|Audi’s electrohydraulic combination brake (EHCB). Click to enlarge.|
Audi recently held a TechDay in Ingolstadt focused on its efforts to electrify the driveline. (Earlier post.) In electric mobility, Audi distinguishes between two groups of vehicles: fully hybrid vehicles, driving a maximum of about 3 km (1.86 miles) under electric power; and high-energy battery vehicles for longer distances. These are the plug-in hybrids, the range extender vehicles and the strictly electrical vehicles.
At TechDay e-mobility, Audi spent time describing the issues and its general approaches for constituent elements of different electric drive systems including power electronics and electric machines (PEEM); battery pack; charging (including contactless inductive approaches); thermal management; energy recuperation; electromechanical brakes; torque distribution; safety; and the use of a cell phone in the human machine interface (HMI).
Power electronics and electric machines. Core components in the electrified driveline are the power electronics and the electric machines—complex sub-systems with very different properties.
Electric motors are “all-rounders”, said Jörg Kerner, Head of Development - Electrical Drive. They are outstanding in their high reliability, relatively low weight and high efficiency in the range of about 93 to 97% through a relatively broad rpm range. Unlike internal combustion engines, electric motors yield their maximum torque at extremely low rpms, practically from the starting position. For this reason, single-staged transmissions usually occur in purely electrical vehicles.
Audi is using two types of motors as drive systems: asynchronous motors (ASM) and permanently excited synchronous motors (PSM). Asynchronous motors (ASM) have no permanent magnets and are simple in design, sturdy, low in maintenance and long-lived. They are only conditionally suitable for transmission-integrated applications.
The higher power density of permanently excited synchronous motors (PSM) allows a more compact and lighter design, depending on the application. In addition, a high torque and efficiency can be attained in the lower rev range.
The performance of an electric motor depends on its design; power and torque can be adapted by varying the diameter and length.
In hybrid vehicles such as the Audi Q5 hybrid quattro (earlier post), Audi employs PSMs specially designed for high torques. They have a large diameter, and yet take up only a few centimeters in length and therefore can be easily integrated between the combustion engine and the transmission. With an optimal rev range between 500 and 5000 rpm, they harmonize with the torque characteristics of a combustion engine.
For vehicles driven electrically either frequently or permanently, power-optimized PSM as well as ASM motors are suitable in their high-rev versions, Audi said. The diameters of these motors are smaller than in the torque-optimized electric motors. On the other hand, they are longer in length and feature a significantly broader usable rev range.
All electric motors used by Audi are liquid-cooled, so as to maintain their reliable operating temperature of at most 180 °C (356 °F). While the thermal factor limits the power potential, short-term violations of the limit are permitted. The electric motor in the Audi A1 e-tron, for example, achieves a continuous power of 45 kW (61 hp), with a peak power of 75 kW (102 hp) briefly available.
A pulse-controlled inverter transforms the DC to AC voltage as required by the electric motor. In the Audi Q5 hybrid quattro, the pulse-controlled inverter has to process up to 40 kW of power at 264 V rated voltage. Its core is a module consisting of several interconnected IGBTs (insulated gate bipolar transistors).
The interior of the power electronics attains temperatures considerably higher than 100°C (212 °F). Fluid cooling in the metal housing dissipates the heat. The latest-generation pulse-controlled inverters used in the Audi Q5 hybrid quattro are lightweight and compact; their volume does not exceed about 6 liters (0.21 cubic ft). It also integrates the DC converter that supplies the 12 V electrical system from the high-voltage system.
Battery. Audi distinguishes between two basic types of lithium-ion battery. High-performance batteries are suited for hybrid vehicles like the Q5 hybrid quattro. The high-energy batteries, on the other hand, are designed for vehicles traversing longer distances under electric power, such as the R8 e-tron.
At present, lithium-ion batteries in the automotive sector have an energy density of about 0.14 kWh per kilogram. While this density exceeds that of lead batteries by a factor of 4, it is about 40% less than the densities of the small lithium-ion batteries used in consumer products such as cell phones, notebooks and similar devices, Audi noted.
High-performance batteries are generally discharged to about 50% state of charge (SOC); high-energy batteries have a lower limit of about 20%. If these limits are maintained, the batteries can undergo several thousand charging and discharging cycles before their power appreciably declines.
