Injector sizing

[Pat Ford]

 great info so far but is there any way to get the bsfc without a dyno?

Previously, you (Stewart Prince) wrote:
{ 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.

cool

{ 
{ 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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--
Pat Ford                           email: pford at qnx.com
QNX Software Systems, Ltd.           WWW: http://www.qnx.com
(613) 591-0931      (voice)         mail: 175 Terence Matthews          
(613) 591-3579      (fax)                 Kanata, Ontario, Canada K2M 1W8

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