2010 BMW Group Innovation Days Mobility of the Future - The Electric Drivetrain.

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Complete Report:
Chapter 1. Why Electromobility?
Chapter 2. Project i.
Chapter 3. The Electric Drivetrain.
Chapter 4. Lightweight design and the LifeDrive concept.
Chapter 5. CFRP - A Material for the Future.

Chapter 3. The Electronic Drive Train

Up to now, driving a car has always meant having a combustion engine for company. However, changes in the environment and within society have shown that using fossil fuels across all areas of daily life comes at an ecological cost. And, of course, the fuels themselves will not be available indefinitely. The BMW Group views vigorously driving forward the technical development of electromobility as a way of combating these issues. But what does the term “e-mobility” actually represent? How does an electric drive system differ from a combustion engine? What is the potential of this technology? And what are the challenges the development engineers still need to overcome?

Emission-free and dynamic – the new generation of propulsion systems.
Powering vehicles solely using electric power opens up totally new avenues in mobility. As an electric motor emits no environmentally damaging gases, the use of electric energy offers a zero-local-emission and therefore eco-friendly route to mobility. The use of renewable energies, meanwhile, allows the energy generation process to be completely emission-free as well. Added to which, e-mobility also delivers a totally new and extremely agile driving experience. A new-generation BMW Group electric motor boasts impressive performance characteristics and quickly removes any associations with niche electric vehicles of the recent past. Only the absence of engine noise will remind you that you’re driving an electrically powered vehicle.

“The power development of an electric vehicle is almost like that of a light switch: you get full power the moment you switch it on.” (Hans-J�rgen Branz)

The BMW Group designs leading-edge electric drive systems, such as the unit for the Megacity Vehicle (MCV), that generate well over 100 kW. But their stand-out attribute is that the driver can already use the motor’s full output when pulling away – rather than having to wait for the power to build up through the engine revs, as with combustion engines. The differentials, slip control systems and gear assembly ensure that every ounce of the motor’s torque is transferred to the road. The ability to drum up peak torque from a standstill imbues electric vehicles with an exceptional level of agility and provides eye-catching acceleration. The rear-wheel drive of the Megacity Vehicle provides the perfect complement to the performance of the electric motor. Dynamic wheel load transfer allows more weight to be placed on the driven wheels when moving off, enabling better traction and power transfer. The electric motor’s impressive torque and rear-wheel drive therefore combine to deliver the driving dynamics for which the BMW Group is renowned.

The BMW Group is also aiming to be the force behind the best drive systems over the years ahead – drive systems whose efficiency, performance and smoothness set them apart from the competition, even if it is electricity rather than fossil fuels that are converted into propulsion. To this end, the BMW Group is vigorously driving forward technical developments in the field of electromobility. The BMW Group’s “electric power station”, its centre of expertise for electric drive systems, brings together development, manufacturing and procurement specialists under one roof. All their efforts are focused on the development and implementation of the new generation of drive systems.

Accelerating without changing gear.
Electric motors operate over a far wider rev band than combustion engines, routinely exceeding the 12,000 rpm mark. This means that electric motors also take a different route to reaching their top speed. The impressive torque of an electric vehicle allows it to accelerate more quickly than a model of comparable output powered by a combustion engine, and the high usable rev band also enables uninterrupted torque delivery across the full rev range. The output of the motor is fed through just a single gear ratio directly to the wheels; there is no need for a multi-speed transmission. All of which means that an electric vehicle sprints from standstill to its top speed in just a single gear. This uninterrupted flow of forward propulsion with steadily increasing revs is a very special experience, one which a combustion engine has so far only been able to deliver through highly sophisticated engineering measures, such as dual-clutch gearboxes.

