Method to control the temperature of the exhaust gases of a supercharged internal combustion engine

1. A method to control the temperature of the exhaust gases of a supercharged internal combustion engine ( 1 ) comprising a number of cylinders ( 3 ), an intake manifold ( 4 ), which is connected to the cylinders ( 3 ), an exhaust manifold ( 5 ), which is connected to the cylinders ( 3 ), as well, and feeds the exhaust gases produced by the combustion taking place inside the cylinders ( 3 ) to an exhaust system, and a turbocharger ( 12 ), which is provided with a turbine ( 13 ) and a supercharger ( 14 ); and a sensor ( 25 ), which measures the air/fuel ratio of the exhaust gases downstream of the exhaust manifold ( 5 ); the supercharged internal combustion engine ( 1 ), furthermore, is designed to permit the passage of air not involved in the combustion taking place inside the cylinders ( 3 ) from the intake manifold ( 4 ) directly into the exhaust pipe ( 10 ); the method comprises the steps of:

determining the flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) and flows directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 );

determining the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) as a function of the flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) and flows directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 );

detecting the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) provided by the sensor ( 25 );

comparing the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) and/or the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) with a number of threshold values (UTV OL _ SCAV , UTV INHIB _ SCAV , UTV LIMIT _ SCAV , LTV OL _ SCAV , LTV INHIB _ SCAV , LTV LIMIT _ SCAV , SV); and

controlling the supercharged internal combustion engine as a function of the comparison between the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) and/or the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) with said number of threshold values (UTV OL _ SCAV , UTV INHIB _ SCAV , UTV LIMIT _ SCAV , LTV OL _ SCAV , LTV INHIB _ SCAV , LTV LIMIT _ SCAV , SV).

2. A method according to claim 1 and comprising the further steps of:

comparing the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) with a first pair of threshold values (UTV LIMIT _ SCAV , LTV LIMIT _ SCAV ); and

limiting the flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) and flows directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 ), if the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) is not comprised within the interval defined by the first pair of threshold values (UTV LIMIT _ SCAV , LTV LIMIT _ SCAV ).

3. A method according to claim 1 and comprising the further steps of:

comparing the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) with a second pair of threshold values (UTV INHIB _ SCAV , LTV INHIB _ SCAV ); and

forbidding the passage of a flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 ), if the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) is not comprised within the interval defined by the second pair of threshold values (UTV INHIB _ SCAV , LTV INHIB _ SCAV ).

4. A method according to claim 2 , wherein the interval defined by the first pair of threshold values (UTV LIMIT _ SCAV , LTV LIMIT _ SCAV ) is comprised within the interval defined by the second pair of threshold values (UTV INHIB _ SCAV , LTV INHIB _ SCAV ).

5. A method according to claim 1 and comprising the further steps of:

comparing the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) with a third pair of threshold values (UTV OL _ SCAV , LTV OL _ SCAV ); and

controlling the air/fuel ratio of the exhaust gases downstream of the exhaust manifold ( 5 ) in open-loop, if the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) is not comprised within the interval defined by the third pair of threshold values (UTV OL _ SCAV , LTV OL _ SCAV ).

6. A method according to claim 1 and comprising the further steps of:

comparing the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) with the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ); and

limiting the flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) and flows directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 ), if the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) is smaller than the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ).

7. A method according to claim 1 and comprising the further steps of:

comparing the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) with a safety value (SV); and

forbidding the passage of a flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 ), if the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) is greater than the safety value (SV).

8. A method according to claim 1 and comprising the further steps of:

determining the reference value of the air/fuel ratio (λ target ) of the exhaust gases as a function of the flow rate (m air _ scav ) of air that is not involved in the combustion taking place inside the cylinders ( 3 ) and flows directly from the intake manifold ( 4 ) into the exhaust pipe ( 10 );

calculating the difference between the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) and the reference value of the air/fuel ratio (λ target ) of the exhaust gases; and

controlling the supercharged internal combustion engine ( 1 ) as a function of the difference between the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) and the reference value of the air/fuel ratio (λ target ) of the exhaust gases.

9. A method according to claim 8 , wherein the reference value of the air/fuel ratio (λ target ) is variable as a function of the engine point and of the thermal state of the internal combustion engine ( 1 ).

10. A method according to claim 8 and comprising the further step of filtering the difference between the air/fuel ratio (λ meas ) of the exhaust gases downstream of the exhaust manifold ( 5 ) detected by the sensor ( 25 ) and the reference value of the air/fuel ratio (λ target ) of the exhaust gases.

11. A method according to claim 1 , wherein the minimum air/fuel ratio (λ min ) of the exhaust gases downstream of the exhaust manifold ( 5 ) is calculated by means of the formula:

λ

m

⁢

⁢

i

⁢

⁢

n

=

1

-

c

p

⁢

⁢

Δ

⁢

⁢

T

ma

⁢

⁢

x

k

i

*

(

1

+

A

st

/

F

st

)

[

11

]

λ min air/fuel ratio that can be obtained in the exhaust gases downstream of the exhaust manifold ( 5 );

c p specific heat of the air;

ΔT max maximum temperature difference in an exhaust system;

k i lower heating value of the fuel being used; and

A st /F st stoichiometric ratio of the fuel being used.