Method for knock regulation in an internal combustion engine

A method for knock regulation in an internal combustion engine, includes using signals from at least one knock sensor to obtain knock values (KW) and using these knock values (KW) to form knock thresholds (KS) with which the knock values (KW) are compared for the purpose of identifying engine knock. Each determined knock value (KW) is compared with a limit value (g) and is replaced by a replacement value (E) if it exceeds the limit value (g), for calculation of a practical knock threshold (KS).

BACKGROUND OF THE INVENTION
 Field of the Invention
 The invention relates to a method for knock regulation in an internal
 combustion engine, wherein knock values are formed from signals of at
 least one knock sensor within a measurement window to be specified for
 each ignition, the knock values are used to form noise values, the noise
 values are used to form knock thresholds with which the knock values are
 compared, and engine knocking is identified if the knock values exceeds
 the knock threshold.
 The starting point for knock regulation of a multicylinder internal
 combustion engine is a knock threshold KS for each individual cylinder
 (which is smaller than or equal to a specified maximum knock threshold
 KS.sub.max applicable to all of the cylinders), which is generally formed
 by knock sensor signals or by knock values KW determined for that cylinder
 from those knock signals (which is referred to below as knock values) and
 with which these knock values are compared. If the knock threshold KS is
 exceeded by the knock values KW, engine knock is inferred and, for
 example, the ignition point for this cylinder is retarded. If the knock
 threshold is not exceeded for a prolonged period, the ignition point is
 slowly advanced again.
 If knock values assume a high value several times in succession, the knock
 threshold that is calculated from them continues to be increased, thereby
 requiring ever higher knock values to exceed that threshold. Knock
 regulation becomes less sensitive (engine knock is not detected) and the
 risk of engine damage increases.
 European Patent Application 0 399 068 A1, corresponding to U.S. Pat. No.
 5,040,510, has disclosed a cylinder-selective knock-regulating method in
 which sensor signals from a knock sensor are sampled at a specified
 sampling frequency in a particular angular range of the engine crankshaft,
 within a measurement window that can be specified for each cylinder. In
 that case, a specified number of the highest sample values from each
 measurement window is used to determine a knock value KW, by averaging for
 example, and a specified number of previous knock values KW is used to
 calculate a variable noise value GW, by sliding average formation, for
 example. A knock threshold KS is determined from that value, e.g.
 KS=(GW+S)*F or KS=GW*F+S, wherein S is a knock summand and F is a knock
 factor. Both variables are either specified values or are stored in tables
 as a function of at least one operating parameter (engine speed n, load L,
 etc.), preferably in characteristic maps as a function of engine speed n
 and load L. Both variables are used to fine-tune the knock threshold to
 ensure good knock detection, even with difficult boundary conditions, e.g.
 a poor signal-to-noise ratio at very low engine speeds.
 The noise value GW can be determined more simply from the current knock
 value KW and the previous noise value GW, which implicitly contains the
 preceding knock values.
 In order to prevent knock regulation becoming increasingly insensitive, as
 described above, provision is made in the case of the subject matter of
 European Patent Application 0 399 068 A1, corresponding to U.S. Pat. No.
 5,040,510, not to use knock values that exceed a specified value for the
 calculation of the noise value GW.
 If those knock values are included in the calculation with the value "0",
 then, in the case of intensive knocking, the knock threshold may become
 lower and lower and engine knock may be detected more and more often even
 though engine knock is not occurring. In that case, the ignition point may
 be retarded to an ever increasing extent.
 However, if, as is more sensible, only knock values which have been
 assessed as free from knock are used, the knock threshold KS may be
 increased to an ever greater extent by a number of knock values KW which
 are just below KS, and knock regulation becomes increasingly insensitive.
