EP1462620A1

EP1462620A1 - Method and apparatus for determining operational mode in a cam profile switching (cps) engine

Info

Publication number: EP1462620A1
Application number: EP03075870A
Authority: EP
Inventors: Alexander Stotsky
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2003-03-27
Filing date: 2003-03-27
Publication date: 2004-09-29
Anticipated expiration: 2023-03-27
Also published as: DE60302429D1; DE60302429T2; EP1462620B1

Abstract

Method and apparatus for determining which of a first cam having a first cam profile and a second cam having a second cam profile taller than said first cam profile currently is acting on a lift mechanism (7) for a gas exchange valve (8) being arranged for controlling gas exchange into or out from a cylinder (3) arranged in a combustion engine (1).

Description

TECHNICAL FIELD

The invention relates to a method for determining which of a first cam having a first cam profile and a second cam, having a second cam profile taller than said first cam profile currently, is acting on lift mechanisms for gas exchange valves according to the preamble of claim 1. Furthermore the invention relates to an apparatus for determining which of a first cam having a first cam profile and a second cam, having a second cam profile taller than said first cam profile, currently is acting on lift mechanisms for gas exchange valves according to the preamble of claim 11.

BACKGROUND ART

Internal combustion engines for use in, for example, vehicles, must be capable of operation at various engine speeds and loads. The timing of the opening and closing of the intake and exhaust valves must be set to optimise the power output and efficiency of the engine over a reasonable range of speeds and loads.
For example, in a high output, multi-valve, spark ignition four stroke engine which is designed to operate at high engine speeds, it is generally desirable to provide means, such as cams, to control the opening of the inlet valves which preferably have a long valve opening period, in order to maximise the combustible charge drawn into the combustion chambers during the suction strokes of the engine. This has the advantage of improving the volumetric efficiency of the engine, thereby increasing the maximum power and torque outputs of the engine.
However, if such an engine is operated at speeds below that at which maximum power is developed, since the inlet valves are open for a relatively long period, some of the combustible charge drawn into each combustion chamber on its suction stroke can be forced back through the valve before it closes. This effect clearly reduces the volumetric efficiency, and hence the output, of the engine. It also causes uneven engine idling and low speed operation, and also makes exhaust emissions more difficult to control. It is therefore desirable to additionally provide a valve control mechanism for use only at low engine speeds which has a relatively short operating or opening period.
Control systems for combustion engines having variable valve control are known from inter alia US 5287830.
In internal combustion engine with variable valve control, it is necessary to monitor the function of this variable valve control at regular intervals. Generally, known monitoring methods determine the position of a component used to adjust the valve control in order to thus determine the current position of the valve control. However a monitoring method of this kind provides only a general idea of how the valve control is being controlled at a given moment.
Since variable valve control, such as cam profile switching (CPS) technology, where different cam profiles are used in order to provide different gas exchange characteristics for the cylinders of the engine at different operating conditions, has significant effect on drivability, exhaust emissions and fuel consumption, there is a requirement to have an effective diagnostic method for identification of the system failure. The valve lift event can not be directly measured and special diagnostic algorithms, based on indirect information about the valve lift, are required to identify a failure in the system.
A couple of methods which allow to detect the CPS state are known. , US 6213 068 B1 discloses a method which is based on the difference in the air charge inducted in the cylinders for different lifts. The inducted air charge measured by Manifold Air Flow (MAF) sensor is compared with the air charge model based on the measured position of the throttle flap, intake manifold pressure and engine speed. The CPS state is associated with the error between measured and modelled air charge. The drawback of the method described in US 6213 068 B1 is that the method does not allow the individual failure detection in the cases where the difference in the volumetric efficiency between low and high lifts is not large. Moreover, the low frequency pressure oscillations induced by the failure of one bank make the failure detection difficult. Although it is claimed that the intake manifold pressure signal can be used for the CPS state detection the use of the high frequency component of the pressure signal which includes the information about the CPS state is not discussed in the US 6213 068 B1.
In US 6 006 152 another method is disclosed which is based on the combustion state monitoring using fluctuations of the engine speed. The method is based on the fact that the combustion state may change considerably during shifting. The method disclosed therein uses the technique of the combustion state monitoring via irregularities of the engine speed. Irregularities are associated with the CPS state. The method allows the cylinder individual failure detection. The method uses the torque estimation technique well known in the literature devoted to the combustion efficiency monitoring functions, see for example US 4 532 798 and references therein.
The drawback of the method proposed in the US 6 006 152 is the fact that the detection is restricted to the steady-state case only. The detection methods described in the US 6 006 1521 can be seen as an indirect method. The detection method based on the engine speed nonuniformity suffers from the dependence of the combustion state of the engine on ignition and Air/Fuel ratio. In other words any problems in the ignition or fuel system affect this diagnostic method. A method which is based on the intake manifold pressure irregularities suffers from the dependence of the irregularities on purge flow, positions of Variable Intake System (VIS) flaps, etc... Therefore there is a necessity to develop the CPS diagnostic method which is based on the direct information about the valve events.

