GB2348299A

GB2348299A - Method for cylinder cut-off and restart

Info

Publication number: GB2348299A
Application number: GB0006952A
Authority: GB
Inventors: Johann Froehlich; Hong Zhang
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1999-03-26
Filing date: 2000-03-23
Publication date: 2000-09-27
Anticipated expiration: 2020-03-23
Also published as: DE19913908C2; FR2791394A1; FR2791394B1; GB0006952D0; GB2348299B; DE19913908A1

Abstract

In order to configure the cut-off of cylinders and the restart of cut-off cylinders (A, B, C, D) in a multiple cylinder internal combustion engine so as to be low with respect to bucking, the cylinders (A, B, C, D) of the internal combustion engine are cut off and restarted in two groups (C, D; A, B). The time period (T/2) between the cut-off or restart of the two groups is selected so that it corresponds to one-half the period duration of a characteristic bucking oscillation of the internal combustion engine.

Description

2348299 METHOD FOR CYLINDER CUT-OFF AND RESTART The invention concerns a

method for cylinder cut-off and restart in a multiple cylinder internal combustion engine, the cylinders of which can be cut off and placed back in operation sequentially.

In order to increase efficiency in the operation of an internal combustion engine, in particular an internal combustion engine in a vehicle, cylinders are customarily cut off during overrun. In this process, individual cylinders or all cylinders of the internal combustion engine are cur off so that ignition of a fuel/air mixture does not occur. For multiple cylinder internal combustion engines, various methods are known for cutting the cylinders off individually, that is, sequentially, and placing them back in operation. Thus, for example, injection processes can be suppressed in individual cylinders or gas exchange valves can be kept closed for an extended period. Cylinder cutoff as a rule takes place using fixed, predefined cylinder cut-off patterns; for example fuel injection is suspended in accordance with the ignition order for all cylinders.

Naturally a degree of bucking is associated with a cylinder cut-off or restart since upon cut-off the load suddenly drops to zero and a drag moment is created. The object of the present invention is to specify a method for cylinder cut-off and restart of cut-off cylinders through which this bucking is reduced.

This object is achieved through the invention defined in Claims I and 2.

2 According to the invention, the characteristic vibration of the drivetrain is taken into consideration in the cylinder cut-off and restart. The underlying idea of the invention is that the cylinder cutoff and restart is divided into two steps. At a point in time in which the cylinder cut-off or restart command is given, only half of the cylinders are cut off and only at an appropriate subsequent point in time are the other half cut off.

In this way the smallest possible stimulation of the drivetrain to undesired bucking vibrations is achieved.

In order to also quickly damp the vibration, the second cylinder group is cut off or restarted in the following turning point of the RPM curve, i.e. when the rotational speed begins to deviate in the opposite direction from the target rotational speed.

Naturally a degree of lead time must be taken into consideration for operating reasons of the internal combustion engine, for example in cutting off or restarting injection so that at the time of the change point of the RPM curve, the corresponding cylinders are in fact cut off or restarted.

The length of time between cut off or restart of the first and the second cylinder groups must be selected based on operating factors since for example the engine rotational speed or the gear selection influence the bucking vibrations.

Since the bucking vibrations are caused by the torque change upon cut off or restart, a bucking function can be obtained from the difference between target and actual engine rotational speed, one-half the period length of which is the time span between cut off of the one cylinder group and of the other 3 cylinder group or between restart of the one cylinder group and of the other cylinder group.

If the internal combustion engine passes through non-steady operating conditions, the predetermined duration must be corrected accordingly, for example through its being measured in segments of the camshaft signal and adjusted by changes with respect to the commencement and duration of injection.

Advantageous embodiments of the invention are characterised in the subclaims.

The invention will be explained below using an exemplary embodiment with reference to the drawings.

Figure I shows a diagram with the curve of an overrun cut-off in segment synchronous representation Figure 2 shows three different variables at initiation of cylinder cut- off Figure 3 shows a time series of an exemplary bucking ftinction.

