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Tribology Trends and Challenges in Electric Vehicles: Short Summary
Electric vehicles, while often considered a new phenomenon, have existed since the 1800s. At that point in time, electric vehicles outnumbered gasoline powered vehicles for many reasons. Electric vehicles did not have the same vibration, noise, and smell of gasoline-powered cars. Changing gears on gasoline-powered cars presented great difficulties, an issue solved with electric cars, which did not require any gear changes. With the invention of the electric starter, mass production, and greatly reduced gas prices, the electric vehicle was reduced to the scrap yard heap by 1935.
In the latter half of the nineteen hundreds, concerns began to arise regarding exhaust emissions and its effect on the environment, sparking a renewed interest in the electric car. As concerns skyrocket, so has the popularity of the electric vehicle, especially when adding in the factor of rising gasoline prices. The automotive industry continues to be pushed towards making changes. As more citizens become concerned about the state of the environment and the future of the planet, emission regulations are changing. Manufacturers are encouraged to build more electric vehicles (EVs) and fewer internal combustion engine (ICE) vehicles. Companies producing EVs like Tesla, or its rival from China, Nio, become the drivers of the stock markets. Consumers are encouraged to buy the more environmentally friendly EVs.
There are quite some differences between EVs and ICEs. Here is a (very) short summary of the two from the tribological perspective.
Drivetrain, transmission and tires
The main difference between EVs and ICE vehicles is the drivetrain. ICE vehicles have an engine and gas tank while EVs have a motor and batteries. Electric motors and ICE engines have very different torque and power curves. An EV motor creates torque right from the start and can provide power over a fairly wide range of speeds. In an ICE powertrain, a clutch is needed so that power can be transferred when the vehicle is not moving.
The number of gear ratios in ICE transmissions has increased over time in order to increase energy efficiency. However, this increased range of gear ratios, in turn, results in a limited range of input shaft speeds. As a result, lubrication for ICE vehicles is not as challenging as it is for EVs. Transmission fluids are more important in electric vehicles because they preserve greater efficiency at cooler temperatures. Consequently, transmission fluids help prevent EV motors from over heating, which decreases the amount of energy lost while simultaneously increasing torque.
And finally, different tires are being developed and used on EVs to help improve mileage. As EVs are heavier, the extra weight puts additional stress on the tires, so tires with a stronger sidewall are beneficial.
An internal combustion engine is a heat engine where the combustion of fuel happens as it is mixed with an oxidizer (usually air). The lubrication system distributes oil to all the moving parts of the vehicle to reduce friction between the different surfaces. Consequently, lubricants play an important role in the expected life span of an automotive engine. Faulty lubrication systems can cause an engine to overheat and seize.
Lubrication is important for friction reduction in EVs since minimizing friction helps maximize mileage. This is a change from ICE vehicles, as is the fact that while lubrication is important for load carrying in ICE vehicles, for EVs, it is important for transferring torque.
In working towards improving the driving range of electric cars, bearings become more important. Reducing parasitic loss by improving bearings can increase the electric cars’ driving range, which consumers typically consider to be one of their weakest aspects. The new and improved bearings must operate quietly at high speeds because electric cars are quieter when running than gasoline-powered vehicles. Additionally, bearing fatigue increases in importance.
Bearing lubricants need a special formulation so that they will not corrode the copper components of EVs. As copper components corrode, flecks of copper are transferred to areas requiring electrical insulation. Therefore, it is essential to reformulate different lubricants. Gearboxes in electric vehicles use a limited amount of grease, usually in the sealed ball bearing. This results in additional challenges, as the grease in the ball bearings is not accessible. Grease with a shorter life results in a decreased lifespan for the vehicle, a problem that needs further research and development.
Active thermal management is key for electric vehicles. All systems require thermal management. Fluids for EVs need to have a lower viscosity. The EV motor, while being more efficient, still generates heat. Better lubricants to make the motors more efficient would help prevent the motor and battery from being damaged from the heat being generated. The alternative would be to cool the motor and battery.
Future Needs and Required Changes
The main issue for consumers regarding EVs concerns their durability and efficiency issues with regards to the moving components of the car. Reducing friction between the parts of the vehicle will help increase the driving range. Electric vehicle lubricants must be compatible with all components of EVs, including advanced polymers, copper wires, electric currents, and magnetic fields. Improving EV performance may require different fluids or additives. New testing methods are needed to determine whether additional fluids would help increase the longevity of electric vehicles.
Engine oils in EVs run at a lower temperature as compared to an internal combustion engine. Therefore, lower viscosity engine oils improve the oil consumption levels and durability of the vehicles.
The key to improving lubricants for EVs is to build more accurate simulation models that provide information regarding how lubricants, mechanical design, and performance of subsystems interact and affect each other.
In the field of tribology, improvements are needed in gear and driveline oils, lubricant fluids including coolants, materials, and component design, and greases and bearings.
For a more detailed review, please take a look at the article “Lubrication and Tribology Trends (and Challenges) in EVs” (https://www.machinedesign.com/markets/automotive/article/21136262/lubrication-and-tribology-trends-and-challenges-in-evs) by Jeanna Van Rensselar.