What you need to know about Brake Specific Fuel Consumption

[UFO]

^^ You might get a little better efficiency, but if the engine is not designed for a lean burn, going just a little too lean will result in detonation and engine damage. Your efficiency will come at a high cost.

[drmiller100]

Originally Posted by Old Mechanic

I always thought that high velocity air flowing through a restriction was cooler after the restriction.
Mech

I'm gonna go around in circles, then hopefully bring it all together.

There is something fancy called the Ideal Gas Law, which has a formula of
PV=nRT

What it does is relate Pressure, Volume, Temperature, and basically the number of little molecules bouncing around.

It creates a simple way to figure out what happens to gases when you change one of the variables, and gives a simple formula for figuring out the end result.

So. Take an air compressor. It takes air floating around us at 14.7 psi. And the compressor changes the VOLUME. It does this by sucking air into the cylinder, and compressing it to a smaller volume.

Same number of molecules, but now the volume is much smaller then it used to be. The formula says the Temperature must rise, or the Pressure must rise, or both.

Does the top of the compressor get hot? Absolutely. It gets hot BECAUSE the air has been compressed.

If We cool the air by letting it sit in the tank, it will COOL OFF, which means because the volume hasn't changed, the pressure will drop.

Likewise. If we take 100 psi air out of our air tank, and squirt it on us, it feels COOLER then the air around us. that is becuase it is cooler - the Pressure went from 100 psi to 14.7 psi, the volume expands and the temperature drops.

So, the venturi itself doesn't cause the temperature change. In order for the air to go across the venturi, there MUST be a pressure difference. If there is not a pressure difference, no air moves.

The Venturi does indeed get very cold, but that is because the PRESSURE changes across the venturi because of the pressure differential.

Harley riders and old volkswagon drivers know this because when the engines are running at about 33 degrees on a rainy, sleety day, the carb will ice up and the engine will run like crap.

The carb ices up because the carb is getting cold because there is a pressure differential at the throttle plate.

[drmiller100]

Originally Posted by E4ODnut

I did some research on "lean burn". It looks to me like a true lean burn engine is designed to run on much leaner fuel mixtures throughout the complete power band and that a major stumbling block to these designs is that of maintaining emissions standards.

Having said that, and emissions concerns aside, I see no reason why conventional gasoline engines can't get improved BSFC at lower engine loads by leaning the mix, adjusting spark timing to suit this leaner mix, and opening the throttle somewhat to maintain the required power. The larger throttle opening will reduce pumping losses or what ever you want to call it, as well as improve the thermal loss. The exact cause of the improvement doesn't really matter, what matters is that the end result should be better low load BSFC.

I see you have a megasquirt. For sure you can do this yourself.

Keep in mind there is a band you do NOT want to run at. If you go a touch leaner then stoich, the engine will get HOT. But if you go leaner yet, the combustion chamber will start to cool off until it gets sooooo cool it won't fire.

Look for TPS openings in the 10 to 20 percent max range. Consider buying an EGT and learning what your engine is like normal. 1200 is considered on the higher end of things for most NA engines. 1500 is already blown up.

consider running more spark advance then you would expect.

consider running EGR valve open during this - it can actually cool the chamber.

and stay away from knock.

[drmiller100]

Originally Posted by Old Mechanic

My point is while he could easily have taken the position of "I am the superior intelligence and you all should bow down to my obviously higher intellect", in fact he did the exact opposite. His best character trait was his ability to explain something to me in a way that I did not need to spend 6 years at MIT to understand what he was talking about.


Mech

Your friend learned a lot of things from you.

Some of us are so passionate about the desire to learn about the physical world our emotional and communciation sets truly and miserably suck.

In other words, I can be a jerk. And I love to learn, and the best way I know to learn is to discuss, and "argue".

When I am trying to understand, define, and solve a problem, there are many confusing side channels and sideroads I head into.

Once I truly "Grok" something, or Understand it, I SHOULD be able to explain it down to basic principals fairly easily.

Right now, I don't think ANYONE has a good way to actually calculate the losses due to throttling.

Once you can calculate it, then you should be able to explain it.

