What you need to know about Brake Specific Fuel Consumption

[user removed]

Consider this a hypothetical situation.

What would be the difference in power produced by the same engine in these two scenarios.

Throttle plate positioned to maintain half of atmospheric pressure.
Throttle plate wide open at an altitude, like 18,000 feet where atmospheric pressure is 50% of sea level.

The difference in power produced between these two scenarios would define the loss due to throttle restriction.

Aircraft engines have had this scenario since planes were capable of reaching 18,000 feet altitude (I did not try to figure out the exact altitude where atmospheric pressure is precisely 50% of sea level).

My point is the principle cause of inefficiency is due to lower in cylinder compression. This is well defined by the different altitude scenario.

Best BSFC is a combination of the ideal RPM combined with the lowest manifold vacuum.
Other than that the atmospheric pressure available is another factor.

Losses are due to throttle restriction (the example above), which I think is fairly minor, heat loss, friction, and the energy required to overcome the changes in velocity inherent in all every reciprocating engine.

Grind two opposing lobes on the camshaft and you have a pump, suction, discharge.

Old timers used to do this with many engines to make them a high capacity pump. Take half the cylinders and grind two lobes on the cam to make half the engine an air pump.

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

[drmiller100]

I know how to measure this.

someone needs to do a coast down test. Engine off. Car in gear with a manual transmission.

One test is throttle at idle, starting at 4000 rpm.
One test is throttle WOT, starting at 4000 rpm.

[user removed]

Originally Posted by drmiller100

I know how to measure this.

someone needs to do a coast down test. Engine off. Car in gear with a manual transmission.

One test is throttle at idle, starting at 4000 rpm.
One test is throttle WOT, starting at 4000 rpm.

Third test pull the plugs out.
Fourth test coast in neutral.

regards
mech

[gone-ot]

plugs out = motoring friction
neutral = rolling friction

[mort]

Ugh, I hate doing this. Usually I can abide by the old saw that if you can't say anything nice don't say anything. But this thread has gone way stupid lately and I feel compelled to comment. So I'll get the nasty part out of the way first. A couple of commenters seem to object to my position that in a normally aspirated Otto cycle engine with a throttle, that at part throttle power operation, like cruise conditions, there are 2 major causes for the drop in efficiency compared to full throttle. There are pumping losses, that is there is power needed to pull the charge into the cylinder and that power, which could have been used to propel the car, is instead wasted in suction across the throttle. And second, a loss of efficiency due to the charge in the cylinder being sparse, the sparse charge results in lower pre-ignition temperature and just like running an engine with a lower compression ratio, the thermodynamic efficiency suffers. My claim is that the thermodynamic efficiency drop swamps pumping power. I will call this view the "reality position."

The 2 main complaints seem to be that I called pumping loss and throttling loss different things (I did!) And something else that I can't decipher. It seems that there is a belief that pumping losses are really important. What I see here is either there is some dishonesty, or ignorance, or maybe a combination: dishognorance. So I will call the other point of view the "dishognorance position."

Now lets get in the time machine and see where this started... suspectnumber961 posted a link to an article form 2008: AutoSpeed.
In the article is this quote:

So what accounts for this terrible decrease in SFC at just the throttle openings the engine will be used at most often? ‘Throttle’ is the key word here – as the engine is increasingly throttled, it has to work harder and harder at drawing air past the throttle blade. This is the reason that there is a measurable vacuum after the throttle blade – the engine is trying to drag in more air than it is being permitted to. Each time a piston is descending on the intake stroke, it’s having to do this extra work. Working internally hard as a vacuum pump means there’s less power available at the flywheel...

This work against the throttle restriction is referred to as ‘pumping losses’.

I objected to this, complaining:

There is a pretty good discussion too, but they ascribe low power inefficiency to pumping losses. Which, I admit is common, but wrong.

And was asked to explain. Does anyone accept that the AutoSpeed article's explanation for loss of efficiency at part throttle being only due to pumping is complete?

