THERMAL MANAGEMENT IN ELECTRIC AND HYBRID VEHICLES

More than 2 million electric cars and plug-in hybrids were sold worldwide for the first time in 2018. With 2.1 million vehicles sold, their market share has risen to 2.4 percent of all new registrations – and the trend continues to rise. (Centre of Automotive Management)
In Norway, for example, the market share is already at 50%! 

According to the International Energy Agency (IEA), the growth of electric and hybrid mobility is driven primarily by government programs such as sales bonuses, local driving bans for cars with internal combustion engines, or targets for clean air. The authority considers e-vehicles to be one of several current drive technologies that can be used to achieve the long-term sustainability goals of reducing emissions. According to a study by management consultants PricewaterhouseCoopers, every third new car registered in Europe in 2030 could be an electric car.

Therefore, it is no longer a question whether vehicles with electric, hybrid or hydrogen technologies will really prevail. They will soon become part of everyday life on our streets. 

These vehicles will also have to be serviced and repaired, and the subject of thermal management will become increasingly complex. The temperature control of the battery and power electronics plays just as important a role as the heating and cooling of the vehicle interior. Air-conditioning components are also required for these types of drives – and their importance is increasing, since the air-conditioning system often has a direct or indirect influence on the cooling of the batteries and electronics. Air-conditioning maintenance will therefore play an even more important role in the future.

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.

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. 

Different management systems are used for the accumulators, which take over the charge and discharge control, temperature monitoring, range estimation and diagnostics. The durability depends essentially on the operating conditions and compliance with the operating limits. Battery management systems including temperature management prevent harmful and possibly safety-critical overcharging or exhaustive discharge of the accumulators and critical temperature conditions. The monitoring of each individual battery cell allows it to react before a failure or damage to other cells occurs. Status information can also be stored for maintenance purposes and, in the event of an error, corresponding messages can be issued to the driver.

Basically, the battery capacity of most electric cars today is sufficient for the majority of all short and medium-length distances. A study published in 2016 by the Massachusetts Institute of Technology came to the conclusion that the range of current standard electric cars is sufficient for 87% of all journeys. However, the ranges fluctuate strongly. The speed of the electric vehicle, the outside temperature and especially the use of heating and air conditioning lead to a significant reduction in the radius of action. However, the ever shorter charging times and the constant expansion of the charging infrastructure make it possible to further increase the action radius of electric cars. 

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! 

• Starting and moving the vehicle: In order to drive a vehicle with a high-voltage system – even if only from or to the workshop – the respective person must be instructed
• Service and maintenance: Service and maintenance work (changing wheels, inspection work) on high-voltage vehicles may only be carried out by persons who have previously been informed of the dangers of these high-voltage systems and instructed accordingly by a "Fachkundiger für Arbeiten an HV eigensicheren Fahrzeugen" (expert for work on HV intrinsically safe vehicles)
• Replacement of high-voltage components: Persons replacing high-voltage components such as an air-conditioning compressor must have the appropriate qualifications ("expert for work on HV intrinsically safe vehicles")
• Replacing the battery: The repair or replacement of live high-voltage components (batteries) requires special qualification.
• Roadside assistance / towing / recovery: Anyone providing breakdown assistance on vehicles with high-voltage systems or towing or recovering them must have received instruction in the structure and functioning of the vehicles and their high-voltage systems. Furthermore, the respective instructions of the vehicle manufacturer must be taken into account in advance. If high-voltage components (battery) are damaged, the fire brigade should be consulted

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.

It is possible to pre-cool the heated interior to the desired temperature before starting the journey. This can be activated via remote control.

This process of cooling while stationary is possible only if there is enough battery charge available. The compressor is controlled with the lowest possible output taking into account the necessary air-conditioning requirements.

In the high-voltage compressors used today, the power is regulated by adjusting the speed in steps of 50 rpm. It is therefore not necessary to have an internal power control.

In contrast to the swash plate principle, which is primarily used in the belt-driven compressor field, the high-voltage compressors use the scroll principle to compress the refrigerant. The benefits are that the weight is reduced by around 20% and there is a reduction in the displacement of the same amount while the output remains identical.

A direct-current voltage of over 200 volts is used to generate the right amount of torque to drive the electrical compressor – a very high voltage in this vehicle sector. The inverter fitted into the electric motor unit converts this direct-current voltage into the three-phase alternating voltage required by the brushless electric motor. The return flow of refrigerant to the intake side facilitates the necessary heat dissipation from the inverter and the motor windings.

Continuous ongoing education is required to maintain and repair the complex thermal management systems found in electric and hybrid vehicles. In Germany, for example, employees working on such high-voltage systems require additional 2-day training as "experts for work on high-voltage (HV) intrinsically safe vehicles".

This course teaches the employee to recognize the risks when working on systems of this kind and how to switch off all the current to the system for the duration of the work. People who have not attended specific training courses are prohibited from working on high-voltage systems and their components. The repair or replacement of live high-voltage components (batteries) requires special qualification.

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.)

• In the event of an accident, in most cases the HV system is switched off when the airbag is deployed. This applies to almost all passenger cars, but not necessarily commercial vehicles.
• In order to be able to work without danger, all measures from the chapter "Basic rules for working on electric and hybrid vehicles" must be taken into account
• Some manufacturers recommend or prescribe that the negative terminal of the 12/24 V DC vehicle electrical system battery be disconnected (further information can also be found in the respective rescue guidelines).
• If HV batteries or HV capacitors (energy storage devices in commercial vehicles) have been damaged or torn out by an accident, this poses a particular hazard. The fire brigade should be called in to help in this case. When handling damaged HV batteries, appropriate personal protective equipment (face protection, protective gloves for working with voltage) is required. 
• Spilled battery fluids may be corrosive or irritant, depending on battery type. Contact should be avoided in any case. After an accident, it cannot be ruled out that the HV batteries may still catch fire later as a result of internal reactions. Damaged vehicles should therefore not be parked in enclosed spaces.