FR3084405A1 - METHOD FOR PREVENTING A PISTON LOGE INFLUENCE IN A CYLINDER OF A HEAT ENGINE - Google Patents

Abstract

The present invention relates to a method of preventing seizure of a piston housed in at least one cylinder of a heat engine, seizure occurring below a predetermined minimum threshold (S1) of play between the piston and said au minus one cylinder. An instantaneous clearance (J) between the piston and said at least one cylinder is calculated from estimates of the instantaneous temperatures of the piston and said at least one cylinder and, when this calculated instantaneous clearance (J) is less than the minimum threshold (S1 ) predetermined clearance (J), a limitation is made (Lim C) of at least one operating parameter of the heat engine contributing to increasing the clearance (J).

Description

METHOD FOR PREVENTING INFLUENCE OF A LOGE PISTON INTO A

CYLINDER OF A THERMAL ENGINE [0001] the present invention relates to a method of preventing seizure of a piston housed in a cylinder of a heat engine. The method can be implemented for all the cylinders of a heat engine, the engine having a cylinder block.

It is known to leave a minimum clearance between a piston and its cylinder which houses it. The clearance between an external wall of the piston and an internal wall of the cylinder is dimensioned so as to allow in all life situations a displacement of the piston without damage by abrasion or seizure between the piston and the cylinder with a determination of a minimum clearance says "on the plane", for example defined at 20 ° C.

In operation, the clearance decreases since the temperature of the piston increases faster than the temperature of the cylinder, which is the case when the engine temperature rises following a macerated engine start. The piston then undergoes greater thermal expansion than the cylinder and the friction between the piston and the cylinder increases.

It is advisable to limit the friction between the piston and the cylinder in order in particular to reduce the fuel consumption of the vehicle and the wear of the faces opposite the piston and the cylinder. A tilting of the piston increases when the clearance between piston and housing is large. This therefore occurs under conditions other than those causing friction. It is the reduction in backlash which limits the appearance of unwanted acoustic effects linked to tilting.

However, this limitation cannot be effected without a compromise between the different functions and services, such a limitation possibly causing impaired operation of the heat engine. The compromise is precisely the balance between a sufficiently low clearance to avoid tilting, noise and oil consumption, and a sufficiently high clearance to avoid high friction and abrasion / seizure.

Document FR-A-2 816 687 describes a device for guiding a moving part inside a fixed part with the interposition of a guide element carried by one of the parts and sliding on the other parts with a guide clearance. Means for adjusting the guide clearance are provided between the guide element and the part which carries it so as to obtain a substantially constant guide clearance throughout the operation of the device.

The adjustment means comprise means for initial adjustment of the deformable guide clearance under the action of a control member and which press on the guide element and means for absorbing expansion of one at least parts and / or the guide element deformable under the action of the initial adjustment means.

This document provides a mechanical solution which has disadvantages concerning a possible seizure by malfunction of the mechanical system for adjusting the clearance, this system also being subjected to overheating.

Therefore, the problem underlying the invention is to manage the clearance between a piston and its cylinder in a heat engine so that the clearance cannot fall below a predetermined minimum threshold, this clearance management being carried out by means other than mechanical.

To achieve this objective, there is provided according to the invention a method of preventing a seizure of a piston housed in at least one cylinder of a heat engine, the seizure occurring below a predetermined minimum threshold clearance between the piston and said at least one cylinder, characterized in that an instantaneous clearance between the piston and said at least one cylinder is calculated from estimates of the instantaneous temperatures of the piston and said at least one cylinder and, when this calculated instantaneous clearance is less than the predetermined minimum clearance threshold, there is a limitation of at least one operating parameter of the thermal engine contributing to increasing the clearance.

The present aims to shift the compromise of geometric dimensioning of the piston / cylinder game towards a benefit on the consumption of lubricating oil consumption and the noise following the tilting of the piston on one side, and the reduction of friction in the engine by reducing the geometric clearance on the plane between piston and cylinder of the other.

