Hydrogen powertrains offer significant advantages for heavy duty commercial vehicles during the tricky transition from fossil fuel to full battery electric power. Afton Chemical discusses why switching from diesel to hydrogen fuel is a step change in every sense and a dedicated hydrogen engine oil is best placed to unlock the full potential of new original equipment manufacturer hardware.
Emissions legislation is forcing commercial vehicle OEMs to leave carbon-based fuel behind, but the road that leads them to full electrification is long. Reaching their destination will require lighter, faster-charging and more durable battery technologies that have yet to be developed. The transition from combustion engine to battery electric power is likely to create a “messy middle” where multiple hardware solutions and fuel choices co-exist alongside rapidly changing infrastructure.
For commercial vehicles with higher weights and daily ranges, such as long-haul trucks, construction equipment and mining vehicles, a hydrogen powertrain offers a promising option for navigating the interim period more cleanly.
Numbers of hydrogen-powered internal combustion vehicles — referred to as H2 ICE vehicles — are small now, but some expect sales to grow fast in percentage terms in coming years. Research firm Markets and Markets forecast in late 2024 that global sales of heavy-duty vehicles with hydrogen-powered internal combustion engines (ICEs) will grow at a cumulative annual rate of 49% from 2031 to 2035, reaching 36,000 units by the end of that period.
Different Fuel, New Challenges
Retaining an ICE makes this territory seem familiar but switching fuels places different demands on the engine and lubricant. Hydrogen and diesel have markedly dissimilar properties and combustion behaviours — and using hydrogen creates three main challenges.
First, hydrogen gas is notable for its very low minimum ignition energy. This makes it highly explosive, creating a far greater risk of pre-ignition and engine damage.
Second, only water is created when 100% hydrogen is burned. High levels of water in exhaust and blow-by will mean more water in the engine oil, increasing corrosion and making it more challenging to form an effective lubricant film.
Third, with its higher adiabatic flame temperature, faster flame speed and shorter quenching distance, hydrogen can respectively increase nitrogen oxide formation, require optimized spark timing and place higher thermal stress on combustion chamber surfaces.
So, what must the lubricant achieve to facilitate use of H2 ICEs?
Protecting Against Preignition
Pre-ignition is a common phenomenon for any fuel with low minimum ignition energy. Low-speed pre-ignition (LSPI) has in recent years been a much-discussed challenge for turbocharged gasoline engines. OEMs can minimise this through hardware design, but the lubricant plays an important role. In 2018, the lubricant industry implemented the half-step passenger car engine oil category upgrade, API SN Plus in order to quickly introduce LSPI testing, rather than waiting for adoption of API SP and ILSAC GF-6. That decision was made due to concerns that LSPI was causing significant damage to some turbocharged gasoline engines.
With its relevant test expertise and insights, Afton began its research with oils already rated as good or bad for gasoline LSPI. Preliminary testing was carried out in a single-cylinder hydrogen engine and generated surprising results: the stronger candidate performed poorly while the weaker candidate proved more effective at suppressing hydrogen pre-ignition.
Afton applied different formulation levers to candidate oils before carrying out preignition testing on H2 ICEs using FEV’s six-cylinder hydrogen engine setup alongside Sequence IX LSPI testing. Once again, LSPI performance did not correlate with previous gasoline test results, indicating that the underlying pre-ignition mechanism may differ. Hardware design plays a part, but oils that perform well in gasoline engines may not be the right choice — or even a good starting point — for H2 ICE lubricants.
What has become clear from collaboration with various OEMs, is that it is essential to understand the unique preignition requirements of each engine set-up. In some hardware the amount of preignition is negligible, but in other cases it can be catastrophic.
Water, Water Everywhere
When burning hydrogen fuel, corrosion becomes a major issue because the air in the crankcase and combustion chamber is much more humid. While the engine is running, oil flows over and protects metal components, but when the engine is stopped, surfaces are more vulnerable to rusting.
Afton carried out humidity cabinet testing, where steel coupons are dipped in oil and placed in a cabinet that cycles through various temperatures and humidity levels. Test results have informed the selection of corrosion inhibitors that remain on surfaces and can protect them over long periods when the engine is not running.
When water mixes with oil, it separates out and if picked up by the oil pump, the liquid circulating through the engine may have insufficient lubricity to protect parts from wear. Commercial vehicles running at highway speeds for long periods, like long-haul trucks, get less water build-up in oil, but construction vehicles or buses with stop-start drive cycles or lots of idling are likely to experience higher levels.
Dispersing this water throughout the oil in a stable emulsion that lasts for an extended time is vital. Afton found that emulsion stability tests over seven days predict performance in hydrogen ICEs better than a typical 24-hour test, and those emulsifiers that boost low-temperature performance are vital for winter cold starts.
Handling the Heat
Hydrogen’s greater flame speed and temperature will also impact the lubricant. Oxidation increases exponentially with heat, doubling with every 10-degree rise in temperature. Engine oil thermal stability and resistance to oxidation must not be forgotten and is at least as important for hydrogen-powered engines as for diesel engines.
Conversely, some aspects of performance may become less important. For example, soot and sludge deposits are massively reduced with carbon-free fuel, so maintaining cleanliness, which requires strong dispersant additives in a diesel engine oil, could be reduced for hydrogen engine lubricants.
Carbon dioxide and particulate formation in hydrogen-powered engines are minimal, coming only from small amounts of burned lubricant, and this enables OEMs to reconsider exhaust aftertreatment strategies. This could remove additive chemistry restrictions currently driven by the sensitivity of diesel aftertreatment systems to phosphorus and sulfated ash.
Finally, the pressure to keep reducing lubricant viscosity to increase fuel economy to meet CO2 emissions targets may ease. Hydrogen is expensive, so fuel economy will remain a total cost of ownership driver for CV operators, but slightly higher viscosities may make it easier to achieve priorities such as emulsion stability with longer-lasting wear protection.
These changing priorities help to open up the formulation space for lubricant additives used in hydrogen-powered vehicles.
Although the potential of hydrogen as a commercial bridging technology is still being determined by legislation, gaining insight into the opportunities afforded by dedicated engine oil will pay dividends for OEMs at every stage of development.
The lubricant drivers for H2 ICEs are not the same as for conventional ICEs, and hardware design will determine how much heavy lifting the engine oil must do to deliver hydrogen-specific LSPI, corrosion, emulsification and antioxidant performance.
Tara Loosmore is a research and development chemist with Afton Chemical’s Engine Oils Global R&D team in Bracknell, United Kingdom.