Understanding micro, mild, full and plug-in hybrid electric vehicles

Most of the vehicles in use worldwide are still powered by internal combustion engines (ICE), either gasoline/petrol or diesel. A Hybrid Electric Vehicle (HEV) has at least two sources of power for propulsion: the internal combustion engine and an electric motor.

There are three main reasons for which the automotive manufacturers are developing and selling HEVs:

  • reduction of the CO2 emissions (by reduction of the fuel consumption)
  • reduction of the exhaust gas toxic emissions
  • improvement of the powertrain dynamics (by increasing total power and torque)

The powertrain of a HEV is quite complex because it contains all the components of an ICE vehicle plus most of the components of a pure electric vehicle (EV). Also, depending on the level of hybridization, it needs two energy sources, the fuel tank for the engine and a battery for the electric machine.

Toyota Prius V Hybrid Powertrain (HEV)

Image: Toyota Prius V Hybrid Powertrain (HEV)
Credit: Toyota

If we have an ICE vehicle, in order to transform it into a HEV, we need to add:

  • a high voltage battery (between 200 and 400 V)
  • a power electronics controller (inverter)
  • an electric machine
  • a DC-DC converter

The main disadvantages of a HEV are: it’s adding more weight to the vehicle due to additional electric components, it’s more difficult to build and the total price of purchasing and ownership increases (compared to a ICE vehicle).

In most of the HEVs the electric propulsion is done using permanent-magnet electric machines. The main advantages of an electric machine, compared to an ICE, are:

  • constant high torque at low speeds
  • very high efficiency
  • instant torque delivery
  • energy recuperation capability
Chevrolet Malibu Hybrid powertrain (HEV)

Image: Chevrolet Malibu Hybrid powertrain (HEV)
Credit: Chevrolet


  1. 1.8 L internal combustion engine and electric drive unit
  2. lithium-ion high voltage battery system

Compared with a conventional powertrain, by putting together an electric machine with an ICE we get the following advantages:

  • by providing torque assistance with the electric motor, the ICE can work in the most fuel efficient point (speed and torque)
  • the ICE can be downsized, retaining in the same time a constant overall torque and power of the powertrain, thanks to the electric motor assistance
  • the kinetic energy of the vehicle during braking can be recovered and stored in the high voltage battery, with the help of the electric machine operating as a generator
  • the torque response of the powertrain can be improved, because of the instant torque delivery of the electric motor
  • the gear ratios of the transmission can be lowered, to keep the engine at lower speed operating points (better fuel efficiency), because the electric motor can deliver instant torque request from the driver

Having two sources of power, the hybrid control system needs to decide what is the torque split between the ICE and electric machine, depending on the driver input and vehicle operating state.

A hybrid electric vehicle can perform at least one or more of the following functions:

  • engine idle stop/start
  • electric torque assistance (fill and boost)
  • energy recuperation (regenerative braking)
  • electric driving
  • battery charging (during driving)
  • battery charging (from the grid)

A hybrid electric vehicle it’s also called a Full Hybrid Electric Vehicle (FHEV), in order to make a distinction with the other types of hybrid electric vehicles (mild and plug-in).

Full Hybrid Electric Vehicle (FHEV) modes

Image: Full Hybrid Electric Vehicle (FHEV) modes

ENG – internal combustion engine
MOT – electric motor (machine)
TX – transmission
BATT – high voltage battery
PE – power electronics module (MOT controller)

Idle stop/start function

When the vehicle is stationary, the stop/start (S&S) function switches off the internal combustion engine, without the intervention of the driver (through the ignition key). This function reduces the overall fuel consumption of the vehicle. When the driver shows the intention to drive (clutch pedal pressed or brake pedal released) the engine is restarted automatically.

Most of the vehicles with idle stop/start function have also some sort of energy management function, which optimizes the consumption of the low voltage (12 V) battery energy. In a conventional ICE vehicle, without any energy management, the primary function of the low voltage battery is to generate the electrical energy required for the engine to start. After the engine is running, the electrical energy for all the electrical consumers is supplied by the alternator (generator), which is putting a load torque on the engine.

