What Are Fuels?

Fuels are substances that release chemical energy through combustion or electrochemical reaction, providing the thermal or mechanical power required by engines, turbines, boilers, and fuel cells. A fuel's utility depends on properties including energy density, ignition characteristics, combustion products, and physical state. The discipline of fuel science draws from chemistry, thermodynamics, and chemical engineering, and it encompasses the characterization, production, and end-use optimization of substances ranging from petroleum distillates to biological hydrocarbons to elemental hydrogen.

The dominant categories in engineering practice are fossil fuels, derived from ancient organic deposits, and alternative fuels, which include biologically sourced liquids, synthetic hydrocarbons, gaseous fuels such as natural gas and hydrogen, and blends designed to improve combustion characteristics or reduce net carbon emissions.

Fossil Fuels and Their Properties

Petroleum-derived fuels remain the most widely used energy carriers in transportation and stationary power. Gasoline, diesel, and jet fuel are complex mixtures of hydrocarbons refined from crude oil, each optimized for its combustion target: gasoline for spark-ignited engines, diesel for compression ignition, and jet fuel for gas turbine combustion stability across a wide range of temperatures and altitudes. Key combustion quality metrics differ by fuel type: gasoline is rated by research octane number (RON), which quantifies resistance to autoignition or knock, while diesel is rated by cetane number, which measures ignitability under compression. Conventional gasoline has a pump octane of 84 to 93, while diesel cetane typically falls in the range of 40 to 55.

Coal gas, produced by the high-temperature treatment of coal in the absence of air, was a historically significant manufactured fuel supplying city gas networks in the nineteenth and early twentieth centuries. Modern coal gasification processes produce synthesis gas (syngas), a mixture of hydrogen and carbon monoxide used both as a fuel and as a feedstock for synthetic liquid fuel production via Fischer-Tropsch processes.

Alternative and Renewable Fuels

Alternative fuels include ethanol, biodiesel, renewable diesel, compressed natural gas, liquefied petroleum gas, and hydrogen. According to the Alternative Fuels Data Center fuel properties comparison, alternative fuels generally have lower volumetric energy densities than petroleum: ethanol delivers approximately 67% of the energy per gallon of gasoline, and methanol approximately 50%. However, many alternatives compensate with superior octane numbers, with hydrogen exceeding 130 and methanol reaching 112, enabling higher compression ratios and improved thermal efficiency in purpose-built engines.

Biofuels are produced from biomass feedstocks including corn, sugarcane, soy, and cellulosic materials. According to the U.S. Energy Information Administration overview of biofuels, first-generation biofuels such as corn ethanol and soy biodiesel are commercially mature and blend with petroleum fuels in percentages regulated by standards such as ASTM D4806 for ethanol and ASTM D6751 for biodiesel. Advanced or second-generation biofuels aim to convert cellulose and hemicellulose from agricultural waste and energy crops into drop-in fuels that are chemically indistinguishable from their petroleum equivalents. Renewable diesel, for instance, is chemically equivalent to petroleum diesel and meets ASTM D975, making it fully compatible with existing infrastructure and engines.

Hydrogen is a zero-carbon fuel when used in fuel cells or combusted in air, producing only water vapor. The U.S. Department of Energy biofuel basics resource situates hydrogen alongside advanced biofuels as part of a long-term strategy to displace petroleum in transportation, contingent on the development of cost-competitive production and distribution infrastructure.

Fuel Sensing and Metering

Fuel injection systems rely on transducers and sensing devices to control the delivery of fuel with precision. Fuel injectors meter fuel based on signals from oxygen sensors, mass airflow sensors, and pressure transducers, collectively enabling the engine control unit to maintain the stoichiometric air-fuel ratio or, in diesel engines, the commanded fuel mass per cycle. Displacement transducers are used in high-pressure fuel metering systems to monitor injector needle lift and confirm injection events.

Applications

Fuels have applications in a range of fields, including:

• Automotive and commercial vehicle propulsion, including internal combustion and hybrid powertrains
• Aviation and aerospace, from piston aircraft to rocket propellants
• Marine shipping using heavy fuel oil, diesel, and liquefied natural gas
• Industrial power generation and combined heat and power plants
• Fuel processing industries for refining, petrochemicals, and synthetic fuel production
• Waste-to-energy conversion using landfill gas, biogas, and municipal solid waste derived fuels