DISCLOSURE OF INVENTION

An object of the invention is to provide a method for CPS diagnostics which is based on the direct information about the valve events.
In the present invention it is suggested to identify the valve lifts based on the estimated camshaft acceleration. This approach is based on the direct information about the valve lift.
Information regarding the valve lift is provided from the fact that different levels of torque are required to drive the inlet camshaft at the different valve lift levels. The CPS state can thus be estimated from information about identified torque levels. Camshaft shall instantaneously accelerate and decelerate at different rates for different values of valve lift. Different levels of torque are required to drive the inlet valve camshaft at the different valve lift levels. With a camshaft position sensor having sufficient resolution, that is preferably with the possibility of detecting at least 60 fixed position per cam shaft revolution, it is possible, with suitable processing, to identify the acceleration/deceleration characteristics of a healthy system and a failed system.
Camshaft torque characteristics has a wave form whose frequency is proportional to the camshaft speed. The positive torque corresponds to the valve closing due to the valve spring energy. The negative torque represent the valve opening process and peaks are at the points which correspond to the maximum resistance to the valve operation. The negative torque represent the work which is required to overcome the cylinder pressure and to open the valves. The method for detecting the valve lift according to the invention is based on the fact that maximum resistance to the valve operation (maximum negative torque) is different for low and high valve lift levels. This difference is an input to the diagnostic algorithms to make a decision about successful cam profile switching.
The camshaft torque is proportional to the camshaft angular acceleration which, in turn, is estimated via the camshaft angle measurements by the differentiation.
In a preferred embodiment of the present invention the camshaft acceleration is estimated via a spline interpolation method which gives the high quality acceleration estimate due to the analytical differentiation. The idea for the spline interpolation method is to fit a polynomial of a second order to the camshaft angle measurements as a function of time in least squares sense and take the derivatives analytically.
The invention also relates to an apparatus for determining which of a first cam having a first cam profile and a second cam, having a second cam profile taller than said first cam profile, currently is acting on lift mechanisms for gas exchange valves according to the characterising portion of claim 11.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in further detail below, with references to appended drawings where:

Fig. 1: show relative camshaft angle produced by simulation and by curve fitting according to the method suggested herein,
Fig. 2: show comparison of derivatives of the camshaft angle produced by simulation and by using a derivative of a least square fitted polynomial according to the method suggested herein,
Fig. 3: show comparison of angular acceleration produced by simulation and angular acceleration calculated according to the method suggested herein,
Fig. 4: show variations for camshaft acceleration for low and high cam lift,
Fig. 5: is a schematic diagram of an internal combustion engine with a variable valve control and a control device therefor constructed in accordance with preferred embodiments of the present invention,
Fig. 6: is a side sectional view of a tappet and valve assembly which may be used in connection with the invention in order to provide a first and a second cam profile, and
Fig. 7: show a flow chart of a method for determining operational mode in a CPS engine.

