[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