EP3581788B1 - Control of glow plugs of an internal combustion engine - Google Patents

Description

The present invention relates to the field of glow plugs for internal combustion engines, in particular diesel type engines.

More particularly, the present invention relates to the field of estimating the temperature of such glow plugs.

In order to start a diesel engine when the outside temperature is below a threshold value, the temperature of the air-fuel mixture must be raised to a certain temperature to improve the self-ignition of the mixture. For this, glow plugs installed in the combustion chamber of the engine and controlled via a preheating unit are used.

There are so-called “quick-ignition” spark plugs, controlled in particular by a control of the Opening Cyclic Ratio type, acronym “RCO”. The use of such candles requires knowing the temperature of the heating element of the candles, so as not to exceed their maximum allowable temperature, which would lead to damage to the heating element of said candles.

The document US 2011/0220073 A1 discloses a control of the heating element according to a pressure prevailing in the combustion chamber.

We can refer to the document FR 2 891 868 - B1 which describes a method for estimating the exhaust temperature on an outlet of a cylinder of an internal combustion engine, from the measured cylinder pressure.

However, this document only provides for the calculation of a temperature in the combustion chamber when the exhaust valve is opened.

We know the theoretical models for estimating the temperature of glow plugs in which an energy balance is carried out at the level of the heating element, here the spark plug rod.

Avec :

• m, la masse du crayon, exprimée en Kg ;
• Cv, la capacité calorifique massique du matériau du crayon, exprimée J.K-1.kg-1 ; et
• T_bougie, la température du crayon de la bougie, exprimée en K.

The variation of internal energy dE of the spark plug is written according to the following equation: $$\mathit{of}=m.\mathit{CV}.{\mathit{dT}}_{\mathit{candle}}$$

With:

• m, the mass of the pencil, expressed in Kg;
• Cv, the specific heat capacity of the rod material, expressed JK-1.kg-1; and
• T _candle , the temperature of the pencil of the candle, expressed in K.

Avec :

• h, un coefficient d'échanges thermiques variant de façon linéaire en fonction de la température des gaz dans la chambre de combustion : h(T_gaZ) = h _0. (1 + KT_gaz ), exprimée en W/(m².K) ;
• S, la surface du crayon en contact avec les gaz dans le cylindre, exprimée en _m ² ;
• T_gaz, la température des gaz internes à la chambre de combustion, exprimée en K;
• t, le temps, exprimé en s ;
• λ, la conductivité thermique exprimée en W/(m².K) ;
• s, la surface d'échange entre la bougie et la culasse, exprimée en m² ; et
• T_culasse, la température de la culasse, approximable à la température du liquide de refroidissement T_eau mesurée par un capteur, exprimée en K.

The lost energy E _{lost is} written according to the following equation and corresponds to cooling by convective exchanges with the gases internal to the combustion chamber, and by conduction between the spark plug and its attachment to the cylinder head of the engine. $$E_{\mathit{lost}}=h\left(T_{\mathit{gas}}\right).S.\left(T_{\mathit{candle}}-T_{\mathit{gas}}\right).\mathit{dt}+\lambda .s.\left(T_{\mathit{candle}}-T_{\mathit{cylinder\; head}}\right).\mathit{dt}$$

With:

• h, a heat exchange coefficient varying linearly as a function of the temperature of the gases in the combustion chamber: h (T _gaZ ) = h _{0 .} (1 + KT _gas ) , expressed in W / (m ² .K);
• S, the area of the rod in contact with the gases in the cylinder, expressed in _m ² ;
• T _gas , the temperature of the gases internal to the combustion chamber, expressed in K;
• t, the time, expressed in s ;
• λ, the thermal conductivity expressed in W / ( m ² .K ) ;
• s, the exchange surface between the spark plug and the cylinder head, expressed in m ² ; and
• T _{cylinder head} , the temperature of the cylinder head, approximate to the temperature of the coolant T _water measured by a sensor, expressed in K.

Avec :

• U, la tension appliquée à la bougie, exprimée en V;
• I, l'intensité parcourant la bougie, exprimée en A; et
• R, la résistance de la bougie variant de façon linéaire en fonction de la température de la bougie : R(T_bougie )=R₀.(1+a.T_bougie ), exprimée en W, avec a, un coefficient d'évolution de la résistance électrique du crayon de la bougie avec sa température.

The energy gained E _{gained is} written according to the following equation and corresponds to the electric power applied to the spark plug. $$E_{\mathit{won}}=U.I.\mathit{dt}=\frac{U^2}{R\left(T_{\mathit{candle}}\right)}.\mathit{dt}$$

With:

• U, the voltage applied to the spark plug, expressed in V;
• I, the intensity traversing the candle, expressed in A; and
• R, the resistance of the spark plug varying linearly as a function of the temperature of the spark plug: R ( T _{spark plug} ) = R _0. (1 + aT _{spark plug} ) , expressed in W, with a, an evolution coefficient of the electrical resistance of the spark plug pencil with its temperature.

The variation in internal energy dE of the spark plug can thus be written according to the following equation: $$\mathit{of}=m.\mathit{CV}.{\mathit{dT}}_{\mathit{candle}}=E_{\mathit{won}}-E_{\mathit{lost}}=\frac{U^2}{R\left(T_{\mathit{candle}}\right)}.\mathit{dt}-h\left(T_{\mathit{gas}}\right).S.\left(T_{\mathit{candle}}-T_{\mathit{gas}}\right).\mathit{dt}+\lambda .s.\left(T_{\mathit{candle}}-T_{\mathit{cylinder\; head}}\right).\mathit{dt}$$

$$\frac{{\mathit{dT}}_{\mathit{bougie}}}{\mathit{dt}}=\frac{1}{{\tau }_{\mathit{chauffe}}}.{\left(\frac{U}{U_{\mathit{ref}}}\right)}^2.\left(\frac{1}{a}-T_{\mathit{bougie}}\right)-\frac{1}{{\tau }_{\mathit{refr}}}.\left(1+K.T_{\mathit{bougie}}\right).\left(T_{\mathit{bougie}}-T_{\mathit{gaz}}\right)+\lambda .s.\left(T_{\mathit{bougie}}-T_{\mathit{eau}}\right).\mathit{dt}$$

Avec :

• Uref, la tension de référence d'alimentation du réseau de bord, généralement 12V, exprimée en V; $\frac{1}{{\tau }_{\mathit{chauffe}}}=\frac{a.{U_{\mathit{ref}}}^2}{m.\mathit{Cv}.R_0},$
  
  une valeur constante, exprimée à s^-1 ; et
• $\frac{1}{{\tau }_{\mathit{refr}}}=\frac{h_0.S}{m.\mathit{Cv}.R_0},$
  
  une valeur constante, exprimée à s^-1 .