MODE(S) FOR CARRYING OUT THE INVENTION

Initially a theoretical background to the invention and preferred embodiments is discussed with references to figures 1- 4.
The method for detecting the valve lift according to the invention is based on the fact that maximum resistance to the valve operation (maximum negative torque) is different for low and high valve lift levels.
This difference is an input to the diagnostic algorithms to make a decision about the successful cam profile switching.
However, camshaft torque depends on number of variables. According to a preferred embodiments those dependencies should be taken into account when making diagnostic decisions.
Below we describe the variables which influence the camshaft torque.
1. Variation of the camshaft torque with the throttle position and VVT positions. When the throttle position, and VVT (intake and exhaust) positions change, intake manifold pressure changes as well, this in turn, changes the work done in opening and closing valves.
2. Variation of the camshaft friction with engine speed.
Typically camshaft friction torque decreases significantly as engine speed increases. This is for instance described in Baniasad S., Emes M. " Design and Development of Method of Valve-Train Friction Measurements ", SAE Technical Paper, 980572, International Congress and Exposition, Detroit, Michigan,Feb.23-26, 1998, which is incorporated by reference.
3. Variation of the camshaft friction with the oil temperature Camshaft friction varies with the oil temperature depending on engine speed. The high speed friction is much less dependent on the temperature than the low speed friction.
The camshaft torque is proportional to the camshaft angular acceleration which, in turn, is estimated via the camshaft angle measurements by the differentiation.
The diagnostic decision making process can be described by the following inequality:
where
is the estimated camshaft angular acceleration. If the absolute value of the camshaft angular acceleration exceeds the ,threshold b(. ), which is the function (mapping) of the throttle angle th, intake and exhaust VVT positions vvt_i, vvt_e, engine speed n_e, and oil temperature T, then the high lift profile is engaged, otherwise the low lift cam profile is used.
The detection method is based on the fact that the maximal value of the angular acceleration is bigger for high cam profile than for low cam profile since the maximal resistance to the valve operation (maximal negative torque) is higher for the high valve lift. The method allows the cylinder individual failure detection and the comparison as shown in inequality (E1) should be done on the cylinder individual basis.
Since the camshaft acceleration profile changes significantly with the engine speed we assume that the method is preferably used for low engine speeds.
In a preferred embodiment of the invention the camshaft acceleration is estimated via the spline interpolation method which gives the high quality acceleration estimate due to the analytical differentiation. The idea for the spline interpolation method is to fit a polynomial of a second order to the camshaft angle measurements as a function of time in least squares sense and take the derivatives analytically.

