SE541273C2

SE541273C2 - Starting an Internal Combustion Engine in a Parallel Hybrid Powertrain

Info

Publication number: SE541273C2
Application number: SE1650835A
Authority: SE
Inventors: Afram Kourie; Peter Carlson
Original assignee: Scania Cv Ab
Priority date: 2016-06-15
Filing date: 2016-06-15
Publication date: 2019-06-04
Also published as: DE102017005406A1; SE1650835A1

Abstract

The invention relates to a method for starting an internal combustion engine (4) in a parallel hybrid powertrain (2) in a vehicle (1) comprising an electric machine (14), a clutch (12) arranged between the electric machine (14) and the internal combustion engine (4) and a gearbox (6) with an input shaft (17) connected to the electric machine (14). The method comprises the steps of: a) shifting gears in the gearbox (6) to a selected gear; b) providing torque (T) by the electric machine (14) sufficient to propeller the vehicle (1) on the selected gear; c) controlling the clutch (12) such that it provides clutch torque (T) for starting the internal combustion engine (4); and d) controlling the electric machine (14) such that the torque (T) provided by the electric machine (14) is increased with an additional torque (T), wherein the additional torque (T) corresponds to the provided clutch torque (T). The invention also relates to a vehicle (1) with a parallel hybrid powertrain (2).

Description

Starting an Internal Combustion Engine in a Parallel Hybrid Powertrain TECHNICAL FIELD The invention relates to a method for starting an internal combustion engine in a parallel hybrid powertrain, a vehicle comprising such a parallel hybrid powertrain, a computer program for starting an internal combustion engine and a computer program product comprising program code according to the appended claims.

BACKGROUND Hybrid vehicles may be driven by a primary propulsion means such as an internal combustion engine and a secondary propulsion means such as an electric machine. The electric machine is equipped with at least one energy storage such as an electrochemical energy storage for storing electrical energy and a control unit to control the flow of electrical energy between the energy storage and the electric machine. The electric machine may thus alternately operate as a motor and a generator, based on the operating condition of the vehicle. When the vehicle is braking, the electric machine generates electrical energy, which is stored in the energy storage. This is usually called regenerative braking, which means that the vehicle is braking by means of the electric machine. The stored electrical energy may be utilized later for the propulsion of the vehicle.

Under certain operating conditions it is desirable to turn off the internal combustion engine, for example in order to save fuel and to avoid cooling of an internal combustion engine exhaust gas after treatment system due to low flow of exhaust gases through the after treatment system. When a torque increase to the hybrid powertrain is needed or when the energy storage must be recharged, the internal combustion engine must start quickly and efficiently. Known solutions for parallel hybrid powertrains commonly require the gearbox to be in neutral in order to be able to start the internal combustion engine by means of the electric machine or a start engine. With the gearbox in neutral no propulsion torque can be provided while the internal combustion engine is started. This affects the driveability of the vehicle and the comfort of the driver is compromised. Also, starting the internal combustion engine by means of the electric machine when a gear in the gearbox is engaged may influence the comfort of the driver. The reason for this is that a reduction of propulsion torque in the powertrain occurs when the internal combustion engine is started.

Document US20130274969 discloses a method to improve hybrid motor torque compensation utilizing active countermeasures to directly react and compensate for the torque disturbances during clutch engagement for start of an internal combustion engine. When a starter motor engages the internal combustion engine negative engine torque occurs. To compensate for the negative engine torque, the torque output of an electric machine is ramped up before the clutch engagement period.

Document WO2013091946 A1 discloses a parallel hybrid powertrain comprising an internal combustion engine, an electric machine, a clutch between the internal combustion engine and the electric machine, an output clutch downstream of the electric machine and a gearbox downstream of the output clutch. The document further describes a method for starting the internal combustion engine while the electric machine is driving the vehicle, wherein the clutch is closed such that the internal combustion engine can be started by the electric machine. At the same time, the output clutch is opened such that the output clutch slips and the output torque maintains constant. This solution requires an additional clutch device and is thus complex and affects the size of the powertrain.

However, the above-mentioned prior art documents do not discuss whether it is convenient or not to start the internal combustion engine by means of the electric machine when the electric machine generates a particular torque and if it may be convenient to connect the internal combustion engine to the powertrain when the electric machine is generating that particular torque.

