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.