EP2590847A1

EP2590847A1 - Energy control system and method for a hybrid vehicle

Info

Publication number: EP2590847A1
Application number: EP11803886.8A
Authority: EP
Inventors: Tomas Selling; Karl Redbrandt
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2010-07-08
Filing date: 2011-06-17
Publication date: 2013-05-15
Also published as: WO2012005655A1; EP2590847A4; SE1050761A1; CN103003117A; BR112012031467A2; SE535514C2

Abstract

Energy control system for a hybrid vehicle with at least one electrical machine and a combustion engine and at least one chargeable energy store, which system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store. The control unit comprises a calculation unit adapted to calculating at a time t₀, inter alia on the basis of the vehicles current speed and weight, a time t₁ which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P, and the control unit is adapted to controlling the offtake of energy from the energy store during the period from t₀ to t₁ so that the charge level of the energy store at time t₁ is below a predetermined low level Q_L.

Description

Title

Energy control system and method for a hybrid vehicle Field of the invention

The present invention relates to an energy control system and a method for a hybrid vehicle, according to the preambles of the independent claims.

Background to the invention

One of the greatest challenges in the heavy vehicle industry is to reduce fuel consumption. Fuel costs represent about 30% of a heavy truck's lifecycle cost. Average distance travelled is about 150,000 km per annum and average fuel consumption is about 32.5 litres per 100 km. A small decrease in fuel consumption therefore results in large decreases in fuel costs. A good way of saving fuel is to regenerate brake energy and employ that energy for propulsion when needed, instead of merely converting the kinetic energy to heat by applying conventional brakes. This is possible by using a hybrid vehicle instead of a conventional vehicle.

A hybrid vehicle is a vehicle with at least two energy sources. For example, an internal combustion engine may be backed up by an electrical machine. The electrical machine may be used as both motor and generator. This makes it possible to treat the electrical machine as a means of reducing the vehicle's speed by using it as a generator, in which case the kinetic energy is used to induce a current which is then used to charge the battery, making it possible for the energy to be saved and employed later, instead of the kinetic energy being converted to heat by applying the conventional brake equipment. In driving situations which involve high fuel consumption, it can be greatly reduced by using the electric motor to back up the engine. Such situations typically occur during acceleration and on uphill runs.

There are various kinds of electric hybrid systems, e.g. series hybrid, parallel hybrid and the combination of them known as power split system.

In a series hybrid system such as illustrated in Figure 1 , the engine drives an electrical generator instead of directly driving the vehicle's wheels. The generator not only charges a battery but also provides energy for an electric motor to propel the vehicle. When large amounts of energy are needed, the engine takes it from both battery and generator.

In parallel hybrid vehicles, the engine and an electrical machine which is used as both generator and motor are mechanically connected via engine shafts. An example of a parallel hybrid system is depicted in Figure 2. The connection may be situated between the engine and the electrical machine, making it possible to run the vehicle purely electrically. As the engine and the electrical machine rotate at exactly the same speed (when the connection is operative), they complement one another and run in parallel. When hybrid systems are to be implemented for buses, a series hybrid system is often used. An urban bus is braked to a halt many times per day.

An important aspect as regards energy saving is maximum utilisation of the energy which is regenerated during braking. There needs to be capacity in the energy store to accommodate the regenerated energy.

This entails having to lower the energy level in the energy store in good time before braking is initiated.

According to the systems currently used, the length of time or distance to the next stopping place is not known, which means that the energy store may well be discharged too slowly, leaving no capacity to accommodate the energy expected to enter the energy store during the next braking. Acceleration under the driving technique currently often applied empties the energy store quickly, resulting in large internal resistive losses. One reason for this is a desire to ensure that the energy store is at a predetermined low level when a coming retardation phase is due to begin, i.e. to ensure that there is sufficient storage capacity for the energy which is then generated.

For buses, so-called supercapacitors are preferably used for energy storage. The advantage of a capacitor as against a battery is that it tolerates a large number of repeated chargings in a short time such as often happen with buses.

For an energy store (capacitors, batteries, etc.) the following relationships generally apply:

U = C x Q

in which U is voltage, C capacitance and Q charge. P = U x I

in which P is the power and I the current.

Pioss = R x l²

in which Pi_oss is the power loss in the energy store and R the internal resistance.

Thus the power losses of the energy store increase with the square of the current, which means that a large current offtake, e.g. during an acceleration phase, is negative from an energy perspective.