Battery pack temperature can be managed with either air or liquid. Many factors have to be taken into account, Audi noted, for example, the fan conveying the air must not be so loud as to disturb the acoustics in the interior. At the same time the air must flow as uniformly as possible and not swirl too soon; and the battery must be neither too large nor too heavy.
The German Automobile Industry Association is working together with automotive manufacturers on uniform cell standards, Audi said. Prismatic cells, which are of primary interest here, generate between 500 W (for hybrid vehicles) and more than 1500 W (for heavy-duty electric vehicles). Their capacity begins at about 130 cm3 (8 in³) and ends at just under 900 cm3 (55 in³). Compared with round cells, flat cells have a large surface area relative to volume, which means that they can dissipate heat very well. They also can be packed together to save space. Besides the cells and the cooling system other components round off the battery system, namely the high-voltage and service connections, the electronic battery management system, the electro-mechanical components, the housing and various sensors and actuators.
Audi expects the price per kWh for Li-ion batteries to be halved over the next ten years.
Charging. Audi distinguishes between standard charging, contactless charging and quick charging. Standard charging by cable is the most obvious option. The transmission capacity depends primarily on the output from the socket. With power drawn from the single-phase German household grid with its output of 230 volts, capacity extends up to 3.6 kW.
With a three-phase high-voltage (400 V) current, capacity reaches 11 or 22 kW, depending on whether the protection per phase is designed for 16 or 32 amps. Audi is consequently working on on-board chargers with an output between 3.6 kW and 22 kW. The on-board charging units convert alternating current into the direct current required by the traction battery.
At an ordinary household single-phase connection the vehicle’s battery can be fully charged in about six hours, according to this model calculation, assuming that 2.5 kW is used for direct charging. Connecting a vehicle with a 22 kW charger to a corresponding AC charging station or to a three-phase high-voltage connection would increase the charging time to about 45 minutes.
At the same time, Audi together with a development partner is pushing ahead with a contactless charging system that uses induction to transmit single-phase current during parking. A conductive loop in the floor produces a magnetic field which also intermingles with a secondary loop, or oscillating circuit, integrated in the vehicle. The induction produces in this secondary coil an alternating current that is uncoupled, rectified and fed to the battery. A particular advantage of this solution as far as the customer is concerned is automatic starting and termination of the charging process. By current estimates, energy transmission with an efficiency of more than 90% is possible.
To distances longer that supported by the onboard battery pack in a single day, Audi is looking to quick charging; it does not favor battery-changing stations because of restrictions in the design and package.
To implement rapid DC charging, Audi is supporting the development of a modular plug-in system consisting of the AC connector IEC 62196-2 type 1 or type 2 and an extension for DC charging. The DC charging stations have various output arrangements. As wall boxes (external chargers) they can supply 11 or 22 kW or, as is already the practice in the Asian market, provide a maximum output of over 50 kW. In theory stations with up to 150 kW are conceivable. Their weight and their size pose no practical problems, the company said; they can be retrofitted at filling stations and similar infrastructure locations.
Generally, however: the lower the charging power, the less the charging process detracts from the battery lifetime. Quick charging will come into its own only when short charging times have priority, Audi said. Audi assumes that different types of charging will become available for different applications. Charging outputs up to 3.6 kW should become available from a household socket or through induction systems. At higher output levels charging with direct current seems more likely than charging with alternating current.
Thermal management. FOr the Q5 hybrid, Audi is using a dual cooling concept. The passive circuit uses the temperate air from the vehicle interior. The air is suctioned by a radial fan placed in front of the battery, and flows through the battery via its internal ducts. In this way, the battery can be cooled in many situations to exactly the ideal operating temperature of 37°C (98.6 °F). Six integrated sensors monitor the heat build-up. The passive circuit delivers about 0.5 kW of cooling power.
If the temperature exceeds the 37 ° mark, the second cooling circuit becomes active, yielding up to 1.4 kW. This circuit is connected to the refrigerant circuit of the vehicle’s air conditioning system and uses its own refrigerant evaporator. The air circulating through the battery becomes heated, is cooled in the evaporator and flows again through the battery. Should the temperature continue increasing, the battery management system increases the refrigerant power. Only if the temperature exceeds the mark of 55 °C (131 °F), does the hybrid feature have to be deactivated. The dual cooling system, which despite its efficiency weighs only about 5 kg (11 lb), has many advantages over simple passive cooling, Audi says—the biggest of which is that the driver can subject the battery to a high load, exacting high charging and discharging currents in both cold and warm weather.
