BMW Plug-in Hybrid Electric Vehicle (PHEV) powertrain architecture

A hybrid electric vehicle (HEV) has two sources of power, an internal combustion engine (ICE) and at least one electric motor. Depending on the level of electric power and on the functions performed by the electric machine, there are several levels of powertrain hybridization. For a detailed information about each HEV architecture, read the article Understanding micro, mild, full and plug-in hybrid electric vehicles.

A HEV is also know as a full hybrid, the main reason being the electric propulsion mode. There are two main categories of full hybrid electric vehicles:

• series hybrid electric vehicles
• parallel hybrid electric vehicles

In the case of a series hybrid electric vehicle, the propulsion is done only through the electric motor. The ICE, connected to an electric generator, is only used to generate electric energy to power the electric motor or to charge the high voltage battery. In case of an series HEV, the ICE is not propel the vehicle directly.

In a parallel hybrid electric vehicle, both the ICE and the electric motor can power the wheels, either independently (one or the other) or together (in the same time, in parallel). Depending on the way the power from ICE and electric motor is send to the wheel axles, we can have two types of parallel hybrid electric vehicles:

• (concentric) parallel hybrid electric vehicle: both ICE and electric motor can power the same axle (front, rear or both)
• “through the road” parallel hybrid electric vehicle: the ICE is powering one axle and the electric motor the other axle

A plug-in hybrid electric vehicle (PHEV), from the architecture and traction modes point of view, is a full hybrid electric vehicle, but with an additional feature, the possibility to charge the high voltage battery from the grid. A PHEV can be either a series or a parallel hybrid electric vehicle.

A battery electric vehicle (BEV) is a vehicle which relies solely on electric motors to power the wheels. A typical BEV architecture is with an electric motor on one of the axles, a fixed step gearbox and a high voltage battery. In a BEV, without a range extender, the energy for traction is supplied 100% from the high voltage battery.

BMW has currently on the market two architectures of plug-in hybrid electric vehicles and one electric vehicle.

BMW i8 is a “through the road” plug-in hybrid electric vehicle, with an ICE on the rear axle and an electric motor on the front axle.

In this article we are going to discuss in detail about the technical aspects of the BMW’s iPerformance plug-in hybrid electric vehicle architecture.

There are two BMW concepts linked to hybrid or battery electric vehicles:

• BMW eDrive: is represented by the package of electrical motor, lithium-ion high-performance battery and intelligent energy management system
• BMW’s “i” technology: refers to electrified powertrain. There are the i8 PHEV and the i3 BEV and the latest iPerformance PHEV family

The latest addition to BMW’s PHEV family is the BMW 740e iPerformance, with the long wheelbase version 740Le iPerformance. The powertrain consists of a four-cylinder petrol engine with BMW TwinPower Turbo technology and an electric motor integrated into the eight-speed Steptronic transmission to generate a system output of 240 kW/326 HP.

The BMW 740Le xDrive iPerformance has an intelligent all-wheel drive (AWD) system which distributes the combined power of the combustion engine and electric motor permanently and as required between the front and rear wheels.

The fuel efficiency figures and electric range (EU test cycle) are notable for all three versions:

	740e iPerformance	740Le iPerformance	740Le xDrive iPerformance
Fuel consumption [l/100 km]	2.0 – 2.2		2.1 – 2.5
Mileage [mpg]	141.2 – 128.4		134.5 – 113
CO2 emissions [g/km]	45 – 50	45 – 51	49 – 56
Electric range [km]	44 – 48		41 – 45

The powertrain has “the best of two worlds”, the electric traction is supplied by ZF’s integrated motor generator ZF8HP hybrid automatic gearbox and the ICE has BMW’s TwinPower Turbo technology. The 2.0 liter gasoline engine is developing a maximum output of 190 kW / 258 HP and it is the most powerful four-cylinder engine ever fitted in a series-produced BMW. The peak torque of 400 Nm (295 lb-ft) is kept constant between 1550 and 4400 rpm.

where:

1. internal combustion engine
2. engine disconnect clutch (K0 clutch)
3. electric machine (integrated in the automatic transmission)
4. automatic transmission
5. power electronics
6. high voltage battery
7. rear differential

The package of technology built into the new engine comprises in:

• TwinScroll turbocharger integrated into the cast-steel exhaust manifold
• High Precision Injection with a maximum fuel pressure of 200 bar
• the latest version of VALVETRONIC variable valve timing
• and variable camshaft control on both the intake and exhaust sides (Double-VANOS).

