E-mobiles – Lubrication and Cooling while Electrified
The motto: “No more petrol!” Quiet, emission-free electric vehicles will dominate the streets in the future. Although e-mobiles no longer run on petrol or diesel, they still need lubricants and functional fluids. However, these lubricants are different from known classics. Tremendous expectations are placed on lubricant manufacturers and oil analysts with regard to the development and monitoring of new products. E-mobility is booming, and the current supply of hydrogen-powered fuel cells or alternative fuels will most likely not cover the demand in the next years. New registrations for battery-powered electric vehicles in Germany are still marginal. Approximately 220,000 electric cars were registered on 1 November 2019. However, numerous promotions and billions of investments from automotive manufacturers support the further development of e-mobility so that Germany no longer lags behind China and the US. The technological goal is ambitious. Vehicle manufacturers, suppliers, politicians and we as end consumers will be confronted with many questions and challenges in the next years. The more e-mobiles are on the road, the higher the demand for electricity. Control over the entire electrical grid must be optimised, and the network of charging stations must be expanded for comprehensive coverage. Charging times for batteries are to be further reduced and ranges maximised. The currently common lithium-ion battery exhibits high specific energy, but the worldwide sought-after metal is highly reactive. Its usage has many negative consequences for the environment and local population. Although used lithium-ion batteries can be recycled, the process remains extremely elaborate. The battery must be operated within a specific temperature range in order to achieve a long service life and optimal effectiveness of the vehicle. Other vehicle components also play a role.
Battery, motor and command centre
Electrical skateboards and hoverboards are among the smallest battery-operated means of transportation. Many engineers have borrowed from the structural design of these little speedsters in order to design their e-vehicles. The heaviest part, the battery, is placed centrally in a relatively low position. The “skateboard design” supports good roadholding, allows for a large wheel distance and ensures more space in the interior of the car. Electricity, sourced from a charging station, provides the power for movement and is stored onboard in one or multiple batteries that supply electricity to the drive. A separate low-voltage battery supplies smaller additional electric motors e.g. for electric windows, windshield wipers, air conditioning or the radio. A synchronous alternating current motor is installed as the electric drive. In simplified terms, the car is propelled in the following manner: The alternating current motor consists of two magnets – the stator and the rotor. The stator is fixed and provides a variable magnetic field by means of the alternating current. The rotating rotor consist of a permanent magnet or provides its magnetic field by means of direct current. Both magnets alternatingly attract and repel each other. The rotor turns, and this motion is transferred to the wheels via the gear. For an electric motor, the maximum torque is available for a comparably high speed range. However, speeds from 0 to over 200 km/h cannot be achieved merely with engine speed. A gear is required. Instead of a transmission with 4 to 10 gears, only a one to two-gear transmission is needed to cover the entire speed range. A battery with a capacity of 150 kWh as in the case of the Audi e-tron supplies energy to the electric motor. Charging a depleted battery with a common 3-kW household electrical socket would take several days. “Filing up” at a quick charging station with 50-kW power supply would still take 30 minutes for a driving distance of approx. 150 km. However, the first ultra-quick charging stations are being established along autobahn service stations at which an 80-% battery charge can be achieved during a coffee break. The engine speed is regulated with power electronics via a “power pedal”. Power electronics are the “command centre of the e-car”, consisting of an inverter, a direct current converter and an electronic control unit. The direct current from the battery is converted into alternating current required by the motor. Power electronics control the alternating current frequency and thereby the engine speed and also regulate the electric amplitude and thus the motor output power. They also play an important role when braking. Kinetic energy is not wasted but recovered when the driver takes the foot off the pedal. The vehicle brakes by “recuperating”. The electric motor functions like a generator during recuperating or regenerative braking. Gained electricity is stored in the battery. The power electronics accordingly adapt the energy supply.
