Bad mpg?

[Antonm]

This example doesn't impact the discussion. We are discussing same power level. 80% throttle in 5.3 will not have same power as 6.2 at 20% throttle. One will be using more fuel and for the same engine speed will also have a much lower afr.

Remember that the original quote you took exception to was that the 5.3 and 6.2 can't make same power at safe afr.

So let me get this straight,,,,,,, You're saying that at the same power level, two engines with the same efficiencies, will use a different amount of fuel if running at the same afr?

Is that what you are saying?
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[Antonm]

You said this wasn't true because both will be using same fuel. I've proven this false.

LOL, you've proven that you can get different amount of energy out of the same amount of fuel without changing the engines efficiency have you,,,, interesting,,,have you also invented a perpetual motion machine machine?

Because getting a different amount of energy out of the same fuel (without changing an efficiency) kinda goes against the law of conservation of energy (just like the perpetual motion machine does)

Lets say these engine are 30% efficient overall (which is in the range of normal), so if you give each of them an "X" amount of the same fuel, the both engines will make the same power of X * 0.3 of fuel's energy.

So, both engines will make the same HP, at the afr, and use the same amount of fuel doing it.

If you have figured out a way around the law of conservation of energy somehow, then the world would very much like to know, as that could revolutionize society.
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[PPV_2018]

While cursing down the road like a normal person,
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All this time i thought i was a little “off” as i cursed down the road.. relieving to know this is what normal people do

 

[Antonm]

All this time i thought i was a little “off” as i cursed down the road.. relieving to know this is what normal people do

lol, damn autocorrect got me again,,, although I have been known to curse from time to time while driving.
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[blanchard7684]

Absolutely , and airflow is dependent on where the drivers right foot is.

So each engine isn't actually taking in its full displacement worth of airflow unless,,,,the driver has it a wide open throttle.

So, at part throttle (where we actually drive) if the smaller 5.3 has its throttle open further such that's its taking in the same amount of air as the 6.2 is, then both engines will be taking in the same volume or air, and the PCM will therefore inject the same amount of fuel,,, and the same amount of power will be produced ,,,,,shocker.
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Changing throttle position changes VE , the nv term in the power equation, which was previously held constant in the comparison between engines.

The only variables were displacement and AFR.

For a given rpm, and a given afr, increasing VE is the only other way to make the power the same between the two engines.

But increasing VE isn't done without also causing other variables to change that would impact the power output desired. RPM for example...

I was strictly looking at AFR and displacement.

But lets look at this anyway as if the engines were on an engine dyno.

We have a 5.3 that is at a throttle position that happens to put the 5.3 at the same air flow as a 6.2 at a lower throttle position.

This would require a lower pressure drop across the throttle and through intake (for the 5.3...putting it closer to atmosphere pressure). Lets assume this is true as well.

Then assume the cylinder heads can flow such that the air flow can match the 6.2 at these throttle positions and pressure drop.

Then the airflow would be the same as if on a flow bench. This is measured in volumetric terms, of course.

So the engine starts unloaded.

Dyno brake is applied to keep rpm the same.

Throttle position is increased to specified point for similar air flow. Further we are setting afr to 14.7. This is also making a huge assumption that spark timing would support this...

Then we are all surprised by the engine producing less power despite having same afr and same fuel flow rate at same rpm...

because power is torque x rpm/constant.

The 6.2 has higher compression and more displacement, it makes more torque.

 

[blanchard7684]

So let me get this straight,,,,,,, You're saying that at the same power level, two engines with the same efficiencies, will use a different amount of fuel if running at the same afr?

Is that what you are saying?
...

5.3 will need more rpm which increases BSFC aka fuel consumption per indicated power.

 

[blanchard7684]

LOL, you've proven that you can get different amount of energy out of the same amount of fuel without changing the engines efficiency have you,,,, interesting,,,have you also invented a perpetual motion machine machine?

Because getting a different amount of energy out of the same fuel (without changing an efficiency) kinda goes against the law of conservation of energy (just like the perpetual motion machine does)

Lets say these engine are 30% efficient overall (which is in the range of normal), so if you give each of them an "X" amount of the same fuel, the both engines will make the same power of X * 0.3 of fuel's energy.

So, both engines will make the same HP, at the afr, and use the same amount of fuel doing it.

If you have figured out a way around the law of conservation of energy somehow, then the world would very much like to know, as that could revolutionize society.
...

