WO2007048959A1

WO2007048959A1 - Internal combustion engine with variable compression rate

Info

Publication number: WO2007048959A1
Application number: PCT/FR2006/051068
Authority: WO
Inventors: Emmanuel Devaud; Pascal Emery
Original assignee: Renault S.A.S
Priority date: 2005-10-25
Filing date: 2006-10-19
Publication date: 2007-05-03
Also published as: FR2892457A1

Abstract

The invention relates to a method of controlling a mobile actuator (2) such as to vary the compression rate of at least one combustion chamber of an internal combustion engine (1). The inventive method comprises the following steps consisting in: receiving a compression rate set value, generating a reference position set value for the actuator, determining the real compression rate of the combustion chamber obtained for the compression rate set value, generating a corrected position set value for the actuator as a function of the determined real compression rate and the reference position set value, and applying the corrected position set value to the actuator (2). The invention is characterised in that the step consisting in generating the reference position set value of the actuator is performed as a function of the pre-stored relations between compression rate set values and position set values of the actuator.

Description

INTERNAL COMBUSTION ENGINE WITH COMPRESSION RATIO

VARIABLE

The invention relates to internal combustion engines, and in particular to internal combustion engines provided with a device for varying their compression ratio.

The anti-pollution standards define increasingly stringent requirements concerning the pollutant emissions of motor vehicle engines. Many compromises in engine design are required to meet these standards while ensuring satisfactory levels of fuel consumption and performance.

Engines equipped with a compression rate variation device called VCR thereafter, have been developed in particular to obtain gains in consumption or pollutant emissions.

The VCR device is usually used as follows:

For a gasoline engine like a diesel engine, the compression ratio is reduced at high load and increased at low load. For a petrol engine, the performance is improved and the rattling is avoided at high load, while the efficiency is optimized at low load. For a diesel engine, the specific power is improved and the NOx emission is reduced at high load and the emissions of HC, CO and CH ₄ are reduced at low load. VCR devices of different types have been developed. These devices comprise a mobile actuator whose position variation makes it possible to modify the compression ratio of one or more cylinders. Developments propose a piston with variable geometry modifying the volume of the combustion chamber for a given position of the piston. Other developments provide a variable position cylinder head to change the volume of the combustion chamber. Other developments propose to modify the kinematics of the piston or to adjust an eccentric on the crankshaft. Other developments propose to vary the volume of a second combustion chamber.

Engines, and in particular those equipped with a VCR device, inevitably suffer from dispersions in the geometry and kinematics of their components. These dispersions lead to dispersions of the compression ratios, which can have a significant amplitude and have a negative impact on consumption, yield and pollutant emissions. In addition, the actuators of the VCR devices may be dimensioned unsuitable with respect to these dispersions.

US Pat. No. 4,834,031 proposes to measure a compression ratio by using in particular a combustion pressure sensor. An oil circuit controlled by the opening of valves makes it possible to modify the compression ratio by rotating an actuator in the form of an eccentric connecting rod support either in a position inducing a high compression ratio, or in a position inducing a reduced compression ratio. A command opens or closes valves in an oil circuit depending on the desired compression ratio. Measurement of the failing compression ratio, engine rotational speed, and cylinder inlet pressure are measured to detect pressure sensor failure. When such a failure is detected, the VCR device goes into safety mode in which the reduced compression ratio is imposed in order to avoid damage to the engine.

The document FR-2,854,436 describes a method for managing an internal combustion engine with a variable compression ratio.

None of the known VCR devices can accurately maintain the compression ratio to a set value. The invention aims to solve this drawback.

The invention thus proposes a method of controlling a mobile actuator so as to vary the compression ratio of at least one combustion chamber of an internal combustion engine, comprising the following steps:

- receive a set of compression ratio;

generate a reference position reference of the actuator;

determining the actual compression ratio of the combustion chamber obtained for said compression ratio setpoint; generating a corrected position reference of the actuator as a function of the actual compression ratio determined and the reference position reference;

- Apply the corrected position setpoint on the actuator (2), characterized in that the step of generating the reference position reference of the actuator is performed as a function of relationships previously stored between compression rate setpoints and position instructions of the actuator.

