Fuel efficiency
Fuel efficiency quantifies the effectiveness with which a device or process converts the chemical potential energy in fuel into useful work, expressed as the ratio of output to input energy, akin to thermal efficiency.[1][2] In transportation, it is commonly measured by distance traveled per unit of fuel, such as miles per gallon (mpg) for gasoline vehicles or gallons per ton-mile for freight.[3][4] Improvements in fuel efficiency reduce operational costs for consumers and businesses by lowering fuel expenditures, while also decreasing reliance on finite fossil fuel resources and mitigating associated environmental externalities like greenhouse gas emissions per unit of travel.[5][6] Empirical data from U.S. light-duty vehicles show average fuel economy rising from about 13.5 mpg in 1975 to over 25 mpg by the 2010s, driven by technological advances including electronic fuel injection, lighter materials, and aerodynamic designs, alongside regulatory mandates like Corporate Average Fuel Economy (CAFE) standards.[7][8] Notable defining characteristics include the influence of operational factors such as vehicle load, driving speed, and maintenance on real-world performance, often diverging from laboratory ratings.[9] Controversies persist around the rebound effect, where efficiency gains enable increased usage—such as more miles driven—partially offsetting energy savings, with estimates ranging from 5% to 30% based on econometric analyses of vehicle travel data.[10][11] Despite this, net reductions in fuel consumption and emissions have materialized from sustained efficiency enhancements, underscoring causal links between engineering innovations and resource conservation.[12][13]Definitions and Fundamentals
Terminology and Measurement Units
Fuel efficiency quantifies the ratio of useful output, such as distance traveled or work performed, to the energy content of fuel consumed.[2] Metrics are categorized as fuel economy, expressing distance or work per unit fuel, or fuel consumption, expressing fuel per unit distance or work.[14] The choice of terminology reflects operational context, with "fuel economy" prevalent in road vehicles and "specific fuel consumption" in engines and propulsion systems.[15] In road transportation, the predominant unit for fuel economy is miles per gallon (MPG), defined as miles traveled per U.S. gallon of fuel, standardized by the U.S. Environmental Protection Agency for passenger vehicles.[14] Equivalent metric units include kilometers per liter (km/L) or the inverse liters per 100 kilometers (L/100 km) for fuel consumption, commonly used in Europe and elsewhere.[16] For electric or hybrid vehicles, miles per gallon gasoline equivalent (MPGe) normalizes energy efficiency to gasoline's energy content.[17] Engine efficiency employs brake-specific fuel consumption (BSFC), measured in grams of fuel per kilowatt-hour (g/kWh), indicating fuel required to produce one kilowatt of brake power for one hour.[18] Typical BSFC values range from 200 g/kWh for diesel engines to 250 g/kWh for gasoline engines.[18] In aviation, thrust-specific fuel consumption (TSFC) uses units like kilograms per kilonewton-second (kg/(kN·s)) or pounds per pound-force-hour (lb/(lbf·h)), while fleet-level metrics include kilograms of fuel per revenue tonne-kilometer (kg/RTK).[15][19] Maritime shipping assesses efficiency through the Energy Efficiency Design Index (EEDI), often in grams of CO2 per tonne-mile, proxying fuel use via carbon content, or directly in megajoules per tonne-kilometer (MJ/t-km).[20] Rail transport metrics favor ton-miles per gallon, with U.S. freight railroads achieving approximately 500 ton-miles per gallon system-wide.[4][21]| Transportation Mode | Common Efficiency Unit | Description |
|---|---|---|
| Road Vehicles | MPG or L/100 km | Distance per fuel volume or fuel volume per distance.[14][16] |
| Internal Combustion Engines | g/kWh (BSFC) | Fuel mass per power-time unit.[18] |
| Aviation | kg/RTK or lb/(lbf·h) | Fuel per payload-distance or per thrust-time.[19][15] |
