Overview: Bosch hybrid systems

Bosch has boldly announced that its full series-parallel hybrid system, used on the Porsche Cayenne and Volkswagen Touareg — which share a platform — is ‘the world’s first parallel full hybrid [system] to enter series production’. So what about all those Toyotas and Lexuses? Well, they use epicyclic power-splitting devices, and Bosch has decided that this detail disqualifies them from achieving ‘full hybrid’ status.

But of course doing away with a component does strike a blow for simplicity and weight-saving. Bosch uses power electronics to manage the interaction between the vehicle’s internal combustion engine and its electric motor without the need for a splitter. The Motronic control unit, which manages the combustion engine, uses an additional CAN bus to exchange data with the hybrid components, power electronics and battery

Florence and the machine: Volkswagen Touareg with Santa Maria del Fiore.

There is inevitably a considerable degree of idea-sharing within Bosch, and some of the techniques used in the Company’s hybrid control system came originally from work on management systems for directly-fuelled petrol engines. While development engineers were busy programming Motronic systems for these engines, they learnt how to switch between different operating points without any change in torque. The basic idea was to allow direct injection systems to switch from homogenous operating mode to lean operation — for example — without any jump in torque. This led to the development of a system for engaging an additional source of torque without any loss of smoothness. And that’s something Lexus could certainly learn from.

Smooth engagement is an obvious issue when the electric motor hands over to the internal combustion engine, when both sources work together, or when the combustion motor temporarily shuts down. These internal manoeuvres are carried out autonomously and automatically, on the basis of how much torque the driver is asking for. The hybrid control unit also determines how much brake torque should be provided by the electric motor — acting as a generator, it can feed a fair amount of negative torque into the drivetrain.

Looking at how the system is designed makes it easier to understand how it works. The electric motor is positioned between the car’s internal combustion engine and its automatic transmission. The hybrid module, which is 145mm in length and 300mm in diameter, encompasses the Integrated Motor Generator (IMG), a separate clutch controlled by actuators, a water cooling system, and the permanent magnets and coils of the rotor and stator. The developers have also incorporated digital sensors to monitor the rotor position. The IMG enables the car to be driven on electric power alone at speeds of up to 35mph. When the driver demands full acceleration, the electric motor and petrol engine are used in parallel, and the torque of the two power units is combined; under steadier driving conditions and when braking, the IMG functions as a generator, supplying electrical power to the power electronics.

Bosch power electronics control box.

The power electronics are integrated in the liquid cooling circuit, as the device incorporates a high-power transformer within its 10-litre casing. Two things happen inside this box. First, the device acts as an interface between the high-voltage — 288V, 370A — network of the hybrid drive, and the vehicle’s 12V ‘domestic’ electrical system. Secondly, bipolar transistors convert the direct current from the battery into three-phase alternating current for the electric motor, and vice versa when the motor is working as a generator.

The operating strategy for these components is controlled by the hybrid control unit. The development team was able to base its work on the Motronic direct injection control technology. They also incorporated expertise from the realm of start-stop systems, which taught the developers how to start up combustion engines quickly in different modes without jarring. One essential step was ensuring that sensors were in place to send a continuous supply of information to the control unit regarding temperatures, piston position and engine speed.

A new feature is the way in which the combustion engine shuts down when the driver releases the accelerator, activating what the engineers call ‘sailing mode’, which can be used at speeds up to around 100mph. When the car is ‘sailing’, the electric motor operates as a generator for the vehicle’s 12V electrical system.

As soon as the combustion engine is needed, the clutch system tracks the point at which the combustion engine starts to transfer torque. It records this touch point in the hybrid control unit as a reference value for the next shift between power sources. Sensor data from the IMG, combustion engine, battery, and other components also funnel through to this control unit, which analyses the flow of data in real time and extrapolates it to provide inferred information about components that are not being directly monitored. This allows the IMG to control the meshing of the drives using the clutch actuator system within just a few milliseconds.

The moment you reach the operating point at which the combustion engine should engage, the clutch engages just far enough to start up the internal combustion engine, and then disengages again. This minimises the energy required to get the combustion engine running. As soon as the combustion engine has reached the same speed as the electric motor, the clutch engages fully. This process is rapid.

Bosch is interested in using its hybrid system on cheaper cars. It is, the Company says, perfectly suited to paving the way for downsizing concepts to enter the mass market. The system’s precision of control could help iron out any lack of smoothness ‘typically seen in highly turbocharged direct injection two-cylinder and three-cylinder engines.’ There is even the suggestion that cutting off individual cylinders (modular operation) might be feasible without sacrificing comfort. (Alfa Romeo tried this in the late seventies — not surprisingly, without success.)

The key to making hybrid systems more widely available clearly lies in reducing costs. Battery packs are currently something of a stumbling-block, with costs still very high. All hybrid systems are constructed in a modular format, which means they can be incorporated in vehicles made by different manufacturers. Increasing unit volumes would lead to lower system costs, which is the only way that hybrid technology can realistically make its way into high-volume models. But not just a long-term vision: Bosch has a number of projects underway with mainstream manufacturers, including a hybrid development cooperative venture with PSA.

In the future, Bosch intends to market itself as a single-source supplier of complete, fully integrated electric drives with lithium-ion batteries. To achieve this, Bosch has embarked on a joint venture with Samsung SDI for building vehicle batteries. The company that Bosch and Samsung have formed — SB LiMotive — is currently setting up its first production facility in South Korea, and will be producing the first batteries for hybrid vehicles from 2011; the following year, the facility wioll begin production of battery packs for fully electric vehicles.

