Injection pulses for depressurization

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Injection pulses for depressurization

Postby Experimenteer » Tue Jul 20, 2010 8:30 pm

I have been led to believe that in many common rail diesel systems, fuel rail depressurisation at engine shut-down is accomplished by pulsing the injector with a very short pulse, sufficient for the valve in the injector servo chamber to open and close (expelling fuel out the injector spill/return line), but with a duration so short that it does not cause the injector needle to lift and inject fuel - i.e. servo is shut before inertia takes effect etc - sort of like PWM.

I was just wondering if anyone has ever observed this on a scope, or whether it happens so fast that the process is finished by the time the engine has stopped rotating - indeed are injectors prevented from firing by a safety interlock unless there is rotation sensed?

Also I am a little confused by earlier posts on injection voltage. I understand that voltages on the two injector lines should be measured relative to ground rather than to each other - is it the case that on opening, wire A is pulsed 50V positive relative to B, and then pulsed 50V negative relative to B to close it? Is this negative voltage just recovery of the collapsing mag field to be held in a capacitor for the next quick field formation stage, or is the injector explicitly driven through a 100V peak to peak, 50 volts either side of mid-way, or what?

I appreciate any clarification or experience people have had on either of these two points.
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Re: Injection pulses for depressurization

Postby Dave Hill » Thu Jul 22, 2010 8:00 pm

Hi Experimenteer (Ahhhgg I hate using Forum names :shock: )

I have noticed this "venting" of fuel pressure that you mention, but only on the Delphi common rail systems that we see on makes such as, Renault, Ford, Jaguar. (in the UK)

You will hear a double "high pitched buzz" at engine switch off, which sounds a little like a Bumble Bee trapped under the engine cover.

I have seen it on many waveforms, using a current clamp, to monitor injector events.

What part of the world are you in?

Cheers

Dave
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Re: Injection pulses for depressurization

Postby Experimenteer » Tue Jul 27, 2010 6:16 pm

Dave Hill wrote:Hi Experimenteer (Ahhhgg I hate using Forum names :shock: )

I have noticed this "venting" of fuel pressure that you mention, but only on the Delphi common rail systems that we see on makes such as, Renault, Ford, Jaguar. (in the UK)

You will hear a double "high pitched buzz" at engine switch off, which sounds a little like a Bumble Bee trapped under the engine cover.

I have seen it on many waveforms, using a current clamp, to monitor injector events.

What part of the world are you in?

Cheers

Dave




I think I saw it first described in a Bosch Patent.

May I ask, how long does it last for? It actually happens even thought the engine has come to a stop RPM = 0 ????

You wouldn't happen to have a waveform captured that you could post? In principle amplitude should be correlated to rail pressure.
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Re: Injection pulses for depressurization

Postby v12col » Fri Aug 06, 2010 7:34 am

Hi Experimenteer,

Frank Massey has an has a video in this months Autoinform (issue 9) http://www.autoinform.co.uk/ It shows the injector / fuel rail de-pressurisation on a Delphi system.

Regards

Colin
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