Types of Hybrid Electric Vehicles (HEV)

A hybrid electric vehicle (HEV) is a vehicle which is using two sources of energy for propulsion, one of them being electrical energy. Most of the road vehicles with hybrid powertrain use an internal combustion engine (ICE) combined with an electric machine (EM).

Compared with a conventional vehicle, powered by an ICE, a hybrid electric vehicle is capable of performing these functions:

• fast ICE stop & start
• energy recuperation during braking (regenerative braking)
• torque assist/boost
• electric driving
• coasting (optional)

For a detailed introduction in hybrid electric vehicles, types and their mode of operation, please read the article What is a Hybrid Electric Vehicle (HEV) ?

In this article we are going to focus on the different types (architectures) of HEV powertrain, highlighting the working principle and the advantages/disadvantages compared with other types.

Parallel hybrid powertrain with two clutches

In a parallel hybrid powertrain, both the internal combustion engine and electric motor can transmit torque to the driving wheel, in a sequence or in the same time. For a rear-wheel drive (RWD) vehicle, a common hybrid powertrain architecture is using an electric machine between two clutches.

where:
ICE – internal combustion engine
EM – electric machine
TRN – transmission
PE – power electronics

The first clutch, between engine and electric motor, allows the engine to be disconnected from the drivetrain and drive in pure EV mode. Also, during deceleration phases, by disconnecting the engine, we can remove its braking effect and have higher efficiency for kinetic energy recuperation.

The second clutch, allows the electric machine to be disconnected from the drivetrain and let the vehicle coast during deceleration. From the implementation point of view, the second clutch is part of the transmission and not as a separate component.

The states of the clutches during each mode of the hybrid powertrain are summarised in the table below.

Mode	Clutch before EM state	Clutch after EM state
(1)Engine stop & start	Open	Open/Closed
Energy recuperation	Open	Closed
Torque assist/boost	Closed	Closed
Electric driving	Open	Closed
Coasting	Open	Open
Charge at standstill	Closed	Open

⁽¹⁾assuming the engine stop/start is performed with an additional electric starter

There are several OEMs which are using this hybrid powertrain architecture for their vehicle. One example is Nissan, with two variants:

• Front Engine Front-Wheel Drive Vehicle Hybrid System (FF HEV)
• Front Engine Rear-Wheel Drive Vehicle Hybrid System (FR HEV)

The FF HEV system is using an internal combustion engine together with an electric machine and a continuously variable transmission (CVT) on the front axle.

The hybrid system improves driving performance and fuel consumption by using the motor to regenerate power during deceleration and store it in the battery, as well as supporting the engine by using the battery to power the motor for low-speed EV driving and acceleration. Further, fuel consumption is enhanced by using the battery to power the car’s electronic components when the vehicle is parked. As the engine can be separated from the drive-train when necessary, the power source for both the engine and motor can be utilised optimally, meaning there is also no loss from engine friction when regenerating with the engine separated from the powertrain or during EV driving.

The system employs Intelligent Dual Clutch Control, combining one motor and two clutches to implement a front-wheel drive hybrid system that is lightweight and compact. Clutch 1 is set up between the engine and motor, while Clutch 2 is installed between the motor and the CVT. Use of the engine or motor can be differentiated according to the circumstances by selecting “on” or “off” for Clutch 1, making for efficient driving.

The hybrid system clutch and motor are designed to fit into the space for the torque converter used in CVT engine vehicles. Accordingly, no specific chassis is required and it can be used in a variety of models. It can also be applied easily to four-wheel drive vehicles and plug-in hybrids. A high-output lithium-ion battery, developed for hybrids and with excellent discharging and charging, has been adopted for the system’s battery.

In the FR HEV system the one-motor two-clutch hybrid system can separate the engine from the drivetrain as necessary. It can utilise the engine and motor as power sources, from running just on the motor to using both motor and engine for full acceleration, achieving a more efficient drive as per the situation. During regeneration and electric-mode driving, the engine is completely disconnected from the drive-train, resulting in zero loss from engine friction.

