Fuel Injection Sizing Formula

After much research it has been found that there are several different formulae used to determine injector size. It appears that most of the various formulae in use are basically identical, with only 2 real variables, being whether or not an 80% duty cycle is accounted for and what the desired Brake Specific Fuel Consumption (BSFC) should be. This is the formula with the correction based on the duty cycle:

Flow rate (lbs/hr) = (Max Hp x BSFC) / (no. injectors x duty cycle)

BSFC = 0.45 – 0.55 for naturally aspirated engines
0.55 – 0.65 for forced induction engines

Conversion from lbs/hr to cc/min = lbs/hr x 10.515

Simplified max Hp x 1.807 (nat) Simplified
max Hp x 2.136 (turbo)

RPM/Angle/Time Formula

With electronic engine management, the concepts of RPM, angle of engine rotation and time are frequently required to be related for calculations that determine maximum injection time or maximum coil charge time.

Where RPM = Revolutions of crank per minute
a = Angle the crank travels through
t = Time in seconds that the cranks takes to travel the angle ‘a’.

RPM = a / (6 x t)

t = a / ( 6 x RPM )

a = 6 x t x RPM

Example:

How much time does it take for the crank angle to travel 10 degrees at 1000RPM

a = 10, rpm = 1000:

t = a / (6 x RPM)

= 10 / (6 x 1000)

= 1.667 ms