Method for determining the mass of gas enclosed in a combustion chamber

Abstract

The method for determining the mass of gas enclosed in a combustion chamber (2) according to the invention is based solely on the measurement of the pressure (MES P) inside the combustion chamber. The method implements a model of the combustion chamber, one of the parameters of which, the frequency of pressure oscillations, is determined (DET f) with the measurement of the gas pressure (MES P) in the combustion chamber.

Description

technical field

[0001] The present invention relates to the field of determining the mass of gas enclosed in a combustion chamber of an internal combustion engine.

[0002] The mass of gas trapped in a combustion chamber is a useful parameter, particularly for the control and testing of internal combustion engines. It allows for the analysis of combustion, the improvement of combustion control strategies, and the enhancement of cycle-by-cycle and inter-cylinder performance. Knowledge of this parameter is especially important for spark-ignition engines and for diesel engines equipped with exhaust gas recirculation (EGR) and / or variable valve timing. Previous technique

[0003] No sensor can directly measure the mass of gas trapped in a combustion chamber. Therefore, several methods have been developed to estimate this parameter. However, no method provides reliable information.

[0004] In particular, some methods are based on intake pressure measurements, which may not reliably represent the physical phenomena occurring within the combustion chamber. Other methods require signal processing techniques that can be complex, time-consuming, and may also introduce inaccuracies in the estimated mass of trapped gas.

[0005] Patent application WO 2007060349 describes a method based on the use of a pressure sensor in the cylinder and a temperature sensor downstream of the exhaust valve. This method therefore requires specific instrumentation for the internal combustion engine. Furthermore, this method reconstructs the gas temperature in the cylinder from the measurement taken by the temperature sensor downstream of the exhaust valve. This reconstruction generates approximations, which affect the accuracy of the estimated trapped gas mass.

[0006] Patent application WO 2015082731 describes a method based on estimating the resonance frequency of the pressure in the combustion chamber using a Fourier transform, which is a processing of the signal from the pressure sensor. This estimation of the resonance frequency lacks the precision required for an accurate estimation of the enclosed gas mass. Furthermore, this method uses the solution of Bessel's equations, assuming that the combustion chamber is a perfect cylinder. However, the combustion chambers of internal combustion engines are not perfectly cylindrical. The estimate obtained by this method is therefore inaccurate due to this assumption. In addition, this method requires oscillations of large amplitudes.Pau Bares' thesis "In-cylinder pressure resonance analysis for trapped mass estimation in automotive engines" (June 30, 2017 (2017-06-30), Valencia) discloses a calculation of the mass of air and fuel in the combustion chamber. Summary of the invention

[0007] The present invention aims to accurately determine the mass of gas trapped in the combustion chamber of an internal combustion engine, using simple and conventional instrumentation and without complex signal processing. To this end, the method for determining the mass of gas trapped in a combustion chamber according to the invention is based solely on measuring the pressure within the combustion chamber. The method employs a model of the combustion chamber, one parameter of which, the pressure oscillation frequency, is determined by measuring the gas pressure in the combustion chamber. Thus, the method according to the invention is accurate, reliable, and requires simple instrumentation.

[0008] The invention according to claim 1 relates to a method for determining the mass of gas enclosed in a combustion chamber of an internal combustion engine, in which the following steps are carried out: a) The pressure P of said gas is measured in said combustion chamber; b) The oscillations of said gas pressure P are determined from said pressure measurement P of said gas; c) The mass of gas enclosed m is determined using the formula m = γPV λ 2 f 2 and by determining the oscillation frequency f of pressure with said oscillations measured, V being the volume of said combustion chamber, λ the wavelength in said combustion chamber, P said measured pressure of said gas in said combustion chamber, and γ the specific heat ratio.

[0009] According to one embodiment, said volume V of said combustion chamber is obtained by means of a mapping as a function of the crankshaft angle of said internal combustion engine.

[0010] According to the invention, said wavelength λ is obtained by means of a mapping as a function of the crankshaft angle of said internal combustion engine.

