Machines run on a wide range of fuels and energy sources, from ancient combustibles like wood and coal to modern options like lithium-ion batteries and hydrogen. The type of fuel depends on what the machine does, where it operates, and how much power it needs. Here’s a breakdown of every major energy source that keeps machines running.
Fossil Fuels: Gasoline, Diesel, and Natural Gas
The three fossil fuels are oil, natural gas, and coal. Oil is refined into more usable forms like gasoline, diesel, and jet fuel before it powers anything. These liquid fuels dominate transportation and portable machinery because they pack an enormous amount of energy into a small, easy-to-carry volume. A gallon of gasoline contains roughly 112,000 to 116,000 BTUs of energy, which is why a relatively small tank can move a two-ton vehicle hundreds of miles.
Gasoline powers most passenger cars, motorcycles, lawnmowers, chainsaws, and portable generators. Diesel fuels heavier equipment: semi-trucks, construction excavators, agricultural tractors, cargo ships, and backup generators. Natural gas runs industrial furnaces, power plant turbines, and some fleet vehicles like city buses. A typical combustion engine converts only about 20% of the available energy in its fuel into useful work, with the rest lost as heat.
Electricity and Batteries
Electric machines draw power either from the grid (through a plug or wired connection) or from onboard batteries. Factory robots, CNC mills, conveyor belts, and household appliances all run on grid electricity. Portable and mobile machines, from electric cars to cordless drills, rely on rechargeable batteries.
Lithium-ion batteries have been the dominant chemistry since Sony first commercialized them in 1991 at around 80 Wh/kg. Today’s best production cells reach 360 Wh/kg, more than four times that original figure. Lab prototypes have hit 711 Wh/kg, pointing toward future applications in electric aviation and humanoid robots where weight matters enormously. The higher the energy density, the longer a machine can operate before recharging.
Solar Power
Some machines bypass fuel entirely by harvesting sunlight through photovoltaic panels. A solar-powered cocoa pod splitting machine in Ghana, for example, uses five 275-watt monocrystalline panels paired with a gel battery bank to produce about 5,844 watt-hours per day, roughly 24% more than the machine actually needs. The battery bank stores enough energy for three days of operation without sun. This approach is especially valuable in off-grid areas where diesel delivery is expensive or unreliable. Solar also powers water pumps, irrigation systems, weather stations, and increasingly, small industrial tools in remote locations.
Hydrogen Fuel Cells
Hydrogen fuels machines through two different paths. It can burn in a conventional combustion engine, much like gasoline. Or it can flow through a fuel cell, a device that converts hydrogen directly into electricity with high efficiency and low energy losses. Fuel cells produce only water as a byproduct, which makes them attractive for forklifts in warehouses, city buses, and experimental aircraft. Hydrogen also has an exceptionally high energy content by weight (about 20,160 BTUs per pound), though storing it requires either high-pressure tanks or cryogenic cooling, which adds bulk and complexity.
Biofuels
Biofuels are made from plant material or waste rather than ancient fossil deposits. They fall into several generations based on their source. First-generation biofuels come from edible oil crops like soybean, corn, and palm oil. Second-generation biofuels use non-edible plants such as jatropha, castor, neem, and cotton. Ethanol, the most common biofuel, is widely produced from sugarcane residue and corn. Third-generation biofuels come from waste materials, while fourth-generation versions are derived from algae.
Most biofuels work in standard engines with little or no modification. Ethanol is blended into gasoline (the “E10” at most gas pumps is 10% ethanol), and biodiesel can replace conventional diesel in trucks, farm equipment, and generators.
Coal, Wood, and Steam
Before gasoline and electricity, machines ran on steam generated by burning coal or firewood in a boiler. Steam engines powered the Industrial Revolution, driving factory equipment, locomotives, and pumps in mines. Coal and wood heated water into high-pressure steam, which pushed pistons or spun turbines to create mechanical motion. Coal-fired steam turbines still generate a significant share of the world’s electricity today, though the machines at the end of that power line are technically running on electricity rather than coal directly.
Nuclear Power
Nuclear energy fuels some of the most remote and long-lasting machines ever built. Spacecraft like the Mars Curiosity rover use radioisotope thermoelectric generators (RTGs), which are essentially nuclear batteries. These devices contain about 4.8 kilograms of plutonium-238 dioxide, which produces roughly 2,000 watts of thermal power and 120 watts of electrical power as the plutonium naturally decays. No moving parts, no refueling. The RTGs on the Pioneer 10 spacecraft operated for three decades, and the Voyager 1 and 2 missions have been running on RTG power since their 1977 launch. These generators are designed for a minimum lifetime of 14 years, making them ideal for deep space exploration where solar panels can’t capture enough light.
On Earth, nuclear reactors use uranium to generate steam that drives massive turbine generators at power plants, producing electricity for millions of machines downstream.
Compressed Air and Pressurized Fluids
Not all machine power comes from a chemical fuel. Hydraulic and pneumatic systems transmit energy using pressurized liquids and compressed air, respectively. When force is applied to a column of confined liquid, that pressure transmits equally in every direction, allowing a small input to generate a large output force. Hydraulic systems power construction excavators, car brakes, aircraft landing gear, and industrial presses. Pneumatic systems, using compressed air from rotary or screw compressors, drive jackhammers, dental drills, nail guns, and factory automation tools.
These aren’t fuels in the traditional sense. They’re energy carriers. Something else, usually an electric motor or diesel engine, powers the compressor or pump that pressurizes the fluid. But from the machine’s perspective, the pressurized fluid is what delivers the force.
Biological Fuel: How Living Machines Work
The human body is itself a machine, and it runs on glucose. Cells break down sugars and fats through a series of chemical reactions, capturing nearly 50% of the available energy and storing it in molecules of ATP. That’s remarkably efficient compared to a car engine’s 20%. A single molecule of glucose yields about 30 molecules of ATP, and a typical cell contains roughly a billion ATP molecules at any given moment, all of which are used up and replaced every one to two minutes.
ATP acts as a universal energy packet, powering muscle contraction, nerve signaling, cell division, and every other process that keeps biological machines running. Every animal on Earth uses this same basic fuel system, from hummingbirds to blue whales.