Heat engines, including car engines, take energy from being warm compared to the surroundings and turn it into motion, and only about 12%-30% of the energy in a conventional car's fuel moves it down the road, as of November 2026. In a gas car, the water pump, fuel pump, oil pump, ignition system and engine control system use energy made by the engine. EVs convert over 77% of the electrical energy from the grid to power at the wheels, as of November 2026, and an EV electric drive system loses 15% to 20% of the energy, compared with 64% to 75% for a gasoline engine, as of November 2026, and when an electric car's battery is charged, some energy is lost in changing AC power from the grid into DC power for the battery.
Most gas engines are four-stroke engines, which need four piston strokes to finish one cycle, and fuel is mixed with air before it is drawn into the cylinder, and a fuel injection system puts fuel into the engine's combustion chambers. In the intake stroke, the intake valve is open, and fuel and air are drawn into the cylinder. In the compression stroke, the piston squeezes the fuel-air mixture into a smaller space, and compressing the fuel-air mixture takes work, just as inflating a bicycle tire with a pump takes work, and in the power stroke, the burning gases push the piston. In the exhaust stroke, the engine clears the cylinder of the used exhaust to get ready for another ignition cycle.
On a pressure-volume diagram, the area enclosed by an engine's Otto cycle is proportional to the work the cycle produces, and an engine's power equals the work of each cycle times the number of cycles per second. Real engines do worse than the ideal cycle: for a real cycle the work is always less than the ideal value because of losses. A piston engine is extremely responsive and more fuel-efficient at low outputs than a turbine, which makes it well suited to vehicles, and a piston engine only delivers power during a stroke, so a flywheel keeps power flowing to the drive train steadily, and a flywheel stores rotational kinetic energy, which is later turned into the motion of the moving car.
A spark plug lights the fuel in a gas engine, and its spark needs high voltage, anywhere from 40 kV to 100 kV, and a spark plug has no moving parts. If fuel ignites too early because of compression, a gas engine knocks, so spark timing must be precise. Higher-octane gasoline helps fuel ignite on time from the spark plug, not early from the piston's compression, and a diesel engine takes in only air, compresses it, and then sprays fuel into the hot air to make it ignite. Spark plugs are part of gasoline engines but not diesel engines, because diesel is ignited by compression.
The crankshaft turns the up-and-down motion of the pistons into spinning motion. The Wright brothers' 1903 engine had four cylinders, with each piston connected to a common crankshaft. In the Wright engine, the crankshaft turned two times for every one turn of the cams, kept in time by a timing chain. The spark came from a make-and-break connection with moving parts inside the combustion chamber, and modern engines use a spark plug instead. The Wright engine lost power as it warmed up, from about 16 horsepower at the start to about 12 horsepower when hot. Early gasoline cars had to be started with a hand crank, and a starter motor replaced the hand crank in 1912 when an electric starter was introduced.
In a gas car, the transmission passes the engine's power on to the wheels, and a gearbox lets a gas engine run at efficient speeds no matter how fast the car is going, and in a gas car with a transmission, a jolt is usually felt when the transmission shifts from one set of gears to another. A torque converter is a fluid-filled device that sends torque to the wheels in an automatic car, letting the engine idle while the car is stopped. A differential lets the wheels spin at different speeds while both are still powered, as when a car turns a corner, and in a rear-wheel drive car, the drive shaft sends power to the drive axle, which holds a final drive gear and a differential, and a long, thin drive shaft runs the length of the car to the rear differential.
Electric motors work by electromagnetism: a force is applied when an electric current is in a magnetic field. Two types of electric motors are used in electric vehicles, the direct current (DC) motor and the alternating current (AC) motor. A DC motor has field coils that create magnetic forces, a rotor (armature) that turns in that field, and a commutating device that reverses the forces. In a DC motor, the commutator reverses the current each time the coil does a half turn so the rotor can spin continuously, and most electric vehicles do not use a multi-speed transmission, and the motors are usually single speed. One electric car layout uses a single large electric motor coupled to the rear wheels through a differential housing, and another layout uses two smaller motors that power each wheel separately through independent drive shafts. With wheel motors, the motors are attached directly to the wheel, which removes driveshafts and differentials.
The onboard charger changes incoming AC electricity from the charge port into DC power for the battery, and drivers can charge at home with Level 1 equipment that comes with the car, or install Level 2 (240 V) equipment for a faster charge. An electric car can be charged from a regular 120 volt outlet, which is the slowest type of charging, as of November 2026. DC fast charging is the fastest of the three types of plug-in charging, as of November 2026, and fully recharging an electric car's battery pack can take 3 to 12 hours, as of November 2026. A typical electric car battery pack may last 12 to 15 years in moderate climates and 8 to 12 years in severe climates, according to a DOE study cited as of November 2026.
When an electric car slows down, its motor can act as a generator and send energy to the battery, and regenerative braking reduces wear on an electric car's brakes, and it can return 10% or more of the energy used by the drive system to the batteries over a single charge. If regenerative braking energy is counted as reused, an EV's energy efficiency can exceed 94% in the city and 77% on the highway, as of November 2026. Highway driving has little braking, so regenerative braking offers little advantage on the highway, and a regenerative brake system cannot do all the braking, so a friction brake system is still needed for heavy braking.
A hybrid electric car has both an internal combustion engine and one or more electric motors, and a hybrid car's regenerative braking uses the forward motion of the wheels to turn the motor, which makes electricity and helps slow the car. In a hybrid car, an electric generator makes electricity from the rotating wheels while braking and sends it to the traction battery pack. Parallel plug-in hybrids connect both the engine and the electric motor to the wheels, and in a series plug-in hybrid, only the electric motor turns the wheels, and the gasoline engine only generates electricity. A plug-in hybrid can cost roughly $4 to $8 thousand more than a comparable non-plug-in hybrid, as of November 2026.
The electric car came from a series of 1800s breakthroughs, from the battery to the electric motor, and the idea of a brake that stores the energy it absorbs for later use dates to the late 1800s. The first successful electric car in the U.S. appeared around 1890, and it was a six-passenger vehicle with a top speed of 14 miles per hour. By 1900, electric cars made up around a third of all vehicles on the road, but steam cars could need up to 45 minutes to start in the cold, and electric cars all but disappeared from U.S. roads by 1935, and the electric Lunar rover became the first manned vehicle to drive on the moon in 1971.
Words to know
- efficiency: energy used well
- octane: fuel rating
- flywheel: energy-storing wheel
- commutator: current reverser
- differential: wheel speed gear
- regenerative: energy-returning
