Methods and systems for dual fuel injection

1. A method for an engine, comprising:

pressurizing fuel in a port injection fuel rail via an engine camshaft-driven high pressure fuel pump;

intermittently sampling fuel pressure in the port injection fuel rail; and

in response to a determination that a port-injection pulse of fuel delivered to the engine is smaller than a threshold, moving the port-injection pulse from an initial timing, asynchronous with the intermittent sampling, to a final timing, synchronous with the intermittent sampling.

2. The method of claim 1 , wherein intermittently sampling fuel pressure in the port injection fuel rail includes a first sampling of the fuel pressure at a first timing followed by a second sampling of the fuel pressure at a second, later timing with no intermediate pressure sampling, a duration elapsed between the first and second timings based on engine firing frequency.

3. The method of claim 2 , wherein the initial timing is based on the first sampling of fuel pressure, the initial timing is after the first timing and after the second timing, and wherein the final timing coincides with the second timing.

4. The method of claim 2 , wherein the moving includes advancing the initial timing to the second timing such that a middle angle of the port-injection pulse is aligned with the second timing.

5. The method of claim 4 , further comprising adjusting an end of injection angle of the port-injection pulse based on the second sampling of the fuel pressure.

6. The method of claim 5 , wherein the adjusting includes advancing the end of injection angle towards the final timing when the fuel pressure at the second sampling is smaller than the fuel pressure at the first sampling, and retarding the end of injection angle away from the final timing when the fuel pressure at the second sampling is larger than the fuel pressure at the first sampling.

7. The method of claim 1 , further comprising adjusting intake port fuel puddle model dynamics based on the moving.

8. The method of claim 7 , further comprising moving an end of injection angle of the port-injection pulse based on the moving and the adjusted intake port fuel puddle model dynamics.

9. The method of claim 1 , wherein the threshold is based on engine speed and load, the threshold lowered as engine speed and load increases.

10. The method of claim 1 , further comprising, in response to a determination that the port-injection pulse is larger than the threshold, maintaining the port-injection pulse at the initial timing.

11. The method of claim 1 , further comprising operating a port fuel injector to deliver the port-injection pulse at the final timing.

12. The method of claim 1 , further comprising pressurizing fuel in a direct injection fuel rail via the engine camshaft-driven high pressure fuel pump.

13. The method of claim 1 , wherein each of the initial and final timings includes closed intake valve injection, and wherein each of the initial and final timings includes engine crank angle degrees.

14. A method for an engine, comprising:

measuring a port injection fuel rail pressure with a frequency, the port injection fuel rail pressurized by an engine-driven high pressure piston fuel pump;

estimating an initial timing and an initial width of a port-injection pulse of fuel injected into the engine based on a first measurement of the port injection fuel rail pressure; and

selectively updating each of the initial timing and the initial width of the port-injection pulse based on a second, immediately subsequent measurement of the port injection fuel rail pressure, the initial timing asynchronous with the first and second measurements, the selectively updated timing synchronous with the second measurement.

15. The method of claim 14 , wherein the selectively updating includes advancing the initial timing to a timing of the second measurement when the initial width of the port-injection pulse is smaller than a threshold and maintaining the initial timing when the initial width of the port-injection pulse is larger than the threshold.

16. The method of claim 15 , wherein the selectively updating further includes, after advancing the initial timing, trimming the initial width of the port-injection pulse based on a difference in fuel pressure between the first and second measurements.

17. The method of claim 16 , wherein advancing the initial timing to the timing of the second measurement includes aligning a middle of injection angle of the port-injection pulse with the timing of the second measurement, and wherein trimming the initial width of the port-injection pulse includes advancing an end of injection angle of the port-injection pulse towards the second measurement.

18. An engine fuel system, comprising:

an engine;

a first fuel rail coupled to a direct injector;

a second fuel rail coupled to a port injector;

a high pressure mechanical fuel pump driven by the engine via a camshaft, the fuel pump delivering fuel to each of the first and second fuel rails, the first fuel rail coupled to an outlet of the high pressure mechanical fuel pump, the second fuel rail coupled to an inlet of the high pressure mechanical fuel pump;

a pressure sensor coupled to the second fuel rail for estimating a fuel pressure; and

a controller with computer readable instructions stored on non-transitory memory configured to:

pressurize the second fuel rail via the high pressure mechanical fuel pump;

perform a first sampling of the fuel pressure in the second fuel rail;

calculate an initial timing and width of a port-injection pulse based on the first sampling; and

if the width of the port-injection pulse is smaller than a threshold,

advance the port-injection pulse from the initial timing to a timing corresponding to a second sampling of the fuel pressure, the second sampling immediately following the first sampling with no further sampling in-between.

19. The system of claim 18 , wherein the controller includes further instructions configured to adjust intake port fuel puddle model dynamics based on the advancing, and adjust the width of the port-injection pulse based on the adjusted intake port fuel puddle model dynamics.