TA-03 Flow Curve

Tech Corner
Article 03

Understanding The Injector Flow Curve

Every fuel injector has its own unique flow curve. Injectors are specified at two different points on the curve: static flow and dynamic flow. Static flow is the flow through an injector when it is energized open and left on for a given period of time, usually one minute. This flow rate is the maximum output of the injector at a given pressure. It is this flow rate that is commonly used to describe and classify injectors (i.e. 19 lbs/hr, 30 lbs/hr).

The dynamic flow rate is the flow through an injector while it is pulsed on and off for a given number of pulses. The time between the start of each pulse is called the period or pulse interval (PI). The time the injector is actually energized during the period is called the pulse width (PW). Figure 1 shows the relationship of PW to PI.

Dynamic flow rate is sometimes expressed in duty cycles which are simply the percentage the injector is turned on during the period.

For example, if the PI is 20msec and PW is 5msec, then the duty cycle is 25%.

(5/20) x 100% = 25%.

Figure 2 shows a basic flow curve for an injector. This particular injector flows 11.5cc/1000 pulses at a PW of 2.5msec and 64.5cc/1000 pulses at a static flow. The static flow value was reduced from its normally measured value of 386.8cc/min to 1000 pulses at a 10msec PW and 10msec PI. This results in the injector being turned on for the duration of 1000 10msec PW’s. The dotted line that parallels the flow curve is the theoretical static flow curve which does not account for lost flow during the opening of the injector.

Injectors should not be operated below 1.5-2.0msec PW’s due to the non-linear portion of the curve. At extremely low PW’s, the injector’s coil does not have time to reach its magnetic potential, thus the opening and closing action of the injector becomes erratic.

At the other end of the flow curve, near static flow, where the duty cycle exceeds 90%, the flow again becomes erratic and non-linear. This time, the opposite is occurring. The injector does not have time to completely close before the coil is re-energized to open once again. Figure 3 shows what happens in the non-linear regions.

Another term widely used when discussing fuel injectors is Dynamic Range. Dynamic Range is the linear portion of the flow curve. It is important to know the Dynamic Range when sizing an injector for a particular application because the same injector is required to supply the minimum fuel requirement for smooth idles and still be able to supply engine demand at wide open throttle. For example, the Dynamic Range of an injector used in a turbocharged 4-cyclinder is much larger than a V-8 engine injector. The 4-cyclinder injector requires a higher static flow rather than the V-8 injector due to the 4-cyclinder’s higher horsepower per cylinder ratio.

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