FR2921114A1 - Turbine's average input pressure determining method for spark ignition engine, involves reading expansion ratio value of turbine from data table, and multiplying ratio value with pressure to determine average input pressure of turbine - Google Patents

Abstract

The method involves evaluating a physical parameter such as opening state of a turbine, temperature and pressure of respective intake and exhaust gas of the turbine, and mass flow rate of a turbine gas. An adapted flow rate (Qadapt) is determined using a specific relationship comprising the parameter such as mass flow rate, gas temperature and pressure. An expansion ratio value of the turbine associated to values formed by the rate (Qadapt) and the opening state, is read from a data table. The ratio value is multiplied with the pressure to determine an average input pressure of the turbine.

Description

The invention relates to the determination of an inlet pressure of a turbocharger turbine equipping a heat engine.

The power of a heat engine depends directly on the amount of air that can be admitted into its combustion chambers, which is mainly conditioned by the density of this air entering the rooms. Thus, the power of a given displacement engine can be increased by supercharging, that is to say by compressing the air before admission into its cylinders. The turbocharger which is the most used supercharging device comprises a turbine located at the output of the engine to recover a portion of the energy contained in the exhaust gas, coupled to a compressor located at the engine inlet to increase the density of the engine. intake air. The operation of the turbine requires a high pressure ratio between the inlet and the outlet of this turbine. It is therefore essential to increase the output pressure of the engine, which thus depends on the flow characteristic of the turbine.

This increase in the pressure at the engine outlet results in an increase in the losses due to the transfer of the gases, and in an increase in the amount of burnt gases that can remain in the combustion chambers. The integration of a turbocharger also causes an increase in the temperature of the gases at the start of combustion, which results in an increase in the temperature of the flue gases. The increase in losses due to the transfer of gases can be offset by the gains provided by the increase of the intake air pressure. But if the boost is used in a wide operating range, the losses are greater than the gains in part of the operating range. As for the flue gas remaining in the combustion chambers, they occupy part of the available space, which reduces the amount of fresh air that can be admitted. In addition, these gases can degrade the combustion of the next cycle, particularly in the case of a spark ignition engine. Thus, the output pressure of the engine, ie at the turbine inlet, has a fundamental effect on the operation of the engine and on its optimization, and this pressure depends on the flow characteristic of the turbine. This pressure depends in particular on the flow characteristic of the turbine and the turbine opening: the turbocharger can be of the variable geometry type, the inclination of the stator vanes which channel the introduction of the gas into the vanes of the rotor. turbine being for example variable, and being driven by a so-called turbine opening control unit. In the case of a turbocharger with fixed geometry, the supercharging circuit may comprise a bypass branch of the turbine, called "bypass" or "waste gate", provided with a solenoid valve. In this case, the gas from the combustion chambers passing through this branch does not pass through the turbine. This solenoid valve is then controlled by a so-called turbine opening control unit, which makes it possible to regulate the flow rate and therefore the turbine inlet pressure.

However, the quantities provided by the manufacturers, representative of the behavior of the turbine, depend on the pressure at its inlet. In other words, the known methods do not make it possible to accurately determine the turbine inlet pressure from manufacturer data.

On the other hand, it is difficult to directly measure this pressure because it varies greatly at a frequency dependent on the engine speed, and that the integration of a suitable pressure sensor is not possible in the region. located between the engine and the turbine. The object of the invention is to propose a method for expressing the inlet pressure of the turbine as a function of physical quantities relating to the engine.