For this, a function integrated into an engine control unit identifies the operating conditions of the engine particularly at risk for seizure and increased friction. These operating phases are generally of low clearance between piston and cylinder, which generates a minimal risk of clearance with the consequences mentioned above.

The method according to the invention applies a temporary reconfiguration of the operation of the engine in order to protect it from these risks. This function is based on an estimate of the instantaneous temperatures of the piston and the cylinder and a calculation of the clearance between the two parts. When a minimum game threshold is reached, the engine benefits are temporarily reduced, advantageously until returning to a game value considered to be "non-risky" above the minimum game.

The implementation of the method according to the present invention allows in all life situations a displacement of the piston without damage by abrasion or seizure. When the engine is running and the temperature rises, the clearance between piston and cylinder decreases since the temperature of the piston increases faster than the temperature of the cylinder block, which occurs in the event of a temperature rise of the engine following a starting a macerated engine.

The present invention makes it possible to limit the friction between the piston and its cylinder in order to reduce the consumption of the vehicle, the clearance between the piston and the cylinder to reduce the consumption of engine oil, to avoid tilting of the piston. and associated unwanted acoustic effects. This is achieved by a compromise between tolerable play and performance of the engine.

Advantageously, said at least one operating parameter of the heat engine is a motor torque. This is the simplest parameter to control to prevent too much reduction of the clearance. This parameter is also not very restrictive on the operation of the engine and does not lead, for example, to degraded combustion conditions.

Advantageously, a second predetermined clearance threshold is set greater than the first predetermined minimum clearance threshold and, when the calculated instantaneous clearance is greater than the second predetermined threshold, the limitation of at least one operating parameter of the heat engine is stopped.

The measures taken on the operation of the engine to combat a reduction in play below a minimum threshold are disadvantageous for the operation of the engine although selected to have the least possible effect on the operation of the engine. They should therefore be suspended as soon as the game has risen above a second threshold high enough so that there is no risk of the game falling again below the first threshold in the very short term. Thus, the engine is controlled to resume its normal operating conditions.

Advantageously, the first predetermined minimum clearance threshold is of a first value of 20 microns with a margin of +/- 15% around this first value and the second predetermined clearance threshold is of a second value of 25 microns with a margin of +/- 15% around this second value.

What will now be stated is one of the possible variants of a mathematical formulation of the piston and cylinder temperature estimators, but this variant is not the only one possible for the implementation of the method.

Advantageously, an estimate of an instantaneous temperature of the piston T_p (t) at an instant t is made according to a first equation:

^MCP ~ ^P dt ~ ^{dt >> XT} ~ ^p ^ ~ ^{dt >> + SHh} -P ( ^N ) ^{x Th <} ÏÏ + SHe_p (N) x Te (t) + ap x <pp (N, C) ^MCP ~ ^P dt ~ ^{+ SHh} ~ ^{p + SHe} ~ ^p φρ (Ν, Ο) being a flow to the piston resulting from combustion in the heat engine depending on the engine operating point with reference to the engine speed N and to the engine torque C, Th being a temperature of a measured engine lubricating oil and Te a temperature of a measured engine coolant, a (p) being a constant serving as a coefficient for correcting the combustion flow at the piston,

Mcp_p (t) being a thermal inertia of the piston given by a second equation:

MCp_p (t) = Ap x T_p (t) + Bp

Ap and Bp being constants relating to the thermal inertia of the piston,

SHh_p (N) being a convective exchange coefficient of the piston with an engine lubricating oil given by a third equation:

/ N \ ° ' ⁸

SHh p (N) = SHh pO + SHh ni X --- \ NrefJ in which SHh_pO and SHh_p1 are respectively convective exchange coefficients of the piston with an engine lubricating oil for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design,

SHe_p (N) being a convective exchange coefficient of the piston with an engine coolant given by a fourth equation:

/ N \ ° ' ⁸

SHe p (N) = SHe pO + SHe pl x (----) \ NrefJ in which SHe_pO and SHe_p1 are respectively convective exchange coefficients of the piston with an engine coolant for two specific preset engine speed values , Nref being a reference engine speed selected during an engine design.

