Pressure across an injector. MAP compensation. etc.

[H2power]

I have a question that is probably so stupid I am embarrassed asking it.

Say you have a turbocharged CNG engine. Say the fuel rail pressure is 70psi across the injector. Say the injector will not open past 100psi at atmospheric pressure. Will the injector open with 30 psi guage manifold pressure. Pressure across the injector would still have to be 70psi (with manifold pressure compensation) but the pressure on the rail side of the injector would be 100psi.

The problem I have is does the injector solonoid "see" Pressures across or fuel rail pressure?

Thanks.

 

[Warpspeed]

The injector only "sees" the differential pressure across itself. That is the pressure it has to open against, and that pressure also determines the flow and spray characteristics once it has opened.

Usually there is a tracking pressure regulator to hold the fuel rail pressure a constant amount above inlet port pressure. As far as the injector is concerned, it is always operating under fixed (pressure) operating conditions.

 

[patdaly]

H2power, rail pressure and the pressure across are effectively the same for this system. The orifice that manifold pressure could affect is so small, you can ignore it.

 

[H2power]

Warpspeed, The way I understand it, by mass of fuel, as long as the delta P across the injector is the same the mass injected will be the same at a set dutycycle and temperature.

The problem I am trying to figure what happens at high fuel rail pressures. If an injector is designed for 70psi operation naturally aspirated (0psig, 15psia) and will not reliably open at 100psi naturally aspirated (0psig, 15psia) will it open at 100psi rail pressure and 30psig or 45psia manifold pressure?

On one hand the delta P across the injector is the same for both conditions at 70psi. On the other hand the fuel rail pressure is 100psi for the second case. What is the important phenomon causing the injector not to open, delta P or fuel rail pressure.

And yes, I know 30psi is high boost!

Thanks.

 

[WilliamH]

H2power,

The answer your question regarding injector solenoid characteristics with very high fuel rail pressure is; yes the injector coil will work harder. You can see this if you connect an oscilloscope to an injector and watch the voltage peaks.

The injector coil will become hotter with very high fuel rail presures and can become very unstable. Also, the resultant high "firing" voltages can cause the ECU to become unstable as well. We are talking relatively here. There are injectors available that are designed to operate at fuel rail presssures of 120psi.

Will

 

[H2power]

WilliamH, The solenoid characteristics you are talking about show up as increased current rise time right?

The problem I am trying to figure out is what happens when you increase the boost to 60psig, or more. I want to run the injector at a constant differential pressure but I fear that as I keep increasing the the fuel rail pressure I will reach a point where the injectors will quit firing. At these large boost numbers if I don't increase the fuel rail pressure I will not be able to flow enough fuel. With 120psi injectors that still leaves me only 60 psi delta P at 60psi map unless I can run the injectors at higher rail pressures.

 

[Warpspeed]

I cannot see this. Boost pressure is trying to force the injector open, and fuel rail pressure is trying to force it shut. The injector actuator only has to work against the differential pressure.

 

[CNGBrick]

Keeping in mind we are talking CNG injectors, the following points should be considered:

1) Mass flow through the injector is determined by rail pressure (mostly) as long as sonic flow conditions exist at the injector metering orifice (neglecting the effects of differential pressures on opening/closing times). When the manifold pressure is high enough that we fall out of sonic flow, the mass flow is then affected by both the rail and manifold pressures.

2) CNG fuel system rail pressure is normally fixed with respect to atmospheric pressure -- there is generally no or only limited tracking of the pressure with the manifold pressure. When some form of manifold reference is used, a sudden large drop in manifold pressure could cause an injector to lock up as the pressure differential spikes, especially if a deceleration fuel cut-out strategy is used(remember, the gas can't "recirculate" back to the tank, it is stuck between the injector and regulator).

3) Running an injector (you haven't identified the type/brand) beyond its intended operating pressure could compromise the integrity of the internal seals when temperature extremes are considered.

That being said, the injector only "sees" the differential between rail pressure and manifold pressure and thus should be able to open and close if that differential is maintained as the rail pressure goes up. Keep in mind point number 1 though as you may fall out of sonic flow condition as the pressures go up.

 

[dgallup]

H2Power: As far as the injector opening, it only sees the differential pressure across it. So if the fuel rail pressure tracts the manifold pressure to keep a constant delta P the injector will open even with high rail pressure. Believe me, I design & test OE fuel injectors.

WilliamH: As far as injector coil heating, this is entirely a funtion of drive circuitry and injector coil design, it is entirely independant of rail pressure. The voltage across an injector is pure battery voltage for a high impendance saturated or ballasted design or is battery voltage up to the current peak & then chopped in a peak & hold design. The ECU does not become "unstable".

CNGBrick: I agree with everything you said although MOST injector seals are OK far above the rail pressures mentioned here.