Method for controlling the quantity of air introduced to the intake of a supercharged internal combustion engine by a single-inlet turbocompressor

Abstract

Method for controlling the quantity of air introduced to the intake of a supercharged internal combustion engine, said engine comprising an exhaust gas outlet (32) connected to an exhaust collector (30), comprising a supercharging device (38) comprising a turbocompressor with a turbine (40) having a single inlet (46) connected to said exhaust gas outlet, and an external air compressor (44), and a duct (64) for the partial transfer of the compressed air from the compressor to the inlet of the turbine. According to the invention, a branch (70) is provided on said partial transfer duct, said branch being connected to the inlet of the turbine and comprising a proportional valve means (74), and in that the flow of the compressed air in this branch is controlled during transitory operating phases in accordance with strategies that are applied to said valve means and are determined according to characteristics of the stabilized phases.

Description

The present invention relates to a method for controlling a device for introducing the quantity of air to the intake of an internal combustion engine supercharged by a single-inlet turbocompressor, in particular a stationary engine or for a motor vehicle or industrial vehicle.

As is widely known, the power delivered by an internal combustion engine is dependent on the quantity of air introduced into the combustion chamber of this engine, a quantity of air which is itself proportional to the density of this air.

Thus, it is the usual practice to increase this quantity of air by means of a compression of the outside air before it is taken into this combustion chamber. This operation, called supercharging, can be performed by any means, such as a turbocompressor or a driven compressor, which can be centrifugal or volumetric.

In the case of supercharging by a single-inlet turbocompressor, the latter comprises a rotary turbine, provided with a single inlet, linked by a shaft to a rotary compressor. The exhaust gases from the engine pass through the turbine which is then driven in rotation. This rotation is then transmitted to the compressor which, through its rotation, compresses the outside air before it is introduced into the combustion chamber.

As is better described in the French patent application No. 2 478 736, there is provided, to be able to significantly boost this quantity of compressed air in the combustion chamber of the engine, to even further increase the compression of the outside air by the compressor.

That is done more particularly by increasing the spin speed of the turbine and therefore of the compressor.

For that, a part of the compressed air outgoing from the compressor is deflected to be taken directly into the inlet of the turbine by being mixed with the exhaust gases. This turbine is then passed through by a greater quantity of fluid (mixture of compressed air and of exhaust gases), which makes it possible to increase the spin speed of the turbine and consequently of the compressor. This increase in speed of the compressor thus makes it possible to increase the pressure of the outside air which will be compressed in this compressor then introduced in the combustion chamber of the engine.

For that, the compressed air has a higher density which makes it possible to increase the quantity of air contained in the combustion chamber.

This type of supercharged engine, although satisfactory, does however exhibit not-inconsiderable drawbacks.

Indeed, the flow rate of the compressed air which is taken into the inlet of the turbine is not correctly controlled, which can lead to a malfunctioning of the engine.

Thus, by way of example, in the case of too great a quantity of compressed air deflected to the inlet of the turbine, the exhaust gases entering into the turbine are cooled too much by this air, and bring about a reduction of the overall efficiency of the supercharging.

The present invention sets out to remedy the abovementioned drawbacks by virtue of a method for controlling a device for introducing a quantity of air to the intake of a supercharged internal combustion engine which makes it possible to address all the power demands of the engine, and in particular in the transition operating phases.

The present invention makes it possible to produce and manage a transfer of the compressed air from the intake to the exhaust even when the average pressure of the compressed air at the intake is lower than that of the gases at the exhaust. All that is required is for there to be phases during the operating cycle of the engine where the pressure at the intake is higher than that at the exhaust.

