Dual fuel engine operating method and control strategy for optimized heat release

1. A method of operating an engine system comprising:

autoigniting a first fuel including a plurality of liquid fuels premixed with air, in a cylinder in an engine, to trigger ignition of a direct-injected main fuel in a first engine cycle;

receiving data indicative of an undesired heat release of combustion of the first fuel in the first engine cycle;

varying, based on the data, relative amounts of the plurality of liquid fuels premixed with air and admitted to the cylinder in a second engine cycle; and

autoigniting the first fuel having the varied relative amounts of the plurality of liquid fuels in the cylinder, to trigger ignition of the direct-injected main fuel in the cylinder in the second engine cycle.

2. The method of claim 1 wherein the first fuel includes a liquid fuel blend injected into a stream of intake air fed to the cylinder.

3. The method of claim 2 wherein the varying relative amounts of the plurality of liquid fuels includes varying a blend ratio of the liquid fuel blend.

4. The method of claim 2 wherein the varying relative amounts of the plurality of liquid fuels includes injecting one of the plurality of liquid fuels separately of the liquid fuel blend into the stream of intake air fed to the cylinder.

5. The method of claim 2 wherein the liquid fuel blend includes an ether, and the direct-injected main fuel includes an alcohol.

6. The method of claim 5 wherein the liquid fuel blend includes dimethyl ether (DME), methanol (MeOH), and water, and the direct-injected main fuel includes MeOH.

7. The method of claim 1 wherein the undesired heat release includes an undesired heat release rate (HRR) modality of a HRR curve.

8. The method of claim 7 wherein the varying relative amounts of the plurality of liquid fuels includes increasing a relative amount of a first one of the plurality of liquid fuels, and further comprising reducing multimodality in a HRR curve of combustion of the first fuel in the second engine cycle.

9. The method of claim 8 wherein the increasing a relative amount of a first one of the plurality of liquid fuels includes increasing a relative amount of MeOH.

10. The method of claim 9 wherein the relative amount of MeOH is increased to approximately 40% by mass or greater.

11. An engine system comprising:

an engine including therein a cylinder, and an intake port extending to the cylinder;

a fuel system including a first fuel supply of a first fuel including a plurality of liquid fuels, a first fuel injector fluidly connected to the first fuel supply and arranged to inject the first fuel into a stream of intake air fed through the intake port, a second fuel supply of a main fuel to be ignited via autoignition of the first fuel, a direct fuel injector fluidly connected to the second fuel supply, and a fueling control unit;

the fueling control unit being structured to:

receive data indicative of an undesired heat release of combustion of the first fuel in a first engine cycle;

vary, based on the data, relative amounts of the plurality of liquid fuels injected into the stream of intake air fed through the intake port, in a second engine cycle; and

reduce the undesired heat release of combustion of the first fuel in the second engine cycle based on the varied relative amounts of the plurality of liquid fuels.

12. The engine system of claim 11 wherein the undesired heat release includes an undesired heat release rate (HRR) multimodality.

13. The engine system of claim 11 wherein the first fuel includes an ether and an alcohol.

14. The engine system of claim 13 wherein the first fuel includes dimethyl ether (DME), methanol (MeOH), and water, and the main fuel includes MeOH.

15. The engine system of claim 14 wherein the varied relative amounts include an increased relative amount of the MeOH.

16. The engine system of claim 15 wherein the increased relative amount of the MeOH includes about 40% MeOH by mass or greater.

17. The engine system of claim 11 further comprising a third fuel injector fluidly connected to the second fuel supply and arranged to inject the main fuel into the stream of intake air.

18. A fuel system comprising:

a first fuel supply for a first fuel including a plurality of liquid fuels;

a first fuel injector fluidly connected to the first fuel supply to inject the first fuel at a first injection location into a stream of intake air fed to a cylinder in an engine;

a second fuel supply for a main fuel;

a direct fuel injector fluidly connected to the second fuel supply to directly inject the main fuel at a direct injection location into the cylinder;

a fueling control unit structured to:

determine an undesired heat release rate (HRR) of the first fuel autoignited in the cylinder; and

output a fuel composition control command to increase a relative amount of one of the plurality of liquid fuels admitted to the cylinder in a second engine cycle based on the determined undesired HRR.

19. The fuel system of claim 18 wherein the one of the plurality of liquid fuels includes the main fuel blended with at least one other fuel to form the fuel blend.

20. The fuel system of claim 19 further comprising:

a fuel composition valve fluidly connected to the second fuel supply, and movable between an open position and a closed position based on the fuel composition control command; and

a fuel mixing conduit extending from the fuel composition valve to a fuel outlet defining one of a fuel-fuel mixing location for mixing the main fuel into a feed of the first fuel to the first fuel injector, or a fuel-air mixing location for mixing the main fuel into the feed of intake air fed to the cylinder.