New technologies, new terms: See the technological highlights Audi e-tron has on board at a glance.
Battery electric vehicles have a battery as the sole source of energy, in contrast to hybrid and range extender vehicles. The Audi R8 e-tron is a BEV.
In a fuel cell oxygen and hydrogen interact in a controlled chemical reaction, yielding water, heat and electrical energy. This energy is used to drive an electric motor.
Fuel cell electric vehicles (FCEVs), e.g. the Audi Q5 HFC concept car, attain higher efficiency compared with combustion engines and emit only water vapor. Implementation of the technology in production still faces a number of problems – the lack of an infrastructure for hydrogen, an insufficient lifetime and costs that are not yet marketable.
Torque is the product of the force acting about the centre of rotation of the rotor bearing of an electric motor, and the distance between the rotor coil and the rotor axle. Unlike combustion engines, electric motors produce their maximum torque at very low rpms; the torque decreases continuously only after the highest output is attained. This is why strictly electrically driven vehicles usually get by with single-speed transmissions.
Electric motors are impressive in their high reliability, low weight and high efficiency. They each consist of a moving part (the rotor) and a fixed component (the stator). The stator generates a rotating magnetic field that exerts a force on the rotor, thereby setting it in motion. Electric drive motors in vehicles are generally fluid-cooled.
Two types of three-phase motors are currently used in vehicles from Audi. Asynchronous motors (ASM) that dispense with permanent magnets are simple in design, sturdy, require little maintenance and have long lifetimes. Permanently excited synchronous motors (PSM), on the other hand, require a more complex sensor system. They are more compact and lightweight in design; they moreover offer advantages in terms of torque, usable rev range and efficiency.
In hybrid vehicles like the Q5 hybrid, Audi uses a high-torque PSM integrated in the transmission. It has a relatively large diameter for a high torque, and yet is short in length; its rated speed lies between approximately 2000 and 7000 rpm. For strictly electrical vehicles the high-rev versions of PSMs and ASMs are suitable – they attain rated speeds between 10,000 and 14,000 rpm and have smaller diameters, and yet are longer in length. One gear can be omitted, however.
The energy density is the amount of energy that a battery can store per unit of mass; it is measured in kilowatt-hours (kWh) per kilogram of weight. At present lithium-ion batteries have an energy density of about 130 kWh per kilogram.
In view of developmental progress, Audi experts expect an energy density of 2500 Wh per kg to be reached by 2020, which is also considered the upper limit for lithium-ion batteries. By comparison: one kilogram of gasoline (about 1.33 liters (0.35 US gallons)) contains about 12,000 Wh of energy.
"e-tron quattro" refers to the quattro drive of the next generation, implemented electrically either in whole or in part. In the R8 e-tron the drive is provided by the four electric motors, one per wheel. The motors can be individually accelerated and decelerated with high precision by an intelligent management system – innovative torque vectoring enhances the driving dynamics and stability to a level never before known. A similarly intelligent distribution of the drive torque can also be achieved if a combustion engine and electric motor are each driving different axles.
"HEV" stands for "hybrid electric vehicle" – a vehicle having a combustion engine combined with an electric motor. A recuperation system, like many standard Audi models have on board, warrants the designation "micro hybrid". If the electric drive serves merely the occasional support of the combustion engine, we speak of a mild hybrid. In full hybrids like the Audi Q5 hybrid quattro the electric motor can itself temporally provide the full drive.
In plug-in hybrids and strictly electrical vehicles the battery is basically recharged externally, apart from brake energy recuperation. The on-board charger converts the alternating current from the public grid to direct current for the battery. With a 400 V high-voltage current and higher charging current the charging time decreases several times over compared with a 230 V household current. As an alternative solution, Audi is working on contactless charging by induction and on quick charging with direct current and high output. This technology allows a further reduction of the charging time.
Hybrid vehicles like the Audi Q5 hybrid quattro are usually powered by the combustion engine outside built-up areas. The hybrid manager controlling the interplay of the drives makes sure that the TFSI temporarily has less of a load in the low rev range than is required for the drive – the load point is shifted to a higher range, and efficiency improves. The excess torque benefits the electric motor, which then serves as a generator and recharges the battery.
The lifetime of a traction battery is greater than ten years, assuming that the battery remains at moderate temperatures. The load profile and the intensity of the charging and discharging cycles critically affect the lifetime. For this reason batteries in hybrid vehicles like the Q5 hybrid quattro are generally discharged only to about 50 per cent of their energy content or state of charge (SOC); in electric vehicles 20 per cent is considered the lower limit. The operating strategy that controls the vehicle manages the reliable load limits.
The special components of an electric or hybrid car unavoidably add to the vehicle's weight. Audi keeps that additional weight within limits, however – the increase is hardly more than 100 kg (220.46 lb) in the Q5 hybrid quattro and A1 e-tron. In developing electric mobility, the Four Rings brand profits from its comprehensive know-how lead in lightweight design. In its competitive environment the Q5 is a benchmark for lightweight design in this vehicle class. The aluminum bodies with the Audi Space FrameAudi Space FrameThe Audi Space Frame is a high-strength aluminium frame structure into which all panels are integrated so that they also perform a load-bearing function.Audi Space Frame (ASF) design and the new fiber-reinforced plastics contain great potential in this regard.
