Hybrid Vehicle Technologies

Hybrid vehicle technologies represent one of the most complex integration challenges in automotive electronics, combining internal combustion engines with electric motors and sophisticated power management systems. These vehicles require seamless coordination between multiple power sources to optimize fuel efficiency, performance, and emissions while maintaining driver expectations for responsiveness and reliability. Understanding hybrid electronics provides insight into advanced control systems, power electronics, and energy management strategies that are reshaping transportation.

Introduction to Hybrid Powertrains

Hybrid vehicles emerged as a bridge technology between conventional internal combustion engines and fully electric vehicles, offering improved fuel economy and reduced emissions without the range limitations of battery-only designs. The fundamental concept involves combining two or more power sources, typically a gasoline or diesel engine with one or more electric motor-generators, to leverage the strengths of each technology while mitigating their individual weaknesses.

The complexity of hybrid systems stems from the need to manage power flow between multiple sources in real-time, optimizing for efficiency across widely varying driving conditions. This requires sophisticated electronic control systems that monitor vehicle state, predict driver intent, and coordinate engine operation, motor control, and battery management simultaneously. The result is a powertrain that can operate more efficiently than either power source could achieve independently.

Modern hybrid architectures range from mild hybrids with small motor-generators providing assist functions to full hybrids capable of electric-only operation, and plug-in hybrids with substantial electric range. Each architecture presents unique electronic design challenges and optimization opportunities that influence vehicle performance, cost, and consumer acceptance.

Power Split Device Control

Planetary Gear System Fundamentals

The power split device, typically implemented using a planetary gear set, enables continuous variable power distribution between the engine, motor-generators, and wheels without requiring a conventional stepped transmission. In the Toyota Hybrid Synergy Drive architecture, the planetary gear's sun gear connects to one motor-generator (MG1), the ring gear connects to the output shaft and a second motor-generator (MG2), and the carrier connects to the engine. This arrangement allows infinite variation in the ratio of engine power flowing to the wheels versus to MG1 for electrical generation.

Electronic Control Strategies

Power split control requires precise coordination of engine speed and torque with motor-generator operations. The hybrid control unit (HCU) determines optimal power distribution based on driver demand, battery state of charge, and efficiency considerations. When the battery is depleted, more engine power flows through MG1 for charging while MG2 provides propulsion assistance. During light loads, the system can operate in electric-only mode or in series mode with the engine driving MG1 as a generator while MG2 provides all propulsion.

Speed and Torque Relationships

The mathematical relationships in a planetary gear set create specific constraints between component speeds. The engine speed is determined by the weighted average of MG1 and ring gear (output) speeds. By controlling MG1 speed, the system can operate the engine at its most efficient speed independent of vehicle speed. This electronic continuously variable transmission (eCVT) functionality eliminates the efficiency losses and shift events of conventional automatic transmissions.

Transient Response Management

Managing rapid changes in power demand requires careful control of torque and speed transitions. The control system must coordinate engine torque changes with motor-generator responses to maintain smooth vehicle acceleration without driveline oscillations. Predictive algorithms anticipate driver inputs based on pedal application rate and vehicle state, pre-conditioning the powertrain for expected demands.

Motor-Generator Unit Management

Permanent Magnet Synchronous Motors

Most hybrid vehicles employ permanent magnet synchronous motors (PMSMs) for their high power density, efficiency, and controllability. These motors use rare-earth magnets (typically neodymium-iron-boron) to create the rotor magnetic field, eliminating rotor copper losses present in induction motors. The stator windings receive three-phase AC power from the inverter, creating a rotating magnetic field that the rotor follows synchronously.

Field-Oriented Control

Motor control uses field-oriented control (FOC), also known as vector control, to independently regulate torque-producing and flux-producing current components. The control algorithm transforms three-phase stator currents into a rotating reference frame aligned with the rotor flux, enabling direct control of torque similar to DC motor control. High-bandwidth current loops maintain precise torque control during transient conditions.

Regenerative Braking Control

During deceleration, motor-generators operate as generators, converting kinetic energy to electrical energy for battery charging. The control system must coordinate regenerative braking torque with friction braking to provide consistent pedal feel while maximizing energy recovery. Blending algorithms distribute total braking demand between regenerative and friction systems based on battery acceptance limits, motor torque capacity, and vehicle stability requirements.

