|Fig. 1: Diesel Availability and Fuel Prices 1980-2012.  (Courtesy of the U.S. Department of Energy.)|
Though still relatively low, in recent years, personal automobile sales of diesel vehicles has been increasing faster than ever in the United States.  Diesel supporters are often quick to acknowledge a maturing market, citing an efficiency superiority of the diesel engine as a primary motivator. One clear fact is that recently in Europe, as many as half of personal automobile sales are of engines running on diesel fuel whereas in the United States that number is still in the sub- two percent range. 
Assuming fossil fuels will be the predominant means of energy usage in personal transportation for the foreseeable future, it is important to understand the advantages and disadvantages of gasoline and diesel fuel as a means of providing energy for this purpose. A natural basis for comparison consists of the traditional aspects: energy output, emissions, and pricing. The goal of this paper is to develop these metrics for comparison of the fuel types and leave the reader to decide which is the "fuel of the future."
In order to establish a uniform basis for comparison of the two fuel types in the context of their respective engine, this paper uses diesel and gasoline to refer to engine types in automobiles (Otto Cycle) for personal transportation unless explicitly stated otherwise. Also, analysis of the two fuel types herein is assumed to be in the absence of extremes such as hot or cold weather, altitude, etc.
There are two significant differences to grasp related to the operation of gasoline and diesel engines; the fuel ignition process and the compression ratio. Diesel engines were invented by German engineer Rudolf Diesel who theorized an exploitation of the heat generated by compressing an air-fuel mixture, according to the ideal gas law PV = nRT.  Gasoline engines, on the other hand, use spark plugs to ignite the air-fuel mixture. Spark plugs allow lower temperatures in the combustion chamber which translates to a lower pressure and/or volume according to the same ideal gas principals.
The higher temperatures in diesel engine translate to more air compression than a gasoline engine. A gasoline engine compresses at a ratio of 6-12:1 whereas a diesel compresses at a ratio of 14-25:1. Greater compression of air means greater compression of oxygen which is the main reactant with either diesel or gasoline. Based on these first principals, diesel engines provide an efficiency that is quite superior to gasoline given current technology.
Relating to energy, another important metric to keep in mind is the energy density of diesel and gasoline. Diesel fuel is heavier and oiler than gasoline and its takes less refining to create, its chemical compound is C14H30. Gasoline on the other hand is C9H20.  When burned, these chemical compounds correspond to energy densities of approximately 155 million Joules per gallon for diesel and 132 million Joules per gallon for gasoline. Thus, in terms of energy density, diesel is clearly chemically ahead.
The thermal efficiency of both gas and diesel engines is readily computed via the ideal gas law as η = 1 - (V1/V2)γ - 1. For the engines in question, the specific heat ratio of the air-fuel mixture γ is approximately 1.28. Therefore, in gasoline engines, η = 1 - (1/8)0.28 = 0.44, which, when adjusted for heat losses, frictional losses, and gas dynamics is reduced to approximately η ≈ 0.25 at best. In diesel engines, η = 1 - (1/15)0.28 = 0.53, which reduces to a best of η ≈ 0.36 upon adjustment.
Due to the less-refined nature of diesel fuel, emissions have been a problem since the engines became widely available in the 1970s. Currently, the United States holds both gasoline and diesel engines to the same standards for emissions of NOx, carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM). Diesel engines characteristically emit less CO and HC but substantially more NOx and PM than their gasoline counterparts, thus demanding more expensive and technically challenging emission control measures. In the European Union, NOx and PM standards are much lower, enabling wider adoption of diesel technology at a price comparable to their gasoline counterparts.
Recently, pricing of diesel and diesel related technologies has led to consumer adoption issues. The US government, including state level, taxes diesel at a rate up to 25% higher than gasoline.  Also, recent EPA mandating of sulfur PM emissions has led to a requirement for ultra-low sulfur diesel (ULSD) to be sold nationwide. This has led to an increase of more than 10 cents per gallon compared to the previously mandated low-sulfur diesel fuels of the early 2000's. These factors together have led to diesel fuel becoming approximately 10% more expensive than gasoline, despite lower refinement. Finally, diesel engines are inherently more expensive than their gasoline counterparts due to both materials cost and the recoup cost for research and development from manufacturers.
|Fig. 2:Breakdown Prices of Gasoline and Diesel (Courtesy of the U.S. Department of Energy.)|
Equipped with a relevant and accurate basis for comparison between gas and diesel, one can begin to develop an opinion as to which is the optimal fuel of the future. On the one hand, gasoline has a lower energy density and runs at a lower thermal efficiency based on the compression ratio in an Otto Cycle. On the other hand, diesel is more expensive in terms of government imposed taxes/emissions standards and material cost for the engines, but less expensive in terms of refining and crude oil cost.
Future potentials of gasoline and diesel as a means to power consumer automobiles include increasing the thermodynamic efficiency of both engines. The lead of the diesel engine is prospectively shrinking due to enhanced hardware like variable value timing, direct fuel injection, and turbochargers in gasoline engines.  Diesel still has room to incrementally increase, however, with advents like independent cylinder combustion control and improved after treatment systems, which should also reduce environmental costs and taxes.
© Isaac Ramos. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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