Methods for determining fuel bulk modulus in a high-pressure pump
Methods are provided for finding the bulk modulus of a fuel used in the direct injection system of an internal combustion engine. A method is needed to continuously monitor and reliably calculate the fuel's bulk modulus during engine operation on-board the vehicle, where the fuel's bulk modulus may be used to infer the ratio of fuels in a fuel mixture or determine the density of supercritical propane when propane is used as the injected fuel. To find the fuel's bulk modulus on-board a vehicle, methods are proposed that involve monitoring and recording fuel rail pressures, high pressure pump duty cycles, and fractional liquid volume pumped values in order to find zero flow relationships.
1. A method, comprising:
adjusting duty cycle of a high pressure pump to measure a bulk modulus of a fuel based on a zero flow function for the high pressure pump, the fuel being pumped through the high pressure pump and the zero flow function based on a change in pump duty cycle relative to a resulting change in fuel rail pressure.
2. The method of claim 1 , wherein determining the zero flow function for the high pressure fuel pump includes:
while not direct injecting fuel into an engine and while the engine is in a stabilized idling condition, commanding a first pump duty cycle;
waiting until fuel rail pressure reaches a steady-state value and then determining a first fuel rail pressure;
then commanding a second, higher pump duty cycle and determining a second fuel rail pressure; and
continue increasing pump duty cycle incrementally and determining fuel rail pressure until an upper duty cycle threshold is reached.
3. The method of claim 1 , wherein determining the zero flow function for the high pressure fuel pump includes:
while direct injecting fuel into an engine to maintain a positive fuel flow rate, commanding a multitude of pump duty cycles corresponding to a multitude of fuel rail pressures and determining a responsive fractional volume of liquid fuel pumped, thereby forming a dataset, wherein the dataset comprises a multitude of operating points, each operating point consisting of a duty cycle, fuel rail pressure, and fractional volume pumped; and
determining a multitude of horizontal-axis intercepts that correspond to zero flow rate data based on a known line slope.
4. The method of claim 3 , wherein the known line slope is a slope of the dataset, wherein a vertical axis is fractional liquid fuel volume pumped and a horizontal axis is pump duty cycle.
5. The method of claim 1 , wherein high pressure pump duty cycle is a measure of a closing time of a solenoid activated check valve that controls an amount of fuel pumped into the fuel rail by the high pressure pump.
6. The method of claim 5 , wherein any reduced current of the solenoid activated check valve is disabled.
7. The method of claim 1 , wherein the high pressure fuel pump ingests liquid fuel with no fuel vapor.
8. The method of claim 1 , wherein the fuel is a mixture of ethanol and gasoline, a mixture of propane and gasoline, or liquid propane.
9. An engine system, comprising:
an engine;
a direct fuel injector configured to direct inject fuel into the engine;
a fuel rail fluidly coupled to the direct fuel injector;
a high pressure fuel pump fluidly coupled to the fuel rail;
a controller with computer readable instructions stored in non-transitory memory for:
adjusting duty cycle of a high pressure pump to measure a bulk modulus of a fuel based on a zero flow function for the high pressure pump, the fuel being pumped through the high pressure pump and the zero flow function based on a change in pump duty cycle relative to a resulting change in fuel rail pressure.
10. The engine system of claim 9 , wherein determining the zero flow function for the high pressure fuel pump includes:
while not direct injecting fuel into an engine and while the engine is in a stabilized idling condition, commanding a first pump duty cycle;
waiting until fuel rail pressure reaches a steady-state value and then determining a first fuel rail pressure;
then commanding a second, higher pump duty cycle and determining a second fuel rail pressure; and
continue increasing pump duty cycle incrementally and determining fuel rail pressure until an upper duty cycle threshold is reached.
11. The engine system of claim 9 , wherein determining the zero flow function for the high pressure fuel pump includes:
while direct injecting fuel into an engine to maintain a positive fuel flow rate, commanding a multitude of pump duty cycles corresponding to a multitude of fuel rail pressures and determining a responsive fractional volume of liquid fuel pumped, thereby forming a dataset, wherein the dataset comprises a multitude of operating points, each operating point consisting of a duty cycle, fuel rail pressure, and fractional volume pumped; and
determining a multitude of horizontal-axis intercepts that correspond to zero flow rate data based on a known line slope.
12. The engine system of claim 11 , wherein the known line slope is a slope of the dataset, wherein a vertical axis is fractional liquid fuel volume pumped and a horizontal axis is pump duty cycle.
13. The engine system of claim 9 , wherein high pressure pump duty cycle is a measure of a closing time of a solenoid activated check valve that controls an amount of fuel pumped into the fuel rail by the high pressure pump.
14. The engine system of claim 13 , wherein any reduced current of the solenoid activated check valve is disabled.
15. The engine system of claim 9 , wherein the high pressure fuel pump ingests liquid fuel with no fuel vapor.
16. The engine system of claim 9 , wherein the fuel is a mixture of ethanol and gasoline, a mixture of propane and gasoline, or liquid propane.
17. A method, comprising:
while not direct injecting fuel into an engine via a high pressure pump and while the engine is in a stabilized idling condition, determining a relationship between high pressure pump duty cycle and fuel rail pressure; and
finding a slope from the relationship to determine a bulk modulus of a fuel.
18. The engine method of claim 17 , wherein determining the relationship includes:
incrementally increasing pump duty cycle and waiting for a period of time before measuring a responsive fuel rail pressure for each pump duty cycle; and
continue incrementally increasing pump duty cycle until an upper threshold duty cycle is reached.
19. An engine method, comprising:
while direct injecting fuel into an engine to maintain a positive fuel flow rate, determining a relationship between high pressure pump duty cycle and fuel rail pressure; and
finding a slope from the relationship to determine a bulk modulus of a fuel.
20. The engine method of claim 19 , wherein determining the relationship further comprises:
selecting a multitude of operating points, each operating point including a pump duty cycle and a fuel rail pressure that correspond to a fractional fuel volume pumped;
regressing each operating point to find a multitude of intersections with a horizontal axis; and
plotting the intersections on a graph.
21. The engine method of claim 20 , wherein regressing each operating point involves finding a slope of a line based on pump duty cycle and fractional fuel volume pumped.