Thermal Management Integration

Motor-generator thermal limits constrain continuous and peak power ratings. Temperature sensors in stator windings and rotor assemblies feed the control system, which reduces power when temperatures approach limits. Liquid cooling circuits maintain motor temperatures within acceptable ranges, with coolant flow modulated based on thermal load. Some designs use oil cooling for improved heat transfer from rotor components.

Position and Speed Sensing

Precise rotor position knowledge is essential for field-oriented control. Resolvers provide robust, high-resolution position feedback immune to the electrical noise present in motor drive environments. Digital signal processors convert resolver signals to position and speed information for the control loops. Sensorless control algorithms estimate position from motor currents during conditions where resolver signals may be unreliable.

Hybrid Control Unit Architectures

Distributed Control Systems

Hybrid powertrains typically employ distributed electronic control with multiple specialized controllers communicating over vehicle networks. The hybrid control unit coordinates overall powertrain operation, interfacing with the engine control module (ECM), motor control units, battery management system, and vehicle systems. This distribution allows specialized controllers optimized for their specific functions while the HCU manages system-level coordination.

Central Processing Requirements

The HCU requires substantial computational capability to execute complex control algorithms in real-time. Modern implementations use automotive-grade microcontrollers with multiple CPU cores, hardware floating-point units, and specialized peripherals for motor control and communication. Execution times for control loops typically range from 100 microseconds for motor current control to 10 milliseconds for vehicle-level power management.

Safety-Critical Architecture

Hybrid control systems must meet functional safety requirements defined by ISO 26262. Safety-critical functions include high-voltage isolation monitoring, motor torque limiting, and graceful degradation during faults. Redundant sensors, diagnostic routines, and fail-safe states ensure safe vehicle behavior when components fail. Hardware and software designs undergo rigorous analysis to achieve appropriate Automotive Safety Integrity Levels (ASIL).

Software Architecture

Control software follows automotive software architectures like AUTOSAR, separating application logic from hardware-dependent layers. This modularity enables software reuse across vehicle platforms and facilitates updates. Model-based development using tools like MATLAB/Simulink generates production code from validated simulation models, reducing development time and improving software quality.

Calibration and Adaptation

Hybrid control systems include numerous calibratable parameters tuned for specific vehicle applications. Development teams optimize these parameters through vehicle testing, balancing performance, efficiency, and drivability objectives. Adaptive algorithms adjust certain parameters based on component aging, environmental conditions, and learned driver behavior, maintaining optimal performance throughout vehicle life.

Energy Flow Optimization

Global Optimization Strategies

Energy flow optimization seeks to minimize fuel consumption over driving cycles by controlling when to use stored electrical energy versus generating power from the engine. Simple rule-based strategies establish operating regions based on power demand and battery state, while advanced approaches use equivalent consumption minimization strategies (ECMS) that assign a virtual fuel cost to battery energy use. Dynamic programming provides globally optimal solutions for known drive cycles, informing real-time heuristic strategies.

Predictive Energy Management

Connected vehicles can leverage route information, traffic data, and terrain maps to predict future power demands and optimize energy management accordingly. Knowing an upcoming descent allows the control system to deplete the battery beforehand, maximizing regenerative braking energy capture. Similarly, anticipating highway driving versus urban conditions influences the charging and discharging strategy. Machine learning algorithms identify patterns in driver behavior and route characteristics to improve predictions.

Engine Operating Point Selection

Internal combustion engines exhibit widely varying efficiency across their operating range. Hybrid systems can select engine operating points more freely than conventional powertrains because motor-generators can absorb or supply the difference between engine output and wheel demand. The optimal strategy typically operates the engine at higher torque, lower speed points where efficiency is greatest, using electrical power to satisfy additional demand.

Battery State of Charge Management

Maintaining appropriate battery state of charge (SOC) balances multiple objectives. Sufficient charge headroom enables electric-only operation during low-speed driving and provides regenerative braking capacity. However, charging the battery requires running the engine, which may be inefficient at light loads. Typical strategies maintain SOC within a target band, adjusting the band based on driving conditions and predicted demands.

Real-Time Optimization Execution

Implementing optimization in real-time requires computationally efficient algorithms executable within control loop timing constraints. Simplified models capture essential system behavior without excessive computational burden. Look-up tables pre-computed from detailed optimization provide rapid access to optimal setpoints. Adaptive algorithms refine these strategies based on measured system response and changing conditions.

