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Author Topic: How do you convert g/kWh to MPG roughly?  (Read 429 times)

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bubblegumpi

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How do you convert g/kWh to MPG roughly?
« on: December 07, 2018, 09:38:00 AM »

I was looking up this gigantic dump truck thing and wondered what it's MPG was, Seems like something exxon mobile would want us to daily drive to the grocery store with. But just for fun how much would it cost to go grocery shopping with it? Before we get into the arbitrary gas costs $/g and the trip is 10 miles what are these funny units? Here is a quote from the Wikipedia artile

"
Instead of a single engine, the Siemens MMT 500 drive system is powered by two MTU 65-litre 16-cylinder four stroke diesel engines, each with 2,300 horsepower. These are coupled to two AC alternators and four AC traction motors (two in each axle)Fuel consumption (according to company data) is 198 g/kWh per engine, with option to run on only one if carrying less than capacity loads. Maximum speed is 64 km/h, and economy maximum speed (when fully loaded and on a 10% gradient) is 40 km/h
"

g/KWh is grams of fuel per kilowatt hour? Diesel fuel is around 0.85 g/ml. So you have to figure out how much KWh each motor makes then use that total number times the g/KWh then apply that to its running speed of 64km/h? The hour in km/h is different then a kWh just to make things more confusing right? No crossing out units for conversion simplicity?

Before we get to metric to imperial how do we make sense of this g/kWh business? I bet the thing gets something like 40 gallons to the mile is my rough guess not doing any math, just based on the fact a tank gets 3 gallons to the mile.

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Corribus

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Re: How do you convert g/kWh to MPG roughly?
« Reply #1 on: December 07, 2018, 10:19:44 AM »

It's not really convertable because they are measures of different things. One is a fuel economy (how far a vehicle can travel on a certain amount of fuel) and the other is an emission standard (how much mass of pollutant is emitted per amount of energy consumed). It makes no sense to basically ask if a car gets so many miles per galon, what is its fuel economy in g/kwh?

To do any kind of conversion, you'd need to account for the energy density of the fuel and so forth. From a practical point of view, you also need to know the efficiency with which the vehicle converts the fuel energy to usable energy, and the efficiency with which the car traps pollutant mass (catalytic converters, filters, etc.). I.e., you need more information.

Boiled to its basic SI units, g/kwh is s2/m2 and mpg is m-2. Just from this fact alone tells you that you need more information to do any kind of conversion.

By the way, an interesting tidbit on the units of fuel economy, which has units of inverse area: https://what-if.xkcd.com/11/

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Enthalpy

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Re: How do you convert g/kWh to MPG roughly?
« Reply #2 on: December 07, 2018, 12:16:03 PM »

198 g/kWh could well be a fuel consumption rather than a pollutant emission, because the number is big. At 44MJ/kg for a petrol-based fuel it would mean 41% energy efficiency, a credible figure.

No conversion to mpg, I agree. It depends on how many J the vehicle takes to run 1m.
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