The invention also relates to an internal combustion engine comprising:

- a combustion chamber;

a movable actuator so as to vary the compression ratio of the combustion chamber; a device for determining the actual compression ratio of the combustion chamber; a device for controlling the mobile actuator, capable of receiving a compression ratio set point, the mobile actuator control device correcting the reference position setpoint as a function of the actual compression ratio determined and applying said setpoint of corrected position on the actuator, characterized in that the control device of the movable actuator comprises a module where are stored the relationship between compression rate setpoints and reference position instructions of one actuator.

Other features and advantages of the invention will emerge from the description which follows in conjunction with the accompanying drawings provided by way of example only and in which:

- Figure 1 illustrates a schematic sectional view of a combustion engine embodying the invention;

FIG. 2 schematically illustrates the method implemented in an exemplary application of the invention;

FIG. 3 illustrates an exemplary algorithm for implementing the method during the operation of an engine.

The invention proposes to control an actuator whose mobility influences the compression ratio of a combustion chamber. The position of the actuator is chosen by a map, which receives a compression ratio setpoint and generates a reference position reference of the actuator as a function of relationships previously stored. By determining the actual compression ratio in the combustion chamber resulting from the compression ratio setpoint, a corrected position setpoint of the actuator is generated and this corrected position setpoint is applied to the actuator.

Thus, the position of the actuator position takes into account the actual operation of the engine, which reduces the impact of manufacturing dispersions or the time evolution of the engine on the accuracy of the control of the compression ratio in the combustion chamber. . Figure 1 illustrates a schematic sectional view of an internal combustion engine 1 at its combustion chamber. The engine 1 comprises a device for varying the compression ratio. The illustrated example relates to a variant in which the kinematics of the moving parts are modified to influence the compression ratio. The illustrated motor comprises an actuator 2 on which a rod 3 is rotatably mounted by any suitable means. The actuator 2 has a guide surface of the substantially cylindrical connecting rod 3. The axis of this cylinder substantially defines the axis of rotation of the connecting rod around the actuator 2. This axis is offset with respect to the axis of the crankpin 11 on which 1 the actuator 2 is mounted. The actuator 2 is pivotably mounted relative to the crankpin 11 about the axis of this crankpin. The actuator can be maintained in different pivoting positions with respect to the crankpin 11. For each pivoting of the actuator 2 relative to the crankpin 11, there corresponds a value between the axis of rotation 4 of the crankshaft and the axis rotation of the big end. Each of these positions of the actuator 2 thus defines a value of the stroke of the piston 5 in the cylinder 6 and consequently a compression ratio of the combustion chamber. The compression rate variation device also comprises an electronic management module 9. The electronic management module comprises an interface for receiving or calculating a compression ratio setpoint. The module 9 stores beforehand relations between compression rate instructions and reference position setpoints of the actuator 2. These relations may for example be in the form of a function for calculating a reference position reference of the actuator as a function of a compression ratio setpoint or in the form of a table comprising pairs of discrete values, each pair associating a compression ratio setpoint with a reference position reference of the actuator 2. The compression ratio values stored in the pairs advantageously comprise values ranging between a maximum value and a minimum value of compression ratio, precise variations in the compression ratio can thus be controlled over most of the operating range of the engine. The use of these previously stored values makes it possible to improve the response time of the VCR system. The module 9 generates from these relationships a reference position reference of the actuator 2 according to a compression ratio setpoint. The engine 1 has a pressure sensor 7, from which it makes pressure measurements in the combustion chamber. This sensor 7 is for example fixed to the cylinder head 10. The measurements provided by the sensor 7 are supplied to the module 9. The module 9 determines a real compression ratio obtained in the combustion chamber, following the reception of a setpoint of compression ratio. From the determined actual compression ratio and the reference position reference, a corrected position reference of the actuator is generated and applied to the actuator, for example by via a control circuit not shown. The actuator is for example slaved to this corrected position command.

