EP2754875A1

EP2754875A1 - Method of operating a combustion engine

Info

Publication number: EP2754875A1
Application number: EP13151306.1A
Authority: EP
Inventors: Vincenzo Rosito; Joachim Paul; Stefan Motz; Sebastian-Paul Wenzel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2013-01-15
Filing date: 2013-01-15
Publication date: 2014-07-16

Abstract

The invention relates to a method of operating a combustion engine (10), wherein an exhaust manifold pressure (p_em) within an exhaust manifold (12) is determined, characterized in that a cylinder pressure (p_c1) of at least one cylinder (13a) of said combustion engine (10) is determined (100) and in that said exhaust manifold pressure (p_em) is derived (110) from said cylinder pressure (p_c1).

Description

Prior Art

The present invention relates to a method of operating a combustion engine, wherein an exhaust manifold pressure within an exhaust manifold is determined.
The invention further relates to a corresponding control unit configured to operate a combustion engine.
Conventional systems and methods of this type employ a dedicated pressure sensor which is located within the exhaust manifold. Apart from being costly due to the extra sensor hardware, the conventional systems do not offer high precision pressure measurements especially in non-stationary operation conditions. Thus, when engine parameters such as e.g. load and/or rpm are rapidly changing, no precise pressure values for the exhaust manifold pressure may be obtained by the conventional systems. Moreover, the exhaust gas temperature affects the reliability of conventional exhaust manifold pressure sensors, which further reduces precision.
It is an object of the present invention to improve the prior art systems.

Disclosure of the Invention

The invention solves this object by a method according to claim 1. As well, the invention solves this object by a control unit according to claim 11.
The method according to the invention comprises operating a combustion engine wherein an exhaust manifold pressure within an exhaust manifold is determined.
According to the present invention, a cylinder pressure of at least one cylinder of said combustion engine is determined and said exhaust manifold pressure is derived from said cylinder pressure.
This advantageously enables to determine an exhaust manifold pressure of the combustion engine without the need of providing a dedicated exhaust manifold pressure sensor in the exhaust manifold. Rather, since the exhaust manifold pressure is derived from the cylinder pressure, the provisioning of at least one cylinder pressure sensor is sufficient for also determining said exhaust manifold pressure. Thus, existing cylinder pressure sensor(s) may advantageously be used to also assess the exhaust manifold pressure. A further advantage of the inventive principle is the fact that the exhaust manifold pressure, when being derived from the cylinder pressure, can be determined with a comparatively high sample rate. I.e., the inventive principle also enables to attain precise values for the exhaust manifold pressure even in non-stationary operation of the combustion engine, which is not possible with existing exhaust manifold pressure sensors that are mounted within the exhaust manifold.
According to an embodiment, a cylinder pressure of more than one cylinder of said combustion engine, preferably of all cylinders of said combustion engine, is determined. In this embodiment, the cylinder pressure values of a plurality of cylinders may be used for determining the exhaust manifold pressure, which ensures increased precision and reliability. I.e., even if due to some error the cylinder pressure of one specific cylinder cannot be determined any more, the cylinder pressure value(s) of further cylinder(s) may be used to perform the inventive determination of the exhaust manifold pressure depending on said cylinder pressure.
According to a further embodiment, said cylinder pressure is determined by means of at least one cylinder pressure sensor which is assigned to a respective cylinder of the combustion engine. Alternatively, the cylinder pressure could also be determined by a corresponding model which simulates the cylinder pressure, as long as the values of such model are sufficiently precise for also deriving the exhaust manifold pressure therefrom.
According to a further embodiment, said cylinder pressure sensor is configured to provide sample values of said cylinder pressure with a rate of about at least one sample per 1 degree of a crankshaft angle of said combustion engine. Thus, a sufficiently high temporal resolution (expressed in samples per time or crankshaft angle) for obtaining the cylinder pressure values is ensured.
According to a further embodiment, said cylinder pressure of a specific cylinder is determined during a phase in which an exhaust valve of said specific cylinder is open, and in which preferably also an intake valve of said specific cylinder is closed. In this phase, according to applicant's analysis, a particularly precise determination of the exhaust manifold pressure based on the cylinder pressure is enabled, since pressure variations due to gas exchange of the cylinder are comparatively low.
According to a further embodiment, said phase is determined such that a change over time of the cylinder pressure is smaller than a predetermined first threshold value. In other words, the "phase" in time for assessing the cylinder pressure in order to derive the exhaust manifold pressure therefrom is characterized by a rather "flat" cylinder pressure curve over time.
According to a further embodiment, said exhaust manifold pressure is determined by averaging said cylinder pressures of one or more cylinders over a predetermined time. I.e., on the one hand, various cylinder pressure values as obtained from one cylinder, i.e. during the abovementioned phase, may be averaged. On the other hand, various cylinder pressure values as obtained from several cylinders, i.e. during the abovementioned phase, may be averaged. It is also possible to combine single cylinder pressure values of several sensors to perform averaging in the sense of the present embodiment.
According to a further embodiment, a pressure difference, particularly drop, between a cylinder the cylinder pressure of which is determined and between a specific position in the exhaust manifold, is taken into consideration when determining said exhaust manifold pressure. Thus, a particularly precise value for the exhaust manifold pressure may be attained which takes into account e.g. the configuration of pipes and the geometry of the exhaust manifold.
According to a further embodiment, said exhaust manifold pressure is determined depending on an averaged cylinder pressure Pavg, wherein said averaged cylinder pressure is determined depending on the following equation: $P_{avg} = \frac{1}{m \cdot n \cdot (α_{end} - α_{start})} \sum_{j = 0}^{m - 1} \{\sum_{k = 0}^{n_{cyl} - 1} [\sum_{θ = α_{start}}^{α_{end}} δ \cdot P_{cyl, k} (θ)]\}$

