Injector sizing

[Greg Hermann]

At 1:35 PM 2/25/01, Tlsalt at aol.com wrote:
>Greg Hermann wrote:
>
>>Pulsewidth is calculated from duty cycle TIMES maximum available cycle
>time.
>
>Hello Greg,
>
>    Thanks for your response.  I've never focused on duty cycle, other than
>to ensure that the injectors are large enough to avoid going over 85% and
>that the pulsewidth at maximum rpm is within the time window.  I've never
>seen pulsewidth calculated from duty cycle.  What is this formula ?  The VE
>based maximum pulsewidth formula has always worked for me.

Cycle time is simply the time for the engine (assuming it is a 4 stroke) to
complete two full revolutions.
 SO-- it is equal to the reciprocal of rpm divided by two and divided by
60. (in seconds) times one thousand, to put it into ms.

Cycle time times duty cycle = pw, OR duty cycle = pw/ cycle time.

Duty cycle is simply the % of the time during each cycle that the injector
is open. So, flow through the injector is (approximately) equal to flow
rating times duty cycle.

>
>>in these numbers, BSFC is assumed to be CONSTANT from 1500 through 4500 rpm.)
>
>I managed to find a BSFC chart for my engine and it relatively flat from
>1500-3500 pm (0.44-0.46) but it begins to rise at 3500 and peaks at 0.55 at
>5000 rpm (hp peak).  I assume you use the BSFC at the hp peak and 80-85% duty
>cycle to size the injectors.

Yep. And then HP at the torque peak (or anywhere else) times BSFC at the
torque peak (or the corresponding anywhere else point) divided by the
number of cylinders and divided by the injector flow rate (corrected for
the difference between ABSOLUTE fuel rail pressure and ABSOLUTE manifold
pressure) mto determine the duty cycle at the torque peak (or the
corresponding anywhere else point). Then--duty cycle times cycle time at
whatever point you are looking at to get the pulse width at that point.

Flow through any orifice (such as an injector) varies with the square root
of the pressure drop across it. So, say an injector is rated 24 lbs/hr AT 3
bars (43.5 psi) pressure drop. (Pretty typical numbers here.)

Say you are supercharged , running 8 psi of boost, at sea level. That means
you MAP would be 22.7 psia. Say your fuel rail pressure reads 40 psi on a
gauge (40 psig) this converts to 54.7 psia. So--the pressure difference
across the injectors = 54.7 - 22.7 = 31 psi. This means that those
injectors that flow 24 lbs/hr at 42.5 psi will flow 24 x ((sq rt 31)/ (sq
rt 42.5)) = 20.5 lbs. per hr. of fuel at a 100% duty cycle under these
operating conditions.

As always--the devil is in the details!
>
Greg


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