Which leads to the following equations: $$\frac{{\mathit{dT}}_{\mathit{candle}}}{\mathit{dt}}=\frac{U^2}{m.\mathit{CV}.R_0}.\frac{1}{\left(1+\mathrm{at}.T_{\mathit{candle}}\right)}-\frac{h_0.S}{m.\mathit{CV}.R_0}.\left(1+K.T_{\mathit{candle}}\right).\left(T_{\mathit{candle}}-T_{\mathit{gas}}\right).+\lambda .s.\left(T_{\mathit{candle}}-T_{\mathit{water}}\right).\mathit{dt}$$

$$\frac{{\mathit{dT}}_{\mathit{candle}}}{\mathit{dt}}=\frac{1}{{\tau }_{\mathit{heated}}}.{\left(\frac{U}{U_{\mathit{ref}}}\right)}^2.\left(\frac{1}{\mathrm{at}}-T_{\mathit{candle}}\right)-\frac{1}{{\tau }_{\mathit{refr}}}.\left(1+K.T_{\mathit{candle}}\right).\left(T_{\mathit{candle}}-T_{\mathit{gas}}\right)+\lambda .s.\left(T_{\mathit{candle}}-T_{\mathit{water}}\right).\mathit{dt}$$

With:

• Uref, the onboard network supply reference voltage, generally 12V, expressed in V; $\frac{1}{{\tau }_{\mathit{heated}}}=\frac{\mathrm{at}.{U_{\mathit{ref}}}^2}{m.\mathit{CV}.R_0},$
  
  a constant value, expressed at s ^-1 ; and
• $\frac{1}{{\tau }_{\mathit{refr}}}=\frac{h_0.S}{m.\mathit{CV}.R_0},$
  
  a constant value, expressed at s ^-1 .

It will be noted that the coefficient of exchange by convection h ₀ is likely to be variable with the pressure and the speed of the gases in the combustion chamber (Nusselt and Planck number). This coefficient may depend on the gas flow admitted by the engine.

The temperature of the candle T (t) as a function of time is calculated by discretizing the equation Eq.i.5 at a temporal recurrence in an on-board computer, according to the following equation: $$T_{\left(t\right)}=T_{\left(t-1\right)}+\frac{1}{{\tau }_{\mathit{heated}}}.{\left(\frac{U_{\left(t\right)}}{U_{\mathit{ref}}}\right)}^2.\left(\frac{1}{\mathrm{at}}-T_{\left(t-1\right)}\right)-\frac{1}{{\tau }_{\mathit{ref}}r}.\left(1+K.T_{\left(t-1\right)}\right).\left(T_{\left(t-1\right)}-T_{\mathit{gas}}\right)+k.\left(T_{\left(t-1\right)}-T_{\mathit{water}}\right)$$

In this equation, the temperature of the gases inside the combustion chamber T _gas is not measured and is difficult to estimate precisely. It can be approximated at an average temperature, either fixed corresponding to the equilibrium temperature of the spark plug, for example during measurements on test benches, or characterized by maps integrated into the computer.

However, the temperature of the gases inside the combustion chamber T _gas depends in particular on the engine operating point, the temperature of the admitted gases, the gas flow rate passing through the engine, the combustion regulation, etc.

It is thus difficult to obtain a reliable estimate of the temperature of the spark plug T (t) in the absence of a determination of the temperature of the gases inside the combustion chamber T _gas .

There is a need to improve methods and systems for estimating the temperature of glow plugs.

The aim of the present invention is therefore to allow a precise and robust estimation of the temperature of the glow plugs and thus to allow the best use of the potential of said plugs to efficiently heat the combustion chamber and improve the starting of the engine, as well as engine stability after starting.

The subject of the invention is a method for controlling the glow plugs according to claim 1 and a control unit for an internal combustion engine according to claim 5. The method comprises estimating the temperature of the rod of the glow plugs. implanted in a combustion chamber of an internal combustion engine, in particular of the Diesel type, comprising at least one cylinder pressure sensor, by measuring the cylinder pressure, for each crankshaft position, for example every crankshaft degree 1 ° crk, in knowing the driving voltage of the spark plug, by initializing the temperature of the spark plug at the first cold engine calculation step, at the temperature of the cylinder head, that is to say at the temperature of the coolant, or at the temperature previous in the event of a hot start, an average temperature value in the cylinder is estimated over a combustion cycle and by iterative calculation steps, the temperature of the spark plug rod is estimated.

Thus, the temperature of the glow plug rods is estimated accurately, using only the physical and geometric parameters of the engine.

A temperature value in the cylinder is calculated for each angular position of the engine as a function of a value of the temperature at the closing of the intake valves, a value of the volume of the combustion chamber at the closing of the intake valves , calculated as a function of the crankshaft angle, the stroke, the bore and the dead volume of the cylinder, a volume value of the combustion chamber, for each angular position of the cylinder, a value of cylinder pressure) on closing intake valves, measured by a cylinder sensor and a cylinder pressure value, for each angular position of the cylinder, measured by the cylinder sensor.

est une constante entre un point considéré et le point de fermeture des soupapes d'admission.Thus, the temperature in the cylinder for each angular position of the engine for which the intake and exhaust valves are closed, can be calculated from the ideal gas equation (PV = nRT), where $\frac{P.V}{T}$

is a constant between a point considered and the point of closing of the inlet valves.

For example, the temperature at the closing of the intake valves is calculated by assimilating this phase to a polytropic, as a function of a temperature value at the opening of the valves intake calculated as a function of temperature, exhaust gas recirculation temperature and fresh air and recirculated gas flow rates measured by sensors, the volume of the combustion chamber at the time (ivo (valve opening intake), ivc (closing the intake valves), evo (opening the exhaust valves) and evc (closing the exhaust valves), and a polytropic coefficient of compression depending on the composition of the gases allowed (EGR exhaust gas recirculation rate).

Alternatively, the temperature at the opening of the intake valves can be measured by a sensor.

An estimate of the average temperature of the gases in the cylinder to which the spark plug rod is subjected during an engine cycle is calculated as a function of the temperature value in the cylinder for each angular position of the engine calculated.

The estimate of the average gas temperature is, for example, calculated as a function of the angular opening time of the intake valves, the angular duration with all valves closed, the angular opening time of the exhaust valves, the average temperature in the cylinder during the intake and expansion phases, assimilated to the average temperature [T _ivo ; T _ivc ]; [T _evo ; T _evc ] and the temperature when the exhaust valves are closed.