Thus, the present invention relates to a method for controlling the quantity of air introduced to the intake of a supercharged internal combustion engine, said engine comprising an exhaust gas outlet linked to an exhaust manifold, comprising a supercharging device comprising a turbocompressor with a turbine having a single inlet connected to said exhaust gas outlet and an outside air compressor, and a duct for the partial transfer of the compressed air from the compressor to the inlet of the turbine, characterized in that the partial transfer duct is linked to the inlet of the turbine, said duct bearing proportional valve means, and in that the circulation of the compressed air is controlled in this duct during transition operating phases in accordance with the strategies applied to said valve means, and determined according to the characteristics of the stabilized phases.

A non-return valve can also be disposed on the duct.

The compressed air which circulates in the transfer duct can be heated.

For a control strategy, the following steps can be performed:

• • from an opening setting state of the valve means for an initial stabilized state,
  • the setting is reduced by a determined gradient, to a value that is maintained for a time t,
  • said setting is increased by a determined gradient to reach the opening setting of the valve means for a final stabilized state.

The initial state can be a low charge state and the final state can be a high charge state.

The opening setting states of the valve means for the initial and final stabilized states can be identical.

The other features and advantages of the invention will emerge on reading the following description, given in a purely illustrative and nonlimiting manner, and to which are attached:

FIG. 1 which illustrates an internal combustion engine with its supercharging device used according to the invention;

FIG. 2 which illustrates a variant of the internal combustion engine with its supercharging device according to FIG. 1;

FIGS. 3 to 9 show examples of management strategy for the valve means;

FIGS. 10 and 11 show examples of operation of a GMP (power train) according to the application of the present invention.

In FIG. 1, the internal combustion engine 10 comprises at least two cylinders, here four cylinders referenced 12₁ to 12₄ from the left of the figure.

Preferentially, this engine is an internal combustion engine with a direct injection, notably of diesel type, but that does not in any way rule out any other type of internal combustion engine.

Each cylinder comprises intake means 14 with at least one intake valve 16, here two intake valves each controlling an intake pipe 18. The intake pipes 18 culminate at an intake manifold 20 fed by a duct 22 for the supply of intake air, such as compressed air.

This cylinder also comprises waste gas exhaust means 24 with at least one exhaust valve 26, here also two valves each controlling an exhaust pipe 28.

In the example illustrated, the exhaust pipes of each cylinder are connected to an exhaust manifold 30 with an exhaust gas outlet 32. This exhaust gas outlet culminates at a turbocompressor 38 for the compression of the air and more particularly to the expansion turbine 40 of this turbocompressor.

As illustrated in FIG. 1, the turbocompressor is a single-inlet turbocompressor, better known by the term “Mono Scroll” or “Single Scroll” turbocompressor.

This type of turbocompressor comprises the expansion turbine 40 scavenged by the exhaust gases and which is linked in rotation by a shaft 42 with a compressor 44.

At the turbine, the latter comprises a single inlet 46 for the exhaust gases which is connected to the exhaust gas outlet 32 of the manifold 30. The gas discharge 50 of the turbine 40 is conventionally connected to the exhaust line 52 of the engine.

The compressor 44 of the turbocompressor 38 comprises an outside air intake 54 fed by a supply duct 56. The compressed air outlet 58 of this compressor is linked to the supply duct 22 of the intake manifold 20 by a duct 60.

Advantageously, provision can be made to place a compressed air cooling radiator 62 on the duct 60, between the compressor and the duct 22.

As can be seen better in FIG. 1, a transfer duct 64 makes it possible to circulate a part of the compressed air outgoing from the compressor 44 to the inlet 46 of the turbine.

More specifically, this partial transfer duct starts from the duct 60, at a point of intersection 66 between the compressor and the supercharging air cooling radiator 62, and then culminates at the inlet 46 of the turbine by its junction 70 with the exhaust gas outlet 32.

This transfer duct bears valve means 74, such as a proportional valve, controlled by control means 78. This type of proportional valve thus makes it possible to control the flow rate of the circulation of compressed air which circulates in the transfer duct 64.

Advantageously, the transfer duct also comprises a non-return valve 80 which prevents the circulation of the compressed air or of the exhaust gases from this duct to the compressor.

