Intricacies of extended-range electric vehicle technologies

IN the previous article, I outlined the extended-range electric vehicle (EREV) operations and architecture.

This week, I delve further into the architecture of the EREV.

The Atkinson-cycle engine design used in EREVs is also utilised in the majority of hybrid electric vehicles (HEVs) and engine applications of plug-in hybrid electric vehicles (PHEVs) because of its wide range of control of the late intake closing events and fuel system delivery strategy.

When using sophisticated engine valve and fuel delivery strategies and electric propulsion, the vehicle operator will not recognise any differences between the EREV and traditional internal combustion engine (ICE) powertrain systems.

Let us begin by looking at the different EREV operation modes.

Operation mode 1 is when an EREV uses “single electric motor driving (no engine assist)”, and operates as a battery electric vehicle (BEV).

The internal combustion engine is not operating, and the vehicle is only propelled by the drive motor, with energy from the battery pack.

As with PHEV technologies, this full electric mode will not utilise the ICE functionality (engine is off).

So, if the throttle is wide open, the driver gets full electric vehicle (EV) motor operation without assistance from the ICE.

Until the battery pack depletes to the minimum state of charge (SOC) operating point, the system will transition into a mode that will crank and start the ICE to operate the generator, which will provide electrical power to drive the motor and sustain battery pack SOC.

In this mode, the power flow involves applying the C1 clutch that locks the planetary ring gear to the transmission case.

This will permit the drive motor (MGB/MG2), which is connected to the sun gear, to force the carrier (pinion gears) to rotate inside the ring gear.

Because the carrier is the planetary member connected to the output, MGB/MG2 torque is transmitted from the differential gear set to the drive axles.

EREV operation mode 2 uses two-motor (load sharing) driving.

The powertrain will use both the generator, motor generator (MGA/MG1), and drive motor (MGB/MG2) to provide propulsion speed and torque.

As vehicle speed is increasing (i.e. greater than 80 kilometres per hour), the drive motor (MGA/MG1)’s efficiency and torque limits are exceeded, which requires a change to happen within the drive unit.

To maintain torque and revolutions per minute (rpm) level, the C1 clutch is unlocked, and the C2 clutch is applied.

This will permit MGB/MG2 to continue applying torque to the carrier from the sun gear.

With the C2 clutch applied, MGA/MG1 can apply torque to the carrier through the ring gear.

The torque from both machines will be combined and transferred to the carrier, and ultimately to the differential and drive axles.

When the C2 clutch is applied, the control system will gradually increase MGA rpm and torque.

At the same time, the control system will reduce MGB/MG2 torque and rpm.

By reducing MGB/MG2 speed and torque, the control system will command it into a higher efficiency and torque range and place it back into an optimal operating state.

By simultaneously applying and increasing MGA/MG1 torque and rpm and reducing MGB/MG2 rpm and torque, the net effect is cancelled and there are no perceptible changes in vehicle operation, resulting in transparent operation to the driver.

 

By using both the drive motor and generator to propel the vehicle, instead of only using the drive motor, the system torque and efficiency can be optimised.

If the system was to only use the drive motor at higher vehicle speeds (greater than 80 kilometres per hour), it will result in reduced levels of motor efficiency and torque.

This would ultimately result in wasted battery pack energy and reduced vehicle range.

As the vehicle operates and continually makes changes from one-motor driving to two-motor driving, the battery pack SOC percentage will decrease to a point of depletion.

This will require the system to transition to another mode to crank and start the engine, so that the electrical power can be generated.

Examples of two-mode systems in HEVs in Zimbabwe are the Toyota Aqua, Toyota Axio, Toyota Prius and Mazda Axela hybrids.

EREV operational mode 3 uses “series mode, engine on”.

In this mode, the engine will be operating, but it will be operating a series hybrid vehicle, which is defined as a “vehicle that utilises the engine to power an electrical generator, but the engine will not transfer propulsion torque to propel the vehicle”.

In simple terms, the engine and generator serve as a “generator set” to propel electrical power to the drive motor and battery pack.

When the EREV system senses that the battery pack is at its depleted SOC percentage, it will command MGA/MG1 to crank and start the engine.

This is accomplished while the vehicle is being driven, but it is nearly imperceptible to the operator.

To accomplish this operating mode, the control system will transition from two-mode driving, unlock the C2 clutch and apply to the C1 clutch.

This will place the system into one-motor driving mode, and with the C2 clutch unlocked, MGA/MG1 cannot apply torque to the planetary gear set.

The control system will also apply the C3 clutch, which will connect the engine and MGA/MG1.

With the C3 clutch applied, the power inverter controller will command MGA/MG1 to engage in approximately 1 000 rpm to crank the engine.

Once the engine has started, the power inverter controller will command the MGA/MG1 to operate as a generator to provide electrical power MGB/MG2 and transfer energy to the battery pack to sustain its SOC to approximately 20 percent.

Because the engine is not connected to the powertrain and is only providing power to the generator, the control software is designed to operate the engine at optimal efficiency state.

Without the engine being connected to the powertrain, there are no road transients being transmitted through the driveline.

This permits the engine to operate at specified rpm bands, instead of being operated throughout an entire rpm range, with constantly changing road loads.

 

*Taurayi Raymond Sewera is ASE and AutoCate 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]