TECHNICAL FIELD

The present invention relates to a method for controlling fuel injection into a combustion chamber. The invention also relates to a control unit for controlling fuel injection into a combustion chamber. The invention is applicable on vehicles, in particularly heavy duty vehicles such as trucks or buses. Although the invention will mainly be described in relation to a truck, it is also applicable for other vehicles utilizing spray controlled combustion, such as cars or working machines, etc.

BACKGROUND

In relation to internal combustion engines, for example internal combustion engines for heavy duty vehicles such as trucks or buses; the demand on the internal combustion engines have been steadily increasing and engines are continuously developed to meet the various demands from the market. Reduction of exhaust gases, increasing engine efficiency, i.e. reduced fuel consumption, and lower noise level from the engines are some of the criteria that becomes an important aspect when choosing vehicle engine. Furthermore, in the field of trucks, there are applicable law directives that have e.g. determined the maximum amount of exhaust gas pollution allowable. Still further, a reduction of the overall cost of the vehicle is important and since the engine constitutes a relatively large portion of the total costs, it is natural that also the costs of engine components are reduced. In order to meet the described demands, various ways of controlling the internal combustion engine have been developed throughout the years.

US 7,007,664 relates to a fuel injection control system of an interna! combustion engine. More specifically, US 7,007,664 relates to a fuel injection control system for achieving improvement of e.g. gas mileage. The control system uses injection timing feedback control such that a cylinder pressure maximum value coincides with a target pressure value.

However, although the fuel injection control system in US 7,007,664 controls the cylinder pressure maximum level, there is still a need to further improve the thermal efficiency of the internal combustion engine, which in turn reduces e.g. fuel consumption.

SUMMARY

It is an object of the present invention to provide a method for controlling fuel injection into a combustion chamber which improves the thermal efficiency of the internal combustion engine compared to the prior art solutions. The object is at least partly achieved by the method according to claim 1. According to a first aspect of the present invention, there is provided a method for controlling fuel injection into a combustion chamber of a spray controlled internal combustion engine, the method comprising the steps of providing fuel into the combustion chamber with a first fuel rate during a first time interval; determining a pressure gradient in the combustion chamber during the first time interval; comparing the pressure gradient during the first time interval with a maximum allowable pressure gradient of the combustion chamber for a combustion cycle for determining a difference between the pressure gradient during the first time interval and the maximum allowable pressure gradient; estimating a second fuel rate as a function of the first fuel rate and the determined difference between the pressure gradient during the first time interval and the maximum allowable pressure gradient; and providing fuel into the combustion chamber, during a second time interval after the first time interval, with the estimated second fuel rate for reducing the difference between the pressure gradient in the combustion chamber and the maximum allowable pressure gradient.

The wording "fuel rate" should in the following be understood to mean that the fuel is injected into the combustion chamber of the internal combustion chamber with a specific fluid velocity and momentum. Hence, during the time interval of which fuel is injected with the specific fuel rate, a specific amount of fuel is injected into the combustion chamber.

Moreover, the determined pressure gradient in the combustion chamber should be understood to be a pressure increase over time within the combustion chamber. The step of determining the pressure gradient may for example include receiving measured pressure values over time for the combustion chamber. The determined pressure gradient may also be received virtually from a virtual engine model as will be described further below.

Furthermore, the wording "maximum allowable pressure gradient" should be understood as the maximum possible pressure gradient for the specific combustion chamber during the specific load cycle of the internal combustion chamber. For example, if the vehicle is driving up a steep slope, the maximum allowable pressure gradient may be different in comparison to the driving scenario of the vehicle driving down the steep slope. According to another example, the volume of the combustion chamber may determine the maximum allowable pressure gradient. Accordingly, it should be readily understood that the maximum allowable pressure gradient may be different for different cylinders as well as for different driving conditions of the vehicle. Examples of parameters controlling the maximum allowable pressure gradient are the strength and durability of the cylinder components, maximum allowable noise level, allowable amount of produced Ox, etc.

Still further, the step of estimating the second fuel rate as a function of the first fuel rate and the determined difference should be understood to mean that the rate of the second fuel rate is dependent on the first fuel rate and an estimation of whether to increase or decrease the fuel rate in relation to the first fuel rate based on the comparison between the maximum allowable pressure gradient and the determined pressure gradient after the first time interval. Hence, the second fuel rate may be increased or decreased relative to the first fuel rate. Also, the step of estimating the second fuel rate may be an iterative process until the pressure level in the

combustion chamber reaches a specific pressure limit, which will be described further below.

