What is a Hybrid Electric Vehicle (HEV) ?

The word “hybrid” means a mix, blend between two different things. In the automotive industry hybrid is used to describe the powertrain of a vehicle. A Hybrid Electric Vehicle (HEV) is a vehicle which is using two energy sources for propulsion, at least one of the energy sources being electrical energy. The vast majority of hybrid electric vehicles are using a combination of petrol (gasoline) engines and electric motor(s).

There are different types of hybrid vehicles, for example: hybrid air vehicles, hybrid kinetic vehicles, etc., but in this article we are going to focus only on hybrid electric vehicles.

Hybrid vehicle principle

Image: Hybrid vehicle principle

From the working principle point of view, a hybrid vehicle is using 2 energy sources, with 2 energy converters. These laws govern the way a hybrid electric vehicle works:

  • there is primary energy source (1) and a secondary energy source (2)
  • there is primary energy converter (1) and a secondary energy converter (2)
  • for a HEV, the primary energy source is the fuel tank and the secondary energy source is the battery
  • the primary energy source has much more energy content than the secondary energy source
  • energy can be transferred from the primary energy source towards the secondary energy source but not vice versa
  • the transfer of energy from the primary source towards the secondary source is done through energy converters
  • for a HEV, the primary energy converter is the internal combustion engine and the secondary energy converter is the electric machine (motor/generator)
  • part of the kinetic energy of the vehicle can be recovered during braking only by the secondary energy converter and stored in the secondary energy source
  • both energy converters can apply traction torque to the wheel in the same time

A hybrid electric vehicle (HEV) is using both the internal combustion engine and at least one electric machine for propulsion.

There are three main drivers for the development of hybrid electric vehicles:

  • reduce fuel consumption and CO2 emissions
  • reduce exhaust gas emissions
  • improve vehicle dynamics by increasing torque and power output

Types of Hybrid Vehicles

The degree of fuel consumption or vehicle dynamics improvement depend on the level of hybridisation. A hybrid electric vehicle is a mix between a internal combustion engine vehicle (ICEV) and a battery electric vehicle (BEV). The level of energy contained in the battery and the power of the electric machine decide the level (type) of the hybrid electric vehicle.

For a more detailed comparison between hybrid electric vehicle types, read the article Understanding micro, mild, full and plug-in hybrid electric vehicles.

The ratio between different energy sources and the ratio between different propulsion devices determines the type of hybrid vehicle.

Types of vehicles function of energy source and propulsion device

Image: Types of vehicles function of energy source and propulsion device

The different types of hybrid vehicles are summarised in the table below.

TypeEnergy sourcePropulsion deviceCharacteristics
ICEV
(Internal Combustion Engine Vehicle)
– 100% fuel (petrol/gasoline, diesel)– 100% internal combustion engine– conventional vehicle
mHEV
(micro Hybrid Electric Vehicle)
– 99% fuel (petrol/gasoline, diesel)
– 1% electrical energy
– 100% internal combustion engine– it has the ability to use the battery energy for electrical systems without drawing power from the alternator
– can modify the charging profile of the low voltage battery (12 V) by increasing the charge rate during vehicle deceleration
MHEV
(Mild Hybrid Electric Vehicle)
– 80-90% fuel (petrol/gasoline, diesel)
– 10-20% electrical energy
– 80-90% internal combustion engine
– 10-20% electric motor
– the electric machine can provide additional torque during vehicle acceleration phases
– the electric machine can recuperate electrical energy during vehicle deceleration
– has two electrical networks and batteries, a low voltage one (12 V) and a high voltage system (48-150V)
– has a DCDC converter to exchage energy between the low voltage and high voltage network
– has a high voltage electric machine, usually controlled by a 3-phase inverter
HEV
(Hybrid Electric Vehicle)
– 70-80% fuel (petrol/gasoline, diesel)
– 20-30% electrical energy
– 70-80% internal combustion engine
– 20-30% electric motor
– additional to the MHEV characteristics:
– the vehicle can drive in EV mode
– the high voltage system can go up to 300-400V
– the high voltage battery has higher energy content
– the electric machine has higher power output
PHEV
(Plug-in Hybrid Electric Vehicle)
– 60-70% fuel (petrol/gasoline, diesel)
– 30-40% electrical energy
– 60-70% internal combustion engine
– 30-40% electric motor
– additional to the HEV characteristics:
– the high voltage battery can be charged from the grid
– the vehicle can be driven in EV mode up to 50-60 km
– the high voltage battery has higher energy content
– the electric machine has higher power output
REEV
(Range Extender Electric Vehicle)
– 80% electrical energy
– 20% fuel (petrol/gasoline)
– 100% electric machine– can be regarded as a series hybrid
– the internal combustion engine is used only as power generator
– has an additional electrical generator connected to the internal combustion engine
BEV
(Battery Electric Vehicle)
– 100% electrical energy– 100% electric machine(s)– full battery electric vehicle
FCEV
(Fuel Cell Electric Vehicle)
– 50% battery energy
– 50% hydrogen energy
– 100% electric machine– fuel cell is used as an energy converter
– uses a hydrogen tank as additional energy source