“Electromobility is by no means a disclaimer on wheels. Electric vehicles are genuinely enjoyable.” (Patrick M�ller)

Having said that, the engineers have chosen not to fully exploit the theoretical top speed of the Megacity Vehicle. Since the MCV will be used primarily in cities and surrounding areas, a maximum of 150 km/h is more than sufficient. Higher speeds would be possible, but not necessarily of discernible benefit. Driving at high speeds uses up very large quantities of energy and, as the vehicle’s speed rises, so – exponentially – does aerodynamic drag, which also pushes up energy consumption. As the battery’s storage capacity only delivers a restricted amount of energy, unnecessarily high speeds would eat into the vehicle’s range significantly. Furthermore, a different gear ratio would be required to reach a higher end speed, and this would considerably reduce the vehicle’s agility in urban traffic. Another way of increasing top speed would be to fit a multi-speed transmission, but that would entail a notably more complex construction, as well as a considerable increase in packaging space and weight.

Braking with the accelerator.
Another attribute of electric vehicles which adds to their very distinctive driving experience is the ability to brake using the accelerator pedal, making it effectively a “driving pedal”. If the driver takes his foot off the accelerator, the vehicle does not continue to coast, but actively brakes. This deceleration force is harnessed for energy recuperation. Under braking the electric motor takes on the role of a generator, producing energy and charging the battery. The underlying principle is similar to that of the Brake Energy Regeneration technology in the EfficientDynamics package, although in the electric vehicle the recuperated energy can be converted directly into propulsion. Extensive use of the motor’s energy recuperation function increases the range of the vehicle by as much as 20%. The “driving pedal” also allows relaxing driving with less frequent footwork. Moreover, it enables quick reactions and is therefore particularly well suited to “going with the flow” in city traffic. Here, up to 75% of deceleration manoeuvres can be carried out without bringing the brake pedal into play.

Powerful and compact – the drive components.
A vehicle powered by an electric drive system offers more than just a rewarding driving experience. The electric motor also boasts a greater power density than a combustion engine – i.e. it generates the same power but takes up less space. For example, the entire drive system in the BMW ActiveE concept vehicle (and, in due course, the MCV) is – minus the energy storage system – the size of two crates of beer. The compact drive assembly can therefore be integrated neatly into the vehicle architecture, with the added benefit that there is no additional drivetrain or complex air intake system to be fitted in. The smaller dimensions and significantly lower mass of the electric drive system reduce the amount of installation space required by up to 50% compared to a combustion engine and transmission. Passengers will be the main beneficiaries of this extra room in future vehicle concepts, thanks to the increase in interior spaciousness.

An electric drive system as a whole consists of several components – the electric motor, the power electronics, a gear assembly and the electric energy storage system – which combine to provide the vehicle with propulsion.

Electric heartbeat – the motor.
The electric drive system is the heartbeat of the electric motor. In simplified terms, the latest-generation electric motor from the BMW Group consists of a tubular stator fixed to the casing and a rotating cylinder inside the stator (the rotor). The rotor is connected to the gear assembly and therefore – through the gear assembly – to the driven wheels. Inside the stator are coils in which a magnetic field is generated through current flow. On the rotor, meanwhile, are one or several magnets with fixed polarity. The electric motor is activated by generating calculated attraction and repulsion forces between the rotor and stator by means of a moving magnetic field (rotating field). To achieve this effect, the system uses the attraction of the opposite poles of a magnet and the repulsion of two identical poles (north and south poles attract each other, two south poles or two north poles repel each other).

Switching on the current causes the south pole of the magnetic field generated in the stator to attract the north pole of the rotor magnet. However, before the north pole of the rotor reaches the south pole of the stator, the south pole is switched onto the next phase. As a result, the rotor also continues to turn and “chases” the alternating magnetic fields produced by the stator. Through its rotational movement the rotor transfers the mechanical energy required for propulsion. The speed with which the rotating field moves around the stator dictates the speed of the vehicle. Torque levels, meanwhile, are controlled by the number of magnets and the current strength: the greater the number of magnets on the rotor and the stronger the current, the more torque the electric drive system is able to generate.

The operating principle described here is for the type of permanently excited, three-phase synchronous motor to be used in the ActiveE concept and the MCV. In this type of synchronous unit, the rotor follows the rotating exciter field on the stator synchronously. In addition, magnets ensure that the magnetic field of the rotor is permanently excited and does not first have to be induced (externally generated). External generation would be significantly more complex, as it would require a second intervention to generate the magnetic field in the rotor. Permanently excited motors currently offer the optimum balance of complexity, function and the ability to meet driver requirements.