For example, the temperature at the closing of the exhaust valves is calculated by assimilating this phase to a polytropic as a function of a polytropic coefficient of the expansion, depending on the composition of the exhaust gases (richness), and on the temperature at the opening of the exhaust valves.

An estimate of the temperature of the spark plug rod is calculated as a function of the average temperature of the gases over a combustion cycle, this temperature being calculated from the temperature values in the cylinder for each angular position of the engine, these values being calculated thanks in particular to the measurements of the cylinder pressure sensor.

This eliminates the imprecision of the characterization of the temperature of the gases by tables or maps in the equation for calculating the temperature of the glow plugs.

Thanks to the invention, the calculation of the estimate of the temperature of the glow plug rod is robust at the engine operating point (speed and torque, or speed and quantity of fuel injected), at ambient temperature or pressure. , the flow admitted by the engine and the combustion settings.

The engine control unit comprises a system for estimating the temperature of the rod of the glow plugs installed in a combustion chamber of an internal combustion engine, in particular of the Diesel type, comprising at least one cylinder pressure sensor , in which as a function of the cylinder pressure measured by the sensor, for each crankshaft position, for example all crankshaft degrees 1 ° crk, by knowing the pilot voltage of the spark plug, by initializing the temperature of the spark plug at the first step when the engine is cold, at the temperature of the cylinder head, i.e. at the temperature of the coolant, or at the previous temperature in the event of a hot start.

The system comprises a module for estimating an average temperature value in the cylinder over a combustion cycle and in steps of iterative calculations, and a module for estimating the temperature of the spark plug rod.

Thus, the temperature of the glow plug rods is estimated accurately, using only the physical and geometric parameters of the engine.

The system comprises a module for calculating a temperature value in the cylinder for each angular position of the engine for which the intake and exhaust valves are closed, as a function of a value of the temperature when the valves are closed. intake, a volume value of the combustion chamber when the valves are closed intake, calculated as a function of the crankshaft angle, the stroke, the bore and the dead volume of the cylinder, a volume value of the combustion chamber, for each angular position of the cylinder, a value of cylinder pressure at the closing of the intake valves, measured by a cylinder sensor and a cylinder pressure value, for each angular position of the cylinder, measured by the cylinder sensor.

est une constante entre un point considéré et le point de fermeture des soupapes d'admission.Thus, the temperature in the cylinder for each angular position of the engine for which the intake and exhaust valves are closed can be calculated from the ideal gas equation (PV = nRT), where $\frac{P.V}{T}$

is a constant between a point considered and the point of closing of the inlet valves.

The inlet valve closing temperature is calculated by assimilating this phase to a polytropic, as a function of a temperature value at the opening of the inlet valves calculated as a function of the temperature, of the recirculation temperature of the exhaust gas and fresh air and recirculated gas flow rates measured by sensors, the volume of the combustion chamber at time X (ivo (opening of the intake valves), ivc (closing of the intake valves), evo (opening of the exhaust valves) and evc (closing of the exhaust valves), and a polytropic compression coefficient depending on the composition of the gases admitted (EGR exhaust gas recirculation rate).

Alternatively, the temperature at the opening of the intake valves can be measured by a sensor.

The module for calculating an estimate of the average temperature of the gases in the cylinder to which the spark plug rod is subjected during an engine cycle is configured to calculate said estimate as a function of the temperature value in the cylinder for each angular position of the motor calculated.

The estimate of the average gas temperature is calculated as a function of the angular opening time of the intake valves, the 9 angular duration with all valves closed, the angular opening duration of the exhaust valves, the average temperature in the cylinder during the intake and expansion phases assimilated to the average temperature [T _ivo ; T _ivc ]; [T _evo ; T _evc ] and the temperature when the exhaust valves are closed.

The temperature at the closing of the exhaust valves is calculated by assimilating this phase to a polytropic according to a polytropic coefficient of the expansion, depending on the composition of the exhaust gases (richness), and the temperature at the opening exhaust valves.

Thus, the module for calculating an estimate of the temperature of the spark plug rod is configured to calculate said estimate as a function of the temperature in the cylinder for each angular position of the engine calculated and of the average temperature of the gases over a cycle of calculated combustion.

This eliminates the imprecision of the characterization of the temperature of the gases by tables or maps in the equation for calculating the temperature of the glow plugs.

Thanks to the invention, the calculation of the estimate of the temperature of the glow plug rod is robust at the engine operating point (speed and torque, or speed and quantity of fuel injected), at ambient temperature or pressure. , the flow admitted by the engine and the combustion settings.

• la figure 1 illustre un système d'estimation de la température des bougies de préchauffage selon l'invention ; et
• la figure 2 illustre les étapes d'un procédé d'estimation de la température des bougies de préchauffage selon l'invention mis en œuvre par le système de la figure 1.

Other objects, characteristics and advantages of the invention will become apparent on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

• the figure 1 illustrates a system for estimating the temperature of the glow plugs according to the invention; and
• the figure 2 illustrates the steps of a method for estimating the temperature of the glow plugs according to the invention implemented by the system of the figure 1 .

A system for estimating the temperature of the glow plugs, referenced 10 as a whole, is intended to be integrated into a control unit of an internal combustion engine of Diesel type (not shown), comprising a combustion chamber in the pistons of the engine, glow plugs located in said combustion chamber and at least one pressure sensor of cylinder P.

Avec :

• T_ivc, la température à la fermeture des soupapes d'admission « intake valve closing temperature » en termes anglo-saxons, calculées selon l'équation Eq.2 ci-dessous, exprimée en K ;
• Vol_ivc, le volume de la chambre de combustion à la fermeture des soupapes d'admission calculé en fonction de l'angle vilebrequin, la course, l'alésage et le volume mort du cylindre, exprimé en m³ ;
• Vol_(angle), le volume de la chambre de combustion, pour chaque position angulaire du cylindre, calculé en fonction de l'angle vilebrequin, la course, l'alésage et le volume mort du cylindre, exprimé en m³ ;
• Pcyl_(ivc), la pression du cylindre à la fermeture des soupapes d'admission, mesurée par un capteur cylindre, exprimée en bar ; et
• Pcyl_(angle), la pression du cylindre, pour chaque position angulaire du cylindre, mesurée par un capteur cylindre, exprimée en bar.

