New technologies, new terms: See the technological highlights Audi e-tron has on board at a glance.
Battery electric vehicle (BEV)
Battery electric vehicles have a battery as the sole source of energy, in contrast to hybrid and range extender vehicles. The Audi Frankfurt-showcar Audi e-tron is a BEV.
Fuel cell electric vehicle (FCEV)
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.
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 specific energy 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 a specific energy 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.
Audi uses the term “e-tron quattro” to denote the next-generation quattro drive system. It combines the strengths of the proven quattro drive with the potential offered by electric mobility. The fully or partially electric quattro drive includes extra-efficient operating modes and opens up new possibilities for quattro handling dynamics.
The e-tron quattro technology demonstrator was designed as a parallel hybrid with plug-in technology. The rear axle is driven by a second electric motor whose power can be divided between both wheels. When braking in a corner, the torque vectoring can freely distribute the recuperation torque between the two rear wheels. The maximum possible recuperation can then be further increased and driving safety in critical situations noticeably improved.
Hybrid vehicle (HEV)
Micro hybrid Energy recovery and start-stop system reduce the CO2 emissions.
Mild hybrid In a mild hybrid, the electric motor supports the combustion engine in order to boost the power output.
In full hybrids such as the Audi Q5 hybrid quattro, the electric motor can propel the car independently for limited periods.
The plug-in hybrid is capable of travelling purely electrically over longer distances. The battery can be charged directly from a mains socket.
Hybrid Fuel Cell (HFC)
The power for the electric drive systems of the future can be drawn from the mains – but it can also be generated on board the vehicles themselves with the help of a fuel cell that uses hydrogen as a fuel. The most sophisticated version to date of a fuel-cell hybrid drive is showcased in the Audi Q5 HFC technology study. The three letters in the study's name stand for "Hybrid Fuel Cell".
Two high-pressure cylinders hold a total of 3.2 kilograms of hydrogen pressurised to 700 bar. The low-temperature hydrogen-based fuel-cell system has an output of 89 kW (120 bhp). The lithium-ion battery used for hybridisation, which is also employed in the Audi Q5 hybrid quattro, has an energy content of 1.3 kWh. Drive power is provided by two electric motors close to the wheels, which can generate a peak joint output of 90 kW (122 bhp) and up to 420 Nm of torque. The Audi Q5 HFC accelerates to 100 km/h from standstill in 13.4 seconds and attains an electronically limited top speed of 160 km/h.
Very efficient use is made of the hydrogen fuel – the fuel cell drive has an efficiency factor of over 50 percent. This results in an operating range of up to 250 kilometres. Once the hydrogen tanks can be appropriately designed and integrated into the vehicle, ranges of 500 km are a realistic possibility. Refuelling takes no longer than on vehicles with conventional drive systems.
In plug-in hybrids and strictly electric vehicles the battery is basically recharged externally, apart from in braking energy recuperation and load point shifting. The on-board charger converts the alternating current from the public grid to direct current for the battery. With a 400 V three-phase current and higher charging current the charging time decreases several times over compared with a 240 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.
Load point displacement
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 (capacity); in electric vehicles 20 per cent is considered the lower limit. The operating strategy that controls the vehicle monitors the permissible load limits.
Lightweight design for hybrid and electric vehicles
The special components of an electric or hybrid car unavoidably add to the vehicle's weight. Audi keeps the overall 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 Frame(ASF) design and the new fiber-reinforced plastics contain great potential in this regard.
Power output of electric motors
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 volume. 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 designation "lithium-ion battery" is a collective term for a technology with much potential. 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.
With parallel hybrid drive, both the combustion engine and the electric motor are connected to the drivetrain. Both drive principles can be used either independently or in tandem.
With serial hybrid drive, the combustion engine’s energy is passed on to the electric motor via an alternator. This motor then propels the vehicle. It is therefore not possible to run the combustion engine independently of the electric motor.
Plug-in hybrid vehicle (PHEV)
A plug-in hybrid electric vehicle (PHEV) is a hybrid vehicle in which the battery can be charged directly from the mains supply. Vehicles of this type can travel longer distances in purely electric mode. The e-tron Spyder concept car, which Audi exhibited at the Paris Motor Show, uses this basic principle.
Extended range electric vehicle (EREV)
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 combustion engine recharges the battery via a generator.
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.
TCNG – TFSI combined with CNG (compressed natural gas)
On the road towards the CO2-neutral mobility of tomorrow, Audi is focusing systematically on renewable energies – the Audi e-gas project is due to set a key milestone along this road. The project has two main components. Wind turbines will generate clean energy, part of which will be used by Audi to build and power its future e-tron cars. A new plant – the second element of the e-gas project – will use the remaining green power to produce hydrogen by means of electrolysis. This hydrogen can be used as a fuel for HFC vehicles, or used to manufacture methane in an additional step by combining it with CO2. Methane produced in this way is also known as synthetic natural gas, but Audi calls it Audi e-gas. It can be used to fuel combustion engines designed to run on natural gas; from 2013, Audi will be putting models with this type of engine into series production with the designation TCNG.
The technology standard-bearer, the Audi A3 TCNG, is able to run on the e-gas produced by Audi in its methanation plant. Its four-cylinder TFSI unit and the catalytic converter in the exhaust system are both designed for operation with natural gas. As with natural gas, e-gas gives off far less CO2 when it burns than premium unleaded fuel. What this means in the context of the e-gas project is that CO2 emissions are very low, not just overall (well-to-wheel) but also at the exhaust pipe (tank-to-wheel). Not a single gram of CO2 is emitted from the exhaust which would not have been previously consumed during the production of e-gas. In other words, the manufacture of the fuel and its combustion together form a closed CO2 cycle.
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.
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. In this respect Audi pursues the goal of achieving as low a CO2 footprint as possible from the generation of the power that is needed to build and operate electric cars.
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.