Observation: An electric machine can be motor or generator, depending on the driving situation. When the vehicle is accelerated and the electric machine provides torque to the wheel it becomes an electric motor. During vehicle deceleration (braking), the electric machine acts as a generator and converts the kinetic energy of the vehicle in electric energy, charging up the battery.

In a hybrid electric vehicle, the higher the electrical energy and power available, the lower the fuel consumption and exhaust emissions. This is possible due to the fact that the electric power can be used for longer periods of time and also at higher vehicle speeds.

The fuel consumption improvement is minimum for a micro hybrid and maximum for a plug-in hybrid.

Fuel economy function of hybridization level

Image: Fuel economy function of hybridization level
Credit: [1]

A Hybrid Electric Vehicle (HEV) is also know as full hybrid or self-charging hybrid. The full hybrid comes from the fact that a HEV can be driven in pure EV mode, by comparison with an MHEV. The self-charging hybrid comes from the fact that the battery in a HEV is only charged onboard, by the internal combustion engine or during energy recuperation. By contrast, the high voltage battery in a PHEV can be charge from the electrical grid.

The main functions of a hybrid electric vehicle are summarised in the table below, function of the level of hybridisation [6].

FunctionsHybrid type
mHEVMHEVHEVPHEV
Engine idle stop/start
Energy recuperation
Electric torque assist
Electric driving
Battery charging from the grid

Hybrid Electric Vehicles (HEV) Architectures

From the energy source and propulsion device points of view, a hybrid electric vehicle is a mix between a conventional ICEV and a BEV.

In a vehicle powered by an internal combustion engine, all the energy for propulsion is stored in a fuel tank. Through a fuel supply line the fuel is fed to the engine which, together with the transmission, powers the driving wheels.

Vehicle powertrain architecture - ICE

Image: Vehicle powertrain architecture – ICE

Vehicle powertrain architecture - EV

Image: Vehicle powertrain architecture – EV

In a pure battery electric vehicle, all the energy for propulsion is stored in a high voltage battery. Through an electric supply line the energy is fed to the electric motor, together with the transmission, powers the driving wheels.

There are several ways to combine the internal combustion engine, electric machine (motor/generator) and the high voltage battery. Four basic hybrid electric vehicle architectures are used :

  • series HEV
  • parallel HEV
  • split HEV
  • series-parallel HEV

In a series HEV, the internal combustion engine never directly powers the vehicle. Instead, the engine drives an electrical generator, and the generator can either charge the batteries or power an electric motor that drives power to the the wheels.

Vehicle powertrain architecture - Series HEV

Image: Vehicle powertrain architecture – Series HEV

Series HEV is the simpler type, where only the electric motor provides all the propulsion power. A downsized internal combustion engine on board drives a generator, which supplements the high voltage battery and can charge it when the state of charge (SOC) falls below a minimum threshold. The power required to move the vehicle is provided solely by the electric motor. Beyond the internal combustion engine and the electrical generator, the propulsion system is the same as in an BEV, making electric motor power requirements the same as for in the BEV.

Advantages of a series HEV are:

  • flexibility in packaging and location of the engine-generator set
  • simple drivetrains
  • simple control strategies for the internal combustion engine (operating at most economical speed and torque)

The disadvantages of a series HEV are:

  • it requires an additional electric machine (generator)
  • without the help of the engine, the electric motor must be designed for the maximum sustained power that the vehicle may require, such as when climbing a high grade; however, the vehicle operates below the maximum power most of the time
  • all three drivetrain components need to be designed for maximum power for long-distance, sustained, high-speed driving; this is required, because the batteries will deplete fairly quickly, leaving the engine to supply all the power through the generator

In a parallel HEV, the internal combustion engine connects to the transmission, as well as the electric motor. So both the engine and the electric machine (generator/motor) can supply power to the wheels, switching back and forth as driving conditions vary.

Vehicle powertrain architecture - Parallel HEV

Image: Vehicle powertrain architecture – Parallel HEV

In a parallel HEV, the internal combustion engine and the electric motor can be connected to the driveshaft through separate clutches. Power requirements of the electric motor in the parallel hybrid are lower than that of an BEV or series hybrid, because the internal combustion engine complements for the total power requirement of the vehicle. The propulsion power may be supplied by the internal combustion engine, by the electric motor, or by the two systems in parallel.

