gruntguru wrote: ↑12 Nov 2019, 23:42
henry wrote: ↑12 Nov 2019, 11:05
Since there is no return for the fuel the flow the FFS sees is only the flows from the injectors. This is anything but steady with peaks when injecting and no flow when not. I would expect the peaks would be much greater than 100kg/hr and the FFS would register them, since the FFS sampling frequency is higher than likely “frequency” of injector events. There must be some process somewhere that sums the FFS samples and averages them to get the value needed for management.
So the FFS is seeing cyclic input and cyclic output. I wonder if it would be possible to trim the peaks by having minimum input coincide with maximum output.
I addressed this in an earlier post.
Of course we are talking about a very small quantity of storage (accumulation) at moderately high frequency eg at 10,500 rpm (and assuming one injection pulse per intake event)
- Fuel qty/cylinder = 0.053g
- frequency of injection events = 525 Hz
A very small accumulator on the fuel rail or even the flexibilty of the lines and fuel would be sufficient to damp these fluctuations so they were invisible to the flow sensor. AFAIK the FIA has defined what constitutes acceptable accumulation for these purposes.
Sorry. Missed it.
Is the fuel qty right? I calculated 0.096 g based on 12000rpm, 3 injection events per rpm/sec.
Edit: you are right. Redid my calc and it’s in line with your value. Apologies for doubting.
However a further point is the the fuel quantity isn’t delivered over the whole of the available time. I don’t know what proportion of a rev an injection cycle takes, or how many there are, but my assumption is that fuel quantity is delivered in a shorter period than simple calculation of the time for 1/3 of a rev. A consequence of this is that the instantaneous flow rate will be higher than the average permitted, potentially much higher. But there will also be periods where there is zero delivery.
The FFS samples at a high enough rate to mean that it will see both these very high rates and the zero rates. Consequently it will need to hand its sample numbers to a downstream process that calculates the average and checks it is compliant.
Some other issues are that firing order is not even, and that as well as the very unsteady nature of the output from the FFS the input is also potentially unsteady due the primer pump frequencies.
As you say the limited accumulator volume mean the FFS must see these variations.
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