hi,

i want to calculate (for some personnal strange reasons ) the injection flow for my rx7
i have an 1989 RX7 (S5) with i mean 550cc/min injectors.
i would like to know the quantity of fuel is injected at each injector pulse
what is the formula ? what is the role of fuel pressure ?
when denso say injector is 550cc/min, ok, but at which duty cycle ? (fully open?) and at which pressure ?

if anyone know the answer....

thank for your help !

 

Duty cycle is the % time the injector is open. I don't know what it is for the Rx-7, but most cars don't have it near 100.

http://users.erols.com/srweiss/table...m#NIPPON_DENSO


???

PSI = lbs / inch^2

cc = cubic centimeters... volume.

Good luck. Maybe an expert will chime in.

 

Calculate injector pulsewidth from airflow

First the CPU determines the air mass flow rate from the sensors - lb-air/min. (The various methods to determine airflow are beyond the scope of this topic. See MAF sensor, or MAP sensor.)

* (lb-air/min) × (min/rev) × (rev/4-strokes-per-cycle) = (lb-air/intake-stroke) = (air-charge)

- min/rev is the reciprocal of engine speed (RPM) – minutes cancel.
- rev/2-revs-per-cycle for an 8 cylinder 4-stroke-cycle engine.

* (lb-air/intake-stroke) × (fuel/air) = (lb-fuel/intake-stroke)

- fuel/air is the desired mixture ratio, usually stoichiometric, but often different depending on operating conditions.

* (lb-fuel/intake-stroke) × (1/injector-size) = (pulsewidth/intake-stroke)

- injector-size is the flow capacity of the injector, which in this example is 24 lb/h if the fuel pressure across the injector is 40 psi.

Combining the above three terms . . .

* (lb-air/min) × (min/rev) × (rev/4-strokes) × (fuel/air) × (1/injector-size) = (pulsewidth/intake-stroke)

Substituting real variables for the 5.0 L engine at idle.

* (0.55 lb-air/min) × (min/700 rev) × (rev/4-strokes-per-cycle) × (1/14.64) × (h/24-lb) × (3,600,000 ms/h) = (2.0 ms/intake-stroke)

Substituting real variables for the 5.0 L engine at maximum power.

* (28 lb-air/min) × (min/5500 rev) × (rev/4-strokes-per-cycle) × (1/11.00) × (h/24-lb) × (3,600,000 ms/h) = (17 ms/intake-stroke)

Injector pulsewidth typically ranges from 4 ms/engine-cycle at idle, to 35 ms per engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices.

[edit] Calculate fuel-flow rate from pulsewidth

* (Fuel flow rate) ≈ (pulsewidth) × (engine speed) × (number of fuel injectors)

Looking at it another way:

* (Fuel flow rate) ≈ (throttle position) × (rpm) × (cylinders)

Looking at it another way:

* (Fuel flow rate) ≈ (air-charge) × (fuel/air) × (rpm) × (cylinders)

Substituting real variables for the 5.0 L engine at idle.

* (Fuel flow rate) = (2.0 ms/intake-stroke) × (hour/3,600,000 ms) × (24 lb-fuel/hour) × (4-intake-stroke/rev) × (700 rev/min) × (60 min/h) = (2.24 lb/h)

Substituting real variables for the 5.0L engine at maximum power.

* (Fuel flow rate) = (17.3 ms/intake-stroke) × (hour/3,600,000-ms) × (24 lb-fuel/hour) × (4-intake-stroke/rev) × (5500-rev/min) × (60-min/hour) = (152 lb/h)

The fuel consumption rate is 68 times greater at maximum engine output than at idle. This dynamic range of fuel flow is typical of a naturally aspirated passenger car engine. The dynamic range is greater on a supercharged or turbocharged engine. It is interesting to note that 15 gallons of gasoline will be consumed in 37 minutes if maximum output is sustained. On the other hand, this engine could continuously idle for almost 42 hours on the same 15 gallons.

 

The formula is:
Fuel quantity injected in cc = 550 * Pulsewidth in minutes

However, I am not sure how much good that will do you since you do not know the pulsewidth.

The fuel pressure is required to inject the fuel. Too little pressure will not allow the fuel to atomize properly, while too much pressure will cause the injectors to lose control. See this link for the relationship of fuel injection pressure and flow rate:
http://www.rceng.com/technical.aspx?...ldsbCSHyZGPGvi

This is covered in the factory service manual, page 4B-71 and 4B-75.