The use of this valve presents another advantage of the invention since it allows for operation of the bypass system even when the average pressure in the transfer duct 64 is lower than the average pressure in the exhaust outlet 32. It is in fact sufficient for the pressure swings in each of these two branches to make it possible to occasionally have a pressure differential favorable to the bypass of the air from the intake to the exhaust for the bypass system to be active, and be so even if the average pressure levels in these two branches oppose a flow from the intake to the exhaust.

This configuration thus makes it possible, during the operation of the engine, to exploit the zones of low exhaust pressure prevailing occasionally in the exhaust manifold to introduce compressed air into the turbine and thus increase the flow rate of this turbine and consequently of the compressor, more commonly called “Boost Turbo”. That also makes it possible to have a more effective supercharging for the low speeds and in particular to manage the transition phases with suitable proportional valve control strategies.

During operation, in case of the need for air in large quantities in the cylinders, the proportional valve 74 is commanded to open to introduce compressed air from the compressor 44 into the turbine 40.

The compressed air outgoing from the compressor 44 circulates in the transfer duct 64 to culminate at the exhaust gas inlet 46 of the turbine 40 and by adding a surplus of fluid to this turbine therein.

Thus, the turbine is passed through not only by the exhaust gases coming from the outlet 32, but also by compressed air which is added to these gases. Because of this, the rotation of the turbine is increased, which leads to an increase in rotation of the compressor and, consequently, an increase in the pressure of the compressed air which leaves this compressor.

The valve 74 is controlled by the control means 78 so as to take the quantity of compressed air into the turbine according to the supercharging needs of the engine, and in particular during the transition phases lying between two stabilized operating phases.

In FIG. 2, which essentially comprises the same elements as those of FIG. 1, the compressed air outgoing from the compressor 44 and circulating in the transfer duct 64 is preheated before being introduced into the turbine 40.

For that, the transfer duct 64 bears means 88 for heating the compressed air, here, a heat exchanger in the form of a heating radiator, placed after the point of intersection 66 and the valve 74. This radiator is crossed by the compressed air which circulates in this duct while being passed through by the exhaust gases from the engine. These exhaust gases originate from the discharge 50 of the turbine and are brought by a duct 90 to the inlet 92 of the radiator. The exhaust gases pass through this radiator, transferring the heat that they contain to the compressed air to then re-emerge from this radiator through the outlet 94 to be directed to the exhaust line of the engine.

Thus, a part of the energy of the exhaust gases is harvested by the compressed air which is introduced into the turbine by the inlet 46.

This heated compressed air thus makes it possible to provide additional energy to the turbine which, consequently, will turn at a higher speed. This high spin speed is then transmitted to the compressor which will produce a higher compression of the outside air.

The operation of the system described above, whether associated or not with an exhaust gas recirculation (EGR) circuit at the intake, needs to be adjusted precisely in order to produce the desired short-circuiting rate.

Generally, the operation of the engine can be likened to a succession of stabilized phases (which can be more or less long) broken up by more or less abrupt transition phases.

The initial pre-positioning of the valve 74 is derived from a previously completed speed/charge mapping. Furthermore, the compressed air flow rate at the inlet of the turbine is estimated by virtue of previously defined measurements and/or estimators.

The present invention is particularly suited to managing the transition operating phases between two stabilized operating phases.

“Stabilized operation” should be understood to mean a state in which either the torque demand does not vary, or the set of actuators of the engine are not modified, for example when the quantities of fuel injected are constant over time, the EGR rate remains constant, the actuator of the turbocompressor is in one and the same position, etc.

Conversely, “transition operation” should be understood to mean the phase during which certain actuators are activated in response to a change of the power demand from the user. It is for example possible to describe the case of a “charge-up” during which the user will ask the engine to switch from a low-charged state (little torque, little power) to a high-charged state (high torque, high power). This charge-up can be carried out either at constant engine speed, or with a simultaneous increase in the charge and the speed. It is also possible to consider the example of a “foot lift”, that is to say a more or less abrupt reduction of the torque demand. In each of these cases, the actuators of the engine must be driven in order to ensure an optimal operation of the power train (GMP).

