# Gasoline and diesel fuel: Carbon pricing and heating values

## Gasoline and diesel fuel: Carbon pricing and heating values

### Introduction

Shortly after New Year’s Day 2017 a radio news item noted that the Province of Ontario’s newly introduced cap and trade1 system for carbon pricing would result in higher pump prices for gasoline and diesel fuel. The increase for diesel fuel would be greater on a per litre basis. Why is this? It is because diesel fuel produces more carbon dioxide on a per litre basis. Diesel fuel also produces more heat energy on a per litre basis when burned. These facts provide teachers with a lead-in to some stoichiometry and thermochemistry calculations, as well as to the concept of carbon pricing. These engineering topics will provide several opportunities for unit conversion exercises.

According to the US Energy Information Administration: “About 19.64 pounds of carbon dioxide (CO2) are produced from burning a gallon (US) of gasoline that does not contain ethanol. About 22.38 pounds of CO2 are produced from burning a gallon of diesel fuel.”2 (See Question 1.)

Gasoline1 and diesel fuel1 are complex mixtures of hydrocarbons, mainly alkanes, and additives. The actual composition of a fuel can vary widely, due to feedstock variability, refinery capability, supply and demand factors, climate and seasonality. For this reason, statements about ‘gasoline’ or ‘diesel fuel’ properties must be understood as average or typical values only.

Petroleum diesel Ethanol-free gasoline
Density 0.832 kilograms per litre
(6.943 pounds per US gallon)
0.745 kilograms per litre
(6.217 pounds per US gallon)
Net heating value 43.1 Megajoules per kilogram 43.2 Megajoules per kilogram
Volumetric energy density

35.86 Megajoules per litre
128,700 British Thermal Unit per US gallon

32.18 Megajoules per litre
115,500 British Thermal Unit per US gallon
CO2 emissions 73.25 grams per Megajoules 73.38 grams per Megajoules

Data from Wikipedia article diesel fuel - the information as of 2010.

The table data shows that diesel and gasoline are very close in heating value (energy content) on a mass basis. This is because the carbon content of diesel and gasoline are very similar; although gasoline alkanes range from C4 to C12, while diesel alkanes range from C10 to C15. The carbon percentage of octane (C8H18), a stand-in for gasoline, and hexadecane (C16H34), a stand-in for diesel, are 84.2% and 85.0% respectively. The amount of carbon dioxide produced from gasoline and diesel and therefore the carbon pricing, are also both very similar on a mass basis. However, the straight chain higher C-number liquid alkanes in diesel pack more densely than the branched chain lower C-number alkanes in gasoline. The difference in liquid densities is some 11%. Diesel produces 11% more heat energy and 11% more carbon dioxide than gasoline on a volume basis. Using the data in the table, one can calculate that diesel produces 2.627 kg of carbon dioxide per litre burned, and gasoline 2.361 kg/L (see Question 2). Transport fuels are pumped, measured and carbon priced by volume, and fuel efficiency is measured using volume units as mpg or L/100 km. For this reason, the carbon price of diesel at the pump will be about 11% greater than that of gasoline. It is interesting to note that diesel engines have an advantage of 11% in fuel efficiency before any other performance factors are considered.