Mild Hybrid Electric Vehicles (MHEV) – examples

A vehicle with conventional powertrain is using an internal combustion engine (gasoline or diesel), a transmission (consisting of clutch/torque converter, gearbox) and a drivetrain (propeller shaft, final drive) to provide torque at the wheels, function of the driver demand (accelerator pedal position).

A mild hybrid electric vehicle (MHEV) has two sources of torque, the internal combustion engine (ICE) and an electric machine (EM). The energy for the ICE comes from the fuel tank and for the EM from a high voltage (HV) battery.

There are also other aspects which need to be considered when comparing a MHEV with a conventional vehicle, such as:

  • the energy flow in the high voltage battery is bidirectional; when the electric machine is in motor state (vehicle accelerates, positive torque), the HV battery supplies the electrical energy; when the electric machine is in generator phase (vehicle decelerates, regeneration/recuperation, negative torque) the HV battery is charged
  • the electric machine can provide a significant amount of negative torque during braking, which means that the friction brakes might not be used, if the electric machine can fulfil the braking torque demand
  • the internal combustion engine is operated at the most efficient point (minimum brake specific fuel consumption) due to the ability of the electric machine to assist with positive/negative torque

Compared with a conventional vehicle, an MHEV has better fuel economy, reduced exhaust gas emissions and increased torque and power.

Mercedes-Benz S 400 HYBRID

S 400 HYBRID is the first passenger vehicle produced by Mercedes-Benz with a hybrid powertrain. It combines a 3.5 liter V6 petrol (gasoline) engine and an electric machine, which certifies on the New European Driving Cycle (NEDC) a combined fuel consumption of 7.9 l/100 km and 186 g/km of CO2.

Mercedes Benz S 400 MHEV

Image: Mercedes-Benz S 400 MHEV
Credit: Daimler

The S 400 HYBRID is an MHEV with a P1 architecture.

The internal combustion engine outputs 279 HP (205 kW) and the electric machine 20 HP (15 kW). This gives a combined maximum powertrain power of 299 HP (220 kW) and a maximum torque of 385 Nm. The maximum electric machine torque is 160 Nm.

The electric machine is positioned between the ICE and the 7-speed automatic transmission (7G-TRONIC). The disc-shaped electric machine performs the following electric motor functions:

  • starter (cold start and fast engine start at brake pedal release)
  • standstill engine stop & start (when the vehicle is stationary)
  • advanced engine stop & start (when the driver brakes and the vehicle speed drops below 15 km/h)
  • torque fill (during acceleration phases)
  • torque assist (during high speed drive)
Mercedes Benz S 400 MHEV - engine and electric machine

Image: Mercedes-Benz S 400 MHEV – engine and electric machine
Credit: Daimler

In generator mode, the electric machine can output electric energy in the following situations:

  • at idle (if the HV battery is depleted)
  • during normal driving (by shifting the operating point of the engine)
  • during overrun (when the accelerator pedal is released but no braking applied)
  • during braking (when the braking pedal is pressed)

The high voltage battery has lithium-ion chemistry and is the energy source for the electric machine and air conditioning compressor. Also, it supplies energy to the 12 V electric network through the DCDC converter.

Mercedes Benz S 400 MHEV - electric machine and automatic transmission

Image: Mercedes-Benz S 400 MHEV – electric machine and automatic transmission
Credit: Daimler

The 3.5-litre V6 petrol engine has variable valve control and works on the Atkinson cycle. The expansion phase is longer than the compression phase. The intake valve is kept open slightly longer between the intake and compression phases, which improves the engine’s thermal efficiency while reducing the specific fuel consumption and exhaust emissions. It also features a new cylinder head, different pistons and a modified camshaft.

Mercedes Benz S 400 MHEV - electric machine

Image: Mercedes-Benz S 400 MHEV – electric machine
Credit: Daimler

The compact electric machine is a 3-phase AC external rotor magneto motor with an operating voltage of 120 V. On top of motor-generator role, the EM acts also as a torsional vibration damper, further reducing the noise and vibration of the powertrain.

The 120 V lithium-ion battery consists of the cell block with its lithium-ion cells and the electronic cell monitoring system, the battery management function, the high-strength housing, the cooling gel, the cooling plate, the coolant feed and the high-voltage connection.

Mercedes-Benz S 400 MHEV - lithium-ion battery

Image: Mercedes-Benz S 400 MHEV – lithium-ion battery
Credit: Daimler

  1. cooling module
  2. lithium-ion cells
  3. battery management system (BMS)
  4. cooling system connection
  5. high voltage connection
  6. cell voltage monitoring circuit

Dedicated control electronics are required to operate the 3-phase AC electric motor in the 120 V high-voltage DC network. The current converter is accommodated in the space formerly occupied by the starter. As the control electronics heat up as a result of electric currents measuring up to 150 A, the system is equipped with its own, additional low-temperature cooling circuit.