The patent specifications listed below refer to various systems and devices within the field of energy regeneration for hybrid vehicles.

US 6,414,401 refers to a control system pertaining to energy regeneration in a hybrid vehicle so that sufficient energy can be stored by regeneration during retardation of the vehicle.

US 2007/0018608 refers to a device for control of the charging of a battery in a hybrid vehicle in order to be able to limit the amount of battery charging during the regeneration phase. US 7,242,159 also refers to a device for controlling the charging of batteries and/or capacitors in a hybrid vehicle.

An object of the present invention is to achieve improved and more optimum energy use for a hybrid vehicle, in order in particular to keep down the internal resistive losses in the capacitors/batteries.

Summary of the invention

The above objects are achieved with the invention defined by the independent claims.

Preferred embodiments are defined by the dependent claims. The invention relates to an energy control system for a hybrid vehicle with at least one electrical machine and a combustion engine and at least one chargeable energy store, which system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store. The control unit comprises a calculation unit adapted to calculating at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P, and the control unit is adapted to controlling the offtake of energy from the energy store during the period from to to ti so that the charge level of the energy store at time ti is below a predetermined low level Q_L.

The invention relates also to a method in an energy control system for a hybrid vehicle, which system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store. The method comprises:

A) calculating at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P,

B) controlling the offtake of energy from the energy store during the period from to to ti so that the charge level of the energy store at time ti is below a predetermined low level Q_L.

According to an important aspect of the invention, the distance to the next stopping place is calculated or determined from, for example, the vehicle builder/operator's paging system, e.g. a so-called bus PC system or the like, or the distance is calculated on the basis of information from the bus PC about the distance between the two bus stops concerned. Information from GPS or the like might also be used.

Applying the present invention makes it possible to lower the energy store level more quickly or in a different way than might otherwise have been applied, before a bus stopping place, in order to gain capacity for the portion of the vehicle's kinetic and potential energy which is expected to be desired to be added to the energy store. Thus knowing the distance to the next scheduled halt at a stopping place and the vehicle's speed makes it possible to calculate when the retardation phase will begin. It also means knowing when the charge level needs to be down to a predetermined low level. The fact that a halt at a stopping place is actually happening becomes known only when the stop button is pressed, which is the time at which the calculations are made.

Brief description of the drawings

Figure 1 illustrates schematically a series hybrid system for a vehicle.

Figure 2 illustrates schematically a parallel hybrid system for a vehicle.

Figure 3 is a block diagram illustrating the present invention.

Figure 4 is a flowchart illustrating the present invention.

Figure 5 is a time diagram illustrating the present invention.

Detailed description of preferred embodiments of the invention

Figure 3, a block diagram illustrating the present invention, will now be described in detail.

The invention thus relates to an energy control system for a hybrid vehicle, which vehicle comprises at least one electrical machine, at least one combustion engine and at least one chargeable energy store.

The hybrid vehicle may be a series or parallel hybrid vehicle or a combination of them.

The energy control system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store. The control unit itself comprises a calculation unit adapted to calculating at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P (see Figure 5). Figure 5 is a time diagram illustrating the present invention. At the top it depicts a vehicle, in this case a bus, on a run between stopping places P_A and P_B. Below there are two time diagrams showing how the charge level Q of the energy store varies depending on how far the vehicle has travelled along the run. The upper time diagram illustrates schematically how the charge level varies according to a currently usual pattern, and the lower time diagram how the charge level of the energy store varies in a vehicle using the energy control system according to the present invention.

During the retardation phase (RET.), the charge level is raised from a low level Q_L, which in the diagram is about 25% of maximum charge level, by the energy which is

regenerated. This portion of the charge curve is similar in the two cases depicted. During the acceleration phase (ACC), the charge level may for example be lowered, as in the upper time diagram, by a large offtake of energy from the energy store.

To reduce the internal losses of the energy store, the control unit is adapted, according to a preferred embodiment, to controlling the offtake from the energy store during the vehicle's acceleration phases so that the internal losses of the energy store are minimised. This is illustrated in the lower diagram in Figure 5, where the charge level between P_A and a location Al (which marks the end of the acceleration phase) remains at a high level.

At time to the calculation unit receives an indication that the vehicle is to halt at location P_B. The indication may for example be provided by the stop button in the bus being pressed and a stop signal being generated and conveyed to the control unit.

