How important are electric and hybrid technologies for the workshop?
Here you will find useful basic knowledge and practical tips on the subject of thermal management in electric and hybrid vehicles.
Important safety note
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.
|Micro hybrid||Mild hybrid||Full hybrid|
|Output of theelectric motor / alternator||2 – 3 KW |
(regenerative braking via alternator)
|10 – 15 KW||> 15 KW|
|Voltage range||12 V||42 – 150 V||> 100 V|
|Achievable fuel savings compared to vehicles with conventional drives||< 10%||< 20%||> 20%|
|Functions that help reduce fuel consumption||– Start-stop function |
|– Start-stop function |
– Boost function
|– Start-stop function |
– Boost function
– Electrical driving
As the overview shows, each of the technologies has various functions that contribute to reducing fuel consumption. These four functions are briefly described below.
If the vehicle comes to a stop, e.g. at traffic lights or in a traffic jam, the internal combustion engine switches off. The engine starts automatically if the clutch is pressed and first gear is engaged to drive off. This means it is ready to start driving again immediately.
Recuperation is a technology that recovers a portion of the energy from braking. Normally, this energy would be lost as thermal energy when braking. In contrast, recuperation uses the vehicle's alternator as an engine brake in addition to the normal wheel brakes.
The energy created by the alternator as the vehicle slows is fed into the accumulator (battery). This process specifically increases the drag torque of the engine, slowing the vehicle.
As the vehicle accelerates, the available torque of the internal combustion engine and electric motor are combined. This means that a hybrid vehicle can accelerate more quickly than a similar vehicle with a conventional drive system.
The boost function is used to help during start-up and allows more power to be delivered when overtaking. This power is generated by an electrical auxiliary drive that only serves these two purposes. An example: in the VW Touareg Hybrid, this means a performance increase of 34 KW.
If less drive power is required, e.g. when driving in the city, only the electric motor is used as a power unit. The internal combustion engine is switched off. One of the benefits of this kind of drive: no fuel consumption and no emissions.
With these technologies in the vehicle, there are also changed conditions that you have to take into account in your daily work.
The requirements and performance levels that the electric drive of an electric/hybrid vehicle needs to satisfy cannot be achieved with voltage ranges of 12 or 24 volts. Much higher voltage ranges are required here.
Vehicles with high-voltage systems are vehicles that operate the drive and auxiliary units with voltages from 30 volts to 1000 volts AC (alternating voltage) or 60 volts to 1500 volts DC (direct-current voltage). This applies to most electric and hybrid vehicles
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".
The video shows components of an electric vehicle as an example, and provides information on how to handle high-voltage systems
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:
The circuit of the refrigerant-based system consists of the main components: Condenser, evaporator and battery unit (battery cells, cooling plate and electric auxiliary heater). It is supplied by the refrigerant circuit of the air-conditioning system and controlled separately via valves and temperature sensors. The functions of the individual components are described in the explanation for the illustration of the coolant and refrigerant-based system (b).
The more powerful the batteries are, the more the use of the comparatively complex coolant and refrigerant-based circuit makes sense.
The entire cooling system is subdivided into several circuits, each disposing of a separate cooler (low-temperature cooler), a coolant pump, thermostat and coolant stop valve. The refrigerant circuit of the air-conditioning system is also integrated via a special heat exchanger (chiller). A high-voltage coolant heater provides sufficient battery temperature control at low outside temperatures.
The coolant temperature for the electric motor and the power electronics is maintained at below 60 degrees Celsius inside a separate circuit (inner circuit on the figure) using a low-temperature cooler. For reaching full performance while ensuring the longest possible service life, it is necessary to always maintain the coolant temperature of the battery between approx. 15 and 30 degrees Celsius. When temperatures become too low, the coolant is heated via an auxiliary high-voltage heater. When temperatures become too high, it is cooled via a low-temperature cooler. Should this not suffice, a chiller integrated into both the coolant and refrigerant circuits, will further reduce the coolant temperature. Here, the refrigerant of the air-conditioning system flows through the chiller and further cools down the coolant, which also flows through the chiller. The entire control occurs via individual thermostats, sensors, pumps and valves.
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!
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.
The storage evaporator comprises two blocks: an evaporator and an accumulator block. Refrigerant flows through both blocks in the start-up phase or when the engine is running. In the meantime, a latent medium in the evaporator is cooled so far that it freezes. This makes it a cold accumulator.
In the stop phase, the engine is switched off and the compressor is not driven as a result. The warm air flowing past the evaporator cools down and a heat exchange takes place. This exchange continues until the latent medium has completely melted. Once the vehicle continues on, the process starts again so that the storage evaporator can start cooling the air again after just one minute.
On vehicles that do not have a storage evaporator, the engine has to be started again even after a short standstill period in very warm weather. This is the only way to maintain interior cooling.
Air conditioning inside the vehicle also includes heating the passenger compartment if required. In full-hybrid vehicles, the internal combustion engine is switched off when driving using the electric motor. The prevailing residual heat in the water circuit is sufficient to heat the interior for a short period of time only. As support, high-voltage air auxiliary heaters are then switched on to take over the heating function. The operation is similar to that of a hair dryer: the air that is drawn in by the interior fan is heated up as it flows past the heating elements and then passes into the interior.
