Alternator Definition: the electrical generator in a vehicle
The power grid in a vehicle with an internal combustion engine, for example a car or truck, is usually fed by a generator, which is often driven by the engine via a belt. Since the operation of the headlights (i.e. the generation of light for lighting purposes) was originally the most important task of generating electricity on board, the term alternator has become commonplace for the generator. In the meantime, almost every car contains a large number of other electrical consumers, which together require considerably more electrical energy than the headlights.
Other names for the alternator are generator and power generator.
Since on the one hand the engine speed and on the other hand the electrical load is variable, a rechargeable battery is generally used as an energy storage device. The same battery serves as a starter battery, i.e. for starting the combustion engine, and for supplying consumers when the combustion engine is switched off. The alternator contains a charge regulator, which enables the battery to be charged properly (with adequate current and avoiding overcharging). It also limits the electrical voltage at high engine speeds to prevent damage to loads.
The regular operating voltage of the vehicle electrical system is quite low in a car: usually about 12 to 14 V, in trucks about twice that. Since the power output in a car can already be several kilowatts, quite high currents are required – sometimes over 200 A. Therefore, the alternator must be connected by quite strong cables, and the contact points must be clean.
If an alternator is defective, this is usually indicated in the car by the charging indicator light continuing to shine even when the engine is running.
Direct current and three-phase alternators
Old alternators, as used until the 1970s, were mostly electrically excited direct current generators. At low engine speeds, at least when idling, the electrical voltage generated was often too low to supply consumers or charge the battery. Nowadays, three-phase generators are almost exclusively used in conjunction with a rectifier, which can deliver considerable power even at low engine speeds. (The transmission ratio for driving a generator is often selected so that the full power can already be delivered at idle speed and two thirds of the full power – which unfortunately means reduced energy efficiency at high engine speeds).
Increasing power demand in vehicles
The alternators of today’s cars are designed for a maximum output of sometimes more than 5 kW, sometimes even for about twice that. In the future, the electrical power requirement is likely to rise even further, since not only are more and more comfort functions being demanded, but also various functions important for the engine itself are increasingly electrified. For example, radiator fans, cooling water pumps, power steering and in some cases even compressors of air conditioning systems are increasingly being operated electrically.
Despite the energy losses in the alternator and electric motors, this can be energetically advantageous, because it enables operation that is better suited to the respective requirements. For example, it is very annoying in car air-conditioning systems that a compressor driven directly by the engine withdraws a certain amount of power from the engine even when the air-conditioning system is not running at all.
Modern starter alternators can replace the alternator and the starter (starter motor); they allow higher power to be produced with greater efficiency than traditional alternators.
As the power output increases, a low on-board voltage of 12 V becomes a limiting factor, because very high electrical currents (e.g. 200 A for a power output of 2.4 kW) occur, and this requires thick and heavy cables. For this reason, significantly higher on-board voltages are likely to be used in the future, and naturally also alternators with a correspondingly higher nominal voltage.
Energy efficiency of alternators
The alternator is driven by the combustion engine and brakes it in the process; it therefore absorbs mechanical energy in order to be able to deliver electrical energy. The mechanical power absorbed can be significantly higher than the power output, since considerable energy losses also occur, especially at high engine speeds – for example, due to friction, for driving the fan (fan wheel) that cools the alternator, and due to ohmic losses in the coil windings and in the rectifier. Even without electrical load, the drive of the alternator requires a certain driving power, which then increases accordingly with electrical load (as a result of magnetic forces).
In old cars, the energy efficiency of the alternator was usually quite low; however, only relatively low power was converted anyway. The efficiency could easily be well below 50 %. More efficient devices would have been possible, but were probably not realized due to cost pressure. In modern vehicles, much higher electrical power is needed to supply a large number of consumers; several kilowatts may be required (see above).
Here, the efficiency of the generator becomes more important in order to keep fuel consumption low and also to prevent the cooling effort for the generator from becoming too high: The power lost there becomes heat, which must be safely dissipated to avoid destruction. Unfortunately, the problem with today’s alternators is that high efficiency for a wide range of speeds and loads is difficult to achieve.
Especially the operation at high speed but low load is quite inefficient. Therefore, even a peak efficiency of e.g. 80%, which some modern alternators can achieve, will probably result in an average practical efficiency of only between 50 and 70%. Modern starter generators in combination with suitable power electronics could be even better.
Recuperation with alternators
To a very limited extent, alternators can also be used for recuperation, i.e. for the recovery of braking energy in micro-hybrid vehicles. For this, it must be possible to increase the power generated during braking in a targeted manner, and the additional power is fed into the vehicle battery. (At other times, less power can then be generated, so that the engine is less loaded and fuel is saved). The recoverable power can be limited by the alternator or by the battery, usually to a few kilowatts.
In vehicles with a true hybrid drive, both units are much stronger, so that higher braking power can be used; in this case, we no longer speak of an alternator, but of a generator, or possibly a powerful electric machine that can be used as an engine or generator. For the transport of corresponding power, considerably higher electrical voltages are then required (several hundred volts), which is why the normal on-board power supply cannot be used for this purpose.
It is also possible to reduce the power output of an alternator for a short time if there is a high demand for drive power (e.g. when accelerating). In this way, acceleration in particular can be slightly improved without increasing the engine power, and with minimal technical effort.