Thus, during the stabilized operating phases, two types of operation can be envisaged:

• • Either the position of the valve 74 corresponding to the flow rate of the initially completed mapping remains in this position as long as the charge/speed demand is unchanged.
  • Or the position of the valve 74 is adjusted throughout the period during which the engine remains in this stabilized phase. In this case, it has to be adjusted to obtain the best trade-off between:
    • the temperature at the exhaust (heating or cooling demand)
    • the richness of the exhaust (regeneration of the particulate filter FAP which makes it possible to control the oxygen ratio at the exhaust)
    • the consumption
    • the pollutant emissions
    • the dynamics requirement
    • the performance levels

The rate of compressed air diverted is therefore adjusted throughout the stabilized operating phase in order to obtain the best operating point that satisfies the trade-off sought. The search for this trade-off is therefore done on the basis of the analysis of all or some of the following measurements or estimators:

• • the speed
  • the charge
  • the temperature at the cylinder head outlet or turbine outlet exhaust: in the case where a maximum temperature criterion is provided
  • the dynamic history (analysis of use): in the context of a highly dynamic use in which the invocations of torques are frequent or very frequent, the position of the valve 74 can be positioned to prioritize the engine response possibly momentarily to the detriment of other parameters
  • the regeneration of the particulate filter (FAP): in the context of an FAP regeneration, the opening of the valve 74 can be prevented, even limited, in order to control the air flow rate on the FAP and not risk the runaway of the regeneration that can lead to an excessively high temperature
  • the post-treatment temperature: in the case where the compressed air intake at the inlet of the turbine may lead to the unpriming of the post-treatment, the use thereof can be limited. However, in the case where the dynamic history shows a significant probability of the stabilized phase being followed by an increase in charge, the criterion of dynamic range may be favored.
  • engine temperature (water temperature/oil temperature)
  • the temperature of the air
  • the stabilization time: based on the time of operation of the stabilized point, the strategy of compressed air intake at the inlet of the turbine can be reviewed. For example, the dynamic history may show a highly “dynamic” operation that the time of stabilization on the point may contradict

The valve is therefore positioned so as to obtain the target flow rate of diverted compressed air, corrected or not by virtue of the estimators.

In addition, provision is made to optimize the dynamic range of the engine during transition operation by controlling the opening and the closing of the valve 74 allowing the passage of the gases from the partial transfer duct for compressed air to the inlet 46 of the turbine 40.

Generally, these transition phases can be defined as being transitional phases between two stabilized states for which the scavenging level, permitted by the proportional valve 74, is stabilized and optimal from the point of view of the engine performance levels expected on each of these operating points. The optimal control of the valve of the partial transfer duct consists in applying a determined strategy for changing the trajectory of closure or opening of the valve in order to switch from one stabilized state to another stabilized state in optimized conditions.

FIG. 3 schematically illustrates this situation of transition between the points A, representative of the stabilized point before the transition phase, and B, representative of the stabilized point after the transition phase. The point A can correspond for example to the start of a charge-up, and the point B to the instant from which the desired torque is reached. That can also illustrate a reduction of charge between A and B. The transition phase will, for its part, be represented in the following figures which describe examples of several strategies for optimally managing the transition between the two stabilized states A and B.

In FIG. 3, “100%” and “0%” respectively mark the opening and the total closure of the passage of the gases in the valve 74. Thus, the method for managing the transition phases consists in applying determined strategies comprising at least one sequence of opening (and/or of closure) of the proportional valve to obtain an optimized operation of the compressor 44 moved by the turbine 40 to switch from one stabilized state to another.

FIGS. 4 to 9 illustrate different examples of management of the scavenging level during the transition phase. “Scavenging level” here designates the quantities of gas introduced by the partial transfer duct 64 for compressed air into the inlet of the turbine.