Start-Stop System Control

Automatic Engine Shutdown

Start-stop systems automatically shut down the engine when the vehicle is stationary and restart it when the driver releases the brake or engages the clutch. This reduces fuel consumption and emissions during idle periods, particularly in urban driving. The control system monitors multiple conditions before allowing shutdown, including engine temperature, battery state, climate control demands, and driver inputs.

Belt-Integrated Starter-Generator

Mild hybrid architectures often implement start-stop functionality using a belt-integrated starter-generator (BSG) replacing the conventional alternator. This motor-generator can start the engine more quickly and quietly than traditional starters, enabling rapid restarts when the driver requests acceleration. The BSG also provides limited torque assist during acceleration and regenerative braking during deceleration.

Restart Speed Requirements

Consumer acceptance of start-stop systems depends critically on restart performance. Target restart times of 300-400 milliseconds require powerful starter motors and optimized engine restart sequences. Direct fuel injection enables combustion restart on the appropriate stroke without waiting for full engine rotation. Some systems maintain engine position during shutdown to facilitate faster restart.

Battery Considerations

Frequent engine cycling increases battery demands compared to conventional vehicles. Enhanced flooded batteries (EFB) or absorbed glass mat (AGM) batteries provide improved cycle life and charge acceptance necessary for start-stop operation. Some systems include a secondary battery dedicated to starting, allowing the main battery to support vehicle electrical loads without interruption during engine-off periods.

Driver Comfort Integration

Climate control is a primary consideration in start-stop operation. Electric auxiliary heaters and coolers maintain cabin comfort during engine-off periods. The control system may prevent shutdown or force early restart when cabin temperatures deviate from setpoints. Integration with navigation and traffic data can predict stop duration, adjusting shutdown decisions based on expected wait times.

Electric Turbocharger Systems

Electrically Assisted Turbocharging

Electric turbocharger systems address turbo lag by adding an electric motor to the turbocharger shaft. This motor can spin the compressor to provide boost pressure before exhaust energy is sufficient to drive the turbine. The combination enables downsized engines that maintain responsiveness while achieving efficiency gains from reduced displacement. Electric assist also enables more aggressive turbocharger sizing optimized for high-speed efficiency.

E-Compressor Configurations

Some systems separate the electric compressor from the exhaust-driven turbocharger, placing an electrically driven compressor in series. This configuration allows independent optimization of each component and can provide boost at any engine condition. The electric compressor activates during transients or low-speed operation where the exhaust turbo is ineffective, deactivating once the turbo provides sufficient boost.

48-Volt System Integration

Electric turbochargers require power levels exceeding conventional 12-volt system capabilities, driving adoption of 48-volt mild hybrid architectures. The higher voltage reduces current requirements for the same power, enabling practical wiring and connector solutions. Typical electric turbocharger motors produce 5-10 kW, requiring currents that would be impractical at 12 volts but manageable at 48 volts.

Control Strategy Integration

Electric turbocharger control integrates with engine management to coordinate boost pressure, airflow, and fuel delivery. Predictive algorithms anticipate driver demands based on pedal inputs and vehicle state, pre-spinning the compressor before boost is requested. Closed-loop boost pressure control adjusts motor speed to maintain target pressures across varying conditions. Coordination with wastegate control optimizes the balance between electric and exhaust-driven boost.

Thermal and Mechanical Challenges

Integrating electric motors with hot exhaust-driven turbochargers presents significant thermal challenges. Motor components must survive temperatures near the turbine housing or require thermal isolation. High rotational speeds exceeding 100,000 RPM demand precision bearings and careful rotor dynamics design. Cooling strategies range from oil lubrication systems to dedicated coolant circuits for the motor components.

Mild Hybrid 48V Systems

System Architecture

48-volt mild hybrid systems add a belt-driven or integrated starter-generator to conventional powertrains, providing start-stop functionality, regenerative braking, and limited electric boost at moderate cost and complexity. The 48V subsystem operates alongside the traditional 12V system, with a DC-DC converter maintaining 12V battery charge and powering conventional loads. This architecture enables significant efficiency improvements without the high-voltage safety requirements of full hybrids.

