Bad mpg?

[Antonm]

View attachment 447855

Lets look at this drastic difference you've called out here. Which is an efficiency difference, thermal efficiency in this case. Which I've said all along the two engines efficiencies' are very similar,,, similar enough to be negligible and considered the same.

Looking at the scales on both axis, not a lot difference between 5.3 and 6.2, but since I'm sitting in front of an internet connected computer (its a work day after all) lets quantity that particular efficiency difference I say is negligible with actual hard numbers shall we.

First lets digitize that graph using this open source (available on the internet) program .

WebPlotDigitizer - Copyright 2010-2024 Ankit Rohatgi

apps.automeris.io

That'll allow us to pull actual numbers off the graph without being accused of interpretation error. So that give us the below points we can throw into excel and re-create the curve.



After a little copy/ paste and graphing in excel, we get this, and we can extrapolate values for 5.3 and 6.2 L from it pretty accurately.


Y axis value of 5.3L = 311
Y axis value of 6.2L = 294
For delta of 17

But those numbers are in gm/KW-hr,,,,so lets do some unit conversions over to freedom units.

Each gallon of gasoline has 3217 grams, so that comes out to 0.0052 gals/ kw-hr

128oz/ gal so, 0.67oz/ kw -hr,,,, but who likes talking in terms of KW, so with 1.341 hp /KW that comes out to a whopping 0.504 oz/ hp-hr.

A tablespoon contains 0.5oz ,,,, so a tablespoon is the BSFC delta between 5.3 and 6.2L in hp-hr.

So lets take this a step further because hp-hr is kind of a strange term that's not really relatable.
Lets say driving down the road at 70 mph it takes 200 hp to keep the car moving (which is within the realm of reason for these heavy bricks) and you do that for one hour. So that means the delta in fuel usage for that hour is 100oz (0.5oz * 200hp for one hour). So in that hour/ 70 miles you used an additional .78 gallons of fuel, so your fuel consumption went up by 0.011 mpg. Better get Greta on the phone, you've fixed all the worlds issues.

Considering the fuel consumption of these engines in general, I'm calling that the same.

So again, burn the same amount of fuel (at the same afr) in two engines with the same efficiencies , get the same amount of power.
...

 

[Antonm]

There is no abandoning the argument, here.

The 5.3 can get to same power as 6.2 at same afr (within reason) with increase engine speed.

You made both of the statements above in one post, yet you say that you also "stand behind" your statement that started this whole argument (see below).



Can't have is both ways my man, both can't be true.

Are you now pivoting to efficiency differences (from the previous afr differences) between the two engines? Because sure there are differences there (talking efficiencies now) in an absolute sense (like if I were to pour a gallon of pure distilled water in the ocean, technically the ocean's salinity/ salt concentration would change ,,, not by enough to make any practical difference, but it would change). Which the about the magnitude of efficiency differences between the same generation 5.3's and 6.2s at part throttle.
...

 

[blanchard7684]

Lets look at this drastic difference you've called out here. Which is an efficiency difference, thermal efficiency in this case. Which I've said all along the two engines efficiencies' are very similar,,, similar enough to be negligible and considered the same.

Looking at the scales on both axis, not a lot difference between 5.3 and 6.2, but since I'm sitting in front of an internet connected computer (its a work day after all) lets quantity that particular efficiency difference I say is negligible with actual hard numbers shall we.

First lets digitize that graph using this open source (available on the internet) program .

WebPlotDigitizer - Copyright 2010-2024 Ankit Rohatgi

apps.automeris.io

That'll allow us to pull actual numbers off the graph without being accused of interpretation error. So that give us the below points we can throw into excel and re-create the curve.

View attachment 447858

After a little copy/ paste and graphing in excel, we get this, and we can extrapolate values for 5.3 and 6.2 L from it pretty accurately.

View attachment 447859
Y axis value of 5.3L = 311
Y axis value of 6.2L = 294
For delta of 17

But those numbers are in gm/KW-hr,,,,so lets do some unit conversions over to freedom units.

Each gallon of gasoline has 3217 grams, so that comes out to 0.0052 gals/ kw-hr

128oz/ gal so, 0.67oz/ kw -hr,,,, but who likes talking in terms of KW, so with 1.341 hp /KW that comes out to a whopping 0.504 oz/ hp-hr.

A tablespoon contains 0.5oz ,,,, so a tablespoon is the BSFC delta between 5.3 and 6.2L in hp-hr.