The system 10 for estimating the temperature of the glow plugs comprises a module 12 for calculating a temperature value in the cylinder T _{cyl (angle)} for each angular position of the engine for which the intake and exhaust valves associated with the combustion chamber are closed, according to the following equation: $$T_{\mathit{cyl}\left(\mathit{angle}\right)}=\frac{T_{\mathit{ivc}}}{{\mathit{Flight}}_{\mathit{ivc}}.{\mathit{Pcyl}}_{\mathit{ivc}}}.{\mathit{Flight}}_{\left(\mathit{angle}\right)}.{\mathit{Pcyl}}_{\left(\mathit{angle}\right)}$$

With:

• T _ivc , the temperature at the closing of the intake valves “intake valve closing temperature” in Anglo-Saxon terms, calculated according to the equation Eq.2 below, expressed in K;
• Vol _ivc , the volume of the combustion chamber when the intake valves are closed calculated as a function of the crankshaft angle, the stroke, the bore and the dead volume of the cylinder, expressed in m ³ ;
• Flight _(angle) , the volume of the combustion chamber, for each angular position of the cylinder, calculated according to the crankshaft angle, stroke, bore and dead volume of the cylinder, expressed in m ³ ;
• Pcyl _(ivc) , the cylinder pressure when the intake valves are closed, measured by a cylinder sensor, expressed in bar; and
• Pcyl _(angle) , the cylinder pressure, for each angular position of the cylinder, measured by a cylinder sensor, expressed in bar.

est une constante entre un point considéré et le point de fermeture des soupapes d'admission (ivc).Thus, the temperature in the cylinder T _{cyl (angle)} for each angular position of the engine for which the intake and exhaust valves are closed, can be calculated from the gas equation perfect (PV = nRT), where $\frac{P.V}{T}$

is a constant between a point considered and the point of closing of the inlet valves (ivc).

The temperature at closing of the intake valves T _ivc is calculated according to the equation below by assimilating this phase to a polytropic: $$T_{\mathit{ivc}}=T_{\mathit{ivo}}.{\left(\frac{{\mathit{Flight}}_{\mathit{ivo}}}{{\mathit{Flight}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}$$

Avec :

• T_ivo, la température à l'ouverture des soupapes d'admission « intake valve opening temperature » en termes anglo-saxons, calculée selon l'équation Eq.3 ci-dessus en fonction de la température T_air, de la température de recirculation des gaz d'échappement T_EGR, et des débits d'air frais Q_air et de gaz recirculés Q_EGR, mesurés par capteurs ; en variante, la température à l'ouverture des soupapes d'admission T_ivo peut être mesurée par un capteur, exprimée en K;
• Vol_X, le volume de la chambre de combustion au moment X (ivo (ouverture des soupapes d'admission), ivc (fermeture des soupapes d'admission), evo (ouverture des soupapes d'échappement) et evc (fermeture des soupapes d'échappement), exprimé en m³ ; et
• y_ivc, un coefficient polytropique de la compression dépendant de la composition des gaz admis (taux de recirculation des gaz d'échappement EGR).

With: $$T_{\mathit{ivo}}=\frac{T_{\mathit{air}}.Q_{\mathit{air}}+T_{\mathit{EGR}}.Q_{\mathit{EGR}}}{Q_{\mathit{air}}+Q_{\mathit{EGR}}}$$

With:

• T _ivo , the temperature at the opening of the intake valves "intake valve opening temperature" in English terms, calculated according to the equation Eq.3 above as a function of the _air temperature T, of the recirculation temperature exhaust gases T _EGR , and flow rates of fresh _air Q air and recirculated gases Q _EGR , measured by sensors; as a variant, the temperature at the opening of the intake valves T _ivo can be measured by a sensor, expressed in K;
• Vol _X , the volume of the combustion chamber at time X (ivo (opening of the intake valves), ivc (closing of the intake valves), evo (opening of the exhaust valves) and evc (closing of the valves of 'exhaust), expressed in m ³ ; and
• y _ivc , a polytropic coefficient of compression depending on the composition of the gases admitted (EGR exhaust gas recirculation rate).

The system 10 for estimating the temperature of the glow plugs further comprises a module 14 for calculating an estimate of the average temperature of the gases Tgas in the cylinder to which the spark plug rod is subjected during a cycle. engine.

Avec:

• α_ivo → α_ivo, la durée d'ouverture angulaire des soupapes admission α_admin, exprimée en [°crk] ;
• α_{ivc → evo}, la durée angulaire toutes soupapes fermées, exprimée en [°crk];
• α_evo → α_evc, la durée d'ouverture angulaire des soupapes d'échappement a_exh, exprimée en [°crk] ;
• moy{Tcyl _(angle)} _αivc→α_evo , la température moyenne dans le cylindre pendant les phases d'admission et de détente assimilée à la moyenne des températures respectivement [T_ivo; Ti_vc] et [T_evo ; T_evc], exprimée en K ; et
• T_evc, la température à la fermeture des soupapes d'échappement « exhaust valve closing temperature » en termes anglo-saxons, calculées selon l'équation Eq.5 ci-dessous, exprimée en K.

Considering the thermal inertia of the spark plug rod, of the order of 1 / 10th of a second, clearly greater than the characteristic time of evolution of the cylinder temperature (or pressure), of the order of a thousandth of a second, the average temperature Tgas (cycle) can be estimated as a temperature value of the gases inside the cylinder averaged over a combustion cycle, and calculated by the module 14 according to the following equation: $$T_{\mathit{gas}\left(\mathit{cycle}\_\mathrm{not}\right)}=\left[\frac{{\alpha }_{\mathit{ivo}}\to {\alpha }_{\mathit{ivo}}}{720°}\cdot \frac{T_{\mathit{ivo}}+T_{\mathit{ivc}}}{2}\right]+\left[\frac{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}{720°}.\mathit{avg}{\left\{{\mathit{Tcyl}}_{\left(\mathit{angle}\right)}\right\}}_{{\alpha }_{\mathit{ivc}}\to {\alpha }_{\mathit{evo}}}\right]+\left[\frac{{\alpha }_{\mathit{evo}}\to {\alpha }_{\mathit{evc}}}{720°}.\frac{T_{\mathit{evo}}+T_{\mathit{evc}}}{2}\right]$$

With:

• α _ivo → α _ivo , the angular opening time of the inlet valves α _admin , expressed in [° crk];
• α _{ivc → evo} , the angular duration with all valves closed, expressed in [° crk];
• α _evo → α _evc , the angular opening time of the exhaust valves at _exh , expressed in [° crk];
• moy { Tcyl _{( angle )} } _αivc → α _evo , the average temperature in the cylinder during the intake and expansion phases assimilated to the average temperature respectively [T _ivo ; Ti _vc ] and [T _evo ; T _evc ], expressed in K; and
• T _evc , the temperature at the closing of the exhaust valve closing temperature in Anglo-Saxon terms, calculated according to the equation Eq.5 below, expressed in K.