SUMMARY OF THE INVENTION Despite known solutions in the field, there is still a need to develop a method for starting an internal combustion engine in a parallel hybrid powertrain, which does not affect the driveability of the vehicle and which entails a great comfort for the driver and passengers of the vehicle.

An object of the present invention is thus to achieve a method for starting an internal combustion engine in a parallel hybrid powertrain, which maintains the driveability of the vehicle.

Another object of the invention is to achieve a method for starting an internal combustion engine in a parallel hybrid powertrain, which maintains the comfort of the vehicle driver and passengers.

A further object of the invention is to achieve a method for starting an internal combustion engine in a parallel hybrid powertrain, which is quick and efficient.

Another object of the present invention is to achieve a new and advantageous computer program for starting an internal combustion engine in a parallel hybrid powertrain.

The herein mentioned objects and other objects are achieved by a method for starting an internal combustion engine in a parallel hybrid, a vehicle comprising such a hybrid powertrain, a computer program for starting an internal combustion engine and a computer program product comprising program code according to the independent claims.

According to an aspect of the present invention a method for starting an internal combustion engine in a parallel hybrid powertrain is provided. The parallel hybrid powertrain comprises an electric machine, a clutch arranged between the electric machine and the internal combustion engine and a gearbox with an input shaft connected to the electric machine and the clutch. The method comprises the steps of: a) shifting gears in the gearbox to a selected gear; b) providing torque by the electric machine sufficient to propeller the vehicle on the selected gear; c) controlling the clutch such that it provides clutch torque for starting the internal combustion engine; and d) controlling the electric machine such that the torque provided by the electric machine is increased with an additional torque, wherein the additional torque corresponds to the provided clutch torque. The method further comprises the steps of, when the internal combustion engine has reached a predetermined speed: e) gradually reducing the clutch torque to zero, wherein the clutch is completely disengaged; f) gradually reducing the additional torque from the electric machine to zero; g) synchronizing the internal combustion engine speed and the electric machine speed; and h) controlling the clutch to a completely engaged position.

When the internal combustion engine is turned off, i.e. not running, in the parallel hybrid powertrain, the electric machine solely provides torque in the powertrain to a propeller shaft arranged in connection to the gearbox and driving wheels of the vehicle. In this situation a gear is engaged in the gearbox and the clutch is disengaged. The clutch is connected to a crankshaft of the internal combustion engine and to the input shaft of the gearbox and thus also to the electric machine. When the electric machine provides the output torque, the input shaft rotates and the crankshaft is standing still. Controlling the clutch such that clutch torque is provided means that the clutch is controlled to a partly engaged state and thereby slips, whereby the crankshaft of the internal combustion engine will start rotating and the clutch torque is thus transferred to the internal combustion engine. The provided clutch torque will thus affect the speed of the internal combustion engine. When the internal combustion engine reaches an idling speed, the internal combustion engine may be controlled such that it ignites and the internal combustion engine is thereby started.

When shifting gears in the gearbox to a selected gear the rotational speed and the torque of the electric machine is decreased, so that no rotational speed and torque are transferred by the gearbox in the powertrain. However, depending on the driving conditions the rotational speed of the electric machine may be increased during shifting gears in order to synchronize rotational speed in the gearbox. After gears have been shifted to a selected gear the gearbox is prepared to transfer torque and rotational speed in the powertrain. Therefore, torque is provided by the electric machine, which is sufficient to propeller the vehicle on the selected gear. In case the internal combustion engine should be started for reasons explained above, the clutch is controlled such that it provides a clutch torque for starting the internal combustion engine. The clutch may be controlled so that it reaches a partly engaged state corresponding to the clutch torque for starting the internal combustion engine. The provided clutch torque depends on the degree of clutch engagement. The more engaged the clutch is, the greater clutch torque may be provided and transferred to the internal combustion engine. When the clutch is completely engaged, the transferred torque to the internal combustion engine is the same as the torque provided by the electric machine, less any torque required for propelling the vehicle. However, when the internal combustion engine is started a reduction of propulsion torque in the powertrain may occur. In order to avoid such a reduction of propulsion torque the electric machine is controlled such that the torque provided by the electric machine is increased with an additional torque, wherein the additional torque corresponds to the provided clutch torque. When starting the internal combustion engine by means of the electric machine, the clutch torque and the torque provided by the electric machine are counteracting. The torque provided by the electric machine to propelling the vehicle must therefore be increased with the additional torque for compensating the clutch torque needed for starting the internal combustion engine. The torque for propelling the vehicle will as a result be maintained. Thus, the driveability of the vehicle and also the comfort of the vehicle driver and passengers will be maintained.

