SE1650837A2

SE1650837A2 - Starting an Internal Combustion Engine in a Parallel Hybrid Powertrain

Info

Publication number: SE1650837A2
Application number: SE1650837A
Authority: SE
Inventors: Afram Kourie; Peter Carlson
Original assignee: Scania Cv Ab
Priority date: 2016-06-15
Filing date: 2016-06-15
Publication date: 2019-01-22
Also published as: SE541413C2; SE1650837A1; DE102017005334A1

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) providing torque (T) by the electric machine (14) to the powertrain (2); b) detecting a first signal (S) indicating that the internal combustion engine (4) should be started; c) starting the internal combustion engine (4) in response to the detected first signal (S); d) detecting a second signal (S) indicating that the clutch (12) should be controlled for connecting the internal combustion engine (4) to the electric machine (14); and e) controlling the clutch (12) in response to the detected second signal (S) for connecting the internal combustion engine (4) to the electric machine (14). 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 and the internal combustion engine. 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 the internal combustion engine exhaust gas after treatment system due to low flow of exhaust gases through the after treatment system. When a torque increase in 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 on 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. Also, starting the internal combustion engine by means of the electric machine when the electric machine runs at a high rotational speed will result in that the rotational speed of the internal combustion engine will increase from a low level to a high level in a short period of time. Such increase of the rotational speed of the internal combustion engine is not comfortable for the driver and passengers in the vehicle. Also, an undesired increase of fuel consumption may take place as a result of the increase of the rotational speed of the internal combustion engine.

Document US201 30291 830 discloses a method of starting rotation of a stopped internal combustion engine if the predicted desired torque after the transmission upshift is greater than a threshold amount of torque. An engine start may be requested via an operator key or pushbutton input that has a sole function of requesting an engine start. Alternatively, an engine restart may be automatically requested by a control unit based on operating conditions. The engine may be started via a starter or via an electric machine. The method judges if it may be desirable to start the engine based on transmission shifting or forecasted transmission shifting.

Document WO201 3091 946 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 dutch 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 is running at a particular rotational speed and if it may be convenient to connect the internal combustion engine to the powertrain when the electric machine is running at that particular rotational speed.

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, which entails a great comfort of the driver and passengers of the vehicle, which save fuel and which also instantly may start the internal combustion engine for delivering torque to the powertrain.

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 is quick and efficient.

A further 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.

Another object of the invention is to achieve a method for starting an internal combustion engine in a parallel hybrid powertrain, which save fuel.

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) providing torque by the electric machine to the powertrain; b) detecting a first signal indicating that the internal combustion engine should be started; c) starting the internal combustion engine in response to the detected first signal; d) detecting a second signal indicating that the clutch should be controlled for connecting the internal combustion engine to the electric machine; and e) controlling the clutch in response to the detected second signal for connecting the internal combustion engine to the electric machine.

When the internal combustion engine is turned off 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 and the dutch is disengaged. The clutch is connected to a crank shaft 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 crank shaft is 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 crank shaft 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.

The first signal according to the invention indicates that the internal combustion engine should be started. There may be different reasons why such a first signal is generated. The electrical energy for the electric machine, which is stored in an energy storage may be empty or is at a low level. Another reason may be that a driver of the vehicle demands higher output torque to the driving wheels than may be delivered by the electric machine. In response to the detected first signal the internal combustion engine is started. However, depending on the rotational speed of the electric engine and the next coming driving conditions of the vehicle it may be more and less suitable to connect the internal combustion engine to the powertrain. If the rotational speed of the electric machine is increased due to shifting of gears in the gearbox it is most likely that the rotational speed of the electric machine is decreased during shifting of gears. Thus, it is not suitable to connect the internal combustion engine to the powertrain when the rotational speed of the electric machine has been increased, because the rotational speed of the internal combustion engine will then increase from a low level to a high level in a short period of time. Thereafter, the rotational speed of the internal combustion engine will decrease to a low level due to shifting of gears. Such increase of the rotational speed of the internal combustion engine is not comfortable for the driver and passengers in the vehicle, especially the sound generated when increasing the rotational speed of the internal combustion engine, is not comfortable for the driver and passengers in the vehicle. Also, an undesired increase of fuel consumption may take place as a result of the increase of the rotational speed of the internal combustion engine.