Advantageously, an estimate of the instantaneous temperature of the cylinder T_c (t) at an instant t is made according to a fifth equation:

^MC P- ^c ^ ~ ^dt>) x T_c (t - dt) + SHh_c (N) x Th (t) + SHe_c (N) x Te (t) + ac x <pc (N, C) ⁼ MCP-Ç (t - Æ) _{+ SHh cm + SHe c (N)} (pc (N, C) being a flow to the cylinder resulting from combustion depending on the engine operating point with reference to engine speed N and engine torque C, Th being a temperature of a lubricating oil of the engine measured and Te a temperature of a coolant of the engine measured, a (c) being a constant serving as a coefficient of correction of the combustion flow to the cylinder,

Mcp_c (t) being a thermal inertia of the cylinder given by a sixth equation:

MCp_c (t) = Ac x T_c (t) + Bc

Ac and Bc being constants relating to the thermal inertia of the cylinder,

SHh_c (N) being a convective oil exchange coefficient given by a seventh equation:

/ N \ ° ' ⁸

SHh c (N) = SHh cO + SHh cl x --- “\ NrefJ in which SHh_cO and SHh_c1 are respectively convective exchange coefficients of the cylinder with an engine lubricating oil for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design.

SHe_c (N) being a convective exchange coefficient for a cooling fluid given by an eighth equation:

/ N \ ° ' ⁸

SHe c (N) = SHe cO + SHe cl x --- “\ NrefJ in which SHe_cO and SHe_c1 are respectively convective exchange coefficients of the cylinder with an engine coolant for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design.

Advantageously, an instantaneous clearance between cylinder and piston Jeu_cp expressed in microns is given from the respective diameters of a respective barrel of the piston D_p and of the cylinder D_c after thermal expansion according to the ninth equation:

Jeu_cp = (D_p - D_c) x1000 the respective diameters of the barrel of the piston D_p and the barrel of the cylinder D_c being given respectively according to the tenth and eleventh equations:

D_c = D_cnomi x [1 + (T_c (t) - Tref) x Cdil_c]

D_p = [p_cnomt - x [1 + (T_p (t) - Tref) x Cdil_p] with Tref a reference temperature, Jeu_ref a reference set between piston and cylinder on the plane at the reference temperature Tref, Cdil_c and Cdil_p being respectively a coefficient of thermal expansion of the cylinder and the piston, D_cnomi a nominal expansion, T_p (t) and T_c (t) being the estimates of the instantaneous temperature respectively of the piston and the cylinder.

The invention also relates to an engine control unit of a thermal engine of a motor vehicle comprising a piston housed in at least one cylinder, the engine control unit implementing such a method of preventing seizure of the piston in said at least one cylinder, characterized in that it comprises means for estimating the instantaneous temperatures of the piston and of said at least one cylinder, means for calculating an instantaneous clearance between the piston and said at least one cylinder on the basis of the estimates of the instantaneous temperatures of the piston and of said at least one cylinder, means for comparing the instantaneous play calculated with a predetermined minimum threshold of play saved in storage means and means for limiting at least one operating parameter of the heat engine when the calculated instantaneous clearance is less than the predetermined minimum clearance threshold.

Piloting by such an engine control unit allows a reduction in the clearances between the piston and its cylinder and therefore an improvement in the benefits and / or functions of oil consumption without penalizing fuel consumption and by controlling the risk of engine seizure.

The invention finally relates to a motor vehicle comprising a heat engine and an engine control unit, characterized in that the engine control unit is as previously mentioned.