Now more specifically about objections offered. Mwebb offers some graphs of cylinder pressure which show that less air gets into the cylinder at idle than at wide open throttle. And then tries to claim that the difference in peak pressure at TDC is somehow representative of pumping power. As I showed, if you used an Atkinson type engine in the same experiment you would get the same graph but no pumping loss. Mwebb posted a different graph showing the same thing and then claiming that "calculated load" was also a good representation of pumping power. It isn't. Calculated load is used to compare volumetric efficiency.
I don't really care if throttle loss and pumping power are used to mean the same thing, if they are used to mean either one, but don't use them to mean something else. If you mean volumetric efficiency, say that, if you mean compression pressure say that.
(I edited this following paragraph)
Pumping loss is a real thing, it can be computed or measured or felt in the seat of your pants. And here I get to drmiller100, and so his suggestion to compare WOT to idle coast down is referring to what is called compression braking. In the ideal case consider the piston is at TDC starting the intake stroke, the throttle is closed completely, so the piston pulls a near vacuum in the cylinder down to BDC then the pressure in the crankcase, 1 atm, pushes the piston back up. Assuming no leakage that is a wash. Now the power stroke and the piston again draws a vacuum in the cylinder. At BDC the exhaust valve opens Pop! air rushes in and then gets pushed out again. Then the cycle starts again. This is the highest pumping loss an engine can have as all the work is thrown away when the exhaust valve opens. In a running engine (ie. actually producing power) some of the suction is reclaimed in the power stroke.
(I had some stuff about Diesel engine braking and got all messed up cutting and pasting intake and exhaust and TDC and BDC = gibberish. Maybe I'll try again later.)

This situation of the engine being rotated by another power source, gravity or inertia for this truck, by an electric motor in a dyno test, is called "motoring." In motoring a Diesel with a Jake Brake or an Otto cycle with the throttle closed the power produced is negative and is the sum of all the mechanical loads, like friction and ancillaries, plus the pumping power.

serialk11r makes a good point about using BMEP, So here is the formula for finding power output given BMEP
P = (BMEP X displacement X RPM) / (60 X 2).
(watts = pascals X cubic meters X Revs per Sec / 2 revs per power cycle)
The formula assumes 100% volumetric efficiency, engines don't develop high VEs in motoring, but most engines show better than 100% VE at some output levels. We will assume 100% VE for engine braking because all the pumping power is wasted.
In motoring against a closed throttle you may use a BMEP of 1 atm = 101 kPa, mwebb's engine is 2 liter = .002 cubic meters. The results are so sad I can't show them. But in the case of an engine powering a car the pumping power isn't all thrown away, it's wasted at approximately the ratio of effective compression ratio at closed throttle compared to mechanical CR. That's the amount of the rarefied charge's pressure that can be recovered during the expansion stroke. It's about the same number as volumetric efficiency for the closed throttle. So if pumping power at idle in a motoring test was 2000 W and the VE at idle was 20% then the recovered suction is 20% so about 1600 W are lost. VE at 1/4 or 1/2 throttle are higher and so more of the suction effort is recovered. And the suction force, manifold vacuum, is lower than idle.
Consider the graphs posted by Old Tele man, in particular the 1/4 throttle at 4000 rpm points. Looking at the graphs about 13 hp are produced at a SFC of 0.9 lb/hp-hr. WOT makes about 60 hp and SFC is about 0.45. So about 20% as much power but about 40% as much fuel. Since the A:F will be about the same this says that the 1/4 throttle setting is using 40% as much air as at WOT. Assuming 1 L displacement, BMEP at 1/4 throttle is about 294 kPa (13 hp) manifold pressure cannot be higher than 101 kPa - 294 kPa / 8 = about -64 kPa (CR 8:1) Now since the chart is in hp I'm converting to conventional units. 64 kPa = 1337 lb/sq ft (about 9.3 psi) and air flow is about 40% of 1 liter X 4000 rpm / 2 or about 13 l/s = 0.47 cu ft/sec
1337 X 0.47 = 628 ft lb/sec 628/550 = 1.1 hp. That's approximately the pumping power. Oops! that's 8% of power output. I'm so sorry I ever said not more than 5%. Where did the other 10 hp go?
(EDIT, also that the graphs pointed out by Old Tele man are from the same article in AutoSpeed)

-mort

[gone-ot]

...for those who don't know it, the graphs that I posted both come directly FROM the Autospeed article that is the subject of this post:

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[serialk11r]

Originally Posted by Old Mechanic

Consider this a hypothetical situation.

What would be the difference in power produced by the same engine in these two scenarios.

Throttle plate positioned to maintain half of atmospheric pressure.
Throttle plate wide open at an altitude, like 18,000 feet where atmospheric pressure is 50% of sea level.

The difference in power produced between these two scenarios would define the loss due to throttle restriction.

Aircraft engines have had this scenario since planes were capable of reaching 18,000 feet altitude (I did not try to figure out the exact altitude where atmospheric pressure is precisely 50% of sea level).

