Mild hybrid and 48-volt technologies

MY previous article focused on micro hybrids that use the start-stop technology (SST) to increase the mileage on internal combustion engines (ICE) within a conventional powertrain.

This week, we are talking about the mild hybrid electric vehicle (MHEV).

This technology utilises an electric motor to assist the ICE operation.

Boosting the ICE with electric propulsion will minimise the fuel used by the engine, while maintaining or exceeding the performance expected by the driver.

A mild hybrid system provides a small amount of electric propulsion assistance to the engine, but not enough to allow the vehicle to propel in only electric power.

We can safely say the MHEV is an upgrade of micro hybrids; in addition to the start-stop system that is found on micro hybrids, the mild hybrids will use power from electric motor (motor-generator) to assist the ICE under hard acceleration and restart the engine seamlessly, after it is switched off.

Unlike micro hybrids, which use ultracapacitors, etcetera, to start the engine during start-stop system operation, the mild hybrids will use electric machine (motor-generator) to start the engine during start-stop system operation.

The MHEV system will also harvest vehicle kinetic energy from the powertrain when the brakes are applied, and capture it in the battery pack/rechargeable energy storage system (RESS), so that it can be utilised in the next acceleration cycle.

This process is also known as regeneration.

On average the MHEV system can provide 10 percent to 15 percent more efficient operation, compared to conventional system.

This type of hybrid drive system enhances a conventional ICE layout, without major internal powertrain or external vehicle changes.

The MHEV system allows the driver the fuel efficiency enhancements of a hybrid drive system.

 

The MHEV system provides an intermediate step towards full electrification and permits the vehicle system to maximise the range of the vehicle, while using the electrical system as a benefit for internal combustion engine (ICE) operation.

Some General Motors mild hybrids, Mercedes MHEV, BMW MHEV, etcetera, utilise e-Assist system.

This system uses a belt alternator-start (BAS)/MGU (motor generator unit) in place of an alternator and starter on the vehicle.

This motor is used to start the ICE, and also as a power generator unit to charge the high voltage battery pack, so the DC-DC converter unit within the high voltage e-Assist system can provide electrical power to 12-volt accessories when the engine is either operating or off.

The use of the MGU to provide propulsion torque to the powertrain and electrical power to 12-volt accessories makes the start-stop system and propulsion assist functions nearly unnoticeable.

Utilising the advanced internal combustion engine (ICE) technologies, such as active fuel management (AFM), variable valve timing (VVT), and the direct injection with the e-Assist system permits an electric assist and start-stop capabilities to the powertrain operation.

When the ICE is commanded to shut off, the e-Assist provides electrical power to the vehicle heating, ventilation, and air condition (HVAC), infotainment and other electrical components through the DC-DC converter system.

Once the driver releases the brake pedal, the motor generator unit cranks the engine, to start it for the vehicle propulsion.

When accelerating, the MGU is commanded by the power train controller to operate as a drive motor and provide torque to the powertrain and a belt-driven air conditioning (AC) compressor.

When the powertrain controller needs high torque from the MGU, it can command the MGU into a generation mode to provide electrical energy to charge the 12-volt battery and operate 12-volt accessories.

If the vehicle operation commands the AC compressor on, the engine will continue to operate and will not enter into the start-stop mode until the AC is commanded off.

The steering is electrically powered and, whether the engine is operating or not, the power steering is not affected.

The onboard BAS AC brushless induction motor (MGU) system can provide an electrical boost to the ICE operation of up to 20 horsepower (15 kilowatts) and 44 pound-feet (lb.-ft) of power during required operation periods.

With this supplemental electric power, the ICE can operate in AFM cylinder disable mode for a long period on the V8 engine application.

As the number of electronic modules and other electrical devices continue to increase in current vehicles, the need to help maintain their operation has required OEMs to raise the power level of the electrical system.

To help reduce the physical size of vehicle wire harnesses to power this electrical/electronic system, raising the vehicle system voltage level to a 48-volt system allows the OEM to reduce component size, while increasing electrical power to all systems and components.

Using an understanding of Ohm’s law – as voltage increases, the current through the circuit increases (provided circuit resistance does not change) – this means that the circuit can handle more electrical loads, without adverse effects on the performance of any one component.

Additionally, when the voltage of an electrical system increases, the total available electrical power increases.

The 48-volt system utilises an electric machine to assist with vehicle propulsion, thereby reducing ICE fuel consumption.

This type of system allows for the reduction of engine size, without affecting vehicle performance and enhancing fuel economy, while using an intermediate voltage system (less than 60 volts DC) that will minimise the possibility of technician electrical injury.

 

*Taurayi Raymond Sewera is ASE and Auto Cate Association-certified World Class Master Technician with 39ASEs, ASE Advanced Level Specialist L1, L2, L3 and L4, AMI-Accredited Master Electric Vehicles and Master Automotive Manager, and ACDC-certified Master Hybrid and Electric Vehicles Technician. He is the founder and CEO of TauRay Automotive. He can be contacted on: +263772341193, +263772357296 or [email protected]