Depending on the generated torque of the electric machine and the next coming driving conditions of the vehicle it may be more or less suitable to connect the internal combustion engine to the powertrain. If the generated torque of the electric machine has been increased and the electric machine deliver torque to the powertrain at a high level it is not suitable to connect the internal combustion engine to the powertrain because the torque delivered by the electric machine cannot be increased or only be increased to a limited degree before the electric machine reaches its maximum torque level.

The method for starting the internal combustion engine in the parallel hybrid powertrain thus maintains the driveability of the vehicle and also maintains the comfort of the vehicle driver and passengers. The method also leads to a quick and efficient starting of the internal combustion engine.

According to an aspect of the invention the shifting gears in the gearbox in step a) is performed when the electric machine provides no torque to the powertrain. When shifting gears in the gearbox to a selected gear the rotational speed and the torque of the electric machine is decreased , so that no rotational speed and torque are transferred by the gearbox in the powertrain. Depending on the driving conditions the rotational speed of the electric machine may be increased during shifting gears in order to synchronize rotational speed in the gearbox.

According to a further aspect of the invention the clutch torque and the torque provided by the electric machine are counteracting. When starting the internal combustion engine by means of the electric machine, the clutch torque and the torque provided by the electric machine are counteracting. The reason for this is that the internal combustion engine is driven by means of the electric machine.

According to a further aspect of the invention the clutch in step c) is controlled so that the clutch reaches a partly engaged state corresponding to the clutch torque for starting the internal combustion engine. Controlling the clutch such that clutch torque is provided means that the clutch is controlled to a partly engaged state and thereby slips, whereby the crankshaft of the internal combustion engine will start rotating and the clutch torque is thus transferred to the internal combustion engine.

According to a further aspect of the invention the clutch in step c) is controlled so that the clutch from a completely disengaged state gradually reaches the partly engaged state corresponding to the clutch torque for starting the internal combustion engine. When the clutch gradually reaches the partly engaged state corresponding to the clutch torque a smooth starting sequence of the internal combustion engine is achieved.

According to a further aspect of the invention the clutch in step c) is controlled only if the torque provided by the electric machine in step b) and the clutch torque are lower than or equal to the maximum torque that can be delivered by the electric machine. If the torque provided by the electric machine and the clutch torque together are lower than or equal to the maximum torque that can be delivered by the electric machine it is possible for the electric machine to deliver enough additional torque to compensate for the provided clutch torque.

According to a further aspect of the invention the clutch in step c) is controlled to provide clutch torque when the torque provided by the electric machine in step b) has reached a level corresponding to less than half of the maximum torque that can be delivered by the electric machine. When the torque provided by the electric machine in step b) has reached a level corresponding to less than half of the maximum torque that can be delivered by the electric machine it is still possible for the electric machine to deliver additional torque which corresponds to the provided clutch torque. After the electric machine has reached a level corresponding to less than half of the maximum torque it may not be possible for the electric machine to deliver additional torque which is enough to compensate for the provided clutch torque.

According to a further aspect of the invention the clutch in step c) is controlled to provide clutch torque when the torque provided by the electric machine in step b) has reached a level corresponding to the maximum torque that can be delivered by the electric machine reduced with a margin torque, which corresponds to or is larger than the additional torque defined in step d). If the torque provided by the electric machine has reached that level or is below that level, the clutch in step c) is controlled to provide clutch torque. Thus, it may be possible for the electric machine to deliver additional torque which is enough to compensate for the provided clutch torque.

According to a further aspect of the invention the steps c) and d) are performed substantially simultaneously. When the clutch is controlled such that it provides the clutch torque for starting the internal combustion engine, the electric machine should be controlled such that the torque provided by the electric machine is increased with additional torque by the electric machine, wherein the additional torque corresponds to the provided clutch torque. When these steps are performed substantially simultaneously a reduction of propulsion torque in the powertrain may be avoided.

