Part 1 of a series on the engineering of electric vehicles
It is safe to say that electric vehicles are the next great transformation of the automobile industry. By eliminating tailpipe emissions altogether (carbon dioxide, carbon monoxide, nitrogen oxides, and particulate matter alike), electric vehicles (EVs) stand to reshape the industry while relieving much of the burden that road transport places on the environment. In this series, we will discuss what EVs are and then examine their principal components one at a time. Throughout, we will read each component against its counterpart in the conventional automobile, because the electric vehicle builds on more than a century of mechanical engineering rather than breaking from it.
That transformation is already well under way. More than 17 million electric cars were sold worldwide in 2024, over one in five new cars sold [1]. To see why this is happening and what it asks of engineers, we have to start with first principles.
Defining the Electric Vehicle
Broadly, an electric vehicle is any vehicle in which an electric machine supplies some or all of the tractive effort. That definition covers several distinct architectures:
- Hybrid electric vehicles (HEVs) pair an internal combustion engine (ICE) with an electric machine and a small battery. All of the energy ultimately comes from fuel; the electrical system recovers braking energy and lets the engine run nearer its efficient operating points.
- Plug-in hybrid electric vehicles (PHEVs) carry a larger battery that can be charged from the grid, which gives them a meaningful all-electric range before the engine takes over.
- Fuel cell electric vehicles (FCEVs) generate electricity on board, usually from hydrogen in a proton-exchange-membrane fuel cell, and drive the wheels with an electric motor.
- Battery electric vehicles (BEVs) draw all of their energy from an on-board battery charged from an external source. They have no combustion engine at all.
This series concentrates on the BEV, because it is the purest expression of the shift from thermochemical to electrochemical propulsion. Unless stated otherwise, “EV” from here on means a battery electric vehicle.
The technology is not new. Around 1900, electric cars made up roughly a third of the vehicles on American roads, competing with steam and gasoline [2]. Three developments sidelined them: Ford’s mass-produced Model T, Charles Kettering’s electric starter of 1912 (which removed the hand crank, the gasoline car’s most unpleasant drawback), and cheap petroleum. EVs returned at scale because of the lithium-ion cell, commercialized in 1991, and its dramatic fall in cost. Average battery pack prices dropped from well over US$1,000/kWh in 2010 to about US$115/kWh in 2024 [3].
The Fundamental Difference: How Energy Becomes Motion
The deepest distinction between the two vehicle types lies in their energy conversion chains, not in any single component.
The heat engine and its thermodynamic ceiling
An internal combustion engine is a heat engine. It releases chemical energy as heat through combustion and converts part of that heat into work through the expansion of a working gas. Its efficiency is therefore bounded by thermodynamics. For the ideal air-standard Otto cycle, thermal efficiency depends only on the compression ratio r and the ratio of specific heats γ:
ηOtto = 1 − 1 / r(γ − 1)
With r = 10 and γ = 1.4, the ideal efficiency is about 60%. Real engines fall well short of that because of heat transfer to the cylinder walls, finite combustion duration, pumping work, friction, and incomplete expansion. Production spark-ignition engines peak at a brake thermal efficiency of roughly 35–40%, and the best diesels and hybrid-optimized engines reach the low-to-mid 40s [4]. Over a real driving cycle, with frequent part-load operation and idling, average efficiency is considerably lower. US government data indicate that a conventional gasoline vehicle delivers only about 12–30% of its fuel energy to the wheels [5].
The electric machine: no Carnot limit
An electric motor is not a heat engine. It converts electrical energy directly into mechanical work through electromagnetic force, the Lorentz force on current-carrying conductors, and the reluctance and permanent-magnet torques that arise in a magnetic circuit. The Carnot limit does not apply. Its losses are copper (I²R) losses, core losses (hysteresis and eddy currents), and mechanical friction and windage. Modern traction motors reach peak efficiencies above 95%, and their inverters exceed 97% [6], [7]. The result is that an EV converts more than 77% of the electrical energy drawn from the grid into power at the wheels [5].
