Fuel economy is a crucial aspect of modern vehicle maintenance and fleet management. With rising fleet fuel costs and ever-tighter environmental regulations, achieving better fuel efficiency has never been so important.
The lubricant in use is often an overlooked factor that plays a significant role; there is an intricate relationship between engine oils and fuel economy. Synthetics, low HTHS viscosity lubricants, and oil detergency can significantly improve fuel efficiency.
The Role of Bulk Viscosity
Viscosity is the thickness of the oil. For almost a generation, the diesel engine world has been dominated by standard semi-synthetic, multigrade 15W-40 engine oils. These oils are largely backward compatible, making lubricant selection a relatively straightforward exercise for fleet operators.
The recent emphasis on fuel economy has led to many OEMs exploring the use of lower viscosity engine oils, with 10W-40 and 5W-30 diesel engine oils hitting the market, being included in 2016’s API FA-4 engine oil spec, and even becoming the factory-fill lubricant for several (mostly European) OEMs.
This has all been done to reduce fluid friction. If viscosity is the resistance to flow, then less viscosity means lower resistance, and therefore, less energy is consumed merely to circulate the oil throughout the engine.
High-Temperature, High-Shear (HTHS) Viscosity
Even though we think of lubricants as incompressible Newtonian fluids (which are relied upon for hydraulic systems), lubricants can exhibit different flow behaviors under pressure, flow, and shear conditions.
High-temperature high-shear (HTHS) viscosity measures an oil’s resistance to flow under high temperature and shear conditions typical of engine operation and better simulates the behavior of oil in loaded cam surfaces or gears.
Oils with lower HTHS viscosity correlate with better fuel economy, providing sufficient lubrication with less drag on the engine. Many “fuel economy” oils exhibit lower HTHS viscosities than their standard cousins.
This property emerges from both the bulk oil viscosity and the behavior of polymers (such as viscosity index improvers) in the high shear zones.
HTHS viscosity is also one of the landmark differences between the standard, backward-compatible CK-4 engine oils (min 3.5 cP) versus the newer FA-4 specification (2.9 – 3.2 cP).
Balancing Economy and Wear
This raises an obvious concern – lower viscosity oils have historically had lower load-carrying capabilities, making the engines and associated components prone to higher wear rates.
For this reason, most 5W-30 diesel engine oils are full-synthetic formulations, with the higher film-carrying capability of Group III and Group IV PAO base oils trading for the reduced bulk viscosity compared with a 15W-40. This maintains wear protection over the long term.
Advances in antiwear additives have also improved wear protection performance in heavily loaded parts of the engine, and modern oil formulations often contain a combination of zinc, boron, and molybdenum additives working together.
HTHS viscosities have also become more consistent over the oil’s life, with modern viscosity index improvers exhibiting substantially improved shear stability relative to their earlier variants.
Converging Technologies
Using all these technologies together ensures that engines stay robust while delivering fuel economy improvements:
- Lower bulk viscosity means more protection at engine startup
- Lower bulk viscosity means less fuel is used to pump the oil around the engine
- Shear stable VI improvers enable lower HTHS, resulting in less resistance
- Improved additive technologies ensure wear protection
“Real-World” Examples
Several notable “real-world” case studies have been used to demonstrate the benefits of modern lower-viscosity oils. In the real world, weather variability, payload, traffic, and the driver’s right foot tend to overcome the marginal improvements observed from changing engine oils.
To truly compare lubricants, it is necessary to artificially control for these variables by completing tests on engine/chassis dynos and test tracks with repeatable driving or else to accumulate so much data as to reduce the variability to a statistically insignificant component.
The Shell Starship Initiative demonstrated how advanced lubricants can be used among a range of other technologies to enhance fuel efficiency. During a coast-to-coast run across the USA, Shell Starship 2.0 achieved a fuel economy of 10.8 mpg, significantly higher than the North American average of 6.6 mpg.
This was made possible by using low-viscosity oils, among other energy-efficient technologies.
When measuring the fuel economy benefits of a low-viscosity engine oil, it was found that switching from a Shell Rotella 15W-40 (synthetic blend) to a similar Rotella 10W-30 improved fuel economy by ~2%. A prototype 0W-20 fully synthetic oil showed an additional 1.7% improvement compared with the 15W-40.
Similarly, the development of Mobil Delvac 1 LE 5W-30 showed 3.4% mpg improvements in city driving and 2.8% for highway driving compared with a standard semi-synthetic 15W-40.
This test was slightly different because it was conducted on a test track, and the entire driveline was converted to run on synthetics, including a synthetic gear oil 75W-90 and synthetic transmission fluid.
The Economics
With fuel comprising up to 30% of some logistics company budgets, plus increased regulatory scrutiny around emissions, a switch to low-viscosity synthetic engine oils might likely make sense.
Even a modest 2% improvement in fuel economy would translate to 0.6% overall savings – which might seem modest until you realize that some trucking companies are spending tens of millions of dollars a year in fuel costs!
Along with the added benefits of more extended oil drains and improved engine cleanliness afforded by synthetics, full synthetic low-viscosity engine oils may well be the industry’s future.
As always, it is necessary to check with the engine OEM if this is appropriate, lest unintended harm occur. Pre-2016 engine models predate the FA-4 engine oil specifications, and some OEMs have slowly adopted these changes.