Other characteristics, objects and advantages of the present invention will appear on reading the detailed description which follows and with regard to the appended drawings given by way of nonlimiting examples and in which:

FIG. 1 is a clearance curve between piston and cylinder in a heat engine without implementing the method for preventing a seizure of a piston housed in a cylinder of a heat engine according to the present invention,

- Figure 2 is a clearance curve between piston and cylinder in a heat engine with the implementation of the method of preventing seizure of a piston housed in a cylinder of a heat engine according to the present invention, this method putting implements a limitation of at least one operating parameter of the thermal engine contributing to increasing the clearance.

The present invention relates to a method of preventing seizure of a piston housed in at least one cylinder of a heat engine, seizure occurring below a predetermined minimum threshold S1 of clearance J between the piston and said at least one cylinder.

Generally between a movable part which is here the piston and a fixed guide part which is a cylinder block with a cylinder housing the piston, in case of reduction of the clearance J guide beyond a certain limit , the oil film can be torn and the moving part will bear directly on the guide part on which the moving part is caused to slide, which will have the consequence of increasing the friction forces which may cause blockage of the moving part or piston in the fixed guide part or cylinder.

According to the invention, an instantaneous clearance J between the piston and said at least one cylinder is calculated from estimates of instantaneous temperatures of the piston and of said at least one cylinder. When this calculated instantaneous clearance J is less than the predetermined minimum clearance threshold S1, there is a limitation Lim C of at least one operating parameter of the thermal engine contributing to increase the clearance J.

Figures 1 and 2 give a clearance J expressed in microns as a function of time (expressed in seconds) shown on the abscissa, respectively without implementation of the method in Figure 1 and with implementation of the method in Figure 2 .

In Figure 1, there is no limitation of at least one operating parameter of the engine contributing to increase the clearance J. It follows that the clearance J can quickly drop below 20 microns and remain below this limit for a long time, only gradually increasing.

In Figure 2, there is shown a first minimum threshold S1, substantially equal to 20 microns. When the calculated instantaneous clearance J is less than this predetermined minimum threshold S1 of clearance J, a limitation Lim C is effected of at least one operating parameter of the heat engine contributing to increase the clearance J. This is shown by the interval double arrow Lim C.

It can be set a second predetermined threshold S2 game J greater than the first minimum threshold S1 predetermined game J. When the instantaneous game J calculated is greater than the second predetermined threshold S2, the limitation Lim C of at least one parameter of the engine is stopped.

As shown in FIG. 2, the first predetermined minimum threshold S1 of clearance J can be of a first value of 20 microns with a margin of +/- 15% around this first value. The second optional predetermined threshold S2 of clearance J is of a second value of 25 microns with a margin of +/- 15% around this second value.

Among the engine parameters selected to help increase the clearance J while being limited, an engine torque can be chosen.

In what follows, a series of equations will be mentioned to determine the temperatures of the piston T_p (t) and of the cylinder T_c (t) as a function of time t. This method of calculating temperatures is not limiting.

With regard to the piston, an estimate of an instantaneous temperature of the piston T_p (t) at an instant t can be made according to a first equation:

^MCP ~ ^P dt ~ ^{dt >> XT} ~ ^p ^ ~ ^{dt >> + SHh} -P ( ^N ) ^{x Th <} ÏÏ + SHe_p (N) x Te (t) + ap x <pp (N, C) ^MCP ~ ^P dt ~ ^{+ SHh} ~ ^{p + SHe} ~ ^p (pp (N, C) being a flow to the piston resulting from combustion in the heat engine depending on the engine operating point with reference to the engine speed N and to the engine torque C, Th being a temperature of a lubricating oil of the engine measured and Te a temperature of a coolant of the engine measured, a (p) being a constant serving as a coefficient of correction of the combustion flow at the piston.