Avec :

• γ_evc, un coefficient polytropique de la détente, dépendant la composition des gaz d'échappement (richesse) ; et
• Tevo, la température cylindre à l'ouverture des soupapes d'échappement, exprimée en K, avec T_evo = Tcyl_(evo)

The temperature at closing of the exhaust valves T _evc is calculated according to the equation below by assimilating this phase to a polytropic: $$T_{\mathit{evc}}=T_{\mathit{evo}}.{\left(\frac{{\mathit{Flight}}_{\mathit{evo}}}{{\mathit{Flight}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{iec}}}$$

With:

• γ _evc , a polytropic coefficient of expansion, depending on the composition of the exhaust gases (richness); and
• Tevo, the cylinder temperature at the opening of the exhaust valves, expressed in K, with T _evo = Tcyl _(evo)

$$T_{\mathit{gaz}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.T_{\mathit{ivo}}.\frac{1+{\left(\frac{{\mathit{Vol}}_{\mathit{ivo}}}{{\mathit{Vol}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}}{2}\right]+\left[\frac{1}{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}.{\displaystyle \underset{{\alpha }_{\mathit{ivc}}}{\overset{{\alpha }_{\mathit{evo}}}{\int }}\mathit{Tcyl}.\mathit{d\alpha }}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}{\mathit{Tcyl}}_{\left(\mathit{evo}\right)}.\frac{1+{\left(\frac{{\mathit{Vol}}_{\mathit{evo}}}{{\mathit{Vol}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{evc}}}}{2}\right]$$

By incorporating the equations Eq.2 and Eq.3 into the equation Eq.4, the average temperature of the Tgas (cycle) over a combustion cycle is written according to the following equations: $$T_{\mathit{gas}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.\frac{T_{\mathit{ivo}}+T_{\mathit{ivc}}}{2}\right]+\left[\left(720°-\left({\alpha }_{\mathit{ad}\min }+{\alpha }_{\mathit{exh}}\right)\right)\mathrm{..}\mathit{avg}{\left\{{\mathit{Tcyl}}_{\left(\mathit{angle}\right)}\right\}}_{{\alpha }_{\mathit{ivc}}\to {\alpha }_{\mathit{evo}}}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}\frac{T_{\mathit{evo}}+T_{\mathit{evc}}}{2}\right]$$

$$T_{\mathit{gas}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.T_{\mathit{ivo}}.\frac{1+{\left(\frac{{\mathit{Flight}}_{\mathit{ivo}}}{{\mathit{Flight}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}}{2}\right]+\left[\frac{1}{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}.{\displaystyle \underset{{\alpha }_{\mathit{ivc}}}{\overset{{\alpha }_{\mathit{evo}}}{\int }}\mathit{Tcyl}.\mathit{d\alpha }}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}{\mathit{Tcyl}}_{\left(\mathit{evo}\right)}.\frac{1+{\left(\frac{{\mathit{Flight}}_{\mathit{evo}}}{{\mathit{Flight}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{evc}}}}{2}\right]$$

The system 10 for estimating the temperature of the glow plugs further comprises a module 16 for calculating an estimate of the temperature of the rod of the spark plug T (t) as a function of the temperature in the cylinder T _{cyl (angle)} for each angular position of the engine calculated in equation Eq.1 and for the average gas temperature Tgas (cycle) over a combustion cycle calculated in equation Eq.7.

Avec :

• T_chauffe, le temps caractéristique de chauffe du crayon de la bougie, exprimé en s ;
• _Trefr, le temps caractéristique de refroidissement du crayon de la bougie, exprimé en s, dépendant du débit d'air aspiré par le moteur, exprimé en kg/s ;
• a, un coefficient d'évolution de la résistance électrique du crayon de la bougie avec sa température ;
• K, un coefficient d'évolution des facteurs thermiques avec la température des gaz ;
• k, un facteur d'échanges conductifs via la culasse égal à λ.s, avec λ, la conductivité thermique exprimée en W/(m².K) et s, la surface d'échange entre la bougie et la culasse, exprimée en m² ;
• U_ref, la tension de référence, exprimée en V ; et
• T_eau, la température de l'eau, exprimée en K.

The estimate of the temperature of the spark plug rod T _{(t) is} written according to the following equation: $$T_{\left(t\right)}=T_{\left(t-1\right)}+\frac{1}{{\tau }_{\mathit{heated}}}.{\left(\frac{U_{\left(t\right)}}{U_{\mathit{ref}}}\right)}^2.\left(\frac{1}{\mathrm{at}}-T_{\left(t-1\right)}-\frac{1}{{\tau }_{\mathit{ref}}r}.\left(1+K.T_{\left(t-1\right)}\right).\left(T_{\left(t-1\right)}-T_{\mathit{gas}\left(\mathit{cycle}\right)}\right)+k.\left(T_{\left(t-1\right)}-T_{\mathit{water}}\right)\right)$$

With:

• T _{heats up} , the characteristic heating time of the candle rod, expressed in s;
• _Trefr , the characteristic cooling time of the spark plug rod, expressed in s, depending on the air flow sucked by the engine, expressed in kg / s;
• a, a coefficient of variation of the electrical resistance of the spark plug rod with its temperature;
• K, a coefficient of evolution of the thermal factors with the temperature of the gases;
• k, a conductive exchange factor via the cylinder head equal to λ.s, with λ, the thermal conductivity expressed in W / ( m ² .K ) and s, the exchange surface between the spark plug and the cylinder head, expressed in m ² ;
• U _ref , the reference voltage, expressed in V; and
• T _water , the temperature of the water, expressed in K.

Thus, by measuring the cylinder pressure P _cyl , for each crankshaft position, for example all the crankshaft degrees by knowing the piloting voltage of the spark plug, by initializing the temperature of the spark plug T (t) at the first cold engine calculation step , at the temperature of the cylinder head, that is to say at the temperature of the coolant, or at the previous temperature in the event of a hot start, it is possible to estimate an average temperature value in the cylinder on a T _{gas combustion cycle (cycle)} . By steps of iterative calculations, the temperature of the glow plug rod T _(t) can then be estimated precisely, using only the physical and geometric parameters of the engine.

This avoids the imprecision of the characterization of the temperature of the Tgas gases by tables or maps in the equation Eq.8 which is in the state of the simplified technique.

Thanks to the invention, the calculation of the estimate of the temperature of the glow plug rod is robust at the engine operating point (speed and torque, or speed and quantity of fuel injected), at ambient temperature or pressure. , the flow admitted by the engine and the combustion settings.

The organization chart shown on the figure 2 illustrates an example of a method 20 implemented by the system shown in figure 1 .