The energy density counterpoint
A rigorous comparison has to acknowledge where the ICE remains superior. Gasoline stores about 12.8 kWh per kilogram. A state-of-the-art lithium-ion cell stores roughly 0.25–0.30 kWh/kg, and the complete pack, with its housing, thermal system, and electronics, stores less. That is a specific-energy gap of about fifty to one. The electric drivetrain’s three- to four-fold conversion advantage narrows the gap but does not close it. This is why a long-range EV carries a battery pack of 400–700 kg and why mass, range, and charging time remain its central engineering constraints.
The environmental case also has to be judged across the whole life cycle, not only at the tailpipe. Emissions from electricity generation and battery manufacturing matter. Even so, a comprehensive life-cycle study found that a medium-size BEV registered in 2021 emits roughly 66–69% less greenhouse gas over its lifetime than a comparable gasoline car in Europe, 60–68% less in the United States, and 37–45% less in China, where coal still dominates the grid [8].
Torque–speed characteristics
The two prime movers also differ in their mechanical output. An ICE produces useful torque only within a limited speed band, typically from about 1,000 to 6,500 rpm, and produces none at standstill. It therefore needs a clutch or torque converter to launch the vehicle and a multi-speed gearbox to keep the engine in its usable band. A traction motor delivers its full rated torque from zero speed up to a base speed. Above base speed, field weakening sustains roughly constant power up to 15,000–20,000 rpm or more [6]. This characteristic closely matches the ideal tractive-effort curve of a road vehicle, and it explains why most EVs use a single fixed reduction ratio.
Anatomy of an Electric Vehicle: What Changes
The table below maps the major subsystems of a conventional vehicle to their counterparts in a battery electric vehicle. Each is introduced briefly here and will get a dedicated article later in the series.
| Conventional vehicle | Battery electric vehicle |
|---|---|
| Fuel tank | Traction battery pack |
| Engine management (ECU) | Battery management system and vehicle control unit |
| Internal combustion engine | Traction motor |
| Fuel injection and ignition control | Inverter (power electronics) |
| Multi-speed gearbox and clutch | Single-speed reduction gear |
| Alternator and starter motor | DC-DC converter |
| Fuel filler neck | Charge port and on-board charger |
| Radiator and engine cooling | Integrated thermal management and heat pump |
| Friction braking only | Regenerative braking blended with friction |
| Exhaust and after-treatment | None |
The traction battery pack. The pack is the EV’s energy reservoir and its most expensive single component. It is a hierarchy of cells, often grouped into modules, held in a structural enclosure with cooling channels, high-voltage busbars, and safety disconnects. The dominant chemistries are nickel-rich layered oxides (NMC, NCA), which offer higher energy density, and lithium iron phosphate (LFP), which offers lower cost, longer cycle life, and better thermal stability. Most packs operate at a nominal 400 V, and 800 V architectures are increasingly common because they allow faster charging and lower current.
The battery management system (BMS). A fuel tank needs little supervision, but a lithium-ion pack needs constant supervision. The BMS measures every cell voltage, pack current, and temperature throughout the pack. It estimates the state of charge and state of health, balances cells, enforces voltage, current, and temperature limits, and isolates the pack when faults occur [9].
The traction motor. The motor is the electromechanical successor to the engine. Most production EVs use permanent-magnet synchronous machines (PMSMs) because of their high efficiency and power density. Induction machines avoid rare-earth magnets, and electrically excited synchronous machines are gaining ground for the same reason [6]. Mechanically, a motor has a rotor, a shaft, and a pair of bearings. It has no valvetrain, pistons, crankshaft, or reciprocating masses to balance. That change alone moves much of the vehicle’s mechanical complexity into the electromagnetic domain.
The inverter. The battery supplies direct current, while the motor needs polyphase alternating current of variable frequency and amplitude. The inverter bridges the two. It switches power semiconductors (IGBTs or, increasingly, silicon carbide MOSFETs) at kilohertz frequencies, and its control algorithm, typically field-oriented control, regulates motor torque precisely [7]. In effect, the inverter takes over the role that throttle, fuel metering, and ignition timing play in an ICE.
The transmission. Because of the motor’s wide speed range, a single-stage or two-stage reduction gear, typically with a ratio between 8:1 and 10:1, feeds the differential. The motor, inverter, and gearbox are increasingly integrated into a single e-axle. The mechanical engineering challenges change rather than disappear: high input speeds, gear whine that combustion noise no longer masks, and bearing currents induced by the inverter.