Motor-Generator Sizing

Typical 48V motor-generators produce 10-15 kW peak power, sufficient for engine starting, light regenerative braking, and modest torque assist during acceleration. Continuous power ratings of 5-7 kW support sustained assist during climbing or aggressive driving. Belt-driven configurations limit torque transmission but simplify packaging, while crankshaft-integrated designs provide higher torque capacity at greater installation complexity.

Battery Technology

48V systems typically use lithium-ion battery packs with 0.5-1.0 kWh capacity, sized for the power requirements rather than significant energy storage. High power-to-energy ratios demand cells optimized for current handling rather than capacity. The battery management system monitors cell voltages, temperatures, and current, protecting against conditions that would degrade performance or safety.

Regenerative Braking Implementation

The 48V motor-generator captures braking energy that would otherwise be dissipated as heat in friction brakes. Energy recovery during typical driving cycles ranges from 0.3-0.5 kWh per 100 km, depending on driving style and terrain. The control system blends regenerative and friction braking transparently, maintaining expected pedal feel while maximizing energy recovery within battery acceptance limits.

Electric Supercharging and Boost

48V systems can power electric superchargers, electric A/C compressors, and electric water pumps, reducing parasitic loads on the engine. These electrically driven accessories can operate independently of engine speed, enabling optimization for efficiency or performance. Electric boost during acceleration supplements engine torque, improving performance from downsized engines optimized for efficiency rather than peak power.

Plug-In Hybrid Charging Systems

Onboard Charger Design

Plug-in hybrid vehicles include onboard chargers converting AC grid power to DC battery charging power. Charger power ratings typically range from 3.3 kW to 7.7 kW for Level 2 charging, with some vehicles supporting higher powers. The charger includes power factor correction, galvanic isolation, and output voltage regulation to safely and efficiently charge the high-voltage battery from various AC sources.

Charging Standards and Protocols

PHEV charging follows standards including SAE J1772 for North American AC charging and IEC 62196 for European markets. These standards define connector pinouts, communication protocols, and safety interlocks. The vehicle and charging station communicate over a pilot signal to negotiate charging current and monitor session status. Some PHEVs support DC fast charging using CCS or CHAdeMO protocols for rapid charging capability.

Charge Scheduling and Optimization

Smart charging features allow users to schedule charging for off-peak electricity rates or to ensure departure with a full battery. The vehicle's telematics system can integrate with utility demand response programs, modulating charging to support grid stability. Climate preconditioning while plugged in uses grid power to heat or cool the cabin before departure, preserving battery energy for driving.

Battery Thermal Preconditioning

Charging efficiency and battery longevity benefit from maintaining appropriate battery temperatures. During charging, the battery thermal management system can heat cold batteries or cool hot ones using grid power rather than stored energy. Preconditioning before departure ensures optimal battery temperature for efficient acceleration and regenerative braking capability.

Vehicle-to-Grid Potential

Bidirectional charging enables PHEVs to return stored energy to the grid during peak demand periods, potentially earning revenue for owners while supporting grid stability. This capability requires bidirectional onboard chargers and communication protocols for utility interaction. The control system must balance grid service with ensuring adequate charge for the owner's driving needs.

Transition Management Algorithms

Mode Transition Control

Hybrid vehicles operate in multiple modes including electric-only, series, parallel, and engine-only operation. Transitions between modes must be executed smoothly to avoid driveline disturbances perceptible to occupants. The control system coordinates motor torques, clutch engagements, and engine operation to maintain consistent output torque during mode changes. Predictive algorithms anticipate needed transitions, pre-positioning actuators for rapid, smooth switching.

Engine Start-Stop Transients

Starting and stopping the engine during driving requires careful torque management to prevent vehicle surge or hesitation. Motor-generators compensate for engine torque fluctuations during starting by applying opposing torques, maintaining smooth vehicle motion. Engine shutdown sequences coordinate fuel cutoff with motor torque adjustments to minimize disturbances as engine friction torque dissipates.

Clutch Control Strategies

Hybrid architectures with clutches between motors, engines, and output shafts require precise engagement control. Slip control during engagement trades some energy loss for smooth torque transfer. Position and torque sensors provide feedback for closed-loop control. Adaptive algorithms learn clutch characteristics as friction materials wear, maintaining consistent performance throughout service life.

