1001001
Well Known Member
I think I understand that in an electronic fuel injection system (with direct injection at each cylinder), the fuel charge is determined by the pressure at the fuel "rail" and the duration of the opening pulse of the fuel injector. This should ensure that fuel only enters the cylinder during the appropriate part of the cycle (intake).
In a mechanical fuel injection system, such as the Airflow Performance system (which I have on the engine package for my -10), the fuel spider is pressurized at all times, but at varying pressures as determined by the fuel servo, so that the flow is determined by the differential pressure across the injector and its orifice.
What (if anything) prevents fuel from flowing during the other parts of the cycle for a given cylinder? Just differential pressure? Does this mean that fuel can flow during all cycle stages, essentially wasting fuel, even in a direct injection system?
For instance, if the fuel pressure is 20 psig (34.7 psia at sea level) (just to pull a number out of lean air) and the intake stroke pressure is, let's say, 20 inches of mercury(about 9.8 psia), we can calculate the flow of fuel through a given orifice. During the compression and power strokes, the pressure will rise well above the fuel pressure, and fuel flow through a direct injector should stop (in fact, cylinder gases should back up through the injector briefly). But when the exhaust stroke occurs, and the cylinder is essentially open to atmosphere again, we would expect a pressure of, let's say at sea level, 14.7 psia. So this would be a 20 psi differential again, and we would expect fuel to flow into the exhausting cylinder.
Does this happen? Am I missing something? Does this kind of system really waste fuel like this?
Forgive my ignorance of fuel injection for I am but a grasshopper.
In a mechanical fuel injection system, such as the Airflow Performance system (which I have on the engine package for my -10), the fuel spider is pressurized at all times, but at varying pressures as determined by the fuel servo, so that the flow is determined by the differential pressure across the injector and its orifice.
What (if anything) prevents fuel from flowing during the other parts of the cycle for a given cylinder? Just differential pressure? Does this mean that fuel can flow during all cycle stages, essentially wasting fuel, even in a direct injection system?
For instance, if the fuel pressure is 20 psig (34.7 psia at sea level) (just to pull a number out of lean air) and the intake stroke pressure is, let's say, 20 inches of mercury(about 9.8 psia), we can calculate the flow of fuel through a given orifice. During the compression and power strokes, the pressure will rise well above the fuel pressure, and fuel flow through a direct injector should stop (in fact, cylinder gases should back up through the injector briefly). But when the exhaust stroke occurs, and the cylinder is essentially open to atmosphere again, we would expect a pressure of, let's say at sea level, 14.7 psia. So this would be a 20 psi differential again, and we would expect fuel to flow into the exhausting cylinder.
Does this happen? Am I missing something? Does this kind of system really waste fuel like this?
Forgive my ignorance of fuel injection for I am but a grasshopper.