FIG. 4 corresponds to the case of a “scavenging” cut suddenly upon starting of the transition phase at A to position it on the setting corresponding to the stabilized operating point B. It is noted that the fact that the “scavenging” is cut suddenly does not mean that the engine reaches its second stabilized operating point B immediately.

FIG. 5 corresponds to the case of a “scavenging” cut gradually, substantially linearly or not, during the transition phase.

FIG. 6 corresponds to the case of a chopped “scavenging” strategy during the transition phase. Here, several sequences of opening and of closure of the proportional valve are performed between the positions of the valve during the stabilized phases A and B.

FIG. 7 corresponds to the case of a “scavenging” cut gradually with levels maintained during the transition phase.

The choice of the method for optimal management of the proportional valve 74 during the transition phase depends essentially on the characteristics of the starting stabilized operating point (point A) and on those of the target stabilized operating point (point B). In other words, the strategy for control of the proportional valve must be optimized to best correspond to the characteristics of the operating points of the engine (charge, speed, flow rate, richness, time spent on each operating point, etc.) and of the supercharging (spin speed, pumping guard, etc.).

It is for example possible to describe a charge-up, that is to say a rapid increase in the demand for power to the engine. For this example, a case of transition operation is considered with three main phases:

• • phase 1: engine stabilized on a low-charge operating point;
  • phase 2: charge-up transition phase;
  • phase 3: engine stabilized on a high-charge operating point (charge greater than that of the first, “low” charge phase).

It is considered that the “scavenging” is active both on the first, low-charge operating point (first phase), and on the second, high-charge operating point with a level which can be different from that of the first phase according to the needs of the engine specific to this charge level. It is also envisaged to authorize the scavenging during the transition phase, at variable levels, according to the examples illustrated previously.

FIG. 8 schematically illustrates the case of charge-up with a simplified management of the scavenging. Initially, the scavenging is first gradually cut at the start of the transition phase (trajectory A-A1), then a level is maintained with a certain level of “scavenging” during the transition phase (trajectory A1-A2), and the “scavenging” is gradually increased at the end of the transition phase (trajectory A2-A3).

FIG. 9 illustrates the example in which the two stabilized phases are operated with one and the same “scavenging” level, that is to say with the same settings of the proportional valve. It is considered that only the level of closure during the transition phase can be modified. This strategy relies on 4 parameters: the scavenging cutoff delay (t1), the cutoff time (i.e. the time spent with scavenging reduced or prevented, t2), the delay in restarting the scavenging (t3) and the “level” of cutoff.

The embodiments show that there is a trade-off to be found between:

• • a rapid restarting of the “scavenging” which can lead to a “hole” in the torque (linked to the intake/exhaust pressure balances, to the rate of residual gases, etc.);
  • a prolonged stoppage of the “scavenging” system which can lead to a reduction of the spin speed of the turbine, a reduction which can then affect all the dynamics when the “scavenging” is once again authorized.

An optimal strategy hitherto identified consists:

• • in cutting the scavenging very rapidly at the start of the transition phase in order to obtain a rapid rise in the charge. Indeed, since the “low-charge” operating point allows the scavenging, the engine has a good richness margin for enriching the mixture rapidly and charging up (without however emitting fumes);
  • in then partially cutting the scavenging for a short time (approximately 0.2 to 0.3 seconds on the simulated cases) so as not to collapse the spin speed of the turbine (a partial cutoff makes it possible a priori to retain a flow rate and maintain the turbine speed during the transition phase);
  • and finally in restarting the scavenging “gently” so as not to cause the torque “hole” to appear.

This optimal strategy and other simulated cases are illustrated in FIG. 10. The “RSS” curve corresponds to the reference operation “single supercharging reference”. The “RDS” curve corresponds to a “double supercharging reference” case, an asymptotic case mentioned in this figure only for indication purposes, the scavenging system a priori allowing an engine with single supercharging to gain in performance levels but without reaching those of an engine with double supercharging.