Considering the fuel consumption of these engines in general, I'm calling that the same.

So again, burn the same amount of fuel (at the same afr) in two engines with the same efficiencies , get the same amount of power.
...

I posted the trend for reference that displacement decreases BSFC.

(Credit due...I'm actually impressed with the graphing and it adds to the discussion. Well done.)

To get a real look at BSFC for these two engines you have to have the specific engine contour plot of mean effective pressure vs engine speed and plot it over incremental power levels. The contours are different levels of BSFC.


It would look something like this:





I'm not claiming a huge difference.

For the same AFR, same rpm, same ve, the power levels will be different based on displacement alone.

For low power demand, will rpm, afr, and ve be close? yep. For high power demand, 5.3 can't get there. For mid-range power the 5.3 will need to increase engine speed or richen fuel mixture--to the point where it's fuel consumption per mile is at a deficit to 6.2.

 

[blanchard7684]

You made both of the statements above in one post, yet you say that you also "stand behind" your statement that started this whole argument (see below).

View attachment 447874

Can't have is both ways my man, both can't be true.

Are you now pivoting to efficiency differences (from the previous afr differences) between the two engines? Because sure there are differences there (talking efficiencies now) in an absolute sense (like if I were to pour a gallon of pure distilled water in the ocean, technically the ocean's salinity/ salt concentration would change ,,, not by enough to make any practical difference, but it would change). Which the about the magnitude of efficiency differences between the same generation 5.3's and 6.2s at part throttle.
...

if all other things are equal, then according to the power equation referenced, there is not a mathematical path to same power with a displacement difference between 5.3 and 6.2, if the afr is the same.

If other variables are opened up for change, then the 5.3 absolutely can make same power at same afr. The same power equation says so.

 

[Antonm]

I posted the trend for reference that displacement decreases BSFC.

(Credit due...I'm actually impressed with the graphing and it adds to the discussion. Well done.)

To get a real look at BSFC for these two engines you have to have the specific engine contour plot of mean effective pressure vs engine speed and plot it over incremental power levels. The contours are different levels of BSFC.


It would look something like this:

View attachment 447877



I'm not claiming a huge difference.

For the same AFR, same rpm, same ve, the power levels will be different based on displacement alone.

For low power demand, will rpm, afr, and ve be close? yep. For high power demand, 5.3 can't get there. For mid-range power the 5.3 will need to increase engine speed or richen fuel mixture--to the point where it's fuel consumption per mile is at a deficit to 6.2.

Splitting hairs again, since both the engines being discussed hear have the exact same stroke length (3.62 inches) , piston speed is identical for a given RPM.

And since they're only 0.9 L worth of displacement different in diameter (little over 1/4 inch bore size) their contour plots will look/ be very similar too because their rotational speed isn't that much different at a given power level.
...

 

[Pressureangle]

if all other things are equal, then according to the power equation referenced, there is not a mathematical path to same power with a displacement difference between 5.3 and 6.2, if the afr is the same.

If other variables are opened up for change, then the 5.3 absolutely can make same power at same afr. The same power equation says so.

The part you missed is that in this scenario neither engine is operating at WOT and minimum manifold vacuum. The only difference between the two at the same AF ratio and RPM is the manifold vacuum and mean cylinder pressure.

 

[Antonm]

if all other things are equal, then according to the power equation referenced, there is not a mathematical path to same power with a displacement difference between 5.3 and 6.2, if the afr is the same.

If other variables are opened up for change, then the 5.3 absolutely can make same power at same afr. The same power equation says so.

You do realize that if you put the same quantity of air in two different volume cylinders, the pressures will be different right?

For a given set amount of air ,,,, say that amount of air needed to perfectly burn a given amount of fuel at stoichiometric ratio, if you put that amount of air in a smaller engine then;




But if you take that same amount of air (the amount needed to burn a given amount of fuel at stochiometric), and put it in a larger engine, then you get;



Maybe I should have mentioned this earlier, but I just kinda assumed this was a common sense thing.

So burning the same amount of fuel, at the same afr (which requires the same amount of air) in two different displacements engines makes the same power ( assuming the engine efficiencies are the same of course).

That lecture you keep referring to even alludes to that later on when talking about ways to raise power, see Pressure (adding boost) at BDC statement below.