My point is the principle cause of inefficiency is due to lower in cylinder compression. This is well defined by the different altitude scenario.

Best BSFC is a combination of the ideal RPM combined with the lowest manifold vacuum.
Other than that the atmospheric pressure available is another factor.

Losses are due to throttle restriction (the example above), which I think is fairly minor, heat loss, friction, and the energy required to overcome the changes in velocity inherent in all every reciprocating engine.

Grind two opposing lobes on the camshaft and you have a pump, suction, discharge.

Old timers used to do this with many engines to make them a high capacity pump. Take half the cylinders and grind two lobes on the cam to make half the engine an air pump.

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

I had never thought of it this way, and you make a good point. Thank you for bringing it up.

If I'm not mistaken though, airplanes have better BSFC at higher altitudes...I am not sure of this though. But the colder intake charge, and greater pressure ratio should improve the thermodynamic efficiency right?

I take back what I said about the "compression ratio" changing. It makes no sense if you think about it in the airplane context. I guess there is a difference, that at high altitudes the air is colder because the throttle effectively heats up the air slightly.

However I know that compression and combustion temperature are important factors in combustion efficiency...idle engines do produce the most emissions afterall.

mort, I think you are right that the combustion efficiency is affected by part loads, but I don't know if it's the main cause for the drop in efficiency...perhaps sometime someone should try to see what BSFC maps would look like if it were just friction/throttling causing the problems. Friction alone seems to consume a very large proportion of power when operating at low loads, so I feel like most of the effects should be attributable to friction. Maybe when I have some time I'll compute a few points using thermodynamic cycle efficiency, friction, and pumping together. If the efficiency drop off isn't as much as in real engines, then it would be reasonable to conclude that combustion efficiency has some significant effect.

[mwebb]

Mwebb offers some graphs of cylinder pressure which show that less air gets into the cylinder at idle than at wide open throttle. And then tries to claim that the difference in peak pressure at TDC is somehow representative of pumping power. As I showed, if you used an Atkinson type engine in the same experiment you would get the same graph but no pumping loss. Mwebb posted a different graph showing the same thing and then claiming that "calculated load" was also a good representation of pumping power. It isn't. Calculated load is used to compare volumetric efficiency.

(1)we are not speaking of an atkinson cycle engine
(2) delta of calculated load under similar conditions DOES reflect the actual difference in fuel consumption assuming the system is operating at stoich , so yes
(using a wee bit of common sense )
you can use calculated load value for more than VE values
and
calculated load will NOT be 100% at all rpm and load conditions even at WOT

for example
look at calculated load with no throttle input and nothing running except the engine ,
then switch on AC and set heater blower to max , switch on hi beams
calculated load will increase significantly ,
simple math will show how much more fuel is used with accessories running or not running .

(2) if peak pressure at TDC is not an indication of how well or poorly the engine system can pump air what is it showing then ?
no intent to start a pissing match here -

(4) EGR is used on many engines used by people in this forum
EGR works to improve FE - EGR degrades thermodynamic efficiency -


we can measure VE

we can use calculated load for many things , including showing an under reporting MAF sensor - these are just distractions , i am attempting to focus on what is in bold

[drmiller100]

the cylinder is only partly full at idle. This is why the engine is at idle. if it were at WOT, the pressures inside the cylinder would be much higher, and the engine would be trying to accelerate.

My argument is it takes energy to create a change in pressure. If we run a compressor, with a leak, and the compressor is running, are you arguing the compressor is not using energy to compress the air which is leaking out?

Likewise, an intake manifold. We have a change in pressure, with a leak called the idle valve. We use energy to maintain that change in pressure.

If the leak were to stop, we could create a perfect vacuum, and energy needed would drop towards zero.

Some of us learn by arguing. Nothing personal. And I'm not TRYING very hard to be obtuse. It comes naturally to me.

[drmiller100]

ok, bear with me. I'm trying to figure out throttling losses.
Assume atmosphere is 14.7 psi.
Assume 5 psi absolute at idle (guess!) inside the intake manifold.

Assume size of engine is 200 cubic inches.
Assume 1000 rpm.

from here
Horsepower required to Compress Air

it shows .05 horsepower for one cubic foot per minute.

One cubic foot is 1728 cubic inches.

so, we have
.05 hp * 1000 rpm * 228 cubic inches / 1728

is 6.5 horsepower. Which passes the big picture bs test.

Interesting is the fact that a 100 cubic inch engine takes half the power - smaller engines burn less gas at idle.

Duh.