The cylinder cut-off of a four cylinder internal combustion engine is described below. The methods according to the invention naturally also apply for any multiple cylinder internal combustion engine and analogously also for the restart of cylinders which have been cut off. In like manner, only a portion of the available cylinders of an internal combustion engine can be cut off and some cylinders can continue to be operated.

In order to reduce or eliminate bucking with sequential cut-off of cylinders of an internal combustion engine, the cylinders are cut off in two groups, one after the other.

4 If an overrun cut-off is to be initiated, one half of the cylinders are first cut off. This is illustrated in Figure 1. In Figure 1, injection events are represented with the symbol designated 1, suppressed injection events are represented with the symbol designated 2, ignitions are represented with the symbol designated 3, a torque-generating work stroke is represented with the symbol designated 4, and a torque -absorbing work stroke without ignition of fuel/air mixture is represented with the symbol designated 5.

In Figure 1, the curve of the cylinder cut-off is reflected for a fourcylinder internal combustion engine with the cylinders being designated A, B, C and D according to their ignition sequence. The commencement of cut-off of the first cylinder group is designated with point in time ti, the commencement of the cut-off of the second cylinder group is designated with the point in time t2, the point in time of the first missing work stroke of the first cylinder group is designated t3, and the point in time of the first missing work stroke of the second cylinder group is designated t4. Curve 6 drawn in the schematic depiction reflects the curve of the bucking function.

If it is detected at time point t, that the overrun cut-off must be started, the next two injections are suppressed and thus a first cylinder group is formed. In the case represented in Figure 1, the first cylinder group is made up of cylinders C and D which are cut off first. At time point t3, the cut-off cylinders of the first cylinder group provide a negative torque. As a result, the actual engine rotational speed of the internal combustion engine begins to deviate from the target engine rotational speed. A downward deviation in engine rotational speed begins to form.

In order to keep the bucking vibration triggered by the cylinder cut-off as small as possible, the second cylinder group, in the example of Figure I the cylinders A and B, continue to be operated past t3 and are not cut off until later. In the example of Figure 1, they perform one additional operating cycle. As a result, the stimulation of the drivetrain to bucking is reduced.

At this time the cut-off of the second cylinder group, i.e. cylinders A and R, is carried out so that they provide their negative moment at a point in time at which the actual engine rotational speed begins to rise due to bucking above the target engine rotational speed.

This can be better understood by considering bucking function 6. It is obtained from the engine rotational speed deviation by subtracting the actual engine rotational speed ftom the target engine rotational speed. An exemplary curve of such a bucking function is presented in Figure 3. The corresponding segment of curve 6 is drawn in on Figure 1. As can be seen, the value of the bucking function rises with the commencement of the overrun cut-off.

The reason for this is that the negative moment of the cut-off cylinder causes a stimulation of the drivetrain.

Now cylinders A and B of the second cylinder group are cut off so that they provide the full drag torque of the internal combustion engine at the zero crossing of bucking function 6, i.e. with the turning point of the engine rotational speed curve over time. As a result, the bucking is quickly damped.

The consequences of this method are shown in greater detail in Figure 2. Figure 2A shows the operating degree of the internal combustion engine in a time series, with I meaning operation of all cylinders and 0 meaning complete overrun cut-off. Figure 2B shows the time curve of the engine rotational speed of the internal combustion engine during the overrun cutoff.

6 As can be seen, at time point t3, the cylinders of the first cylinder group are cut off. In the example of Figure 1, this was cylinders C and D. The operating degree of the internal combustion engine drops to 0.5, and the engine rotational speed depicted in curve I I begins to fall, i.e. it drops below the target engine rotational speed. The second cylinder group is now cut-off at t4 so that its drag torque develops at the moment at which the actual engine rotational speed begins to rise above the target engine rotational speed. For this reason, the operating degree of the internal combustion engine is set to 0 at time point t4, as curve 10 in Figure 2A shows. As curve I I in Figure 2B reflects, the bucking vibration in the drivetrain of the internal combustion engine, which is reflected in fluctuations in engine rotational speed, is quickly damped. Curve I I shows as a comparison the engine rotational speed curve if the operating degree of the internal combustion engine was set at 0 at time point t3, i.e. all cylinders were cut off at the same time.