[0011] According to the invention, said mapping of said wavelength λ is constructed beforehand digitally.

[0012] According to the invention, said mapping of said wavelength λ is constructed by a three-dimensional finite element approach.

[0013] According to one aspect of the invention, said specific thermal ratio γ is determined by taking into account the composition of said enclosed mass, and by means of the thermal capacity of the components of said enclosed mass.

[0014] In accordance with a characteristic, the said mass of gas enclosed is determined in real time.

[0015] According to the invention, the pressure oscillation frequency f is determined by implementing the following calibration steps: i) Consider at least two predefined enclosed gas masses m₁, ..., mn; ii) Determine a specific heat ratio γ for each predefined enclosed gas mass m₁, ..., mn; iii) Determine using said formula m = γPV λ 2 f 2 a pressure oscillation frequency f for each predefined enclosed gas mass m 1 , ..., mn as a function of said measured gas pressure P, said combustion chamber volume V, said wavelength λ and said determined specific heat ratio γ; and iv) Each determined pressure oscillation frequency f is compared with said measured pressure oscillations, and said enclosed gas mass is determined as the predefined mass for which the pressure oscillation frequency f minimizes said comparison.

[0016] Advantageously, each specific heat ratio γ of each predefined enclosed mass of gas m1, ..., mn is determined by implementing the following steps: (1) The composition of said enclosed mass is determined; (2) A relationship is determined which links said specific heat ratio γ and the capacity of the gas cp in said combustion chamber: γ = C p C p − R with R the gas constant, and the gas capacity cp being a function of the temperature of said gas and of said composition of said enclosed mass; (3) The temperature of said gas is determined by means of the ideal gas law applied to each predefined enclosed mass m 1 , ..., mn; and (4) The specific heat ratio γ is deduced for each predefined enclosed mass m 1 , ..., mn.

[0017] Alternatively, the pressure oscillation frequency f is determined by means of an average over a time window of said determined oscillations.

[0018] According to one implementation, a wavelength λ is determined for each mode of oscillation of said pressure of said gas.

[0019] Other features and advantages of the process according to the invention will become apparent from the following description of non-limiting examples of implementations, with reference to the figures attached and described below. List of figures

[0020] There figure 1 illustrates a cylinder of an internal combustion engine. The figure 2 illustrates the steps of the process according to one embodiment of the invention. figure 3 illustrates the steps for calibrating the pressure oscillation frequency according to one embodiment of the invention. figure 4 illustrates a graph of the relative mass of gas estimated by the process according to the invention for an example. Description of the implementation methods

[0021] The present invention relates to a method for determining the mass of gas enclosed in a combustion chamber of an internal combustion engine.

[0022] Typically, an internal combustion engine comprises at least one cylinder, a piston sliding within that cylinder in a reciprocating linear motion, means for inhaling an oxidizer (gas), means for expelling burnt gases, a combustion chamber, and injection means for injecting fuel into the combustion chamber. The combustion chamber is located in the upper part of the cylinder, and its volume varies with the movement of the piston within the cylinder. figure 1This figure schematically and without limitation illustrates a cylinder 1 of an internal combustion engine according to an embodiment of the invention. In this figure, the intake, exhaust, and injection means, and any ignition means, are not shown. Within cylinder 1, a piston 6 moves in a reciprocating linear motion. The combustion chamber 2 is the area where combustion occurs; it is bounded by the upper part of the piston 6, the lateral wall 4 of the cylinder, and the cylinder roof 5. This area corresponds to the white area of ​​the figure 1 Furthermore, for the process according to the invention, the cylinder includes a pressure sensor 3, which measures the gas pressure in the combustion chamber 2.

[0023] The internal combustion engine can be of any type, spark-ignition or auto-ignition, with or without exhaust gas recirculation, with or without turbocharging. However, the process according to the invention is particularly suited to spark-ignition engines and auto-ignition engines equipped with exhaust gas recirculation (EGR) and / or variable valve timing.

