Injector sizing

[Stewart Prince]

Although the injector pulse width is larger at peak torque, the injector is
usually sized on peak power because this is where fuel flow is maximum.

 fuel flow (lbm per hour)=bsfc(lbm per hp-hr) x power (hp)

the fuel flow rate equation for an injector is

fuel flow (lbm per hour)=injector max flow(lbm per hour) x pulse width (sec) /
injection period (sec)/ #injectors

this equation should be corrected for pulse width offset, which is usually about
1 millisecond.  injection period could be the engine period, or the cam period,
depending on how many times you inject per cycle.
For instance, if the engine (assume 4 cyl) is to produce 100 hp and the bsfc is
.5 lbm per hp-hr, the fuel flow would be 50 lbm per hour.  Since most injectors
should open at a duty cycle of 85% at max power, we get about 15 lbm per hour
static flow, per injector.  A more precise calculation would include the injector
offset, which does affect the sizing equation.  It is true that the pulse width
would be longer at peak torque, but since peak torque always occurs at an rpm
lower than peak power, the injector duty cycle is actually LOWER here, thus one
should size the injector at peak power, not peak torque.

Tlsalt at aol.com wrote:

> Greg Hermann wrote:
>
> the .55 bsfc does it. Try looking at the HP AT the torque peak to figure
> out the pw (and duty cycle) you will need at the torque peak. HP is work
> per unit time, and injector flow rate is a part of the determination of
> fuel flow per unit time. They are consistent units. Torque has no time
> factor in it.
>
> Hello Greg:
>
>     I am still puzzled how a 20% increase in BSFC can account for a 50%
> increase in torque.  If torque is proportional to the mass of air consumed
> doesn't the amount of fuel increase proportionally for a given air fuel
> ratio.  Is there a thermal efficiency difference between NA and supercharged
> to explain it ?  The pulsewidth at peak torque is higher than at peak
> horsepower, so I don't see how horsepower at peak torque is used to calculate
> peak torque pulsewidth.  Do you have a formula ?  I used the VE based formula
> to calculate the mass of air at peak torque and then a 13:1 AFR to calculate
> the injector puslewidth as a base setting for the Haltech fuel curve. Since
> the puslewidth is relate to a single event and the amount of air inhaled per
> event is greatest at peak torque, pulsewidth at peak torque should be the
> longest in the fuel curve ? Do you agree ?  I understand the time bit and the
> maximum flow rate, duty cycle etc. for the ultimate fuel delivery in lbs/hr
> and the highest duty cycle I've seen in a data log was 84% at WOT at redline
> rpm.  I am having a hard time with this partially because the original torque
> curve of this engine was a peak at 1500 rpm and a flat line to 4500 rpm.  The
> problem my be entirely in my head.
>
> Paul
>
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--
STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE


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