Balancer shafts with rolling bearing supports in the aluminium crankcase enhance the engine’s refinement, while the close-coupled arrangement of the catalytic converter and an electrically controlled wastegate for the turbocharger optimize its emissions. Moreover, electronically switchable engine mounts provide the connection with the body. These mounts smooth out the vibrations caused by the engine (which vary according to load) as required, ensuring that all occupants experience exceptionally low vibration levels and the engine delivers outstanding refinement.

where:

1. power electronics module
2. high voltage battery charger controller
3. ZF8HP hybrid automatic transmission
4. high voltage battery

Since the plug-in hybrid variants of the luxury sedan come with a specific, extremely powerful starter motor for the combustion engine and a compressor for the climate control system that operates using energy from the high-voltage battery, the petrol engine is equipped with a special belt drive. This removes the need to integrate the generator and refrigerant compressor as required in conventionally powered vehicles. A large proportion of the engine’s output and torque is therefore available to be converted into dynamic performance.

The hybrid transmission (ZF8HP) is provided by ZF (Germany). Based on the principle of a permanently excited synchronous motor, the electric drive system stands out with its linear power delivery up into the higher reaches of the speed range and its exceptionally high level of efficiency. Its maximum output is 83 kW/113 HP. And, as is characteristic for electric drive systems, it serves up its peak torque of 250 Nm (184 lb-ft) from the word go. The boost effect for the combustion engine provides thrilling, instantaneous responsiveness and power build-up with absolutely no delay when accelerating from stationary, as well as far more dynamic performance whenever a quick burst of speed is required. Furthermore, the electric motor also takes on the role of a generator – powered, depending on the hybrid function selected, either by recuperating energy on the overrun and under braking or by efficiently raising the engine’s load points. It then feeds the energy it generates into the high-voltage battery.

where:

ICE – internal combustion engine
DMF – dualmass flywheel
K0 – engine connect/disconnect clutch
EM – electric machine
IAE – launch clutch
pump – mechanical oil pump
E-pump – electric oil pump

The K0 clutch is used to disconnect and shutdown the engine from the wheels, when the vehicle is driving in Electric mode or when it’s Coasting. When launching the vehicle, from standstill, the IAE clutch is used to connect the electric machine and the engine to the planetary gearbox. When the engine or electric machine are providing torque, the mechanical pump is generating the necessary oil flow and pressure for the automatic transmission to work. When the vehicle is in Stop & Start or Coasting state, the E-pump takes over to maintain the oil pressure in the hydraulic system.

The electric motor is fully integrated into the car’s eight-speed Steptronic transmission. This ensures that pure-electric driving, the dynamics-enhancing electric boost function and the recuperation of braking energy all happen extremely efficiently. The integration of the electric motor also means the transmission does not require a torque converter (enabling a reduction in weight), while the hybrid module takes over the function of the starter element. The eight-speed Steptronic transmission combines efficiency with shift dynamics and shift comfort at a high level. The transmission’s broad spread of ratios helps the car to pull away quickly under acceleration but also promotes low energy consumption when driving at higher speeds. The small rev intervals on upshifts and downshifts benefit both sports performance and driving comfort.

Gearshift paddles on the steering wheel are available as an option, enabling the driver to make manual gear selections with exceptional swiftness and the greatest of ease. Working together intelligently, the combustion engine and electric drive system bring a system output of 240 kW/326 HP and combined peak torque of 500 Nm (369 lb-ft) to the table. Immediate response to every movement of the accelerator and sustained delivery of dynamic power allow the BMW 740e iPerformance to dash from 0 to 100 km/h (62 mph) in 5.4 seconds and the BMW 740Le iPerformance to complete the same sprint in 5.5 seconds. The top speed of both models is electronically limited to 240 km/h (149 mph).

where:

1. electric machine
2. IAE (launch) clutch
3. planetary gearbox

The high-voltage lithium-ion battery stands out with its compact construction and has a gross capacity of 9.2 kWh (net capacity: 7.4 kWh). It consists of 96 cells arranged in six modules and has its own cooling system integrated into the climate control system for the interior. The high-voltage battery was developed specifically for this model and is accommodated underneath the rear seat bench in a space-saving position that also ensures optimum crash safety.

The energy flow between the high-voltage battery, electric motor and charger is controlled by power electronics likewise developed specifically for these plug-in hybrid models. The power electronics also regulate the supply of energy from the high-voltage battery to the 12V onboard electrical system via a voltage transformer.