, wherein m is a maximum number of engine cycles considered for averaging, n_cyl is a maximum number of cylinder pressure sensors, δ is a constant crankshaft angle value representing a sample rate for the cylinder pressure, wherein δ is preferably about 1° of crankshaft angle, wherein α_start is a start crankshaft angle for averaging, wherein α_end is an end crankshaft angle for averaging, wherein P_cyl,k(θ) is a cylinder pressure of cylinder k at crankshaft angle θ, and wherein P_avg is said averaged cylinder pressure.
According to a further embodiment, said averaged cylinder pressure Pavg is modified to take into account pressure drops between the pressure in a cylinder and the pressure in the exhaust manifold, particularly by adding a correction value ΔP which represents said pressure drops.
A further solution to the object of the present invention is given by a control unit for operating a combustion engine, wherein said control unit is configured to determine an exhaust manifold pressure within an exhaust manifold, characterized in that said control unit is further configured to determine a cylinder pressure of at least one cylinder of said combustion engine and to derive said exhaust manifold pressure from said cylinder pressure.
Advantageous embodiments are given by the dependent claims.
Further features, possible uses, and advantages of the invention will become apparent from the ensuing description of exemplary embodiments of the invention, which are shown in the drawing figures. All the features described or shown are the subject of the invention on their own or in arbitrary combination, regardless of how they are summarized in the claims or of their dependency and regardless of how they are worded or shown in the specification and the drawings, respectively.
In the drawings:

Fig. 1: depicts a schematic block diagram of an internal combustion engine for performing the method of the invention;
Fig. 2: schematically depicts a block diagram of a method according to the embodiments,
Fig. 3: shows a simplified flow chart of one embodiment of the method of the invention; and
Fig. 4: depicts a cylinder pressure graph over time.