When the internal combustion engine has been started and reached a predetermined speed, which may be the idle speed of the internal combustion engine, the clutch torque is gradually reduced to zero, wherein the clutch is completely disengaged. Thereafter, the additional torque generated by the electric machine is gradually reduced to zero, while rotational speed of the internal combustion engine speed is accelerated and synchronized to the speed of the electric machine. When the rotational speed of the internal combustion engine and the electric machine are synchronized the clutch is controlled to a completely engaged position.

According to an aspect of the invention a computer programme product is provided, comprising a programme code stored on a computer-readable medium for performing the method steps according to the herein mentioned method for starting an internal combustion engine, when said computer programme is run on an electronic control unit or a computer connected to the electronic control unit.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following details, and also by putting the invention into practice. Whereas the invention is described below, it should be noted that it is not restricted to the specific details described. Specialists having access to the teachings herein will recognise further applications, modifications and incorporations within other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment of the invention, Figure 2 schematically illustrates a parallel hybrid powertrain according to an embodiment of the invention, Figure 3 illustrates a flow chart for a method for starting an internal combustion engine according to an embodiment of the invention, Figure 4a illustrates a diagram of rotational speed variations during a method for starting an internal combustion engine according to an embodiment of the invention, Figure 4b illustrates a diagram of torque variations during a method for starting an internal combustion engine according to an embodiment of the invention, and Figure 4c illustrates a diagram of a clutch position during a method for starting an internal combustion engine according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a side view of a vehicle 1 which comprises a parallel hybrid powertrain 2 according to an embodiment of the invention. The hybrid powertrain 2 comprises an internal combustion engine 4 connected to a gearbox 6. The gearbox 6 is also connected to the driving wheels 8 of the vehicle 1, through a propeller shaft 10.

Figure 2 schematically shows a parallel hybrid powertrain 2 in a vehicle 1 according to an embodiment of the invention. The powertrain 2 comprises an internal combustion engine 4, a clutch 12, an electric machine 14 and a gearbox 6. The crankshaft 16 of the internal combustion engine 4 is connected to the clutch 12 and the electric machine 14 is connected to the clutch 12. The clutch 12 is thus arranged between the internal combustion engine 4 and the electric machine 14. The electric machine 14 and the clutch 12 are connected to an input shaft 17 of the gearbox 6. An output shaft of the gearbox 6 is connected to the propeller shaft 10, which in turn is connected to the driving wheels 8 of the vehicle 1. Only two driving wheels 8 are illustrated in Figure 2, however, any number of driving wheels 8 may be driven by the hybrid powertrain 2 within the scope of the invention.

The internal combustion engine 4, the clutch 12, the electric machine 14 and the gearbox 6 are arranged in connection to a control unit 18. The control unit 18 is adapted to control the internal combustion engine 4, the clutch 12, the electric machine 14 and the gearbox 6, for example for starting the internal combustion engine 4 while the vehicle 1 is propelled by the electric machine 14. A computer 20 may be connected to the control unit 18. The control unit 18 and/or the computer 20 comprises a computer program P, which can include routines to control the start of the internal combustion engine 4 of the invention. A program P may be stored in an executable form or compressed form in a memory M and/or in a read/write memory. A computer program product may be provided, which comprises a program code stored on a computer readable medium for performing the start of the internal combustion engine 4, when said program is run on the control unit 18 or the computer 20 connected to the control unit 18. Said code may be non-volatile, stored in said computer readable medium.

In order to drive the driving wheels 8 and thus propel the vehicle 1, the internal combustion engine 4 and/or the electric machine 14 generates a torque which is transferred via the gearbox 6 to the propeller shaft 10. The torque on the propeller shaft 10, called output torque, is the torque that propels the vehicle 1. When the internal combustion engine 4 provides output torque on the propeller shaft 10 and propels the vehicle 1, the clutch 12 is engaged and a gear is engaged. The electric machine 14 may in this case either provide additional output torque on the propeller shaft 10 or it may operate as a generator. In some situations it may be desirable to turn off the internal combustion engine 4 and propel the vehicle 1 by means of the electric machine 14. For instance, in order to save fuel and to avoid cooling of an internal combustion engine exhaust gas after treatment system. In these situations the clutch 12 is disengaged and a gear is engaged in the gearbox 6. When, however, the internal combustion engine 4 is needed again it is important that it can be started in a quick and efficient way, and so the driveability of the vehicle and the comfort of the vehicle driver and passengers are maintained.