Vehicles with full-hybrid technology use electric high-voltage compressors that do not depend on the internal combustion engine running. This innovative drive concept allows functions to be carried out which further increase the comfort of the air-conditioning system in the vehicle.
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.
The battery is essential for the operation of an electric and hybrid vehicle. It must provide the high amount of energy required for the drive quickly and reliably. Most of these are lithium-ion and nickel-metal hybrid high-voltage batteries. This further reduces the size and weight of the hybrid vehicle batteries.
It is imperative that the batteries that are used are operated within a defined temperature window. Service life decreases at operating temperatures of +40 °C or higher, while efficiency drops and output is lower at temperatures below -10 °C. Furthermore the temperature differential between the individual cells must not exceed a particular value.
Brief peak loads in connection with high current flows, such as from recuperation and boosting, lead to a significant increase in the temperature of the cells. Also, high outside temperatures in the summer months can mean that the temperature quickly reaches the critical 40 °C level.
When the temperature is exceeded, the result is faster aging and the associated premature failure of the battery. Vehicle manufacturers strive to ensure that the calculated battery lifetime is 1 car life (approx. 8-10 years). Therefore, the aging process can only be countered with a corresponding temperature management system.
Until now, three different temperature management options have been used.
Air is drawn in from the air-conditioned vehicle interior and is used to cool the battery. The cool air drawn in from the vehicle interior has a temperature of less than 40 °C. This air circulates around the accessible surfaces of the battery pack.
Disadvantages of this are:
To avoid this risk, the intake air is filtered. Alternatively, air cooling can also be effected by a separate small air-conditioning unit similar to the separate rear air-conditioning systems in luxury-class vehicles.
A special evaporator plate inside the battery cell is connected to the air-conditioning system in the vehicle. This is done using what is known as the splitting process on the high-pressure and low-pressure side via pipelines and an expansion valve. This means that the evaporator inside the vehicle and the evaporator plate in the battery, which works like a normal evaporator, are connected to the same circuit.
The different tasks for the two evaporators result in different requirements for refrigerant flow accordingly. While the interior cooling system aims to satisfy the comfort demands of the passengers, the high-voltage battery must be cooled to varying degrees of intensity depending on the driving situation and the ambient temperature.
These requirements are the defining factors for the complex control of the quantity of evaporated refrigerant. The special design of the evaporator plate and its resulting integration into the battery offer a large contact surface for the heat exchange. This means it is possible to guarantee that the critical maximum temperature of 40 °C is not exceeded.
When the outside temperatures are very low, increasing the battery temperature to the ideal temperature of the battery may require an increase of at least 15 °C. However, the evaporator plate cannot help in this situation. A cold battery is less powerful than one that has the right temperature. It is also difficult to charge the battery when temperatures are significantly below freezing. In a mild hybrid, this can be tolerated: in extreme cases, the hybrid function is only available in a limited capacity. It is, however, still possible to drive with the internal combustion engine. On the other hand, a battery heater needs to be fitted in purely electric vehicles so that the vehicle can be started and driven whatever the situation in the winter.
Evaporator plates that are directly integrated into the battery cannot be individually replaced. Therefore, the whole battery needs to be exchanged in the event of damage.
The correct temperature plays a key role for batteries with higher capacities. Therefore, an additional battery heater is required at very low temperatures to ensure the ideal temperature range is achieved. This is the only way to achieve satisfactory range when in the "electric driving" mode.
To enable this additional heating, the battery is integrated into a secondary circuit. This circuit ensures that the ideal operating temperature of 15 °C – 30 °C is maintained at all times.
Coolant, made of water and glycol (green circuit), flows through a cooling plate integrated into the battery block. At lower temperatures, the coolant can be quickly heated by a heater in order to reach the ideal temperature. The heater is switched off if the temperature in the battery rises when the hybrid functions are being used. The coolant can then be cooled via a battery cooler located in the vehicle front end or low-temperature cooler using the airstream from the vehicle driving forward.
If the cooling by the battery cooler is not sufficient at high outside temperatures, the coolant flows through a special heat exchanger. In it, refrigerant from the vehicle air-conditioning system is evaporated. In addition, heat can be transferred from the secondary circuit to the evaporating refrigerant in a very compact space and with a high power density. An additional re-cooling of the coolant is performed. Thanks to the use of the special heat exchanger, the battery can be operated within the most efficient temperature window.
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.
The situation is also special when performing routine inspections and repair work (such as on exhaust systems, tyres, shock absorbers, oil change, tyre change, etc.). These may only be performed by employees who have been trained by an "expert for work on HV intrinsically safe vehicles"
on the dangers of these high-voltage systems and instructed accordingly. Furthermore, it is imperative that tools are used that comply with the specifications of the vehicle manufacturer!
Motor vehicle companies are required to instruct all employees involved in the operation, maintenance and repair of electric and hybrid vehicles. Please note the respective country-specific conditions.
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.)
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