Nissan replaced an existing 7-speed automatic transmission’s torque converter with a motor and two clutches in a compact configuration. Using a one-motor system to drive the wheels and regenerate electricity allows for a reduction in the number of parts and a lighter weight.

The two clutches transfer energy mechanically to the engine and motor. While being efficient and with little energy loss compared to a normal torque converter, the system also has intuitive and responsive acceleration. Through integrated control of this system and transmission using high-level control technology it achieves a drive that responds to a range of driving conditions.

The hybrid car lithium-ion battery can discharge high currents in a short time. In this way, the proportion of running the motor increases, and it is possible to recover braking energy frequently. Being able to use electricity effectively means the consumption of gasoline fuel decreases and contributes accordingly to better mileage.

Parallel hybrid powertrain with double-clutch transmission

In some vehicle applications, especially those which have front engine and front-wheel drive, components packaging is critical due to lack of space. The installation of a clutch between the engine and electric machine is probably not possible since it will require additional space, which is not available.

In this case it is desirable to use an existing powertrain solution and make it hybrid. One solution is to use a double clutch transmission (DCT) and integrate an electric machine on one of the input shafts, after the clutch.

In this architecture, the electric machine is integrated in a sub-unit of the transmission, on the odd or even gears. Thanks to the two input shafts and output shafts for odd and even gears, the input torque of the internal combustion engine can be transmitted to the output by a mechanical path different from the torque path of the electric machine. The torques are summed up at the ring gear that is connected to the differential.

Getrag has converted one of their own 7-gear double clutch transmission 7DCT300 into a hybrid transmission, by integrating an electric machine on the even gears shaft. The 7DCT300 front-transverse transmission has seven forward gears and one reverse gear. The even (2, 4, 6, R) and odd-numbered gears (1, 3, 5, 7) are divided into two sub-transmissions. The engine and the two partial transmissions are connected by means of a wet double (dual) clutch.

The result, 7HDT300, is a flexible hybrid transmission capable for integration in mild, full and plug-in hybrid electric vehicles. The full hybrid variant of the transmission integrates a 40 kW electric machine, which allows EV driving up to a vehicle speed of 50-60 kph (depending on application).

Image: Getrag hybrid dual-clutch transmission 7HDT300 Credit: Getrag

Image: Getrag hybrid dual-clutch transmission 7HDT300 electric machine integration Credit: Getrag

This solution allows the disconnection of the engine by opening both clutches and pure EV driving. The disconnection of the electric machine from the rest of the drivetrain is also possible by disengaging the gear synchronises within the transmission.

The states of the clutches during each mode of the hybrid DCT powertrain are summarised in the table below.

Mode	Clutch odd gears state	Clutch even gears state
(1)Engine stop & start	Open	Open
Energy recuperation	Open	Open
Torque assist/boost	Closed/Open	Open/Closed
Electric driving	Open	Open
Coasting	Open	Open
Charge at standstill	Open	Closed

⁽¹⁾assuming the engine stop/start is performed with an additional electric starter

The main benefit of this solution is that the electric machine needs no additional installation space in the transmission case, allowing high flexibility for vehicle integration and applications. This hybrid architecture can accommodate different levels of power for the electric machine (15-110 kW), working at different levels of voltage (48 – 360 V). For this reason can be easily integrate in different types of hybrid vehicles, from mild hybrids to full and plug-in hybrids.

Axle-split parallel hybrid powertrain

Another type of parallel hybrid electric vehicle is the axle-split hybrid. In this architecture one axle (usually front) is driven by the internal combustion engine and a transmission, the second axle being driven by an electric motor and a fixed ratio gear.

This solution requires an automatic transmission on the front axle, which will make possible engine disconnection during EV driving. The advantage of this solution is that the vehicle has all-wheel drive as long as the battery is not depleted.

Compared with the previous parallel hybrid solutions, the axle-split hybrid can not charge the battery while the vehicle is stationary. To overcome this issue, and also to allow continuous all-wheel drive, the engine is equipped with a secondary belt integrated electric machine. The additional electric machine, permanently connected with the engine (P0 architecture), allows for electrical energy generation during standstill and during driving.

