Mild Hybrid Electric Vehicle (MHEV) – architectures

In this article we are going to go through different system architectures for Mild Hybrid Electric Vehicles (MHEV), looking into the positioning of the components on the vehicle, the possible control functions and advantages and disadvantages in terms of fuel efficiency and driveability.

To recall the different types of hybrid electric vehicles and what makes a mild hybrid electric vehicle, read the following articles:

By architecture, topology or configuration we understand the positioning of the main components of the hybrid electric system on the vehicle. Since the only mechanical link between the electrical system and the rest of the vehicle is done through the electric machine, the MHEV architecture is basically defined by the position of the electric machine and the type of connection with the powertrain / drivetrain (belt, integrated or gear mesh).

The 48V electrical system has become the industry standard for mild hybrid MHEV applications for several reasons:

  • it’s relatively simple to integrate on a vehicle
  • it’s modular, safe and compact
  • the mass of the components is relatively small so the impact on the total weight of the vehicle is limited
  • the system cost to performance (fuel efficiency, torque boost) ratio is very competitive

With the except of the crankshaft-mounted integrated starter generator (Honda IMA and Mercedes Benz BlueHybrid), all the mild hybrid vehicle architectures presented in this article are based on 48V systems.

The powertrain configuration of the MHEV system has significant impact on the performance and characteristics of the vehicle, in terms of:

  • integration cost
  • fuel efficiency
  • dynamic performance (powertrain torque enhancement)

Automotive OEMs and automotive Tier 1 system suppliers are currently analyzing and evaluating several major powertrain architectures for MHEVs. The electric machine can be positioned, relative to the other powertrain components, in five major points:

MHEV powertrain architectures

Image: MHEV powertrain architectures

The brief description of the electric machine connection points is done in the table below.

P0The electric machine is connected with the internal combustion engine through a belt, on the front end accessory drive (FEAD)
P1The electric machine is connected directly with the crankshaft of the internal combustion engine
P2The electric machine is side-attached (through a belt) or integrated between the internal combustion engine and the transmission; the electric machine is decoupled from the ICE and it has the same speed of the ICE (or multiple of it)
P3The electric machine is connected through a gear mesh with the transmission; the electric machine is decoupled from the ICE and it’s speed is a multiple of the wheel speed
P4The electric machine is connected through a gear mesh on the rear axle of the vehicle; the electric machine is decoupled form the ICE and it’s located in the rear axle drive or in the wheels hub

Notice that P0 and P1 architectures do not allow the mechanical disconnection of the electric machine from the engine. On the other side P2, P3 or P4 configurations disconnects the electric machine from the engine through a clutch.

Belt Starter Generator Architecture (P0)

Also know as BiSG from Belt integrated Starter Generator, this mild hybrid topology is the most cost effective due to the limited impact of the 48V system on the existing vehicle architecture.

On a hybrid electric vehicle application, there are two major cost drivers: the impact on the existing powertrain components and the high voltage battery. To minimize the integration costs, the vehicle and transmission architecture should be kept the same as for a conventional vehicle. Thus, the easiest way of achieving a minimum cost is to integrate the 48V electric machine into the already existing engine accessories belt drive, by replacing the 12V alternator (generator).

MHEV P0 architecure - Belt Starter Generator (BSG)

Image: MHEV P0 architecure – Belt Starter Generator (BSG)

In the BiSG architecture, the internal combustion engine (ICE) and the electric machine can not be separated, they are mechanically linked through the accessory belt. Therefore, one of the disadvantage of this configuration is that, the engine friction torque will be a parasitic loss for the electric machine when it gives boost torque and when it’s recuperating electrical energy.

The main characteristics of the BiSG MHEV architecture are summarized in the table below.

Electric machine performanceMaximum torque (at crankshaft): up to 50 Nm (with belt pulley ratio multiplication, e.g. 2.8)
Maximum power: 12 … 14 kW
Continuous power: 2.5 … 3.5 kW
Efficiency: up to 85%
Fuel EfficiencyNew European Driving Cycle (NEDC): 10 … 12 %
Worldwide harmonized Light vehicles Test Procedure (WLTP): 7 … 9 %
Hybrid modes (functions)Idle Stop & Start
Moving Stop & Start
Engine load shift
Torque assist (fill)
Torque boost
Sailing / Coasting
Energy recuperation
Brake regeneration
AdvantagesLow cost of integration
Air or liquid cooled electric machine
Integrated inverter (with electric machine)
Size modularity for the electric machine
Speed / torque ratio possible between electric machine and ICE results in lower Power demand from the electric machine
DisadvantagesLimited torque capacity due to belt drive
Energy recuperation affected by engine friction losses
Overall characteristicsTorque Boosting Capability: Medium (limited by belt slip, durability)
Electrical Energy Recuperation: Medium (due to engine losses)
Driveability Improvements: Medium (due to limited torque boost)
Electrical Creep / Drive: Not possible (due to limited torque and belt drive)
Packaging: Easy components integration with limited impact on other components
System Efficiency: Medium (mainly due to belt-drive integration on the FEAD)