As storage batteries become lighter and more efficient, Bosch sees hybrid technology as offering even greater potential to make savings.

Porsche
Cayenne
S Hybrid
Volkswagen
Touareg
Hybrid
Body type 5-door
SUV
5-door
SUV
Cylinders 6V 6V
Valves 4 4
Bore/stroke 84.5/89.0 84.5/89.0
Swept volume 2995cc 2995cc
Compression
ratio
10.5:1 10.5:1
PS/rpm
E-motor
Combined
333/5500
47
380/5500
333/5500
47
380/5500
Nm/rpm
E-motor
Combined
440/3000
300
580/1000
440/3000
300
580/1000
High V battery:
Type
NiMH NiMH
High V battery:
Nominal voltage
288 288
Capacity 6Ah 6Ah
Maximum speed 150 149
0-100km/h 6.5s 6.5s
Urban MPG
(l/100km)
32.5
(8.7)
32.5
(8.7)
Combined MPG
(l/100km)
34.4
(8.2)
34.4
(8.2)
CO2 g/km 193 193
Emissions EU5 EU5
Transmission A8 A8
Driven wheels All All
Torque F:R 40:60
Wheels
Tyres
8.0Jx18
205/55 XL
TBA
Brakes: front
Brakes: rear
360x36
330x28
TBA
Kerb mass* 2240 2245
Length 4846 4795
Width 1939 1940
Height 1705 1732
Wheelbase 2895 2893 †
Cd 0.36 TBA
CdA 1.008 TBA
Track: front
Track: rear
1655
1669
1650 (1)
1670 (1)
Fuel tank 85l 85l
* DIN. EU kerb mass = DIN + 75kg.
† Difference in quoted figures not explained.
(1) Difference results from wheel widths.

Technoglossary: Hybrids

Power electronics

Function
The link between the battery (which requires DC) and the electric motor, which requires or generates — depending on its mode of operation — three-phase AC. A pulse inverter ‘chops’ the DC using bipolar transistors — switching elements that switch on and off at a very fast rate. In addition, a DC-DC converter acts as an interface between the vehicle’s 12V electrical system and the 288V network of the electric drive.

Challenge
Tight on space. Having started out at a volume of 15l, it has now shrunk to 10l in the Cayenne and Touareg. Bosch is currently working on a 5l version. The devices must be capable of withstanding tough conditions in the vehicle (-40°C-120°C) and must keep working for the entire useful life of the vehicle. Although cooling is necessary, there is a thin line between applying what is needed and simply wasting energy. Temperature sensors in the power electronics ensure that the cooling water pump only runs when it’s required.

Miscellany
The core components are the pulse inverters. The load on these switches — which are actually semiconductors — is enormous. The only way of enhancing performance is by boosting the chip area, but developers are actually trying to do exactly the opposite. The aim is to make the inverter smaller rather than even bigger. Key techniques to achieve this are cooling concepts, new semiconductor materials, and new assembly and packaging technologies. Design skill also plays an important role. The inverter is extremely tightly packed.

Integrated Motor Generator (IMG)

Function
Bosch uses disc motors. The motor is around 30cm in diameter and positioned between the internal combustion engine and the transmission. It is a water-cooled, permanent magnet synchronous machine with an inner rotor. It starts up the internal combustion engine, enables the vehicle to be driven on electric power alone, can be operated in tandem with the combustion engine, and acts as a generator to convert kinetic energy and combustion engine torque into electricity.

Challenge
Tight on space. Between the combustion engine and the transmission there is extremely limited space if you want to avoid having to reposition the (longitudinal) engine further forward. Transverse engine installations are even more space-critical. High-density power in a limited space requires cooling, especially with the combustion engine radiating heat. That is why cooling ducts traverse the outer stator, which are also used to dissipate the heat emitted by the spinning rotor inside. Leakage currents are another problem caused by the tight fit, since there is very little distance between live parts and metal components. Developers are forced to maintain a balance between minimum distances and maximum power density. Another difficulty is activating the IMG in a way that ensures the rotor immediately turns in the right direction. To achieve this, three digital sensors monitor the position and speed of the rotor. In the future, the sensors will also be able to determine the direction in which the engine is moving; this is because fully electric vehicles do not have a transmission, and the control unit needs to know whether it is going forwards or backwards.

Hybrid Control Unit

Function
The control unit draws together sensor data from the electric and internal combustion engine powertrains. The flow of data allows the control unit to infer the state of other components that are not directly monitored by sensors. All this happens in real time and provides the basis for (ideally) efficient and seamless interaction between the combustion engine and electric motor. One of the key input variables comes from interpreting the driver’s wishes by monitoring the pressure on the accelerator and brake pedal.

Challenge
Achieving maximum comfort using software-based power splitting without mechanical elements. Smooth transitions between electric-powered, hybrid, and combustion engine-powered driving without jolts or jarring and without any sudden surges in torque.

Miscellany
The operating strategy — determining when and how the car travels on electric power — is determined by the driver. Bosch offers its own strategies, but many automakers are keen to design this aspect themselves to tailor the feel of the system. An additional CAN bus ensures that the hybrid control unit can access all the required sensor data in real time. Bosch already has some experience in controlling transitions in a drivetrain without sudden changes in torque through its work on conventional drives. For example, when the catalytic converter heating comes on in direct-injection engines, the Motronic balances out any potential fall in torque.

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