For this purpose, the subject of the invention is a method for determining, in a supercharged heat engine, an average pressure P3 at the inlet of a turbine of a turbocharger fitted to this engine, this engine comprising an opening control of turbine such as a control of a solenoid valve for regulating the flow of gas entering the turbine, in which a quantity representative of the turbine opening is evaluated by measurement and / or calculation, the temperature T3 of the gas entering the turbine, the turbine, the pressure P4 of the gas leaving the turbine, and the mass flow rate Qgb of the gas passing through the turbine, in which a suitable flow Qadapt with the relation Qadapt = Qgb is determined from these data. T3 / P4, in which is read in a data table a value of the expansion rate Pit of the turbine associated with the pair of values formed by the adapted flow Qadapt and the turbine opening, and in which the rate of expansion is multiplied Pit by the P4 turbine outlet pressure to determine the turbine inlet pressure P3. Thus, it appears that the rate of expansion of the turbine depends solely on the adapted flow rate and the turbine opening. It is therefore possible, by expressing the adapted flow rate as indicated above, and a quantity representative of the state of the turbine opening control, to determine the turbine inlet pressure, from operating data external to the turbine. this turbine. The invention also relates to a method as defined above, in which the outlet pressure of the turbine P4 is determined from the evaluation, of the mass flow rate of gas Qgb, of an evaluation of the atmospheric pressure P0, d a CPE back pressure value at a reference flow rate Qref, with the relationship P4 = PO + CPE * Qgb z. The invention also relates to a method as defined above, in which the pressure P4 at the turbine outlet is determined by direct measurement with a pressure sensor. The invention also relates to a method as defined above, wherein the turbine opening value is derived from a turbine opening control unit. The invention also relates to a method as defined above, in which the flow rate of Qair gas entering the compressor is determined from the flow of gas Qgb leaving the turbine and a factor k representative of the ratio between the Qgb gas flow leaving the turbine and Qair gas flow entering the compressor, with the relation Qgb = k.Qair. The invention also relates to a method as defined above, in which the data table giving the rate of the Pit compression of the turbine as a function of the adapted Qadapt flow rate and the opening of the turbine is established by carrying out tests. of the turbine at different operating points.

The invention will now be described in more detail and with reference to the appended figures.

Figure 1 is a functional representation of an engine equipped with a turbocharger; FIG. 2 is a graph for determining a turbine expansion ratio from its adapted power and its adapted flow rate.

The idea underlying the invention is to estimate, in a heat engine equipped with a turbocharger, the pressure, denoted P3, at the output of this engine, ie at the inlet of the turbine of the turbocharger. This estimate is made from a functional representation of the turbocharger, the need for supercharging, the pressure at the turbine outlet and the energy supplied by the engine. The engine is modeled in three functional entities appearing in Figure 1: a block located upstream of the turbine including the engine, the compressor, the intake air lines, the compressor and the organs increasing the density of the air , and the air lines located between the engine and the turbine inlet; the turbine itself; and the exhaust line between the turbine and the outlet. Figure 1 also comprises a representative block of the turbine opening which also conditions the operating point of the powertrain.

The operation of the engine conditions or defines the following variables: the fresh air flow, noted Qair; the fuel flow noted Qcarb; and the temperature at the outlet of the engine, that is to say at the inlet of the turbine, which is denoted T3. To obtain this air flow, the compressor generates a certain compression ratio, denoted Pic, which is the ratio between the pressure P1 at the inlet of the compressor, and the pressure P2 at the outlet of the compressor. The compressor has a performance that depends on the temperature Ti at its input, the engine air flow and the compression ratio, which defines the power Pwc needed to perform the supercharging. This operation results in the temperature T2 at the outlet of the compressor.

To obtain a desired power level at the compressor, the turbine opening control is controlled accordingly: the flow rate passing through the turbine can be increased or reduced by changing the opening of the solenoid valve of the branch branch of the turbine or the inclination of the vanes of the turbine stator.

The sum of the fresh air flow and the fuel flow admitted into the engine corresponds to the flow Qgb evacuated by the exhaust line. Given the permeability of the exhaust line which is located at the outlet of the turbine, it is possible to estimate the pressure P4 at the turbine outlet.

This functional representation proposed makes it possible to isolate the unknown of the problem, namely the pressure P3 at the turbine inlet. For this, we introduce the notations Qtred and Qcred which respectively correspond to the corrected flow rate of the turbine and to the corrected flow rate of the compressor: Qtred = Qgb * T3 and Qcred = Qair * Tl [1] P3 Pl We also introduce the quantity Qadapt which is homogeneous at the actual flow rate by means of the temperature T3 and the pressure P4. This magnitude is intrinsic to the turbine, whereas the temperature T3 is solely conditioned by the operation of the engine, and the pressure P4 depends only on the pressure drop of the exhaust line:

Qadapt = Qred.PR = QgP4T3 [2] The pressure P4 can be expressed as a function of the exhaust flow and the permeability of the line, with the following relation in which PO designates the atmospheric pressure, and CPE the back pressure at the exhaust for a reference flow rate noted Qref: z P4 = PO + CPE * Qgb [3] Qref This pressure P4 can also be evaluated differently or be measured. The turbine opening control is then adjusted to obtain the power required by the compressor, in accordance with the following relationship in which Cpair denotes the heat capacity of the air:

