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Old 04-09-2016, 10:25 AM   #1 (permalink)
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Gasoline energy content calculations - does this look right?

Was just doing some back-of-the-envelope calculations, and had some questions, so I thought I'd post here.

According to Wikipedia, gasoline contains about 42.4MJ/kg of energy, or 33.3kWh/gal. The EPA uses a figure of 33.7kWh/gal for their eMPG numbers, so I'm inclined to trust this.

Wikipedia also claims that the density of gasoline is around 0.74kg/liter (0.71-0.77) or 2.80kg/gal. 42.4MJ = 11.8kWh/kg, * 2.8 = 33.0kWh/gal, which is in the ballpark. My butchering of sigfigs may account for the differences here.

At 50mph, if I'm getting around 100mpg, in one hour I've traveled 50 miles and consumed half a gallon of gas, or 16.7kWh, using 33.3kWh/1gal as my number for how much energy is in a gallon of gas.

We know internal combustion engines aren't 100% efficient, but how efficient is this exactly?

If I've traveled 100 miles in an hour at 100mpg, I would've been drawing 16.7kW for 1 hour. 746w = 1HP, so 16.7kW = 22.4HP. That's before considering that an ICE will only extract maybe 40% of the usable energy from gasoline in ideal situations. So, while 16.7kW of energy might have been released from the combustion of gasoline, only perhaps 6.68kW (optimistically) was actually being used to drive the car, which works out to 8.96HP.

Just for comparison, the EPA rates the Nissan leaf as consuming 30kWh/100 miles. I understand this is "mixed driving", but how the heck does the EPA consider 30kWh/100miles to be 114MPGe, when 16.7kW worth of gasoline is 100mpg?

~

BSFC chart for my car claims that between 1750 and 2250RPM, at roughly 60-80% load, it has a BSFC somewhere between 200 and 215g/kWh.

Using 33.3kWh/gal and 1gal gasoline = 2.80kg, we work out that a kilogram of gasoline contains 11.9 kWh of energy. 11.9kWh/kg is equivalent to 84g/kWh, which would suggest a BSFC of 200 is extracting around 42% of the energy in gasoline, and turning it into mechanical energy.

Another Wikipedia article claims gasoline to have an energy content of 12,200Wh/kg. This is equivalent to 82.0g/kWh, which gives a reasonable "200g/kWh BSFC = 41% efficient", which is around what Honda claimed for this engine.

Do all of my numbers look reasonable?

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Old 04-10-2016, 03:02 AM   #2 (permalink)
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I haven't re-run your numbers to check for accuracy, and don't know much about BSFC. Is that grams per kilowatt hour?

What's the significance of kWh/100miles? EVTV use a metric of Watts per pound-mile. One emphasizes time; the other, weight.
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Old 04-10-2016, 08:22 AM   #3 (permalink)
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Regarding EVs:

https://www.fueleconomy.gov/feg/noframes/34918.shtml

Quote:
EPA MPG
MPGe:Miles per Gallon Equivalent
1 gallon of gasoline=33.7 kWh


Combined MPG:114

City MPG:126

Highway MPG:101

30 kWh/100 miles
^ Was going off of the metrics published by fueleconomy.gov.


Yes, BSFC is grams per kilowatt hour, used to measure the total energy released from the gasoline that is converted into useful mechanical energy.

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Old 04-10-2016, 12:47 PM   #4 (permalink)
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MPGe works well as a shorthand, but it doesn't tell the whole story. Let's look at how the EPA calculates MPGe.

The agency needed some way to compare two very different power sources. Gasoline and electricity use completely different units (gallons versus kilowatt-hours) so you can't just shove them into the same formula. Then someone came up with a very clever solution: If you burned a gallon of gasoline, it would generate 115,000 British thermal units of heat. So, how much electricity would it take to generate the same amount of heat? Answer: 34 kilowatt-hours (kWh). We now have our connection. One gallon of gasoline produces the same amount of energy as 34 kWh.

With this new comparison point, the EPA can now figure out how far an electric car will travel on only 34 kWh. In the case of the Mitsubishi i, that much electricity will carry it for 126 miles. So, it's 126 miles per the equivalent of a gallon of gas. Voila! You have MPGe.

Source
Electric cars 101: What does MPGe mean, exactly? - CSMonitor.com
So your car has a rating of 179 MPGe

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