It can be seen from Figure 2A as well as from Figure I that the time span between the cut-off of the first cylinder group and the cut-off of the second cylinder group is of critical importance for the low-bucking cutoff of cylinders. This time period is designated in Figures I and 2 with T/2. It represents one-half the period duration of the bucking function depicted in curve 6 of Figure 3.

This half period duration of the drivetrain vibration is naturally dependent upon the properties of the drivetrain, in particular upon the gear ratio.

In an advantageous embodiment, the time duration depending on the gear selected and the engine rotational speed is saved in a table in the operating control device of the internal combustion engine. In the event of a 7 cylinder cut-off, it is read from the table and time point t2 for the second cylinder group is determined from it.

Optionally this time duration taken from the table can be adaptively corrected in the operation of the internal combustion engine through the period duration of the bucking vibration being measured from the curve of the bucking ftinction 6 depending, for example, on the gear, and converted into a correction value with which the corresponding value stored in the table is corrected.

If the cylinder cut-off is undertaken through interruption of the fuel injection, the half period duration T/2 of the drivetrain vibration must already be determined prior to initiation of the overrun cut-off since the interruption of fuel injection at t, or t2 must take place somewhat before time point t3 or t4 at which time point the particular cylinder provides a drag torque and accordingly influences the bucking. The adaptive adaptation of a correction factor which is applied to the value stored in the table for the period duration T/2 is therefore especially advantageous in order to react to long-time changes in the drivetrain of the internal combustion engine.

Claims

1. Method for cylinder cut-off in a multiple cylinder internal combustion engine, the cylinders of which can be cut off individually, in which the cut-off of the cylinders takes place in two steps so that in a first step a portion of the cylinders of the internal combustion engine are cut off and afterwards in a second step an additional portion of the cylinders of the internal combustion engine are cut off whereby between the initiation of the first and the initiation of the second step a predetermined period of time is waited, which is selected based on operating parameters.

2. Method for the restart of cut-off cylinders in a multiple cylinder internal combustion engine, the cylinders of which can be restarted individually, in which the restart of cut-off cylinders takes place in two stages so that in a first stage a portion of the cylinders of the internal combustion engine are restarted and afterwards in a second stage an additional portion of the cylinders are restarted whereby between the initiation of the first and the initiation of the second step a predetermined period of time is waited, which is selected based on operating parameters.

3. Method according to one of the preceding claims, characterised in that the predetermined period of time is one-half of the period duration of a bucking vibration which was stimulated through the cut-off or restart in the first stage.

4. Method according to one of the preceding claims, characterised in that the predetermined time period is stored in a characteristic diagram 9 which extends at least over the rotational speed of the internal combustion engine.

5. Method according to one of the preceding claims, characterised in that the predetermined time period is one-half of the period duration of a bucking function which was determined from the difference between a target engine rotational speed and the actual engine rotational speed of the internal combustion engine.

6. Method according to Claims 4 and 5 characterised in that by means of the half period duration obtained according to Claim 5 and the predetermined time duration taken from the characteristic diagram according to Claim 4, a correction value is adaptively adapted with which the predetermined time period taken from the characteristic diagram according to Claim 4 is corrected in order to obtain a corrected value for the predetermined time duration.

7. Method according to one of the preceding claims, characterised in that the predetermined time period is selected depending on the gear ratio.

8. Method according to one of the preceding claims, characterised in that in an internal combustion engine for the cylinder cut-off of which injection is cut off, the predetermined time period for non-steady operating conditions is corrected with respect to changes of the specification for commencement of injection and/or duration of injection.

9. Method according to one of the preceding claims, characterised in that the predetermined time period is expressed in segments of a camshaft signal.