legacycentral bbs

Electronic Fuel Injection - Operating Modes wrote: Under normal operation, the fuel ratio will be kept at 14.7:1 (for gasoline). This stoichiometric ratio will yield the most complete combustion and results in near peak exhaust gas temperature.

This allows the catalytic converter to operate efficiently and reduce emissions. Spark timing is at its minimum as the fuel mixture burns fastest at 14.7:1. The mixture is constantly adjusted using closed loop feedback from the oxygen sensor.

There are also other operating modes for certain engine conditions, described below.


Engine Cranking (not running)

Conditions:

* Engine speed below minimum for idle

While the starter is cranking the engine, the ECM will command a rich fuel ratio at 10:1 and less. On cars with distributors, the ECM will not have control of the spark timing and it will default to the base setting (position of the distributor cap). Once a minimum engine speed is reached, the ECM will transition to Warm-up Mode.


Warm-up

Conditions:

* Low engine run time
* Low coolant temperature

For a short time after the engine has been started, components are cold and need time to reach operating temperature. More fuel is added to aid combustion as the cold mixture will not burn completely. The slow-burning cold and rich mixture will require more spark advance to extract as much energy as possible. Once the oxygen sensor and coolant have reached operating temperature, the ECM can switch to Normal Mode with closed loop control.


Lean Cruise

Conditions:

* Vehicle speed above minimum (typically highway speeds)
* Constant load (little acceleration/deceleration)
* Low to medium throttle

Some ECM’s have the capability to operate under a Lean Cruise Mode. While the engine is operating under steady conditions (usually while driving on the highway at steady speed) the fuel mixture can be leaned out improve fuel economy. The fuel ratio can be leaned past 20:1 depending on how much spark energy the ignition system can deliver. Lean operation will lower engine temperature and reduce combustion speed. As a result, the ECM will advance the spark timing to minimize the power reduction. Vehicles which use a narrowband oxygen sensor will typically intermittently switch back to Normal Mode update the fuel correction values.


Power Enrich

Conditions:

* Heavy engine load
* Throttle position above threshold

Under heavy acceleration, the ECM will enter Power Enrichment Mode. This is to increase the power output of the engine while preventing excessive combustion temperature and pressure which could cause detonation. More fuel is added until the ratio is in the range of 12.5:1 – 13.8:1. This mixture is best for maximum power. The extra fuel cools the combustion chamber while the conversion of hydrocarbons to carbon monoxide (CO) releases more energy than conversion to carbon dioxide (CO2). Spark timing can be advanced to extract more power, but sometimes detonation will occur before peak power can be achieved. In this case, decreasing the engine’s octane requirement or using higher octane fuel will allow more spark advance and therefore more power.


Deceleration

Conditions:

* Throttle closed
* Low engine load
* Engine speed above idle limit

Under engine deceleration, no power is necessary so fuel ratio can be leaned out and eventually the fuel injectors can be turned off completely. Spark timing is set to transfer combustion heat to the rest of the engine to slow heat loss instead of expelling it out the exhaust. Deceleration mode will eventually be disabled under prolonged deceleration to prevent the engine from cooling too much, which would require another warm-up period.

skid542 wrote:Also, still wandering about what our cars run stock AFR at idle and light cruise.