Mild Hybrid Electric Vehicle (MHEV) – components (Continental)

In this article we are going to explain the electrical and mechanical parameters of the main components of a mild hybrid electric vehicle (MHEV). This article is focused on the 48V MHEV components supplied by Continental.

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

• Understanding micro, mild, full and plug-in hybrid electric vehicles
• Mild Hybrid Electric Vehicle (MHEV) – introduction
• Mild Hybrid Electric Vehicle (MHEV) – architectures

A 48V mild hybrid electric vehicle systems is made up only by a few core components:

• electric machine
• ACDC inverter
• DCDC converter
• battery

Adding more electrical components, on the 48V electrical network, allows for further improvements in terms of fuel efficiency and vehicle driveability. Depending on the vehicle application, these components can be electrified and plugged in the 48V network:

• engine oil pump
• engine vacuum pump
• transmission oil pump
• engine water pump
• A/C compressor
• intake air compressor
• engine coolant heaters

Higher battery voltage and energy capacity gives the opportunity for improved technologies for the exhaust gases after-treatment systems. One example is:

• Electrically Heated Catalyst EMICAT® from Continental

Electric machine and inverter

Currently, the most common mild hybrid topology is the P0 architecture, in which the electric machine is integrated in the Front End Accessory Drive (FEAD). In this case, the electric machine is replacing the alternator in terms of role and packaging.

The belt-integrated starter generator (BiSG or e-machine) can an asynchronous or synchronous electric machine, which has two main functions:

• provide torque to the powertrain in motor mode
• produce electricity in generator mode

As example, a BiSG can provide 4-6 kW of nominal power and around 14-16 kW of peak power. The output torque can be around 60 Nm which can reach up to 160 Nm at the crankshaft when amplified by the belt pulley ratio.

1. pulley
2. housing
3. rotor
4. stator
5. integrated inverter

Most of the 48V starter generators are 3-phase alternating current (AC) electric machines with an integrated inverter. The inverter has two roles:

• convert the direct current (DC) supplied by the the battery in AC, to power the electric machine when it’s in motor mode
• convert the AC generated by the electric machine (in generator mode) to DC, which can be stored in the battery

The general electrical and mechanical parameters of a 48V belt starter-generator with integrated inverter are summarized below:

• the inverter and electric machine are integrated in the same housing (therefore there are no external cables between inverter and e-machine)
• output torque: 60 Nm (up to 160 Nm at crankshaft)
• mechanical peak power (available for around 2 s): 14-16 kW
• mechanical continuous power: 4-6 kW
• maximum speed (continuous): 20000 rpm
• water cooled
• the rotor can be claw-pole type for synchronous e-machines or copper/aluminium type for asynchronous/induction e-machines

For comparison, in case of a P1 mild hybrid architecture, the e-machines need to be flat, in order to be positioned between the internal combustion engine and transmission. For these applications, in order to have a high torque output, the e-machines need to have high power density. Therefore permanent magnet synchronous e-machines are used instead of induction asynchronous e-machines.

Transmission-mounted electric machines, or P2 mild hybrid architectures, can be integrated into the transmission’s case or mounted aside, belt-driven.

The belt-driven design proposed by Continental is very compact and facilitates integration in the limited space between the internal combustion engine and transmission. The main benefit of this configuration is that the engine can be decoupled from the electric machine, which improves the recuperation of electrical energy (no engine losses involved). Also, if engine-off coasting/sailing is implemented, the electric machine can provide a limited amount of torque and also recuperate electrical energy during braking.

Image: 48V P2 hybrid module Credit: Continental

Image: 48V belt drive system Credit: Continental

1. 48V electric machine
2. decoupling tensioner
3. drivetrain pulley
4. belt
5. A/C compressor

If the cooling of the P2 e-machines is done through the transmission oil, the rotor type must be brushless, either permanent magnet or induction. In case of packaging constraints (limited space), a permanent magnet machine (higher torque density) is preferred instead of a induction machine.

DCDC converter

A 48V mild hybrid electric vehicle has two electric networks: a low voltage (12 V) network and a high voltage (48 V) network. Electrical energy can be produced only by the 48V e-machine, therefore we need a DCDC converter to transfer energy from the 48V system to the 12V system.