The constant a (p) as a correction coefficient makes it possible to correct the combustion fluxes. This parameter is used when the model is calibrated to improve the digital / physical correlation and to have a model correctly readjusted compared to the actual temperatures measured.

Mcp_p (t) is a thermal inertia of the piston which can be given by a second equation:

MCp_p (t) = Ap x T_p (t) + Bp [0041] Ap and Bp being constants relating to the thermal inertia of the piston, these constants Ap and Bp making it possible to describe the thermal inertia of the piston, which varies linearly in function of the material temperature.

SHh_p (N) is a convective exchange coefficient of the piston with an engine lubricating oil given by a third equation:

/ N \ ° ' ⁸

SHh p (N) = SHh pO + SHh ni x --- \ NrefJ in which SHh_pO and SHh_p1 are respectively convective exchange coefficients of the piston with an engine lubricating oil for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design. For example, Nref is the reference engine speed, in which the heat exchanges are characterized and known. In general, the standard speed is chosen to be 4,000 rpm.

SHe_p (N) is a convective exchange coefficient of the piston with an engine coolant given by a fourth equation:

/ N \ ° ' ⁸

SHe p (N) = SHe pO + SHe pl x (----) \ NrefJ in which SHe_p0 and SHe_p1 are respectively convective exchange coefficients of the piston with an engine coolant for two specific preset engine speed values , Nref being a reference engine speed selected during an engine design.

As regards the cylinder, an estimate of the instantaneous temperature of the cylinder T_c (t) at an instant t is made according to a fifth equation:

^MCp ~ ^C ^. ~ ^dt>) x T_c (t - dt) + SHh_c (N) x Th (t) + SHe_c (N) x Te (t) + ac x <pc (N, C) ^{= MCr} - <'- ^de > + SHh_ _cm + SHesW (pc (N, C) being a flow to the cylinder resulting from combustion depending on the engine operating point with reference to engine speed N and engine torque C, Th being a temperature of an engine lubricating oil measured and Te a temperature of a measured engine coolant, a (c) being a constant serving as a coefficient of correction of the combustion flow to the cylinder.

The constant a (c) as a correction coefficient makes it possible to correct the combustion fluxes. This parameter is used when the model is calibrated to improve the digital / physical correlation and to have a model correctly readjusted compared to the actual temperatures measured.

Mcp_c (t) is a thermal inertia of the cylinder given by a sixth equation:

MCp_c (t) = Ac x T_c (t) + Bc

Ac and Bc being constants relating to the thermal inertia of the cylinder, these constants Ac and Bc making it possible to describe the thermal inertia of the cylinder which varies linearly as a function of the material temperature.

SHh_c (N) is a convective oil exchange coefficient given by a seventh equation:

/ N \ ° ' ⁸

SHh c (N) = SHh cO + SHh cl X --- “\ NrefJ in which SHh_c0 and SHh_c1 are respectively convective exchange coefficients of the cylinder with an engine lubricating oil for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design.

of the material temperature.

SHe_c (N) is a convective exchange coefficient for a cooling fluid given by an eighth equation:

/ N \ ° ' ⁸ He c (N) = SHe cO + SHe cl x --- “xNrefs in which SHe_cO and SHe_c1 are respectively convective exchange coefficients of the cylinder with an engine coolant for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design.

An instantaneous clearance between cylinder and piston Jeu_cp expressed in microns is given from the respective diameters of a respective barrel of piston D_p and cylinder D_c after thermal expansion according to the ninth equation:

Jeu_cp = (D_p - D_c) x1000 The respective diameters of the barrel of the piston D_p and the barrel of the cylinder D_c are given respectively according to the tenth and eleventh equations:

D_c = D_cnomi x [1 + (T_c (t) - Tref) x Cdil_c] (jQu rcfx

D_cnomi --- ^ qqq) ^x [1 + (T_p (t) - Tref) x Cdil_p] with Tref a reference temperature, Jeu_ref a reference play between piston and cylinder on the plane at the reference temperature Tref, advantageously 20 ° C, Cdil_c and Cdil_p being respectively a coefficient of thermal expansion of the cylinder and the piston, D_cnomi a nominal expansion, T_p (t) and T_c (t) being the estimates of the instantaneous temperature respectively of the piston and the cylinder.