[0024] The terms gas or oxidizer include air at ambient pressure, supercharged air, or a mixture of air (supercharged or not) with burnt gases.

[0025] The method for determining the mass of trapped gas according to the invention uses a single sensor: a pressure sensor in the combustion chamber. Using a single sensor reduces the need for engine instrumentation. Furthermore, measuring the pressure in the combustion chamber provides direct information on the gas behavior within the chamber, unlike a sensor located at the intake or exhaust, which would require reconstructing the combustion chamber pressure. Such a sensor also has the advantage of being readily available on engine test benches and in the internal combustion engines of modern vehicles, particularly new auto-ignition engines, thus simplifying implementation and reducing costs.

[0026] The process according to the invention comprises the following steps: Measurement of gas pressure in the combustion chamber; Determination of oscillations of measured gas pressure; and Determination of the mass of gas enclosed by determining the frequency of oscillations with the measurements.

[0027] These steps will be detailed later in the description.

[0028] According to one embodiment of the invention, these steps can be implemented in real time, in order to determine in real time the mass of gas enclosed, facilitating the exploitation of this parameter.

[0029] Furthermore, these steps can be implemented online, on a vehicle's internal combustion engine, or on an engine test bench.

[0030] The steps of determining the oscillations of the gas pressure and of determining the mass of gas enclosed can be implemented by computer means, in particular a computer equipping the internal combustion engine.

[0031] There figure 2 This illustrates, schematically and without limitation, the steps of the process according to the invention. The first step consists of measuring the gas pressure MES P in the combustion chamber using a pressure sensor. The second step consists of determining the gas pressure oscillations OSC P from the gas pressure measurement performed previously. Then, the mass of trapped gas m is determined by determining DET f the pressure oscillation frequency f from the pressure oscillations determined in the previous step. 1) Gas pressure measurement

[0032] During this step, the gas pressure in the combustion chamber is measured using a gas sensor placed in the combustion chamber. 2) Determination of pressure oscillations

[0033] In this step, the oscillations of the gas pressure over time are determined from the gas pressure. This allows for a precise understanding of the physical phenomena occurring in the combustion chamber.

[0034] In one embodiment, this step can be implemented using a filter, preferably a bandpass filter around the frequency of interest. As a non-limiting example, a bandpass filter between 5 and 9 kHz can be used. A Butterworth filter can also be used. 3) Determination of the mass of gas enclosed

[0035] During this step, the mass of enclosed gas m is determined using the formula m = γPV λ 2 f 2 (also called the combustion chamber model) and by determining the pressure oscillation frequency f with the measured oscillations determined in the previous step. In this formula, V is the volume of the combustion chamber (variable over time), λ is the wavelength in the combustion chamber, P is the measured gas pressure in the combustion chamber, and γ is the specific heat ratio.

[0036] The formula m = γPV λ 2 f 2 is obtained from the ideal gas law: PV = mRT, of the equation for the frequency of oscillations f = c λ and the equation for the speed of sound c = γRT , where c is the instantaneous speed of sound in the combustion chamber. Thus, the formula is representative of the physical phenomena of the gas in the combustion chamber, which allows for the precise determination of the mass of gas enclosed.

[0037] According to one embodiment of the invention, the volume V of the combustion chamber can be obtained by means of a map or table of the combustion chamber volume as a function of the crankshaft angle of the internal combustion engine. This map reflects the variations in volume related to the movement of the piston. The use of such a map makes it possible to determine the combustion chamber volume quickly and in real time.

[0038] The wavelength λ depends on the piston position (and is therefore time-varying) and the shape of the combustion chamber. According to one embodiment of the invention, the wavelength λ can be obtained by mapping the wavelength λ as a function of the crankshaft angle of the internal combustion engine. Using such a map makes determining the wavelength λ rapid and applicable in real time.

[0039] For this implementation, the wavelength λ map can be constructed beforehand and numerically. The numerical determination of the wavelength λ allows for the determination of a wavelength λ for any shape of the combustion chamber.