Battery cooling in the Q5 hybrid quattro represents the first step Audi is taking in this area. Engineers are working on new solutions for the future, such as flood cooling. It requires only a low drive output, takes up less space and manages the temperature level in the individual cells of the battery even more efficiently and uniformly. The A1 e-tron demonstrator already has such a solution on board.
The technical field of thermal management extends beyond the aspect of battery cooling to encompass the entire vehicle. The system in the Q5 hybrid quattro already includes the electric motor and power electronics. Further progress concerns the main air conditioning system: it operates without restriction even if the 2.0 TFSI in the Q5 is deactivated. Instead of the usual reciprocating piston compressor that is belt-driven by the combustion engine, a component with an electric drive is at work here. The demand-based control of this scroll compressor affords a highly efficient mode of operation.
Audi is developing approaches for purely electric vehicles such as the R8 e-tron. Theoretically, conventional electrical heating elements (PTCs, positive temperature coefficients) or thermo-electrical heat exchangers can be used for heating the interior. However, the efficiency of these elements is not satisfactory in practice, according to Audi. When a PTC heater with 6 kW heating capacity is operating at an outdoor temperature of -20°C (-4°F), the range of the vehicle will decrease by about half, and by almost 30% at -10 °C (14 °F) and a 4 kW output.
Audi’s solution is the heat pump. The design of the heat pump is based on the classical refrigerant circuit, supplemented by an additional condenser, among other things. In the condenser the refrigerant compressed to high pressure and temperature levels is cooled, condensed and super-cooled. Following expansion the coolant absorbs the heat from the surroundings in the other condenser, which acts as an evaporator.
For operating heat pumps efficiently, a complex and highly variable control strategy is required. Audi will be obtaining its initial experiences with the pump in the e-tron concept car, and the new technology should be ready for production in a few years.
In all their developmental work the engineers had recourse to a broad range of test rigs, including a climatic wind tunnel in which the technology demonstrator and prototypes can run through arbitrary driving cycles at low minus as well as high plus temperatures.
This modular simulation system has also been part of the portfolio for about two years now. This platform allows the behavior of all important components from the areas of climate control, flow dynamics, drive system, driving performance and the electrical system to be played in detail through the computer. All simulations are closely networked with one another, so that all developers have access to all the data at all times.
Energy recuperation. In recuperation the vehicle’s energy of motion is converted to electrical energy during deceleration and stored in the drive system battery, so as to be available for subsequent acceleration. Recuperation utilizes all components of the electrified driveline. In the deceleration phases the electric drive motor functions as a generator, converting the mechanical power from the wheels into a three-phase current; the power electronics then turns this current into a high-voltage DC current, which it then stores in the high-voltage battery. Software controls the process.
The central input variable for recuperation is the measure of deceleration desired by the driver. To register this quantity, the Q5 hybrid quattro as well as all other hybrid and electric vehicles from Audi have a position sensor at the brake pedal; another sensor analyzes the position of the accelerator. Part of the energy is recovered as soon as the driver releases the right pedal. During braking the recovered energy continues to increase accordingly.
The recuperation software controls the distribution of deceleration between the one or more electric motors and the mechanical wheel brakes. The electric motors always receive the greatest possible share. Nevertheless, the brakes also play an important role in hybrid and electric vehicles, for several reasons.
At a standstill the electric motors cannot generate any static braking torque without a supply of energy. In sharp deceleration, as in a maximum brake application, their braking torque does not suffice. In addition, the availability of electrically motorized braking depends on numerous factors, such as the driven speed and the current energy level of the traction battery—a fully charged battery would suffer damage in recuperation.
The crucial criterion for the driver in recuperation braking is predictable, constant deceleration behavior corresponding to the pressure of the foot on the pedal. In the Q5 hybrid quattro, Audi has installed a newly developed brake system that is precisely tuned to recuperation.
Audi is also working on new concepts for further improving the distribution of the deceleration torque between the electric motors and the wheel brakes. Depending on the vehicle, these systems can come with fully electrical braking or with electro-mechanical brake servos. In both cases, brake operation is uncoupled from the mechanical brake—enabling the free distribution of torque and ultra-precise fade-in and thus the smooth transition from electrical to mechanical braking.
