Toyota and Tesla: Exploring viscosity’s role in engine and drivetrain efficiency
Viscosity, the measure of a fluid’s resistance to flow, is an important consideration in determining lubricant performance across various applications. Researchers have spent decades fine-tuning viscosity levels to reduce frictional losses, enhance overall efficiency and reduce emissions. A trend towards low-viscosity engine oils aims to strike a balance between adequate engine protection and maximising fuel efficiency.
The traditional belief is that thicker oil provides better engine protection, but this is only true up to a certain point. The Stribeck curve illustrates this concept by showing the relationship between lubrication regimes, viscosity, and friction. As viscosity increases, so does the lubrication film thickness, which can reduce friction and wear. However, beyond a certain viscosity, the friction may actually increase due to higher fluid resistance. The Stribeck curve helps in selecting the optimal oil viscosity for efficient engine performance and protection.
Japanese automakers pioneered the shift towards low-viscosity engine oils primarily due to their focus on fuel efficiency and reducing emissions. The development of ultra-low viscosity oils, such as SAE 0W-8 and SAE 0W-12, was driven by advances in engine technology and the need to meet stringent environmental regulations. These low-viscosity oils help to reduce frictional losses in the engine, thereby improving fuel economy and reducing CO2 emissions. Japanese engines are often designed with tighter tolerances and higher precision, which can benefit more from the use of low-viscosity oils compared to engines from other regions. This focus on fuel efficiency and emissions reduction aligns with the global shift towards more sustainable automotive practices.,
Viscosity’s role in engine efficiency
A collaboration between the Japan Automobile Manufacturers Association (JAMA), the Petroleum Association of Japan (PAJ), and the Society of Automotive Engineers of Japan (JSAE) was the driving force behind the introduction of JASO GLV-1, an ultra-low-viscosity gasoline passenger car engine oil specification which was published in 2019 to improve fuel efficiency without compromising an engine’s durability.
Speaking at F+L Week 2024 at the Intercontinental Saigon in Ho Chi Minh City, Vietnam, on March 7-8, Kazuo Yamamori of Toyota Motor Corporation unveiled for the first time in an industry gathering JASO GLV-2, the revision to JASO GLV-1, during his presentation on Standardisation of Engine Oil that Contributes to Carbon Neutrality. Yamamori is the current chair of the Engine Oil Sub-Committee of the Japanese Automotive Standards Organization (JASO). JASO is an organisation that sets automotive standards in Japan, including those for engine oils, fuels, and other automotive-related products.
Toyota is the world’s top-selling automaker in 2023, marking the fourth consecutive year it has held this title. The company sold a record 11.2 million vehicles, driven by its diverse lineup, including gasoline-electric hybrids, which accounted for about a third of its sales. Battery electric vehicles (BEV) made up less than 1% of its sales.
The ratio of next-generation vehicle sales in Japan has risen to 45%. Nevertheless, Yamamori emphasised that carbon neutrality cannot be attained solely through electric vehicles (EV). He highlighted the significance of combustion engine-powered vehicles, including hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV), which he believes will remain mainstream in the immediate term.
Yamamori underscored the importance of the new JASO standard in improving fuel efficiency and reducing emissions in both new and in-use vehicles. During his presentation, Yamamori showed that engine oils have the potential to save 14.2 million tons of CO2 emissions annually based on 2021 data of 1.1 billion in-use vehicles.
JASO GLV-2 is categorised as a “flat viscosity oil,” offering superior performance with minimal temperature dependency. It effectively reduces viscosity across the entire temperature range. Engine oils with viscosity beyond SAE 0W-8 are “technically limited,” he said, citing examples such as oil pressure dropping at high temperatures. Thus, the GLV-2 specification, unlike GLV-1, specifies the use of SAE 0W-16 and SAE 0W-20.
Yamamori outlined key revisions in the latest JASO specification, including a new Shear Stability test method and several changes to bench and engine test limits. A key advancement was the need to improve the NOACK Volatility Test, also known as ASTM D5800. The test measures the evaporation loss of lubricants and engine oils under high-temperature conditions. It is an important parameter in evaluating the quality of an engine oil, as excessive volatility can lead to oil consumption, a decrease in oil volume, and potential engine deposits. In the test, a sample of oil is heated to 250 degrees Celsius (°C) for 60 minutes, and the percentage of oil that evaporates is measured. Lower NOACK values indicate better oil performance, as they suggest less oil evaporation and, therefore, better oil stability and lubrication retention under high temperatures.
Toyota observed a poor correlation (R2=0.78) with the actual engine oil consumption at 250°C, prompting a change to 150°C to match the oil film temperature of the engine bore wall surface. A new test duration of 12 hours was also implemented to improve test accuracy.
JASO GLV-2 will come into effect in October 2024, he said.