It shouid be readily understood that the invention is applicable for applications where fuel injection is continuously provided into the combustion chamber, as well as applications where the fuel is injected as multiple injections, i.e. injected into the combustion chamber at intervals. The former alternative, i.e. where fuel is injected continuously, can thus control the rate of the injected fuel to be changed and adapted continuously in order to reduce the difference between the pressure gradient in the combustion chamber and the maximum allowable pressure gradient. Advantages of the present invention are that the thermal efficiency of the internal combustion engine is utilized to its approximate optimum limits. The invention is based on the insight that by controlling the pressure gradient within the combustion chamber during a combustion cycle such that the pressure gradient is as close to the maximum allowable pressure gradient as possible will increase the thermal efficiency of the internal combustion engine. At the same time, the noise level of cylinders of the internal combustion engine will be within allowable limits. Accordingly, by controlling the pressure gradient of the combustion chamber during a combustion cycle will increase the thermal efficiency and keep the noise level acceptable.

According to an example embodiment, the method may further comprise the steps of receiving a maximum allowable pressure limit of the combustion chamber for the combustion cycle; and reducing the rate of provided fuel into the combustion chamber when a measured pressure of the combustion chamber is within a first predetermined pressure range from the maximum allowable pressure limit.

The "maximum allowable pressure limit" of the combustion chamber should be understood as the maximum possible pressure limit for the specific combustion chamber during the specific load cycle of the internal combustion chamber. By means of the same example as described above, if the vehicle is driving up a steep slope, the maximum allowable pressure limit may be different in comparison to the driving scenario of the vehicle driving down the steep slope. According to another example, the volume of the combustion chamber may determine the maximum allowable pressure limit. Accordingly, it should be readily understood that the maximum allowable pressure limit may be different for different cylinders as well as for different driving conditions of the vehicle. Accordingly, different maximum allowable pressure limits may be received depending on different engine parameters as well as engine operation parameters. Examples of parameters controlling the maximum allowable pressure limit are, for example, the strength and durability of the cylinder components, cylinder temperature, etc.

An advantage is that the thermal efficiency will be further increased since the internal combustion engine will operate the load cycle at a maximum limit. Hereby, the combustion chamber is controlled to operate at a maximum limit. Furthermore, the provided fuel rate can be reduced when the pressure in the combustion chamber is within the first predetermined pressure range from the maximum allowable pressure limit, i.e. before the maximum allowable pressure is reached, since the inventors have realized that the pressure in the combustion chamber will continue to increase during a time interval after the fuel rate is reduced. The first predetermined pressure range may be different depending on the specific combustion chamber as well as the specific load cycle. The first predetermined pressure range may, for example, be zero or close to zero, such that a reduction of the fuel rate is not executed until the maximum allowable pressure limit is reached.

According to an example embodiment, the method may further comprise the step of providing fuel into the combustion chamber with a third fuel rate when the measured pressure of the combustion chamber is within a second predetermined pressure range from the maximum allowable pressure limit.

The second predetermined pressure range should be understood as a range in which it is considered that the pressure in the combustion chamber is at the maximum limit. The second predetermined pressure range may hence be zero, or close to zero, such that when the pressure in the combustion chamber is within the second predetermined pressure range, the pressure is at the maximum allowable pressure limit.

As described above, the fuel rate is reduced when the pressure in the combustion chamber is within the first predetermined pressure range from the maximum allowable pressure limit. When the pressure in the combustion chamber is within the second predetermined pressure range from the maximum allowable pressure limit, the pressure is at a maximum limit. By providing fuel with the third fuel rate when the pressure in the combustion chamber is at its maximum level will keep the pressure within the combustion chamber at the maximum level for a further duration of time. Accordingly, the pressure level in the combustion chamber is kept at the maximum allowable limit for an increased time interval which further increases the thermal efficiency of the internal combustion engine.

According to an example embodiment, the combustion cycle may be configured to utilize a predetermined total amount of fuel and wherein the total amount of fuel is provided into the combustion chamber after the provision of fuel with the third fuel rate is provided.

Hereby, all fuel for the combustion cycle is supplied into the combustion chamber of the internal combustion engine after the provision of fuel with the third fuel rate is provided. An advantage is that the amount of fuel for the combustion cycle is optimally supplied to the internal combustion chamber.

According to an example embodiment, the rate of provided fuel into the combustion chamber may be reduced to zero when the measured pressure of the combustion chamber is within the first predetermined pressure range from the maximum allowable pressure limit.