Estimation of Acceleration from the Camshaft Angle Measurements

Numerical calculation of the derivatives of a signal is an old problem in numerical analysis and digital signal processing. The backward difference method gives one of the earliest and simplest numerical differentiators. Despite it is quite common in engineering applications the behavior of the derivative is very often accompanied with peaking phenomena. The peaks can be filtered out by a simple lowpass filter, this however may lead to a loss of information. The compromise between the quality of the signal and the quality of the derivative is difficult to achieve with backward difference method.
According to a preferred embodiment of the invention a spline interpolation method is proposed. A spline interpolation method is based on on-line least-squares polynomial fitting over the moving window of a size W. The advantage of this method over the backward difference method is its nice transient behavior.
The idea for the spline interpolation method is to fit a polynom of a certain order as a function of time in least squares sense and take the derivatives analytically. A background to this method is described in Diop S., Grizzle J., Moraal P., Stefanopoulou A., " Interpolation and Numerical Differentiation for Observer Design ", Proc. American Control Conference, Baltimore, Maryland, June, 1994, p.p. 1329 -1333., which is incorporated by reference.
The problem statement is the following. Camshaft angular acceleration should be estimate from the camshaft angle measurements.
As an example of a second order polynomial is chosen: α (t) = c1 + c2t + c3t2 where α and (t) is the estimate of the camshaft angle α(t), t is time, and c_i, i=1, 2, 3 are coefficients to be updated.
Since the measurements of the camshaft angle are performed at discrete time intervals a discretized version of equation (E2) need to be introduced α k= c1k + c2kkΔt + c3k(kΔt)2 where k, k =1,...,∞ is a step number, Δt is a step size.
The objective is to find coefficients c_1k , c_2k and c_3k such that the following sum becomes minimal at every step
where W is the size of the moving in time window. Notice that, in order to minimize S_k the coefficients c_1k, c_2k and c_3k should be updated at every step k. The minimum of S_k is achieved when equating to zero partial derivatives of S_k with respect to c_ik, i = 1,...,3 , i.e., ∂Sk ∂cik = 0
Three equations (E5) can be written as follows:
Presenting equations (E6) - (E8) in the matrix form we get: A c = b where
c^T =( c_1k, c_2k, c_3k), b^T =( b₁, b₂, b₃), where
In order to find spline coefficients c_ik the matrix equation (E9) should be solved with respect to c at every step. Then the derivative ( rotational velocity) is calculated as follows:
and the rotational acceleration is calculated as follows
This algorithm has one parameter only to be calibrated which is the size of the moving window. If the derivative of the camshaft angle changes slow it is advisable to have relatively large window size to filter out measurement noise. If the rotational velocity changes fast, the window size should be chosen small enough to capture corresponding fast changes in the derivative. The price for that is the noise in the estimated signal. Ideally, the windows size should be adjustable so that it is small enough during transients in order to capture fast changes in the derivative of the signal, and big enough under steady-state conditions so that to filter out measurement, space-discretization noise. This approach is currently under investigation.
Simulations of the suggested approach using results from TYCON simulation programme which is multibody analysis software (AVL Code) for valve train dynamics. Simulation results are shown on Figures 1-4.
In fig. 1 a line marked with + signs represents relative camshaft angle (rad) produced by TYCON system is shown. A line marked with the sign o represents the result of curve fitting of the curve (E3). During the first number of flanks the curve fitting is not performed yet. This is represented by the absence of a curve during the first number of flanks. The period of absence of a correct signal corresponds to the window size. After the curve fitting is completed, the curve fitting gives an identical output as the TYCON system.
In figure 2 a comparison of the derivatives of the camshaft angle ( rad / sec). A line represented by + signs is the derivative of camshaft angle simulated by TYCON system and dashed line is the derivative calculated from (E10). The lines are almost completely identical.
Camshaft angular accelerations calculated with (11) ( rad / sec²) and with TYCON are presented in Fig. 3, lines numbered (1) and (2) respectively. Simulations show relatively good agreement of the method proposed here and the TYCON simulation system.
The method was evaluated by using measurements from a V6 engine. High resolution encoder was mounted on the camshaft. The encoder signal was converted into the rotational velocity. The velocity measurements were loaded in MATLAB. The rotational acceleration is computed in MATLAB via the spline interpolation method, described above.
Fig. 4 shows the difference in the variation in the camshaft angular acceleration ( rad / sec²) for different CPS modes ( for low and high lift), (the line marked (1) corresponds to the camshaft acceleration variation for low lift and the line marked (2) is the camshaft acceleration variation for high lift ).