When the internal combustion engine 4 is turned off in the parallel hybrid powertrain 2, the electric machine 14 solely provides torque in the powertrain 2 to the propeller shaft 10 arranged in connection to the gearbox 6 and for driving the driving wheels 8 of the vehicle 1. The torque provided by the electric machine 14 may be increased by increasing the current from an energy storage 19 to the electric machine 14. In this situation a gear is engaged and the clutch 12 is disengaged. When the electric machine 14 provides the output torque, the input shaft 17 rotates and the crankshaft 16 is standing still, i.e. not rotating. Controlling the clutch 12 such that clutch torque Tcis provided means that the clutch 12 is controlled to a partly engaged state and thereby slips, whereby the crankshaft 16 of the internal combustion engine 4 will start rotating and the clutch torque Tcis thus transferred to the internal combustion engine 4. The provided clutch torque Tcwill thus affect the speed of the internal combustion engine 4. When the internal combustion engine 4 reaches an idling speed the internal combustion engine 4 may be controlled such that it ignites and the internal combustion engine 4 is thereby started.

The provided clutch torque Tcfor starting the internal combustion engine 4 may be gradually increased.

Figure 3 shows a flowchart for a method for starting an internal combustion engine 4 in a parallel hybrid powertrain 2 according to an embodiment of the invention. The parallel hybrid powertrain 2 may be configured as described in Figure 2. The method comprises the steps of: a) shifting gears in the gearbox 6 to a selected gear; b) providing torque Teby the electric machine 14 sufficient to propeller the vehicle 1 on the selected gear; c) controlling the clutch 12 such that it provides clutch torque Tcfor starting the internal combustion engine 4; and d) controlling the electric machine 14 such that the torque Te provided by the electric machine 14 is increased with an additional torque Ta, wherein the additional torque Tacorresponds to the provided clutch torque Tc.

When shifting gears in the gearbox 6 to a selected gear in step a) the torque of the electric machine 14 is decreased, so that no torque is transferred by the gearbox 6 from the electric machine 14 to the driving wheels 8 of the vehicle 1. Also, depending on an upshift or a downshift in the gearbox 6, the rotational speed of the electric machine 14 will decrease or increase. After gears have been shifted to the selected gear, the gearbox 6 is prepared to transfer torque and rotational speed in the powertrain 2. In step b) torque Teis provided by the electric machine 14, which is sufficient to propeller the vehicle 1 on the selected gear. After the gear shifting to the selected gear, the torque Teprovided by the electric machine 14 is increased to a torque Tewhich is sufficient to propeller the vehicle 1. In this situation the clutch 12 is disengaged and the internal combustion engine 4 is turned off, i.e. not running. When a torque increase to the powertrain 2 is needed or when the energy storage must be recharged, the internal combustion engine 4 is started and connected to the powertrain 2.

In step c) the clutch 12 is controlled such that it provides a clutch torque Tcfor starting the internal combustion engine 4. The clutch 12 may be controlled so that it reaches a partly engaged state corresponding to the clutch torque Tcfor starting the internal combustion engine 4. The provided clutch torque Tcdepends on the degree of clutch engagement. The more engaged the clutch 12 is, the greater clutch torque Tcmay be provided and transferred to the internal combustion engine 4. When the clutch 12 is completely engaged, the transferred clutch torque Tcto the internal combustion engine 4 is the same as the torque Teprovided by the electric machine 14, less the torque required for propelling the vehicle.

However, when the internal combustion engine 4 is started, a reduction of propulsion torque in the powertrain 2 may occur. In order to avoid such a reduction of propulsion torque, the electric machine 14 is in step d) controlled such that the torque Te provided by the electric machine 14 is increased with additional torque Taby the electric machine 14, wherein the additional torque Tacorresponds to the provided clutch torque Tc. When starting the internal combustion engine 4 by means of the electric machine 14, the clutch torque Tcand the torque Te provided by the electric machine 14 are counteracting. The torque Teprovided by the electric machine 14 for propelling the vehicle 1 must therefore be increased with additional torque Tafor compensating the clutch torque Tcneeded for starting the internal combustion engine 4. The torque for propelling the vehicle 1 will as a result be maintained. Thus, the driveability of the vehicle 1 and also the comfort of the vehicle 1 driver and passengers will be maintained.