The other curves correspond to an engine provided with single supercharging equipped with a partial transfer system for compressed air or so-called “scavenging” system, according to the invention. The “RBE” curve (external scavenging reference) corresponds to the case allowing the scavenging continuously, that is to say, on the low-charge point, during the transition phase, and on the high-charge point.

The curve “1RBE-OFF 0.01-STAB 2.0-ON 0.01” corresponds to a case where the scavenging cutoff delay is 0.01 second, the scavenging prevention time is 2.0 seconds, and the scavenging restart time is 0.01 second. It can be seen in this case that a torque “hole” appears upon the restarting of the scavenging because said scavenging was stopped for too long.

The curve “2RBE-OFF 0.01-STAB 0.3-ON 0.2” finally corresponds to an optimal case where the scavenging cutoff delay is 0.01 second, the scavenging prevention time is 0.2 second, and the scavenging restart time is 0.2 second. As can be seen in this figure, this scavenging control strategy makes it possible to obtain a rapid charge-up.

FIG. 11 illustrates the trend of the speed of the turbine 40. The slight drop in speed can in particular be seen, corresponding to the case which prevents the scavenging for too long.

As indicated previously, the optimal control strategy varies from one engine to another, the values of the different control parameters indicated here are therefore supplied purely as an indication.

Claims

1.-7. (canceled)

8. A method for controlling a quantity of air introduced to an intake of a supercharged internal combustion engine, comprising an exhaust gas outlet linked to an exhaust manifold, a supercharger including a turbocompressor having a turbine with a single inlet connected to the exhaust gas outlet and to an outside air compressor, a duct for partially transferring compressed air from the outside air compressor to the inlet of the turbine, and a partial transfer duct linked to an inlet of the turbine, which includes a proportional valve, the method comprising steps of: determining characteristics of stabilized phases of the engine and controlling circulation of the compressed air during transition between operating phases of the engine in accordance with a control strategy applied to control the proportional valve which is determined according to the determined characteristics of stabilized phases of the engine.

9. The method as claimed in claim 8, comprising a non-return valve disposed on the partial transfer duct which prevents circulation of compressed air or exhaust gas back to the compressor.

10. The method as claimed in claim 8, comprising heating the compressed air circulating in the partial transfer duct.

11. The method as claimed in claim 9, comprising heating the compressed air circulating in the partial transfer duct.

12. The method as claimed in claim 10, comprising the heating the compressed air circulating in the partial transfer duct is by heat exchange with the exhaust gases.

13. The method as claimed in claim 11, comprising the heating the compressed air circulating in the partial transfer duct is by heat exchange with the exhaust gases.

14. The method as claimed in claim 8, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

15. The method as claimed in claim 9, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

16. The method as claimed in claim 10, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

17. The method as claimed in claim 11, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

18. The method as claimed in claim 12, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

19. The method as claimed in claim 13, comprising forming a control strategy by steps of: starting from an opening setting of the proportional valve which provides an initial stabilized state of the engine;reducing an opening setting of the proportional valve with a determined gradient for reducing the opening setting of the proportional valve by following the gradient to a setting of the valve that is maintained for a time t; andthen increasing the opening setting of the valve with a determined gradient for reaching the opening setting state of the valve to provide a final stabilized state.

20. The method as claimed in claim 14, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

21. The method as claimed in claim 15, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

22. The method as claimed in claim 16, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

23. The method as claimed in claim 17, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

24. The method as claimed in claim 18, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

25. The method as claimed in claim 19, wherein the initial stabilized state is a low charge state and the finalized final state is a high charge state.

26. The method as claimed in claim 14, wherein the opening setting state of the valve for each of the initial and final stabilized states are identical.

27. The method as claimed in claim 20, wherein the opening setting state of the valve for the initial and final stabilized states are identical.