Avec :

• T_ivc, la température à la fermeture des soupapes d'admission « intake valve closing temperature » en termes anglo-saxons, calculée selon l'équation Eq.2 ci-dessous, exprimée en K ;
• Vol_ivc, le volume de la chambre de combustion à la fermeture des soupapes d'admission calculé en fonction de l'angle vilebrequin, la course, l'alésage et le volume mort du cylindre, exprimé en m³ ;
• Vol_(angle), le volume de la chambre de combustion, pour chaque position angulaire du cylindre, calculé en fonction de l'angle vilebrequin, la course, l'alésage et le volume mort du cylindre, exprimé en m³ ;
• Pcyl_(ivc), la pression du cylindre à la fermeture des soupapes d'admission, mesurée par un capteur cylindre, exprimée en bar ; et
• Pcyl_(angle), la pression du cylindre, pour chaque position angulaire du cylindre, mesurée par un capteur cylindre, exprimée en bar.

In a first step 21, a temperature value is calculated in the cylinder T _{cyl (angle)} for each angular position engine for which the intake and exhaust valves are closed according to the following equation: $$T_{\mathit{cyl}\left(\mathit{angle}\right)}=\frac{T_{\mathit{ivc}}}{{\mathit{Flight}}_{\mathit{ivc}}.{\mathit{Pcyl}}_{\mathit{ivc}}}.{\mathit{Flight}}_{\left(\mathit{angle}\right)}.{\mathit{Pcyl}}_{\left(\mathit{angle}\right)}$$

With:

• T _ivc , the temperature at the closing of the intake valves “intake valve closing temperature” in Anglo-Saxon terms, calculated according to the equation Eq.2 below, expressed in K;
• Vol _ivc , the volume of the combustion chamber when the intake valves are closed calculated as a function of the crankshaft angle, the stroke, the bore and the dead volume of the cylinder, expressed in m ³ ;
• Flight _(angle) , the volume of the combustion chamber, for each angular position of the cylinder, calculated according to the crankshaft angle, stroke, bore and dead volume of the cylinder, expressed in m ³ ;
• Pcyl _(ivc) , the cylinder pressure when the intake valves are closed, measured by a cylinder sensor, expressed in bar; and
• Pcyl _(angle) , the cylinder pressure, for each angular position of the cylinder, measured by a cylinder sensor, expressed in bar.

est une constante entre un point considéré et le point de fermeture des soupapes d'admission (ivc).Thus, the temperature in the cylinder T _{cyl (angle)} for each angular position of the engine, for which the intake and exhaust valves are closed, can be calculated from the ideal gas equation (PV = nRT), or $\frac{P.V}{T}$

is a constant between a point considered and the point of closing of the inlet valves (ivc).

Avec : $$T_{\mathit{ivo}}=\frac{T_{\mathit{air}}.Q_{\mathit{air}}+T_{\mathit{EGR}}.Q_{\mathit{EGR}}}{Q_{\mathit{air}}+Q_{\mathit{EGR}}}$$

Avec :

• T_ivo, la température à l'ouverture des soupapes d'admission « intake valve opening temperature » en termes anglo-saxons, calculée selon l'équation Eq.3 ci-dessus en fonction de la température T_air, de la température de recirculation des gaz d'échappement T_EGR, et des débits d'air frais Q_air et de gaz recirculés Q_EGR, mesurés par capteurs ; en variante, la température à l'ouverture des soupapes d'admission T_ivo peut être mesurée par un capteur, exprimée en K;
• Vol_x, le volume de la chambre de combustion au moment X (ivo (ouverture des soupapes d'admission), ivc (fermeture des soupapes d'admission), evo (ouverture des soupapes d'échappement) et evc (fermeture des soupapes d'échappement), exprimé en m³ ; et
• y_ivc, un coefficient polytropique de la compression dépendant de la composition des gaz admis (taux de recirculation des gaz d'échappement EGR).

The temperature at closing of the intake valves T _ivc is calculated according to the equation below by assimilating this phase to a polytropic: $$T_{\mathit{ivc}}=T_{\mathit{ivo}}.{\left(\frac{{\mathit{Flight}}_{\mathit{ivo}}}{{\mathit{Flight}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}$$

With: $$T_{\mathit{ivo}}=\frac{T_{\mathit{air}}.Q_{\mathit{air}}+T_{\mathit{EGR}}.Q_{\mathit{EGR}}}{Q_{\mathit{air}}+Q_{\mathit{EGR}}}$$

With:

• T _ivo , the temperature at the opening of the intake valves "intake valve opening temperature" in English terms, calculated according to the equation Eq.3 above as a function of the _air temperature T, of the recirculation temperature exhaust gases T _EGR , and flow rates of fresh _air Q air and recirculated gases Q _EGR , measured by sensors; as a variant, the temperature at the opening of the intake valves T _ivo can be measured by a sensor, expressed in K;
• Vol _x , the volume of the combustion chamber at time X (ivo (opening of the intake valves), ivc (closing of the intake valves), evo (opening of the exhaust valves) and evc (closing of the intake valves) 'exhaust), expressed in m ³ ; and
• y _ivc , a polytropic coefficient of compression depending on the composition of the gases admitted (EGR exhaust gas recirculation rate).

During step 22, an estimate is calculated of the average temperature of the gases T _gases in the cylinder to which the spark plug rod is subjected during an engine cycle.

Avec :

• α_ivo → α_ivo, la durée d'ouverture angulaire des soupapes admission α_admin, exprimée en [°crk] ;
• α_{ivc →evo}, la durée angulaire toutes soupapes fermées, exprimée en [°crk] ;
• α_evo → α_evc, la durée d'ouverture angulaire des soupapes d'échappement a_exh, exprimée en [°crk] ;
• moy{Tcyl _(angle)]α_ivc → α_evo , la température moyenne dans le cylindre pendant les phases d'admission et de détente assimilée à la moyenne des températures respectivement [T_ivo; T_ivc] et [T_evo ; T_evc], exprimée en K ; et
• T_evc, la température à la fermeture des soupapes d'échappement « exhaust valve closing temperature » en termes anglo-saxons, calculées selon l'équation Eq.5 ci-dessous, exprimée en K.