The DC-DC converter. An EV has no alternator. A DC-DC converter steps the high-voltage bus down to supply the 12 V (or, increasingly, 48 V) low-voltage network that runs lighting, infotainment, and control electronics.
The charging system. For alternating-current charging, an on-board charger rectifies grid AC and regulates charging current, typically at 7–22 kW. DC fast charging bypasses the on-board charger and supplies the pack directly at hundreds of kilowatts, under standards such as CCS, GB/T, CHAdeMO and, in North America, SAE J3400 (NACS) [10], [11].
Thermal management. An ICE wastes most of its fuel energy as heat, so its cooling system mainly rejects heat. An EV produces far less waste heat, and it has to manage temperature in both directions. Lithium-ion cells perform and age best within a narrow window of roughly 15–35 °C, and cabin heating can no longer come from engine waste heat. Modern EVs therefore integrate the battery, motor, power-electronics and cabin circuits around a heat pump, a refrigeration-cycle device whose coefficient of performance can exceed unity.
Regenerative braking. In a conventional vehicle, the friction brakes turn kinetic energy into heat and lose it. An EV can run its motor as a generator, recovering a substantial share of braking energy back into the battery. The friction brakes remain essential. The engineering task is to blend the two systems seamlessly while preserving stability and pedal feel.
What Remains: The Enduring Mechanical Foundation
It would be a mistake to treat the EV as purely an electrical product. Suspension kinematics, steering geometry, tyre mechanics, body structure, crashworthiness, and noise, vibration and harshness (NVH) all remain within mechanical engineering, and electrification makes several of them harder. The battery’s mass, often a quarter of curb weight, raises suspension and tyre loads and accelerates tyre wear. The pack is mounted low between the axles in the now-common “skateboard” platform, which lowers the centre of gravity and improves handling, but the pack must also be protected in side impacts. Some manufacturers now make the pack part of the vehicle structure. Without engine noise, road and wind noise become far more noticeable, and so does the high-frequency whine of the gears.
The electric vehicle therefore does not make mechanical engineering obsolete. It moves the discipline’s focus and requires it to work closely with electrochemistry, power electronics and control theory. That combination is the subject of this series.
The Road Ahead
The articles that follow take each subsystem in turn. We begin with the traction battery: its electrochemistry, cell formats and pack architecture, and the mechanical and thermal engineering that makes it safe and durable. We then move through the battery management system, traction motor, power electronics, transmission, thermal management, regenerative braking, charging, and the vehicle architecture that ties them together.
References
[1] International Energy Agency, Global EV Outlook 2025. Paris: IEA, 2025.
[2] U.S. Department of Energy, “The History of the Electric Car,” Energy.gov, 2014.
[3] BloombergNEF, “Lithium-Ion Battery Pack Prices See Largest Drop Since 2017, Falling to $115 per Kilowatt-Hour,” Dec. 2024.
[4] J. B. Heywood, Internal Combustion Engine Fundamentals, 2nd ed. New York: McGraw-Hill Education, 2018.
[5] U.S. Department of Energy and U.S. Environmental Protection Agency, “All-Electric Vehicles,” FuelEconomy.gov.
[6] M. Ehsani, Y. Gao, S. Longo, and K. M. Ebrahimi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, 3rd ed. Boca Raton, FL: CRC Press, 2018.
[7] X. She, A. Q. Huang, Ó. LucÃa, and B. Ozpineci, “Review of Silicon Carbide Power Devices and Their Applications,” IEEE Transactions on Industrial Electronics, vol. 64, no. 10, pp. 8193–8205, 2017.
[8] G. Bieker, “A Global Comparison of the Life-Cycle Greenhouse Gas Emissions of Combustion Engine and Electric Passenger Cars,” International Council on Clean Transportation (ICCT), White Paper, July 2021.
[9] L. Lu, X. Han, J. Li, J. Hua, and M. Ouyang, “A Review on the Key Issues for Lithium-Ion Battery Management in Electric Vehicles,” Journal of Power Sources, vol. 226, pp. 272–288, 2013.
[10] M. Yilmaz and P. T. Krein, “Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles,” IEEE Transactions on Power Electronics, vol. 28, no. 5, pp. 2151–2169, 2013.
[11] SAE International, SAE J3400: North American Charging System (NACS) for Electric Vehicles. Warrendale, PA: SAE International, 2024.
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