The high-voltage battery can be topped up with energy from any domestic power socket, a Wallbox designed for higher currents or public charging stations. The vehicles come as standard with a charging cable, which is stored in a bag and can be held in place using a tension belt on the left-hand side of the boot. The battery can be charged to full capacity in under four hours from a domestic power socket, while tailor-made solutions enabling safe, straightforward and rapid charging at home are available through BMW 360° ELECTRIC.

The BMW i Wallbox Pure uses its output of 3.7 kW (16 A/230 V) to charge the high-voltage battery in under three hours. The progress of the charging process can be tracked via a graphic in the car’s instrument cluster or the BMW Remote app on a smartphone. A second version of the charging station, the BMW i Wallbox Pro, comes with features including a seven-inch touchscreen, which indicates how charging is progressing and allows customers to choose their own settings governing load management and self-produced electricity.

The powertrain can be operated in five modes. These are summarized in the table below.

Powertrain mode	ICE state	EM state	K0 clutch state	IAE clutch state
Electric Drive (EV mode)	Off	Motor mode (positive torque)	open	closed
Parallel (torque assist)	On (positive torque)	Motor mode (positive torque)	closed	closed
Regeneration	Off / On (negative torque)	Generator mode (negative torque)	open / closed	closed
Hold (charge)	On (positive torque)	Generator mode (negative torque)	closed	closed
Coasting	Off	Off	open	open / closed

If the state of charge of the high voltage battery is above the minimum threshold (around 10%), from standstill, the vehicle is launched in Electric Drive (EV mode). During this mode, the K0 clutch is open and the ICE is shutdown. Function of several operating conditions (environmental temperature, high voltage battery state of charge, etc.) the maximum vehicle speed, in EV mode, can reach up to 140 kph.

The Parallel mode is used when there is a high torque demand from the powertrain. If the driver is pressing heavily on the accelerator pedal, both the ICE and EM are producing torque. In this mode the electric machine is assisting the power output of the powertrain.

During braking, a part of the kinetic energy of the vehicle is regenerated (recovered) through the electric machine. When the driver is pressing the brake pedal, the electric machine is switching to generator mode and it’s using the inertia of the vehicle to generate electric current. If the brake pedal position is relatively small (e.g around 10%) all the vehicle braking can be performed through the electric machine and the ICE brake (friction) torque, the foundation (hydraulic) brakes being inactive. If there is a high braking demand, the foundation brakes are activated in order to to slow/stop the vehicle efficiently.

The Hold mode is active when the high voltage battery reaches the minimum allowed state of charge. In this mode, the vehicle propulsion is performed mainly by the ICE. A small amount of the engine torque is used by the electric machine (generator) to generate electricity and hold the state of charge of the high voltage battery at a desired level. In this mode the electric machine is allowed to enter in motor mode and boost the powertrain torque but only for very short periods of time (couple of seconds).

The Coasting mode is used to improve the mileage (reduce fuel consumption) of the vehicle by shutting down the engine during long deceleration. When the driver is slowly removing the foot from the accelerator pedal, at medium and high vehicle speeds, the K0 clutch is disconnected and the engine is shutdown. In this mode the vehicle will coast down, slowly decelerating mainly due to road losses and wheel friction losses.

During Coasting, because the IAE clutch is open, it is not possible to regenerate electrical energy. Coasting is efficient only in long deceleration scenarios. For short, more intensive deceleration, Regeneration mode being more efficient.

The eDrive button on the center console allows the driver to select from the AUTO eDRIVE, MAX eDRIVE and Battery Control settings, and so determine the operating approach of the plug-in hybrid drive system. AUTO eDRIVE, which is automatically activated when the vehicle starts up, uses intelligent energy management to ensure the combustion engine and electric motor work together to the best possible effect in terms of both efficiency and dynamics.

The driver can switch to the MAX eDRIVE operating function at the touch of a button. In this setting, the car is powered exclusively by the electric motor, although the combustion engine can be activated at any time by pushing the accelerator into kickdown (fully pressed).

Regardless of the setting selected via the eDrive button, when the gearshift lever is moved into the S (sport) gate the engine comes to life as required to ensure the maximum combined output of the two drive systems is permanently on tap.

At the touch of a button, the driver can activate a vehicle setting that optimises dynamics, comfort or efficiency. Their selection adjusts the characteristics of the accelerator and steering, the shift profile of the Steptronic transmission and the responses of the Dynamic Damper Control system. The range of characters encompassed by the ECO PRO, COMFORT and SPORT modes are even more clearly defined than with conventionally powered vehicles.