 SUMMARY OF THE INVENTION
 It is accordingly an object of the invention to provide a method for knock
 regulation in an internal combustion engine, which overcomes the
 hereinafore-mentioned disadvantages of the heretofore-known methods of
 this general type in such a way that neither an impermissible increase in
 a knock threshold due to excessively high knock values, nor a lowering of
 the knock threshold due to nonallowance for high knock values, occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 With the foregoing and other objects in view there is provided, in
 accordance with the invention, a method for knock regulation in an
 internal combustion engine, which comprises forming knock values KW from
 signals of at least one knock sensor within a measurement window to be
 specified for each ignition; using the knock values KW to form noise
 values GW; using the noise values GW to form knock thresholds KS with
 which the knock values KW are compared; identifying engine knocking if the
 knock value KW exceeds the knock threshold KS; comparing each determined
 knock value KW with a limit value g; and, if it exceeds the limit value g,
 replacing the knock value KW for calculating the noise value GW and the
 knock threshold KS, with a replacement value E: for KW&gt;g.fwdarw.KW=E.
 Like the knock summand S and the knock factor F, the limit value g is
 either a specified value or is stored in tables as a function of at least
 one operating parameter (engine speed n, load L, etc.), preferably in
 characteristic maps as a function of the engine speed n and load L.
 Alternatively, in accordance with another mode of the invention, the limit
 value g is recalculated each time, for example from the value of a maximum
 knock threshold KS.sub.max which is applicable to all of the cylinders and
 of the knock factor S and the knock summand S, which are
 operating-parameter-dependent in accordance with the following formulae:
 ##EQU1##
 The replacement value E, with which the determined knock value KW is
 replaced if the latter is higher than the limit value g, is either a
 specified value or is stored in tables as a function of at least one
 operating parameter (engine speed n, load L, etc.), preferably in
 characteristic maps as a function of the engine speed n and the load L, or
 it is simply set so as to be equal to the limit value g: E=g.
 In the case described, where limit values g are stored in tables or
 characteristic maps, comparison of the determined knock value KW with the
 limit value g taken from a table or a characteristic map can be performed
 quickly and in a simple manner. Otherwise, the limit value g has to be
 calculated.
 In accordance with a further mode of the invention, the replacement of the
 knock value KW by the replacement value E can also be carried out rapidly
 and in a simple manner since the replacement value E is either taken from
 a table or a characteristic map or is set so as to be equal to the limit
 value g.
 The respective points in the characteristic maps for the knock factor F,
 the knock summand S, the limit value g and the replacement value E in this
 case correspond to the operating point of the cylinder when the knock
 value KW was determined.
 In accordance with an added mode of the invention, the maximum knock
 threshold KS.sub.max is either a specified value or is stored as a
 function of at least one operating parameter (engine speed n, load L,
 etc.) in a characteristic map (or a table) applicable to all of the
 cylinders.
 In accordance with a concomitant mode of the invention, the variables knock
 factor F, knock summand S, limit value g and replacement value E are
 stored as a function of at least one operating parameter (engine speed n,
 load L etc.) either in tables or characteristic maps (or tables)
 applicable to all of the cylinders or characteristic maps (or tables) for
 individual cylinders.
 This ensures, in the case of every cylinder, that the noise value GW
 represents the noise behavior of the cylinder without assuming excessively
 high or excessively low values, i.e. both the noise value GW and the knock
 threshold KS dependent on it assume practical values, even if there is an
 assumed relation of high knock values KW.
 In the case of characteristic maps or tables which are not
 cylinder-selective, the values are calculated or specified as for a single
 cylinder and applied to all of the other cylinders.
 Other features which are considered as characteristic for the invention are
 set forth in the appended claims.
 Although the invention is described herein as embodied in a method for
 knock regulation in an internal combustion engine, it is nevertheless not
 intended to be limited to the details presented, since various
 modifications may be made therein without departing from the spirit of the
 invention and within the scope and range of equivalents of the claims.
 The method of operation of the invention, however, together with additional
 objects and advantages thereof will be best understood from the foregoing
 description of specific embodiments.