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Old 01-24-2013, 06:58 AM   #1 (permalink)
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BSFC map. What am I looking at?

Just as an example, here a legible BSFC map. What is the scale of the Iso lines and how can they be shown as higher than the torque curve? Do any of the lines necessarily show 100% throttle? I am assuming the 250 line is centered around 95% throttle lets say, and the 275 line is the constant fuel consumption Iso with 100% throttle across the top which happens to blend to the same consumption with lesser torque output and a lesser throttle opening of 85% (or something) around it's lower portion.
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Old 01-24-2013, 12:35 PM   #2 (permalink)
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What is the scale of the Iso lines and how can they be shown as higher than the torque curve?
By scale do you mean 275 whats? - the top of the chart says "BSFC (gr.kW.hr)" that is grams of fuel needed to produce a kilowatt hour of work.
The maximum torque at 3500 rpm is about 158 Nm but the specific fuel consumption is about 270 gr/kw hr. This is how you show that if you only have iso lines at multiples of 25.

Do any of the lines necessarily show 100% throttle?
No.

I am assuming the 250 line is centered around 95% throttle lets say, and the 275 line is the constant fuel consumption Iso with 100% throttle across the top which happens to blend to the same consumption with lesser torque output and a lesser throttle opening of 85% (or something) around it's lower portion.
Kinda like that, the iso lines are not throttle positions. Only in the most general way can you say that the part of an iso line near full output is near full throttle - but otherwise the graph give no information about throttle.

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Old 01-24-2013, 01:19 PM   #3 (permalink)
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Quote:
Originally Posted by mort View Post
By scale do you mean 275 whats?
I understand the 275 as you said it is described as the fuel used per kWh. I also see that the horizontal aspect of the iso lines is rpm. What is the vertical aspect of the iso lines? It can't be torque as it extends above the maximum torque curve on this particular graph although most other graphs always stop at the torque curve.
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X and Y of the bold torque curve are clearly labeled but all of the BSFC maps I have seen never label the Y axis of the iso lines. Unless this graph is just an oddball.
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Old 01-24-2013, 01:47 PM   #4 (permalink)
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Are you sure that's the torqure curve? AFAIK, most cars don't have two torque peaks.
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Old 01-24-2013, 01:55 PM   #5 (permalink)
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Stays kind of high toward redline to be torque doesn't it. Maybe I should just pick a different graph.
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Old 01-24-2013, 02:27 PM   #6 (permalink)
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Quote:
Originally Posted by sendler View Post
What is the vertical aspect of the iso lines? It can't be torque as it extends above the maximum torque curve on this particular graph although most other graphs always stop at the torque curve.
Hi sendler,
Yes, the vertical axis for the fuel consumption iso lines is also torque. In this case Nm = Newton meters. That's fairly common, but you will also see ft lbs. Both are measured at the flywheel. But many graphs show cylinder pressure, BMEP = brake mean effective pressure, in bars or rarely psi. That is convertible to torque.
As I said, the iso lines seem to go above the maximum torque, simply to show what the fuel consumption was on the maximum torque line. It isn't as if the engine could get to that iso line. It's handy to know that at maximum torque at, say 3500 rpm, the fuel consumption is hardly better than 275. But it would be useful different information if that 275 iso line zoomed off showing that at 3500 rpm the fuel consumption was almost 250. Put it another way, showing an iso line outside the engine operating area allows you to determine the bsfc at points where the engine does operate.
-mort
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Old 01-24-2013, 02:58 PM   #7 (permalink)
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Quote:
I wish there was a way to create a "live BSFC map" for drivers that would show & update the engine's current location on the BSFC given the current RPM and engine load. I guess seeing instantaneous MPG is a decent enough indicator, but the map would help conceptually.
YESYESYES

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