Lubricants and coolants reinvented
Specific clearances are provided by many automotive manufacturers for lubricants and coolants that are used in vehicles with petrol or diesel motors. Their requirements are taken into account in the specifications of the ACEA (European Automobile Manufacturers Association) and API (American Petroleum Institute). Although e-vehicles do not require motor oils, gear oils, lubricant oils and greases as well as coolants, e.g. for the battery, remain indispensable. However, these products must fulfil very specific requirements. No e-vehicle manufacturer to date has defined a generally valid clearance list with specifications for lubricants and coolants. Furthermore, OEMs are still pursuing different approaches for the design of their e-mobiles. Therefore, lubricant manufacturers cannot yet engineer oils and coolants that are universally compatible with all e-vehicles.
|New lubricants – new challenges|
An e-mobile requires battery coolant, automatic gear oil, brake fluid for the disc brakes and low-viscosity greases for the roller bearings of the electric motor but also for other small components such as windshield wipers, the seat adjustment or the central locking system. The number of existing types is not too great – but wholly new demands are placed on lubricants and coolants. Supply for the drive and auxiliary aggregates ranges from 30 volts to 1,000 volts AC (alternating current) or 60 volts to 1,500 volts DC (direct current). Utilised fluids and greases sometimes come in direct contact with electrical and/or electronic vehicle components and must be reliably insulated for the entire usage time in order to prevent short-circuits and sparking. Moreover, they must be compatible with copper, a variety of plastics and sealing materials.
Battery requirements are demanding
The top priority is to keep the battery temperature in an optimal range so that e-cars can be operated at a high efficiency level. Desired chemical reactions within the battery and the battery power decrease at temperatures below 0 °C. However, the battery should also not become too hot. Batteries age quickly at temperatures above 30 °C, and irreparable damage can occur when exposed to temperatures over 40 °C. Only batteries that are operated in a moderate temperature range from 15 to 30 °C can perform adequately over a long period of time, thereby fulfilling up to approximately 70 % of the range stated in the brochure. A well-functioning thermal management system is indispensable in order to ensure that the battery can even go that far. Vehicle manufactures pursue different paths in order to achieve this goal:
- Thermal management of the battery management or battery pack can be realised by means of an air conditioner coolant circuit. A cooling plate is installed for the condenser and evaporator of the air conditioner. Its supply with air conditioner coolant is controlled separately via valves and temperature sensors. A power-consuming auxiliary heater ensures that the temperature does not sink too low in the winter.
- A complex cooling and coolant system with several coolant circuits and separate components is often used for highly powerful batteries over 100 kWh. The coolant circuit of the air conditioner is integrated in the cooling of the battery. An additional heater heats the coolant and battery in low temperatures. The system is most often filled with a water-glycol mixture whose formulation deviates from previously known coolants.
- Tesla is pursuing a different path: More than 7,000 individual cylinder-shaped lithium-ion cells are installed in the Model S with an 85-kWh battery pack. The internal chemical composition and the linking as well as the charging and discharging of individual cells are specifically designed for long-distance electric cars used by Californians. Individual cells are directly flooded with special coolant.
Nowadays, classic coolants that are based on glycol concentrates and mixed with lots of water are only seldom used. The industry has developed new cooling fluids for the special conditions of e-mobiles that, e.g., contain water mixed with paraffin, glycol and surfactants. The direct cooling of battery cells is optimised with a higher storage density and transport capacity.