Audi A8 Hybrid

Audi A8 is a luxury sedan available for first time with standard electrified powertrain. The vehicle benefits from a mild hybrid (MHEV) P0 architecture. The basis is the 48 V electrical system, which
features for the first time in the new Audi A8 as the primary electrical system.

Audi A8 MHEV - components (1)

Image: Audi A8 MHEV – components (1)
Credit: Audi

  1. internal combustion engine
  2. 12 V starter
  3. DCDC converter
  4. 12 V battery
  5. 48 V battery
  6. 48 V electric machine
Audi A8 MHEV - components (2)

Image: Audi A8 MHEV – components (2)
Credit: Audi

The Audi A8 hybrid drive comprises two main components:

  • the 48 V belt alternator starter (BAS), watercooled, which complements the conventional 12 V pinion starter (only required for cold engine starts)
  • the lithium-ion battery in the luggage compartment to act as the energy accumulator for the MHEV (mild hybrid electric vehicle) with a 10 Ah charge carrier capacity and a 48 V system

The fourth-generation A8 can enter the noiseless coasting mode within the speed range of 55 to 160 km/h (34.2 to 99.4 mph) . The automobile can then coast with zero emissions for up to 40 seconds with the engine off altogether. As soon as the driver steps on the gas again, the BAS prompts a swift, very smooth restart. In addition, start & stop (advanced) operation is actually active from 22 km/h (13.7 mph).

The powertrain management system uses route data and information from its network of sensors (which include the front camera), to decide whether the vehicle is in coasting or energy recuperation mode. It recovers energy not just from brake applications or when coasting, the Audi A8 also does so if it’s in danger of getting too close to a preceding vehicle while coasting.

Dodge Ram 1500 eTorque

The Dodge Ram 1500 is a P0 architecture MHEV with a Pentastar V-6 petrol engine and a 48 V electric system (eTorque). The hybrid powertrain can output a maximum of 305 HP and 365 Nm of torque.

The eTorque mild-hybrid system replaces the traditional alternator on the engine with a belt-driven motor generator unit that performs several functions. The motor generator unit works with a 48 V battery pack to enable improved fuel efficiency, driveability, performance and value with quick, seamless engine stop & start functions and energy recovery with brake regeneration used to support increases in towing capacity and payload.

Dodge Ram MHEV

Image: Dodge Ram MHEV
Credit: Dodge

With the engine running, eTorque’s motor generator unit feeds 48 V current to a 430 Wh lithium-ion nickel manganese cobalt (NMC)-graphite battery. The battery pack includes a 3 kW DCDC converter to maintain the battery’s state of charge and convert 48 V to 12 V to power the Ram 1500’s accessories and charge its conventional 12 V lead-acid battery.

The Pentastar V-6 engine with eTorque retains a conventional 12 V starter motor, used for cold starts and the first start of the day due to its greater efficiency in extreme temperatures. eTorque delivers seamless and nearly unnoticeable engine restarts by sending up to 122 Nm of supplemental torque to the crankshaft during stop-start maneuvers. Torque to the wheels travels within 400 milliseconds, more than twice as fast as many starter-motor engine stop-start systems.

In addition to spinning the engine for restarts to help launch the vehicle, eTorque also recaptures energy during deceleration and braking to charge the battery pack. eTorque also enhances the driving experience by adding torque to the crankshaft during gear changes to minimize noise, vibration and harshness (NVH). The liquid-cooled eTorque motor generator is mounted on the front of the Pentastar engine and employs a pair of belt tensioners to keep the eight-rib drive belt tight when the unit is generating electricity or adding torque to the crankshaft.

Range Rover Evoque

The 2nd generation Range Rover Evoque is a P0 architecture MHEV. The powertrain consists of an Ingenium petrol or diesel engine combined with a powerful 48 V electric machine.

The 48 V lithium-ion battery can store up to 200 Wh of energy for the MHEV system without compromising cabin space. Separate from the normal vehicle battery, the electric machine generates up to 140 Nm of torque to help during vehicle acceleration. When the driver accelerates, this stored energy is redeployed to assist the engine. The vehicle harvests the energy normally lost during deceleration and stores it in the underfloor battery.

Range Rover Evoque MHEV

Image: Range Rover Evoque MHEV
Credit: Jaguar Land Rover

  1. Ingenium engines: a range of petrol and diesel units, with maximum power from 150 HP to 300 HP
  2. 48 V battery: 14 lithium-ion pouch cells, with a capacity of 8 Ah, can provide up to 200 Wh of electrical energy
  3. DCDC converter: converts energy from the 48 V battery to the 12 V network in order to support 12 V battery charging and ancillaries
  4. 48 V belt-integrated starter generator (BiSG): recuperates energy during vehicle deceleration and redeploys at the crankshaft (torque fill and assist) during vehicle acceleration

At speeds below 17 km/h (11 mph), while under braking, the engine will shut off (advanced stop & start) to increase efficiency, restarting as soon as it’s needed. This technology reduces fuel consumption and tailpipe emissions, now as low as 50.4 mpg (5.6 l/100 km) and CO2 emissions of 149 g/km (NEDC equivalent).

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