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POLICIES

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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 forthe duration of 1000 10msec PWs. 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.

Figure 2

Injectors should not be operated below 1.5-2.0msec PWs due to the non-linear

portion of the curve. At extremely low PWs, the injectors coil does not have time to

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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.

Figure 3

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 isrequired 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-cyclinders higher horsepower per cylinder ratio.