Short Term Fuel Trim question

[Geoc]

I am having a slight confusion regarding fuel trims. Since engine stoich is approximately 14.7, when the STFT reports RICH, does that mean the actual A/F is < 14.7? And A/F >14.7 for LEAN?

Or does STFT reports the compensation so that the engine is at stoich(=14.7)?

[Lomac]

Rich(er) would be something like 10:1
Lean(er) would be something like 20:1

[?uestlove]

Its supposed to go rich lean rich lean for the cat converter to work
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[Geoc]

So it varies constantly from >14.7 (ie. 15) and <14.7 (ie. 13)? From what I understand anything above 14.7 is categorized lean and anything under is rich, the intensity of it is not relevant to me at this moment(I am looking at normal engine operating parameters).

[dangonay]

Your engine will still be running as close to 14.7 under most conditions.

Your ECU will expect that at a certain RPM/load combination it will need to add x amount of fuel (determined by injector pulse width). However, all fuel injectors do not flow exactly the same and they change over time. Likewise, fuel pressure regulators can also be out by a small amount. So can your MAF or MAP sensor. Your engine can also change mechanically affecting combustion quality.

Fuel trims are a way for an ECU to compensate for changes to your engine. The factory program for the ECU might say that at a specific engine RPM/load combination it needs to use a fuel injector pulse width of 3.0ms. However, when the ECU reads the oxygen sensor it might find the mixture was slightly lean (meaning you need a longer pulse width than 3.0ms). This could be caused by a dirty injector (for example). So the ECU needs to add additional time to the pulse width to keep the mixture correct. This is known as fuel trims.

Your ECU is constantly comparing the oxygen sensor readings with the amount of fuel it added and when the difference exceeds a certain amount it modifies the fuel trim. It does this continually for the life of the vehicle. However, if you clear codes with some generic scan tools (or specifically delete the fuel trims/adaptations on a scan tool that allows this function) then the ECU is reset to "factory" and needs to learn the fuel trims all over. This can cause your engine to run a little less than optimal until it relearns (perhaps 10-100 km's depending on the vehicle).


Now as to your engine running rich/lean - well this isn't actually the case. An oxygen sensor might be showing rich/lean, but that's only on cars with narrow band sensors. That's because these sensors show large voltage swings for very small changes in the mixture. Your ECU can keep your engine running very close to 14.7 even with a narrow band sensor. The way it does this is as follows:

Let's say the ECU is using a pulse width of 5.0ms. The oxygen sensor swings to rich. Next cycle the ECU changes the pulse width to 4.8ms. The oxygen sensor reads lean. The ECU changes the pulse width to 4.9ms for the next cycle. The oxygen sensor might stay at stoich, or it could still swing slightly lean/rich. The point is that although the oxygen sensor might seem to be wildly swinging from lean to rich, the ECU is only making very small adjustments to the injector pulse width to achieve those swings. So your engine is getting exactly the right amount of fuel, and even when it's off it's only by a very small amount.

[?uestlove]

Yes it varies constantly. If you look at the primary oxygen sensor wave form you'll see it going up and down (rich and lean rich and lean). Normally I see STFT as a number (points of correction to the fuel map)not AFR/rich/lean but it depends on what you're working on

Mad props to Alatar for teaching me things when I was learning this stuff
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[Geoc]

I see, but if the engine is not warmed up and is in open loop, will the STFT report a consistently positive value(rich) until the engine is warmed up? This would seem logical as a car naturally consumes more gas when operating in a cold start state, am I wrong?

Here's a question, an engine is running with the MAF reporting 100g/s and STFT is +10 percent, assume spec. g of gas is 0.70g/mL, to find fuel consumption at that very second

MAF/stoich * grav. * STFT offset = 100/14.7 * 0.70 * 1.10 = 5.23mL/s

Is this correct?

[Muskys SS]

When in open loop there is no fuel correction taking place. STFT will only take place when the vehicle is in closed loop mode and the ecu will make fuel corrections based on the desired air/fuel ratio. When a narrowband o2 sensor is being used this value is 14.7:1.
When the STFT value is +3% that means that the vehicle is running leaner than 14.7:1 so the ecu is adding 3% fuel.
When the STFT value is -3% that means that the vehicle is running richer than 14.7:1 so the ecu is subtracting 3% fuel.

As for your question. There is a slightly easier way to look at it. If the vehicle is running correctly meaning the o2 sensors are functioning properly and the MAF reading is accurate you can base the fuel consumption on the 14.7:1 ratio. Since most modern cars will target 14.7AFR during cruising you can assume that if the MAF reads 100g/s of air, then the ecu will be trying to supply the engine with 6.8g/s of fuel. (100/14.7).

However if the MAF and o2 sensors are out of calibration you must calculate fuel consumption another way. You will have to use injector pulse width, fuel pressure, and fuel temperature to calculate the fuel consumption accurately.