In most cases of practical automotive operation these brake systems allow deceleration by the drive motors alone. The wheel brakes then come into play in only a few situations: during very sharp deceleration, at low speeds and at a standstill. The recuperation function cooperates closely with the ESP stabilization system. Should the vehicle threaten to become unstable, the electric motor operated braking is immediately reduced; the individually controlled wheel brakes handle deceleration and stabilization.
The driver is precisely informed about the amount of energy recovery at any given time; the power meter display in Audi hybrid vehicles includes a recuperation indicator. The information on the intensity of the mechanical brake intervention is also displayed. Consequently, the driver is always able to utilize the recuperation potential to its full extent and thereby reduce fuel consumption.
|Audi’s electrohydraulic combination brake system. Click to enlarge.|
Electromechanical brake. Following years of preliminary development at Audi, the EHCB (electrohydraulic combination brake) system uses hydraulic brakes operating at the front axle, and electro-mechanical units at the rear axle. A small electric motor controlled by a control unit integrated in the caliper is fed current from the 14 V electrical system. The motor acts via transmission gearing on a so-called ball screw that presses the lining against the disk. The process is completed within a hundredth of a second.
The basic design of the electro-mechanical brake is that of a floating caliper brake—the housing encompasses the brake disk like a fist; the brake caliper is mounted flexibly on the carrier and is moved in the axial direction during braking. Floating caliper brakes manage with one brake piston per side, allow highly dynamic control, and operate quietly and with little vibration.
In recuperation, the EHCB brake system uncouples the build-up of power at the wheel from the brake pedal, allowing fine automatic transitions between electric recuperation and mechanical braking. Regardless of how lightly or forcefully the driver steps on the accelerator, he or she always senses a continuous and finely differentiated response.
Electric actuation of the rear wheel brakes—brake by wire—offers further advantages: fast response, high power and ultra-precise control, even with intervention by the ESP stability system. Moreover, there’s no residual torque, that slight scraping of the linings against the disks, as can occur with hydraulic brakes.
The brake power distribution and the pressure are precisely tuned to the given situation, for example to a high payload or trailer operation. The system manages without hydraulic lines, and can integrate the functions of a hill-start assist and a parking brake; when the vehicle is parked, the brake disk is locked by an additional integrated mechanism.
Torque distribution. Electrified vehicles of the future will offer new possibilities in four-wheel drive; the R8 e-tron concept car demonstrates the potential of this technology. Four motors—two each on the front and rear axles—drive the wheels of the Audi R8 e-tron via short shafts. The torque is distributed variably, although as a rule in favor of the rear axle. As in a mid-engined sports car, normally about 70% of the power flows to the rear, while 30% reaches the road through the front axle.
Intelligent driving dynamics and traction software continually analyzes the current driving state. When the situation changes, it appropriately adapts the torque distribution within a few milliseconds. Even before slip occurs at a wheel, the distribution changes not only for the selected axle, but also for the selected wheel, maximizing both driving stability and traction.
In asymmetrically controlling the drive motors of an axle, the computer imposes on the R8 e-tron a yaw moment about the vertical axis. This torque vectoring function can occur simultaneously at the front and rear axles, thereby counteracting either understeering or oversteering at its onset. The electric high-performance sports car behaves with exemplary neutrality even at maximum lateral acceleration, taking the curves as if on rails. The gain in driving dynamics also increases driving safety.
The four electric motors not only drive the wheels individually, but brake them as well. This allows an active influence on the driving dynamics on the one hand and increases the electric range through recuperation of the deceleration energy on the other.
When the driver presses down on the pedal only lightly or moderately, the electric motors take over the deceleration work themselves. Stronger braking results in a continuous transition to mechanical braking. This solution ensures the optimal braking power distribution in any driving situation. For this functional sequence the electric motors are networked in a novel way with the wheel brakes.
Safety. With regard to electrified vehicles, priority is given to the networking of user safety, functional safety and crash safety. In Europe, legal regulations on the crash properties of hybrid and electric vehicles are currently under discussion, while in the USA and Japan relevant standards and requirements are already in place. Audi engineers participate in working groups.