In order to avoid these shortcomings a second signal is generated and detected, which indicates that the clutch should be controlled for connecting the internal combustion engine to the electric machine. The second signal may be generated after shifting of gears and when the rotational speed of the electric engine is low.

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

The inventors have realized that the starting of the internal combustion engine and connecting the internal combustion engine to the powertrain may take place at different occasions in time. The internal combustion engine may be started by means of a starter motor arranged at the internal combustion engine when the clutch is disengaged or by means of the electric machine when the clutch is partly engaged, as discussed above, and thereafter disengaged when the internal combustion engine has been started. However, the internal combustion engine may thereafter, at a suitable occasion in time, be connected to the powertrain in order to avoid inconvenient increase of the rotational speed of the internal combustion engine.

After step c) and before step d) the method may further comprise the step of: f) running the internal combustion engine on idle speed. When the first signal has been detected and the internal combustion engine has been started, the internal combustion engine may be running at idle speed until it is suitable to connect the internal combustion engine to the powertrain. Depending on the driving conditions of the vehicle, it may also be possible to increase the rotational speed of the internal combustion engine above the idle speed when waiting for the suitable moment to connect the internal combustion engine to the powertrain. A suitable moment to connect the internal combustion engine to the powertrain may be after shifting of gears in the gearbox, when the rotational speed of the electric machine has decreased to a rotational synchronizing speed between the internal combustion engine and the electric machine.

According to an aspect of the invention the internal combustion engine in step c) is started when the speed of the electric machine is predicted to substantially reach a rotational synchronizing speed between the internal combustion engine and the electric machine. It may be suitable to delay the start of the internal combustion engine even if the first signal is detected at a substantially earlier point of time. Thus, fuel may be saved. The point of time when the internal combustion engine is started may be when the speed of the electric machine is predicted to substantially reach a rotational synchronizing speed between the internal combustion engine and the electric machine. During shifting of gears in the gearbox, the rotational speed of the electric machine will be different from the rotational speed before shifting of gears. The rotational speed of the electric machine after shifting of gears may be predicted when having information of the gear ratio in the gearbox. Also, the time when the electric machine reaches the rotational speed after shifting of gears may be predicted. The rotational synchronizing speed between the internal combustion engine and the electric machine should be the rotational speed of the electric machine after shifting of gears. Also, it may be suitable to connect the internal combustion engine to the powertrain shortly after the internal combustion engine has been started.

According to an aspect of the invention the internal combustion engine is in step c) started by means of a starter motor arranged at the internal combustion engine. When starting the internal combustion engine by means of the starter motor the clutch is open. Thus, the internal combustion engine may be started independent of the condition of the electric machine or the driving condition of the vehicle.

According to an aspect of the invention the internal combustion engine is started by controlling the clutch in step c) such that it provides a clutch torque generated by the electric machine. The clutch may be controlled so that it reaches a partly engaged state corresponding to the clutch torque required 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 dutch 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.

According to an aspect of the invention the clutch is disengaged when the internal combustion engine has been started. Since the speed of the electric machine may be different from the idle speed of the internal combustion engine the clutch is disengaged and the internal combustion engine runs independently of the electric machine and waits for a suitable moment to be connected to the powertrain.

According to an aspect of the invention 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 may therefore be increased with 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.

According to an aspect of the invention the first signal in step b) is generated when the torque provided by the electric machine cannot be maintained or is not enough for the driving characteristics demanded by the vehicle. The electrical power for the electric machine, which is stored in the energy storage may be empty or at a low level or the driver of the vehicle demands higher output torque to the driving wheels than may be delivered by the electric machine. There may also be other reasons why the internal combustion engine should be started. Under such conditions the first signal indicating that the internal combustion engine should be started is generated.

According to an aspect of the invention the second signal in step d) is generated when the torque provided by the electric machine cannot be maintained or is not enough for the driving characteristics demanded by the vehicle. In a situation when the electrical power in the energy storage is running out in a very short period of time, or when the driver of the vehicle immediately demands higher output torque to the driving wheels than may be delivered by the electric machine, the internal combustion engine may be connected to the powertrain immediately even though it may not be comfortable for the driver or the passengers in the vehicle.