Toyota continues to recognise the significance of low-viscosity fluids, a sentiment echoed by many in the automotive industry. However, Tesla, the world’s largest electric vehicle maker in terms of market share, seems to hold a slightly different perspective on fluid viscosity.
Viscosity’s role in drivetrain efficiency
Speaking at F+L Week 2024, Dr. Wenyang Zhang, senior staff mechanical design engineer, Drive System Engineering at Tesla Inc., emphasised that lower viscosity does not necessarily lead to higher efficiency and may compromise system durability.
Tesla, a pure EV automaker, delivered 1.8 million battery electric vehicles in 2023. Although the growth rate may not be as rapid as in previous years, Tesla’s sales continue to increase. Tesla first delivered its Cybertrucks to customers in November 2023 which utilises the Tesla-formulated KAF I fluid for cooling and gearbox lubrication. This fluid was initially introduced with the 4th Generation of Tesla Drive Units in 2022.
In a presentation on the System Design Approach of Tesla KAF I and Comparison with Major EV Fluids in Ho Chi Minh City, Zhang offered insights into the fundamental principles guiding Telsa’s fluid design. He suggested there is little benefit in low-viscosity oils, emphasising the importance of understanding a fluid’s contribution to each aspect of the system design.
The complexity of gear train lubrication highlights the importance of adopting a meticulous fluid system design approach, says Zhang. The development of KAF I lubricants involved a delicate balance of key fluid parameters. In a comparative analysis, KAF I, which has a viscosity of ~6 cSt at 100•C, outperformed other major OEM EV fluids in terms of system efficiency, despite its higher viscosity, he said.
KAF I was formulated to optimise system durability, efficiency, thermal conductivity and provide electrically induced bearing damage (EIBD) protection. Fluids used in these studies from other OEMs have ~40 to 68% of KAF I’s kinematic viscosity at 100°C. The Tesla representative underlined 0.5% to 2% efficiency gains in most areas compared to other OEM EV fluids. A much higher KAF I lambda ratio across the drive cycle also indicated superior gear train protection compared to other OEM fluids. The lambda ratio is important for optimising EV drivetrain performance and ensuring system efficiency.
Zhang stressed that the drive unit, comprising the motor, gearbox and inverter, contributes only a small percentage of total vehicle losses (~16%). Within the drive unit, a fluid’s primary contribution is in the gearbox. Among gearbox losses, fluid formulation dominates torque-dependent loss. Torque-dependent loss refers to the power losses in a vehicle’s drivetrain that are proportional to the amount of torque being transmitted. These losses are primarily due to friction and other resistive forces in the transmission, differential, and other components of the drivetrain. This is an important factor to consider when designing and optimising a vehicle’s drivetrain for efficiency and performance.
Viscosity contributes to parasitic loss, which only accounts for around 20% of gearbox losses (hardware and lubrication system design dependent). Parasitic losses refer to the energy losses that occur in a vehicle’s systems and components, which are not directly involved in producing useful work for propulsion. These losses can occur due to various factors such as:
- Friction: Mechanical friction in the engine, transmission, drivetrain, and other moving parts can lead to energy loss.
- Aerodynamic drag: The resistance of the vehicle body against the air as it moves through it can cause energy loss.
- Rolling resistance: The friction between the tires and the road surface can lead to energy loss.
- Accessory loads: Energy used by accessories such as air conditioning, power steering, and electrical systems like lights and entertainment systems can also contribute to parasitic losses.
- Pumping losses: Energy used to pump air and fluids (such as oil and coolant) through the engine and other vehicle systems.
While lower viscosity proves advantageous in certain conditions, it does not necessarily guarantee better overall efficiency. Merely reducing viscosity won’t significantly enhance overall system efficiency, Zhang said. He emphasised the importance of designing fluids alongside hardware to optimise performance and comprehensively understanding how fluids contribute to various aspects of system design, such as minimising losses, protecting gears, extending fluid life, and other factors.
Zhang also discussed Tesla’s vision and future direction regarding the cooling and lubrication of its vehicle fleet. He highlighted development efforts on the next generation of Tesla lubricants, known as KAF II. In a full film lubrication regime this experimental formulation is touching superlubricity level (below 0.01 in friction coefficient), he said. Superlubricity refers to an extreme state of low friction between two surfaces in relative motion, where virtually no measurable resistance occurs.
Footnotes:
1 The Development of JASO GLV-1 Next Generation Low Viscosity Automotive Gasoline Engine Oils Specification (https://www.sae.org/publications/technical-papers/content/2020-01-1426/)
2 Automakers and oil industry divided on ultra-low viscosity oils (https://www.fuelsandlubes.com/automakers-and-oil-industry-divided-on-ultra-low-viscosity-oils/)
3 JASO GLV-2: New engine oil standard to be published in March (https://www.fuelsandlubes.com/jaso-glv-2-new-engine-oil-standard-to-be-published-this-month/)
