Bad mpg?

Antonm said:

The pressure at BDC is a result how every 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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The feet thing is hilarious.

Bolded part isn't true here is why:

The pressure starts at a maximum of atmospheric and decreases from there.

At WOT you might get back to 90% VE. (Racing applications can get above 100% VE.) This is from acoustic tuning. You are tuning the pressure pulsations to coincide with the desired crank angle or crank position.

The pressure at BDC is highly dependent on pressure drops along the flow path. This is true for any system flowing a gas or liquid.

If you have a pipe of a gas with a pressure drop across it due to a valve, pressure on one side will be higher than the other side and this drop is dependent on the flow characteristics of the valve.

The air flow into an engine is no different.

blanchard7684 said:

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.

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blanchard7684 said:

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.

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See post #161 above, the difference came out to 0.011 mpg (at sustained 200hp usage) based off the difference in BSFC. Note, that is 0.011 mpg.

There is not a significant difference in getting 18.02 mpg compared to 18.01 mpg ( that difference would be 0.055%, seems like a hair to me.)
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Antonm said:

See post #161 above, the difference came out to 0.011 mpg (at sustained 200hp usage) based off the difference in BSFC. Note, that is 0.011 mpg.

There is not a significant difference in getting 18.02 mpg compared to 18.01 mpg ( that difference would be 0.055%, seems like a hair to me.)
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The trend showed the effect of displacement on BSFC.

CR is another variable that improves BSFC.

And engine speed is another.

You combine these into a contour plot.

The bullet points on this slide elude to the other ways BSFC is affected.

blanchard7684 said:

The feet thing is hilarious.

Bolded part isn't true here is why:

The pressure starts at a maximum of atmospheric and decreases from there.

At WOT you might get back to 90% VE. (Racing applications can get above 100% VE.) This is from acoustic tuning. You are tuning the pressure pulsations to coincide with the desired crank angle or crank position.

The pressure at BDC is highly dependent on pressure drops along the flow path. This is true for any system flowing a gas or liquid.

If you have a pipe of a gas with a pressure drop across it due to a valve, pressure on one side will be higher than the other side and this drop is dependent on the flow characteristics of the valve.

The air flow into an engine is no different.

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Did you intentionally miss the part that says " Once its in there"? or are you just blindly leaving that part out on purpose?

Yes all those factors you mentioned affect how much air may get in the cylinder, but once you trap an given volume/ cubic feet of air, how that air got there makes no difference on the pressure it'll create once trapped.

This isn't a hard concept to grasp, there is even a real simple equation for it;

Pressure at point one (P1) times(x) volume at point 1 (V1) = Pressure at point 2 (P2) times (x) volume at point 2 (P2) or shorthand of P1 * V1 = P2 * V2.
Please tell me you've heard of this very basic equation before, its like high school, maybe even middle school, level.

And what does this basic equation tell you if you trap the same amount of air in two different size cylinders?

And most (if not all) automotive engineers use psia when referring to air pressure in naturally aspirated engines, makes the calculations easier when you don't have to cross zero all the time or use negative values. There is even a sensor called the MAP sensor, why don't you goggle up what the letters M.A.P. stand for in that sensor real quick. Maybe then you'll understand how easy calculations can be even when using pressure below atmospheric.
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blanchard7684 said:

The trend showed the effect of displacement on BSFC.

CR is another variable that improves BSFC.

And engine speed is another.

You combine these into a contour plot.

The bullet points on this slide elude to the other ways BSFC is affected.

Click to expand...

Are we back to arguing efficiencies now?

You jump around a lot as to what point your trying to make, and what parameter you think is doing what, its hard to keep up with what crazy/ misguided/ twisted thing you're trying to say is doing what, reminds me of when I kids where real little kinda.

Are you done with afr and moving over to the small differences in efficiencies now (like that HUGE difference in BSFC)???
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Antonm said:

Did you intentionally miss the part that says " Once its in there"? or are you just blindly leaving that part out on purpose?

Yes all those factors you mentioned affect how much air may get in the cylinder, but once you trap an given volume/ cubic feet of air, how that air got there makes no difference on the pressure it'll create once trapped.