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 another 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, and Figure 4b illustrates a diagram of torque variations 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 , via a propeller shaft 10.

Figure 2 schematically shows a parallel hybrid powertrain 2 of a vehicle 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 connected 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 in the gearbox 6. 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 4 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.

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 and drives 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 in the gearbox 6 and the clutch 12 is disengaged. When the electric machine 14 provides the output torque, the input shaft 17 rotates and the crank shaft 16 is standing still, i.e. not rotating. Controlling the dutch 12 such that a clutch torque Tc is provided means that the dutch 12 is controlled to a partly engaged state and thereby slips, whereby the crank shaft 16 of the internal combustion engine 4 will start rotating and the clutch torque Tc is thus transferred to the internal combustion engine 4. The provided clutch torque Tc will thus affect the rotational speed of the internal combustion engine 4. The provided clutch torque Tc for starting the internal combustion engine 4 may be gradually increased. 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. According to the invention the clutch 12 may be disengaged when the internal combustion engine 4 has started and be engaged later at a suitable moment. Alternatively, it is possible to start the internal combustion engine 4 by means of a starter motor 22 arranged at the internal combustion engine 4.

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) providing torque Te by the electric machine 14 to the powertrain 2; b) detecting a first signal S1indicating that the internal combustion engine 4 should be started; c) starting the internal combustion engine 4 in response to the detected first signal S1; d) detecting a second signal S2indicating that the dutch 12 should be controlled for connecting the internal combustion engine 4 to the electric machine 14; and e) controlling the clutch 12 in response to the detected second signal S2for connecting the internal combustion engine 4 to the electric machine 14.

In step a) the internal combustion engine 4 is turned off, i.e. not running, and the electric machine 14 solely provides torque Tein the powertrain 2 to the propeller shaft 10 and to the driving wheels 8 of the vehicle 1. 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 is rotating and the crank shaft 16 is standing still. In step b) the first signal S1according to the invention indicates that the internal combustion engine 4 should be started. There may be different reasons why the first signal is generated. For example, the electrical energy for the electric machine 14, which is stored in the energy storage 19 may be empty or is at a low level. Another reason may be that a driver of the vehicle 1 demands higher output torque to the driving wheels 8 than may be delivered by the electric machine 14. In step c) the internal combustion engine 4 is started in response to the detected first signal S1. The internal combustion engine 4 may be controlled to ignite when the internal combustion engine 4 has reached a predetermined rotational speed, suitably an idling speed. The control unit 18 may control the internal combustion engine 4 such that it ignites at a certain internal combustion engine rotational speed nee. The internal combustion engine 4 is thereby started. In step d) a second signal S2is detected, which indicates that the clutch 12 should be controlled for connecting the internal combustion engine 4 to the electric machine 14. The second signal may be generated after shifting of gears and when the rotational speed of the electric engine is low. In step e) the clutch 12 is controlled in response to the detected second signal S2for connecting the internal combustion engine 4 to the electric machine 14. In response to the detected first signal S1the internal combustion engine 4 is started. Depending on the rotational speed 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 rotational speed of the electric machine 14 is increased due to shifting of gears in the gearbox it is most likely that the rotational speed of the electric machine 14 is decreased during shifting of gears. Thus, it is not suitable to connect the internal combustion engine 4 to the powertrain 2 when the rotational speed of the electric machine 14 has been increased, because the rotational speed of the internal combustion engine 4 will then increase from a low level to a high level in a short period of time. Thereafter, the rotational speed of the internal combustion engine 4 will decrease to a low level due to shifting of gears. Such increase of the rotational speed of the internal combustion engine 4 is not comfortable for the driver and passengers in the vehicle 1. Also, an undesired increase of fuel consumption may take place as a result of the increase of the rotational speed of the internal combustion engine 4. In order to avoid these shortcomings a second signal S2is generated and detected, which indicates that the clutch 12 should be controlled for connecting the internal combustion engine 4 to the electric machine 14.

After step c) and before step d) the method further comprises the step of: f) running the internal combustion engine 4 at idle speed. When the first signal S1has been detected and the internal combustion engine 4 has been started, the internal combustion engine 4 may be running at a rotational speed that substantially corresponds to the idle speed nidleuntil it is suitable to connect the internal combustion engine 4 to the powertrain 2.

