Electric Car: Efficient under the Hood

If it were just a matter of aesthetics, an e-motor could hardly compete against the elegance of a six-cylinder. Since it’s basically comprised of compact housing, magnets, copper wire, and a shaft – the potential for a grand spectacle is rather limited. E-motors have to impress with their inner values. And they have plenty of those.

Electric motors impress with their efficiency. Photo: Shutterstock – Herr Loeffler

“One of the electric motor’s great advantages is the efficiency with which it converts energy into mechanical drive power. Especially in city traffic, it beats a combustion engine hands down,” says Andreas Richter, engineer at the DEKRA Competence Center for Electromobility. From a technological point of view, there’s no reason why you shouldn’t use an electric car to pick up rolls at the bakery. Unlike a combustion engine, the electric car has no problem with cold starts and wear. As Andreas Richter explains, electric car engines have a very high degree of efficiency, which can exceed 90 percent. Most of this energy is used for driving. The balance for internal combustion engines is much worse – in the city, the efficiency can be less than ten percent, while it reaches efficiencies in the range of 25 to 40 percent at medium to high loads. The rest of the energy is lost as unused heat.

Whether electric car or washing machine – the basic structure of the motor is the same

Electric motors are a technology that has been tried and tested in a wide range of applications for many decades. Therefore, the basic design of the motor in an electric car is hardly any different from that of a washing machine. Alternating current (AC) motors are used in most cases, or more precisely: three-phase motors. This means that the alternating current flows to the motor housing via three separate conductors (phases). Inside are the drive’s two key players, which, through the interaction of electrical and magnetic forces, transform the energy called up from the battery into mechanical power for propulsion. The stator is basically the boss inside the housing and is responsible for power and efficiency. The rotor, in turn, is mounted for rotation inside the cylindrical stator and is firmly connected to a steel shaft for power transmission. The interaction between the two begins the moment the vehicle is started.

The interplay of magnetic forces causes the motor shaft to rotate

During electric operation, the alternating current flows to the coils in the stator via the connections on the motor housing. These then continually generate a new magnetic field at short periodic intervals. However, the magnetic fields at the various coils are always generated with a temporal offset from one another – this develops the so-called rotating field inside the stator. But how does the rotor’s rotary motion come about? That depends on the electric motor’s design.

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DEKRA experts check the regulations and safety measures to be complied with electric vehicles. Photo: DEKRA Automobil

In synchronous motors, the rotors generate their own magnetic field. Magnets with a permanent magnetic field are used – this is referred to as a permanent-magnet synchronous motor (PSM). However, the rotor can also be turned into an electromagnet with the aid of direct current – the system is then called a DC-excited synchronous motor (FSM). In both cases, the magnetic fields of the stator and rotor interact by attraction and repulsion of their poles. This produces a rotary motion in which the rotor turns synchronously with the rotary field of the stator.

A different principle is applied in asynchronous motors. Here, the rotor usually neither has magnets nor its own power supply. Instead, the rotating field of the stator induces a current in the conductor bars of the rotor, which then build up a magnetic field. In this system, the rotor always rotates a tad slower than the stator’s rotating field – hence the name ‘asynchronous’ motor. This design is considered to be particularly robust and scores with high stability at high speeds. Synchronous motors, on the other hand, offer advantages in terms of power density and efficiency.

Power electronics take over the power supply management

One challenge for engine developers is to match the vehicle and power unit to the desired driving profile. This may be easier for a compact car than for an SUV with much wider usage. In both cases, however, power electronics are a key player in the drive concept. They’re the entity responsible for managing the engine’s power supply, among other things. If the vehicle is to accelerate, for example, the power electronics determine how much additional energy is needed based on the accelerator pedal’s position. Since the battery only emits direct current, the electronics must provide the current in the right form, strength, and frequency. In the case of recuperation, on the other hand, it takes on the task of converting braking energy into direct current electrical energy and feeding it into the battery. In addition, the power electronics keep an eye on engine speed and power at all times. They know the battery cells’ state and communicate with the charging stations during charging.

Good to know: E-motors can also operate in generator mode. In this case, they convert mechanical energy into electrical energy during deceleration, thus charging the battery. This so-called recuperation increases the range of the electric car. This is particularly efficient where braking is required more frequently – for example on routes with downhill gradients or in city traffic with frequently changing speeds. Experienced drivers can achieve up to 20 percent more range by cleverly using recuperation, estimates DEKRA expert Andreas Richter.

The electric motor’s performance capability becomes evident on the road

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Power measurement: Theoretically, an electric motor can also play to its full potential when reversing. Photo: DEKRA Automobil

People who use an e-car as a second car or pure city vehicle could be content with less power. Even with a nominally weak engine, speedy driving in city traffic is quite possible. “This is because the maximum available torque of an electric motor is almost completely available when accelerating from a standstill,” says Andreas Richter. On country roads or the highway, however, a smaller motor’s forward momentum sooner or later runs out of steam. They then run through their maximum torque across the available rev range – but only until they’ve reached maximum power. At this point, the acceleration power decreases significantly. However, people need power if they value maximum high speeds or a dynamic intermediate sprint when overtaking, which is possible with higher electric motor power. If there were a fun formula for the electric car, it would be: “Power can only be replaced by even more power.”

Good to know: Efficient motor performance in all driving situations. Theoretically, an electric motor can also demonstrate its full performance capability when reversing or recuperating. However, as DEKRA expert Richter explains, manufacturers design the characteristic curves of their electric motors in such a way that safe driving behavior is always possible and the technology can be used with little wear. For this reason, electric motor power is usually greatly reduced immediately when when reversing and recuperating. Energy-efficient use of the electric motor is also easy to achieve on the highway. All that is needed is to reduce speed – this reduces air resistance, which increases quadratically with speed.

The transmission is an important player in the powertrain

To ensure that the mechanical power reaches the wheels in the best possible way, the transmission works as a third player alongside the motor and power electronics. Unlike with an internal combustion engine, there’s no need for a gearshift to keep torque and power in the optimum speed range at all times, since electric motors provide their power over a wide speed range. Nevertheless, electric cars also have a transmission on board. This is because the rotor shaft can turn at enormously high speeds. The drive shaft for transmitting the mechanical power to the wheels, however, has to turn much more slowly. To achieve this, carmakers usually rely on a single-stage transmission that reduces the speed. However, there is leeway in the design of the transmission. The Porsche Taycan, for example, has a two-speed transmission that enables maximum acceleration and high top speeds. High-performance sedans could also benefit from a two-speed transmission. Automotive supplier ZF believes that this could improve the electric drive efficiency by five percent. In practice, this would mean an increase in range. However, it would also be conceivable to forego the range and install a smaller battery in the vehicle instead. But what about an electric drive’s reverse gear? Engineers are happy to dispense with it. It’s enough to simply change the direction of the electric motor’s rotation in order to reverse, after all.

Good to know: The transmission is becoming increasingly important in the e-car. Volkswagen equips the ID3 with a single-stage transmission. Since the electric car reaches a top speed of 160 kilometers per hour at a maximum of 16,000 revolutions per minute, a solution was needed to achieve a transmission ratio to slow-speed for the revolutions of the drive shaft at the wheel. To save installation space, engineers use two smaller gears instead of one large gearwheel, which function as an intermediate gear ratio. Automotive suppliers are also coming up with their own developments. Bosch, for example, has just joined forces with Eindhoven University of Technology to develop an automatic transmission that continuously adjusts the speed and torque of the e-motor to the speed of the vehicle.

In the next installment of our series, we’re taking a closer look at the electric car’s battery.

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