The BiSG MHEV architecture has a significant impact on the design of the Front End Accessory Drive (FEAD). The belt durability needs to be increased to sustain higher torque and more engine off/on cycles. The variable belt tensioners have to provide:

  • increase tension during cranking and boost (torque from electric machine to engine)
  • increase tension during recuperation (torque from engine to electric machine)
  • reduce tension during normal driving (in order to reduce friction losses)

There is also a significant impact on the noise, vibrations and harshness (NVH) of the engine and on the durability of the main bearing of the engine’s crankshaft.

Currently, all the 48V BiSG MHEV applications still use the 12V starter. The reason is that the cold engine start, especially after a long period of inactivity, demands a high electric machine torque (due to high engine friction). This is a limitation on the BiSG because the amount of torque which can be transmitted is limited by the belt slip. With an improved design of the FEAD belt and increased durability, the 12V starter can be removed and all its functions performed by the 48V electric machine.

An example of MHEV is the new Audi A8, which features a 48V electric system.

Audi A8 48V MHEV

Image: Audi A8 48V MHEV
Credit: Audi

  1. DC/DC converter
  2. low voltage battery (12 V)
  3. high voltage battery (48 V)
  4. 48V belt-drive starter-generator
  5. 3.0 TFSI internal combustion engine

There are also a significant number of Tier 1 suppliers, which can provide 48V P0 mild hybrid systems ready to be integrated in automotive applications.

Valeo’s 48V system MHEV

Valeo Hybrid4All 48V MHEV system components

Image: Valeo Hybrid4All 48V MHEV system components
Credit: Valeo

  1. Powertrain Control Unit (PCU)
  2. 14V battery sensor
  3. Belt Starter Generator (BSG) 8 – 12 kW / 55 Nm Peak, with integrated inverter
  4. DC/DC converter, 60 V / 12 V, 2 kW
  5. 48V battery, 200 – 600 kJ

Bosch’s 48V system MHEV

Bosch 48V MHEV - components of the boost recuperation system

Image: Bosch 48V MHEV – components of the boost recuperation system
Credit: Bosch

  1. low voltage battery (12 V)
  2. DC/DC converter
  3. electric machine (motor & generator)
  4. high voltage battery (48 V)

Continental’s 48V system MHEV

Continental 48V MHEV system components

Image: Continental 48V MHEV system components
Credit: Continental

  1. electric machine with integrated inverter
  2. DC/DC converter (48 V / 12 V)
  3. Li-Ion battery

Delphi’s 48V system MHEV

Delphi's 2nd Generation 48V MHEV system

Image: Delphi’s 2nd Generation 48V MHEV system
Credit: Delphi

48-VOLT LAUNCH ASSIST
A. Electric motor/generator: starts engine during Stop & Start operation and provides some power to the wheels; later, it charges the battery during braking
B. E-charger (electric air compressor): provides boost while driving away from stop and complements the exhaust turbocharger, eliminating the turbo lag

48-VOLT SYSTEM
1. 48-volt inverter: changes DC battery current to AC current to power the electric motor
2. 48-volt lithium-ion battery: stores energy regenerated during braking to be used later to power vehicle
3. Battery controller: regulates the state of charge in the battery
4. Power distribution box: contains built-in fuses

12-VOLT SYSTEM
5. DC/DC converter: changes 48 volts to 12 volts
6. 12-volt battery – your old friend, but smaller
7. 12-volt electrical distribution center: powers the center console, seats and windows as well as other 12-volt devices

48V P0 mild hybrid architecture is the mainstream technology adopted by the automotive manufacturers for MHEVs, because it combines a relatively low integration cost and considerable benefits in terms of CO2 emissions reduction and dynamic performance boost.

Crankshaft mounted electric machine (P1)

The P1 architecture, with the electric machine connected directly to the crankshaft, is the solution adopted by Honda on their first generation Integrated Motor Assist (IMA) technology. The electric motor functions as a generator, during vehicle deceleration, as an engine starter, and as a motor (to assist the engine) during vehicle accelerations.

