Measuring fuel by energy content instead of volume
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Joined: Nov 2006
Posts: 127
From: Great White North
Measuring fuel by energy content instead of volume
I ran into a discussion of another board titled "How should the MPG of electric cars be measured?" which got me thinking, and led to the following reply. Not only would this way allow people to directly compare between vehicles with different power sources (ie: gasoline/diesel/electricity) but it would also show the differences between burning (for example) straight gasoline vs a gas/ethanol mix.
The original thread was brought about by this article: http://money.cnn.com/2008/09/15/auto...ings/index.htm
What do you guys think?
Originally Posted by JCLW
In my opinion:
The fundamental problem is that traditionally we're measuring the wrong thing. We should be measuring Energy (kW), not Volume (litres or gallons).
It takes a certain amount of Energy to move my car 100km. When I go to fill it up, I'm buying energy. I don't really care how much space the energy takes (within reason of course).
Energy content of common liquid fuels:
Diesel: 41 MegaJoules/litre = 11.4 kiloWatthours/litre
Gasoline: 32 MJ/l = 8.9 kWh/l
E10: 28 MJ/l = 7.8 kWh/l
E85: 22 MJ/l = 6.1 KWh/l
E100: 20 MJ/l = 5.6 kWh/l
I drive a Honda Fit, which we'll say (for example) can run on gasoline or E10.
The fuel tank holds 40 litres, which is 356kWh of gasoline, or 312kWh of E10.
My lifetime fuel economy is roughly 7.3 litres/100km using gasoline, or 65.0kWh/100km.
So I can run 547km on a tank of gasoline, or 480km on a tank of E10.
When I go to fill up, I see one station selling gasoline for $1.20/litre = $1.20/8.9kWh = $0.135/kWh
Across the street, I see another station selling E10 for $1.15/litre = $1.15/7.8kWh = $0.147/kWh
Obviously the gasoline is a better deal when you look at energy content/volume, even when $/volume is more expensive.
Now let's say electricity costs $0.120/kWh. With an 88% efficient charging system your cost to put electricity in the car is $0.136/kWh. In terms of price per energy, you may notice that this is almost exactly the same price as gasoline.
But the electric drive system is much more efficient then the internal combustion engine. Using rough figures from the Prius PHEV (Plug-in Hybrid Electric Vehicle) kits, let's say my Fit is offered with a PHEV system that runs 50km on a 5kWh battery pack. So on electric drive I would get 10kWh/100km.
This is 6.5 times more efficient then the internal combustion engine.
So how do I think the marketing should work?
Gasoline Fit should read:
Energy efficiency (1.5l spark ignition engine): 65.0kWh/100km
Energy capacity / range (gasoline): 356kWh / 547km
Energy capacity / range (E10): 312kWh / 480km
Energy capacity / range (E85): 244kWh / 375km *(This line only if the engine could run on E85)*
Diesel Fit should read (if one existed):
Energy efficiency (1.2l compression ignition engine): 62.0kWh/100km
Energy capacity / range (diesel): 456kWh / 735km
Hybrid Fit should read:
Energy efficiency (1.5l spark ignition engine): 65.0kWh/100km
Energy capacity / range (gasoline): 356kWh / 547km
Energy capacity / range (E10): 312kWh / 480km
Energy capacity / range (E85): 244kWh / 375km
+
Energy efficiency (electric motor): 10.0kWh/100km
Energy capacity / range (electricity): 5kWh / 50km
Electric Fit should read:
Energy efficiency (electric motor): 10.0kWh/100km
Energy capacity / range (electricity): 16kWh / 160km *(I'm assuming an electric only car would have a bigger battery pack)*
Also, I believe gas stations should have to list guaranteed energy content for fuel, and sell it by $/kWh.
So you'd see signs such as:
Diesel (41MJ/l) = $0.119/kWh
Regular 85 (32MJ/l) = $0.139/kWh
Supreme 89 (32MJ/l) = $0.147/kWh
E10 (28MJ/l) = $0.143/kWh
E85 (22MJ/l) = $0.145/kWh
This would be a big change for a lot of people, but it is the only way I can see to accurately compare different kinds of cars. It also stops gas stations from selling blended fuel for the same price as gasoline (and then people wonder why their fuel economy drops).
And you could still do it in terms of MJ/gallon & kWh/mile or mile/kWh or whatever.
The fundamental problem is that traditionally we're measuring the wrong thing. We should be measuring Energy (kW), not Volume (litres or gallons).
It takes a certain amount of Energy to move my car 100km. When I go to fill it up, I'm buying energy. I don't really care how much space the energy takes (within reason of course).
