Method for controlling the speed of an internal combustion engine supercharged by means of a turbocharger

1. A method for controlling an internal-combustion engine ( 1 ) supercharged by a turbocharger ( 12 ) and including a turbine ( 13 ) and compressor ( 14 ), said control method comprising steps of:

determining a pressure objective downstream of the compressor ( 14 );

determining a critical threshold (M critica ) of a reduced-mass-flow rate (Q AHR ) that delimits on a “reduced-mass-flow rate/compression ratio” plane a critical area substantially close to achievement of sonic conditions; and

filtering by a first filter the pressure objective downstream of the compressor ( 14 ) when a current reduced-mass-flow rate (Q AHR ) is higher than the critical threshold.

2. A control method as set forth in claim 1 , wherein said control method comprises further steps of:

determining the current reduced-mass-flow rate (Q AHR ) of the compressor ( 14 ); and

filtering by a second filter the current reduced-mass-flow rate (Q AHR ) when the current reduced-mass-flow rate (Q AHR ) is higher than the critical threshold (M critica ).

3. A control method as set forth in claim 1 , wherein at least one of the first filter is a low-pass filter of the first order and the second filter is a low-pass filter of the first order.

4. A control method as set forth in claim 1 , wherein said control method comprises further steps of:

determining a safety threshold (M max — turbo ) of the reduced-mass-flow rate (Q AHR ) that is higher than the critical threshold (M critica );

delimiting on a “reduced-mass-flow rate/compression ratio” plane a portion of the critical area substantially closest to the achievement of the sonic conditions; and

imposing that the reduced-mass-flow rate (Q AHR ) of the compressor ( 14 ) has to be lower than the safety threshold (M max — turbo ) of the reduced-mass-flow rate (Q AHR ).

5. A control method as set forth in claim 4 , wherein said control method comprises further steps of:

determining a safety threshold (N max — turbo ) of speed of the internal-combustion engine ( 1 ) according to the safety threshold (M max — turbo ) of the reduced-mass-flow rate (Q AHR ); and

imposing that the speed of the internal-combustion engine ( 1 ) has to be lower than the safety threshold (N max — turbo ) of the speed.

6. A control method as set forth in claim 5 , wherein the safety threshold (N max — turbo ) of the speed of the internal-combustion engine ( 1 ) is calculated by the following equation:

N max — turbo =M max — turbo ·√{square root over ( T o — rif /T o )}·P o /P o — rif ·1/( m· 30 ·N cil )

wherein

N max — turbo =the safety threshold of the speed of the supercharged internal-combustion engine ( 1 );

M max — turbo =the safety threshold of the reduced-mass-flow rate (Q AHR );

T o =absolute temperature upstream of the compressor ( 14 );

P o =absolute pressure upstream of the compressor ( 14 );

T o — rif =absolute reference temperature;

P o — rif =absolute reference pressure;

N cil =number of cylinders ( 3 ) of the internal-combustion engine ( 1 ); and

M=air mass taken in by each cylinder ( 3 ) of the internal-combustion engine ( 1 ).

7. A control method as set forth in claim 1 , wherein said control method comprises further steps of:

establishing a predetermined limit speed (N) of the compressor ( 14 );

calculating a reduced limit speed (N R ) of the compressor ( 14 ) by using the predetermined limit speed (N) of the compressor ( 14 ) and an absolute temperature (T o ) upstream of the compressor ( 14 ); and

determining at least one of the critical threshold (M critica ) of the reduced-mass-flow rate (Q AHR ) and a safety threshold (M max — turbo ) of the reduced-mass-flow rate (Q AHR ) according to the reduced limit speed (N R ).

8. A control method as set forth in claim 1 , wherein said control method comprises further steps of:

establishing on the “reduced-mass-flow rate/compression ratio” plane at least one limit curve ( 29 , 30 ) of operation of the compressor ( 14 );

determining a supercharging-limit value by using the limit curve ( 29 , 30 ) according to the current reduced-mass-flow rate (Q AHR ); and

determining the pressure objective downstream of the compressor ( 14 ) according to the supercharging-limit value and the pressure objective downstream of the compressor ( 14 ) obtained by an engine control.

9. A control method as set forth in claim 8 , wherein said control method comprises further steps of:

determining a current limit-compression ratio (RC) of the compressor ( 14 ) according to the current reduced-mass-flow rate (Q AHR ) and limit curve ( 29 , 30 );

detecting a current pressure value upstream of the compressor ( 14 );

determining the supercharging-limit value downstream of the compressor ( 14 ) by multiplying the current pressure value upstream of the compressor ( 14 ) by the current limit-compression ratio (RC);

determining, regardless of the supercharging-limit value and according to an engine point, an objective-pressure value downstream of the compressor ( 14 ); and

limiting the pressure objective downstream of the compressor ( 14 ) to the supercharging-limit value in case the objective-pressure value downstream of the compressor ( 14 ) obtained by the engine control is higher than the supercharging-limit value.