High-voltage systems also inherently carry high voltages outside the battery, for example in the cables of the power electronics and the drive unit. In a collision, the high-voltage system is disconnected from the battery and the link-circuit capacitor in the power electronics is discharged in a controlled manner. This ensures that all components outside the battery are free of high voltages.
In the entire high-voltage system the conductive components are electrically isolated, i.e. there is no electrically conductive connection with the rest of the vehicle. The high-voltage lines are routed with crash optimization measures in order to prevent short circuits and electric arcs as far as possible. Their orange sheathing conveys a clear visible signal.
All connectors are specially coded and each fits only in the designated socket. Locking plates ensure that the plugs are not inadvertently pulled out.
Audi prioritizes battery protection with respect to crash safety. The crucial factor is a protected installation position. In the Audi Q5 hybrid quattro the lithium-ion battery is located in the area in front of the rear axle. In the R8 e-tron and A1 e-tron this area is shaped like a T, filling the rear section of the center tunnel and the space in front of the rear axle. In the event of a collision, deformation in these areas is routinely less than in other areas of the vehicle.
The A6 hybrid and A8 hybrid employ special, strong metal structures to protect the lithium-ion batteries and stiffen the luggage compartment floor. Generally all future requirements are incorporated at the outset in the design of new model series. In calculating their crash behavior, the extra weight due to the additional components plays an important role.
Data display and cell phones. Because the operating ranges of current electrically driven vehicles are still considerably lower than those of their combustion engine counterparts, Audi is working on new display concepts for ensuring mobility that is amenable to planning. The first step has yielded a solution for informing the driver in detail about the current range.
A further step focuses on networking the charging management system in the vehicle with the customer’s cell phone, so that charging and other services can be remotely programmed and monitored.
The new concept for displaying the range is based on an analysis of possible alternative routes. The navigation system computer also examines these possibilities according to curve and elevation characteristics as well as according to speed limits. From these data the computer determines the probable energy consumption. The current driving situation as well as stored data records from the past enter into the calculation of this self-teaching driver assistance system.
In the display on the HMI (human-machine interface) monitor, the operating range appears as a light-colored zone on a darkened map. The range moves in the direction of the car’s motion, and becomes bigger or smaller depending on whether the energy consumption drops or rises. In a future expansion stage environmental data will also be available to the vehicle’s energy management system. With detailed knowledge of the route, the onboard computer of an electric vehicle can derive a driving and recuperation strategy for optimally utilizing the battery’s energy.
Further, under the working title “Electric Mobility Planning”, Audi specialists have developed an app that currently runs on an iPhone and will soon run also on smart phones with other operating systems. The app connects the cell phone to a receiving module in the electric car; contact is made via a secure Audi server. The module in the car itself communicates with the charging management system and with other systems.
The owner can query the current status of the vehicle through the cell phone network. Information about a low windshield washing fluid, whether the vehicle is locked, the windows are still open or the charge level of the battery is displayed on the cell phone. The information appears in easily understandable form as brief texts, numbers and graphics. A road map depicts the current range. The location of a parked vehicle is also shown on request.
When the electric car stands at a charging station, the driver can track the most important data on his or her cell phone while sitting in a café, for example. The same is true when the vehicle is connected to the power supply in the garage at home. If the owner enters the desired range, he or she can also determine the minimum charging required for the battery. Once this level is reached, the owner is notified on the cell phone. Even when the vehicle is not connected to the charging infrastructure the customer can access the vehicle status and range at any time.
Electric mobility planning moreover includes a calendar feature that routinely manages charging at the home socket. A person driving approximately the same route every day to work can pre-program regular charging. Should problems arise, perhaps because the person forgot to plug in the charging cable, a message is promptly displayed.
Other parameters allow different charging profiles to be specified. By means of off-peak electricity the vehicle can be charged at optimal cost. Another possible feature heats up the interior of the vehicle during charging. This preconditioning reduces the load on the battery during driving, since less energy is required for climate control. This process also increases the range of the vehicle.
A fleet test with the Audi A1 e-tron in Munich has put the first subsidiary functions into practice. Vehicles to be put on the road there by mid-2011 will have the necessary hardware and software on board. This fleet test is a broad-based study. Besides gathering experience with the car a second technical area is of primary concern: data transfer between the vehicle and the driver.