I think that maybe you've not be seeing the forest for the trees and focusing on just the equation as symbols, and not what the equation is actually doing/ telling you. Which happens even in the adult / professional world every now and then.
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[blanchard7684]

You do realize that if you put the same quantity of air in two different volume cylinders, the pressures will be different right?

For a given set amount of air ,,,, say that amount of air needed to perfectly burn a given amount of fuel at stoichiometric ratio, if you put that amount of air in a smaller engine then;

View attachment 447886


But if you take that same amount of air (the amount needed to burn a given amount of fuel at stochiometric), and put it in a larger engine, then you get;

View attachment 447887

Maybe I should have mentioned this earlier, but I just kinda assumed this was a common sense thing.

So burning the same amount of fuel, at the same afr (which requires the same amount of air) in two different displacements engines makes the same power ( assuming the engine efficiencies are the same of course).

That lecture you keep referring to even alludes to that later on when talking about ways to raise power, see Pressure (adding boost) at BDC statement below.

View attachment 447891

I think that maybe you've not be seeing the forest for the trees and focusing on just the equation as symbols, and not what the equation is actually doing/ telling you. Which happens even in the adult / professional world every now and then.
...

Points for creativity. It made me think.

However...

The pressure at bottom dead center is the result of the pressure drop from atmospheric pressure in intake housing, across the throttle blade, through the intake runner, through intake ports, and across the intake valve.

I'm assuming the pressure drop has been the same for the sake of discussion.

These intake systems are acoustically tuned and not just for WOT performance. The 6.2 has 2.12 intake and 87 mm throttle body. The 5.3 has 1.9 intake valve and 80 mm throttle body. Not sure about the intake manifold volume between the two engines. They have the same stroke so GM likely made the intakes designed to have max pressure pulse at same crank angle. So in effect they should be the same at BDC.

So I can't get into the idea that the cylinder pressure at BDC is so much different that it takes out the effect of displacement.

 

[Antonm]

Points for creativity. It made me think.

However...

The pressure at bottom dead center is the result of the pressure drop from atmospheric pressure in intake housing, across the throttle blade, through the intake runner, through intake ports, and across the intake valve.

I'm assuming the pressure drop has been the same for the sake of discussion.

These intake systems are acoustically tuned and not just for WOT performance. The 6.2 has 2.12 intake and 87 mm throttle body. The 5.3 has 1.9 intake valve and 80 mm throttle body. Not sure about the intake manifold volume between the two engines. They have the same stroke so GM likely made the intakes designed to have max pressure pulse at same crank angle. So in effect they should be the same at BDC.

So I can't get into the idea that the cylinder pressure at BDC is so much different that it takes out the effect of displacement.

The pressure at BDC is a result how ever many cubic feet of air you have trapped in a given size cylinder.

How that air got there, via what size throttle body, or intake valve , naturally aspirated or forced, is irrelevant to the the pressure it'll create once its in there.

Simply put, if you add "X" amount of cubic feet off air in a set volume, you'll get "Y" amount of pressure every time. Increase that volume, the pressure will go down for that same cubic feet of air (and the reverse is also true).

And to burn a given amount of fuel at the same ratio, you need the same cubic feet of air, not the same pressure, the same volume/ the same cubic feet.

Sorry you can't wrap you head around the idea. I can't wrap my head around the idea that there are women making millions of dollars selling pictures of their feet on the internet, but that doesn't mean it isn't true, or that it isn't happening, just means I can't understand it. It would appear the same goes for you understanding this issue I suppose.
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[blanchard7684]

Splitting hairs again, since both the engines being discussed hear have the exact same stroke length (3.62 inches) , piston speed is identical for a given RPM.

And since they're only 0.9 L worth of displacement different in diameter (little over 1/4 inch bore size) their contour plots will look/ be very similar too because their rotational speed isn't that much different at a given power level.
...

I had a whole reply typed out but it got timed out somehow...

I appreciate the work you put into the graph and the calculation.

The trend was for reference.

I have done contour mapping before with an engine that had 0.8L difference in displacement and there were a few spots on the map where BSFC was different (with the larger displacement showing a lower bsfc). I can't post IP so all I have is an open source plot showing effect of displacement as well as another plot showing effect of rpm and CR on BSFC (which favors the 6.2 in this discussion).

As far as power vs speed, the 6.2 does make quite a bit more torque at lower rpm ranges than 5.3. Point being I think there is a larger than "a hair's" difference going on.