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Tec3r Manual Version 1.11
- Page 78 -
©2017 Electromotive, Inc.
Example:
Engine Horsepower = 300
BSFC = 0.50
300 HP x 0.50 lbs-per-hr/HP = 150 lb/hr
This engine would require a total of
150 lbs/hr
of fuel flow.
The Injector Recovery Time is the time that is takes for an injector to open once it receives a voltage
signal. Typically, a low-impedance injector will have an IRT of around 1.3 milliseconds. A high impedance
injector will generally have an IRT of 1.5 ms. This number determines two things:
The minimum time that the injector can be pulsed
The maximum duty cycle
If an injector is given a pulse width that is shorter than its IRT, it will flutter. This will cause erratic
fuel metering, and ultimately horrible engine performance. This scenario sometimes occurs at idle on high-
revving, high-horsepower engines with large injectors. If the IRT is not accounted for in high-rpm
situations, the duty cycle will be too great, and the injector will flutter. This scenario occurs on engines with
injectors that are too small.
To properly size an injector, the following sequence of equations should be followed:
HP x BSFC = Total Fuel Flow Requirement for the Engine (
TFFR
)
TFFR
(# of Injectors) = Uncorrected Injector Size (
UIS
)
60000
(Peak RPM) = Available Injection Time (ms) per Revolution (
AIT
)
(AIT – Injector Recovery Time in ms)
AIT = Maximum Duty Cycle (
MDC
)
UIS
MDC =
Required Injector Size
The above calculation assumes an injection scenario in which an injector is held open for a maximum
of one engine revolution (typical phase sequential setup). For other firing options in which the injector is
held open for 2 revolutions, multiply AIT by 2.
Example:
Engine Setup: 500HP 8-cylinder with 8 injectors
Phase Sequential Injector Firing
High Impedance Injectors (1.5ms IRT)
Peak HP occurs at 6500 rpm
BSFC = 0.5
Calculations:
TFFR = 500 x 0.5 = 250 lb/hr
UIS = 250
8 = 31.25 lb/hr
AIT = 60000
6500 = 9.231ms
MDC = (9.231 – 1.5)
9.231 = 0.8375
Required Injector Size = 31.25
0.8375 =
37.3 lb/hr (392 cc/min)
Central to the topic of injector sizing is the RPM at which the engine will need the injectors to fire.
High RPM’s shorten the amount of time between TDC events, thus shortening the maximum injector pulse
