Hybrid architectures: series, parallel and power-split

Degrees of hybridisation and how series, parallel (P0–P4) and power-split or series–parallel hybrids route engine and electric power, with series-path efficiency, parallel power and planetary power-split speed, torque and power-balance numericals.

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Why it matters

Hybrids combine an engine with one or more electric machines and a battery so the engine can run near its best efficiency, shut off when not needed, and be assisted or replaced by the motor, while braking energy is recovered. Strong hybrids and mild hybrids are now common in Indian showrooms, and the architecture — series, parallel or power-split — decides how much fuel is saved, what it costs and how it drives. Understanding power flow through each layout, especially the planetary power-split device, is a frequent interview and exam topic.

Key ideas

Degree of hybridisation.

  • Micro hybrid: start-stop only, using an enhanced starter or belt starter-generator.
  • Mild hybrid: a belt- or crank-mounted motor-generator (typically 12–48 V, a few kW to about 15 kW) for start-stop, torque assist and modest regeneration; cannot drive electrically for any distance.
  • Full (strong) hybrid: a high-voltage motor large enough to move the car on electric power alone at low speed.
  • Plug-in hybrid (PHEV): a full hybrid with a larger battery charged from the grid, giving tens of kilometres of electric driving.
  • Range-extended EV: an EV with a small engine-generator (a series hybrid).

Series hybrid. The engine drives only a generator; a traction motor alone drives the wheels; the battery buffers the difference.

  • The engine is decoupled from road speed, so it can run at its most efficient operating point or switch off.
  • Every kW from the engine is converted twice (mechanical → electrical → mechanical), so at steady highway speed the conversion losses can exceed the gain. Three machines are needed, and the traction motor must be sized for full vehicle power.
  • Suits stop-go duty: city buses, range extenders, some passenger cars that drive in series mode most of the time.

Parallel hybrid. Engine and motor both drive the wheels mechanically; their torques add on a common shaft.

  • Classified by motor position: P0 belt starter-generator on the engine's accessory drive, P1 motor on the crankshaft, P2 motor between engine and gearbox with a disconnect clutch (allows EV driving), P3 motor on the gearbox output, P4 motor on the other axle ("through-the-road" hybrid, also giving all-wheel drive).
  • Efficient at steady speed (direct mechanical path), needs only one motor, but the engine speed is still tied to vehicle speed through the gearbox.
  • At a given shaft speed, peak powers add only if both units can deliver their peak at that speed and the battery can supply the motor.

Series–parallel and power-split hybrids.

  • Power-split (e.g. Toyota's system): a planetary gear set with the engine on the carrier, generator MG1 on the sun and the output (with traction motor MG2) on the ring. Engine power divides into a mechanical path (to the ring) and an electrical path (MG1 generates, MG2 motors). By controlling MG1 speed the engine speed is set independently of vehicle speed — an electronic CVT — with no clutch or gear shifts.
  • Clutch-switched series–parallel (e.g. Honda's system): mainly series drive, with a clutch that locks the engine to the wheels at cruising speed.
  • These combine the engine-speed freedom of series with the direct path of parallel, at the cost of complexity.

Operating modes and control. EV mode, engine-only, power assist (both), engine charging, regenerative braking, and engine-off coasting. A full hybrid is normally charge-sustaining: the energy-management controller keeps battery SoC in a middle window (it never lets it run flat), while a PHEV first runs charge-depleting. Strategies range from rule-based maps to equivalent-consumption minimisation.

Formulas

η_series = η_gen × η_rect × η_inv × η_mot

  • Engine-shaft-to-motor-shaft efficiency of the direct electrical path in a series hybrid (dimensionless); add battery charge and discharge efficiencies for energy that passes through the battery.

T_shaft = T_e + T_m, P = T_shaft × ω (parallel, both on the same shaft)

  • T_e, T_m: engine and motor torques at that shaft (N·m), ω (rad/s).

HF = P_mot / (P_mot + P_eng)

  • Hybridisation factor (dimensionless).

Z_s ω_s + Z_r ω_r = (Z_s + Z_r) ω_c

  • Planetary (Willis) speed equation: Z_s, Z_r: sun and ring teeth; ω_s, ω_r, ω_c: sun, ring and carrier speeds (any consistent unit, with sign).

T_r = T_c × Z_r / (Z_s + Z_r), T_s = T_c × Z_s / (Z_s + Z_r)

  • Steady-state, lossless split of carrier (engine) torque T_c into ring torque T_r and the reaction torque T_s that MG1 must hold (N·m).

η_e = 3600 / (bsfc × LHV)

  • Engine brake efficiency (dimensionless) with bsfc in g/kWh and LHV in MJ/kg.

