Basic info on Fuel Injection

I'm not sure I understand exactly how fuel injection works. I'm aware of the following pieces of the system, listed in order downstream from the fuel tanks:

1. Electric boost pump for fuel (provides fuel pressure indepedent of engine speed)
2. Engine-driven fuel pump (provides fuel pressure dependent on engine speed)
3. Fuel control unit (meters fuel based on throttle position)
4. Fuel manifold valve (meters fuel based on mixture position)
5. Fuel injector nozzles (inject fuel into the airflow from the throttle, just upstream of the input valve for each cylinder)

I know the engine-driven pump is dependent the engine turning. Is the manifold valve tied in any way to the engine turning or the position of the pistons? I know in diesel engines the injection happens at high pressure at an exact point in the combustion cycle. Do non-diesel fuel injection systems also time the injection to the correct time in the combustion cycle? If so, I suppose there must be a mechanical connection from the turning engine to the fuel manifold valve to provide the timing.

Please clarify for me. Thanks.

Hi Tom.....

....our little injected engines are not timed or pulsed to coordinate with TDC, they're constant flow. All the injectors flow simultaneously. GM tried timed injector pulses on some racing Chevy engines years ago but quit.

Ross may chime in about how the Subies work.

Regards,

Fuel Injection timing

The timing is accomplished by the intake valve and the spark. The injectors spray the fuel into the manifold, and when the intake valve opens, the fuel & air enter the cylinder. Then the timed spark lights it off...
What kind of engine?

TThurston said:

I'm not sure I understand exactly how fuel injection works. I'm aware of the following pieces of the system, listed in order downstream from the fuel tanks:

1. Electric boost pump for fuel (provides fuel pressure independent of engine speed)
2. Engine-driven fuel pump (provides fuel pressure dependent on engine speed)

Click to expand...

Well not really, it can provide the maximum required pressure at quite low rpm, essentially a constant pressure variable flow pump

TThurston said:

3. Fuel control unit (meters fuel based on throttle position)

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Meters fuel based on sensed air mass flow (its what the 4 small tubes are for in the front), which is dependent on throttle position and air density (altitude, etc), and also on mixture position.

TThurston said:

4. Fuel manifold valve (meters fuel based on mixture position)

Click to expand...

Not sure what you are referring to here - perhaps the 'spider' on top of the engine? This just splits the fuel into 4 or 6 and does no metering.

TThurston said:

5. Fuel injector nozzles (inject fuel into the airflow from the throttle, just upstream of the input valve for each cylinder)

Click to expand...

The injector nozzles run at a fairly low pressure (say 2 to 25 psi), so also have an air inlet to help vaporize the fuel, they are positioned just upstream of the inlet valve. The fuel continuously flows out of the injector nozzle and is sucked into the cylinder when the inlet valve opens.

TThurston said:

I know the engine-driven pump is dependent the engine turning. Is the manifold valve tied in any way to the engine turning or the position of the pistons? I know in diesel engines the injection happens at high pressure at an exact point in the combustion cycle. Do non-diesel fuel injection systems also time the injection to the correct time in the combustion cycle? If so, I suppose there must be a mechanical connection from the turning engine to the fuel manifold valve to provide the timing.

Please clarify for me. Thanks.

Click to expand...

Manifold valve is not linked to engine rotation, fuel flows continuously from all injector nozzles. This system is very basic (read reliable), but works surprisingly well.

Pete

FI 101

The boost pump provides a back up to the engine driven fuel pump. Basically if your talking about Airflow Performance or Bendix (Precision) fuel injection systems, the fuel pressure generated by the boost pump is typically 2-5 PSI higher than the engine driven fuel pump setting. The minimum fuel pressure is determined by the total flow required by the engine at maximum power, the total number of injector nozzles and the orifice size of the nozzle + the other pressure drops in the system (fuel control, flow divider). So fuel pressure does not influence the fuel metering as long as there is enough pressure to satisfy the pressure drops in the system for the given power setting. It is true the system meters fuel on engine airflow consumption, but the four tubes (impact tubes) in a Bendix venturi is not what controls the fuel flow. More correctly the pressure differential created by the venturi (choke) and the impact tubes create a metering suction differential that is applied to a diaphragm in the fuel regulator. It is actually the venturi suction that does most of the work. So in essence it?s really the velocity of air flowing through the venturi that creates the metering signal. Most of the units that all the home-built folks use are not density compensating, but still because of the design of the regulator system the fuel metering is effected by the square root of the change in density. In simple terms you have about 5000 feet of altitude change before you have to reset the mixture. The flow divider (spider) is there only to divide the fuel at idle, it does no metering. If your interested on the theory and operation of Airflow Performance fuel injection systems we have classes twice a year. Typically the first weekend in March and the first weekend in November. These classes really dispel the myths of aircraft fuel injection. We will discuss the differences on Continental and some EFI systems also. If you want more info on these classes you can email me at [email protected]. It?s a good time and the food is great too.

Don

AFP's FI 101

Don at Airflow said:

...If your interested on the theory and operation of Airflow Performance fuel injection systems we have classes twice a year. Typically the first weekend in March and the first weekend in November. These classes really dispel the myths of aircraft fuel injection. We will discuss the differences on Continental and some EFI systems also. If you want more info on these classes you can email me at [email protected]. It?s a good time and the food is great too.

Don

Click to expand...

I attended this class last November and really enjoyed it. I highly recommend it. And yes, the food was great!