Two very different machines move most cars on the road today. One burns liquid fuel inside metal cylinders; the other uses electricity to spin magnets. Understanding what happens under the hood explains why these vehicles feel, sound and maintain so differently.
How an internal combustion engine works
According to the U.S. Department of Energy, an internal combustion engine (ICE) consists of a fixed cylinder and a moving piston. Fuel combusts inside the cylinder, and the expanding gases push the piston, which rotates the crankshaft and ultimately drives the vehicle's wheels.
Most ICE vehicles run on a four-stroke cycle, the Department of Energy explains: intake, compression, combustion/power, and exhaust. During intake, the piston moves down as an intake valve opens to draw in air and fuel. During compression, the valve closes and the piston moves up, squeezing the air-fuel mixture. In the power stroke, the compressed mixture ignites — pushing the piston back down — and in the exhaust stroke, the piston rises again as a second valve opens to expel spent gases.
The Department of Energy distinguishes two main engine designs: spark-ignition gasoline engines, where a spark plug ignites the fuel-air mixture, and compression-ignition diesel engines, where fuel self-ignites in hot, highly compressed air. ICEs can also run on alternative fuels such as natural gas, propane, biodiesel or ethanol, and can be combined with electric components in hybrid or plug-in hybrid systems.
How an electric car motor works
All-electric vehicles replace the engine with one or more electric traction motors powered by a rechargeable battery pack, according to the U.S. government's fueleconomy.gov and the Department of Energy's Alternative Fuels Data Center (AFDC). The AFDC lists the core components: a traction battery that stores energy, a charge port and onboard charger that convert incoming AC electricity to DC for charging, a DC/DC converter that steps down voltage for accessories, a power electronics controller that manages motor speed and torque, and the electric traction motor itself, which drives the wheels (in most designs through a transmission).
Powered by the traction battery, the power electronics controller manages how much electrical energy reaches the electric traction motor and controls the motor's speed and the torque it produces, according to the AFDC. A separate thermal system keeps the battery, motor and electronics within their proper operating temperature range, the AFDC notes.
Regenerative braking: the motor working in reverse
A feature with no equivalent in a conventional ICE vehicle is regenerative braking. As Mazda's consumer technology pages describe it, citing Department of Energy estimates, when the driver lifts off the accelerator, brakes, or coasts downhill, the electric motor reverses roles: instead of consuming battery power to spin the wheels, the spinning wheels turn the motor, which now acts as a generator and sends electricity back into the battery. The Department of Energy estimates, per Mazda, that this process recovers about 22% of driving energy in electric vehicles during combined city and highway driving (versus 5–9% in hybrids). Because regenerative braking cannot provide the stopping force needed in emergencies, conventional friction brakes remain in place as a backup, Mazda notes.
Why the two systems differ in efficiency
Fueleconomy.gov states that electric vehicles convert over 77% of the electrical energy drawn from the grid into power at the wheels, while conventional gasoline vehicles convert only about 12% to 30% of the energy stored in gasoline into power at the wheels — the rest is lost mainly as heat and through engine friction. Fueleconomy.gov also notes that EV batteries may last 12 to 15 years in moderate climates and 8 to 12 years in severe climates, and that a full recharge typically takes 3 to 12 hours, with fast charging reaching 80% capacity in about 30 minutes.
Internal combustion engine vs. electric motor: key differences| Feature | Internal combustion engine | Electric motor |
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| Energy source | Gasoline, diesel or alternative liquid/gas fuels | Electricity stored in a battery pack |
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| Core process | Four-stroke combustion cycle (intake, compression, power, exhaust) | Power electronics controller directs battery current to the traction motor |
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| Energy-to-wheel efficiency | About 12%–30% (fueleconomy.gov) | Over 77% (fueleconomy.gov) |
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| Refueling/recharging | Minutes at a fuel pump | 3–12 hours full charge; ~30 min fast charge to 80% (fueleconomy.gov) |
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| Braking energy recovery | Not applicable; energy lost as heat | Regenerative braking recovers about 22% of driving energy (DOE, via Mazda) |
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Common terms explained
- Crankshaft: the rotating shaft that converts the piston's up-and-down motion into rotary motion, per the Department of Energy.
- Traction battery: the rechargeable battery pack that stores energy for the electric traction motor, per the AFDC.
- Power electronics controller: the component that manages how electrical energy flows from the battery and controls the motor's speed and torque, according to the AFDC.
- Regenerative braking: using the electric motor as a generator during deceleration to send energy back to the battery, as described by Mazda, citing Department of Energy estimates.
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