The calculation unit then, as mentioned above, calculates, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked to a halt at a predetermined location P, and the control unit is adapted to controlling the offtake from the energy store during the period from to to ti so that the charge level of the energy store at time ti is below a predetermined low level Q_L. This is clearly illustrated in the lower diagram, in which the charge level drops to a low level Q_L.

According to an embodiment, the low charge level Q_L is between 20 and 35% of the energy store's maximum charge level. A preferred level is 25%, as also indicated in Figure 5. During the period up to the beginning of the retardation phase at a location A2, offtake from the energy store is prioritised so that the charge level is lowered to a low level QL. This is done primarily by using electrical machine to power the vehicle, but the energy might also conceivably be used for other purposes, e.g. it may be more advantageous to run auxiliary systems etc. during the period.

According to a preferred embodiment, the control unit is adapted to controlling the energy offtake during the vehicle's retardation phases so that offtake from the engine is prioritised as against offtake from the electrical machine.

According to a further preferred embodiment, the control unit is adapted to controlling the offtake from the energy store during the vehicle's retardation phase so that there is little offtake from the energy store and the charge level at the end of the retardation phase is above a predetermined high level QH.

According to an embodiment, the predetermined high level QH is between 70 and 100% of the energy store's maximum charge level. A preferred level is 80%, as also indicated in Figure 5.

The energy store is thus charged during the retardation phase.

Citing figures for the percentage levels of QL and QH is difficult in that they depend on the vehicle's current speed and weight, the size of the energy store and the performance of the hybrid components. Even where performance and energy store size are constant, QL and QH still depend on the vehicle's current speed and weight.

If for example the vehicle is accelerated to a speed x, it is necessary to ensure that there is capacity in the energy store for the kinetic energy which it is possible to regenerate when the driver begins braking to a halt. If the speed is increased to 2x, the kinetic energy increases by a factor of 4, so more energy store capacity is needed when the driver begins braking to a halt. It is nevertheless not certain that there will be four times as much capacity in the energy store, since components may be power-limited, making it impossible to regenerate all of the extra kinetic energy. In cases where the energy store is quite small, this means that the percentage limits are modified quickly by, for example, change of vehicle speed. Even when braking from 40 km/h, the energy level needs to be about 25% to make it possible to capture all of the energy which can be regenerated. If the vehicle speed is "only" 20 km/h, a not unreasonable cruising speed for an urban bus in traffic, then on the above reasoning the kinetic energy will only be a quarter of that at 40 km/h. In such cases the limit for QL is rather 80%, since there is no further energy which can be regenerated during braking. If the energy store level was then 25%, this is certainly too low and the energy store level would be about 40%> when the vehicle was stationary.

The ranges indicated above for Q_L and QH should be regarded as preferred examples cited by way of illustration, but it will generally be the case that QL and QH are calculated on the basis inter alia of the vehicle's speed and weight according to the above reasoning. The control unit comprises, according to a preferred embodiment, a memory unit in which predetermined locations for vehicle halts, such as stopping places, are stored, e.g. in the form of an electronic card. The location of the vehicle relative to the stopping places is then easy to determine, since time and vehicle speed are known. An alternative is to use various types of positioning systems, e.g. GPS, whereby the vehicle's current location obtained via GPS can be charted against an electronic map image and the distance to the next stopping place can then be calculated.

As mentioned above, the energy store takes preferably the form of one or more capacitors, and so-called supercapacitors are often used.

The invention comprises also a method in a system for a hybrid vehicle with an electrical machine, a combustion engine and a chargeable energy store, which system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store.

With reference to Figure 4, the method comprises: A) calculating, at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P,

B) controlling the offtake of energy from the energy store during the period from to to ti so that the charge level of the energy store at time ti is below a predetermined low level QL.

According to a variant, step A is performed if a stop signal is received which indicates a demand for the vehicle to halt. The step A calculation is preferably done continuously, i.e. not as a consequence of, for example, a button being pressed, and the value of ti used is that which applies when a button is pressed, i.e. at time t₀. The energy store is then charged during the retardation phase.

Preferably, the offtake from the energy store is also controlled during an acceleration phase of the vehicle so that the internal losses of the energy store are minimised. This may for example be done by controlling the vehicle's energy offtake during an

acceleration phase so that offtake from the engine is prioritised as against offtake from the electrical machine. More specifically, this may be done by the offtake from the energy store being controlled so that there is little offtake from it during the acceleration phase and the charge level at the end of the acceleration phase is above a predetermined high level QH.

The following is an example of an application of the present invention, inter alia with reference to Figure 5.

