    
     DETAILED DESCRIPTION 
     According to various embodiments, in a method for determining the fuel quality in an auto-igniting internal combustion engine, a defined fuel quantity which differs by a specific amount from the fuel quantity required for the operating state is injected at defined crankshaft angles during a deceleration fuel cutoff phase of the internal combustion engine, the crankshaft torque contribution effected thereby is recorded, and an absolute or relative measure for the fuel quality is calculated therefrom. 
     According to another embodiment, a device for an auto-igniting internal combustion engine may perform the aforesaid method. According to various embodiments, the effect that a fuel injection has on the crankshaft torque is greatly dependent on the fuel quality. If a favorable operating point at which the operating parameters of the internal combustion engine are otherwise constant is thus chosen it is possible to determine the fuel quality by injecting fuel and converting the difference in the crankshaft torque from the crankshaft torque that would result in the case of a standard fuel into a deviation from the standard fuel quality or by absolutely calculating directly from the crankshaft torque an absolute measure for the fuel quality. Determining the torque contribution of an injection is known in the prior art and is now used for determining the fuel quality. 
     Just a single sample injection can suffice to record a metric for the fuel quality. For improved measurement accuracy it is to be preferred to repeat the method while the operating parameters remain unchanged within defined limits and to perform an averaging or a suitable statistical evaluation for the crankshaft torques or fuel quality measured values then recorded. 
     If it transpires that at the selected, defined crankshaft angle the generated crankshaft torque or the determined measure for the fuel quality leads to possibly erroneous or implausible values, it is to be preferred to repeat the method at a changed defined crankshaft angle which may be less or greater than the angle previously used. This will be done in particular in the case of significant deviations from the standard fuel quality or in the case of a measure for the fuel quality which indicates an unusually poor- or good-quality fuel. By means of the crankshaft torques then obtained for different crankshaft angles or measured values for the fuel quality it is then possible to use a linear regression in order to obtain an improved value for the fuel quality. 
       FIG. 1  is a block diagram schematically illustrating a method for determining the fuel quality in an auto-igniting internal combustion engine. After the method is started at a step S 0  it is first queried at a step S 1  whether the internal combustion engine is in a deceleration fuel cutoff mode of operation or in another mode of operation in which the injected fuel mass is constant within defined limits. If no such operating state is present (N branch), the method is terminated at a step S 2 . Determining the fuel quality only takes place (Y branch) if such an operating state is present. 
     Then, at a step S 3 , a defined fuel quantity is injected at a defined crankshaft angle. Said fuel quantity is different from the quantity otherwise provided for the operating state (zero in the case of the deceleration fuel cutoff operating mode). The difference in quantity leads to a specific change in the crankshaft torque which is recorded at a step S 4 . From said torque difference according to step S 4 , either an absolute measure for the fuel quality is determined or a relative measure is calculated taking into account the deviation from a standard value that would result in the case of a standard fuel. The method is then terminated (step S 2 ). 
     If the method is operating in a deceleration fuel cutoff phase, the change in torque is an absolute torque contribution due to the injection of the defined fuel quantity. 
     In order to improve the measure for the fuel quality said method can be modified in a way such as is shown in  FIG. 2 .  FIG. 2  shows the extract part of the method according to  FIG. 1  from steps S 3  to S 5 . 
     In this case a counter is incremented (step S 6 ) so that a statistical evaluation can be carried out by way of the determination of the torque or the torque difference. Step S 6  is in this case arranged in the representation scheme of  FIG. 2  between steps S 2  and S 4 , though it can also be placed before step S 3  or after step S 4 . The main thing is that it precedes a step S 7  which is in turn arranged after the determination of the torque or the torque difference. A check is made at step S 7  to determine whether the counter has reached a specific maximum value. 
     If this is not the case, a sliding averaging, for example, is performed at a step S 8  over the recorded change in crankshaft torque or its deviation from standard fuel conditions. The concluding step S 5  in the determination of the measured value for the fuel quality, which step is not reached until the averaging includes a defined number of loop iterations, then makes use of the averaged value for the torque or, as the case may be, the torque difference. In this way a more accurate determination of the fuel quality is reached. 
     In the embodiment variant shown in  FIG. 2  the averaging can, of course, also include a statistical evaluation. 
     An averaging/statistical evaluation can also be performed on the basis of the measure for the fuel quality, instead of on the basis of the crankshaft torque or the crankshaft torque difference. 
     In that case step S 5  will then come before step S 7  and the statistical evaluation or averaging at step S 8  will make use of the measure for the fuel quality. 
     A further embodiment of the method is shown in  FIG. 3 . This serves to vary the defined fuel quantity and/or the crankshaft angle at which said fuel quantity is injected. This is based on the knowledge that there are specific time instants (referred to the crankshaft angle) at which the fuel quality has a particularly strong impact on the torque contribution of an individual injection. 
     Steps of the method according to  FIG. 3  which correspond to those of the method described with reference to  FIG. 1  are labeled with the same reference signs and, to the extent that it is not necessary, are not explained again. Moreover,  FIG. 3  represents only an extract of the method, which extract starts only at step S 3 , which is, of course, in turn preceded by steps S 0  and S 1  as well as S 2 . 
     Characteristic of the method according to  FIG. 3  is a query step S 9  arranged after steps S 3  and S 4  (and, depending on embodiment, also S 5 ) to determine whether the determined change in torque or, as the case may be, torque difference (or the measure for the fuel quality, if step S 5  is also executed) lies within a certain tolerance range around standard values. If this is the case, the measure for the fuel quality is determined at step S 10 , analogously to step S 5 , or alternatively step S 10  contains no further steps if step S 5  preceded step S 9  (dashed variant of  FIG. 3 ). 
     If, however, a deviation from standard values is present which points to a particularly unusual fuel quality—because e.g. the determined torque difference or, as the case may be, torque change indicates a similar situation or (if step S 5  was executed) the measure for the torque or, as the case may be, torque difference points thereto, a modified specification for the defined crankshaft angle and/or change in fuel quantity which is used at step S 3  is set at a step S 11 . At the same time the value obtained at step S 4  (or step S 5 ) is assigned to the previously used defined value for the crankshaft angle and stored. Subsequently steps S 3  and S 4  (and, where applicable, S 5 ) are executed once more and the query at step S 9  is then skipped. 
     Based on the two defined crankshaft angles or changes in fuel quantity present as well as on the assigned values from step S 4  (and, where applicable, S 5 ), step S 10  then performs a linear regression in which model data is used which expresses a relationship between fuel quality and torque contribution of an injection as a function of the crankshaft angle. By this means an improved indication of the fuel quality can be obtained. It is, of course, possible to perform not just two iterations of the loop of steps S 3  and S 4  with two different crankshaft angles/fuel quantities, but also a higher number, which then improves the linear regression.