[0040] Thus, the determination of the enclosed mass does not involve any approximation related to the shape of the combustion chamber.

[0041] According to a preferred aspect of this implementation, the wavelength λ map can be constructed using a three-dimensional finite element approach (applied to the combustion chamber) to solve the three-dimensional wave equation at each crankshaft angle, and thus determine the relationship between the pressure oscillation frequency and the imposed speed of sound. The inputs to the finite element approach can be: The geometry of the combustion chamber at each crankshaft angle, the wave equation to be solved: Δ P = 1 c 2 δ 2 P δt 2 with the speed of sound c fixed, the boundary conditions: ∇ P. n = 0, n being the normal vector to the wall of the cylinder, The initial conditions: source, that is to say a localized pressure gradient used to initialize the calculation.

[0042] Solving the wave equation allows monitoring of pressure oscillations at different locations in the combustion chamber; subsequently, the oscillation frequencies can be derived via simple spectral density analysis and the wavelengths associated with each oscillation mode can be calculated, for each crankshaft angle.

[0043] Alternatively, the wavelength λ can be determined by other methods, for example analytically.

[0044] Preferably, a wavelength λ can be determined for each mode of oscillation of the gas pressure, in order to obtain more precise information.

[0045] The specific heat ratio depends on time and varies from one operating point to another of the internal combustion engine.

[0046] According to one embodiment of the invention, the specific heat ratio γ can be determined by taking into account the composition of the mass enclosed in the combustion chamber, and by means of the heat capacity of the components of the mass enclosed in the combustion chamber.

[0047] For this embodiment, one can consider the volume fraction of the fuel components injected into the combustion chamber (for example, the volume fraction of ethanol in gasoline), the ratio of air to fuel in the combustion chamber relative to the stoichiometric ratio, and the proportion of unburned gases in the combustion chamber. Furthermore, this embodiment can implement the following steps: The coefficients of the fuel's chemical formula are determined, for example, the coefficients x, y, z for a fuel of the form CxHyOz (when the fuel is not a mixture; when the fuel is a mixture, the coefficients of the basic fuel components can be weighted according to the volume fraction of the fuel components). The heat capacity of the unburned mixture in the combustion chamber is determined, taking into account the coefficients of the fuel's chemical formula (for example, x, y, z) and the ratio of air to fuel in the combustion chamber relative to the stoichiometric ratio. The overall combustion equation (to determine the concentration of the air and fuel components) and a heat capacity equation Cp for each component of the overall equation can be used; this equation can be of the form CP ( T ) = a + b × T + c × T 2< + d× T 3< + e × T4< with T the temperature in the combustion chamber, and a, b, c, d, and e coefficients obtained from tables (e.g., the Janaf table) for each component of the unburned mixture. The heat capacity of the burned mixture in the combustion chamber is determined by taking into account the coefficients of the fuel's chemical formula (e.g., x, y, z). The methodology can be identical to that described for the heat capacity of the unburned mixture. The overall combustion equation (to determine the concentration of the components of the burned mixture) and the heat capacity equations Cp for each component can be used.The heat capacity of the fuel mixture is determined by combining the heat capacity of the burned mixture and the heat capacity of the unburned mixture. According to one embodiment of this step, we can, for example, consider that the fuel mixture comprises 50% of the burned mixture and 50% of the unburned mixture (or any other distribution). The heat capacity ratio γ is determined by the equation. γ = c p C p − R with cp the heat capacity of the fuel mixture determined in the previous step, and R the ideal gas constant.

[0048] For the implementation of this embodiment, the temperature T of the mixture can be deduced from the ideal gas law. PV = mRT by selecting at least one hypothetical enclosed gas mass, P, V and R being known from elsewhere.

[0049] According to a second embodiment, the heat capacity ratio can be obtained by a formula of the type γ = − PdV VdP based on a hypothesis of adiabatic behavior.