The internal combustion engine 4 is in step c) started when the speed of the electric machine 14 is predicted to reach a rotational synchronizing speed nsyncafter changing gear ratio in the gearbox 6. Also, it may be suitable to delay the start of the internal combustion engine 4 even if the first signal is detected earlier. Thus, fuel may be saved. The point of time when the second signal S2is generated may occur shortly after the internal combustion engine 4 has been started.

Thus, according to the invention the internal combustion engine 4 may be started immediately after receiving the first signal S1or the start of the internal combustion engine 4 may be delayed to start just prior to, or when the second signal S2is received.

According to an embodiment of the invention, the internal combustion engine 4 is in step c) started by means of the starter motor 22 arranged at the internal combustion engine 4. The clutch 12 is open when starting the internal combustion engine 4 by means of the starter motor 22. Thus, the internal combustion engine 4 may be started independent of the condition of the electric machine 14 or the driving condition of the vehicle 1.

According to a further embodiment of the invention the internal combustion engine 4 is in step c) started by controlling the clutch 12 such that it provides clutch torque Tc generated by the electric machine 14. The clutch 12 may be controlled so that it reaches a partly engaged state corresponding to the dutch torque Tc for starting the internal combustion engine 4. The provided clutch torque Tc depends on the degree of clutch engagement. The more engaged the clutch 12 is, the greater clutch torque Tc may be provided and transferred to the internal combustion engine 4. When the clutch 12 is completely engaged, the transferred torque to the internal combustion engine 4 is the same as the torque provided by the electric machine 14, less any torque provided to the drive wheels 8. The control unit 18 may control the clutch 12, such that the clutch torque Tc for starting the internal combustion engine 4 is provided.

The clutch torque Tcdepends on the degree of clutch engagement. The more engaged the dutch 12 is the greater clutch torque Tcmay be provided and transferred to the internal combustion engine 4. A certain dutch state thus corresponds to a certain clutch torque Tc. The control unit 18 suitably controls the clutch 12 such that a partly engaged state is achieved and the clutch 12 slips, whereby the crank shaft 16 of the internal combustion engine 4 will start rotating and a clutch torque Tcis thus transferred to the internal combustion engine 4. The provided clutch torque Tc will thus increase the speed of the internal combustion engine 4. The clutch torque Tc provided for starting the internal combustion engine 4 may be gradually increased.

The clutch 12 is disengaged when the internal combustion engine 4 has been started. Since the speed of the electric machine 14 may be different from the idle speed of the internal combustion engine 4 the clutch 12 is disengaged and the internal combustion engine 4 runs independently of the electric machine14 and waits for a suitable moment to be connected to the powertrain 2.

According to an aspect of the invention the electric machine 14 is 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 starting the internal combustion engine 4 by means of the electric machine 14, the clutch torque Tc and the torque Teprovided by the electric machine 14 are counteracting. When the clutch 12 slips, the crank shaft 16 will have a braking effect on the input shaft 17 connected to the electric machine 14. The torque Teprovided by the electric machine 14, i.e. the output torque on the propeller shaft 10 in this case, will thereby be reduced. The torque Teprovided by the electric machine 14 to 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.

The first signal S1in step b) is generated when the torque Teprovided by the electric machine 14 cannot be maintained or is not enough for the driving characteristics demanded by the vehicle 1. The electric power for the electric machine 14, which is stored in the energy storage 19 may be empty or at a low level or the driver of the vehicle 1 demands higher output torque to the driving wheels 8 than may be delivered by the electric machine 14.

The second signal S2in step d) may be generated when the torque Teprovided by the electric machine 14 cannot be maintained or is not enough for the driving characteristics demanded by the vehicle 1. The second signal S2in step d) may alternatively be generated when a suitable rotational speed is provided by the electric machine 14 to the input shaft 17 of the gearbox 6, e.g. when the rotational speed to the input shaft 1 7 is lowered in connection with shifting gears in the gearbox 6, as discussed above and below.