MHEV P1 architecture - Integrated Starter Generator (ISG)

Image: MHEV P1 architecture – Integrated Starter Generator (ISG)

One of the biggest advantage of this solution is that the electric motor can provide higher torque than the BiSG, since there is no belt limitation (due to slip). However, since there is no speed / torque ratio between the electric machine and crankshaft, the torque requirements on the electric motor can be quite demanding.

Two examples of P1 MHEV architectures are:

  • Honda Insight Hybrid 2009 (with Integrated Motor Assist technology)
  • Mercedes Benz S400 Bluehybrid 2010
Honda IMA mild hybrid electric vehicle (MHEV)

Image: Integrated Motor Assist (IMA) for mild hybrid
Credit: Honda

For example, the main characteristics of the crankshaft-mounted electric machine for mild hybrid Honda Insight 2009 are summarized in the table below.

Electric machine performanceMaximum torque (at crankshaft): up to 34 Nm
Maximum power: 10 kW
Efficiency: up to 94%
Hybrid modes (functions)Idle Stop & Start
Moving Stop & Start
Engine load shift
Torque assist (fill)
Torque boost
Sailing / Coasting
Energy recuperation
Brake regeneration
AdvantagesHigher efficiency
The 12V starter can be removed
DisadvantagesOutput torque limited by the size of the electric machine
Energy recuperation affected by engine friction losses
High impact on existing vehicle architecture
Higher overall cost of the electrical components
Air cooling not possible for the electric machine
Overall characteristicsTorque Boosting Capability: High (if electric machine is capable)
Electrical Energy Recuperation: Medium (due to engine losses)
Driveability Improvements: Medium (depending on the torque capability of the electric machine)
Electrical Creep / Drive: Possible (depending on the torque capability of the e-machine)
Packaging: Difficult components integration (powertrain specially designed for this architecture)
System Efficiency: High (no gear mesh or belt losses)
Mercedes Benz S400 Bluehybrid (MHEV)

Image: Mercedes Benz S400 BlueHybrid (MHEV)
Credit: Daimler

The primary advantage of a P1 mild hybrid architecture, compared with P0, is the removal of the belt drive. This means that the efficiency increases a bit (no more belt losses) and the electric machine torque can be higher in terms of amplitude and response (no more belt slip).

The functions (modes) performed by this mild hybrid topology are similar with those of a BiSG (P0), but, overall, P1 configurations have two big disadvantages: higher cost and higher impact on the the existing vehicle architecture. Therefore, vehicle manufacturers and system suppliers are not investing in the further development of crankshaft-mounted integrated starter generator solution for MHEV applications.

Driveline side electric machine MHEV architectures

Both P0 and P1 mild hybrid configurations have the electric machines on the engine side, without the possibility of mechanical disconnection. This makes torque boosting and energy recuperation not very efficient because of the torque losses. Moreover, recuperating electrical energy with the engine off, during coasting, is not possible.

The P2, P3 and P4 mild hybrid architectures are better in terms of energy flow efficiency, mainly because of the positioning of the electric machine. In these types of configurations, the electric machine is positioned after the driveline connecting device (clutch), on the input shaft of the transmission (P2), on the output shaft of the transmission (P3) or on the rear differential (P4).

MHEV P2 architecture - side electric motor

Image: MHEV P2 architecture – side EM

MHEV P2 architecture - integrated electric motor

Image: MHEV P2 architecture – integrated EM

In a P2 configuration, the electric machine can be side attached to the transmission, connected through a belt, or integrated in the transmission, connected through a gear mesh.

The main advantage of P2 architecture is the increased energy recuperation potential and the availability of additional hybrid control functions (electric creep/drive or energy recuperation during coasting).

The main disadvantage is the higher integration cost of such a system.

MHEV P3 or P4 architecture

Image: MHEV P3 or P4 architecture

In the P3 mild hybrid architecture the electric motor is attached on the transmission, on the output shaft. In the P4 architecture, the electric motor is mounted on the rear axle drive or wheel hubs.

The main advantage of the P3 or P4 topology is the highest energy recuperation potential. Compared with the P0, P1 and P2 configurations, the engine and transmission losses, when the driveline is disconnected, are not taken into account during energy regeneration.

The P3 and P4 architectures are have also the potential for electric driving mode (creep) if fitted with a high torque electric machine. The P4 architecture gives the vehicle four-wheel drive capabilities, with the front axle powered by an internal combustion engine and the rear axle powered by an electric motor.

Due to the fact that the electric machine is on the driveline side, for any P2, P3 or P4 architecture, in order to perform engine Stop & Start, another electric machine has to be fitted on the engine side. This function can be achieved with a standard reinforced starter (12 V) or a belt-integrated starter generator (12 V or 48 V).