Worked examples

Example 1 (standard). (a) A series hybrid runs its engine at 60 kW with bsfc 230 g/kWh (LHV 43 MJ/kg). Generator 94 %, rectifier 97 %, motor inverter 97 %, motor 93 %. Find the power at the motor shaft, engine efficiency, fuel flow and overall fuel-to-motor-shaft efficiency (direct path). (b) A parallel hybrid has an 80 kW engine and a 50 kW motor on the same shaft. Find the peak combined power and hybridisation factor.

  1. η_series = 0.94 × 0.97 × 0.97 × 0.93 = 0.823; P = 0.823 × 60 = 49.4 kW.
  2. η_e = 3600 / (230 × 43) = 0.364; fuel flow = 230 × 60 = 13,800 g/h = 13.8 kg/h.
  3. Overall = 0.364 × 0.823 = 0.299.
  4. Parallel: P_peak = 80 + 50 = 130 kW (if both peak at the same shaft speed and the battery can supply 50 kW); HF = 50 / 130 = 0.385.

Answer: 49.4 kW, 36.4 %, 13.8 kg/h, about 30 % overall; 130 kW and HF ≈ 0.38.

Example 2 (GATE level). In a power-split hybrid the sun has 30 teeth and the ring 78. The engine (carrier) runs at 2000 rpm delivering 120 N·m while the ring (output) turns at 3000 rpm. Find the MG1 (sun) speed, the torques on the ring and sun, and the power balance. Neglect losses.

  1. Willis: 30 ω_s + 78 × 3000 = 108 × 2000 → ω_s = (216,000 − 234,000) / 30 = −600 rpm (opposite to the engine).
  2. T_r = 120 × 78 / 108 = 86.7 N·m; T_s = 120 × 30 / 108 = 33.3 N·m.
  3. Engine power = 120 × (2000 × 2π / 60) = 120 × 209.4 = 25.13 kW.
  4. Ring power = 86.7 × (3000 × 2π / 60) = 86.7 × 314.2 = 27.23 kW.
  5. MG1 power = 25.13 − 27.23 = −2.09 kW: MG1 must motor, supplying 2.09 kW, because it turns backwards while holding the reaction torque. The electrical energy comes from MG2 or the battery — a "negative split" that occurs when the ring overspeeds the engine.

Answer: MG1 at −600 rpm; 86.7 N·m on the ring and 33.3 N·m on the sun; MG1 supplies about 2.09 kW.

Common mistakes

  • Adding engine and motor power for a series or power-split hybrid. In a series hybrid only the traction motor drives the wheels; in a power-split the generator's power is not extra output.
  • Forgetting the double conversion loss in the series path.
  • Mixing up planetary members: in the common power-split layout the engine is on the carrier, MG1 on the sun and the output on the ring.
  • Dropping signs in the Willis equation; a negative result means opposite rotation.
  • Thinking a full hybrid runs its battery flat. It is charge-sustaining and keeps SoC in a window.
  • Assuming a mild hybrid can drive on electricity alone.

For GATE ME

Hybrid questions test theory-of-machines and power-flow skills: epicyclic gear train speed and torque relations, power balance with signs, efficiency chains and engine bsfc–efficiency conversions. Practise the tabular or Willis method for epicyclic trains and always check that powers balance.

Quick check

  1. In which architecture is the engine never mechanically connected to the wheels?
  2. Where is the motor in a P2 parallel hybrid?
  3. Sun 30, ring 78, carrier 1500 rpm, ring 1200 rpm: what is the sun speed?
  4. Why is a series hybrid attractive for city buses?
  5. What does "charge-sustaining" mean?

Answers: 1. Series. 2. Between the engine and the gearbox, with a clutch to disconnect the engine. 3. (108 × 1500 − 78 × 1200) / 30 = 2280 rpm. 4. Stop-go duty with lots of regeneration, and the engine can run at its best point or switch off. 5. Over a drive cycle the battery ends at about the same SoC it started with; all net energy comes from fuel.

Interview questions

All Automotive Electronics and Electric Vehicles interview questions

Try answering each one aloud before you open it.

  1. 1.What is a series hybrid architecture in electric vehicles?Concept

    In a series hybrid architecture, the internal combustion engine (ICE) is connected to a generator, which produces electricity to charge the battery or power the electric motor. The electric motor is the sole source of propulsion for the vehicle, meaning the ICE does not directly drive the wheels. This setup allows for the ICE to operate at its most efficient range, improving fuel efficiency.