FIG. 5 shows in schematic form an internal combustion engine 1 equipped with a variable valve control 2. The variable valve control 2 is arranged to control gas exchange into or out from a plurality of combustion chambers, preferably in the form of cylinders 3 of the combustion engine 1 by selection of cam shaft profile of a cam shaft 4. The cam shaft 4 has a first cam having a first cam profile and a second cam having a second cam profile taller than said first cam profile, as will be described in further detail below. The variable valve control 2 includes an actuating device 5, which is controlled by an electronic control unit 6. The actuating device 5 manoeuvres the cam shaft in order to set which cam profile is currently is acting on lift mechanisms 7 for gas exchange valves 8. The variable valve control, which in the embodiment shown is arranged on the intake valve, can also be arranged on the exhaust valve. The cam profile which is currently is acting on lift mechanisms, is the cam profile, which during a certain operation condition of the engine is controlling the movement of a valve. In other words, by the phrase "cam profile which is currently is acting on lift mechanisms" is intended the cam profile that currently is active.
The variable valve control may optionally be applicable for variable valve control 2 which also is arranged to control the position of a camshaft 4, which is variable with respect to the angular position of a crankshaft 8 by means of an adjusting device.
The actuating device 5 for change of camshaft mode is controlled by a valve control unit 10 arranged in the electronic control unit 6. The control is performed in a manner know to a person skilled in the art in order to provide switching of camshaft mode in dependence of engine operating condition.
The electronic control unit 6 furthermore includes an evaluation device 11, and a monitoring device 12. Evaluation device 11 and monitoring device 12 together with camshaft sensor 9, which in this case acts as a sensing device to detect nonuniformity of the rotational speed of the camshaft 4, together constitute an apparatus for monitoring the which of a first cam having a first cam profile and a second cam having a second cam profile taller than said first cam profile currently is acting on lift mechanisms 7 for gas exchange valves 8.
Evaluating device 11 receives from camshaft sensor 9 a signal corresponding to the angular position of camshaft 4. In the present embodiment, this signal consists of a pulse train, with each pulse corresponding to a specific section of an angle swept by camshaft 4. The pulse train may be created by use of a dented wheel 13, which is rotated by the camshaft 4. The sensor 9 senses the proximity of the wheel 13 whereby an output signal indicating the position of the camshaft is provided. Sensors of this type are well known to persons skilled in the art and will therefore not be described in further detail. Preferably at least 60 indentations are provided on the wheel 13.
The evaluating device 11 includes means 14 for determining camshaft angular acceleration from the signal provided from the camshaft sensor 9. The camshaft angular acceleration may be determined by any known method but is preferably determined according to the method of least square fitting of a polynomial according to what is disclosed above. The evaluating device 11 may according to a preferred embodiment of the invention furthermore include means 15 for estimating a maximum value of said camshaft angular acceleration. The maximum value may be determined in a conventional manner by comparing discrete values. In a still further preferred embodiments the absolute value of the maximum value is determined in said means 15 for estimating a maximum value.
The monitoring device 12 includes means 16 for comparing said camshaft angular acceleration with a threshold value, and means 17 for determining which of said first and second cam profiles is acting on said lift mechanism by using said camshaft angular acceleration. In a preferred embodiment the means for determining which of said first and second cam profiles is acting on said lift mechanism is arranged to decide that said second cam profile is acting on said lift mechanism in the event said camshaft angular acceleration exceeds said threshold value and that said first cam profile is acting on said lift mechanism in the event said camshaft angular acceleration does not exceed said threshold value.
In order to determine the threshold value, the monitoring device includes means 18 for determining the threshold value. The threshold b( . ), is the function (mapping) of the throttle angle th, intake and exhaust WT positions vvt_j, vvt_e, engine speed n_e, and oil temperature T. The map for these variables are determined by experiments. The means 18 for determining the threshold value receives input data from an throttle sensor 19, an engine speed sensor 20, an oil temperature sensor 21 and a camshaft rotational phase detector (not shown) in the event variable cams are used information from a phase shift detector. The camshaft rotational phase detector may be of the type described in EP 1 229 215, which is incorporated by reference.
In a preferred embodiment of the invention the means 14 for determining camshaft angular acceleration is arranged to determine camshaft acceleration by an on line least square polynomial fitting. In this event the means 14 for determining camshaft angular acceleration includes: means 14A for assigning a polynomial representing camshaft angle position, said polynomial being characterised by a set of model coefficients, means 14B for determining said model coefficients, and means 14C for using a second derivative of said polynomial as a representation for angular acceleration.
All the different means included in the means for evaluating 11 and the means for monitoring 12 are constituted by programs running in a microcontroller having processing means and storage areas. The microcontroller is programmed to execute calculation of formulas (E4) - (E11) by use of information provided from the camshaft sensor 9.