Torque Blending

Coordinating torque contributions from engine and motor-generators requires careful bandwidth matching. Motors can respond to torque commands in milliseconds, while engine torque response is limited by intake manifold dynamics and combustion timing. The control system uses motors to provide fast response while the engine follows at its natural rate, with motor torque ramping down as engine torque builds up.

Drivability Optimization

Beyond avoiding discrete disturbances, drivability optimization ensures the hybrid powertrain responds naturally to driver inputs. Torque interpretation algorithms translate accelerator pedal position to driver intent, considering current vehicle state and recent input history. Response characteristics can be calibrated for different driving modes, from efficiency-focused gradual response to sport-mode aggressive acceleration.

Hybrid Battery Thermal Management

Temperature Impact on Performance

Lithium-ion battery performance degrades significantly at temperature extremes. Cold batteries exhibit increased internal resistance, reducing available power and charging acceptance. Hot batteries face accelerated degradation of electrolyte and electrode materials, shortening service life. Optimal operating temperatures typically range from 20-35 degrees Celsius, requiring active thermal management to maintain this range across ambient conditions from -30 to +45 degrees Celsius.

Liquid Cooling Systems

High-performance hybrid batteries use liquid cooling for effective heat removal during charging and discharging. Coolant circulates through channels integrated into the battery pack, absorbing heat from cells and transporting it to a radiator or chiller. Flow rate modulation based on battery temperature and power levels optimizes cooling effectiveness while minimizing pump energy consumption. Some systems share cooling circuits with power electronics or motors for packaging efficiency.

Air Cooling Approaches

Lower-power hybrid batteries may use air cooling, drawing cabin or ambient air across cell surfaces. While simpler and lighter than liquid systems, air cooling provides less effective heat transfer and may introduce temperature gradients across the pack. Careful air path design ensures adequate airflow to all cells, with filters preventing dust accumulation that would degrade cooling effectiveness over time.

Heating for Cold Operation

Cold-weather operation requires battery heating to enable full performance and protect cells from damage during high-power charging. Resistance heaters embedded in the battery pack or coolant circuit warm cells before operation. Waste heat from power electronics or motors can supplement dedicated heaters. Preconditioning while connected to chargers uses grid power for heating, preserving stored energy for driving.

Thermal Runaway Prevention

Battery thermal management systems include safety functions to prevent and respond to thermal runaway conditions. Temperature monitoring at multiple points within the pack detects abnormal heating indicating internal cell faults. Control systems respond by reducing power, activating maximum cooling, and in extreme cases, isolating affected modules. Pack designs include features to prevent propagation of thermal events between cells.

Thermal Model-Based Control

Sophisticated thermal management uses mathematical models predicting cell temperatures based on current, ambient conditions, and cooling system state. These models enable predictive control that anticipates thermal needs rather than reacting to measured temperatures. State estimation algorithms combine model predictions with sensor measurements to estimate internal cell temperatures not directly measurable.

Power Electronics Integration

Inverter Design

Inverters convert DC battery power to AC motor power using power semiconductor switches arranged in a three-phase bridge configuration. Modern designs use insulated gate bipolar transistors (IGBTs) or increasingly silicon carbide (SiC) MOSFETs for improved efficiency. Switching frequencies from 5-20 kHz balance motor current ripple against switching losses. Advanced gate drive circuits optimize switching transitions for minimum losses while ensuring reliability.

DC-DC Converter Functions

Hybrid vehicles require DC-DC converters to interface systems operating at different voltages. High-voltage to 12V converters power conventional vehicle loads and maintain auxiliary battery charge. In 48V mild hybrids, bidirectional converters transfer power between 48V and 12V systems. Boost converters increase battery voltage to motor inverter levels in some architectures, enabling smaller battery packs or higher motor voltages.

High-Voltage Safety

Hybrid high-voltage systems typically operate at 200-400V for conventional hybrids or 400-800V for plug-in variants. Safety measures include orange-colored cables identifying high-voltage circuits, interlocks disabling high voltage when connectors are separated, and insulation monitoring detecting ground faults. Service disconnect switches allow technicians to de-energize high-voltage systems before maintenance.

EMC Considerations

Power electronic switching generates electromagnetic interference that can disrupt vehicle communication systems and external radio services. Shielded cables, filtered connectors, and careful layout minimize radiated emissions. Input and output filters attenuate conducted interference on power and signal lines. EMC testing validates compliance with regulatory limits before vehicle production.

