We typically see heats of combustion in units of MJ/lb or MJ/mole. When dealing

Question

We typically see heats of combustion in units of MJ/lb or MJ/mole. When dealing with transportation fuels, we also are interested in knowing the combustion energy available for every unit volume (since fuel tanks are typically a fixed volume). Determine the energy density (in units of MJ/L) for the following pure components: n-heptane, cyclohexane, isooctane, benzene, and ethanol. Determine also the specific energy in units of MJ/kg. Sketch a plot of specific energy versus energy density. Include gasoline, natural gas, and iron on your plot. To calculate the heat of combustion for iron, using heats of formation, assuming the combustion reaction is Fe + 3/4 02 rightarrow 1/2 Fe203 Describe the optimal location for a fuel on this plot. What effects do we expect if we switch our cars from being powered by E10 gasoline to being powered by E85? (E10 = 10% ethanol, E85 = 85%). How many kg of C02 are produced per gallon for each fuel? Also, calculate the grams of C02 produced per MJ of energy output.

Explanation / Answer

Energy density is the amount of energy per mass. The greater the value the more beneficail the source is. The common unit of measurement for energy density is megajoules per kilogram (MJ/kg). The energy density of ethanol is about 25 MJ/kg.

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