In figure 6 a valve assembly is shown, which may be used in connection with the invention in order to provided two different cam modes.
Referring to FIG. 6 there is shown a valve 110 having a head 111 which is movable in an axial direction to seal the passageway 105. The valve 110 is slidably mounted in a bore 112 in cylinder block 113 and passes through a cavity 114. In the cavity 114 around valve 110 there is located a spring 115 one end of which rests against a lower surface of said cavity 114 and the other end of which is located in a collar 116 mounted on the valve 110 so as to generally bias the valve 110 in an upwards direction.
Mounted on an upper end of valve 110 is a tappet assembly 118. The tappet assembly 118 comprises a co-axial inner tappet 120 and outer tappet 121. The inner tappet bears on a hydraulic lash adjustment element 122 of known type which in turn bears on the upper end of valve 110. The tappet assembly 118 is slidably mounted within bore 119 which extends from the cavity 114 to the upper surface of the cylinder block 113. A cylinder head cover may be positioned over and secured to the upper surface of the cylinder block 113.
Located above the cylinder block 113 is a rotatable camshaft 130, which is drivable in the usual arrangement 131, which comprises a pair of outer cam lobes 126 in between which is situated a central cam lobe 123. The central cam lobe 123 has a profile designed to optimise engine performance over a selected portion of engine speed and load range. Although the central cam lobe 123 is illustrated as having a generally eccentric form it is envisaged that this cam lobe can be a circular form allowing valve deactivation while under control of this cam lobe. The outer cam lobes 126 are of a substantial identical profile to each other and are designed to optimise engine performance over another portion of engine speed and load range.
The camshaft 130 is located such that in low speed conditions an upper surface 120a of the inner tappet 120 is driven by the central cam lobe via finger follower 124. The upper surface 121 a of outer tappet 121 is kept in contact with the outer cam lobes 126 by means of a spring 125 which is coaxially positioned around spring 115 and which locates at one end in recesses 132 in the lower end surface of outer tappet 121. At its lower end spring 125 bears on the lower surface of cavity 114
Cam profile selection is achieved by either connecting the inner tappet 120 and outer tappet 121 so that they move together which allows the outer tappet 121 and outer cam lobes 126 to control the valve 110 or by disconnecting the inner tappet 120 and outer tappet 121, which allows the inner tappet 120 and inner cam lobe 123 to control valve 110. For further details of a valve assembly, which may be used in connection with the invention in order to provided two different cam modes, it is referred to US 5287830, which is incorporated by reference.
In figure 7 a flow chart of determining which of a first cam having a first cam profile and a second cam having a second cam profile taller than said first cam profile currently is acting on lift mechanisms for gas exchange valves is shown.
In a first method step 210 the camshaft angular acceleration is determined.
In a second step 220 the camshaft angular acceleration is compared with a threshold value.
In a third method step 230 it is determined which of said first and second cam profiles is acting on said lift mechanism 7 by using information regarding camshaft angular acceleration. Characteristics of the representation of the camshaft angular acceleration are used to determine which cam shaft is acting on the lift mechanism. This is possible since the different cam profiles has different influence on the cam shaft angular acceleration. Thus it is possible to decide which cam profile is currently active by comparing obtained information about the cam shaft angular acceleration with reference values stored in a memory. This can be done by a comparator unit, which is arranged for performing a comparison between obtained information about the cam shaft angular acceleration and said reference values. In a particularly preferred embodiment the step of determination 230 is performed by deciding that the second cam profile is acting on said lift mechanism in the event said camshaft angular acceleration exceeds said threshold value and that said first cam profile is acting on said lift mechanism in the event said camshaft angular acceleration does not exceed said threshold value.
In the event a negative threshold value is use, which would be the case
where negative acceleration is used for determination of which cam lobe is currently active, the statement that the cam shaft acceleration is exceeding a threshold value should be interpreted as that the cam shaft acceleration has a negative value of greater amplitude than the threshold value. This means that for instance a value (- 4) of cam shaft acceleration is exceeding a threshold value of (-2).
In the event a positive threshold value is use, which would be the case where positive acceleration is used for determination of which cam lobe is currently active, the statement that the cam shaft acceleration is exceeding a threshold value should be interpreted as that the cam shaft acceleration has a positive value of greater amplitude than the threshold value. This means that for instance a value (4) of cam shaft acceleration is exceeding a threshold value of (2).
According to a preferred embodiment of the invention the first method step 210 determines camshaft acceleration by an on line least square polynomial fitting. In this event the first method step includes the following substeps:

