Method for operating a compression ignition engine

1. A method of operating a compression ignition engine, the engine having a cylinder and a piston movable in the cylinder, said method comprising:

forming a combustible mixture by mixing substantially homogeneously a first fuel and air, and introducing the combustible mixture of the first fuel and air into the cylinder;

compressing the combustible mixture with the piston in a compression stroke;

injecting a second fuel into the combustible mixture at an injection-time of the second fuel during the compression stroke but before start of combustion to create a cylinder charge, the second fuel being capable of auto-igniting earlier than the first fuel;

continuing the compression stroke until combustion starts in the cylinder at locations where at least one of (i) concentration of the second fuel is highest and (ii) temperature of the combustible mixture is the highest;

monitoring emissions of the cylinder caused by the combustion by differentiating between NOx and HC emissions; and

if said monitoring determines that HC emissions are above a predetermined HC emissions threshold value individually for the cylinder, then performing:

increasing an amount of the second fuel injected;

retarding an injection timing of the second fuel; and

increasing a temperature of the cylinder charge.

2. The method according to claim 1 , wherein the first fuel is natural gas or a mixture of natural gas and CO2 such that an amount of CO2 and CH4 is greater than 80% of a total amount of the first fuel.

3. The method according to claim 1 , wherein the second fuel has a cetane number between 30 and 70.

4. The method according to claim 3 , wherein the second fuel has a cetane number between 40 and 60.

5. The method according to claim 1 , further comprising monitoring mechanical stress of the cylinder caused by the combustion; and

if mechanical stress is above a respective threshold value individually for the cylinder, then performing at least one of:

changing the amount of the second fuel injected;

changing the injection timing of the second fuel; and

changing the temperature of the cylinder charge, such that mechanical stress goes down below the respective predetermined thresholds.

6. The method according to claim 5 , wherein at least one of said monitoring emissions of the cylinder and said monitoring mechanical stress of the cylinder comprises measuring signals characteristic for the combustion in the cylinder.

7. The method according to claim 5 , wherein said changing the temperature of the cylinder charge, if performed, comprises lowering the temperature of the cylinder charge.

8. The method according to claim 7 , wherein said lowering the temperature of the cylinder charge, if performed, comprises lowering an intake temperature of the first fuel and air.

9. The method according to claim 1 , further comprising monitoring mechanical stress of the cylinder caused by the combustion, wherein at least one of said monitoring emissions of the cylinder and said monitoring mechanical stress of the cylinder comprises measuring signals characteristic for the combustion in the cylinder.

10. The method according to claim 1 , wherein at least one of said monitoring emissions of the cylinder and said monitoring mechanical stress of the cylinder comprises measuring signals characteristic for the combustion in the cylinder, said measuring signals characteristic for the combustion in the cylinder comprises determining at least one of (i) a characteristic position in time of the combustion event and (ii) a duration of the combustion event.

11. The method according to claim 5 , wherein said changing the amount of the second fuel injected, if performed, comprises reducing the amount of the second fuel.

12. The method according to claim 1 , further comprising controlling an in-cylinder temperature by an internal EGR-rate kept in a combustion chamber during a gas exchange process or by an external EGR rate recirculated in the intake system.

13. The method according to claim 1 , wherein the engine comprises a variable valve train configured to individually vary at least one of (i) an exhaust/intake valve timing and (ii) valve lift curves with respect to the cylinder.

14. The method according to claim 13 , wherein said increasing the temperature of the cylinder charge is performed by the variable valve train by closing an exhaust valve to increase the internal EGR and thereby increase the temperature of the cylinder charge.

15. The method according to claim 14 , wherein increasing the temperature of the cylinder charge is performed by the variable valve train by re-opening a closed exhaust valve in an intake phase of the piston to thereby increase the temperature of the cylinder charge.

16. The method according to claim 13 , wherein increasing the temperature of the cylinder charge is performed by the variable valve train by re-opening a closed exhaust valve in an intake phase of the piston to thereby increase the temperature of the cylinder charge.

17. The method according to claim 13 , wherein increasing the temperature of the cylinder charge is performed by the variable valve train by re-opening a closed intake valve during an exhaust stroke of the piston thereby increasing the temperature of the cylinder charge.

18. The method according to claim 1 , wherein said increasing the temperature of the cylinder charge comprises increasing the back-pressure to increase the temperature of the cylinder charge.

19. The method according to claim 1 , wherein said increasing the temperature of the cylinder charge comprises an additional injection of the second fuel and combustion at TDC between an exhaust stroke and an intake stroke while the intake valve and exhaust valves are closed.

20. A compression ignition engine comprising:

a cylinder;

a piston moveable in said cylinder;

an injector to inject the second fuel; and

an electronic control unit configured to perform the method according to claim 1 .