Pwc = Qair * Cpair * (T2 ù Tl) [4] This allows to determine the reduced power of the turbine, noted Pwtred, and the reduced power of the compressor, noted Pwcred: Pwtred = Pwc and Pwcred = Pwc [5] T3 * P3 Tl * Pl The reduced turbine power is also determined by: 1 Pwtred = Qgb * Cpgb * T3 * P4 i) * T 3 * P3 The Cpgb designates the heat capacity of the turbine upstream gases. The 7 corresponds to the polytropic coefficient of the turbine upstream gases. The ~ 7t corresponds to the efficiency of the turbine.

The adapted power is determined from the following relation:

Padapt = Pwtred * PR = Pwc [6] T3 * P4 This quantity is also homogeneous with the real power, by means of the temperature T3 and the pressure P4 which depend respectively on the operation of the motor and the pressure loss of the line of exhaust, while remaining intrinsic to the turbine.

It follows that the operation of the turbine is entirely characterized by two curves bundles obtained by scanning the corrected regimes, and respectively expressing the adapted flow rate and the appropriate power, depending on the rate of expansion. The turbine characterization expressed by the turbine speed, the turbine expansion rate, the turbine flow rate and the turbine efficiency under reference conditions have therefore been transformed by a new characteristic which is expressed by a corrected turbine speed, the expansion ratio, the adapted flow rate and adapted power. The notations adapted to the turbine are to compare, by their structure, to the notations corrected to the compressor. Indeed by using an abuse of language, we can consider that the compressor generates P2 and T2 and undergoes the P1 and Ti imposed by the intake line, and similarly, that the turbine generates P3 and the T4 and undergoes the T3 and P4 imposed by the cylinders and the exhaust line.

A judicious choice of parameters consists in making implicit the variables P2, T2, P3, T4 which are conditioned by the operation of the turbocharger, and making explicit the variables P1, Ti, P4, T3 which are conditioned by the admission lines and exhaust and cylinders. The correspondence between the functional representations of the air lines, the compressors, the cylinders, and the turbine is then done in a natural way since the outputs of one are the inputs of the others. In stabilized operation the real power and the actual speed are equal to the compressor and the turbine: Pwc = Pwt, and Nc = Nt. In addition, the actual flow rate of gas passing through the turbine is equal to the actual flow rate of gas passing through the compressor multiplied by a factor k integrating the injected fuel, as well as other phenomena having an effect on these flow rates. In particular, the fraction of flow passing through the bypass branch, that is to say not passing through the turbine, can be expressed as a function of the state of the turbine opening control, to be taken into account. in the factor k. Qgb = k * Qair The characteristic curves of the compressor can be plotted in the adapted turbine reference system, by using the following relations in which N designates the real speed of the turbocharger, and Tref a reference temperature at which this regime is measured: Pwc Pwc * . \ Tl * Pl = * Tl * Plpwcred T3 * P4 Tl * Pl T3 P4 T3 P4 Qgb * T3 k * Q air * Tl .iT3 Pl T3 Pl Qadapt = _ * ù * ù = Qcred * k * ù * [8] P4 Pl Tl P4 Tl P4 10 Ncorr = N * / Tref N * IT Tl f * ~ Tl T3 Ncorrcomp * I T3 [9] Thus from manufacturer data representative of the behavior of the compressor, an assumption of P1 / P4, from a T1 / T3 hypothesis, from a hypothesis of ratio k between compressor and turbine flow rates, we can obtain the intersection of the turbine and compressor characteristics for each iso regime and reduce the two-curve turbine beam. in the benchmarks Adapted power - Adapted flow rate and Relaxation rate - Adapted flow rate For each state of the control opening. Indeed, from the turbine characteristic there is a characteristic to express the power adapted to the flow rate adapted for a given regime. For the same regime, the compressor has its own characteristic also dependent on the power adapted according to the adapted flow as described above. It is by crossing the 2 characteristics that we get the intersection mentioned above. When the operating speeds of the turbocharger assembly are swept, a unique characteristic is thus obtained, expressed by the turbine expansion ratio, the adpated flow rate and the adapted power. These curves are quasi-perfect quadratics particularly robust to the assumptions of P1 / P4, T1 / T3 and k, and they are intrinsic to Padapt = [7] the association of a turbine and a compressor. They can be approximated directly from manufacturer data if the compressor used for the characterization corresponds to that of the application studied.