Figure 1 depicts a schematic block diagram of an embodiment of a combustion engine 10 according to the invention. The combustion engine 10 comprises four cylinders only two of which are designated by reference numerals 13a, 13b in Fig. 1. An intake manifold section 11 is provided to supply to the cylinders a fuel/air mixture in a per se known manner. Also provided is an exhaust manifold 12 which receives exhaust gases from the cylinders 13a, 13b, .. in a per se known manner. At an output 12a of the exhaust manifold 12, a turbocharger and/or an after treatment system for the exhaust gases may be arranged, which is not depicted by Fig. 1.
For the first cylinder 13a, two exhaust valves 16a are exemplarily depicted. Of course, the further cylinders may also be equipped with such exhaust valves. As is per se known, the exhaust valves control the flow of exhaust gases from the cylinder 13a to the exhaust manifold 12. Also depicted in Fig. 1 is a cylinder pressure sensor 14, which is arranged directly in the cylinder 13a and which as such enables to directly measure a cylinder pressure in said cylinder 13a, especially during combustion. The further cylinders may also be equipped with cylinder pressure sensors. However, for the operation of the invention, which is explained in detail below, a single cylinder pressure sensor 14 is sufficient.
The operation of the combustion engine 10 is controlled by a control unit 20, which may also receive sensor signals such as a cylinder pressure signal of sensor 14a and the like.
Fig. 2 schematically depicts a block diagram of a method according to the embodiments. A cylinder pressure p_c1 of at least one cylinder 13a (Fig. 1) of said combustion engine 10 is determined, e.g. by means of sensor 14a, and an exhaust manifold pressure, which is present in the exhaust manifold 12, is derived from said cylinder pressure p_c1. Thus, advantageously, the exhaust manifold pressure may be determined without requiring a dedicated pressure sensor in said exhaust manifold 12.
Fig. 1 also depicts a dedicated pressure sensor 15, mounted at a position P within the exhaust manifold 12, which is an exhaust manifold pressure sensor. As explained above, this dedicated sensor 15 may be avoided because the present invention enables to derive the exhaust manifold pressure p_em (Fig. 2) from the cylinder pressure p_c1 as obtained by cylinder pressure sensor 14a. In other words, a combustion engine according to the embodiments is not required to have a dedicated exhaust manifold pressure sensor 15.
However, if such dedicated exhaust manifold pressure sensor 15 is present, the inventive method that considers the cylinder pressure p_c1 to determine the exhaust manifold pressure may be used to refine or verify the measurements of sensor 15.
The exhaust manifold pressure p_em as obtained according to the embodiments may e.g. be used to check operational conditions for a turbocharger (not shown) or an exhaust gas after treatment system (not shown), which may be arranged at the output 12a of the exhaust manifold and which are thus exposed to the exhaust manifold pressure. For example, the control unit 20 (Fig. 1) may perform the method according to the embodiments and may monitor the exhaust manifold pressure. If the exhaust manifold pressure is increasing too much, to avoid damages of the turbocharger due to excessive exhaust manifold pressure, the control unit may perform some control routines to reduce the exhaust manifold pressure or shift the combustion engine 10 to a safety condition (e.g., reducing injected fuel quantity).
Fig. 3 depicts a simplified flow chart of the method according to an embodiment. In step 100, the cylinder pressure p_c1 (Fig. 2) is determined by means of the cylinder pressure sensor 14a (Fig. 1). In step 110 (Fig. 3), the exhaust manifold pressure p_em is derived 110 from said cylinder pressure p_c1. The method may e.g. be performed by calculation means (processor) of the control unit 20.
Fig. 4 depicts a cylinder pressure graph over time for a specific cylinder 13a, which may e.g. be obtained by collecting a plurality of cylinder pressure values over time or a crankshaft angle, respectively, the values e.g. being obtained from the sensor 14a (Fig. 1). The time interval T1 of Fig. 4 denotes a phase in which the exhaust valves 16a are open. The time interval T2 of Fig. 4 denotes a phase in which the intake valves (not shown) are open. Between the intervals T1, T2, a compression and work cycle is indicated by a corresponding rise and fall of the cylinder pressure p_c1 in a per se known manner.
In order to obtain precise values for the exhaust manifold pressure p_em (Fig. 2), according to an embodiment, said cylinder pressure p_c1 of said specific cylinder 13a is determined during a phase ph (Fig. 4) in which the exhaust valve 16a of said specific cylinder 13a is open, and in which preferably also an intake valve of said specific cylinder 13a is closed. In the present example of Fig. 4, this phase ph is located in a crankshaft angle range between about 240 degrees and about 320 degrees. I.e., according to an embodiment, only the cylinder pressure values within said phase ph are considered for deriving the exhaust manifold pressure, in order to avoid errors due the cylinder pressure changes during compression/work cycle substantially not correlating with the exhaust manifold pressure.
According to a further embodiment, said phase ph is determined such that a change over time of the cylinder pressure p_c1 is smaller than a predetermined first threshold value. E.g., the gradient of cylinder pressure p_c1 over time may be determined, and only if it is sufficiently small, it is concluded that phase ph is reached.
Although Fig. 4 only depicts the cylinder pressure of one specific cylinder 13a, according to an embodiment, a cylinder pressure of more than one cylinder of said combustion engine 10, preferably of all cylinders of said combustion engine 10, is determined, for deriving said exhaust manifold pressure.
According to a further preferred embodiment, said exhaust manifold pressure p_em (Fig. 2) is determined depending on an averaged cylinder pressure Pavg, wherein said averaged cylinder pressure is determined depending on the following equation: $P_{avg} = \frac{1}{m \cdot n \cdot (α_{end} - α_{start})} \sum_{j = 0}^{m - 1} \{\sum_{k = 0}^{n_{cyl} - 1} [\sum_{θ = α_{start}}^{α_{end}} δ \cdot P_{cyl, k} (θ)]\},$