The advantages of a parallel HEV are:

  • it needs only two propulsion components: engine and electric machine (motor/generator)
  • a smaller engine and a smaller motor can be used to get the same performance, until batteries are depleted. For short-trip missions, both can be rated at half the maximum power to provide the total power, assuming that the batteries are never depleted. For long-distance trips, the engine may be rated for the maximum power, while the motor/generator may still be rated to half the maximum power or even smaller.

The disadvantages of a parallel HEV:

  • the control complexity increases significantly, because power flow has to be regulated and blended from two parallel sources
  • the power blending from the engine and the electric motor requires a complex transmission.

In a split HEV, the engine drives one axle and the electric motor drives the other. There is no connection between the engine and the electric components except “through the road”.

Vehicle powertrain architecture - Split HEV

Image: Vehicle powertrain architecture – Split HEV

In the split type HEV architecture, both the internal combustion engine and the electric motor can power the vehicle in the same time, on different axles. If the battery needs charging, the engine will deliver the necessary torque for both propulsion and spinning the electric machine (generator) on the separate axle.

Compared to a parallel HEV, the advantage of the split HEV is that it has a simple transmission, since the electric machine is on a separate axle. The disadvantage is that when charging the battery, a lot of power is wasted since it is transferred “through the road”, thus has lower energy efficiency.

For practical road vehicles, the best architecture is a combination of the series and parallel HEV configurations. In this series-parallel HEV architecture, the engine is also used to charge the battery and power the drive wheel.

Series-Parallel hybrid powertrain

Image: Series-Parallel hybrid powertrain

In a series-parallel HEV, a power split device (PSD) allocates power from the ICE to the front wheels through the driveshaft and the electric generator, depending on the driving condition. The power through the generator is also used to charge the high voltage battery. The electric motor can also deliver power to the front wheels in parallel with the engine.

The inverter is bidirectional and is used to charge the batteries from the generator or to control the power delivered by the electric motor. For short bursts of speed, power is delivered to the driveshaft from the engine and the electric motor. A central control unit regulates the power flow for the system using multiple feedback signals from the various sensors. Use of the engine to charge the high voltage battery should be minimised when maximising efficiency. Energy is always lost while charging and discharging the battery and during the power flow through the inverter.

Hybrid powertrain system comparison

Image: Hybrid powertrain system comparison
Credit: [2]

The series-parallel hybrid architecture combines the advantages of the series and parallel HEVs. For this reason, is the most used architecture for production hybrid electric vehicles.

Advantages of Hybrid Electric Vehicles (HEV)

Compared with a conventional ICEV, a hybrid electric vehicle has the following advantages:

  • energy loss reduction: the hybrid system automatically stops the idling of the engine (idling stop), thus reducing the energy that would normally be wasted
  • energy recuperation: the kinetic energy of the vehicle that would normally be wasted as heat during deceleration and braking is recovered as electrical energy, which is later used by the electric motor
  • electric motor assist: the electric motor assists the engine during transient operations (acceleration), thus improving the dynamic response and reducing the exhaust gas emissions
  • high-efficiency engine operation: by using the electric machine(s) as a motor or generator, the engine operating point (torque and speed) can be kept in the most economical region
  • pure electric driving: at low speed, the vehicle can be driven in EV mode, thus having zero exhaust emissions and fuel consumption

Compared to the internal combustion engine, an electric motor can deliver instant torque and has high energy efficiency. Due to this advantages, the electric motor can assist the ICE during acceleration phases and also provide extra torque for brief periods of time.

HEV powertrain impact on transient vehicle speed

Image: HEV powertrain impact on transient vehicle speed

Hybrid powertrain dynamic response

Image: Hybrid powertrain dynamic response
Credit: [4]

An internal combustion engine can operate in a steady state (constant torque and speed) or a transient state (variable torque and speed). The transient operation occurs during vehicle acceleration and deceleration. The deceleration phase is usually with the acceleration pedal lifted off, therefore in fuel cut operation (no combustion). The vehicle acceleration phase requires the engine to increase torque and speed, which has negative impact on the fuel consumption and/or exhaust gas emissions. In this situation an electric motor is very helpful because it can deliver part of the torque demanded by the driver and allows the engine to operate more efficiently.

Toyota Hybrid System (THS)

One of the first and most iconic hybrid electric vehicle (HEV) is Toyota Prius, which has a series-parallel hybrid powertrain architecture. Toyota’s hybrid powertrain is called Toyota Hybrid System (THS) and combines an internal combustion engine, two electric machines, a power split device (planetary gear) and a reduction gear.