The invention also relates to an engine control unit of a thermal engine of a motor vehicle comprising a piston housed in at least one cylinder, the engine control unit implementing a method of preventing the piston from seizing up. in said at least one cylinder as previously mentioned.

The engine control unit comprises means for estimating the instantaneous temperatures of the piston and of said at least one cylinder, means of calculating an instantaneous clearance J between the piston and said at least one cylinder from estimates. instantaneous temperatures of the piston and of said at least one cylinder, means for comparing the instantaneous clearance J calculated with a predetermined minimum threshold S1 of clearance J saved in memory means and means for limiting Lim C of at least one parameter of operation of the heat engine when the calculated instantaneous clearance J is less than the predetermined minimum clearance threshold S1.

The invention finally relates to a motor vehicle comprising a heat engine and an engine control unit, the engine being controlled by such an engine control unit.

The invention is in no way limited to the embodiments described and illustrated which have been given only by way of examples.

Claims (9)

1. Method for preventing seizure of a piston housed in at least one cylinder of a heat engine, seizure occurring below a predetermined minimum threshold (S1) of clearance (J) between the piston and said au at least one cylinder, characterized in that an instantaneous clearance (J) between the piston and said at least one cylinder is calculated from estimates of the instantaneous temperatures of the piston and of said at least one cylinder and, when this clearance (J) calculated instant is less than the predetermined minimum clearance threshold (S1), there is a limitation (Lim C) of at least one operating parameter of the thermal engine contributing to increasing the clearance (J). 2. Method according to the preceding claim, wherein said at least one operating parameter of the engine is an engine torque. 3. Method according to any one of the two preceding claims, in which there is fixed a second predetermined threshold (S2) of play (J) greater than the first minimum predetermined threshold (S1) of play (J) and, when the play ( J) calculated instant is greater than the second predetermined threshold (S2), the limitation (Lim C) of at least one operating parameter of the heat engine is stopped. 4. Method according to the preceding claim, in which the first predetermined minimum threshold (S1) of clearance (J) is of a first value of 20 microns with a margin of +/- 15% around this first value and the second threshold (S2) predetermined clearance (J) is of a second value of 25 microns with a margin of +/- 15% around this second value. 5. Method according to any one of the preceding claims, in which an estimate of an instantaneous temperature of the piston T_p (t) at an instant t is made according to a first equation: ^MCP ~ ^P dt ~ ^{dt >> XT} ~ ^p ^ ~ ^{dt >> + SHh} -P ( ^N ) ^{x Th <} ÏÏ + $ He_p (N) x Te (t) + ap x <pp (N, C) ^MCP ~ ^P dt ~ ^{+ SHh} ~ ^{p + SHe} ~ ^p φρ (Ν, Ο) being a flow to the piston resulting from combustion in the heat engine depending on the engine operating point with reference to the engine speed N and to the engine torque C, Th being a temperature of a lubricating oil of the engine measured and Te a temperature of a coolant of the engine measured, α (ρ) being a constant serving as a coefficient of correction of the combustion flow at the piston, Mcp_p (t) being a thermal inertia of the piston given by a second equation: MCp_p (t) = Ap x T_p (t) + Bp Ap and Bp being constants relating to the thermal inertia of the piston, SHh_p (N) being a convective exchange coefficient of the piston with an engine lubricating oil given by a third equation: / N \ ° ' ⁸ SHh p (N) = SHh pO + SHh ni X --- \ NrefJ in which SHh_pO and SHh_p1 are respectively convective exchange coefficients of the piston with an engine lubricating oil for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design, SHe_p (N) being a convective exchange coefficient of the piston with an engine coolant given by a fourth equation: / N \ ° ' ⁸ SHe p (N) = SHe pO + SHe pl x (----) \ NrefJ in which SHe_pO and SHe_p1 are respectively convective exchange coefficients of the piston with an engine coolant for two specific preset engine speed values , Nref being a reference engine speed selected during an engine design. 