| Maritime Shipping | MJ/t-km or g CO2/t-mile | Energy or emissions per cargo-distance. |
| Rail Freight | Ton-miles per gallon | Cargo-distance per fuel volume.[4] |
Energy Content and Properties of Fuels
The energy content of fuels, often expressed as the lower heating value (LHV), represents the quantity of heat released during combustion per unit mass (gravimetric energy density, in MJ/kg) or volume (volumetric energy density, in MJ/L), excluding the latent heat of water vapor condensation. This metric is fundamental to fuel efficiency, as it determines the maximum theoretical energy available for conversion into mechanical work in engines, with actual efficiency limited by thermodynamic cycles (typically 20-40% in internal combustion engines). Gravimetric density is critical for weight-sensitive applications like aviation, while volumetric density influences range in road vehicles constrained by tank volume.[22][23] Fossil-derived liquid fuels dominate transportation due to their high energy densities compared to alternatives. Gasoline, refined from crude oil, has an LHV of approximately 44 MJ/kg and a density of 0.74 kg/L, yielding about 32.6 MJ/L; diesel fuel offers a similar 42-45 MJ/kg but higher density of 0.83-0.85 kg/L, resulting in 35-38 MJ/L, which supports diesel engines' superior efficiency (often 30-50% thermal) through better combustion completeness and higher compression ratios.[24][25] Natural gas, primarily methane, provides 50 MJ/kg but requires compression or liquefaction for vehicular use, achieving 8-10 MJ/L as compressed natural gas (CNG) at 200-250 bar, limiting its volumetric advantage.[22][26] Biofuels and synthetics exhibit trade-offs. Ethanol, produced via fermentation of biomass, delivers 27 MJ/kg and 0.79 kg/L (21.3 MJ/L), roughly 70% of gasoline's volumetric energy, necessitating engine adjustments like increased fuel flow to maintain power, which reduces efficiency gains from blending (e.g., E10 gasoline blends yield 3-4% lower energy content).[26][24] Biodiesel, from vegetable oils or animal fats, matches diesel's 37 MJ/kg but has variable density (0.86-0.90 kg/L), providing comparable volumetric density while improving lubricity.[26] Hydrogen gas boasts exceptional gravimetric density at 120 MJ/kg but negligible liquid density without cryogenic storage (8-10 MJ/L at -253°C), imposing efficiency penalties from compression (20-30% energy loss) or liquefaction (30-40% loss).[25][24] Key combustion properties modulate how effectively energy content translates to work output. For spark-ignition engines, octane number (typically 87-93 for regular gasoline, measured against iso-octane) resists auto-ignition, enabling compression ratios up to 12:1 versus 8-10:1 for lower-octane fuels, boosting thermodynamic efficiency by 5-10% per ratio increment via reduced heat loss and improved expansion.[27][28] In compression-ignition diesel engines, cetane number (40-55 standard) indicates ignition delay; higher values (e.g., 50+) promote rapid, complete combustion, minimizing unburned hydrocarbons and enhancing efficiency by 2-5% while curbing emissions, though excessive density (>0.86 kg/L) can increase particulate formation.[29][30] Viscosity (2-4 cSt at 40°C for diesel) and sulfur content (<10 ppm in ultra-low sulfur variants) further influence atomization and injector durability, indirectly supporting sustained efficiency.[31]| Fuel | LHV (MJ/kg) | Density (kg/L) | LHV (MJ/L) | Key Property Impacting Efficiency |
|---|---|---|---|---|
| Gasoline | 44 | 0.74 | 32.6 | Octane (87-93): Enables high compression ratios for better thermal efficiency.[27] |
| Diesel | 43 | 0.84 | 36.1 | Cetane (40-55): Shortens ignition delay for complete combustion.[29] |
| Ethanol | 27 | 0.79 | 21.3 | Lower energy density requires richer mixtures, reducing volumetric efficiency.[26] |
| Natural Gas | 50 | ~0.18 (CNG) | ~9 | Compressibility losses (20-30%) erode effective density.[22] |
| Hydrogen | 120 | ~0.07 (gas) | ~8.4 (liq) | High gravimetric but storage penalties limit practical efficiency.[25] |