Hereby, the supply of fuel to the combustion chamber is shut off for a short duration of time.

According to an example embodiment, the step of determining the pressure gradient in the combustion chamber after the first time interval may be executed by measuring the increase in pressure during the first time interval.

Hereby, a pressure sensor may be used for measuring the increase in pressure. An advantage is that an accurate and reliable measured value is received.

According to an example embodiment, the maximum allowable pressure gradient may be a linear pressure gradient. An advantage of using a linear pressure gradient is that it is a well defined gradient that is relatively simple to correlate to. tn other words, using a linear pressure gradient simplifies the step of estimating the second fuel rate as a function of the first fuel rate and the determined difference between the pressure gradient during the first time interval and the maximum allowable pressure gradient. However, the maximum allowable pressure gradient may be provided in other geometric shapes and according to an example embodiment, the maximum allowable pressure gradient may be a parabolic pressure gradient.

According to an example embodiment, the step of providing fuel into the combustion chamber may be executed by means of pulse injection. An advantage is that it is easier to increase/decrease the pressure gradient in the combustion chamber. Hence, the pressure gradient in the combustion chamber may be increased/decreased relatively quickly by using pulse injection. Other type of injectors may however also be applicable for the present invention. The main criteria for the injectors are that they can relatively easily and quickly change rate of injection of fuel. A typical injection type is an injector using so-called rate shaping.

According to an example embodiment, a unit injector may be provided for injection of fuel into the combustion chamber.

According to an example embodiment, the method may be provided for a virtual model based internal combustion engine and the steps of providing fuel into the combustion chamber is executed virtually.

Hereby, the internal combustion engine can be pre-calibrated since the load cycles have already been virtually executed. An advantage is that the interna! combustion engine is pre-programmed such that the fuel rates are determined in advance for a specific combustion cycle.

According to an example embodiment, the estimated second fuel rate for the load cycle may be stored in a memory unit.

According to an example embodiment, the memory unit may be providing the estimated second fuel rate to a look-up table, for generating a look-up table comprising estimated fuel rates for different load cycles.

The advantage is that a rapid process is achieved in which a control unit of the internal combustion can control the injector to provide fuel into the combustion chamber with a fuel rate that is recommended from the look-up table for a specific driving condition.

According to a second aspect of the present invention, there is provided a control unit for controlling fuel injection into a combustion chamber of a spray controlled internal combustion engine, the control unit being connected to a fuel injection system of the combustion engine, wherein the control unit is configured to provide fuef into the combustion chamber with a first fuel rate during a first time interval; determine a pressure gradient in the combustion chamber during the first time interval; compare the pressure gradient during the first time interval with a maximum allowable pressure gradient of the combustion chamber for a combustion cycle for determining a difference between the pressure gradient during the first time interval and the maximum allowable pressure gradient; estimate a second fuel rate as a function of the first fuel rate and the determined difference between the pressure gradient during the first time interval and the maximum allowable pressure gradient; and provide fuel into the combustion chamber, during a second time interval after the first time interval, with the estimated second fuel rate for reducing the difference between the pressure gradient in the combustion chamber and the maximum allowable pressure gradient. According to an example embodiment, the control unit may be further configured to provide the estimated second fuel rate to a memory unit.

According to an example embodiment, the memory unit may be configured to provide the estimated second fuel rate to a look-up table, such that a look-up table comprising estimated fuel rates for different load cycles is generated.

Further effects and features of the second aspect of the present invention are largely analogous to those described above in relation to the first aspect of the present invention,

According to a third aspect of the present invention, there is provided a vehicle comprising an internal combustion engine, wherein the internal combustion engine comprises a control unit as described above in relation to the second aspect of the present invention.

Effects and features of the third aspect of the present invention are largely analogous to those described above in relation to the first aspect of the present invention. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention,

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of an exemplary embodiment of the present invention, wherein:

Fig. 1 is a side view of a vehicle provided with an internal combustion engine utilizing the method according to an example embodiment of the present invention; Figs. 2a - 2b show an example embodiment of injection of fuel into the combustion chamber for controlling pressure gradient and the maximum pressure in the combustion chamber;

Fig. 3 is an example embodiment of an internal combustion engine comprising a control unit according to an example embodiment of the present invention; and

Fig. 4 is a flow chart of a method according to an example embodiment of the present invention. DETAIL DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which an exemplary embodiment of the invention is shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, the embodiment is provided for thoroughness and completeness. Like reference character refer to like elements throughout the description.