Electric motors achieve their maximum output early on in the rev spectrum and keep that output constant over a wide range. The rated power is the power that is continually and uniformly outputted over a larger rev range. Maximum output is available for varying lengths of time, depending on different parameters such as acceleration and braking/recuperation, and on the system configuration.
The power density of a battery is the ratio of power output to mass. Current lithium-ion batteries achieve power densities of 800 to 2600 W per kilogram, depending on the types of materials used. The electrical power output is the product of the voltage and current strength.
The power electronics consists of the so-called pulse-controlled inverter, which serves as a controller between the battery and the electric motor. It transforms the battery's direct current into alternating current – into a so-called rotating field, as the motor requires. A DC converter couples the 12 V electrical system to the high-voltage grid; in some cases it is integrated in the pulse-controlled inverter. The heat lost by the power electronics is dissipated by water cooling.
The designation "lithium-ion battery" is a collective term for a technology with much potential. The developmental work for its use in cars is in full swing; up to 40 possible materials are available for each of the cell components – the anode, the cathode, the separator and the electrolyte.
Each variant affords an entire raft of advantages. Lithium-ion batteries are distinguished by an energy density roughly twice as high as that of nickel metal hybrid batteries and nearly four times higher than that of lead batteries. They supply a largely constant voltage and are thermally stable over wide ranges. They have little self-discharge and are not subject to any memory effects.
Audi distinguishes between two basic types of lithium-ion batteries. The high-performance batteries are suitable for use in hybrid vehicles like the Q5 hybrid quattro; the high-energy batteries are reserved for those vehicles covering longer distances under electric power. Both basic types share elaborate thermal and safety management systems.
Parallel hybrids are vehicles with a combustion engine and an electric motor. Both can drive the vehicle either separately or in combination. The Audi Q5 hybrid quattro follows this highly efficient concept. A decoupler interconnects the 2.0 TFSI, the electric motor and the eight-speed automatic transmission, while the so-called hybrid manager controls the interplay of the mechanical units.
A plug-in hybrid vehicle (PHEV) is a hybrid vehicle with a battery that can be recharged from the general electric grid. Vehicles of this type can cover longer distances under electric power. The e-tron Spyder concept car, which Audi presented at the Paris Automobile Salon, embodies this basic idea.
A range extender is a mechanical unit that extends the operating range of an electrically driven vehicle beyond the intrinsic battery range (hence: extended range electric vehicle (EREV)). Range extenders are most often efficient, smaller combustion engines. In the A1 e-tron an especially compact single-disk rotary engine recharges the battery via a generator.
The e-tron models from Audi are already capable of long ranges. The A1 e-tron covers 50 km (31 miles) under electric power, while its range extender ensures an additional operating range of 200 km (124 miles). At its market launch, the Audi e-tron, which operates under electric power alone, will even attain up to 250 km (155 miles) on one battery charging.
Recuperation is the recovery of braking energy. In the start-stop technology from Audi, recuperation proceeds via the generator of the 12 V electrical system, recovering electric power in the coasting and braking phases which is then buffered in the starter battery.
In hybrid and electric vehicles, recuperation occurs via the drive motor(s) that act as generators in such situations. In lighter brake applications they perform the entire deceleration, while in more sudden braking the hydraulic wheel brakes come into play. When these drive motors are decoupled from the brake pedal in a later stage of development, it will be possible to distribute the torque and control the transitions between hydraulic and electric braking even more finely.
The current flowing in each charging and discharging process generates heat, so that traction batteries must be cooled. The cooling system keeps the battery within the suitable temperature range of about 25 to 45°C (77 to 113°F). It moreover largely equalizes the temperatures of the individual cells. The temperature can be reduced either with air or with fluid; temperature sensors supply the necessary information.
The Audi Q5 hybrid quattro has a sophisticated air cooling system for the battery on board – cooling occurs passively or actively as required. This technology increases the range of electrical operation of the performance SUV.
Besides the batteries, the electric motor and power electronics must also be cooled; here a fluid is the medium. Strictly electric vehicles with no combustion engines generating waste heat for heating require entirely new concepts with regard to thermal management.
The heat pump comes from building heating systems – it can heat and cool by absorbing heat from the surroundings. In the vehicle the heat pump is based on the tried-and-tested refrigerant circuit of the air conditioning, to which a second condenser is added.
Audi is working intensively on the use of this technology for its electric vehicles. To air condition the interior, the heat pump utilizes the waste heat from the battery, the electric motors and the power electronics. Thanks to its highly efficient method of operation it requires very little energy – its operation shortens the range of the vehicle only insignificantly.
The catch phrase "well-to-wheel" ("from the primary energy source to the wheel") refers to the comprehensive analysis of energy from an environmental perspective, from generation to utilization in the vehicle. Since the power for electric cars in Germany is for the most part generated in coal-fired power plants, their CO2 emissions are included in the well-to-wheel balance sheet. The same applies to hydrogen as the power source for fuel cell vehicles.
Efficiency is a parameter for the effectiveness of the conversion of one form of energy into another. It is defined as the ratio (in per cent) of energy delivered by a system to the energy absorbed by it. Electric motors in passenger cars achieve an efficiency of up to 97 per cent over large performance ranges – up to three times as much as the most efficient combustion engines.
Cycle stability is defined in terms of the number of charging and discharging cycles that a battery can run through before its capacity falls below a certain percentage of the initial value. Today's lithium-ion traction batteries generally achieve several thousand cycles.