Thermal management in electric and hybrid vehicles

The term "hybrid" as such means a mix or a combination. With respect to vehicle engineering, this term means that an internal combustion engine with standard drive technology has been combined with elements of an electric vehicle in one vehicle.

Hybrid technology has three stages of complexity: from micro-hybrid to mild-hybrid up to full-hybrid technology. Despite technical differences, one thing all the technologies have in common is that the battery used is charged by recovering braking energy.

are usually equipped with a standard internal combustion engine, a start/stop automatic system, and a braking energy recovery (recuperation) system.

in contrast also have an additional (small) electric motor and a more powerful battery. The electrical auxiliary drive is only used as assistance when starting and for greater power delivery when overtaking, a concept known as "boosting".

can not only "boost"; they can also drive purely on electricity. To this end, they are fully equipped with an electric drive train. However, this requires a much more powerful battery than a mild hybrid.

allow the batteries to be charged overnight, for example. The positive side effect of this vehicle type is that, at the same time, the passenger compartment can be brought to a desired temperature before the vehicle is driven. This means that the vehicle is immediately ready for use the following morning. The plug-in hybrid is a form of full hybrid.

Current representatives that typify full-hybrid vehicles include the Toyota Prius, the BMW ActiveHybrid X6 (E72), and the VW Touareg Hybrid. In contrast, the BMW ActiveHybrid 7 and the Mercedes S400 (F04) are examples of mild hybrids.

As the overview shows, each of the technologies has various functions that contribute to reducing fuel consumption. These four functions are briefly described below.

By definition, an electric vehicle is a motor vehicle driven by an electric motor. The electrical energy required for its movement is obtained from a drive battery (accumulator), i.e. not from a fuel cell or a range extender. Since the electric car itself does not emit any relevant pollutants during operation, it is classified as an emission-free vehicle.

In electric vehicles, the wheels are driven by electric motors. Electrical energy is stored in accumulators in the form of one or more drive or supply batteries.
The electronically controlled electric motors can deliver their maximum torque even at standstill. Unlike internal combustion engines, they usually do not require a manual transmission and can accelerate strongly even at low speeds. Electric motors are quieter than petrol or diesel engines, almost vibration-free and emit no harmful exhaust emissions. Their efficiency of more than 90% is very high.

The relatively high weight of the accumulators is offset by the weight saving due to the elimination of the various components (engine, transmission, tank) of the combustion engine. Electric vehicles are therefore usually heavier than corresponding vehicles with combustion engines. The capacity of the battery(s) has a great influence on the vehicle weight and the price.

In the past, electric vehicles had short ranges with one battery charge. Recently, however, the number of electric cars that can reach distances of several hundred kilometres is increasing, for example: Tesla Model S, VW e-Golf, Smart electric drive, Nissan Leaf, Renault ZOE, BMW i3.
In order to further increase the range of electric vehicles, additional devices (usually in the form of an internal combustion engine) are sometimes used to generate electricity. This is referred to as the "range extender".

To be able to operate an electric vehicle with a particularly high level of efficiency, it is necessary to maintain an optimal temperature range for the electric motor, the power electronics and the battery. This requires a sophisticated thermal management system:

Due to their high efficiency, electric drives emit little heat to the environment during operation and no heat at all when stationary. In order to heat the car in the event of low outside temperatures or to defrost the windows, booster heaters are necessary. These represent additional energy consumers and are very significant due to their high energy consumption. They consume part of the energy stored in the battery, which has a considerable effect on the range, especially in winter. Electric auxiliary heaters integrated in the ventilation system are a simple, effective but also very energy-intensive form. Energy-efficient heat pumps are therefore now also being used. In summer they can also be used as an air-conditioning system for cooling. Seat heaters and heated windows bring the heat directly to the areas to be heated and thus also reduce the heating requirement for the interior. Electric cars often spend their downtimes at charging stations. There, the desired vehicle temperature can be achieved before the start of the journey without loading the battery. On the go, considerably less energy is required for heating or cooling. Smartphone apps are also now being offered with which the heating can be controlled remotely.

Electric and hybrid vehicles necessitate the installation of high-voltage components. These are clearly identified with uniform warning signs. Also, all manufacturers mark high-voltage lines with a bright orange colour.

The following procedure applies when working on vehicles with high-voltage systems:

1. Completely switch off the electrical system

2. Secure against current being switched on again

3. Check there is no voltage present

Please observe the specifications of the vehicle manufacturers and our workshop tips!

In standard drive concepts with internal combustion engines, the air conditioning inside the vehicle directly depends on the engine operation due to the mechanically driven compressor. Compressors with belt drives are also used in vehicles that only have a start-stop function (referred to as micro hybrids by experts). The problem here is that when the vehicle is at a standstill and the engine is switched off, the temperature at the evaporator outlet of the air-conditioning system starts to increase after just 2 seconds. The associated slow rise in the temperature of the air blown in by the ventilation and the increase in humidity can be annoying for passengers.

In order to counter this problem, newly developed cold accumulators, so-called storage evaporators, can be used.

Drivers of vehicles with high-voltage (HV) systems are not exposed to any direct electrical hazards – not even in the event of a breakdown. A large number of measures taken by vehicle manufacturers secure the HV system.

Breakdown assistance for vehicles with HV systems is also harmless as long as no intervention in the HV system is necessary to eliminate faults.

However, there are dangers in the event of a breakdown or towing of vehicles damaged in an accident or which have to be towed from snow and water. Although the intrinsic safety of the vehicles to protect against hazards from electric shock or arcing is very high, there is no complete or 100% safety for every case of damage. In case of doubt, the respective information from the vehicle manufacturer must be taken into account or requested.

Breakdown assistance for electric and hybrid vehicles may be provided by anyone who has been specially qualified for this purpose. Anyone providing breakdown assistance therefore receives instruction in the design and operation of vehicles with high-voltage systems. The respective country-specific requirements and conditions for "non-electrical work" apply. (For Germany, DGUV Information 200-005 "Qualifizierung für Arbeiten an Fahrzeugen mit Hochvoltsystemen" (Qualification for work on vehicles with high-voltage systems) (previously BGI 8686) applies. Please note the respective country-specific conditions.)

Be sure to follow the manufacturer's instructions. Only a few vehicles can be jump started via the 12/24 V DC vehicle electrical system. After switching off, dangerous residual voltages may be present which are not discharged via continuous discharge resistors. Before opening, observe the instructions in the operating manual and/or technical information from the vehicle manufacturer.

• Undamaged vehicles can generally be loaded onto a recovery vehicle (platform vehicle).
• When towing with a rod or cable, the manufacturer's specifications must be observed.
• In order to recover vehicles safely, all measures from the chapter "Safe assistance with electric cars" must be taken into account.
• If the vehicle is towed/recovered with a winch, no HV components may be located in the area of the attachment points and be damaged. The same applies when lifting with a jack or loading crane.