Considering the thermal inertia of the spark plug rod, of the order of 1 / 10th of a second, clearly greater than the characteristic time of evolution of the cylinder temperature (or pressure), of the order of a thousandth of a second, the average temperature gas T _{gas (cycle)} over a combustion cycle is calculated by the module 14 according to the following equation: $$T_{\mathit{gas}\left(\mathit{cycle}\_\mathrm{not}\right)}=\left[\frac{{\alpha }_{\mathit{ivo}}\to {\alpha }_{\mathit{ivo}}}{720°}.\frac{T_{\mathit{ivo}}+T_{\mathit{ivc}}}{2}\right]+\left[\frac{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}{720°}.\mathit{avg}{\left\{{\mathit{Tcyl}}_{\left(\mathit{angle}\right)}\right\}}_{{\alpha }_{\mathit{ivc}}\to {\alpha }_{\mathit{evo}}}\right]+\left[\frac{{\alpha }_{\mathit{evo}}\to {\alpha }_{\mathit{evc}}}{720°}.\frac{T_{\mathit{evo}}+T_{\mathit{evc}}}{2}\right]$$

With:

• α _ivo → α _ivo , the angular opening time of the inlet valves α _admin , expressed in [° crk];
• α _{ivc → evo} , the angular duration with all valves closed, expressed in [° crk];
• α _evo → α _evc , the angular opening time of the exhaust valves at _exh , expressed in [° crk];
• moy { Tcyl _{( angle )} ] α _ivc → α _evo , the average temperature in the cylinder during the intake and expansion phases assimilated to the average temperature respectively [T _ivo ; T _ivc ] and [T _evo ; T _evc ], expressed in K; and
• T _evc , the temperature at the closing of the exhaust valve closing temperature in Anglo-Saxon terms, calculated according to the equation Eq.5 below, expressed in K.

Avec :

• γ_evc , un coefficient polytropique de la détente, dépendant la composition des gaz d'échappement (richesse) ; et
• Tevo, la température à l'ouverture des soupapes d'échappement, exprimée en K, avec T_evo = Tcyl_(angle).

The temperature at closing of the exhaust valves T _evc is calculated according to the equation below by assimilating this phase to a polytropic: $$T_{\mathit{evc}}=T_{\mathit{evo}}.{\left(\frac{{\mathit{Flight}}_{\mathit{evo}}}{{\mathit{Flight}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{iec}}}$$

With:

• γ _evc , a polytropic coefficient of expansion, depending on the composition of the exhaust gases (richness); and
• Tevo, the temperature at the opening of the exhaust valves, expressed in K, with T _evo = Tcyl _(angle) .

$$T_{\mathit{gaz}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.T_{\mathit{ivo}}.\frac{1+{\left(\frac{{\mathit{Vol}}_{\mathit{ivo}}}{{\mathit{Vol}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}}{2}\right]+\left[\frac{1}{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}\cdot {\displaystyle \underset{{\alpha }_{\mathit{ivc}}}{\overset{{\alpha }_{\mathit{evo}}}{\int }}\mathit{Tcyl}.\mathit{d\alpha }}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}{\mathit{Tcyl}}_{\left(\mathit{evo}\right)}.\frac{1+{\left(\frac{{\mathit{Vol}}_{\mathit{evo}}}{{\mathit{Vol}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{evc}}}}{2}\right]$$

By incorporating the equations Eq.2 and Eq.3 into the equation Eq.4, the average temperature of the Tgas (cycle) over a combustion cycle is written according to the following equations: $$T_{\mathit{gas}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.\frac{T_{\mathit{ivo}}+T_{\mathit{ivc}}}{2}\right]+\left[\left(720-\left({\alpha }_{\mathit{ad}\min }+{\alpha }_{\mathit{exh}}\right)\right)\mathrm{..}\mathit{avg}{\left\{{\mathit{Tcyl}}_{\left(\mathit{angle}\right)}\right\}}_{{\alpha }_{\mathit{ivc}}\to {\alpha }_{\mathit{evo}}}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}\frac{T_{\mathit{evo}}+T_{\mathit{evc}}}{2}\right]$$

$$T_{\mathit{gas}\left(\mathit{cycle}\right)}=\left[{\alpha }_{\mathit{ad}\min }.T_{\mathit{ivo}}.\frac{1+{\left(\frac{{\mathit{Flight}}_{\mathit{ivo}}}{{\mathit{Flight}}_{\mathit{ivc}}}\right)}^{{\gamma }_{\mathit{ivc}}}}{2}\right]+\left[\frac{1}{{\alpha }_{\mathit{ivc}\to \mathit{evo}}}\cdot {\displaystyle \underset{{\alpha }_{\mathit{ivc}}}{\overset{{\alpha }_{\mathit{evo}}}{\int }}\mathit{Tcyl}.\mathit{d\alpha }}\right]+\left[{\alpha }_{\mathit{exh}}\mathrm{..}{\mathit{Tcyl}}_{\left(\mathit{evo}\right)}.\frac{1+{\left(\frac{{\mathit{Flight}}_{\mathit{evo}}}{{\mathit{Flight}}_{\mathit{evc}}}\right)}^{{\gamma }_{\mathit{evc}}}}{2}\right]$$

During step 23, an estimate of the temperature of the spark plug rod T (t) is calculated as a function of the temperature in the cylinder T _{cyl (angle)} for each angular position of the engine calculated in equation Eq.1 and the average gas temperature T _{gas (cycle)} over a combustion cycle calculated using equation Eq.7.

Avec :

• T_chauffe, le temps caractéristique de chauffe du crayon de la bougie, exprimé en s ;
• τ_{refroidissement} , le temps caractéristique de refroidissement du crayon de la bougie, exprimé en s, dépendant du débit d'air aspiré par le moteur, exprimé en kg/s ;
• a, un coefficient d'évolution de la résistance électrique du crayon de la bougie avec sa température ;
• K, un coefficient d'évolution des facteurs thermiques avec la température des gaz ;
• k, un facteur d'échanges conductifs via la culasse égal à λ.s, avec λ, la conductivité thermique exprimée en W/(m².K) et s, la surface d'échange entre la bougie et la culasse, exprimée en m² ;
• U_ref, la tension de référence, exprimée en V ; et
• T_eau, la température de l'eau, exprimée en K.

The estimate of the temperature of the spark plug rod T _{(t) is} written according to the following equation: $$T_{\left(t\right)}=T_{\left(t-1\right)}+\frac{1}{{\tau }_{\mathit{heated}}}.{\left(\frac{U_{\left(t\right)}}{U_{\mathit{ref}}}\right)}^2.\left(\frac{1}{\mathrm{at}}-T_{\left(t-1\right)}\right)-\frac{1}{{\tau }_{\mathit{ref}}r}.\left(1+K.T_{\left(t-1\right)}\right).\left(T_{\left(t-1\right)}-T_{\mathit{gas}\left(\mathit{cycle}\right)}\right)+k.\left(T_{\left(t-1\right)}-T_{\mathit{water}}\right)$$

With:

• T _{heats up,} the characteristic heating time of the candle rod, expressed in s;
• τ _cooling , the characteristic cooling time of the spark plug rod, expressed in s, depending on the air flow sucked by the engine, expressed in kg / s;
• a, a coefficient of variation of the electrical resistance of the spark plug rod with its temperature;
• K, a coefficient of evolution of the thermal factors with the temperature of the gases;
• k, a conductive exchange factor via the cylinder head equal to λ.s, with λ, the thermal conductivity expressed in W / ( m ² .K ) and s, the exchange surface between the spark plug and the cylinder head, expressed in m ² ;
• U _ref , the reference voltage, expressed in V; and
• T _water , the temperature of the water, expressed in K.