If we can explain it, but can't back it with math, and we cannot verify with real world examples, then we are talking out our asses.

[user removed]

Originally Posted by PaleMelanesian

Back to the start of the discussion. If you take this chart to heart and drive so that you're in the red zone most or all of the time, you will get amazing mileage. Feel free to argue about the details that lead up to it, but you can use it as-is to get real-world results.

Like this.

Precisely, but if you design the vehicle so it accomplishes the same as an individual effort without any specific driver input, then you have changed the way fuel is consumed on a planetary scale.

Which is more effective, one hypermiler driving a car to maximise it's efficiency or a billion vehicles all over the Earth driving themselves in a way that optimizes the efficiency of every drop of fuel they consume.

Hopefully food for thought, not meant as any point of contention.

regards
Mech

[user removed]

Originally Posted by drmiller100

Your friend learned a lot of things from you.

Some of us are so passionate about the desire to learn about the physical world our emotional and communciation sets truly and miserably suck.

In other words, I can be a jerk. And I love to learn, and the best way I know to learn is to discuss, and "argue".

When I am trying to understand, define, and solve a problem, there are many confusing side channels and sideroads I head into.

Once I truly "Grok" something, or Understand it, I SHOULD be able to explain it down to basic principals fairly easily.

Right now, I don't think ANYONE has a good way to actually calculate the losses due to throttling.

Once you can calculate it, then you should be able to explain it.

If we can explain it, but can't back it with math, and we cannot verify with real world examples, then we are talking out our asses.

drmiller, my comment about acting like a jerk, was not directed at you personally or directly at anyone else individually. The point I was trying to make is I see many rational discussions degrade into shooting matches where one member jumps all over another members post and tries to tear it to pieces to prove some point of superiority. Instead of this tactic I personally would much prefer to have my mistake corrected with a rational counterpoint and some information to support that counterpoint.

It follows the general rules of any organized debate to do so, and it keeps those who may not be blessed with the same level of specific knowledge, interested and possibly educated to a greater level than they were when they came to this or any other thread.

Anyone here had a teacher or professor who went out of their way to help with understanding certain facts, and in doing so changed their student from one who learned with difficulty into a voracious gatherer of knowledge.

God bless those teachers, who faced students, every day, that were of lesser education, and inspired them to a much higher level of understanding.
I doubt if intimidation had any part of that interaction.

regards
Mech

[serialk11r]

Yea the temperature is lower after the throttle plate, but if you took a volume of gas and adiabatically expanded its volume via a piston, the temperature would be even lower. The heat addition just doesn't "make up" for the expansion effects, but there is a heat addition.

drmiller, let's do a real world data comparison. I've seen 60% claimed fuel consumption decrease from variable intake duration systems...I think that matches the <1hp estimate pretty well.

[PaleMelanesian]

Originally Posted by Old Mechanic

Precisely, but if you design the vehicle so it accomplishes the same as an individual effort without any specific driver input, then you have changed the way fuel is consumed on a planetary scale.

Which is more effective, one hypermiler driving a car to maximise it's efficiency or a billion vehicles all over the Earth driving themselves in a way that optimizes the efficiency of every drop of fuel they consume.

Hopefully food for thought, not meant as any point of contention.

regards
Mech

You're absolutely right. That's why I'm so excited about the GM 1.4 turbo Cruze and Ford's Ecoboost series. A smaller engine moves cruising load higher up the chart, making it more efficient. A turbo (or hybrid-type electric boost) makes up the lost power, but only when needed.

I just choose to make a difference in my own consumption right now, and not wait for "them" to do it for me. After buying a scangauge, the driving techniques are free.

[gone-ot]

FWIW, I've asked GM for a BSFC chart for the 1.4LT engine in the wife's Cruze LTZ, but never received a reply back.

[drmiller100]

Originally Posted by serialk11r

Y

drmiller, let's do a real world data comparison. I've seen 60% claimed fuel consumption decrease from variable intake duration systems...I think that matches the <1hp estimate pretty well.

60 percent seems rather high.

do you mean 6 percent?????

variable cam is a different effect. the two should be additive.