What you need to know about Brake Specific Fuel Consumption

[mort]

Originally Posted by drmiller100

Maybe I am calculating it wrong.

How would you calculate it????? Engine displacement is one of the variables.

I THINK I have identified all the variables.

??????

Hi drmiller100,
Here is a screen shot of that power to compress air page:

It says "volume of compressed air at atmospheric pressure," the air is not at atmospheric, it's at manifold pressure. You guessed 5 psi, so the apparent VE is .34

1.1 hp.

-mort

[user removed]

Originally Posted by Old Mechanic

Pumping losses have been debated years ago on this site. In my opinion the term "pumping losses" is not very precise as far as what is actually a pumping loss and what is other losses. I guess the debate will continue, without resolution.

regards
mech

In 1982 when I was working for Mercedes Benz, they changed the naturally aspirated diesels, like the 240D, from a throttle plate to no throttle plate.
They had to add a vacuum pump in order for the heater and central locking systems to work (vacuum operated). The pump was driven off the timing chain.

MB's service bulletins on the then new model car claimed a 7% improvement in efficiency over the previous model year. The problems with the throttled model from the previous year was when you developed a vacuum leak in the central locking system or heater controls, the engine could only be shut down by opening the hood and pushing the shut off lever on the fuel injection pump.

I predicted this debate would go downhill earlier in this thread (see quote).
I am not an educated engineer, but I do like the idea of testing different losses in IC engines to determine their relative significance. Arguing hypotheticals and bashing other members credentials, intelligence, or telling them to go and study a specific field of education, is EXACTLY what my prior prediction was prefaced on, knowing that knowledge and the possibility of learning something would soon degrade into a pissing match.

Tried to start a thread a long time ago about defining individual losses in reciprocating engines, including the losses attributable to the changing in inertia in the reciprocating parts of an engine as well as the valve train losses.

A exercise in futility, when the same type of pissing match began.

Rational discussion destroyed by what?

Stupidity disguised as superiority.

Good Luck Gentlemen (using the term very lightly)
Mech

[drmiller100]

Originally Posted by mort

It says "volume of compressed air at atmospheric pressure," the air is not at atmospheric, it's at manifold pressure. You guessed 5 psi, so the apparent VE is .34

1.1 hp.

-mort

in the little field which says "P2" type in the number 10.

You will get a negative number.

[serialk11r]

Originally Posted by drmiller100

you are saying a process is adiabatic, but not reversible, only true with non idea gasses.

Adiabatic means delta Q=0, reversible means delta S=0. I got it mixed up too, mort corrected me. Pulling air past a throttle is converting kinetic energy of the gas into heat, that is an increase in entropy. Adiabatic processes need not be isentropic.

I never said it's not a lot of power, stop putting words in my mouth.

Mech I hope you don't think I'm in that category...I am trying my best to be respectful.

[George Tyler]

Originally Posted by serialk11r

Perhaps BMEP would be the best way to see how the losses compare...
Throttling losses are not 100% of the energy it takes to create the vacuum, since the vacuum helps pull the cylinder back up a little. At idle I believe a typical figure for manifold pressure is a bit over 0.3 bar (absolute). Let's say 1/10 of this is recovered during compression. That puts the throttling loss at about 60kPa maximum. At idle the engine burns off the rest of the energy by overexpansion I believe, but I'm not sure on this.

I can't find very good figures for friction, but for a F1 engine spinning at 18k, the FMEP was about 1/10 of the BMEP which is on the order of 1.3MPa or so (typical good naturally aspirated engines produce a bit over 100Nm/L specific torque), and I imagine even with their super long rods, advanced coatings, and less accessories, 18k rpm is not going to be friendly for friction at all. I saw some chart that indicated around 50kPa FMEP at idle though, so I'd say 50-150kPa seems like a pretty good range for specific friction.

So pumping losses are at worst around the same as friction losses.

If you run your engine at say 20% load, that means you're producing perhaps 250kPa useful specific torque. The air theoretically needed would be 20% of peak, but you obviously need quite a bit more than this because you need to overcome friction and pumping, as well as the lower compression ratio sending more heat to waste. So say you need to throttle the engine down to 0.4bar manifold pressure. Then you're wasting about 50kPa on pumping, about 100kPa on friction, and producing 250kPa useful specific torque. That is, 3/8 of the useful work produced is wasted. But the useful work was produced at a pathetic compression ratio where the ideal efficiency was lowered significantly.