Integration and Packaging

Power electronics integration reduces system volume, weight, and cost while improving reliability by eliminating external connections. Integrated power units combine inverters, DC-DC converters, and chargers in single housings with shared cooling. Direct mounting of power modules to motors eliminates high-current cables and connectors. Advanced packaging technologies enable operation at higher temperatures, reducing cooling requirements.

Diagnostics and Fault Management

OBD-II Hybrid Extensions

Hybrid vehicle diagnostics extend traditional OBD-II functionality with hybrid-specific data and fault codes. Standardized parameter IDs provide access to battery voltage, current, state of charge, and motor parameters. Hybrid-specific diagnostic trouble codes identify faults in motor control, battery management, and powertrain coordination systems. Enhanced diagnostic tools access manufacturer-specific information for detailed troubleshooting.

Battery Health Monitoring

Battery management systems continuously monitor cell health through voltage, current, and temperature measurements. Impedance estimation during operation detects degraded cells before complete failure. Capacity fade tracking compares measured energy throughput to expected values, estimating remaining battery life. Predictive algorithms forecast when battery replacement may be needed, enabling proactive service planning.

Fault Response Strategies

Hybrid control systems implement graduated responses to detected faults based on severity and safety implications. Minor faults may set codes for later diagnosis while allowing continued normal operation. More serious faults trigger limp-home modes with reduced power but continued mobility. Critical faults requiring immediate shutdown implement safe states protecting occupants and emergency responders.

Isolation Monitoring

High-voltage system integrity is continuously verified through insulation resistance monitoring. The isolation monitor detects degraded insulation before hazardous leakage currents develop. Response to detected faults ranges from warning indicators for minor degradation to system shutdown for severe insulation failure. Regular isolation checks during key-on sequence verify safe conditions before enabling high-voltage systems.

Future Trends in Hybrid Technology

Advanced Battery Technologies

Next-generation battery technologies promise improved energy density, charging speed, and longevity. Solid-state batteries eliminate flammable liquid electrolytes while enabling higher energy density. Silicon anode materials increase capacity compared to graphite anodes. Advanced thermal management enables faster charging without accelerated degradation. These improvements will enhance hybrid vehicle capability while reducing battery size and cost.

Wide-Bandgap Power Semiconductors

Silicon carbide and gallium nitride power devices offer improved efficiency, higher operating temperatures, and faster switching compared to silicon. These characteristics enable smaller, lighter power electronics with improved efficiency. While currently more expensive than silicon, manufacturing scale and competition are reducing costs toward parity. Widespread adoption will improve hybrid vehicle efficiency and packaging.

Increased Electrification

The trend toward greater electrification continues, with plug-in hybrids offering increasing electric range. Extended range electric vehicles (EREVs) use engines primarily for charging rather than direct propulsion. Some markets are transitioning directly from mild hybrids to battery electric vehicles, while others maintain hybrid options for infrastructure or use case reasons. The optimal balance between battery capacity and engine power continues evolving.

Connected Vehicle Integration

Vehicle connectivity enables sophisticated optimization based on route, traffic, weather, and grid conditions. Cloud-based computing can perform complex optimizations impractical in vehicle controllers, downloading optimal control strategies for predicted driving conditions. Over-the-air updates improve vehicle efficiency and capability throughout the ownership period. Integration with smart grid systems optimizes charging for cost, carbon intensity, and grid stability.

Conclusion

Hybrid vehicle technologies represent a sophisticated integration of mechanical, electrical, and electronic systems working in concert to improve transportation efficiency and reduce environmental impact. From the precise control of power split devices enabling electronic continuously variable transmission to the complex energy optimization algorithms balancing multiple power sources, these systems demonstrate the power of electronic controls to transcend the limitations of individual components.

The progression from simple start-stop systems through mild hybrids to full and plug-in hybrids illustrates increasing levels of electrification, each bringing enhanced capability and complexity. Understanding these systems requires knowledge spanning power electronics, electric machines, battery technology, and embedded control systems, making hybrid technology an excellent case study in modern automotive electronics integration.

As battery technology improves and costs decline, the line between hybrids and electric vehicles continues to blur. However, hybrids remain relevant for applications where charging infrastructure is limited, driving patterns are unpredictable, or range requirements exceed current battery capabilities. The control strategies, power electronics, and system integration techniques developed for hybrids form the foundation for continuing electrification of transportation.