Thus, by measuring the cylinder pressure P _cyl , for each crankshaft position, for example all one crankshaft degree knowing the spark plug control voltage, by initializing the temperature of the spark plug T (t) at the first cold engine calculation step, at the cylinder head temperature, i.e. at the temperature of the coolant, or at the previous temperature in the event of a hot start, it is possible to estimate an average temperature value in the cylinder on a T _{gas combustion cycle (cycle)} . By steps of iterative calculations, it is then possible to estimate the temperature of the glow plug rod T _(t) , precisely, using only the physical and geometric parameters of the engine.

This avoids the imprecision of the characterization of the temperature of the Tgas gases by tables or maps in the equation Eq.8 which is in the state of the simplified technique.

Thanks to the invention, the calculation of the estimate of the temperature of the glow plug rod is robust at the engine operating point (speed and torque, or speed and quantity of fuel injected), at ambient temperature or pressure. , the flow admitted by the engine and the combustion settings.

Claims (5)

1. Method for controlling glow plugs installed in a combustion chamber of an internal combustion engine, notably of diesel type, comprising at least one cylinder pressure sensor, in which said plugs are controlled as a function of an estimation of a temperature (T_(t)) of the rod of the glow plugs in which:

   - a cylinder pressure (P_cyl) is measured for each crankshaft position,

   - a temperature value in the cylinder (T_cyl(angle)) is calculated for each angular position of the engine as a function of a value of a temperature (T_ivc) on closure of the intake valves, of a volume value (Vol_ivc) of the combustion chamber on closure of the intake valves calculated as a function of a crankshaft angle, of a stroke, of a bore and of a dead volume of the cylinder, of a volume value (Vol_(angle)) of the combustion chamber for each angular position of the cylinder, of a cylinder pressure value (Pcyl_(ivc)) on closure of the intake valves measured by the cylinder pressure sensor and of a cylinder pressure value (Pcyl_(angle)) for each angular position of the cylinder, measured by the cylinder pressure sensor,

   - an average temperature value in the cylinder over a combustion cycle (T_gaz(cycle)) is estimated as a function of the temperature value in the cylinder (T_cyl(angle)) for each angular position of the engine, and

   - the estimation of the temperature (T_(t)) of the rod of a plug is calculated by iterative calculation steps as a function of the temperature value in the cylinder (T_cyl(angle)) for each angular position of the engine, of the average temperature value in the cylinder over a combustion cycle (T_gaz(cycle)) and of a plug control voltage value, by initializing the temperature of the rod of the plug (T_(t)) on the first calculation step with the engine cold, at a value of a temperature of the cylinder head, or at a preceding value of the estimated temperature of the rod of the plug in the case of a hot start.
2. Method according to Claim 1, wherein the temperature on closure of the intake valves (T_ivc) is calculated by likening this phase to a polytropic, as a function of a temperature value (T_ivo) upon opening of the intake valves calculated as a function of a temperature T_air (T_air), of an exhaust gas recirculation temperature (T_EGR) and of the cool air (Q_air) and recirculated gas (Q_EGR) flow rates measured by sensors, of a volume (Vol_x) of the combustion chamber at a given moment (X) and of a polytropic coefficient (_Yivc) of the compression dependent on the composition of the gases taken in.
3. Method according to Claim 1 or 2, wherein the estimation of the average gas temperature (T_gaz) is calculated as a function of a duration of angular opening of the intake valves (α_ivo→α_ivo ), of an angular duration with all valves closed (α_ivc→_evo ), of a duration of angular opening of the exhaust valves (α_evo→α_evc ), of an average temperature (moy{Tcyl _(angle) } α_ivc→evo ) in the cylinder during the intake and expansion phases likened to the average of the temperatures upon opening of the intake valves, upon closing of the intake valves, upon opening of the exhaust valves and upon closing of the exhaust valves ([T_ivo; T_ivc] ; [T_evo; T_evc]) and of the temperature (T_evc) upon closing of the exhaust valves.
4. Method according to Claim 3, wherein the temperature on closure of the exhaust valves (T_evc) is calculated by likening this phase to a polytropic as a function of a polytropic coefficient (γ_evc) of the expansion, dependent on a composition of the exhaust gases (richness), and of the temperature (T_evo) upon opening of the exhaust valves.
5. Control unit of an internal combustion engine, notably of diesel type, comprising a combustion chamber, glow plugs installed in said chamber and at least one cylinder pressure sensor configured to measure a cylinder pressure (P_cyl) for each crankshaft position, said unit comprising a system (10) for estimating the temperature (T_(t)) of the rod of the glow plugs and a system for controlling the glow plugs as a function of said estimation of temperature of the rod of the glow plugs, the estimation system (10) comprising:

   - a module (12) for calculating a temperature value in the cylinder (T_cyl(angle)) for each angular position of the engine as a function of a value of a temperature (T_ivc) upon closure of the intake valves, of a volume value (Vol_ivc) of the combustion chamber upon closure of the intake valves calculated as a function of a crankshaft angle, of a stroke, of a bore and of a dead volume of the cylinder, of a volume value (Vol_(angle)) of the combustion chamber for each angular position of the cylinder, of a cylinder pressure value (Pcyl_(ivc)) upon closing of the intake valves measured by the cylinder pressure sensor and of a cylinder pressure value (Pcyl _(angle)) for each angular position of the cylinder, measured by the cylinder pressure sensor,

   - a module (14) for estimating an average temperature value in the cylinder over a combustion cycle (T_gaz(cycle)) as a function of the temperature value in the cylinder (T_cyl(angle)) for each angular position of the engine, and

   - a module (16) for estimating the temperature (T_(t)) of the rod of a plug by iterative calculation steps as a function of the temperature value in the cylinder (T_cyl(angle)) for each angular position of the engine, of the average temperature value in the cylinder over a combustion cycle (T_gaz(cycle)) and of a plug control voltage value, by initializing the temperature of the rod of the plug (T_(t)) on the first calculation step with the engine cold, to a value of a temperature of the cylinder head, or to a preceding value of the estimated temperature of the rod of the plug in case of a hot start.

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