Depending on the generated torque of the electric machine 14 and the next coming driving conditions of the vehicle 1 it may be more and less suitable to connect the internal combustion engine 4 to the powertrain 2. If the generated torque of the electric machine 14 has been increased and the electric machine 14 delivers torque to the powertrain 2 at a high level, it is not suitable to connect the internal combustion engine 4 to the powertrain 2 because the torque Tedelivered by the electric machine 14 cannot be increased or only be increased to a limited degree before the electric machine Tereaches its maximum torque level. However, if the internal combustion engine 4 must be started and connected immediately to the powertrain 2 the internal combustion engine 4 can be stared without the compensation of additional torque provided by the electric machine 14 or be started by means of a starter motor 22, see Fig. 2.

When shifting gears in the gearbox 6 to a selected gear in step a) the rotational speed and the torque Teof the electric machine 14 may be decreased, so that no rotational speed and torque are transferred by the gearbox 6 in the powertrain 2. As mentioned above, decreasing the rotational speed and the torque Teof the electric machine 14 depends on if an upshift or a downshift in the gearbox 6 will take place and also on the driving conditions of the vehicle 1.

The clutch torque Tcand the torque Te provided by the electric machine 14 are counteracting when starting the internal combustion engine 4 by means of the electric machine 14. The reason for this is that the internal combustion engine 4 is driven by means of the electric machine 14 during the starting sequence of the internal combustion engine 4.

According to an embodiment of the invention, the clutch 12 in step c) is controlled so that the clutch 12 reaches a partly engaged state corresponding to the clutch torque Tcfor starting the internal combustion engine 4. Controlling the clutch 12 such that clutch torque Tcis provided means that the clutch 12 is controlled to a partly engaged state and thereby slips, whereby the crankshaft 16 of the internal combustion engine 4 will start rotating and the clutch torque Tcis thus transferred to the internal combustion engine 4.

According to a further embodiment of the invention the clutch 12 in step c) is controlled so that the clutch 12 from a completely disengaged state gradually reaches the partly engaged state corresponding to the clutch torque Tcfor starting the internal combustion engine 4. When the clutch 12 gradually reaches the partly engaged state corresponding to the clutch torque Tc, a smooth starting sequence of the internal combustion engine 4 is achieved.

According to a further embodiment of the invention the clutch 12 in step c) is controlled only if the torque Teprovided by the electric machine 14 in step b) and the clutch torque Tcare lower than or equal to the maximum torque Tmaxthat can be delivered by the electric machine 14. If the torque Teprovided by the electric machine 14 and the clutch torque Tctogether are lower than or equal to the maximum torque Tmaxthat can be delivered by the electric machine 14, it is possible for the electric machine 14 to deliver enough additional torque Tato compensate for the provided clutch torque Tc.

According to a further embodiment of the invention the clutch 12 in step c) is controlled to provide clutch torque Tcwhen the torque Te provided by the electric machine 14 in step b) has reached a level corresponding to less than half of the maximum torque Tmaxthat can be delivered by the electric machine 14. When the torque Te provided by the electric machine 14 in step b) has reached a level corresponding to less than half of the maximum torque Tmaxthat can be delivered by the electric machine 14 it is still possible for the electric machine 14 to deliver additional torque Tawhich corresponds to the provided clutch torque Tc. After the electric machine 14 has reached a level corresponding to less than half of the maximum torque Tmaxit may not be possible for the electric machine 14 to deliver additional torque Tawhich is enough to compensate for the provided clutch torque Tc.

According to a further embodiment of the invention the clutch 12 in step c) is controlled to provide clutch torque Tcwhen the torque Teprovided by the electric machine 14 in step b) has reached a level corresponding to the maximum torque Tmaxthat can be delivered by the electric machine (14) reduced with a margin torque Tmargin, which corresponds to or is larger than the additional torque Tadefined in step d). If the torque Teprovided by the electric machine has reached that level or is below that level, the clutch 12 in step c) is controlled to provide clutch torque Tc. Thus, it may be possible for the electric machine to deliver additional torque which is enough to compensate for the provided clutch torque.