I'd prefer to say...one engine has higher compression, and larger displacement (via bore size), which makes it lower in BSFC.

Thermal efficiency is highly dependent on compression ratio and power is heavily dependent on displacement.

 

[blanchard7684]

Note that Brake Specific Fuel Consumption is dependent on compression ratio, engine speed, and displacement.

As CR drops BSFC increases...reduced efficiency
As engine speed is varied it reaches a minimum around 2500-3500 rpm
And as engine size increases, BSFC drops.

So in practice, the 5.3 needs to drop a gear to keep power at same level as 6.2 if it is desired to keep same afr.
Although the BSFC drops (which is good) it will still increase fuel use per mile.

Or the 5.3 can richen the mixture up and decrease AFR to keep same power. This too will increase fuel used per mile driven.

 

[Antonm]

Changing throttle position changes VE , the nv term in the power equation, which was previously held constant in the comparison between engines.

The only variables were displacement and AFR.

For a given rpm, and a given afr, increasing VE is the only other way to make the power the same between the two engines.

But increasing VE isn't done without also causing other variables to change that would impact the power output desired. RPM for example...

I was strictly looking at AFR and displacement.

But lets look at this anyway as if the engines were on an engine dyno.

We have a 5.3 that is at a throttle position that happens to put the 5.3 at the same air flow as a 6.2 at a lower throttle position.

This would require a lower pressure drop across the throttle and through intake (for the 5.3...putting it closer to atmosphere pressure). Lets assume this is true as well.

Then assume the cylinder heads can flow such that the air flow can match the 6.2 at these throttle positions and pressure drop.

Then the airflow would be the same as if on a flow bench. This is measured in volumetric terms, of course.

So the engine starts unloaded.

Dyno brake is applied to keep rpm the same.

Throttle position is increased to specified point for similar air flow. Further we are setting afr to 14.7. This is also making a huge assumption that spark timing would support this...

Then we are all surprised by the engine producing less power despite having same afr and same fuel flow rate at same rpm...

because power is torque x rpm/constant.

The 6.2 has higher compression and more displacement, it makes more torque.

Now you're talking about changing efficiency terms.

Yes, changing compression ratio changes some efficiencies. But the argument is (or was at least) about afr.

Have you now conceded that if you burn the same amount of fuel, at the same afr, that it'll release the same amount of energy/ the same power?

Efficiency is how much of that released energy we actually get to put to work. The 6.2 and 5.3 are very similar in efficiencies (because they're really very similar engines and even the scale on your graph shows no significant difference for 0.9 liters of change), but there are slight differences (like the 0.5 point of compression ratio you mentioned).

So if you've now abandoned the afr argument, are you now trying to argue efficiencies ?
...

 

[blanchard7684]

Now you're talking about changing efficiency terms.

Yes, changing compression ratio changes some efficiencies. But the argument is (or was at least) about afr.

Have you now conceded that if you burn the same amount of fuel, at the same afr, that it'll release the same amount of energy/ the same power?

Efficiency is how much of that released energy we actually get to put to work. The 6.2 and 5.3 are very similar in efficiencies (because they're really very similar engines and even the scale on your graph shows no significant difference for 0.9 liters of change), but there are slight differences (like the 0.5 point of compression ratio you mentioned).

So if you've now abandoned the afr argument, are you now trying to argue efficiencies ?
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Actually if you read these posts, you were the one who initiated the "changing throttle to match air flow" argument only after i pointed out that 200 hp worth of throttle in a 5.3 will have a lower afr than 70 hp of throttle in 6.2.

There is no abandoning the argument, here.

Same VE, same RPM, same AFR.

Bigger engine will make more power.

The specific argument you took exception to was " 5.3 can't make same power at same afr as 6.2"

This was assuming everything else is the same.



the only variable that is changing is the displacement in blue.

This stemmed from an assumption that 14.7 was "forced" by the ecm.

Opening up VE as a change by opening throttle is a new discussion. It is a new variable.

To answer why the 6.2 gets better fuel mileage, it is because it can make same power at higher afr, has higher compression and more displacement. These all reduce BSFC.

The academic discussion has supported this, very,very well.

The 5.3 can get to same power as 6.2 at same afr (within reason) with increase engine speed. This comes from increased throttle. In practice you can't decouple these...even before a downshift, increased throttle increases engine speed.

But this situation only causes more fuel use per mile.