With particular reference to Fig. 1, there is provided a vehicle 1 with an internal combustion engine 100 according to an embodiment of the present invention. The vehicle 1 depicted in Fig. 1 is a truck for which the inventive internal combustion engine 100, which will be described in detail below, is particularly suitable for.

Turning now to Figs. 2a and 2b, there are depicted an example embodiment of injection of fuel into a combustion chamber of the internal combustion engine 100 for controlling the pressure gradient and the maximum pressure in the combustion chamber during an injection cycle. In further detail, Fig. 2a illustrates an example embodiment of the variation of fuel rate injection into the combustion chamber of the internal combustion engine 100. Hence, Fig. 2a illustrates how the fuel rate can differ during a combustion cycle according to an example embodiment of the present invention. Fig. 2b on the other hand depicts how the pressure varies in the combustion chamber during the combustion cycle. Figs. 2a and 2b should hence be read and studied together since they relate to the same combustion cycle. When the piston of the internal combustion engine 100 has reached a certain position within the cylinder, here illustrated as approximately 350 degrees, injection of fuel into the combustion chamber is initiated. The fuel is provided with a first fuel rate 102 during a first time interval 104. Hereby, a specific amount of fuel is provided into the combustion chamber, which specific amount of fuel is configured to be combusted. The provided amount of fuel with the first fuel rate 102 during the first time interval 104 gives rise to a pressure gradient 106 in the combustion chamber. More precisely, the pressure in the combustion chamber increases as the piston moves towards the top dead center of the combustion cylinder at 360 degrees. The pressure gradient 106 of the combustion chamber during the first time interval 104 is compared to a maximum allowable pressure gradient 108 of the combustion chamber. Hereby, a difference 110 between the actual pressure gradient and the maximum allowable pressure gradient is determined. In order to increase the thermal efficiency as much as possibie for the internal combustion engine, it is advantageous to provide a pressure gradient within the combustion chamber that is as close as possible to what is allowed, i.e. as close as possible to the maximum allowable pressure gradient 08.

Therefore, by means of the comparison made, a second fuel rate 12 is estimated. The estimated second fuel rate 112 in Fig. 2a is illustrated as being lower than the first fuel rate 102. However, based on the comparison, the estimated second fuel rate may equally as well be higher than the first fuel rate 102. When the second fuel rate 112 is estimated it is provided into the combustion chamber.

When the second fuel rate 112 is provided into the combustion chamber, the pressure gradient 106 can once again be compared to the maximum allowable pressure gradient 108 by determining a difference between the pressure gradient 106, when the second fuel rate is provided into the combustion chamber, and the maximum allowable pressure gradient 108. Although not depicted in Figs. 2a - 2b, the process of providing fuel rate and compare pressure gradient 106 in the combustion chamber with the maximum allowable pressure gradient 08 can continue until a measured pressure in the combustion chamber is within a first predetermined pressure range 116 from a maximum allowable pressure limit 114 within the combustion chamber. Furthermore, it should be noted that even though the measured pressure gradient 106 in Fig. 2b continuously follows the maximum allowable pressure gradient 108 relatively smoothly, the measured pressure gradient 106 may, before being controlled and adjusted, equally as well deviate from the maximum allowable pressure gradient 108 during the time interval when the piston moves towards the top dead center of the combustion chamber. For example, at some points in time the pressure gradient may be higher than the maximum allowable pressure gradient 108 and at some points it may be lower.

As the piston moves towards the top dead center of the combustion chamber, the pressure in the combustion chamber thus continuously increases. When the pressure is within a first predetermined pressure range 116 from the maximum allowable pressure limit 114, the fuel rate provided into the combustion chamber is reduced. In the depicted embodiment, the fuel rate is reduced to zero 122. However, the fuel rate does not have to be reduced to such a low amount, it may suffice that the provided fuel rate is reduced to an amount which is less than what was provided at an instant moment before the pressure in the combustion chamber reached the first predetermined pressure range 16.

As the piston continues to move towards the top dead center, the pressure in the combustion chamber is continuously increasing for a further period of time, despite the fact that the fuel rate is reduced. Hereby, the pressure in the combustion chamber will reach the maximum allowable pressure limit 114 of the combustion chamber. More precisely, the pressure in the combustion chamber will reach a limit where it is within a second predetermined pressure range 120 from the maximum allowable pressure limit 14, within which second predetermined pressure range 120 it is determined that the pressure in the combustion chamber is at the maximum limit.

