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Old 12-10-2007, 07:36 AM   #1 (permalink)
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Pulse and Glide Fuel Economy Calculator 4 valve / Cylinder

Attached is my updated fuel economy calculator for generic 4 valve EFI engines. I've tidied it up a bit, and automated it so that most people here should be able to figure out how it works - I've really dumbed it down this time, no looking up BSFC values, you don't even have to google an air density chart, all you need is temperature and altitude. I assume dry air, but I did the calculations to adjust density for humidity and decided not to bother. Even at 30 deg C, and 100 degrees C, the difference is only 1.6% in air density. At 25 degrees, it's only 1.1%.

Note that it is available either as an ODS file in a nod to open formats, or in reverse engineered xls (MS Excel format). Either should work. Created in Ubuntu using Openoffice. Note that EVERYONE can use this spreadsheet for free, whether they have Windows, Mac or Linux. I suggest trying either gnumeric or openoffice.

Figures you will need to know:
Average speed during pulse and glide (assumes constant acceleration)
Total drop in speed until the next pulse starts
rpm@100kph
Max Power of your car (just google it)
Drag Coefficient (google it, or do a coast down test)
Frontal Area
Coefficient of Rolling Resistance (use 0.06 for heavily pumped tyres)
Total Mass of Car (include driver etc)
Temperature
Altitude above sea level
Displacement of engine (or the idle fuel burn rate)

Note that you can unhide the rows to see my workings out and verify my equations, which I'd strongly encourage. Read my provisos as well.

Attached Files
File Type: zip pg4vlvods.zip (60.6 KB, 120 views)
File Type: zip pg4vlvxls.zip (54.3 KB, 128 views)
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Last edited by newtonsfirstlaw; 12-11-2007 at 05:27 PM..
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Old 12-10-2007, 06:18 PM   #2 (permalink)
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No comments yet? Let me know if anyone desires a two valve per cylinder version. It's a bit of a pain because the BSFC is not linear along a line of best fit over the best BSFC/rpm curve. The 4 valve per cylinder model was a gift.
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Old 12-10-2007, 06:23 PM   #3 (permalink)
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I just got a chance to play with it, very cool! Seems like gearing makes a large difference in my case (as I would expect).

Think about making a web version?
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Old 12-10-2007, 06:45 PM   #4 (permalink)
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Quote:
Originally Posted by SVOboy View Post
I just got a chance to play with it, very cool! Seems like gearing makes a large difference in my case (as I would expect).
Are you sure it's the gearing? Gearing is basically rpm @100kph. Did you change it from the base 4200?

My car is insanely highly geared (the defaults are for my car, and me), and a best case gearing for my car would be financially impractical IMO, the best I could do would be to increase tire radius by 15%, which might net a 9% gain at 100kph, lessening as I go slower. Going the whole hog would net me about 15% from memory.

Easier and cheaper to just take 20% longer to get to my destination, and travel at 80-90kph, and 100kph when I can't help it. At that speed, everything is working for me. Gearing is better, drag is way down, coasts are nice and long. It feels like riding around on a bike!

I can't help but think that there ought to be dedicated slow lanes on highways, where people can travel 80kph and not be pestered. It would open things up to vehicles like the Honda Cub etc.

If businesses were granted a fixed number of allotments where people could come to work and leave during morning and evening rush hours, traffic would be vastly improved and more efficient highway speeds would not result in more congestion. I used to hate the nanny state, but in this day and age it makes sense. At the moment the general population is behaving like a bunch of idiots, having feasts at the beginning of winter with the seed grain and the vegetables in the root cellar.

I wouldn't mind, but I happen to be sharing the same house with those idiots.

Anyway, I was thinking that gearing losses at speed might be useful. It would just be Loss = 100*[BSFCcurrent - BSFCbase)/BSFCbase]
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Think about making a web version?
Not really, I like the open nature of ODF, and the fact that other people can double check my calculations (i.e. basic Open Source theory) so that other people can build on my work and everyone can benefit. Of course, someone else is free to do so. The equations are there.
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Old 12-10-2007, 06:49 PM   #5 (permalink)
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Interesting observations indeed. Of course speed was a huge factor for my car (more so than acceleration), but I was surprised at how much difference playing with gearing at 100kmh while driving at 100kmh changed things around.

The only problem with dedicated slows lanes is that they would have to be cross by traffic to get in/off ramps and things, where the speed difference might make things unsafe, I think.
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Old 12-10-2007, 06:54 PM   #6 (permalink)
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Quote:
Originally Posted by newtonsfirstlaw View Post
Let me know if anyone desires a two valve per cylinder version.
Um, yes please. Will try out the 4-valve version shortly...

Ben: you've got 3 valves per, don't you?
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Old 12-10-2007, 07:04 PM   #7 (permalink)
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Quote:
Originally Posted by SVOboy View Post
Interesting observations indeed. Of course speed was a huge factor for my car (more so than acceleration), but I was surprised at how much difference playing with gearing at 100kmh while driving at 100kmh changed things around.
One thing I forgot to add, this assumes that we are accelerating using 62% of available power, whatever throttle that is at. Deviate too far either way and there will be FE penalties. See the BSFC graph. Pick an rpm, and vary the load on the engine. The BSFC is efficiency, lower = better.
Quote:
The only problem with dedicated slows lanes is that they would have to be cross by traffic to get in/off ramps and things, where the speed difference might make things unsafe, I think.
I think this is done in Germany already. Might be better to just slow down everybody to about 50mph/80kph. I almost can't believe I'm writing this - I used to go a bit above the speed limit religiously. But that's a very good compromise speed - lions share of the benefit, and still a fast speed.
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Old 12-10-2007, 07:07 PM   #8 (permalink)
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I prefer slowing down to 285 kmh, by train!

Anywho, Darin, I see that you're making a joke, I just don't get it,
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Old 12-10-2007, 07:21 PM   #9 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Um, yes please. Will try out the 4-valve version shortly...
Urg. That's a bit of work. If someone feels like printing out the 2 valve engine map in the spreadsheet, drawing a line between (2000, 102) and (5200, 125), marking every gradation from 0.40 (estimated) at (2000,102) and 0.565 (estimated) at (5200, 125), i.e. 0.40, 0.41, 0.42, ... 0.56 by interpolating between the lines, and measuring where they are in terms of rpm, I'd be more than happy to write a lookup function to lookup the correct value.

I might do it later sometime though.

Until then, a useful kludge would be to go into the hidden columns and add 0.05 to the BSFC formula, for rpm at actual speeds greater than 3000rpm
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Old 12-10-2007, 07:27 PM   #10 (permalink)
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I'm OK with the kludge, too. Will report in later.

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