  2. 2.Explain the parallel hybrid architecture used in electric vehicles.Concept

    In a parallel hybrid both the engine and the electric motor are mechanically connected to the driveline, so their torques add and either or both can drive the wheels. Hybrids are classified by motor position: P0 is a belt starter-generator, P1 on the crankshaft, P2 between engine and gearbox with a disconnect clutch so the car can drive electrically, P3 on the gearbox output and P4 on the other axle. The direct mechanical path is efficient at steady speed and only one motor is needed, but engine speed remains tied to road speed through the gearbox.

  3. 3.Describe the power-split hybrid architecture and its advantages.Concept

    A power-split hybrid uses a planetary gear set with the engine on the carrier, generator MG1 on the sun and the output shaft with traction motor MG2 on the ring. Engine power is split into a mechanical path straight to the ring and an electrical path in which MG1 generates and MG2 motors. By controlling MG1's speed, the controller sets engine speed independently of vehicle speed, so the system acts as an electronic CVT with no clutch or gear shifts. The engine can therefore stay near its most efficient operating line, and the system can also drive electrically, regenerate and run in combined modes; the cost is two machines and more complex control.

  4. 4.Why is a series hybrid architecture often used in urban buses?Application

    Series hybrid architecture is often used in urban buses because it allows the internal combustion engine to run at its most efficient speed, reducing emissions and fuel consumption. The electric motor provides smooth and quiet operation, which is beneficial in urban environments with frequent stop-and-go traffic. Additionally, regenerative braking can be effectively utilized to recharge the battery, further enhancing efficiency.

  5. 5.What happens if the battery in a parallel hybrid vehicle is completely discharged?Application

    In a normal full or mild parallel hybrid the battery is never allowed to run completely flat: the energy-management controller keeps state of charge within a middle window and, when SoC gets low, loads the engine slightly more so the motor-generator recharges the battery. If SoC reaches its lower limit, electric assist and EV mode are disabled and the car drives on the engine alone, with reduced performance and fuel economy, until charge is rebuilt. A genuinely dead or faulty high-voltage battery would set fault codes, and depending on the design the car may run in a limp mode or be unable to start if the motor-generator is also the starter.

  6. 6.How does regenerative braking work in a power-split hybrid vehicle?Application

    In a power-split hybrid vehicle, regenerative braking works by using the electric motor as a generator during braking. When the driver applies the brakes, the kinetic energy of the vehicle is converted into electrical energy by the motor-generator. This energy is then stored in the battery for later use, improving overall efficiency by recovering energy that would otherwise be lost as heat.

  7. 7.Calculate the total power output if a parallel hybrid vehicle's internal combustion engine produces 80 kW and the electric motor produces 50 kW.Numerical

    If both units drive the same shaft and can deliver their peak power at the same speed, the peak combined power is 80 + 50 = 130 kW. In practice the sum is reached only where both power curves peak together and the battery can supply 50 kW at that moment; engine and motor peaks often occur at different speeds. The hybridisation factor is 50/130 ≈ 0.38. This simple addition does not apply to series or power-split hybrids, where the generator's power is not additional output.

  8. 8.In a series hybrid vehicle, if the generator efficiency is 85% and the electric motor efficiency is 90%, what is the overall efficiency from the engine to the wheels?Numerical

    On the direct electrical path, overall efficiency is the product of the conversion efficiencies: 0.85 × 0.90 = 0.765, or 76.5 %, from engine shaft to motor shaft. That ignores the power electronics, which may be about 97 % each for the rectifier and inverter, and the final drive, so the real figure is lower. Energy routed through the battery suffers its charge and discharge losses as well. This double conversion is why a series hybrid is less efficient than a parallel one at steady highway speed.

  9. 9.Why might a power-split hybrid be more suitable for a passenger car compared to a series hybrid?Application

    A power-split hybrid might be more suitable for a passenger car because it offers greater flexibility and efficiency across a range of driving conditions. It can seamlessly switch between series and parallel modes, optimizing fuel economy and performance. This adaptability makes it well-suited for varying driving patterns, such as highway cruising and city driving, which are common in passenger cars.

  10. 10.Explain how the planetary gear set functions in a power-split hybrid system.Concept

    The planetary set has three members linked by the Willis equation Z_s·ω_s + Z_r·ω_r = (Z_s + Z_r)·ω_c, so fixing two speeds fixes the third. With the engine on the carrier, MG1 on the sun and the output on the ring, vehicle speed fixes the ring, and MG1's speed then sets engine speed continuously. MG1 provides the reaction torque, which is a fraction Z_s/(Z_s + Z_r) of engine torque, while the ring receives Z_r/(Z_s + Z_r) of it mechanically. MG1's power is routed electrically to MG2 or the battery. If MG1 runs backwards it must motor, giving a negative split.

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