What you need to know about Brake Specific Fuel Consumption

[Ecky]

Originally Posted by Old Tele man

...but, also notice how "sometimes" during half-throttle operation, the two bottom load lines "cross" one another (at about 3,500 rpm), meaning that "sometimes" half-throttle operation is actually better than "full-throttle" operation (bottom graph)!

Does that motor have fuel enrichment?

[drmiller100]

Originally Posted by Old Tele man

...but, also notice how "sometimes" during half-throttle operation, the two bottom load lines "cross" one another (at about 3,500 rpm), meaning that "sometimes" half-throttle operation is actually better than "full-throttle" operation (bottom graph)!

The graph in question actually shows horsepower per mass of fuel.

fuel maps are almost always richer at WOT for any given condition then partial throttle. The idea is if you are WOT, you want max power. The minor amount of extra fuel provides a minor amount of extra power.

[mwebb]

well since there is no combustion taking place yet there is a huge difference in the amount of pressure in the cylinder between
wide open throttle and closed throttle , it would seem the pressure difference can be attributed to change in the ability of the engine to pump air into the combustion chamber

with a closed throttle as opposed to wide open throttle

with extreme suction throttling losses or almost no suction throttling loss

that difference clearly exceeds 5% .

back pressure at higher flow is measured at under 5psi but back pressure varies with flow and is negligible at flow under 50%

suction throttling losses also vary with flow
but even at MAX flow with over 4 psi measured back pressure there is over 4x more pressure in the cylinder without the suction throttling loss imposed by the closing of the throttle plate .

Originally Posted by mort

Hello mwebb,
I've studied your graph and read your posting, but I can't tell what you are asking. Please elaborate.
-mort

[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.

[mort]

Originally Posted by mwebb

it would seem the pressure difference can be attributed to change in the ability of the engine to pump air into the combustion chamber

with a closed throttle as opposed to wide open throttle

with extreme suction throttling losses or almost no suction throttling loss

that difference clearly exceeds 5% .

Hello mwebb,
You are confusing pumping losses with throttling. They aren't the same. Your graph dosen't show any information about pumping losses.
-mort

[drmiller100]

I use the terms pumping losses and throttling losses interchangably.

what is the difference?

[Olympiadis]

Originally Posted by drmiller100

I use the terms pumping losses and throttling losses interchangably.

what is the difference?

Given the same throttle position, you can increase spark timing or fuel burn-speed and increase pumping losses independent of the throttle.
Scavenging and reversion also come into play in a running engine.

[mort]

Originally Posted by drmiller100

I use the terms pumping losses and throttling losses interchangably.

what is the difference?

Hello drmiller100,
I do too, sometimes. But in context, mwebb's graph shows that at idle less air gets into the cylinder. That is due to the throttle being closed, and can be called throttling loss. But how much power is required to suck the air in, that is called pumping. It is power from the engine that cannot be used to move the car.

In a text book or even a piece of automotive journalism, pumping refers to the power used to suck air through the throttle. It is quite a slug of power at cruise and if you could eliminate it you'd notice the better FE. But it isn't anything like the power hog some believe. In the amazing graphs posted by Old Tele man, the FE for 1/4 throttle at 4000 rpm is half the FE of full throttle. At that rpm 1/4 throttle makes about 12 or 13 hp but is burning enough gas to make 25. The pumping power might be about 0.6 hp and the other 12(or so) hp is lost thermodynamic efficiency. But many writers seem to believe the opposite and that is one of my pet peeves. For lots of throttled operation the pumping power is a fraction of the power lost to thermodynamic inefficiency. (conventional Otto cycle, CR around 8 or 9, blah blah blah)

There are pumping losses that can be improved and some are really hard to reduce. All the air goes through the little annular space around the intake valve. Not much can be done about that. All the exhaust goes through the exhaust port, and then there is the whole exhaust piping and muffler. But you do your best. The air cleaner is a place where there is room for improvement, but a measurable improvement is going to be big and expensive. Better intake piping? In any case these are mostly important at high power out, and won't make any difference at cruise. And except for that exhaust port, all are an order of magnitude smaller than trying to suck air through the closed throttle. And the piping arangements are designed to improve performance under certain conditions so alterations to reduce pumping may not be any improvement at all.

In the case of an Atkinson cycle, as seen these days (Prius like), under cruise conditions the throttle will be open and the charge is limited by allowing some of the intake mixture to get pushed out the intake valve after the compression stroke has started, after BDC. But if the engine could idle, and you used the same pressure measuring setup in mwebb's graph, then the cylinder pressure at TDC would be the same. The graph would look almost the same. The same lower amount of intake air, but no throttle and no pumping.


-mort

[drmiller100]

Mort, Tomato, tomato. still the same thing.

The throttle, the air filter, the intake valve create restrictions to slow down the air. This creates a vacuum.

To maintain a vacuum with a leak requires work over time, which by definition is POWER.

if you can have less vacuum, it requires less work.

I guess one could argue Atkinson increases pumping losses (shoving the air back and forth across the intake valve) to decrease throttle losses, but when done, it is all exactly the same affect.

[mwebb]

Originally Posted by mort

Hello mwebb,
You are confusing pumping losses with throttling. They aren't the same. Your graph dosen't show any information about pumping losses.
-mort

above is a brief overveiw of what i was showing a trace of in the earlier screen cap , it is combustion chamber pressure over time culled from a large capture with a pressure transducer and a hi res scope . the positive peaks occur at TDC 0 and 720 degrees of crankshaft rotation ----
running compression test
------------------------------
i can take one cylinder out of the equation by disconnecting spark and injector
same conditions

calculated load delta becomes the measurement of the pumping loss and frictional loss of the non contributing cylinder
in percent

compare calculated load , not absolute calculated load but calculated load , now one cylinder is nothing but pumping and frictional loss
the difference exceeds 5% every time
do not take my word for it

do it your self
post your results

KISS

the discussion around thermodynamic efficiency may work real well in the lab and in computer models - so EGR which degrades thermodynamic efficiency should also degrade FE
but
it (EGR when in use )
improves FE ,
by reducing suction throttling losses / pumping losses and by reducing the amount of air fuel mixture per combustion event

i had not considered the difference between pumping and throttling loss - for the purposes of this discussion
is there a difference ?

real world 5% is not the value

I posted some stuff in a thread Doug Miller started about efficiency.
To sum up. Pumping losses are less than 5% of shaft hp upto WOT, where they go toward zero. The loss of efficiency is simply thermodynamics. For an engine throttled down to 20% and a compression ratio of 8.5 the effective CR is 1.7:1. So at full throttle the maximum (Carnot) efficiency would be about 57% but throttled about 20%