Such a method is illustrated schematically in FIG. 2. A subtracter 27 receives a compression ratio setpoint Ctc and a measurement of the actual compression ratio Mtc. The difference between these two signals is supplied to the correction device 22. As a function of predefined relationships between determined differences and correction values, a correction value δA is calculated and applied to an adder 28. A control map 21 receives by elsewhere the compression ratio setpoint CTc. By using the relationships previously stored, the map 21 generates an actuator reference position reference CPaO, applied to the adder 28. By summing the signals CPaO and δA, the adder forms a corrected position setpoint CPac applied to the actuator 23. The actuator 23 then takes a real position RPa which leads to a real compression ratio Rtc of the motor 24. A device 25 determines the value Mtc on the basis of measurements made on the engine 24.

For a four-stroke engine, the actual compression ratio can be determined by the calculation on the basis of different pressure measurements provided by the sensor 7. By measuring the maximum pressure Pmax on an engine cycle and the average pressure Pmoy during a period where the valves close the combustion chamber the following formulas can be used to calculate a real compression ratio Ter: Tcr = A * (Pmax / Pmoy) ² + B (Pmax / Pmoy) + C, or Tcr = [(B * Pmax / Pmoy) / (AC * Pmax / Pmoy)] ^{1 / γ} , A, B and C being predetermined constants and γ being the ratio of heat capacities to constant pressure and volume.

By measuring the maximum pressure Pmax and the minimum pressure Pmin on a motor cycle, the following formula can be used to determine the actual compression ratio Ter: Tcr = (Pmax / Pmin) ^{1 / Y} '

Advantageously, an engine control device 26 receives a combustion interruption request Rint or determines that combustion interruption conditions are necessary to determine the actual combustion rate in the chamber. The device 26 then generates a control Cint interrupting the introduction of fuel or switching off the controlled ignition and transmits it to the engine 24. The use of this interruption of combustion will be detailed more specifically with reference to FIG. .

The algorithm of FIG. 3 represents the implementation of the control method. After a beginning step 31, it is verified in step 32 whether updating conditions of the corrected position command of the actuator are fulfilled. The conditions that can be envisaged for an update are, for example, a predetermined duration separating two successive updates, a difference between the compression ratio setpoint and the actual compression ratio determined greater than a threshold or a variation greater than a threshold of the rate. of actual compression determined. It can be expected that the time between two successive updates is a function of the rotational speed of the combustion engine.

If the update conditions are not met, the last updated corrected position command continues to be applied to the actuator until the next update. The algorithm then ends in step 37. If the corrected position command updating conditions are fulfilled, the algorithm proceeds to step 33.

During step 33, it is checked whether the engine is in specific operating conditions, in which it is possible to update the corrected position reference of the actuator. If this is the case, we go to step 34. If this is not the case, we advantageously go to step 38, during which the combustion is interrupted in the chamber for at least one cycle, before passing in step 34. The interruption of the combustion carried out in step 38 allows a measurement of the compression ratio more accurate because performed off combustion. Indeed, during a combustion, the increase of the pressure in the cylinder is mainly due to the explosion and not only to the decrease of volume in the combustion chamber. The reliability of the determination of the compression ratio is thus increased because the measurement of the compression ratio is not "polluted" by an increase in pressure due to combustion.

During step 34, the actual compression ratio is determined. During step 35, a new corrected setpoint based on the actual compression ratio determined and the determined reference setpoint, as detailed previously.

In step 36 it is determined whether the engine is stopped or not. If the engine is stopped, the algorithm ends in step 37. If the engine is not stopped, it loops back to step 32.

The predefined engine operating conditions taken into account in step 33 may be as follows:

- The engine runs in the absence of combustion, for example due to a drive by a starter;

- the engine is idling or the user does not depress the accelerator pedal at all; the actuator is maintained in a substantially constant position for at least a predetermined period;

- The rotational speed of the motor is within a range of predetermined values for at least a predetermined period.

