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?
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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?