According to a further embodiment of the invention the steps c) and d) are performed substantially simultaneously. When the clutch 12 is controlled such that it provides clutch torque Tcfor starting the internal combustion engine 4 the electric machine 14 should be controlled such that the torque Teprovided by the electric machine 14 is increased with additional torque Taby the electric machine 14, wherein the additional torque Tacorresponds to the provided clutch torque Tc. When these steps are performed substantially simultaneously a reduction of propulsion torque in the powertrain 2 may be avoided.

According to a further embodiment of the invention the method comprising the further steps of, when the internal combustion engine 4 has reached a predetermined speed: e) gradually reducing the clutch torque Tcto zero, wherein the clutch 12 is completely disengaged; f) gradually reducing the additional torque Tafrom the electric machine 14 to zero; g) synchronizing the internal combustion engine speed nceand the electric machine speed ne; and h) controlling the clutch 12 to a completely engaged position.

When the internal combustion engine 4 has been started and reached a predetermined speed, which may be the idle speed of the internal combustion engine 4, the clutch torque Tcis gradually reduced to zero, wherein the clutch 12 is completely disengaged. Thereafter, or substantially simultaneously, the additional torque Tagenerated by the electric machine 14 is gradually reduced to zero. Thereafter, or at least partially simultaneously the rotational speed of the internal combustion engine speed nceis accelerated and synchronized to the speed neof the electric machine 14. When the rotational speed of the internal combustion engine 4 and the electric machine 14 are synchronized the clutch 12 is controlled to a completely engaged position. The idle speed of the internal combustion engine may be 300-600 rpm.

Fig. 4a shows a diagram of rotational speed variations during a method for starting an internal combustion engine 4 according to the invention. Fig. 4b shows a diagram of torque variations during a method for starting an internal combustion engine 4 according to the invention. Fig. 4c shows diagram of a clutch position during a method for starting an internal combustion engine 4 according to an embodiment of the invention. The method for starting an internal combustion engine 4 is described in Figure 3 and is here further illustrated by the diagrams over the rotational speed variations, torque variations and clutch positions over time measured in seconds.

The upper solid curve in fig. 4a represents the rotational speed of the electric machine 14. The dashed curve in fig. 4a represents a rotational speed of the internal combustion engine 4. At time t1indicated in fig. 4a the speed of the electric machine 14 starts to decrease due to the shifting gears in the gearbox 6 to a selected gear. At the time t1the internal combustion engine 4 is not running and the rotational speed of the internal combustion engine 4 is therefore zero. The clutch 12 is disengaged when the internal combustion engine 4 is not running, which is indicated in fig. 4c. The clutch is completely disengaged when the curve is at level 1 in fig. 4c and is completely engaged when the curve is at level 0 in fig. 4c. When shifting gears in the gearbox 6 the torque Teprovided by the electric machine 14 decreases before time t1and is reduced to zero at time t1which is indicated in fig. 4b.

Between t1and t2the gearbox 6 is shifted to a neutral state, wherein the gearbox 6 does not transfer any torque. Between t1and t2rotating components are synchronized before shifting to a selected gear. At t2shifting gears in the gearbox 6 to a selected gear takes place. The torque Teby the electric machine 14 is negative, which means that the electric machine 14 is driven and may generate power to the energy storage 19. At time t2the shifting of gears to the selected gear has been completed and the electric machine 14 starts to accelerate and torque Tefrom the electric machine 14 is provided to the powertrain 2. When increasing the torque Teprovided by the electric machine 14, the clutch 12 is partly engaged in order to start the internal combustion engine 4. This will take place between t2and t3in figures 4a - 4c. At t3the crankshaft 16 of the internal combustion engine 4 starts to rotate due to the clutch torque Tctransferred by the partly engaged clutch 12.