And most (if not all) automotive engineers use psia when referring to air pressure in naturally aspirated engines, makes the calculations easier when you don't have to cross zero all the time or use negative values. There is even a sensor called the MAP sensor, why don't you goggle up what the letters M.A.P. stand for in that sensor real quick.
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The induction system will change the quantity of air in cylinder. There is pressure drop and flow restriction.

Saying it is the same isn't correct.

Post 171 covers this as well.

Antonm said:

Are we back to arguing efficiencies now?

You jump around a lot as to what point your trying to make, and what parameter you think is doing what, its hard to keep up with what crazy/ misguided/ twisted thing you're trying to say is doing what, reminds me when I kids where real little kinda.

Are you done with afr and moving over to the small differences in efficiencies now (like that HUGE difference in BSFC)???
...

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Nope I cautioning on using that trend for anything more than explaining how displacement affects bsfc.

It isn't an actual contour map for 5.3 or 6.2.

blanchard7684 said:

View attachment 447895

The induction system will change the quantity of air in cylinder. There is pressure drop and flow restriction.

Saying it is the same isn't correct.

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Not true, we (the operators/ drivers/ whoever is pressing the go pedal) determines how much air is drawn in. And if we want to make X amount of power, we draw in the air required to make that power

Lets get some basic statements we can agree on, read the following at tell me if you agree or not.

1.) the equation P1 *V1 = P2 * V2 is real/ accurate.

2.) to burn a given amount of fuel at stoichiometric ratio its takes a given cubic feet of air.

These are very, very basic concepts, do you acknowledge them?
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blanchard7684 said:

Nope I cautioning on using that trend for anything more than explaining how displacement affects bsfc.

It isn't an actual contour map for 5.3 or 6.2.

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The contour maps for two engines that are virtually identical except for 1/4 inch of bore diameter are very, very similar. The differences are on the same (and insignificant) order of magnitude already shown.

Side note, its getting late in the day for me, time to go home/ get away from the computer,,, think real hard overnight and I'll check back in the morning to see what wonders you've discovered.
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Antonm said:

Not true, we (the operators/ drivers/ whoever is pressing the go pedal) determines how much air is drawn in. And if we want to make X amount of power, we draw in the air required to make that power

Lets get some basic statements we can agree on, read the following at tell me if you agree or not.

1.) the equation P1 *V1 = P2 * V2 is real/ accurate.

2.) to burn a given amount of fuel at stoichiometric ratio its takes a given cubic feet of air.

These are very, very basic concepts, do you acknowledge them?
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Click to expand...

When the throttle is actuated it is changing the effective flow coefficient through the entire induction system. It is the main limiting pressure drop, but not all of it.

Again there is pressure restriction from the intake box, throttle, intake manifold, intake runner, and intake valve.

The mass of air will be different, the volume will be different, and the volume inside the cylinder will be different.

Flow through orifice more accurately models flow through throttle body at part throttle. Flow through venturi would be more accurate model for flow at wot.

Your original comment about an "ideal quantity" (mass or volume...you didn't specify) of air raising pressure in a 5.3 cylinder vs dropping pressure in 6.2, that would/could impact the power equation, is not an accurate or reasonable description of what is going on.

1) if mass is same. Boyle is constant mass. The mass flow rate is determined by the throttle, not conditions at bottom dead center. This is bernoulli, not gas laws.

Alternatively, if you are saying V1 is 5.3 and V2 is 6.2 then the error is even more severe.

If I take 5.3 L at a specified pressure , it will also have a specific mass.

Then if I put this into a 6.2 L container, the pressure will decrease by Boyle’s law.

All this does is prove Boyle’s law, it doesn’t prove your original point about pressure at BDC being higher in 5.3.

In fact all this does, in the inverse, is prove how forced induction works.

If you cram 6.2L into 5.3, you make more power than the 5.3 would at atmospheric conditions.

The reason why this exercise is not applicable is because the mass flow is different between the two engines.

This stems again from how the induction system is configured.

Just think: an 80 mm throttle, 1.9 intake valves, with 5.3L pulling air through them will move less air mass than a 6.2 L engine with 87 mm throttle and 2.16 intake valves.

5.3 has smaller intake runners than 6.2, to go along with this...

2) it takes a given mass of air. Remember you can get stiochiometric balance in any size cylinder, with different masses of fuel and air, which create different levels of power when combusted, at same afr.