wherein m is a maximum number of engine cycles considered for averaging, n_cyl is a maximum number of cylinder pressure sensors, δ is a constant crankshaft angle value representing a sample rate for the cylinder pressure, wherein δ is preferably about 1° of crankshaft angle, wherein α_start is a start crankshaft angle for averaging, wherein α_end is an end crankshaft angle for averaging, wherein P_cyl,k(θ) is a cylinder pressure of cylinder k at crankshaft angle θ, and wherein P_avg is said averaged cylinder pressure.
For example, with the situation of Fig. 4, the following values may be chosen: α_start = 240, α_end = 320, n_cyl=1 (i.e., only one cylinder pressure sensor signal), and P_cyl,k(θ) may correspond to p_c1 of Fig. 4 for cylinder number k=0.
As indicated by the sum over index variable j, more than one engine cycle (i.a. up to m many cycles) may be considered for averaging cylinder pressure values to obtain the averaged cylinder pressure Pavg.
According to a further preferred embodiment, said averaged cylinder pressure Pavg is modified to take into account pressure drops between the pressure in a cylinder 13a and the pressure in the exhaust manifold 12, particularly by adding a correction value ΔP which represents said pressure drops.
I.e., if the exhaust manifold pressure at position P (Fig. 1) of exhaust manifold 12 is to be determined, a pressure drop between the cylinder 13a (=location of cylinder pressure sensor 14a) and the position P (due to exhaust gas valve, pipes of the manifold 12) may be considered. The value of the correction value ΔP is implementation specific and may e.g. be determined during a development phase. For example, during development, a fast pressure sensor may be mounted at position P (Fig. 1) to determine the actual exhaust manifold pressure, and the above equation(s) as well as the value of the correction value ΔP may be derived from differences in the cylinder pressure p_c1 and the actual exhaust manifold pressure as determined by the fast pressure sensor. After development, the fast pressure sensor may be removed from the exhaust manifold, and the inventive method ensures a precise determination of the exhaust manifold by means of the cylinder pressure only.
For example, said correction value ΔP may be determined by calibrating a map in the control unit 20 (Fig. 1), i.e. by means of corresponding software. The map may provide said correction value ΔP depending on engine speed, inner torque and delta pressure measured by two sensors (wherein delta pressure characterizes a pressure difference as obtained e.g. according to equation for Pavg above and by the fast pressure sensor temporarily arranged at position P during development).
Another way to determine said correction value ΔP comprises calibrating two parameters of linear curve regression, between Pavg and the signal from a fast pressure sensor temporarily arranged at position P during development.
I.e., by calibrating a specific combustion engine and using a fast pressure sensor temporarily arranged at position P during development it is possible to find a relationship, valid at least for constant engine speed, like P3 = a • Pavg + b, wherein P3 corresponds with the pressure signal of the fast pressure sensor temporarily arranged at position P during development, and wherein parameters a, b denote the regression coefficients.
With a collection of measurement, for example at different engine inner torque, the values "a" and "b" could be calculated and fulfilled in two different curves. I.e. for four different engine speeds of : 1000 rpm, 2000 rpm, 3000 rpm, 4000 rpm, four different values a1, a2, a3, a4 for parameter "a" may be found. Likewise, for said four different engine speeds listed above, four different values b1, b2, b3, b4 for parameter "b" may be found.
Thus, after calibration, Pavg could be determined according to equation 1 above, and the actual exhaust manifold pressure p_em' at position P could be obtained by: p_em' = a • Pavg + b, wherein a, b are chosen from values a1, a2, a3, a4, b1, b2, b3, b4 obtained during calibration as mentioned above.
According to a third variant, it is proposed to measure the exhaust flow (for example with an existing software function of the control unit 20) and to determine Pavg depending on equation 1 above. Under the hypothesis of validity of a regression equation of the typ "p_em' = a • Pavg + b" as explained above and a flow across a nozzle $Q = k • \sqrt{ΔP}$
it is possible to correlate the exhaust manifold pressure to the exhaust flow Q. Thus, from experimental measurement of Q and ΔP (e.g. during a development or calibration phase) e.g. a quadratic regression could be established: ΔP =a'•Q² + b'•Q + c', where a', b', and c' are constants coming from data regression. For series application for well-defined coefficients a', b' and c' and with the determination of Pavg according to equation 1 above and Q from an existing control unit function, the exhaust manifold pressure may also be determined. However, this third variant is less accurate due to the influence of the exhaust flow rate Q being calculated from a control unit software model.
The present invention advantageously enables to avoid dedicated exhaust manifold pressure sensors 15 during regular engine operation of the combustion engine, because the exhaust manifold pressure is derived from cylinder pressure values. Since cylinder pressure sensors 14a are usually faster, i.e. yield sensor data values at a higher data rate as compared to existing exhaust manifold pressure sensors 15, even under fast dynamic operating conditions more precise exhaust manifold pressure values may be obtained by the invention. Thus, particularly turbocharger damages may be avoided because overpressure conditions may promptly be detected using the invention and countermeasures may promptly be activated.
According to an example, cylinder pressure sensors 14a of the "glow plug combustion pressor sensor" type may be used.