An example of axle-split parallel hybrid is Peugeot 3008 hybrid. This hybrid powertrain system was developed by the PSA group in collaboration with Bosch.

The front axle is powered by a 2.0 litre diesel engine, coupled with a 6-gear automated manual transmission. The rear axle contains a 20/27 (continuous/peak) kW permanent magnet electric machine coupled to a single-speed gearbox. The secondary 8 kW electric machine is belt-integrated with the engine, is assuring the engine Stop & Start function and can also generate the electricity needed for operation of the electric motor under all circumstances if required (all-wheel drive mode).

Series hybrid powertrain

In a series hybrid powertrain, the internal combustion engine is not providing torque directly to the drive wheels. Instead, the engine is powering an electrical generator which provides electrical energy to the traction electric motor. A series hybrid is using two electric machines:

• an electric generator (connected to the engine)
• an electric motor (connected to the wheels through a single step gearbox and a differential)

There are also other noticeable differences between a parallel hybrid and a series hybrid. Since there is no direct mechanical connection between the engine and the drive wheels, there is no need for a transmission. This is an advantage in terms of packaging since less space is required for the engine and generator fitting.

Also, there is no clutch between the electric motor and the drive wheels. This is an advantage in terms of packaging but it doesn’t allow the vehicle to Coast since the electric motor will be always connected to the drive wheels.

In terms of powertrain functions, a series hybrid is only capable of:

• Engine stop & start
• Energy recuperation
• Electric driving
• Charge at standstill

Both electric machines need to have similar power rating as the internal combustion engine. Since the vehicle speed does not depend on the engine speed, the operating point of the engine (speed and torque) can be set in order to achieve maximum fuel efficiency. This is an advantage at low vehicle speed but a disadvantage a higher vehicle speeds.

Another disadvantage of the series hybrid is the double energy conversion. If the battery is depleted, the energy required at the wheel is coming from the internal combustion engine, after it is converted into electrical energy. This decreases even further the overall efficiency of the powertrain when the engine is providing the main energy source.

Since a series hybrid is not flexible in terms of power output for a wide range of vehicle speeds, it is not suitable as hybrid powertrain architecture for road vehicles. Nevertheless, by fitting a smaller engine, with a smaller rated power than the traction electric motor, a series hybrid becomes a Range Extended Electric Vehicle (REEV).

Series-parallel hybrid vehicle

A series hybrid becomes a series-parallel hybrid by adding a mechanical connection (clutch) between the two electric machines. The advantage of this architecture is that at low speeds, with the clutch open, the powertrain behaves as a series hybrid, running the engine at the most efficient operating point. At high vehicle speed, the clutch is closed and the engine can transmit torque to the driving wheel thus the powertrain becoming a parallel hybrid.

Compared with the series hybrid, the series-parallel hybrid has the advantage of a smaller power rating of the generator since the excess power of the engine can be transferred directly to the drive wheels. The disadvantage is that, by adding a mechanical connection (clutch), we lose the flexibility in terms of packaging.

In terms of powertrain functions, a series-parallel hybrid is capable of:

• Engine stop & start
• Energy recuperation
• Torque assist/boost
• Electric driving
• Charge at standstill

Compared with a parallel hybrid, a series-parallel hybrid uses two electric machine and performs the same tasks. For these reasons, the series-parallel powertrain architecture with a clutch connection between the two electric machine is not widely used by automotive manufacturers.

Power-split hybrid powertrain

A hybrid vehicle with a power-split architecture combines the characteristics of a parallel hybrid with those of a series hybrid. A power-split hybrid powertrain mechanically links an internal combustion engine and two electric machines using a power split device (PSD). The power-split device is usually a single planetary gear set (PSG) or multiple PSGs.

The internal combustion engine has the highest rated power, compared with the electric machines. A part of the engine power is converted by one electric machine into electrical energy, the excess power being transmitted to the drive wheel, in parallel with the second electric machine power.