On high compression engines this is something like going from 12:1 to 5:1, which in ideal situations is a loss of 25% efficiency or so. But on your run of the mill 10:1 compression ratio engine, the difference is bigger. I believe this is the reason why manufacturers keep trying to bump the compression ratio up despite only very small gains at full power.

So the biggest 2 culprits are reduced compression and friction, and throttling is a close 3rd. Diesels have more respectable efficiency at low loads since they have no throttling losses, and maintain good charge compression. At extremely low loads though (such as idle) they still do horribly since overcoming friction is significant.

I have been thinking about the first principles behind the cycle efficiency for a while in another context, compression ignition vs spark ignition. Carnot efficiency is related the ratio of the temperature the heat is accepted at to the temperature it is rejected at. the heat added varies with the amount of fuel in the cylinder when it fires, which depends on the trottle setting, the temperature rise depends on this and also on the thermal capacity of the charge, which also varies with trottle setting. this means that the temperature rise is constant with varying throttle, all else being equal! in a diesel, it still has full charge at low load, but less fuel, so temperature rise is less. I think if it was not for this the difference in part load efficiency between a diesel and spark ignition engine would be greater. Anyway, this means that efficiency is not actually lost because of reduced effective compression ratio?

[serialk11r]

Yea I think so, was it Old Mechanic who gave the analogy with an aircraft running at high altitude where the air density was much lower? With the throttle plate closed it's almost imitating those conditions except the throttle restriction is adding a little bit of heat to the charge.

Never thought of it in that context before, thanks.

[user removed]

My point is.

In my life I have met a few people whose knowledge was astounding. I have met people whose educations were some of the best in the world. One example was a friend who works for NASA. He has a degree from MIT, a Doctorate in Theoretical Aerodynamics.
Not sure what his IQ was but I would bet it was in the top .1% of humanity.

His therapy was to come out to the auto repair shop and put on a greasy set of coveralls and work on his cars, or to help with customer problems. When I got in a nasty car wreck, just after buying the shop, and was fairly badly injured, he took off a week to come out and help in the shop while I recovered. Never asked for a dime.

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.

I bought the shop from him and two of his partners. The shop was going bankrupt and could not even cover the salaries and tax deposits due to the IRS and state. That's a Felony charge people.

I ran the shop successfully for 14 years before I sold it, so you could argue that my abilities in that narrow context were superior to his. I succeeded where he had failed.

So who was the superior intellect? I succeeded at running a business that he failed at running.

I really don't know or care who was the superior intellect. I know his was astounding, and doubt in his field I would have been the least bit competitive, so he won that example.

So you are really really smart, that's admirable, but can you explain something to a person who does not have the background of education that you have achieved?
That my friends is the most admirable trait of all, not the ability itself, but the ability to share your (self perceived) vastly superior knowledge with another human being is such a way that THEY
benefit from your intellect, while not making them SUFFER from the experience, due to YOUR perceived superiority.

No one here knows me well enough to make that assumption, and I refuse to assume that any member here is more or less smart than I am. It's really more a matter of knowledge, intelligence, and environment that provides one with greater education than the other, but that circumstance, in and of itself, is NOT the deciding factor in any rational comparison of intelligence.

regards
Mech

[user removed]

Originally Posted by serialk11r

Yea I think so, was it Old Mechanic who gave the analogy with an aircraft running at high altitude where the air density was much lower? With the throttle plate closed it's almost imitating those conditions except the throttle restriction is adding a little bit of heat to the charge.

Never thought of it in that context before, thanks.

I always thought that high velocity air flowing through a restriction was cooler after the restriction. I also look at the atmosphere as a column of air that extends from sea level to the edge of space, that weighs a nominal 14.7 pounds, and is trying to fill any void due to mass and gravity.

I know that the grilles on buildings in the middle east serve as a form of passive cooling of the air as it passes through those grilles.

Bottom line for me is the Mercedes SB that claimed 7%. I never studied engineering in any formal classroom scenario, I did consider Physics for a short time, but ended up as an apprentice in a body shop at just past 18, and maxed out my Social Security contribution barely 4 years later. Pop could not afford to finance the education of his 4 sons. Today he is closing in on 91and can easily finance the education of his grandsons, if that is what they want.

regards
Mech

[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.

[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.