Method to determine the injection pattern in the intake stroke of the combustion cycle of the cylinders of a direct-injection internal combustion engine

1. A method for determining an injection pattern in an intake stroke (INT) of a combustion cycle of cylinders ( 2 ) of a direct-injection internal combustion engine ( 1 ), said method comprising the steps of:

determining an overall quantity (Q fuel ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) to satisfy a request for torque (C) to be delivered;

determining a quantity (Q fuel _ int ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the intake stroke (INT) as a function of the overall quantity (Q fuel ) of fuel to be injected for each combustion cycle of each cylinder ( 2 );

determining a maximum quantity (QF max _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) as a function of a speed (V engine ) of the internal combustion engine ( 1 ), of a load (L engine ) of the internal combustion engine ( 1 ), and of a value of the start of injection angle (SOI); and

determining the injection pattern in the intake stroke (INT) as a function of the quantity (Q fuel _ int ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the intake stroke (INT) and of the maximum quantity (QF max _ int ) of fuel to be injected for each partial injection during the intake stroke (INT), which further includes the sub-steps of:

determining an objective quantity (QF obj _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) by determining a ratio between the quantity (Q fuel _ int ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the intake stroke (INT) and a number of partial injections to be performed in the intake stroke (INT);

determining the number of partial injections (i) to be performed in the intake stroke (INT) so that the ratio between the quantity (Q fuel _ int ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the intake stroke (INT) and the number (i) of partial injections to be performed in the intake stroke (INT) is either lower than or equal to the maximum quantity (QF max _ int ) of fuel to be injected for each partial injection in the intake stroke (INT);

wherein the maximum quantity (QF max _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) is calculated as a function of a maximum quantity (T warm ) of fuel to be injected for each partial injection in the intake stroke (INT) when the internal ( 1 ) combustion engine is at least 90° C., and as a function of a maximum quantity (T cold ) of fuel to be injected for each partial injection in the intake stroke (INT) when the internal ( 1 ) combustion engine is at 20° C. or less;

determining the maximum quantity (T warm ) of fuel to be injected for each partial injection in the intake stroke (INT) when the internal combustion engine ( 1 ) is at least 90° C. as a function of the speed (V engine ) of the internal combustion engine ( 1 ) and of the load (L engine ) of the internal combustion engine ( 1 );

determining the maximum quantity (T cold ) of fuel to be injected for each partial injection in the intake phase (INT) when the internal combustion engine ( 1 ) is 20° C. or less as a function of the speed (V engine ) of the internal combustion engine ( 1 ) and of the load (L engine ) of the internal combustion engine ( 1 ); and

calculating the maximum quantity (QF max _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) as a weighted mean between the maximum quantity (T warm ) of fuel to be injected for each partial injection in the intake stroke (INT) when the internal combustion engine ( 1 ) is at least 90° C. and the maximum quantity (T cold ) of fuel to be injected for each partial injection in the intake stroke (INT) when the internal ( 1 ) combustion engine is 20° C. or less.

2. The method according to claim 1 , wherein an amount of time elapsing between two subsequent partial injections of the intake stroke (INT) is constant.

3. The method according to claim 1 , wherein the objective quantity (QF obj _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) progressively either increases or decreases and an amount of time elapsing between two subsequent partial injections of the intake stroke (INT) progressively increases.

4. The method according to claim 1 , wherein the weighted mean for calculating the maximum quantity (QF max _ int ) of fuel to be injected for each partial injection in the intake stroke (INT) is determined as a function of the distance of the current temperature of the internal combustion engine ( 1 ) from 20° C. or less, and from at least 90° C., respectively.

5. The method according to claim 1 , wherein a temperature of the internal combustion engine ( 1 ) is equal to a temperature (T coolant ) of a cooling fluid of the internal combustion engine ( 1 ).

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

determining, as a function of the overall quantity (Q fuel ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ), a quantity (Q fuel _ comp ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during a compression stroke (COMP) and/or a quantity (Q fuel _ esp ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during an expansion stroke (ESP);

determining the quantity (Q fuel _ int ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the intake stroke (INT) as a function of the difference between the overall quantity (Q fuel ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) and the quantity (Q fuel _ comp ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the compression stroke (COMP) and/or the quantity (Q fuel _ esp ) of fuel to be injected for each combustion cycle of each cylinder ( 2 ) during the expansion stroke (ESP).