Hybrid electric vehicle (HEV)

Definition

A hybrid electric vehicle (HEV) is a road vehicle that combines a conventional internal combustion engine (ICE) with one or more electric traction motors and an onboard battery to propel the vehicle. Standard (non–plug-in) HEVs are fueled by gasoline or diesel and do not require external charging; their traction battery is charged internally via regenerative braking and engine-driven generation.

Key technical characteristics

  • Hybrid powertrain architectures
    • Series: the ICE drives a generator only; the electric motor provides all wheel torque, supplied by the battery and generator.
    • Parallel: both ICE and electric motor can drive the wheels through a common drivetrain.
    • Series–parallel (power-split): a power-split device (often a planetary gearset) blends series and parallel behavior, allowing engine drive, electric drive, combined drive, and engine-driven charging.
  • Energy sources and management
    • Dual energy sources: chemical energy from fuel (tank to ICE) and electrical energy from a traction battery (battery to motor).
    • A hybrid control unit manages engine on/off, torque blending, regenerative braking, and battery state of charge (SOC) within a calibrated window (charge-sustaining operation).
  • Electric drive and storage
    • Motors/generators: typically permanent-magnet synchronous or AC induction machines.
    • Power electronics: inverter for motor control and a DC/DC converter to supply the 12 V system.
    • Traction battery: historically NiMH, increasingly lithium-ion (e.g., NMC, LFP); oriented for high power and cycle life rather than large energy capacity. Typical HEV pack voltage is about 100–300 V and energy is usually under ~2 kWh. Charging occurs only via regeneration and the ICE.
  • Transmission and driveline
    • eCVT/power-split gearsets in many full hybrids; dedicated hybrid transmissions (DHT) or modified automatic/manual gearboxes with integrated electric machines in parallel systems.
  • Thermal and safety systems
    • Thermal management for engines, inverters, and sometimes batteries (air or liquid cooling).
    • High-voltage safety: contactors with pre-charge, isolation monitoring, HV interlock loops, orange HV cabling, and a battery management system (BMS) with cell balancing and diagnostics.

Operating modes

  • Engine stop-start and engine-off coasting/idle.
  • Electric-only propulsion at low speed or light load (full hybrids and some architectures).
  • Electric assist/torque fill during acceleration to reduce ICE load and enable downsizing.
  • Regenerative braking to recapture kinetic energy.
  • Engine load-point shifting to operate the ICE in more efficient regions while maintaining battery SOC.

Relevance and role

  • Improves fuel economy and reduces tailpipe CO2 and pollutant emissions versus comparable ICE-only vehicles, without relying on charging infrastructure.
  • Serves as a transitional technology that industrializes core electrified-powertrain components (motors, inverters, batteries, power-split gearsets) also used in PHEVs and BEVs.
  • Helps manufacturers meet fleet-average efficiency and emissions targets and enables high-volume learning in electrified systems, thermal management, and high-voltage safety.
  • Trade-offs: added complexity and cost, limited electric-only operation, and continued dependence on liquid fuels (so “self-charging” is a marketing term; the energy ultimately comes from fuel).

Related terms and classifications

  • Full hybrid (HEV): can propel the vehicle on electric power alone under some conditions; typical HV battery around 100–300 V.
  • Mild hybrid (MHEV): usually 12–48 V; provides start-stop, regeneration, and torque assist but generally cannot drive the wheels electrically on its own (often belt-integrated starter-generator).
  • Micro-hybrid: start-stop with minimal or no traction assist; not a true hybrid drivetrain.
  • Series, parallel, and series–parallel (power-split): architectural subtypes of HEVs.
  • Plug-in hybrid electric vehicle (PHEV): closely related but includes a larger, externally chargeable battery for extended electric-only range.
  • Range-extended electric vehicle (REEV): a series-dominant hybrid where the engine primarily generates electricity (e.g., some “e-POWER” implementations).

Examples

  • Toyota Prius (series–parallel/power-split)
  • Ford Escape/Explorer Hybrid (power-split)
  • Honda Insight (parallel)
  • Hyundai Ioniq Hybrid and Kia Niro Hybrid (parallel)
  • Nissan e-POWER models (series-dominant)

Materials and manufacturing snapshot

  • Electric machines: laminated electrical steels for stators/rotors; copper windings (round wire, hairpin, or bar); NdFeB permanent magnets (sometimes with dysprosium/terbium for high-temperature performance); housings in aluminum or magnesium castings.
  • Traction battery: NiMH or Li-ion cells (NMC, LFP, etc.) with copper/aluminum current collectors; module/pack assemblies with busbars (copper or aluminum), thermal interface materials, sensors, and BMS; enclosures typically in steel or aluminum for structure and EMI shielding.
  • Power electronics: silicon IGBTs or MOSFETs and increasingly SiC devices on ceramic substrates (DBC on alumina or aluminum nitride); liquid or air-cooled heat sinks; encapsulated modules and engineering-plastic housings.
  • Power-split/transmission components: carburized and hardened alloy-steel planetary gearsets; precision machining, hobbing, grinding; aluminum housings to reduce mass.
  • HV wiring and connectors: multi-strand copper conductors with high-temperature polymer insulation; orange sheathing for identification; tin/nickel-plated copper or aluminum busbars and high-CTI connector plastics.
  • Body/chassis: high-strength steels, aluminum, and composites to offset hybrid component mass; processes include hot stamping, hydroforming, extrusion, adhesive bonding, and laser welding.