Hybrid Vehicles
Hybrids combine an engine with electric motors and a battery. Learn micro, mild, full and plug-in hybrids, series, parallel and power-split architectures, regenerative braking, engine-operating-point optimisation and energy-management strategies.
A hybrid uses electric drive to cover the engine's weak spots: it recovers braking energy, lets the engine switch off at stops, adds torque at low speed, and shifts the engine to efficient operating points. The result is 20–40 % less fuel in city driving.
Plug-in hybrids add a larger battery for daily electric driving while keeping the engine for long trips.
An analogy
A hybrid is like a cyclist with an e-bike assist: the motor helps on starts and hills, recovers energy on descents, and the rider (engine) pedals only where they're most efficient.
Levels of hybridisation
Micro (start-stop only), mild (48 V belt starter-generator; assist and small regen), full hybrid (can drive on electricity at low speed — e.g. power-split systems), plug-in hybrid (PHEV — 30–100 km electric range, charged from the grid), range-extender (series with small engine).
Architectures
Series: the engine drives only a generator; motors drive the wheels (efficient in city, losses at highway). Parallel: engine and motor both drive the wheels mechanically (efficient at highway). Power-split / series-parallel: a planetary gear set blends both paths continuously (e-CVT).
Regenerative braking and start-stop
During deceleration the motor becomes a generator, charging the battery instead of heating brake discs. Typical recovery is 50–70 % of braking energy at moderate decelerations. Engine-off at idle and low-speed electric creep save fuel in traffic.
Energy management
The controller decides every moment how to split power between engine and battery while keeping SoC within limits. Rule-based strategies (thresholds) are common; optimisation methods — equivalent consumption minimisation (ECMS), dynamic programming, and predictive control using navigation data — approach the theoretical best fuel economy.
Formulas
- Braking energy available:
E = ½ m (v₁² − v₂²) - Recovered energy:
E_rec = η_regen · E - Fuel energy saved:
m_fuel = E_rec · BSFC / 3600 [g, E in kJ] - Power split (planetary):
ω_s z_s + ω_r z_r = ω_c (z_s + z_r)
Worked example: Regeneration from one stop
Given: 1500 kg from 60 km/h to 0; Recovery 60 %
Find: Energy recovered.
- E = ½ × 1500 × 16.67² = 208 kJ
- E_rec = 0.6 × 208 = 125 kJ = 0.035 kWh
Answer: ≈ 125 kJ per stop
Worked example: Fuel saved in city driving
Given: 30 such stops per 10 km; Recovered energy reused, engine BSFC 250 g/kWh
Find: Fuel saved per 10 km.
- E_rec = 30 × 125 = 3750 kJ = 1.04 kWh
- Fuel = 1.04 × 250 = 260 g ≈ 0.35 L
Answer: ≈ 0.35 L per 10 km (a large share of city consumption)
Common mistakes
- Wrong: Assuming hybrids help most on the motorway. — Right: Gains are largest in stop-go driving; highway gains are smaller.
- Wrong: Using PHEV lab figures without charging. — Right: Real-world benefit depends on regular charging.
Applications
- Full hybrids: Power-split e-CVT taxis and family cars with ~4 L/100 km city economy.
- Mild hybrids: 48 V systems adding torque fill and smoother start-stop at low cost.
- PHEVs & range extenders: Daily electric commuting with engine backup; series hybrids in buses and trains.
Key takeaways
- Micro → mild → full → plug-in hybrids.
- Series, parallel, power-split architectures.
- Regen + engine-point optimisation save fuel.
- Energy management decides the power split.
Going further
- Planetary power split: Speed relations of sun (MG1), carrier (engine) and ring (output/MG2) let the engine speed float independently of vehicle speed.
- Predictive energy management: Navigation and traffic data plan SoC usage over a route (e.g. save battery for city zones).
- Real-driving emissions: Hybrids must manage catalyst temperature during frequent engine stops.