Energy content of common liquid fuels:
Diesel: 41 MegaJoules/litre = 11.4 kiloWatthours/litre
Gasoline: 32 MJ/l = 8.9 kWh/l
E10: 28 MJ/l = 7.8 kWh/l
E85: 22 MJ/l = 6.1 KWh/l
E100: 20 MJ/l = 5.6 kWh/l
I drive a Honda Fit, which we'll say (for example) can run on gasoline or E10.
The fuel tank holds 40 litres, which is 356kWh of gasoline, or 312kWh of E10.
My lifetime fuel economy is roughly 7.3 litres/100km using gasoline, or 65.0kWh/100km.
So I can run 547km on a tank of gasoline, or 480km on a tank of E10.
When I go to fill up, I see one station selling gasoline for $1.20/litre = $1.20/8.9kWh = $0.135/kWh
Across the street, I see another station selling E10 for $1.15/litre = $1.15/7.8kWh = $0.147/kWh
Obviously the gasoline is a better deal when you look at energy content/volume, even when $/volume is more expensive.
Now let's say electricity costs $0.120/kWh. With an 88% efficient charging system your cost to put electricity in the car is $0.136/kWh. In terms of price per energy, you may notice that this is almost exactly the same price as gasoline.
But the electric drive system is much more efficient then the internal combustion engine. Using rough figures from the Prius PHEV (Plug-in Hybrid Electric Vehicle) kits, let's say my Fit is offered with a PHEV system that runs 50km on a 5kWh battery pack. So on electric drive I would get 10kWh/100km.
This is 6.5 times more efficient then the internal combustion engine.
So how do I think the marketing should work?
Gasoline Fit should read:
Energy efficiency (1.5l spark ignition engine): 65.0kWh/100km
Energy capacity / range (gasoline): 356kWh / 547km
Energy capacity / range (E10): 312kWh / 480km
Energy capacity / range (E85): 244kWh / 375km *(This line only if the engine could run on E85)*
Diesel Fit should read (if one existed):
Energy efficiency (1.2l compression ignition engine): 62.0kWh/100km
Energy capacity / range (diesel): 456kWh / 735km
Hybrid Fit should read:
Energy efficiency (1.5l spark ignition engine): 65.0kWh/100km
Energy capacity / range (gasoline): 356kWh / 547km
Energy capacity / range (E10): 312kWh / 480km
Energy capacity / range (E85): 244kWh / 375km
+
Energy efficiency (electric motor): 10.0kWh/100km
Energy capacity / range (electricity): 5kWh / 50km
Electric Fit should read:
Energy efficiency (electric motor): 10.0kWh/100km
Energy capacity / range (electricity): 16kWh / 160km *(I'm assuming an electric only car would have a bigger battery pack)*
Also, I believe gas stations should have to list guaranteed energy content for fuel, and sell it by $/kWh.
So you'd see signs such as:
Diesel (41MJ/l) = $0.119/kWh
Regular 85 (32MJ/l) = $0.139/kWh
Supreme 89 (32MJ/l) = $0.147/kWh
E10 (28MJ/l) = $0.143/kWh
E85 (22MJ/l) = $0.145/kWh
This would be a big change for a lot of people, but it is the only way I can see to accurately compare different kinds of cars. It also stops gas stations from selling blended fuel for the same price as gasoline (and then people wonder why their fuel economy drops).
And you could still do it in terms of MJ/gallon & kWh/mile or mile/kWh or whatever.
What do you guys think?
Member
Joined: Feb 2008
Posts: 1,049
From: lake worth FL
I have looked at this for plug in's.
IMHO it's best to keep it in MPG when talking cars. It's the true std for now
Find out the kw-hrs/mile is and the cost per kw-hr.
Once you have cost per mile, convert that to MPG based on the going cost of gas.
I did this for the GM Volt and it's around 220mpg with ele at $.15/kw-hr and gas at $3.5/gal
IMHO it's best to keep it in MPG when talking cars. It's the true std for now
Find out the kw-hrs/mile is and the cost per kw-hr.
Once you have cost per mile, convert that to MPG based on the going cost of gas.
I did this for the GM Volt and it's around 220mpg with ele at $.15/kw-hr and gas at $3.5/gal
Member
Joined: May 2008
Posts: 174
From: Ottawa, Canada
That's pretty interesting. I've read that this line of thought is precisely why most metric countries measure fuel efficiency in L/100 km, and not km/L. I myself don't like using it; it's awkward to say, and you want to have *lower* numbers, which goes against most performance instincts where higher/bigger is better.
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