Bad mpg?

[blanchard7684]

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.

 

[blanchard7684]

No, IMHO you're just trolling and trying to be a jerk with technicality . You knew what he meant, and so did everyone else that read it.
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is technicalities a synonym for semantics?

 

[Antonm]

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 .

Really?

You don't think a 5.3 towing a trailer up a hill at 80% throttle (with an AFR of 14.7), will be making way more power than a 6.2 at driving on flat ground at 20% throttle (with that same 14.7 AFR)?

But maybe you meant to also include "at the same throttle position", and I'm just pointing out little things.
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[Antonm]

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).

Not true, the 6.2 is not always drawing in more air than a 5.3 because there is this little thing called a throttle body at the front of the intake manifold that restricts air.


Physics says to move an "X" amount of mass at a given velocity, it take a "Y" amount of energy to do so.
So assuming the engine efficiencies aren't drastically different between a truck 5.3 and 6.2, then both engines would be using the same quantity of fuel when cruising down the road making similar horsepower at part throttle.

Yeah the 6.2 may be at 20% throttle when the 5.3 needs to be at 30% throttle, but if both are moving the same mass (or accelerating the same mass), at the same speed (or same rate of acceleration), then both would be making the same horsepower at that time and using the same amount of fuel. The only thing that could possibly change this is if the engines efficiencies where different at whatever power level that given velocity (or the given rate of acceleration) happened to be at.
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[Antonm]

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

This is undoubtingly true in an absolute sense. Just like if I were to pour a 20oz bottle of fresh water into the ocean, in an absolute sense the ocean's salinity/ salt concertation changed because I added 20oz of fresh water to it,,,but in any practical sense the oceans salinity didn't really change.

In those places where the 6.2 might be more efficient (say due to less pumping losses because it can make the same torque at a lower RPM compared to the 5.3) the differences are really negligible.
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[Marky Dissod]

the 6.2L may be at 20% throttle when the 5.3L needs to be at 30% throttle,
but if both are moving the same mass (or accelerating the same mass) at the same speed (or same rate of acceleration),
then both would be making the same horsepower at that time, and using the same amount of fuel.

'and using the same amount of fuel' cannot ALWAYS be true,
otherwise GM would not have been willing to accept the tooling costs difference between 5.3L and 5.7L in the late 90s, followed by 6.0L, followed by 6.2L.
(Also note the low uptake rate of, as well as the eventual demise of, the 4.8L.)
CAFE MpG testing scores are obviously a thorn in GM's side, not only because CAFE MpG results and real-world results have predictable differences.

 

[blanchard7684]

Not true, the 6.2 is not always drawing in more air than a 5.3 because there is this little thing called a throttle body at the front of the intake manifold that restricts air.


Physics says to move an "X" amount of mass at a given velocity, it take a "Y" amount of energy to do so.
So assuming the engine efficiencies aren't drastically different between a truck 5.3 and 6.2, then both engines would be using the same quantity of fuel when cruising down the road making similar horsepower at part throttle.

Yeah the 6.2 may be at 20% throttle when the 5.3 needs to be at 30% throttle, but if both are moving the same mass (or accelerating the same mass), at the same speed (or same rate of acceleration), then both would be making the same horsepower at that time and using the same amount of fuel. The only thing that could possibly change this is if the engines efficiencies where different at whatever power level that given velocity (or the given rate of acceleration) happened to be at.
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If this is true then why offer higher displacement engines at all? Based on this displacement means nothing.

Same mass, same acceleration, same power to make acceleration happen, ergo same fuel use.

Why not have 1.0 L engines in the suburban, then?

The “ how” in how engines make power matters here.

For the smaller engine, it will have to add incrementally more fuel to mixture to make the same power demand.

Peak power is at 12.8-13.1 afr, not 14.7.

This is based from the adiabatic flame temp curve that is depending on afr.

You can make the same power as a 6.2 with smaller displacement. But the engine has to be operating at higher and higher VE levels and more fuel has to be added ( beyond 14.7 afr) to make the power.

And also for the same throttle position ( depending on throttle body design) the 6.2 will be drawing in more air. 6.2 has 87 mm TB, 5.3 has 80mm. So for the same pressure drop through intake runners, the larger area flows more.

 

[Antonm]

If this is true then why offer higher displacement engines at all? Based on this displacement means nothing.

Because we want to either move more mass, or accelerate a given faster at a faster rate.

While cursing down the road like a normal person, you may be using/ making 50hp, but when you want to pass another car, while going uphill, and towing a trailer, you want all the 420 hp the 6.2 has to offer, that's why. And because its fun.
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[Antonm]

For the smaller engine, it will have to add incrementally more fuel to mixture to make the same power demand.

Nope, you have to add more fuel AND more air to burn that more fuel. Power comes from the fuel, the air is needed to burn the fuel.

If you add enough fuel to make a given power, you also have to add the equivalent portion of air to burn that fuel, regardless of engine displacement.

An easy example of this is small displacement forced induction engines that are popular now. Like the little 2.7 four banger that's in the Silverado making 310 hp. That engine injects enough fuel into the cylinder to make 310hp, then crams enough air into that same small *** cylinder via a turbocharge to burn all that fuel.
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[Antonm]

Peak power is at 12.8-13.1 afr, not 14.7.

This is based from the adiabatic flame temp curve that is depending on afr.

This is not always true, has a lot to do with head/ cylinder swirl, but usually max power does happen in the low 13 AFR range for pure gasoline that is true (which no is really running pure gasoline on the street anymore, most are running at least E10, which has a stoichiometric ratio of 14.1 BTW) .

But the tree huggers don't like that because it increases emissions, so the OEMs tune for stoichiometric to make the huggers (and the EPA) happy and knowingly give up a small amount of power doing so.
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