Background
This study evaluates the practicality of using hydrogen as a fuel for heavy and medium-duty internal combustion engines (ICEs) in the effort to reduce carbon emissions. H2 has unique properties, such as faster flame speed and lower minimum ignition energy, which can make engines prone to knocking. However, it also has a wider flammability limit, making it a strong candidate for lean combustion, which can yield higher thermal efficiency and significantly lower NOx emissions.
To achieve power outputs comparable to spark ignition (SI) or compression ignition (CI) engines, H2 ICEs need to operate at relatively richer equivalence ratios (0.5 or higher), increasing the risk of stochastic pre-ignition (SPI). SPI is a significant issue in downsized and boosted gasoline direct injection (GDI) SI ICEs and can be triggered by hot spots, oil droplets, and hot residual gases. Additives in lubricants, including metallic elements like calcium and zinc dialkyldithiophosphate (ZDDP), contribute to increased SPI risk in gasoline engines. However, the specific mechanism of SPI in hydrogen-lean premixed SI engines remains unknown.
Approach
In this study, four oils with different compositions were evaluated using well-established SPI procedures and insights from previous gasoline SPI testing. The engine was operated at a fixed speed of 1200 rpm, representative of typical H2 ICE operations at high loads, to observe SPI events. Each high-load segment lasted 20 minutes, recording 12,000 engine combustion cycles. Data was then statistically analyzed to identify outlier pre-ignition cycles. High-load segments were interspersed with periods of low-load testing to avoid prolonged engine stress and to collect independent data points for statistical analysis.
Recent studies have alluded to the fact that oil volatility differences correlate well with SPI in engines. The impact of typical SPI contributors found in gasoline engine lubricants is expected to be minimal in H2 ICEs, primarily due to the elevated temperatures and pressures these engines operate under, creating conditions suitable for lubricant auto-ignition regardless of the additive packages used. This has already been observed in current test results.
Accomplishments
Three out of the four test oils have been evaluated. These oils display similar volatilities, as expected from fully formulated diesel oils, but have significantly different calcium contents. The recent tests have shown that the test protocol is sufficiently repeatable and no statistically significant differences in SPI activity among the oils have been observed, confirming the low impact of oil additives on SPI activity. The final oil to be tested is a high-volatility oil, anticipated to exhibit higher SPI activity. It remains to be seen whether this will be the case.