Electronics boost performance – the power electronics.
The fundamental ingredient in a functioning electric motor when it comes to providing optimum performance is the correct rotation of the magnetic field around the stator. In order to achieve revs of over 12,000 rpm, the magnetic fields in each phase have to be switched extremely quickly and precisely. This important task is carried out by a special power electronics control unit, which ensures the rotating field is switched at the desired speed and with the necessary field strength. In this way it ensures that the rotor spins at the required speed and delivers the desired torque.

The battery – the electric vehicle’s “fuel tank”.
An extremely strong electric current is required to drive the motor of an electric vehicle. Currents of up to 400 amperes are activated for each phase, which equates to roughly 25 times that of a domestic power outlet. At up to 400 volts, voltages are also almost double those of the conventional power supply of everyday devices. A package of newly developed lithium-ion storage cells is used to store this energy and make it available as required. This lithium-ion technology has already demonstrated its exceptionally high storage capacity and cycle life in a large number of applications – e.g. mobile phones and laptops. A single lithium-ion cell for automotive usage is roughly the size of a notebook and has a rated voltage of approximately 3.7 volts. The usable voltage range of a cell lies between 2.7 and 4.1 volts, so in order to create a high-voltage battery meeting the 400 volt requirement, around 100 of these cells are connected in series.

There are, though, one or two specific factors to bear in mind in the use of battery cells. For example, lithium-ion cells do not work in the same way in different temperatures. Indeed, only their optimum operating temperature of around 20 degrees Celsius will ensure that the car’s maximum range is achievable. With this in mind, the temperature of the energy storage system is adjusted as required using additional heating elements and active cooling.

Having said that, there is substantially greater scope in the usage temperature range of the cells for vehicles than might be familiar from other battery cells. Some laptop cells, for example, should not be charged at temperatures below zero, where the machine’s performance would also be greatly diminished. While the cells used by the BMW Group do suffer from a drop-off in performance at low temperatures, a different composition of chemicals inside the battery means that this is much less pronounced. Preconditioning of the battery – during charging and as part of the need-based temperature control process while on the move – eliminates this potential drawback.

Safety is paramount.
The other main consideration in the development and design of the energy storage system concerned passenger safety. There is a certain risk potential inherent in the energy storage system, given the strong electric currents involved and the chemicals used (which react on contact with one other, possibly causing them to ignite). However, a host of measures are in place to eliminate the possibility of electric shock or the system catching fire. To start with, the compositions of chemicals used in battery cells for vehicles are much more “forgiving” than those in laptop batteries, for example. Plus, the vehicle body offers reliable protection for the battery modules to guard against damage in a crash. Meanwhile, coolant, sophisticated monitoring algorithms and on-board sensors ensure that the battery does not overheat when in use or during charging. Cut-off mechanisms secure the energy storage system against excessive discharge or overcharge, and measures have even been taken to ensure that there are no critical consequences should the energy storage system be pierced by metal objects.

The lifespan of a vehicle.
The BMW Group development engineers are currently working on ensuring that the capacity of the energy storage systems is maintained for as long as possible. Here, various factors need to be taken into consideration which can impact on the system’s service life. A battery ages in two dimensions: in calendar terms – i.e. as it gets older its performance and maximum usable energy content declines – and in response to a range of other factors which affect the service life of a cell. For example, the depth of discharge or temperature at which the battery is used are important criteria which influence how long it can continue to operate. The validation tests conducted by the BMW Group ensure that the cells meet customer requirements in terms of both service life and what is known as “cycling potential” over the vehicle’s entire lifespan. And, as far as sustainability is concerned, batteries which are no longer usable for a vehicle can be reused elsewhere. For example, although the battery capacity may no longer suffice to power a vehicle, it will still have enough left in it to serve as a stationary energy storage system for numerous other applications.