Toyota hybrid powertrain

Image: Toyota hybrid powertrain

Toyota hybrid powertrain - components

Image: Toyota hybrid powertrain – components

There are several versions of THS, in this article we are going to focus on THS II, explaining the components and how it works. The main components of the THS II are:

  • a high-efficiency gasoline engine (working on the Atkinson cycle, which is a high-expansion ratio cycle)
  • permanent magnet AC synchronous motor
  • permanent magnet AC synchronous generator
  • high voltage nickel-metal hydride (Ni-MH) battery
  • a power electronics control unit
  • a power split device (planetary gear set)
  • a metallic chain and a reduction gear

This power electronics control unit contains a high-voltage power circuit for raising the voltage of the power supply system for the motor and the generator to a high voltage of 500 V, in addition to an AC-DC inverter for converting between the AC current from the motor and the generator and the DC current from the hybrid battery. Other key components include a power split device, which transmits the mechanical torque from the engine, the motor and the generator by allocating and combining them. The power control unit precisely controls these components at high speeds to enable them to cooperatively work at high efficiency.

Hybrid Electric Vehicles (HEV) Driving Modes

By comparison with a conventional ICEV, a HEV has at least two sources of torque for traction. Also, during vehicle deceleration, the kinetic energy of the vehicle can be recuperated and converted into electrical energy by the generator. All these situations brings complexity in the behaviour of a HEV in particular in the management of the on-board energy.

The main driving modes of a Toyota Prius are summarised in the table below, but they are standard for most HEVs. Depending on the HEV architecture, the driving (operating) mode might be different, nut not entirely.

Power flowDescription

Toyota hybrid powertrain mode - startup charge

Image: THS operating mode – startup charge

Start-up charge
Depending on the state of charge of the battery, at vehicle start-up, the ICE might start and charge the battery. This mode is available before the transmission is pun into Drive mode.

Toyota hybrid powertrain mode - EV driving

Image: THS operating mode – EV driving

EV driving
The ICE is kept off and the battery is providing all the necessary energy for vehicle launch and driving. This mode is available for limited time and vehicle speed less than 15-25 kph. This mode is also available in Reverse.

Toyota hybrid powertrain mode - engine and motor drive

Image: THS operating mode – engine and motor drive

Engine and motor drive
At low vehicle speed and driver torque demand, the engine is providing partial torque for the driving wheels and partial torque for the generator. The electric motor is also providing drive torque using the energy output from the generator.

Toyota hybrid powertrain mode - engine drive and battery charging

Image: THS operating mode – engine drive and battery charging

Engine drive and battery charging
At low vehicle speed and drive torque demand, if the battery state of charge is low, the engine is providing torque for both vehicle propulsion and battery charging. In this mode the electric motor doesn’t output any torque in order to save the electrical energy and charge up the battery.

Toyota hybrid powertrain mode - engine and motor drive and battery charging

Image: THS operating mode – engine and motor drive and battery charging

Engine and motor drive and battery charging
In this mode the engine provides torque to the driving wheels, torque for the generator to produce electricity for the electric motor and to charge the battery, all in the same time.

Toyota hybrid powertrain mode - full power

Image: THS operating mode – full power

Full acceleration
In an acceleration pedal kick-down scenario, the engine delivers torque for the driving wheel and generator, and the battery powers the electric motor. In this mode, maximum torque is provided by both engine and electric motor.

Toyota hybrid powertrain mode - energy recuperation

Image: THS operating mode – energy recuperation

Energy recuperation
During vehicle deceleration (acceleration pedal lift-off or brake pedal slightly pressed), the electric motor becomes a generator and converts the kinetic energy of the vehicle into electrical energy, charging up the high voltage battery.

Summary

By definition, a hybrid vehicle combines any two power sources for vehicle propulsion. A special case within that broad definition is the hybrid electric vehicle (HEV). Combining the components from a pure electric vehicle (EV) and the conventional, pure internal combustion engine vehicle (ICEV) gives a HEV.

References

[1] John M. Miller, Hybrid Propulsion Systems: The Gasoline-Electric Strong Hybrid, 2005.
[2] Toyota Hybrid System – THS II, Toyota Motor Corporation, 2002.
[3] Chris Mi, Emerging Technology of Hybrid Electric Vehicles, University of Michigan – Dearborn.
[4] Christoph Luttermann, The Full-Hybrid Powertrain of The New BMW Active Hybrid 5, Aachen Colloquium China, 2011.
[5] Iqbal Husain, Electric and Hybrid Vehicles Design Fundamentals, CRC Press, 2005.
[6] Bosch Automotive Handbook, 9th Edition, Wiley, 2014.

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