The combustion chamber has at this point in time not been provided with its total amount of fuel for the specific combustion cycle. Therefore, in order to keep the pressure within the combustion chamber at the maximum allowable pressure limit 114, or at least within the second predetermined pressure range 120, for an increased time period, the remaining amount of fuel for the combustion cycle is provided into the combustion chamber. The remaining amount of fuel is provided into the combustion chamber with a third fuel rate 1 18. Hereby, the time interval of which the pressure in the combustion chamber is at, or near, the maximum allowable pressure limit is increased which thus further improves the thermal efficiency of the internal combustion engine.

In summary, during the initial provision of fuel into the combustion chamber, the pressure gradient within the combustion chamber is controlled to be as close to the maximum allowable pressure gradient as possible. Hence, at this point in time, the combustion process is controlled by means of controlling the pressure gradient in the combustion chamber. When the pressure in the combustion cylinder has reached a specific predetermined pressure limit, which is described above as the first predetermined pressure range 116 from the maximum allowable pressure limit 114, the fuel rate is reduced. After a certain time interval, the fuel rate of provided fuel into the combustion chamber is increased such that the total amount of fuel for the combustion cycle is provided to the combustion chamber and the pressure in the combustion chamber is kept at the maximum level for a further time interval. Hence, at this point in time, the combustion process is controlled by means of controlling the pressure limit in the combustion chamber.

Reference is now made to Fig. 3, which illustrates an example embodiment of an internal combustion engine 100 which is connected to a control unit 300 configured to execute the above described steps of the present invention. The control unit 300 is connected to the internal combustion engine 100 and to at least one sensor 302 configured to measure the pressure gradient 106 within the combustion chamber of the internal combustion engine 100. The control unit can further be programmed to also set the pressure limit within the combustion chamber of the internal combustion engine 100. The control unit 300 may, for example, be connected to a fuel injection system (not shown) of the internal combustion engine for controlling the fuel injection into the combustion chamber. The control unit 300 may however also be connected to other components which in turn is connected to the fuel injection system.

Furthermore, the at least one sensor is, as described above, arranged to measure the pressure gradient. Although the at least one sensor is depicted as connected to the exterior surface of the internal combustion engine 100, it should be understood that the sensor is arranged at a position of the engine 100 where it is possible to measure the different quantities desired. This position may, for example, be at the inside surface of the cylinder head, such that the sensor is facing the combustion chamber. In order to sum up, reference is finally made to Fig. 4 which is a flow chart of the method according to an example embodiment of the present invention.

Firstly, fuel is provided S1 into the combustion chamber. The provided fuel is provided with a first fuel rate 102 during a first time interval 104. A pressure gradient 106 in the combustion chamber is determined S2 during the first time interval 104. The pressure gradient 106 can either be determined during the first time interval 104 or after the first time interval has lapsed.

Thereafter, the determined pressure gradient 106 during the first time interval 104 is compared S3 to the maximum allowable pressure gradient 108 of the combustion chamber. Hereby, a difference between the actual pressure gradient in the combustion chamber and the maximum allowable pressure gradient is determined. A second fuel rate 112 is thereafter estimated S4. The second fuel rate is estimated as a function of the provided first fuel rate 102 and the determined difference between the pressure gradient after the first time interval and the maximum pressure gradient. The estimated second fuel rate 112 is provided S5 into the combustion chamber for reducing the difference between the actual pressure gradient and the maximum allowable pressure gradient 108. The maximum allowable pressure limit 114 is determined S6 and when the measured pressure in the combustion chamber is within the first predetermined pressure range 116 from the maximum allowable pressure limit 114, the rate of fuel provided into the combustion chamber is reduced S7. Finally, fuel is thereafter provided S8 into the combustion chamber with a third fuel rate 118. The third fuel rate 118 is an increase in fuel rate in comparison to the fuel rate provided after the step S7 of reducing the fuel rate into the combustion chamber. The third fuel rate 118 is, as also described above, provided in order to keep the pressure in the combustion chamber at the maximum allowable pressure limit for an extended period of time.

Although the above description has been directed to a physical internal combustion engine, it should be understood that the method is equally applicable for a virtual model of an internal combustion engine. Hereby, a plurality of load sessions can be virtually evaluated and the provided fuel rates for these load sessions can be stored in a look-up table or the like such that it can be implemented into a control unit of an internal combustion engine. Hereby, the control unit of the internal combustion engine is preprogrammed to provide specific fuel rates to the combustion chamber for a combustion cycle based on the specific load session of the vehicle.

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.