Claims

Method of operating a combustion engine (10), wherein an exhaust manifold pressure (p_em) within an exhaust manifold (12) is determined, characterized in that a cylinder pressure (p_c1) of at least one cylinder (13a) of said combustion engine (10) is determined (100) and in that said exhaust manifold pressure (p_em) is derived (110) from said cylinder pressure (p_c1).
Method according to claim 1, wherein a cylinder pressure (p_c1, p_c2, ..) of more than one cylinder (13a, 13b, ..) of said combustion engine (10), preferably of all cylinders of said combustion engine (10), is determined.
Method according to one of the preceding claims, wherein said cylinder pressure (p_c1, p_c2, ..) is determined by means of at least one cylinder pressure sensor (14a) which is assigned to a respective cylinder (13a) of the combustion engine (10).
Method according to claim 3, wherein said cylinder pressure sensor (14a) is configured to provide sample values of said cylinder pressure (p_c1) with a rate of about at least one sample per 1 degree of a crankshaft angle of said combustion engine (10).
Method according to one of the preceding claims, wherein said cylinder pressure (p_c1) of a specific cylinder (13a) is determined during a phase (ph) in which an exhaust valve (16a) of said specific cylinder (13a) is open, and in which preferably also an intake valve of said specific cylinder (13a) is closed.
Method according to claim 5, wherein said phase (ph) is determined such that a change over time of the cylinder pressure (p_c1) is smaller than a predetermined first threshold value.
Method according to one of the preceding claims, wherein said exhaust manifold pressure (p_em) is determined by averaging said cylinder pressures (p_c1) of one or more cylinders over a predetermined time.
Method according to one of the preceding claims, wherein a pressure difference, particularly drop, between a cylinder the cylinder pressure (p_c1) of which is determined and between a specific position (P) in the exhaust manifold, is taken into consideration when determining said exhaust manifold pressure (p_em).
Method according to one of the preceding claims, wherein said exhaust manifold pressure (p_em) is determined depending on an averaged cylinder pressure Pavg, wherein said averaged cylinder pressure is determined depending on the following equation: $P_{avg} = \frac{1}{m \cdot n \cdot (α_{end} - α_{start})} \sum_{j = 0}^{m - 1} \{\sum_{k = 0}^{n_{cyl} - 1} [\sum_{θ = α_{start}}^{α_{end}} δ \cdot P_{cyl, k} (θ)]\}$

, wherein m is a maximum number of engine cycles considered for averaging, n_cyl is a maximum number of cylinder pressure sensors, δ is a constant crankshaft angle value representing a sample rate for the cylinder pressure, wherein δ is preferably about 1° of crankshaft angle, wherein α_start is a start crankshaft angle for averaging, wherein α_end is an end crankshaft angle for averaging, wherein P_cyl,k(θ) is a cylinder pressure of cylinder k at crankshaft angle θ, and wherein P_avg is said averaged cylinder pressure.
Method according to claim 9, wherein said averaged cylinder pressure Pavg is modified to take into account pressure drops between the pressure in a cylinder and the pressure in the exhaust manifold, particularly by adding a correction value ΔP which represents said pressure drops.
Control unit (20) for operating a combustion engine (10), wherein said control unit is configured to determine an exhaust manifold pressure (p_em) within an exhaust manifold (12), characterized in that said control unit is further configured to determine (100) a cylinder pressure (p_c1) of at least one cylinder (13a) of said combustion engine (10) and to derive (110) said exhaust manifold pressure (p_em) from said cylinder pressure (p_c1).
Control unit (20) according to claim 11, wherein said control unit is configured to perform the method according to one of the claims 1 to 10.