Bad mpg?

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blanchard7684

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How O2 Sensors Work
O2 sensors work like mini generators, producing their own voltage as they get hot.
Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb.
The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

The outside of the bulb is exposed to hot exhaust gases while the inside of the bulb is vented internally through the sensor body to the outside atmosphere.
Older style O2 sensors actually have small holes in the body shell so air can enter the sensor.
Newer style O2 sensors "breathe" through their wire connectors and have no vent holes.
Hard to believe, but the tiny space between the insulation and wire provides enough room for air to seep into the sensor
(this is why grease should never be used on O2 sensor connectors, because it can block air flow).
Venting the sensor through the wires reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail.
The difference in O2 levels between exhaust and outside air in the sensor causes piezoelectrically generated voltage to flow through the ceramic bulb.
The greater the difference, the higher the voltage reading.

O2 sensors typically generate up to about 0.9 Volts when the fuel mixture is rich / when there is little unburned O2 in the exhaust.
When the mixture is lean, the O2 sensor's output voltage drops down to about 0.1 Volts.
When the Air/Fuel Ratio is stoichiometric (about 14.7:1 for pure petrol), the O2 sensor reads about 0.45 volts.

When the ecm gets a rich signal (high voltage) from the O2 sensor, it enleans the fuel mixture in the hope of reducing the sensor's reading.
When the ecm gets a lean signal (low voltage) from the O2 sensor, it enriches the fuel mixture in the hope of raising the sensor's reading.

O2 sensors must be hot enough to generate voltage, so many O2 sensors have a small heating element inside to help them hit operating temperature sooner.
The heating element can also prevent the sensor from cooling off too much during prolonged idle or DFCO, which would cause the system to revert to open loop.

Heated O2 sensors are used mostly in newer vehicles and typically have 3 or 4 wires. Older single wire O2 sensors lack heaters.

This ain't no 101 schidt, and it's nothing to do with what I 'think'.
Neither is the fact that some ecms allow tuners to adjust the 'voltage swingpoint', and a few even let tuners adjust the outer bounds as well.
Ok great. Now we are getting somewhere…

Now that we know how o2 sensors create an input signal to the ecm, how does this information apply to the discussion?

My original claim ( worded differently however) was that in some situations the 5.3 will get worse fuel consumption than 6.2 because the 5.3 will need more fuel ( to accelerate the same mass ).

You said this no longer applies because the af ratio is “forced”.

I’m genuinely curious.

If you have tuning experience where this has shown to be the case I’m all about learning.

In my view a smaller 5.3 engine operating at 14.7 af ratio can’t possibly make the same power as a 6.2 at same afr .

Ergo for part load driving the 5.3 will need to be in an afr range that is lower than 14.7. How much? Not sure but if it is any indication my instantaneous mpg reading gets slammed with any touch of the pedal.

Peak power is at 12.8 to 13.1 afr. Peak efficiency is 14.6 -15.0 afr.

In between these two extremes is where a considerable amount driving is done.

Tip in for moderate passing on highway; maintaining speed into heavy headwind; small inclinations.

Afr is a ratio of mass, not volume.

Each cycle , each of the 8 cylinders in a 6.2 is drawing in a larger volume of air and hence a larger mass of air. ( density) .So it can be mixed with a smaller mass of fuel to make the power needed for the above conditions ( relative to 5.3).

So in these driving situations the 6.2 can use a higher afr to reach the same power needed for these maneuvers vs 5.3.

Put another way… Bsfc of a 6.2 is likely lower ( less fuel for same power) than a 5.3. Can’t find anything published but this is an inherent feature of every engine. The 6.2 is a bored out 5.3 so friction losses aren’t nearly as bad as an engine with longer stroke.

Thus it would not surprise me if the 6.2 had better fuel mileage in some driving conditions vs the 5.3.

The published fuel mileage for both 5.3 and 6.2 (in suburban at least ) are virtually identical.

I’m arguing that there are some conditions where the 6.2 can be more efficient than a 5.3.
 
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