Since Hungarian Ányos Jedlik invented the first electric car in 1828, the automotive industry has increasingly pursued the dream of a world transported by electrically powered vehicles. Although the sale of electric cars is growing, mass adoption remains a pipe dream. And it will remain so until electric cars deliver the same mileage per battery charge as a tank of gasoline.
The pressure is on to find a solution, as the quest for ever-lower CO2 emissions increases and the desire to retain vehicle access in some of the world’s most-populous cities becomes paramount. Electrically driven vehicles, therefore, need to become the norm rather than a novelty.
According to a report by the International Energy Agency (IEA), 3.1 million electric vehicles roamed the roads globally in 2017. However, this is expected to grow exponentially with estimates ranging from 125 million to 220 million electric cars worldwide by 2030.
One key to this growth is the availability of components that enable electric vehicles to operate more like conventional vehicles, providing drivers the independence and reliability they expect.
Unfortunately, electric vehicles tend to have shorter travel ranges than gasoline and diesel vehicles. Battery charging takes a lot longer than filling up with traditional fuel. And shortcomings lead to range anxiety — the fear that a vehicle has insufficient range to reach its destination, stranding the vehicle and its occupants. The solution to this is to build the charging infrastructure, improve charging efficiency, and increase battery capacity to extend the distance that cars can travel on a single charge.
In electric vehicles, the e-axle is a critical component that enables the vehicle to perform like a gasoline-powered vehicle. The e-axle is an electro-mechanical propulsion system contained in an axle structure that houses an electric motor, power electronics, and some form of gearing/differential. All of this fits within the traditional engine space. The motor and gearbox are directly coupled. And while the gearbox requires efficient lubrication, it is essential that the motor remain dry. To facilitate this, a highly reliable seal is required between these two components.
The difficulty in sealing this system is that electric motors run most efficiently at high speeds. Gasoline engines normally run at 2,000 to 4,000 revolutions per minute (rpm). The electrically-driven transmission runs up to eight times faster, typically at 16,000 rpm. In the future, this is likely to increase significantly.
The rotational speed limit for traditional seals in today’s e-axle is around 100 ft per second. However, to maximize efficiency, the theoretical optimal rotational surface speed of the e-axle would be greater than 200 ft per second — a speed that is currently impossible to achieve.
This limits electric cars to traveling short distances. But since most electric cars are generally small and used for short journeys in urban areas, it’s acceptable for the electric drive unit to operate at a relatively low speed.
However, if electric cars are going to challenge internal combustion vehicles, they will need to be capable of traveling 250 to 300 hundred miles on a single charge — the equivalent of gasoline-powered vehicles — rather than the 175-mile average achieved today. The challenge facing e-Mobility seal manufacturers is increasing operating rotational speed to support electric vehicle manufacturers’ mission to extend traveling distance from one charge to the next.
Trelleborg Sealing Solutions has been involved in electric vehicle development since its inception and has recognized that a specialized sealing solution was needed for e-Mobility applications. The company created a cross-functional agile product development team — R&D, engineering, product management, manufacturing, and sales — to rapidly find a solution to support the evolving electric vehicle sector.Defining clear, narrow goals, including an aggressive manufacturing cost target to meet market expectations, led to the team’s success.
The team consulted with customers and conducted a detailed market analysis, establishing market requirements and identifying 27 development opportunities, which were whittled down to seven for additional screening. Team members selected the two most promising opportunities for in-depth testing, which engineers conducted at the company’s Bridgewater, U.K.; Stuttgart, Germany; and Fort Wayne, Ind., R&D facilities concurrently to deliver rapid results.
In the end, the team developed two seals, both of which met or exceeded the team’s target speed of at least 130 ft per second. The seals performed comparably relative to critical torque and power consumption and exhibited zero leakage, despite the highly demanding sealing conditions. One seal reduced friction 75 percent compared to a standard seal of its type. And it proved capable of operating at 200 ft per second.
We commercialized both seals, offering solutions for a range of requirements, including run out, dry running, fluid compatibility, high temperatures, and high pressure. HiSpin PDR RT is the best option for dry running, wider fluid compatibility, higher temperature, and high-pressure applications. HiSpin HS40 is better suited to applications that require run out and ease of assembly.
To ensure tests were meaningful and representative of market requirements, the team developed key parameters and test procedures taking into consideration a range of customer requirements, reviews of multiple sets of application data, and discussions with major automatic transmission fluid (ATF) and bearing manufacturers. The team tested seals on a 38-mm shaft at rotational speeds of up to 21,000 rpm (130 ft per second) in temperatures ranging from -40°F to +302°F (-40°C to +150°C) in ATF in oil mist conditions for a duration of a 500-hour accelerated load cycle test that represented real driving conditions, including reversing, city driving, stopping and starting in traffic jams, and high-speed highway driving. In addition to the 500-hour accelerated load cycle test, each seal was also subjected to more than 3,000 hours of endurance testing.
Both HiSpin PDR RT and HiSpin HS40 passed the accelerated load cycle test with no leakage and no wear on the sealing lip or shaft. In fact, the wear on the running surface was barely noticeable and finite element analysis (FEA) results all proved positive.
HiSpin PDR RT is made of Turcon QD1 polytetrafluorethylene (PTFE) perfluoroelastomer-based material, which is compatible with virtually all media. Therefore, no additional material tests were required for this seal. Manufactured from proprietary XLT FKM elastomers, HiSpin HS40 underwent long-term immersion in ATF commonly used in electric drive systems for 168 hours at +284°F and, 500 and 1,000 hours at +257°F. Compared to terpolymer FKM, ethylene acrylic rubber (AEM), and acrylic rubber (ACM) materials, the XLT FKM demonstrated significantly less volume change and excellent retention of chemical and mechanical properties.
Today, the seals are undergoing extended testing by several electric systems and vehicle manufacturers. Provided the seals continue to perform as expected in e-Mobility e-axles, they could help noticeably extend the distance traveled by electric vehicles between charges. And that will likely lead to a lot more electric vehicles on the road.
The team’s work however, does not stop there. The agile product development team continues to focus on improving the efficiency of electric vehicles, making them a viable option for widespread use.
Harlan Hart is technical manager, e-Mobility for Trelleborg Sealing Solutions.
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