COMFORT mode – which is automatically engaged when the driver starts the car – provides the plug-in hybrid with a smoothly balanced overall set-up. In this mode, the electric motor delivers relaxed driving with the lowest possible energy consumption. In SPORT mode the power of both the combustion engine and the electric motor teams up with stiff damper settings, instantaneous responses to movements of the accelerator, sharper shift dynamics and direct steering characteristics to maximize dynamic performance. The electric boost function is stepped up a level in SPORT mode, as is recuperation performance under braking and on the overrun.

The hybrid-specific version of the ECO PRO mode supports an operating strategy which aims to maximize the car’s electric range. As a result, the electric boost function supplementing the power of the combustion engine is only activated if the accelerator is pushed into kickdown. Energy recuperation on the overrun also takes a limited form. Instead, a hybrid-specific coasting function is triggered at speeds between 40 and 160 km/h (25 – 99 mph). As soon as the driver releases the accelerator, the combustion engine is switched off. In addition, recuperation of braking energy is reduced to the level required by the onboard power supply. The car therefore runs on the minimum possible energy consumption and with the lowest possible mechanical resistance. Furthermore, as with the other models in the new BMW 7 Series line-up, ECO PRO mode features precisely gauged power control for electrically operated convenience functions, such as the climate control system, seat heating and exterior mirror heating.

In ADAPTIVE mode the car’s responses noticeably adapt to the driver’s style and route profile, taking their cues from movements in the steering wheel, accelerator and brake pedal, as well as from the position of the gearshift lever in the D or S gate. In addition, the navigation system’s map data is used to ensure the right set-up option is made available when the driver switches from city driving to the motorway, for example, as well as on twisty sections of road or when approaching a junction.

Technical specifications:

		BMW 740e iPerformance	BMW 740Le iPerformance
Body
No of doors/seats		4 / 5	4 / 5
Length/width/height	mm	5098 / 1902 / 1467	5238 / 1902 / 1479
Wheelbase	mm	3070	3210
Track, front/rear	mm	1617 / 1646	1617 / 1646
Ground clearance	mm	135	135
Turning circle	m	12.3	12.8
Fuel tank capacity	l	46	46
Engine oil capacity	l	5.25	5.25
Weight, unladen, to DIN/EU	kg	1900 / 1975	1940 / 2015
Max load to DIN	kg	685	660
Max permissible weight	kg	2585	2600
Max axle load, front/rear	kg	1140 / 1470	1160 / 1475
Max roofload/towbar download	kg	100 / –	100 / –
Luggage comp capacity	l	420	420
Air resistance	cd [-] / A [m2]	0.25 x 2.41	0.25 x 2.42
Drive system
Drive concept		Full hybrid drive, torque vectoring to rear wheels from one or both units
System output	kW/hp	240 / 326	240 / 326
System torque	Nm	500	500
Weight-to-power ratio (DIN)	kg/kW	7.9	8.1
Combustion engine
Config/#cyls/valves		in-line / 4 / 4	in-line / 4 / 4
Engine technology		BMW TwinPower Turbo technology: TwinScroll turbocharger, High Precision Injection, VALVETRONIC fully variable valve control, Double VANOS variable camshaft control
Effective capacity	cm3	1998	1998
Stroke/bore	mm	94.6 / 82.0	94.6 / 82.0
Compression ratio	:1	10.2	10.2
Fuel grade		min RON 91	min RON 91
Power output @ speed	kW/hp @ rpm	190 / 258 @ 5000 – 6500
Power output @ speed	Nm @ rpm	400 @ 1550 – 4400
Output per litre	kW/l	95.1
Electric motor
Motor technology		BMW eDrive technology: synchronous electric motor integrated in 8-speed Steptronic transmission, generator function for energy recuperation for the high-voltage battery
Max power output @ speed	kW/hp @ rpm	83 / 113 @ 3170
Max torque output @ speed	Nm @ rpm	250 @ 0 – 3170
Recuperation output	kW	20
High-voltage battery
Storage technology / installation		lithium-ion / underneath rear seat
Voltage	V	351
Capacity (gross)	kWh	9.2
Charging time for 100% charge		2.7 h at 3.7 kW (16 A / 230 V)
Transmission
Type of transmission		8-speed Steptronic (ZF8HP hybrid)
Gear ratios	1	4.714
	2	3.143
	3	2.106
	4	1.667
	5	1.285
	6	1.000
	7	0.839
	8	0.667
	R	3.317
	Final drive	3.077

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

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