Since the clutch torque Tcand the torque Teprovided by the electric machine 14 are counteracting when the internal combustion engine 4 is started, a reduction of propulsion torque in the powertrain 2 may occur. In order to avoid such a reduction of propulsion torque, the electric machine 14 is between time t3and t4controlled such that the torque Te provided by the electric machine 14 is increased with an additional torque Ta, wherein the additional torque Tacorresponds to the provided clutch torque Tc. In fig. 4b this increase in torque is represented by the continuous curve. The curve with dots in fig. 4b represents a situation when no additional torque Tais added. Between time t3and t4the clutch 12 is partly engaged and the rotational speed of the internal combustion engine 4 is increased. At time t4the clutch 12 is disengaged when the internal combustion engine 4 has started and has reached a rotational speed corresponding to the idle speed. At time t4the additional torque Tais decreased. When the clutch 12 is disengaged the speed of the internal combustion engine 4 is synchronized with the speed of the electric machine 14. This takes place between time t4and ts. Thereafter, when speed of the internal combustion engine 4 has been synchronized with the speed of the electric machine 14 the clutch 12 is engaged and the torque provided by the internal combustion engine 4 may be increased. At the same time ts the torque Teprovided by the electric machine 14 may be decreased, as indicated in fig. 4b.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for starting an internal combustion engine (4) in a parallel hybrid powertrain (2) in a vehicle (1) comprising an electric machine (14), a clutch (12) arranged between the electric machine (14) and the internal combustion engine (4) and a gearbox (6) with an input shaft (17) connected to the electric machine (14), the method comprising the steps of: a) shifting gears in the gearbox (6) to a selected gear; b) providing torque (Te) by the electric machine (14) sufficient to propeller the vehicle (1) on the selected gear; c) controlling the clutch (12) such that it provides clutch torque (Tc) for starting the internal combustion engine (4); d) controlling the electric machine (14) such that the torque (Te) provided by the electric machine (14) is increased with an additional torque (Ta), wherein the additional torque (Ta) corresponds to the provided clutch torque (Tc), and when the internal combustion engine (4) has reached a predetermined speed: e) gradually reducing the clutch torque (Tc) to zero, wherein the clutch (12) is completely disengaged; f) gradually reducing the additional torque (Ta) from the electric machine (14) to zero; g) synchronizing the internal combustion engine speed (nce) and the electric machine speed (ne); and h) controlling the clutch (12) to a completely engaged position.

2. The method according to claim 1, wherein shifting gears in the gearbox in step a) is performed when the electric machine (14) provides no torque (Te) to the powertrain (2).

3. The method according to claim 1, wherein the clutch torque (Tc) and the torque (Te) provided by the electric machine (14) are counteracting.

4. The method according to any of the preceding claims, wherein the clutch (12) in step c) is controlled so that the clutch (12) reaches a partly engaged state corresponding to the clutch torque (Tc) for starting the internal combustion engine (4).

5. The method according to claim 4, wherein the clutch (12) in step c) is controlled so that the clutch (12) from a completely disengaged state gradually reaches the partly engaged state corresponding to the clutch torque (Tc) for starting the internal combustion engine (4).

6. The method according to any of claims 1-3, wherein the clutch (12) in step c) is controlled only if the torque (Te) provided by the electric machine (14) in step b) and the clutch torque (Tc) are lower than or equal to the maximum torque (Tmax) that can be delivered by the electric machine (14).

7. The method according to any of claims 1-3, wherein the clutch (12) in step c) is controlled to provide clutch torque (Tc) when the torque (Te) provided by the electric machine (14) in step b) has reached a level corresponding to less than half of the maximum torque (Tmax) that can be delivered by the electric machine (14).

8. The method according to any of the claims 1 - 3, wherein the clutch (12) in step c) is controlled to provide clutch torque (Tc) when the torque (Te) provided by the electric machine (14) in step b) has reached a level corresponding to the maximum torque (Tmax) that can be delivered by the electric machine (14) reduced with a margin torque (Tmargin), which corresponds to or is larger than the additional torque (Ta) defined in step d).

9. The method according to any of the preceding claims, wherein step c) and d) are performed substantially simultaneously.

10. A vehicle with a parallel hybrid powertrain (2), characterized in that the powertrain (2) comprises an internal combustion engine (4) which is started according to the method in any of the claims 1-9.

11. A computer program (P), wherein said computer program comprises program code for causing an electronic control unit (18) or a computer (20) connected to the electronic control unit (18) to perform the steps according to any of the claims 1-9.

12. A computer program product comprising a program code stored on a computerreadable medium for performing the method steps according to any of claims 1-9, when said computer program (P) is run on an electronic control unit (18) or a computer (20) connected to the electronic control unit (18).