Except Coasting, a power-split hybrid powertrain perform all the functions of a full hybrid:

• Engine stop & start
• Energy recuperation
• Torque assist/boost
• Electric driving
• Charge at standstill

Since the engine is always connected to the drive wheels through the planetary gear set, its speed can only be adjusted within certain limits, depending on the PSG gear ratios and the speed of the two electric machines. The combination of the internal combustion engine, two electric machines and the PSG gives the overall behaviour of a continuously variable transmission (CVT). For this reason a power-split hybrid electric vehicles is advertised as having an electric continuously variable transmission (ECVT).

The Hybrid Synergy Drive system from Toyota is an example of power-split hybrid powertrain.

The front axle is powered by a power-split hybrid powertrain.

At low and medium vehicle speeds, the power-split hybrid can behave as s series hybrid, where the engine powers one electric machine (generator), which in turn generates electricity and powers the second electric machine (motor). At high vehicle speed, where there is a high power demand, part of the engine power is transmitted directly to the drive wheels, through the planetary gear set, in this case the power-split hybrid being a parallel hybrid.

Image: Toyota hybrid powertrain (1) Credit: Toyota

Image: Toyota hybrid powertrain (2) Credit: Toyota

In a power-split hybrid there is also a mechanical link between the internal combustion engine and the electric machines. Because of this, their operating points (torque and speed) are always interdependent and can not fully separate the engine from the electric motor. A power-split hybrid can substantially reduce the fuel consumption at low-medium vehicle speed, but with minimum benefit at high vehicle speed.

The table below summarises the functions of the different types/architectures of hybrid electric vehicles, highlighting their advantages and disadvantages.

	Parallel HEV with two clutches	Parallel HEV with DCT	Axle-split HEV	Series HEV	Series-parallel HEV	Power-split HEV
Engine Stop & Start	Yes	Yes	Yes	Yes	Yes	Yes
Energy recuperation	Yes	Yes	Yes	Yes	Yes	Yes
Torque assist/boost	Yes	Yes	Yes	No	Yes	Yes
Electric driving	Yes	Yes	Yes	Yes	Yes	Yes
Coasting	Yes	Yes	No	No	No	No
Charge at standstill	Yes	Yes	(1)Yes	Yes	Yes	Yes
Power ratings	– engine power much higher than electric machine power	– engine power much higher than electric machine power	– engine power higher than electric machine power	– similar power rating between engine and electric machines	– similar power rating between engine and one electric machine – lower power rating of second electric machine	– similar power rating between engine and one electric machine – lower power rating of second electric machine
Advantages	– flexible powertrain configuration – can be easily integrated with an existing conventional powertrain – only one electric machine required for full hybrid performance	– requires minim additional installation space for the electric machine – only one electric machine required for full hybrid performance	– (1)continuous all-wheel drive capability	– flexibility in installation due to non existing mechanical link between electric machines – engine can operate in most efficient area regardless of the vehicle speed – does not require multi-step transmissions	– engine can transmit torque directly to the drive wheels – does not require multi-step transmissions – can switch between series and parallel hybrid	– compact installation due to usage of planetary gear set – energy efficient at low-medium vehicle speed
Disadvantages	– requires a fully automatic transmission to be also integrated	– the electric machine torque is only multiplied by half (odd or even) of the gear ratios	– Coasting not possible due to permanent connection or electric machine – requires additional installation space on the rear axle	– requires two electric machines of similar power rating – the vehicle dynamic performance depends entirely on the electric machine torque and power – energy inefficient due to double conversion of engine power (mechanical-electrical-mechanical)	– requires two electric machines – engine and electric machines are mechanically linked which requires additional installation space	– requires two electric machines – no fuel efficiency benefit at high engine speed

⁽¹⁾requires an additional belt integrated starter generator on engine side

References

[1] Bosch Automotive Handbook, 9th Edition, Wiley, 2014.
[2] Uli Christian Blessing, et al, Scalable hybrid dual-clutch transmission, AutoTech Review, September 2015, Volume 4, Issue 9.
[3] Uli Christian Blessing, 7HDT300 – How much electrical power does the combustion engine need?, International VDI-Congress 2013, Friedrichshafen.