Comparing Diesel to Gasoline
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Percent market share of new gasoline- and diesel-powered light-duty vehicles in the European Union. The remaining few percent are powered by electricity, natural gas, or unidentified sources. Commission of the European Communities 2006.
Diesel fuel is denser than gasoline (petrol) and contains about 11% more energy per volume, yet both fuels cost about the same per volume to extract and refine. Diesel engines are also inherently more efficient than gasoline engines in converting the energy in the fuel into mechanical work because they operate at higher pressures (higher compression ratios) and temperatures. Altogether, diesel engines obtain roughly 40% higher fuel efficiency per volume of fuel than gasoline engines of the same power. [1] Large trucks and agricultural vehicles have diesel engines to take advantage of this higher efficiency. Many countries, in consideration of the role of such vehicles in their economy, do not tax diesel fuel as heavily as gasoline.
Still, small diesel engines have been plagued with performance issues such as being noisier, generating more vibrations, being more difficult to start, emitting thick black smoke in their exhaust, and exhibiting slower acceleration than their gasoline counterparts. Technical advances during the last few decades—principally, computer-controlled electronic ignition and turbocharged direct fuel injection—have largely overcome these performance issues. In response to these improvements and the rising costs of petroleum fuels, the market share of diesel-powered light-duty vehicles in the European Union has doubled during the past decade and now accounts for half of all new vehicles.
Comparing the greenhouse gas emissions of diesel-powered and gasoline-powered light-duty vehicles is tricky. Diesel fuel contains 11% more energy per volume of fuel combusted but also releases 15% more CO2 per volume of fuel combusted. [2] Diesel engines weigh more than gasoline engines of the same power because diesels require a larger cylinder displacement (volume swept by the pistons) for complete combustion and heavier components to withstand higher pressures and temperatures. In small vehicles, the engine contributes a greater percentage of the total vehicle weight, and the weight penalty of a diesel engine diminishes its advantage in fuel efficiency from 40% to approximately 20%. Moreover, people tend to purchase diesel-powered vehicles with a larger engine (compared to the size of a gasoline-powered engine) because of diesel’s higher fuel efficiency. [3] The net result is that the higher fuel efficiency of diesel engines in light-duty vehicles sometimes barely compensates for their higher CO2 emissions, heavier weights, and larger cylinder displacements: In total, therefore, diesel-powered light-duty vehicles emit around 5% to 30% less greenhouse gases per distance traveled than their gasoline equivalents.
Diesel and gasoline vehicles have larger differences in their emissions of other atmospheric pollutants. [4] Diesel engines, in comparison with gasoline engines emit less carbon monoxide (CO) but over 20 times more nitrogen oxides (NOX) that include N2O, a greenhouse gas, and NO2, which produces photochemical smog. Catalytic converters can remove NOX from diesel exhaust as they do in modern gasoline vehicles, but many diesel fuels contain sulfur at concentrations that foul catalytic converters. Regulations in Europe and the United States now require refineries to remove most of this sulfur. This will enable most diesel vehicles to be equipped with catalytic converters.
Diesels also release potentially harmful particles of carbon soot. Newer diesel vehicles, which have electronic ignition systems that compensate for cold starts and have self-cleaning particle filters, are much better in this regard than older models. Further technical advances in both gasoline-powered and diesel-powered vehicles should soon minimize differences between their engine emissions.
Gasoline and diesel engines also differ in their ability to accommodate alternative fuels such as natural gas (methane) and biofuels. Gasoline engines built after 1980 can use mixtures of gasoline and up to 10% ethanol without modification. With modifications (i.e., fabricating the engine from materials that are resistant to ethanol or methanol corrosion and installing sensors that detect the fuel mixture and allow the electronic ignition control unit to adjust engine timing), gasoline vehicles can use mixtures that contain up to 85% ethanol or methanol and 15% gasoline. Converting a gasoline engine to use compressed natural gas (CNG) entails the addition of a pressure regulator and an electronic multi-point gas injection system similar to a gasoline injection system. Diesel engines are more flexible in their fuel mixtures. Rudolf Diesel, the inventor of the device that bears his name, ran his engines on vegetable oils and became an advocate for independence from petroleum fuels.
Today’s diesel engines, with minor modifications (larger fuel lines and more frequent replacement of the fuel filter), can switch freely between petroleum-based diesel fuel (petrodiesel) and mixtures of petrodiesel and biodiesel. Biodiesel, the product of the reaction between vegetable oils or animal fats with ethanol or methanol, has a viscosity similar to petrodiesel. Some, and perhaps many, diesel vehicles can operate on unadulterated biodiesel.
[1] Heavenrich, R. M. (2006) Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through 2006, U.S. Environmental Protection Agency, Office of Transportation and Air Quality, Washington, D.C., http://www.epa.gov/otaq/cert/mpg/fetrends/420r06011.pdf.
[2] U.S. Environmental Protection Agency (2005a) Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel. http://www.epa.gov/otaq/climate/420f05001.htm#calculating, accessed July 17, 2007.
[3] Schipper, L., C. Marie-Lilliu, and L. Fulton (2002) Diesels in Europe - Analysis of characteristics, usage patterns, energy savings and CO2 emission implications. Journal of Transport Economics and Policy 36:305-340.
[4] ECOtravel (2007) Petrol vs. Diesel. Merseytravel Environmental Information Office, http://www.ecotravel.org.uk/fuels_5.html, accessed July 16, 2007.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