[0050] According to one implementation of the invention, the pressure oscillation frequency can be determined by implementing a calibration using the following steps: At least two predefined enclosed gas masses m₁, ..., mn are considered. These are at least two possible enclosed gas masses, n being an integer greater than or equal to 2. According to one aspect of the invention, the predefined enclosed gas masses m₁, ..., mn are contained in a list predefined prior to the execution of the process. A specific heat ratio γ is determined for each predefined enclosed gas mass m₁, ..., mn. For example, a temperature T in the combustion chamber can be deduced from each predefined enclosed gas mass m₁, ..., mn, and by applying the steps described above, the heat capacity of the mixture cp can be deduced, which is then used to obtain the specific heat ratio γ as a function of the crankshaft angle. The formula is used to determine m = γPV λ 2 f 2 a pressure oscillation frequency f for each predefined enclosed gas mass m₁, ..., mn as a function of the measured pressure P and the determined parameters, the volume V of the combustion chamber and the wavelength λ, and the specific heat ratio γ determined in the previous step; in other words, the formula is reversed m = γPV λ 2 f 2 to obtain an equation f = γPV mλ 2 where all parameters are determined (P, V, y, λ) or predefined (m), each pressure oscillation frequency f obtained in the previous step is compared with the measured pressure oscillations (determined in step 2), and the trapped gas mass is determined as one of the predefined trapped gas masses m₁, ..., mn, for which the pressure oscillation frequency f minimizes the comparison. In other words, the trapped gas mass determined by the method is one of the predefined trapped gas masses m₁, ..., mn: the one for which the difference between the pressure oscillation frequency f and the measured pressure oscillations is minimal. The comparison may consist of a comparison of the oscillations. Preferably, only the frequency of the measured pressure oscillations is used for calibration; their amplitude is not relevant for the comparison.Preferably, this step can consist of generating pressure oscillations with the different determined frequencies, then comparing the oscillations obtained with those determined in step 2, and choosing the signal for which the signal oscillation is comparable to the measured pressure oscillations.

[0051] There figure 3 This illustrates, schematically and without limitation, this implementation of this step of the invention. First, at least two enclosed gas masses m₁, ..., mn are predefined. From these, at least two thermal ratios γ₁, ..., γₙ are deduced, corresponding to the predefined enclosed gas masses m₁, ..., mn. Then, at least two pressure oscillation frequencies f₁, ..., fₙ, corresponding to the predefined enclosed gas masses m₁, ..., mn, are calculated using the equation f = γPV mλ 2 which stems from the formula m = γPV λ 2 f 2 These values ​​are compared (COMP) with the measured pressure oscillations OSC to select the oscillation frequency f from among the pressure oscillation frequencies f₁, ..., fₙ that most closely approximates the measured pressure oscillation frequency OSC. The enclosed gas mass determined by the method according to the invention is then the enclosed gas mass from among the enclosed gas masses m₁, ..., mₙ that corresponds to the selected pressure oscillation frequency f.

[0052] Alternatively, the oscillation frequency f can be obtained by any other calibration method, for example by a method of minimizing an objective function.

[0053] Alternatively, the oscillation frequency can be determined by averaging over a time window the oscillations determined in step 2).

[0054] As can be understood, the invention is not limited to the forms of implementation of the steps of the process described above by way of example, but rather encompasses all variant implementations. Example

[0055] The characteristics and advantages of the process according to the invention will become clearer upon reading the application example below.

[0056] The example relates to comparing the mass of trapped gas determined according to the method of the invention with the mass of trapped gas determined by LES simulation. For this example, a turbocharged internal combustion engine with a rotational speed of 5500 rpm is considered, and 23 critical cycles identified as having varying pressure oscillation levels are considered. In this example, the combustion chamber of the internal combustion engine is equipped with two pressure sensors at different positions within the combustion chamber.