A series can thus be defined as the tabulated data of the expansion ratio, the adapted flow rate and the power adapted for a turbine opening or a supercharging system. It is recommended to obtain this series for a turbine and a compressor from the functional reduction of the turbine and the manufacturer's data relating to the compressor.

One of the main interests of the series representation is to be able to draw on the same graph the iso relaxation rates for the various openings of a supercharging system. In the graph of FIG. 2, each solid line corresponds to a series, that is to say to a state of the turbine opening control. The iso-rate relaxation curves are represented in dashed lines. By series association, it is therefore possible to simulate any supercharging architecture. Therefore starting from an engine operating point, for which the flow rate Qgb of gas passing through the turbine, the state of the turbine opening control, the temperature T3 of the inlet gas of the turbine, and the pressure are known. P4 of the gas at the outlet of the turbine, it is possible to translate these data into a suitable quantity Qadapt and a quantity representative of the state of the turbine opening control. In the graph of FIG. 2, the value Qadapt corresponds to a vertical straight line, and the value of the state of the opening control, or turbine opening, corresponds to the choice of one or the other of the curves in FIG. Full line. It is thus possible to represent the operating point of the motor, and therefore to deduce the value of the expansion ratio of the turbine for this operating point, from the iso-relaxation curves.

The accuracy of the result, ie the rate of expansion, can be improved by interpolations. All of these data are stored, for example, in the form of a two-input table, namely the Qadapt flow rate and the turbine opening, and an output quantity, namely the expansion ratio. Since the expansion ratio is defined by Pit = P3 / P4, the turbine inlet pressure can be expressed as: P3 = Pit.P4.

This method makes it possible to anticipate without measure or to check the measurement of the turbine inlet pressure from the manufacturer data representative of the behavior of the compressor and the turbine. It makes it possible to check the correspondence between the measurement and the nominal operation of the supercharging machine in order to anticipate a possible malfunction of the engine. This pressure can be used in the context of an engine diagnosis, in order to optimize its operation, where it constitutes a functional quantity. It can also be operated by an engine control unit to continuously optimize its operation.

An implementation of this method can consist in determining by direct measurement the temperatures Ti, T2 and T3, and the pressure P4 of the gas leaving the turbine. The value of the flow Qgb is then deduced from these values by using the relation [3], the turbine opening is for example directly from a motor control computer or from a turbine opening control unit.

Claims (6)

1. Method for determining, in a supercharged heat engine, a mean pressure (P3) at the inlet of a turbine of a turbocharger fitted to this engine, this engine comprising a turbine opening control such as a control of a solenoid valve for regulating the flow of gas entering the turbine, in which a quantity representative of the turbine opening, the temperature (T3) of the gas entering the turbine, the pressure, are evaluated by measurement and / or by calculation; (P4) of the gas leaving the turbine, and the mass flow (Qgb) of the gas passing through the turbine, in which a suitable flow (Qadapt) with the relation Qadapt = Qgb is determined from these data. T3 / P4, in which is read in a data table a value of rate of expansion (Pit) of the turbine associated with the pair of values formed by the adapted flow (Qadapt) and the turbine opening, and in which one multiplies the rate of expansion (Pit) by the turbine outlet pressure (P4) to determine the turbine inlet pressure (P3). 2. The method of claim 1, wherein the turbine outlet pressure (P4) is determined from the evaluation, the gas mass flow (Qgb), an evaluation of the atmospheric pressure (P0), a CPE counter pressure value at a reference flow rate Qref, with the relationship P4 = PO + CPE * 2 Qgb Qref 3. Method according to claim 1 or 2, wherein the pressure (P4) at the turbine outlet is determined by direct measurement with a pressure sensor. 4. The method of claim 3, wherein the turbine aperture value is from a turbine aperture control unit. The method of claim 4, wherein the gas flow rate (Qair) entering the compressor is determined from the gas flow (Qgb) exiting the turbine and a factor (k) representative of the ratio of the gas flow (Qgb) leaving the turbine and the gas flow (Qair) entering the compressor, with the relation Qgb = k.Qair. 6. Method according to one of the preceding claims, wherein the data table giving the rate of compression (Pit) of the turbine as a function of the adapted flow rate (Qadapt) and the turbine opening is established by carrying out tests. of the turbine at different operating points.