Maximus
12-18-05, 01:27 AM
Pulsewidth Calculation
These calculations are based on a 4-stroke-cycle, V-8, gasoline engine. The data used are real.
Calculate injector pulsewidth from airflow.
First, the CPU determines airflow from the sensors. (The various methods to determine airflow are beyond the scope of this topic. See MAF sensor, or MAP sensor.)
* 1) (lb-air/min) × (min/rev) × (rev/4-intake-stroke) = (lb-air/intake-stroke) = (air-charge)
Min/revolution is the reciprocal of engine speed (RPM) – minutes cancel.
Factor in the number of induction events per engine rev, minding whether its a 2-stroke or 4-stroke engine.
* 2) (lb-air/intake-stroke) × (fuel/air) = (lb-fuel/intake-stroke)
Fuel/air is the desired mixture ratio, usually stoichiometric, but often different depending on engine conditions.
* 3) (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-lbs/hour.
All three terms above combined . . .
* (lbs-air/min) × (min/rev) × (rev/4-intake-stroke) × (fuel/air) × (1/injector-size) = (pulsewidth/intake-stroke)
Substituting real variables for the 5.0L engine at idle.
* (0.55 lb-air/min) × (min/700 rev) × (rev/4-intake-stroke) × (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-intake-stroke) × (1/11.00) × (h/24-lb) × (3,600,000 ms/h) = (17.3 ms/intake-stroke)
Injector pulsewidth typically ranges from 2 ms/engine-cycle at idle, to 20 ms/engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices. The final pulsewidth will change if fuel line pressure changes, which effectively changes injector flow capacity.
To calculate a 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.
* (2.0 ms/intake-stroke) × (hour/3,600,000 ms) × (24lb-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, and minding the units.
* (17.3 ms/intake-stroke) × (hour/3,600,000-ms) × (24 lb/h fuel) × (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 naturally aspirated passenger car engines. The dynamic range is greater on supercharged or turbocharged engines. 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.
These calculations are based on a 4-stroke-cycle, V-8, gasoline engine. The data used are real.
Calculate injector pulsewidth from airflow.
First, the CPU determines airflow from the sensors. (The various methods to determine airflow are beyond the scope of this topic. See MAF sensor, or MAP sensor.)
* 1) (lb-air/min) × (min/rev) × (rev/4-intake-stroke) = (lb-air/intake-stroke) = (air-charge)
Min/revolution is the reciprocal of engine speed (RPM) – minutes cancel.
Factor in the number of induction events per engine rev, minding whether its a 2-stroke or 4-stroke engine.
* 2) (lb-air/intake-stroke) × (fuel/air) = (lb-fuel/intake-stroke)
Fuel/air is the desired mixture ratio, usually stoichiometric, but often different depending on engine conditions.
* 3) (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-lbs/hour.
All three terms above combined . . .
* (lbs-air/min) × (min/rev) × (rev/4-intake-stroke) × (fuel/air) × (1/injector-size) = (pulsewidth/intake-stroke)
Substituting real variables for the 5.0L engine at idle.
* (0.55 lb-air/min) × (min/700 rev) × (rev/4-intake-stroke) × (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-intake-stroke) × (1/11.00) × (h/24-lb) × (3,600,000 ms/h) = (17.3 ms/intake-stroke)
Injector pulsewidth typically ranges from 2 ms/engine-cycle at idle, to 20 ms/engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices. The final pulsewidth will change if fuel line pressure changes, which effectively changes injector flow capacity.
To calculate a 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.
* (2.0 ms/intake-stroke) × (hour/3,600,000 ms) × (24lb-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, and minding the units.
* (17.3 ms/intake-stroke) × (hour/3,600,000-ms) × (24 lb/h fuel) × (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 naturally aspirated passenger car engines. The dynamic range is greater on supercharged or turbocharged engines. 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.