In a situation that the electrical power in the energy storage 19 is running out in a very short period of time or when the driver of the vehicle 1 immediately demands higher output torque to the driving wheels 8 than may be delivered by the electric machine 14 the internal combustion engine 4 may be connected to the powertrain 2 immediately even though it may not be comfortable for the driver or the passengers in the vehicle 1.

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. The method for starting an internal combustion engine 4 is described in Figure 3 and is here further illustrated by the diagrams over the speed variations and torque variations 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 according to the prior art, and which should be avoided according to the present invention. The lower dotted curve in fig. 4a represents the rotational speed of the internal combustion engine 4 when started shortly after the first signal S1is detected. This will occur at time t1indicated in fig. 4a. The lower fat marked curve in fig. 4a represents the rotational speed of the internal combustion engine 4 when the start is delayed after the first signal S1is detected.

If the internal combustion engine 4 is started and connected to the powertrain 2 shortly after the first signal S1has been detected, and if the rotational speed of the electric machine 14 is increased due to shifting of gears in the gearbox 6, it is likely that the rotational speed of the electric machine 14 is decreased during shifting of gears. Thus, it is not suitable to connect the internal combustion engine 4 to the powertrain 2 when the rotational speed of the electric machine 14 has been increased, because the rotational speed of the internal combustion engine 4 will then increase from a bw level to a high level in a short period of time. Thereafter, the rotational speed of the internal combustion engine 4 will decrease to a low level due to shifting of gears. Such increase of the rotational speed of the internal combustion engine 4 is not comfortable for the driver and passengers in the vehicle 1 due to the sound generated by the internal combustion engine 4. Also, an undesired increase of fuel consumption may take place as a result of the increase of the rotational speed of the internal combustion engine 4.

When the first signal Si at time ti has been detected and the internal combustion engine 4 has been started, the internal combustion engine 4 may be running on idle speed nidleuntil it is suitable to connect the internal combustion engine 4 to the powertrain 2. Depending on the rotational speed 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 rotational speed of the electric machine 14 is increased due to shifting of gears in the gearbox it is most likely that the rotational speed of the electric machine 14 is decreased during shifting of gears. Thus, it is not suitable to connect the internal combustion engine 4 to the powertrain 2 when the rotational speed of the electric machine 14 has been increased, because the rotational speed of the internal combustion engine 4 will then increase from a low level to a high level in a short period of time. Thereafter, the rotational speed of the internal combustion engine 4 will decrease to a low level due to shifting of gears, in order to avoid these shortcomings a second signal S2is generated and detected, which indicates that the clutch 12 should be controlled for connecting the internal combustion engine 4 to the electric machine 14. The second signal S2may be generated when a suitable rotational speed is provided by the electric machine 14 to the input shaft 17 of the gearbox 6, e.g. when the rotational speed to the input shaft 17 is lowered in connection with shifting gears in the gearbox 6.

Alternatively, the second signal S2may be generated when the torque Teprovided by the electric machine 14 cannot be maintained or is not enough for the driving characteristics demanded by the vehicle 1.

The starting of the internal combustion engine 4 may alternatively be delayed to a suitable moment for the starting of the internal combustion engine 4. This suitable moment occurs at time t2, which is shown in figure 4a. The lower fat marked curve represents the rotational speed nbof the internal combustion engine 4 when the start is delayed after the first signal S1is detected. The internal combustion engine 4 may be connected to the powertrain 2 when the rotational speed of the electric machine 14 is substantially equal to a rotational synchronizing speed nsyncafter changing gear ratio in the gearbox 6. This will occur at time t3in fig. 4a. It may be suitable to delay the start of the internal combustion engine 4 even if the first signal S1is detected at the earlier time ti. Thus, fuel may be saved. The time f2when the internal combustion engine 4 is started may be before shifting gears in the gearbox 6 is predicted to take place and when the rotational speed of the electrical machine 14 is lowered in connection with the shifting of gears in the gearbox 6. Also, it may be suitable to connect the internal combustion engine 4 to the powertrain 2 shortly after the internal combustion engine 4 has been started. The second signal S2, which is generated shortly after the internal combustion engine 4 has been started indicates that the clutch 12 should be controlled for connecting the internal combustion engine 4 to the electric machine 14.