A DCDC converter can be operated in buck-mode (step down), when converts from high voltage to low voltage (48 V to 12 V) or boost-mode (step up), when converting from low voltage to high voltage (12 V to 48 V). In MHEV applications, most of the time, the DCDC is operated in buck mode but they are capable of performing both modes.

The parameters of a typical DCDC converter, used for automotive applications, are summarized below:

• continuous power in buck-mode: up to 3 kW
• continuous current: 215 A (buck-mode), 58 A (boost-mode)
• efficiency: higher than 95 %
• input voltage (buck-mode): 24 – 54 V
• output voltage (buck-mode): 6 – 16 V

Electric compressor (e-Compressor)

Turbocharged engines have a so called “turbo lag”, which is basically a delay in vehicle acceleration from the moment the driver tipped-in on the accelerator pedal. This happens because of the inertia of the intake/exhaust gases and the compressor’s rotor.

A 48V network gives the opportunity to use electrical compressors which have faster response time than conventional turbochargers. Continental manufacturers an electrically driven radial compressor for automotive applications, suitable for 48V mild hybrid architectures.

The e-Compressor is improving the overall torque transient response of the engine and the low-end torque characteristic, which enables further downsizing of the engine for a mild hybrid application.

The main characteristics of the 48V e-Compressor supplied by Continental are summarized below:

• electric motor: permanent-magnet synchronous
• supply voltage: 48 V
• peak shaft power: 5 kW
• maximum speed: 70000 rpm
• response time t₉₀: less than 0.25 s (t₉₀ represents the time to reach 90% of the target speed)
• water-cooled
• integrated electronics with CAN interface
• high speed bearing system
• application-specific compressor design (can be customized for different engine specifications)

Electrically Heated Catalyst EMICAT®

Internal combustion engines require after-treatment systems to be able to meet exhaust gas emissions standards (Euro 6, SULEV, etc.). Catalytic converters are widely used in the ICE after-treatment systems. One of the disadvantage of the catalytic converter is that they need to reach a high temperature to be able to operate efficiently.

On a three-way catalytic converter, used for spark ignited (gasoline) engines, the light-off temperature (the value from which they start to operate) is around 300 °C. The most efficient conversion rate is obtained at the nominal temperature, which is between 400 – 800 °C.

In order to reduce as much as possible the amount of emissions, the catalyst should reach as fast as possible the light-off temperature. Hybrid vehicles make this situation worse because engine stop situations occur more frequently during stop & start or coasting with the engine off.

Image: EMICAT® Credit: Continental

Image: EMICAT® and catalyst Credit: Continental

The Electrically Heated Catalyst EMICAT® supplied by Continental solves this problem by heating up the catalytic converter in a very short time. This technology allows repeated and prolonged engine-off phases, reducing in the same time the amount of harmful emissions (by reaching the catalyst light-off temperature in a shorter time). It can be integrated in gasoline and diesel passenger cars after-treatment systems as well as heavy-duty applications including a high effective support catalyst.

The main characteristics of the EMICAT® systems are:

• operation voltage: 12 V or 24 V or 48 V
• power rating for 12 V: 0.3 – 3.6 kW
• power rating for 24 V: 1 – 4 kW
• power rating for 48 V: 2 – 4 kW
• diameter (depending on vehicle application): 50 – 342 mm

In the image above you can see the temperature response and the hydrocarbons (HC) emissions reduction, function of the power rating of the heated catalyst, compared to a unheated catalyst. The best performances are obtained for 3 kW electrical power, which are suitable for 48V electrical networks (mild hybrid vehicles).

An electrically heated catalyst brings several benefits into the after-treatment system, like:

• faster reach of catalyst light off temperature
• reduced catalyst cooling during no-load or engine off phases
• potential for precious metal reduction (cost reduction)
• additional energy added to the exhaust improves vaporization of liquids
• improved low-temperature SCR (Selective Catalytic Reduction)
• intelligent activation for lower heating power
• use of recuperation energy without intermediate storage (no cycling through the battery)
• support of RDE (Real Driving Emissions) challenges

For any questions or observations regarding this tutorial please use the comment form below.

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