One of the bus stopping places in Figure 5 is at location P_B. Let us assume that the driver brakes fairly equally each time he brakes to a halt and that the minimum limit for normal retardations is at A2, i.e. the earliest location where the vehicle has to start braking if braking is to meet passenger comfort requirements. There is a maximum permissible retardation during the retardation phase, determined inter alia with reference to driver and passenger comfort. It is approximately of the order of 2 m/s². With the retardation limit as above, the shortest expected distance from commencement of braking to stationary is calculated for the vehicle's speed at the time, i.e. how far the bus has travelled between to and ti. The time available for gaining capacity for more energy, i.e. to to ti, then becomes the remaining distance divided by the vehicle's speed at the time.

The advantage of the solution according to the invention is that in certain driving situations it is possible to save more fuel by tapping more energy from the energy store. Knowing how much time there is until braking before a stopping place begins means knowing better how much time is available for gaining capacity for the energy.

Even if the stop button is not pressed, the calculations are preferably performed so that the offtake of energy takes place in such a way that the charge level is at a predetermined low level when any retardation begins, e.g. when there are people at the stopping place waiting to board the bus.

With a modified altered driving strategy whereby there is not maximum utilisation of the energy store but the engine is allowed to run instead, the internal power losses in the energy store (the capacitor/battery) will be lower than when the electrical machine is used to the maximum during the retardation phase, resulting in a more energy-efficient system.

The present invention is not confined to the preferred embodiments described above. Sundry alternatives, modifications and equivalents may be used. The above embodiments are therefore not to be regarded as limiting the invention's protective scope which is defined by the attached claims.

Claims

1. An energy control system for a hybrid vehicle with at least one electrical machine and a combustion engine and at least one chargeable energy store, such that the system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store, and the control unit comprises a calculation unit adapted to calculating at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P,

c h a r a c t e r i s e d in that the control unit is adapted to controlling the offtake of energy from the energy store during the period from to to ti so that the charge level of the energy store at time ti is below a predetermined low level Q_L.

2. An energy control system according to claim 1, in which the control unit is adapted to receiving a stop signal which indicates a demand for the vehicle to halt, and the calculation of time ti takes place when the stop signal has been received.

3. An energy control system according to claim 1 or 2, in which the control unit is adapted to controlling the offtake of energy from the energy store during a retardation phase of the vehicle so that the internal losses of the energy store are minimised.

4. An energy control system according to any one of claims 1-3, in which the control unit is adapted to controlling the energy offtake during a retardation phase of the vehicle so that offtake from the engine is prioritised as against offtake from the energy store.

5. An energy control system according to any one of claims 1-4, in which the control unit is adapted to controlling the energy offtake during a retardation phase of the vehicle so that there is little offtake from the energy store and the charge level at the end of the retardation phase is above a predetermined high level Q_H.

6. An energy control system according to any one of claims 1-5, in which the energy store is charged during the retardation phase.

7. An energy control system according to any one of claims 1-6, in which the control unit comprises a memory unit in which predetermined locations for vehicle halts are stored.

8. An energy control system according to any one of claims 1-7, in which said energy store comprises one or more capacitors.

9. A system according to claim 5, in which said predetermined low level Q_L and high charge level Q_H are calculated on the basis inter alia of the vehicle's speed and weight.

10. A method in an energy control system for a hybrid vehicle with an electrical machine, a combustion engine and a chargeable energy store, which system comprises a control unit and a charge level meter which is adapted to measuring the charge level of the energy store, comprising the step of

A) calculating, at a time to, inter alia on the basis of the vehicle's current speed and weight, a time ti which denotes the beginning of a retardation phase during which the vehicle is braked in order to halt at a predetermined location P,

c h a r a c t e r i s e d in that the method comprises

1 1. A method according to claim 10, in which step A is performed if a stop signal is received which indicates a demand for the vehicle to halt.

12. A method according to claim 10 or 1 1 , in which the offtake of energy from the energy store during a retardation phase of the vehicle is controlled so that the internal losses of the energy store are minimised.

13. A method according to any one of claims 10-12, in which the energy offtake during a retardation phase of the vehicle is controlled so that offtake from the engine is prioritised as against offtake from the energy store.

14. A method according to any one of claims 10-13, in which the energy offtake during a retardation phase of the vehicle is controlled so that there is little offtake from the energy store and the charge level at the end of the retardation phase is above a

predetermined high level QH.

15. A method according to any one of claims 10-14, in which the energy store is charged during the retardation phase.