What you need to know about Brake Specific Fuel Consumption

[suspectnumber961]

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And now we have the killer. Here each dot shows the speed and load for a typical mid size car at 1 second intervals during the US fuel economy test. Of the time the car takes to do the test, just 5 seconds are in the island of best BSFC. Quite a few of the dots (the authors say that they overlay) are at worst BSFC – idling at zero load with the car stationary!




So...this is HOW driving techniques show their advantage?

[Ecky]

I believe this is posted in a sticky'd thread in General Efficiency, but it's interesting and probably worth posting again for those who haven't seen it.

[PaleMelanesian]

A good reminder. This is how/why Pulse & Glide works. You use the engine in the red zone, then off (or idle) until you need it again. You make the most output per fuel input by running it at the peak efficiency part-time, rather than medium efficiency all the time.

[oil pan 4]

Messing with the final gearing will move most of that cluster up and to the left.

[mort]

Originally Posted by suspectnumber961

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So The graphs are nice and really pretty. There is a pretty good discussion too, but they ascribe low power inefficiency to pumping losses. Which, I admit is common, but wrong. They also use the term "pseudo-CVT" referring to the Prius. What's that about? So I'd say a good discussion by somebody who didn't know what they were talking about.

-mort

[Daox]

Originally Posted by mort

So The graphs are nice and really pretty. There is a pretty good discussion too, but they ascribe low power inefficiency to pumping losses. Which, I admit is common, but wrong. They also use the term "pseudo-CVT" referring to the Prius. What's that about? So I'd say a good discussion by somebody who didn't know what they were talking about.

-mort

So if low power inefficiency isn't due mainly to pumping losses what is it due to?

He probably just used pseudo-CVT since the Prius CVT is quite different from a conventional CVT and didn't want to confuse readers. The guy owns a 1st gen Prius and is quite knowledgable about how they work. He has written several articles on and about the car.

[mort]

Originally Posted by Daox

So if low power inefficiency isn't due mainly to pumping losses what is it due to?

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%

Originally Posted by Daox

He probably just used pseudo-CVT since the Prius CVT is quite different from a conventional CVT and didn't want to confuse readers. The guy owns a 1st gen Prius and is quite knowledgable about how they work. He has written several articles on and about the car.

Well, that's good to know. You can tell, I bristle whenever it looks like someone is disrespecting Prius engineering.

-mort

[mwebb]

here is a real live running compression waveform
1996 BMW
idle then WOT snap followed by decel back to idle , pressure amplitude in psi in the vertical scale , time in horizontal scale

engine is running but there is no combustion in this cylinder as pressure transducer is threaded into spark plug thread .
injector is disconnected

pressure peak at WOT exceeds 4x pressure peak at idle
the pressure transducer is calibrated to local ambient baro so 0 psi = local ambient baro

this real time test seems to disagree with your theory regarding pumping losses ....
real world EGR gains to FE also disagree

why ?




[QUOTE=mort;288351]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%

[mort]

Originally Posted by mwebb

this real time test seems to disagree with your theory regarding pumping losses ....
real world EGR gains to FE also disagree

why ?

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

[gone-ot]

...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)!