Three key components
Battery-fed power electronics, the e-motor and automatic transmission – these are the three key components of an electric drive train. It makes sense to combine these within a common casing in order to reduce costs and weight. This also makes it possible to supply them more easily with a single fluid, an innovative e-drive fluid. However, this is easier said than done. The gear must be lubricated to ensure low wear and abrasion. The other components especially require heat dissipation. Some renowned lubricant manufacturers have already developed special fluids with a cooling function that also lubricate the gear. These e-drive fluids must be extremely thin as efficient thermal dissipation is needed but also because the input speed of the gear is most often higher than 10,000 revolutions per minute. Its viscosity roughly corresponds to that of diesel fuel. General information about the formulation of these products cannot be currently provided. Engineers are feverishly working on developing innovative e-drive fluids. However, mineral oils or mineral-oil mixable synthetic oils are hardly used as a basis for pilot projects, but rather water mixtures with more than 50 % consisting of other components, silicone oils or glycols. E-drive fluids must cope with a whole range of electrical, thermal, tribologist and chemical challenges. They must perform under high voltage while directly contacting copper components, elastomers of seals and insulating varnish within the e-motor. These fluids must not absorb water in order to retain a high dielectric strength and thereby prevent electric arcing between live parts. Special challenges arise with respect to compatibility between liquid and various materials – most of all copper. Copper’s high electric conductivity makes it the most important but also critical component for all live lines as well as for the coils in the e-motor. E-drive fluids are to be highly compatible with copper. Not only the batteries, but also the power electronics and the electric motor must work within a moderate temperature range. It is imperative that e-drive fluids provide efficient heat dissipation for temperatures up to 180 °C. Operation above the maximum temperature necessarily reduces the service life, efficiency level and range of the vehicles. However, extremely low-viscous e-drive fluid is not only responsible for the electric motors, but also contributes to securing the power transfer via the transmission. Many requirements must be fulfilled. Reliable lubrication, protection against wear and corrosion, high ageing stability, high material compatibility and a minimal tendency for foam formation must be ensured. Thus far, only lubricating oils whose viscosity is more than 10 times higher than the newly developed e-oils have exhibited these qualities.
Braking saves power
Modern electric vehicles are predominately moved over longer distances with the accelerator pedal. If the driver takes the foot off the pedal, the vehicle brakes automatically as kinetic energy is recovered. Since braking is somewhat time-delayed, e-mobiles are additionally equipped with conventional disc brakes. According to the manufacturer’s specifications, these require the classic brake fluid DOT 4 or DOT 5.1, which mainly consists of temperature-stable polyglycolic compounds. In some case, the silicone-containing brake fluid DOT 5 is to be used, which may not be mixed with other types.
New challenges for lubricating greases
Most bearings, gear motors, hinges, guides or other small components in a vehicle only work with a service-life grease. An e-mobile requires approximately 2 to 3 kilograms of various lubricating greases. The greases must fulfil very specific conditions at some grease points especially if they contact electronic components, electric currents and electromagnetic fields. Furthermore, compatibility with protective varnishes, soft plastics and copper is additionally required at some grease points. Lubricating grease for roller bearings within electric motors is subject to especially high requirements due to high temperatures and speeds. The grease must not only provide reliable lubrication, but also reduce noise and vibrations!
The automotive industry is currently reinventing itself. The environment of the supplier industry is changing dramatically. Lubricant manufactures are also affected. The demand for customary motor oils will significantly decrease in the next years. At the same time, manufacturers must adapt parts of their production to totally new lubricants and coolants for e-vehicles. However, standardised specifications for e-mobile lubricants have not yet been defined. Various approaches are being pursued that do not allow for standardised specifications. Long-term experience values in regard to the behaviour of new lubricants and coolants based on long-term usage are not yet available.
Lubricant analytics plays a decisive role under these conditions. Comprehensive expertise, practical experience and the highest degree of flexibility is demanded of OELCHECK. OELCHECK is actively participating in the new DIN committee “Electric properties of oils”. The committee deals with changes to operating fluids due to electrical properties that affect lubricants in the area of e-mobility.
We are also contributing our expertise for the research project “High-voltage suitable e-drive oil” of FVA (Forschungsvereinigung Antriebstechnik e.V.). Although standardised analysis and evaluation methods for lubricant changes of e-mobiles have not yet been developed, we are already providing significant assistance to OEMs with respect to batteries and gears, test station operators and lubricant manufacturers. Discussions are currently being held about expected speeds, temperature stresses, insulation capacities as well as wear and additional requirement scenarios. We will soon have significantly more practical usage data at our disposal with regard to the behaviour of e-drive fluids, greases and coolants. Based on the results of our laboratory examinations, practical experience and the expertise of our tribologists, we will then decide which of our test methods is to be adapted for the evaluation of e-mobile lubricants and operating materials and which threshold values apply for the estimation of remaining usage time.
OELCHECK is still focused on lubricants and operating fluids for electric cars. However, initial consultations with manufacturers of electrically operated omnibuses and lorries are already ongoing.