If the vehicle has an energy management function, even if the engine is running, the battery supplies electrical energy to the consumers. In this way, the alternator doesn’t have to produce electrical energy, the load torque of the alternator is nearly zero, and the fuel consumption is reduces. Further, the battery is recharged when the engine is working in the most fuel efficient points or when the vehicle is braking (through energy recuperation).

Renault 1.6 dCi engine (Micro Hybrid)

Image: Renault 1.6 dCi engine (Micro Hybrid)
Credit: Renault

An example for idle stop/start and energy management functions is the Renault engine 1.6 dCi. It comes with an Energy Smart Management (ESM) function which allows energy created under braking and deceleration to be stored in the low voltage battery, helping to further reduce fuel consumption.

Vehicles that have idle stop/start and energy management functions are called Micro Hybrids.

Electric torque assistance

The electric motor can provide additional torque to the wheel, improving the overall torque response of the powertrain. There are two types of torque assistance:

  • torque fill
  • torque boost

When the drive is pressing the accelerator pedal, it requests more torque from the powertrain. An internal combustion engine (especially diesel) has a certain delay in delivering the requested torque. The torque response delay of the internal combustion engine has several causes:

  • the inertia of the air in the intake manifold
  • the mechanical inertia of the moving parts
  • the torque limitation (to prevent smoke in the exhaust)

In these situations, called torque transients (engine is changing the operating point), the electric motor can assist, providing additional torque, which compensates for the engine torque response delay. This function is called torque filling.

Honda IMA powertrain (MHEV)

Image: Honda IMA powertrain (MHEV)
Credit: Honda

An internal combustion engine has a maximum torque capability, which depends on the engine speed. By adding the electric motor torque, additional to the engine torque, the maximum overall torque of the powertrain is increased (positive offset). This function is called torque boost and can be supplied only for a short duration of time (order of seconds) due to battery depletion.

The function of electric torque assistance is usually provided by mild hybrid electric vehicles (MHEV), full hybrid electric vehicles (FHEV) and plug-in hybrid electric vehicles (PHEV).

When both, the engine and the electric motor, are providing torque for vehicle acceleration, the vehicle is in hybrid/parallel mode.

Energy recuperation (regenerative braking)

When the driver is pressing the brake pedal, the vehicle needs to slow down. Basically we need a braking torque at the wheels in order to reduce the vehicle speed. The total required braking torque at the wheels can be achieved in several ways:

  • only through the foundation brakes (hydraulic brakes)
  • through the foundation brakes plus the powertrain

If the vehicle has a conventional powertrain, only with internal combustion engine, when the driver brakes, the fuel injection is interrupted (fuel-cut) and the engine overruns (engine braking). The amount of engine brake is equal with the total torque losses of the engine (friction torque + pumping losses + auxiliary devices).

Audi Prologue concept (MHEV)

Image: Audi Prologue concept (MHEV)
Credit: Audi


  1. 48V electric machine
  2. 48V lithium-ion battery
  3. 12V battery
  4. DC-DC converter (bidirectional)
  5. 12V electrical system

In an hybrid electric vehicle, when the driver brakes, a negative torque can be requested from the electric machine, enhancing the braking capability of the powertrain. In all hybrid electric vehicles, during vehicle braking, the electric machine is in generator mode. The kinetic energy of the vehicle spins the rotor of the generator, overcoming its negative torque, and electrical energy is generated. The amount of electrical energy generated (harvested) during braking (recuperation/regeneration) depends on the power of the electrical machine.

Electric driving

If the electric machine is powerful enough, the vehicle can be driven in electric mode (EV). In this mode, the internal combustion engine is switched off and the electric motor is proving all the necessary torque for the propulsion of the vehicle.

In the case of full hybrid electric vehicles, the electric mode is only possible up to vehicles speeds of 5 – 10 kph, due to limited energy available in the battery. In the case of plug-in hybrid electric vehicles, the high voltage battery has higher capacity, the EV mode being possible up to speeds of 90 – 100 kph.

Battery charging (during driving)

Every battery has a minimum state of charge (SOC) which needs to be maintained in order to avoid permanent damage. The state of charge represents the theoretical amount of electrical energy available in the battery. If the SOC of a battery is 100% means that there is a maximum theoretical amount of electrical energy which can be used. If the minimum SOC for the battery is 20% we can only use 80% of the theoretical maximum.