- assigning in a first substep 211 a polynomial representing camshaft angle position, said polynomial being characterised by a set of model coefficients,
- determining said model coefficients in a second substep 212,
- forming a second derivative of said polynomial in a third substep 213 and using said second derivative as a representation for angular acceleration

The model coefficients are preferably decided by least square optimising using equation (E4). The model coefficients can be obtained by solving the equation system (E9).
The threshold value is calculated in the means for determining a threshold value by interpolation in a matrix. The matrix is obtained by experiments.
The invention should not be restricted to the embodiments disclosed above, but may be varied within the scope of the appended claims.

Claims

Method for determining which of a first cam (123) having a first cam profile and a second cam (126) having a second cam profile taller than said first cam profile currently is acting on a lift (7) mechanism for a gas exchange valve (8) being arranged for controlling gas exchange into or out from a combustion chamber (3) arranged in a combustion engine (1), characterised in that the following method steps are executed:

determining (210) camshaft angular acceleration, and

determining (230) which of said first and second cam profiles is acting on said lift mechanism (7) using said camshaft angular acceleration.
Method according to claim 1, characterised in that the method further includes the method step of comparing (220) said camshaft angular acceleration with a threshold value and that said method step of determining (230) which of said first and second cam profiles is acting on said lift mechanism (7) using said camshaft angular acceleration is performed by deciding that said second cam profile is acting on said lift mechanism (7) in the event a magnitude of said camshaft angular acceleration exceeds said threshold value and that said first cam profile is acting on said lift mechanism in the event said magnitude of said camshaft angular acceleration does not exceed said threshold value.
Method according to claim 2, characterised in that a maximum value of said camshaft angular acceleration is determined and compared with said threshold value (E1).
Method according to claim 3, characterised in that an absolute value of said maximum value of said camshaft angular acceleration is formed before comparing said maximum value of said camshaft angular acceleration with a threshold value.
Method according to any of claims 2 - 4, characterised in that comparison of camshaft angular acceleration and said threshold value is performed under a valve opening phase of said gas exchange valve (8).
Method according to any of the claims 2 - 5, characterised in that said threshold value is dependent on throttle position, lubricating oil temperature and/or engine speed.
Method according to claim 6, characterised in that said threshold value is furthermore dependent on a shift angle of an intake and/or exhaust variable valve timing cam.
Method according to any of the preceding claims, characterised in that said camshaft acceleration is determined from a cam shaft angle sensor (9).
Method according to claim 8 characterised in that said camshaft acceleration is determined by an on line least square polynomial fitting.
Method according to claim 9 characterised in that said on line least square polynomial fitting includes the following method steps:

assigning (211) a polynomial representing camshaft angle position, said polynomial being characterised by a set of model coefficients,

determining (212) said model coefficients,

using (213) a second derivative of said polynomial as a representation for angular acceleration.
Apparatus for determining which of a first cam (123) having a first cam profile and a second cam (126), having a second cam profile taller than said first cam profile, currently is acting on a lift mechanism (7) for a gas exchange valve (8) being arranged for controlling gas exchange into or out from a cylinder (3) arranged in a combustion engine (1), characterised in that said apparatus includes means (14) for determining camshaft angular acceleration, and means (17) for determining which of said first and second cam profiles is acting on said lift mechanism (7) by using said camshaft angular acceleration.
Apparatus according to claim 11, characterised in that said apparatus further includes means (16) for comparing said camshaft angular acceleration with a threshold value and that said means (17) for determining which cam profile is acting on said lift mechanism (7) is arranged to decide that said second cam profile is acting on said lift mechanism (7) in the event a magnitude of said camshaft angular acceleration exceeds said threshold value and that said first cam profile is acting on said lift mechanism in the event said magnitude of said camshaft angular acceleration does not exceed said threshold value.
Apparatus according to claim 12, characterised in that said apparatus further includes means (15) for estimating a maximum value of said camshaft angular acceleration, and that said means (16) for comparing said camshaft angular acceleration with a threshold value is arranged to compare said maximum value with said threshold value.
Apparatus according to claim 13, characterised in that said means (15) for estimating said maximum value of said camshaft angular acceleration is arranged to form an absolute value of said maximum value of said camshaft angular acceleration.
Apparatus according to any of claims 11 - 14, characterised in that said means (16) for comparing said camshaft angular acceleration and said threshold value is arranged to perform said comparison under a valve opening phase of said gas exchange valve (8).
Apparatus according to any of claims 11 - 15, characterised in that said threshold value is dependent on throttle position, lubricating oil temperature and/or engine speed.
Apparatus according to claim 16, characterised in that said threshold value is furthermore dependent on a shift angle of an intake and/or exhaust variable valve timing cam.
Apparatus according to any of claims 11 - 17, characterised in that a cam shaft angle sensor (9) is arranged to determine said camshaft acceleration.
Apparatus according to claim 18, characterised in that said means (16) for determining camshaft angular acceleration is arranged to determine camshaft acceleration by an on line least square polynomial fitting.
Apparatus according to claim 19, characterised in that means (16) for determining camshaft angular acceleration is arranged to perform said on line least square polynomial fitting using the following method steps:

assigning a polynomial representing camshaft angle position, said polynomial being characterised by a set of model coefficients,

determining said model coefficients,

using a second derivative of said polynomial as a representation for angular acceleration.