[0057] There figure 4This is a graph illustrating the mass m of gas trapped in the combustion chamber as a function of the cycle number N. The mass m used in this graph is a relative value corresponding to the ratio of the mass of gas trapped in the combustion chamber obtained by the method according to the invention to the mass of gas trapped in the combustion chamber obtained by simulation. Thus, a value close to 1 indicates a good correlation between the simulated values ​​and the values ​​obtained by the method according to the invention. Each N corresponds to two trapped gas masses, each obtained by one of the two pressure sensors used. It can be seen that the values ​​(represented by the dots) and their average, represented by the dashed line, are very close to the value of one. Thus, the method according to the invention makes it possible to accurately determine the mass of gas trapped in the combustion chamber.Furthermore, the points and averages of the two pressure sensors are close. Therefore, the method according to the invention is indeed independent of the sensor's position in the combustion chamber.

Claims

1. Method for determining the enclosed mass of gas in a combustion chamber (2) of an internal-combustion engine, wherein the following steps are carried out: a) the pressure P of said gas in said combustion chamber (2) is measured (MES P); b) the oscillations of said gas pressure P are determined (OSC P) from said pressure measurement P of said gas; c) said enclosed mass m of gas is determined using the formula m = γPV λ 2 f 2 and by determining (DET f) the frequency f of pressure oscillations using said measured oscillations, V being the volume of said combustion chamber, λ the wavelength in said combustion chamber (2), P said measured pressure of said gas in said combustion chamber, and γ the specific heat ratio, the frequency f of pressure oscillations is determined (DET f) by implementing the following calibration steps: i) at least two predefined enclosed masses m1, ..., mn of gas are considered; ii) a specific heat ratio γ is determined for each predefined enclosed mass m1, ..., mn of gas; iii) said formula m = γPV λ 2 f 2 is used to determine a frequency f of pressure oscillations for each predefined enclosed mass m1, ..., mn of gas as a function of said measured gas pressure P, said volume V of the combustion chamber, said wavelength λ and said determined specific heat ratio γ; and iv) each determined frequency f of pressure oscillations is compared (COMP) with said measured pressure oscillations (OSC), and said enclosed mass m of gas is determined as the predefined mass for which the pressure oscillation frequency f minimizes said comparison, said wavelength λ being obtained from a map based on the crankshaft angle of said internal-combustion engine, said map being constructed numerically in advance using a three-dimensional finite element approach.

2. Method for determining the enclosed mass of gas according to Claim 1, wherein said volume V of said combustion chamber (2) is obtained from a map based on the crankshaft angle of said internal-combustion engine.

3. Method for determining the enclosed mass of gas according to one of the preceding claims, wherein said map for obtaining said wavelength λ is constructed numerically in advance using a three-dimensional finite element approach with the following inputs: - the geometry of the combustion chamber at each crankshaft angle, - the wave equation to be solved P = 1 c 2 δ 2 P δt 2 with a fixed speed of sound c, - the boundary conditions ∇P.n = 0 , n being the vector normal to the cylinder wall, - the initial conditions: a localized pressure gradient used to initialize the calculation.

4. Method for determining the enclosed mass of gas according to one of the preceding claims, wherein said specific heat ratio γ is determined by taking into account the composition of said enclosed mass, and by means of the thermal capacity of the components of said enclosed mass.

5. Method for determining the enclosed mass of gas according to one of the preceding claims, wherein said enclosed mass of gas is determined in real time.

6. Method for determining the enclosed mass of gas according to one of the preceding claims, wherein each specific heat ratio γ of each predefined enclosed mass m1, ..., mn of gas is determined by implementing the following steps: (1) the composition of said enclosed mass is determined; (2) a relationship linking said specific heat ratio γ and the capacity cp of the gas in said combustion chamber is determined: γ = c p c p − R where R is the gas constant, and the capacity cp of the gas being a function of the temperature of said gas and of said composition of said enclosed mass; (3) the temperature of said gas is determined by means of the ideal gas law applied to each predefined enclosed mass m1, ..., mn ; and (4) said specific heat ratio γ for each predefined enclosed mass m1, ..., mn is deduced therefrom.

7. Method for determining the enclosed mass of gas according to one of the preceding claims, wherein a wavelength λ is determined for each oscillation mode of said pressure of said gas.

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