According to measurements, the average efficiency of a combustion engine in real operation is around 20 percent. 80 percent is lost as waste heat, of which only a small part is useful as cabin heating in winter. In such a system, increases in the efficiency of an electric car are very evident. Electric drives convert more than 80 percent of the energy in the storage system into propulsion. Because there is hardly any waste heat, electric cars need heating energy from the battery in winter. There are also around 10 percent charging losses. These significant differences in technology are the cause of the differences in the consumption patterns experienced. The electric motor has existed longer than the piston engine. Despite its long history, there have recently been new design ideas specifically for use in vehicles.
Electric motors: Power consumption has many influencing factors
The electric motors themselves convert 90 to over 95 percent of electrical energy into kinetic energy, so generally only little is lost in the motor. When it comes to the efficiency of the car as a whole, there are many small adjustment wheels that are complex and depend on each other. So when the efficiencies of motors are discussed, keep in mind that they are just cogs and that the final power consumption depends more on the battery, converter, tires and above all on the aerodynamics than on the motor. An efficient engine design can easily be controlled inefficiently, and there are good reasons to build less efficient designs anyway.
Mr. Berghammer shows the combination housing of the BMW drive as it comes out of the injector sand casting at the Landshut plant. It will later contain the motor, converter and gearbox.
Packaging Benefits
Compared to reciprocating engines, electric motors are very compact and very powerful. Their comparatively small volume with manageable waste heat dissipation makes it possible to pack them close to the axles and integrate them with other components there. That is then called “integrated e-axis” and contains at least a motor, converter and transmission – a popular solution with car manufacturers as well as with suppliers such as Bosch or Schaeffler. Sometimes an axle differential is added (e.g. at ZF). Due to their enormous packaging advantages, e-motors have brought, in addition to the well-known efficiency, another advantage that not everyone appreciates but which benefits everyone: the return of rear-wheel drive to the mainstream.
Schematic: motors on the axles of the BMW iX. A cardan tunnel with transfer case on a reciprocating engine would need significantly more space.
Driving the rear axle of a motor has enormous advantages in terms of steering, driving dynamics and therefore driving safety. The front-wheel drive was always just a concession to utility, because the design “pack donkey in front – cargo area in the back” offers a lot of freedom for loading. Anyone who knows the loading height of some station wagons with rear-wheel drive compared to pack donkey competitors knows that for sure. In the electric car, however, the battery needs space in the substructure anyway, which reduces the advantages of front-wheel drive. Instead of distributing the torque of an engine to four wheels, the electric motor also allows each axle to have its own engine. A few particularly powerful cars like the Audi e-tron GT or Tesla’s Model S Plaid even use three motors, two of them on the rear axle to distribute the torque – also known as torque vectoring.
Common features of all electric cars
Electric motors work with coils that (when powered) create a magnetic field. This magnetic field in the “stator” (because it is relative to the vehicle) moves a “rotor” that is influenced by it (because it rotates relative to the vehicle). The rotor is usually on the inside, but can also be on the outside in flat, wide designs such as wheel hub motors. There are different ways of building electric motors, but only three-phase motors are used in e-cars today.
Taycan motor stator. In the mirror: the cooling channels of the water jacket
“Three-phase” means: Alternating current on three phases, each with voltage amplitudes offset by 120 degrees. You may be familiar with three-phase current from home technology: three phases with 230 V against the neutral conductor (or with 400 V against each other), each phase-shifted by 120 degrees to one another, this is how houses are connected to the low-voltage network in Germany. For an even torque output during motor operation, the phases on coils are divided into multiples of three and the rotor’s magnetization properties are adjusted accordingly. In the engine of the Mini Cooper SE, for example, the drawing-in machine rolls 18 coils to which the three phases are distributed.
A high power density is achieved by winding the coils as tightly as possible without crossing wires. This “groove fill” should be high because it determines engine performance. Therefore use e.g. Porsche, BMW and Hyundai, for example, use rectangular coil wires, which increases the degree of filling from 45 to 50 percent with conventional winding to almost 70 percent. With a high degree of slot filling, the motor can also dissipate heat better.
In the engine of the Porsche Taycan, rectangular coil wires ensure denser slot fillings.
Electric Car Motors: Types of Excitation
The most common motor in electric cars is the “permanently excited synchronous motor” (PSM). “Permanently excited” simply means that the rotor rotates a permanent magnetic field. So it consists of permanent magnets. PSMs are characterized by high power density and efficiency, which is why Formula E teams, for example, use this design. Usually, to build the rotor, many small, strong magnets are put into a field-carrying stack of soft iron sheets, which is pressed onto a steel drive shaft.
Disadvantages: A permanent magnet cannot be switched off, so when it rotates it always induces a voltage in the coils of the stator, which can cause problems in the event of a fault (e.g. when towing with dead control electronics). In addition, the strong permanent magnets are made of neodymium-iron-boron. China dominates the neodymium world market with 95 percent and also prefers to build the magnets itself.
BMW’s separately excited synchronous motor, as used in the iX, iX3 and i4. Pay attention to the power supply of the rotor!
Many manufacturers are aware of this dependency and therefore offer “separately excited” synchronous motors. The permanent magnet is replaced by an electromagnet, i.e. by coils with a soft iron core. The rotor coils are powered either by sliding contacts or inductively. Good designs lose only one to two percent in efficiency compared to variants with permanent magnets. The advantage of the higher raw material independence is that the motor control can dose the field strength of the rotor via current supply, which makes this motor design very easy to control (and causes fewer problems with towing). Disadvantages: The additional construction effort for the external excitation makes the motor more complex and sliders are wearing parts.
synchronicity
The “synchronous” of the PSM, in turn, relates to the magnetic field in the stator in relation to the speed of the rotor. These two yaw rates run synchronously with one another (at a constant load). The driving resistances that the motor overcomes result in an angle that the rotor lags behind the leading magnetic field. This “load angle” or “rotor angle” must not be too large, otherwise the synchronous guidance breaks down and the motor “stalls” (thus no longer delivers any torque). For a very simple motor with only one pair of magnetic poles this happens with a mechanical load angle greater than 90 degrees, for more common designs in cars the (mechanical) angle depends on the number of pole pairs. In generator operation, the situation is reversed: the rotor runs ahead of the voltage pattern induced in the stator by a load angle that correlates with the resistance of the power generation.
