MHEV: Optimize automotive powertrain to improve efficiency and reduce costs

In this article, I will discuss 48V light-hybrid electric vehicles (MHEV) and explain how this technology can achieve approximately two-thirds of the advantages of full-hybrid electric vehicles at approximately one-third of the cost.

Vehicle emission standards are becoming stricter year after year, and it is difficult for internal combustion engine (ICE) car manufacturers to meet the requirements. In order to reduce emissions, one of the manufacturers’ tasks is to partially or fully electrify the transmission system to improve the effective efficiency of the engine and partially or completely reduce the dependence on the engine (see Figure 1).

Of course, achieving electrification has a price, and it involves a long-standing design problem: how to balance cost with other design requirements?

In this article, I will discuss the 48V light-hybrid electric vehicle (MHEV) and explain how this technology can achieve approximately two-thirds of the advantages of a full-hybrid electric vehicle at approximately one-third the cost.

MHEV: Optimize automotive powertrain to improve efficiency and reduce costs
Figure 1: List of commonly used electric drive system topologies

System adds C MHEV and full hybrid electric vehicle

MHEV uses a 48V battery to realize many functions of a full hybrid electric vehicle at a small additional cost. Figure 2 compares the hardware and functions of ICE, MHEV, and full hybrid electric vehicles. A typical full hybrid electric vehicle integrates an electric motor and a 200V to 400V high-voltage battery with a capacity of approximately 1kWh to 2kWh. MHEV uses smaller motors and smaller 48V batteries or similar supercapacitors with a capacity of less than 1kWh. Compared with full-hybrid electric vehicles, this smaller motor and significantly smaller battery greatly reduces the cost and weight of light-hybrid electric vehicles (by improving fuel economy). Compared with a full hybrid electric vehicle, the hardware requirements are reduced, but the performance provided is not as powerful, but as you will see, MHEV can still provide most of the advantages while reducing costs.

MHEV: Optimize automotive powertrain to improve efficiency and reduce costs
Figure 2: Comparison and summary of ICE, MHEV and full hybrid electric vehicles

Start-stop function

After turning off the ICE under the above conditions, start-stop can greatly improve the fuel economy in city driving or other start-stop environments. The energy stored by MHEV batteries is indeed less than that of high-voltage batteries in full-hybrid electric vehicles, but its capacity is sufficient to start and stop in most cases. Using a 12V battery can achieve start and stop, but using a 48V battery can withstand less stress, which can extend the battery life. There may be a voltage drop when using 12V to start, but there is no such problem for 48V to start and stop.

Regenerative braking

As the name implies, regenerative braking recovers the kinetic energy of the vehicle when the driver brakes. This energy is usually dissipated as heat through the brake pads, but regenerative braking can use the light-hybrid or full-hybrid power system motor as a generator to charge the light-hybrid or full-hybrid power system battery. Regenerative braking (such as start-stop) is widely used in start, stop, acceleration and deceleration environments. The reduced battery capacity of the light hybrid system will limit the effectiveness of regenerative braking in some cases, but it is sufficient to recover energy without running out of capacity in most urban driving conditions.

Torque assist

The hybrid power system motor is connected to the transmission system and uses the energy of the hybrid power system battery to increase torque when accelerating. Torque assist is good for acceleration, which means that the vehicle can achieve the same acceleration performance with a smaller ICE. This can reduce engine cost and weight, thereby further improving fuel economy. The light hybrid system motor is smaller and lower performance than the full hybrid system motor, which will limit the amount of increased torque, but in many cases, the light hybrid system motor can still provide most of the torque assistance advantages.

Reduce the weight and cost of wiring harnesses-MHEV’s unique advantages

In terms of wire harness cost and weight, light hybrid power systems have more advantages than full hybrid power systems. Because 48V is not a particularly high voltage, a 12V power supply device can be converted to 48V without much modification. Downgrading from hundreds of volts to 5V or 3.3V requires more expensive and complex power conversion designs, but replacing the 12V to 5V buck regulator with a 48V to 5V buck regulator requires very few changes.

In addition, it is very easy to modify other systems (such as heaters and blowers) to adapt to the 48V voltage. With the same power supply, using 48V instead of 12V to power the device can reduce the required current by 75%, thereby significantly reducing the thickness of the wiring harness, and even (for example) it is possible to switch from copper to aluminum while reducing weight. This not only reduces costs, but also improves fuel economy.

Concluding remarks

MHEV can improve fuel economy and is the future of cost-sensitive vehicles. Some models have already used the light hybrid power system, and proved that the system can be used as a good transition between the ICE and the full hybrid system motor. Fuel economy requirements will only become more stringent, so it is expected that more MHEV vehicles will be on the road in the near future.

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