6. Method according to the preceding claim, in which an estimate of the instantaneous temperature of the cylinder T_c (t) at an instant t is made according to a fifth equation: ^MC P- ^c ^ ~ ^dt>) x T_c (t - dt) + SHhc (N ') x Th (t) + SHe_c (N) x Te (t) + ac x <pc (N, C) ^{= MCr} - f _t - ^dt) + SHhsW + SHesW rpc (N, C) being a flow to the cylinder resulting from combustion depending on the engine operating point with reference to engine speed N and engine torque C, Th being an oil temperature measured engine lubrication temperature and Te a measured engine coolant temperature, a (c) being a constant serving as a correction coefficient for the combustion flow to the cylinder, Mcp_c (t) being a thermal inertia of the cylinder given by a sixth equation: MCp_c (t) = Ac x T_c (t) + Bc Ac and Be being constants relating to the thermal inertia of the cylinder, SHh_c (N) being a convective oil exchange coefficient given by a seventh equation: / N \ ° ' ⁸ SHh c (N) = SHh cO + SHh cl x --- “\ NrefJ in which SHh_cO and SHh_c1 are respectively convective exchange coefficients of the cylinder with an engine lubricating oil for two specific values engine speed N preselected, Nref being a reference engine speed selected during an engine design, SHe_c (N) being a convective exchange coefficient for a cooling fluid given by an eighth equation: / N \ ° ' ⁸ SHe c (N) = SHe cO + SHe cl x --- “\ NrefJ in which SHe_cO and SHe_c1 are respectively convective exchange coefficients of the cylinder with an engine coolant for two specific values of engine speed N preselected, Nref being a reference engine speed selected during an engine design. 7. Method according to the preceding claim, in which an instantaneous clearance (J) between cylinder and piston Jeu_cp expressed in microns is given from the respective diameters of a respective barrel of the piston D_p and of the cylinder D_c after thermal expansion according to the ninth equation : Jeu_cp = (D_p - D_c) x1000 the respective diameters of the barrel of the piston D_p and the barrel of the cylinder D_c being given respectively according to the tenth and eleventh equations: D_c = D_cnomi x [1 + (T_c (t) - Tref) x Cdil_c] D_p = [p_cnomt - x [1 + (T_p (t) - Tref) x Cdil_p] with Tref a reference temperature, Jeu_ref a reference set between piston and cylinder on the plane at the reference temperature Tref, Cdil_c and Cdil_p being respectively a coefficient of thermal expansion of the cylinder and the piston, D_cnomi a nominal expansion, T_p (t) and T_c (t) being the estimates of the instantaneous temperature respectively of the piston and the cylinder. 8. Motor control unit of a motor vehicle thermal engine comprising a piston housed in at least one cylinder, the engine control unit implementing a method of preventing the piston from seizing in said at least one cylinder according to any one of the preceding claims, characterized in that it 5 comprises means for estimating the instantaneous temperatures of the piston and of said at least one cylinder, means of calculating an instantaneous clearance (J) between the piston and said at least one cylinder from estimates of the instantaneous temperatures of the piston and of said at least one cylinder, means for comparing the instantaneous clearance (J) calculated with a predetermined minimum threshold (S1) of clearance saved in storage means 10 and limitation means (Lim C) of at least one parameter of operation of the heat engine when the calculated instantaneous clearance (J) is less than the predetermined minimum clearance threshold (S1) (J). 9. Motor vehicle comprising a heat engine and an engine control unit, characterized in that the engine control unit is according to the preceding claim.