The main characteristics of P2, P3 and P4 mild hybrid architectures are summarized in the table below.

Electric machine performanceMaximum torque (at crankshaft): up to 50 Nm
Maximum power: 21 kW
Efficiency: up to 95%
Fuel EfficiencyNew European Driving Cycle (NEDC): 19 … 22 %
Worldwide harmonized Light vehicles Test Procedure (WLTP): 14 … 16 %
Hybrid modes (functions)Idle Stop & Start (if additional electric machine on engine side)
Moving Stop & Start (if additional electric machine on engine side)
Engine load shift (through the road)
Torque assist (fill)
Torque boost
Sailing / Coasting
Energy recuperation
Brake regeneration
Electrical driving (creep)
AdvantagesHighest efficiency
Electric driving (creep)
Four/All Wheel Drive Mode (P4 architecture)
DisadvantagesRequires an additional electric machine for engine Stop & Start
High impact on existing vehicle architecture
Higher overall cost of the electrical components
Air cooling not possible for the electric machine
Overall characteristicsTorque Boosting Capability: High (if electric machine is capable)
Electrical Energy Recuperation: High (not affected by engine losses, partly by transmission losses)
Driveability Improvements: Medium (depending on the torque capability of the electric machine)
Electrical Creep / Drive: Possible (depending on the torque capability of the e-machine)
Packaging: Difficult components integration (powertrain specially designed for this architecture)
System Efficiency: High (only gear mesh losses)

Getrag is developing a dual clutch transmission with integrated electric motor for high-torque mild-hybrid powertrain.

For example, the hybrid variation 6HDT451 is based on the Getrag Powershift 6DCT451 and uses the integrated electric motor to allow for additional reduction in CO2 emissions by more than 10 % in the New European Driving Cycle (NEDC).

The Mild-Hybrid 6HDT451 was developed for front transverse installation in the middle class and upper middle class segments and in sport utility vehicles (SUV). Total torques of up to 450 Nm can be transferred with simultaneous combustion engine and electric motor operation.

The Getrag Powershift transmission 6HDT451 can be easily combined with all-wheel drive and guarantees start/stop functionality without additional components for the transmission.

Getrag Hybrid Double Clutch Transmission

Image: Getrag Hybrid Double Clutch Transmission

The transmission is integrating a high-speed electric machine with the following characteristics:

  • scalable performance by using different electric machine lengths, from mild to plug-in hybrid architectures
  • electric machine power between 26 – 65 kW (from mild to plug-in hybrid architecture)
  • electric machine voltage between 48 – 400 V (from mild to plug-in hybrid architecture)

Valeo is also working to consolidate its world leading position in electrical systems and further accelerate its expansion in this field.

Valeo’s 48V electrified powertrain solutions can be applied across all vehicle segments, to both gasoline and diesel engine models. They are particularly well-suited to urban cars and compact sedans, which are the top-selling vehicles on the European market.

Valeo 48V Electric Rear Axle Drive (ERAD)

Image: Valeo 48V Electric Rear Axle Drive (ERAD)

At the 2016 Paris Motor Show, Valeo introduced its 48V e4Sport mild hybrid system, a new solution designed to increase the dynamic performance of a vehicle while reducing the fuel consumption and CO2 emission of the internal combustion engine. The 48 V system consists of several main components:

  • an integrated belt starter generator (BiSG)
  • an electric supercharger
  • an electric rear axle drive (ERAD)
  • a high voltage battery (48 V)
  • a DC/DC converter (48 – 12 V)

The 48V e4Sport recovers a maximum amount of braking energy and stores it in a 48V battery for various
uses, such as:

  • increasing engine torque, via the starter-generator
  • maximizing engine power during acceleration and boost phases, using the electric supercharger
  • driving in all-electric mode and, when on low-grip surfaces, improving performance by connecting the rear axle to the 48V eRAD, effectively transforming the vehicle into a four-wheel drive

The MHEV systems is continuously evolving, going towards 48V P4 architectures with integrated electrical supercharging.

Valeo's roadmap for 48V mild hybrid systems

Image: Valeo’s roadmap for 48V mild hybrid systems
Credit: Valeo

Some key aspects to be retained regarding MHEV architectures:

  • first mild hybrid systems introduced by vehicle manufacturers were P1 but they are currently being replaced by P0, P3 and P4
  • the industry standard for MHEV is based on the 48 V electric network
  • the P0 architecture is the current most used mild hybrid solution
  • the market is evolving towards P4 architectures due to higher benefits in terms of CO2 reduction and efficiency
  • electrical supercharging is going to be integrated with most of the 48 V MHEVs

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

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