These various parameters will be adapted by the skilled person according to the specific needs of each engine.

In order better to take into account the dispersions and differences in operation in each combustion chamber, the engine advantageously comprises several actuators, each of which can vary the compression ratio of a respective combustion chamber. The mapping adjustment process is then performed independently for each of the actuators, the mapping can thus control the compression ratio of each combustion chamber with high accuracy.

Claims

A method of controlling a movable actuator (2) to vary the compression ratio of at least one combustion chamber of an internal combustion engine (1), comprising the steps of:

- receive a set of compression ratio;

generate a reference position reference of the actuator;

determining the actual compression ratio of the combustion chamber obtained for said compression ratio setpoint;

- Generate a corrected position setpoint of the actuator according to the actual compression ratio determined and the reference position reference; - apply the corrected position instruction on

1 'actuator (2), characterized in that the step of generating the reference position reference of the actuator is performed as a function of relations previously stored between compression ratio and position instructions of the actuator .

The method of claim 1, wherein the correction comprises:

determining the difference between the compression ratio setpoint and the actual compression ratio determined;

calculating a correction value as a function of predefined relationships between determined differences and correction values; - Generate the corrected position setpoint by summing the calculated correction value and the reference position reference of the actuator.

3. The method of claim 1 or 2, wherein the actual compression ratio of the combustion chamber is determined from pressure measurements in the combustion chamber.

4. The method of claim 3, wherein: the motor (1) is of the four-stroke type;

the maximum pressure Pmax is measured on an engine cycle, the average pressure Pmoy is measured during a period when the valves close the combustion chamber; the real compression ratio Ter is determined by one of the following formulas:

Tcr = A * (P max / P mean) ² + B (Pmax / Pavg) + C, or Tcr = [(B * Pmax / Pavg) / (AC * Pmax / Pavg)] ^{1 / γ,} A, B and C being predetermined constants and γ being the ratio of heat capacities to constant pressure and volume.

The method of claim 3, wherein:

the engine (1) is of the four-stroke type;

the maximum pressure Pmax and the minimum pressure Pmin are measured on an engine cycle;

the real compression ratio Ter is determined by the following formula:

Tcr = (Pmax / Pmin) ^{1 / Y} , where γ is the ratio of heat capacities to constant pressure and volume.

The method of any one of the preceding claims, wherein the relationships stored memories include pairs each associating a discrete value of compression ratio setpoint and a discrete reference position reference value of one actuator.

7. Method according to claim 6, characterized in that the compression ratio values stored in the couples range between a maximum value and a minimum value of compression ratio.

8. Method according to any one of the preceding claims implemented when specific operating conditions of the engine (1) are met, said conditions being chosen from the group comprising: a rotation of the engine in the absence of combustion, a rotation of the engine at idle, a constant actuator position for a predetermined time, a motor rotation speed within a predetermined range of values for a predetermined time in normal operation.

The method according to claim 8, wherein, if said engine specific operating conditions are not met, the introduction of fuel is interrupted during a cycle in the combustion chamber and the actual compression ratio of the combustion chamber. is determined during this interruption.

10. A method according to any one of the preceding claims, wherein a corrected position setpoint is updated episodically, the last updated corrected position instruction being applied to the actuator (2) between two updates.

11. A method of controlling a plurality of mobile actuators so as to vary the compression ratio of respective combustion chambers, wherein the control method according to any one of the preceding claims is applied independently for each of the actuators.

12. Internal combustion engine (1) comprising:

- a combustion chamber;

- a movable actuator (2) so as to vary the compression ratio of the combustion chamber;

a device (9) for determining the actual compression ratio of the combustion chamber;

- A device (9) for controlling the mobile actuator, adapted to receive a compression ratio setpoint, the device (9) for controlling the mobile actuator correcting the reference position reference as a function of the actual compression ratio. determining and applying said corrected position setpoint on the actuator, characterized in that the device (9) for controlling the mobile actuator comprises a module in which are memorized relations between compression rate setpoints and setpoint positions. reference of

1 'actuator.