Figure 4b shows a diagram of the torque variations during the method for starting an internal combustion engine 4 according to the invention. The slim solid cun/e in fig. 4b represents the torque provided by the electric machine 14. The lower dashed curve in fig. 4b represents torque provided by the internal combustion engine 4 which should be avoided. The dotted curve in fig. 4b represents torque provided by the internal combustion engine 4 when started shortly after the first signal S1is detected. This will occur at time t1indicated in fig. 4b. The torque generated by the internal combustion engine 4 is substantially steady when the internal combustion engine 4 is running at idle speed nidle. When the second signal S2is generated and detected, the clutch 12 is controlled for connecting the internal combustion engine 4 to the electric machine 14. The torque generated by the internal combustion engine 4 follows the fat marked curve in fig. 4b after the second signal S2has been generated.

The fat marked curve in fig. 4b represents also torque provided by the internal combustion engine 4 when the start is delayed after the first signal S1is detected. At f2, when the internal combustion engine 4 is started, the torque generated by the internal combustion engine 4 increases up to the point of time when the clutch 12 is controlled at S2. Again, the torque generated by the internal combustion engine 4 follows the fat marked curve in fig. 4b after the second signal S2has been generated. The curve combined with dashes and dots represents the torque provided by the electric machine 14 when the internal combustion engine 4 is started at t2.

A target torque Ttargetis indicated with the horizontal upper dashed line, which has a step between t2and t3in fig. 4b. However, the target torque Ttargetcan also be substantially equal to the torque before shifting gears.

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 comprises the steps of: a) providing torque (Te) by the electric machine (14) to the powertrain (2); b) detecting a first signal (S1) indicating that the internal combustion engine (4) should be started; c) starting the internal combustion engine (4) in response to the detected first signal (S1); d) detecting a second signal (S2) indicating that the clutch (12) should be controlled for connecting the internal combustion engine (4) to the electric machine (14); and e) controlling the clutch (12) in response to the detected second signal (S2) for connecting the internal combustion engine (4) to the electric machine (14).

2. The method according to claim 2, wherein after step c) and before step d) the method further comprises the step of: f) running the internal combustion engine (4) at idle speed (nidle).

3. The method according to claim 1, wherein the internal combustion engine (4) in step c) is started when the speed of the electric machine (14) is predicted to reach a rotational synchronizing speed (nsync) after changing gear ratio in the gearbox (6).

4. The method according to any of the preceding claims, wherein in step c) the internal combustion engine (4) is started by means of a starter motor (22) arranged at the internal combustion engine (4).

5. The method according to claim 4, wherein the clutch is open during the internal combustion engine (4) is started in step c).

6. The method according to any of claims 1 - 3, wherein in step c) the internal combustion engine (4) is started by controlling the clutch (12) such that it provides a clutch torque (Tc) generated by the electric machine (14) for starting the internal combustion engine (4).

7. The method according to claim 6, wherein the clutch (12) is disengaged when the internal combustion engine (4) has started.

8. The method according to claim 6, wherein the electric machine (14) is controlled such that the torque (?e) 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).

9. The method according to claim 8, wherein the clutch torque (Tc) and the torque (Ts) provided by the electric machine (14) are counteracting.

10. The method according to any of claims 6 - 9, 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).

11. The method according to any of the preceding claims, wherein the first signal (S1) in step b) is generated when the torque (Te) provided by the electric machine (14) cannot be maintained or is not enough for the driving characteristics demanded by the vehicle (1).

12. The method according to any of the preceding claims, wherein the second signal (S2) in step d) is generated when the torque (Te) provided by the electric machine (14) cannot be maintained or is not enough for the driving characteristics demanded by the vehicle (1).

13. The method according to any of claims 1 - 11, wherein the second signal (S2) in step d) is generated when the speed of the electric machine (14) is predicted to reach a rotational synchronizing speed (nsync) after changing gear ratio in the gearbox (6).

14. 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 1-13.

15. A computer program (P), wherein said computer program comprises programme 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 -13.

16. A computer programme product comprising a programme code stored on a computer-readable medium for performing the method steps according to any of claims 1 -13, when said computer programme is run on an electronic control unit (18) or a computer (20) connected to the electronic control unit (18).