Fuel Injection Detailed function

Note: These examples specifically apply to a modern EFI gasoline engine. Parallels to fuels other than gasoline can be made, but only conceptually.



Typical EFI components

* Injectors
* Fuel Pump
* Fuel Pressure Regulator
* ECM - Engine Control Module; includes a digital computer and circuitry to communicate with sensors and control outputs.
* Wiring Harness
* Various Sensors (Some of the sensors required are listed here.)

* Crank/Cam Position: Hall effect sensor
* Airflow: MAF sensor, sometimes this is inferred with a MAP sensor
* Exhaust Gas Oxygen: O2 Sensor, Oxygen sensor, EGO sensor, UEGO sensor



Functional description

A contemporary EFI system comprises a digital computer "engine control module" (ECM) and a number of sensors to measure the engine's operating conditions. The ECM interprets these conditions in order to calculate the amount of fuel, among numerous other tasks. The desired "fuel flow rate" depends on several conditions, with the engine's "air flow rate" being the fundamental factor.

The electronic fuel injector is normally closed and opens to flow fuel as long as an electric pulse is applied to the injector. The pulse's duration (pulsewidth) is proportional to the amount of fuel desired. The pulse is applied once per engine cycle, which permits pressurized fuel to flow from the fuel supply line, through the open injector, into the engine's air intake, usually just ahead of the intake valve.

Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the 4-stroke-cycle engine has discrete induction (air-intake) events, the ECM calculates fuel in discrete amounts. The injected fuel mass is tailored for each individual induction event. In other words, every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulsewidth based on that cylinder's fuel requirements.

It is necessary to know the mass of air the engine "breathes" during each induction event. This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. The amount of air inducted in each intake event is known as "air-charge", and this can be determined using one of several methods, but this is beyond the scope of this topic.

Note: The right pedal is not the gas pedal; it is the air pedal. The throttle pedal determines the air, and in turn, the air mass determines the fuel mass. The same is true for carburetors, only carburetors were volume, not mass based devices. With some recent systems, the right pedal isn't even an "air pedal"... it has evolved to a "power demand pedal" - it isn't connected to the throttle at all, it signals the CPU how far the driver has depressed the pedal, and the CPU determines how far to open the throttle using an electric motor. This has many benefits some of which include: controlling emissions during transients, cruise control, traction control, engine start/cranking, driveline clunk, idle speed control, air conditioning load compensation, etc.

The three elemental ingredients for combustion are fuel, air and ignition. However; complete combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers.

To achieve stoichiometry, the air mass flow into the engine is measured and multiplied by the stoichiometric air/fuel ratio 14.64:1 (by weight) for gasoline. The required fuel mass that must be injected into the engine is then translated to the required pulse width for the fuel injector.

Deviations from stoichiometry are required during non-standard operating conditions such as heavy load, or cold operation, in which case, the mixture ratio can range from 10:1 to 18:1 (for gasoline).

Note: The stoichiometric ratio changes as a function of the fuel; diesel, gasoline, ethanol, methanol, propane, methane (natural gas), or hydrogen.

Also, final pulsewidth is inversely related to pressure difference across the injector inlet and outlet. For example, if the fuel line pressure increases (injector inlet), or the manifold pressure decreases (injector outlet), a smaller pulsewidth will meter the same fuel. Fuel injectors are available in various sizes and spray characteristics as well. Compensation for these and many other factors are programmed into the ECM's software.

In summary, the vehicle operator opens the engine's throttle (right pedal), atmospheric pressure forces air into the engine past sensors that indicate air mass flow. The ECM interprets these signals from the sensors, calculates the desired air/fuel ratio, and then outputs a pulsewidth providing the exact mass of fuel for optimal combustion. This process is repeated every time an intake valve opens.

The modern EFI system treats each injection as a discrete event, which when all strung together, perform one smooth seamless experience. An oversimplified analogy is that it is like a motion picture that appears to move, made from a series of individual images.


Sample pulsewidth calculations

Note: These calculations are based on a 4-stroke-cycle, 5.0L, V-8, gasoline engine. The variables used are real data.


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-intake-stroke) = (lb-air/intake-stroke) = (air-charge)

- min/rev is the reciprocal of engine speed (RPM) – minutes cancel.
- rev/4-intake-stroke 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-lbs/hour if the fuel pressure across the injector is 40 psi.

Combining the above three terms . . .

* (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.

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/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 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.