Challenges for the future.
The future of e-mobility lies in the ongoing development of energy storage systems. For this reason, the BMW development engineers are working intensively on ways of making them more compact and lighter, and at lower cost. However, the primary focus is on carrying as much energy as possible on board to give the vehicle a long range. The energy density of the energy storage system in the electric vehicle has not yet reached levels comparable with that of a full tank of fossil fuel. A high-voltage battery with 22 kWh contains energy equivalent to approximately 2.5 litres of premium unleaded petrol – and the distances possible in electric cars are currently much less as a result. Having said that, an electric motor has an efficiency level of up to 96%, much higher than a combustion engine, which is capable of 40% at best. So an electric vehicle with only this small amount of energy on board will take you much further than a similarly powered vehicle fitted with a combustion engine. This extraordinarily high level of efficiency means that the distances possible in an electric vehicle are already sufficient for many people in day-to-day use. The first wave of results from the usage studies with the MINI E show that 90% of the test users have been able to maintain full mobility with the range currently possible.

Range-extending measures.
But the central question remains: how can range be further extended? One possibility would be to increase battery capacity. The problem is that a larger battery would make the vehicle heavier, and that would limit range once again. Simply enlarging the battery on an open-ended basis is therefore not an option because from a certain point on, the extra weight of the battery cancels out the increase in range. Consequently, the BMW Group engineers are looking at ways of exploiting the available battery capacity as effectively as possible. The key here is to minimise the vehicle weight through the application of lightweight design principles wherever possible and the intelligent use of materials (see also ch. 4). In addition to this, the batteries are discharged as far as possible. The usage range of the BMW Group’s battery cells is between 400 and 250 volts, which equates to around 85% of the available energy in the battery. Further discharge is not possible, as excessive discharge triggers chemical-physical processes which would damage the battery cells irreparably.

“We’re using every kilowatt-hour produced by the battery extremely carefully. Our aim is to ensure that the system works as efficiently as possible.” (Patrick M�ller)

As well as the drive system of the vehicle, functions such as the lights, climate control and infotainment systems also need energy to work. While these auxiliary consumers barely register in a vehicle powered by a combustion engine, they have a very noticeable impact on the range of an electric vehicle. For example, in city driving a vehicle requires only about 2.5 kW on average for propulsion, whereas the air conditioning can use up to 5 kW under full loads. For this reason, the engineers are exploring ways of using intelligent charge control techniques and efficient operating strategies to reduce energy consumption as far as possible. This means that the temperature inside the vehicle can be adjusted while it is charging and the full capacity of the battery is therefore available almost exclusively for propulsion during the journey. A pleasant side-effect of this intelligent charging strategy is the comfort value of being able to get into a pleasantly cooled or heated vehicle, as the summer or winter weather demands. Two ways of increasing the range of the vehicle during a journey might be the deactivation of functions which are not required and the option of being able to let the vehicle “glide” at the right times (using the vehicle’s own momentum to coast along without requiring the motor for propulsion). However, the development engineers see the longer-term future in the further development of energy storage systems to increase energy density.

Range extender – small engine, long range.
One particular approach to increasing a vehicle’s range comes in the form of a “range extender” strategy. Here, a combustion engine produces electricity via a generator in order to charge the battery during a journey or maintain it at a constant charge. This could add a considerable distance to the vehicle’s range. With a full-size electric motor already fitted, this combustion engine only needs to be relatively small. Studies show that, on average, output of 20 to 30 kW is ample for normal driving. A range extender of this size would therefore supply enough energy to fulfill the customer’s driving requirements without burning unnecessary quantities of fossil fuels. The compact electric drive system components and new vehicle architectures would make the range extender easy to integrate.

Although a thoroughly conceivable short-term answer to the issue of increasing vehicle range, the range extender remains a compromise solution for the BMW Group. Looking further ahead, the BMW Group development engineers are focusing firmly on the further development of battery technology. The low energy density of the energy storage system – and, therefore, its lower range – combined with its relatively high weight remain limiting factors in e-mobility. However, with energy storage technology for vehicles gaining in momentum we can look forward to further significant advances in development.

“Over the next few years we can expect further leaps forward in development. Soon, smaller and lighter batteries will be capable of taking vehicles greater distances. We are currently midway through a process of development which still has much potential left to exploit.” (Patrick M�ller)