Mitsubishi Outlander powertrain (PHEV)

Image: Mitsubishi Outlander powertrain (PHEV)
Credit: Mitsubishi

Depending on the size, power and chemistry of the battery, the minimum SOC is different. In the table below there is synthesis of the minimum SOC battery function of the type of hybrid electric vehicle:

Type of hybrid electric vehicleMicro (S&S)Mild (MHEV)Full (FHEV)Plug-in (PHEV)
Minimum battery SOC [%]80 … 9040 … 6030 … 5010 … 20
Battery voltage [V]1248 / 160200 – 300300 – 400
Battery chemistrylead-acidlithium-ion / nickel – metal hydridelithium-ionlithium-ion

In any hybrid electric vehicle, function of the SOC level, the battery can be is several states:

  • charge deplete
  • charge sustain
  • charging
BMW i8 powertrain (PHEV)

Image: BMW i8 powertrain (PHEV)
Credit: BMW

When the battery is fully charged, the electrical energy is available for use. In this case the battery is in charge deplete mode. When the SOC of the battery reaches the minimum level, the internal combustion engine is responsible to charge the battery so that the SOC doesn’t go under the minimum level. In this case the battery is in charge sustain mode. When the vehicle is braking, the kinetic energy of the vehicle is converted in electrical energy and stored in the battery. In this case, the battery is in charging mode.

Battery charging (from the grid)

In terms of battery charging, the main difference between a full hybrid electric vehicle and a plug-in electric vehicle is that the PHEV can also be charged by connecting it to a power socket. The power electronics control module of a plug-in hybrid electric vehicle contains a rectifier, which converts the alternating current (AC) of the power socket into direct current (DC) and stored into the high voltage battery.

BMW 330e PHEV charging from the grid

Image: BMW 330e PHEV charging from the grid
Credit: BMW

Depending on the functions that can be handled by the electric system, we distinguish the following types of hybrid electric vehicles:

FunctionsType of hybrid electric vehicle
Micro (S&S)Mild (MHEV)Full (FHEV)Plug-in (PHEV)
idle stop/start
electric torque assistance
(fill and boost)
energy recuperation
(regenerative braking)
electric driving
(EV mode)
battery charging
(during driving)
battery charging
(from the grid)

In the case of mild hybrid electric vehicles (MHEV) there are two distinct types:

  • with a belt-integrated starter generator
  • with a crankshaft-integrated motor generator

The belt-integrated starter generator (BiSG) is using an electric machine mounted on the front end accessory drive (FEAD), being connected with the internal combustion engine through a belt. This is the most commons solution used by the automotive manufacturers for mild hybrid electric vehicles. Valeo has developed a BiSG system for MHEV, which is used by several vehicle manufacturers.

Valeo Belt Starter Generator system (MHEV)

Image: Valeo Belt Starter Generator system (MHEV)
Credit: Valeo

The crankshaft-integrated motor generator (CiMG) is using an electric machine fitted on the crankshaft, between the engine and transmission. An example of CiMG system is the Integrated Motor Assist (IMA) technology from Honda. The main difference between the BiSG and CiMG is that the crankshaft-integrated motor generator solution is using a more powerful electric machine and an battery with higher voltage and power.

In the table below you can find a synthesis of the types of hybrid electric vehicles, in term of battery voltage, electric machine power and potential fuel consumption benefit:

ParameterMicro HybridMild HybridFull HybridPlug-in Hybrid
Battery voltage [V]1248 – 160200 – 300300 – 400
Electric machine power [kW]
2 … 310 … 1530 … 5060 … 100
Electric machine power [kW]
< 310 … 1230 … 4060 … 80
EV mode range [km]005 … 10< 50
CO2 estimated benefit [%]5 … 67 … 1215 … 20> 20

In future article we will describe each hybrid electric vehicle type in more detail.

For any questions, observations and queries regarding this article, use the comment form below.

Don’t forget to Like, Share and Subscribe!


  1. Tony
  2. Valentine
  3. Michal Sztorc
  4. Adrianus

Leave a Reply

Ad Blocker Detected

Dear user, Our website provides free and high quality content by displaying ads to our visitors. Please support us by disabling your Ad blocker for our site. Thank you!