The combustion concept at the heart of the work is reactivity-controlled compression ignition, or RCCI, a strategy that has attracted intense attention as a way to squeeze diesel-like efficiency out of engines while slashing the nitrogen oxide and soot emissions that plague conventional diesel combustion. In an RCCI engine, two fuels of different reactivity are blended inside the cylinder: a low-reactivity fuel premixed with the intake air, and a high-reactivity fuel injected directly. The reactivity gradient that forms across the charge controls when and where ignition occurs, taming the violent pressure rise that makes conventional diesel combustion noisy and NOx-rich. In the hydrogen-diesel variant studied here, hydrogen serves as the low-reactivity premixed fuel while diesel provides the direct-injected ignition trigger.

Hydrogen would seem an ideal partner in this arrangement. Its high diffusivity promotes excellent mixing with air, and its wide flammability limits give engineers enormous latitude in setting the mixture composition. Burning hydrogen produces no carbon dioxide at the point of use, and the resulting combustion is inherently cleaner with respect to soot precursors. But hydrogen carries a hidden complication: its chemical activity is so high that it interferes with the delicate radical chemistry that diesel combustion depends on. The Indian team set out to understand exactly how this interference plays out as the hydrogen energy ratio—the fraction of the total fuel energy supplied by hydrogen—is pushed higher and higher.

To do this, the researchers built a computational model of the hydrogen-diesel RCCI combustion process coupled to a detailed chemical kinetics mechanism, meaning the simulation tracks not just bulk heat release but the individual elementary reactions that consume and produce the reactive intermediates governing ignition. After validating the model against experimental data, they used it as a microscope into the cylinder, examining how the chemistry shifts as the hydrogen energy ratio climbs. The model revealed a specific and consequential reaction: hydrogen competes fiercely for hydroxyl radicals through the reaction H2 + OH, yielding water and a hydrogen atom. Hydroxyl radicals are the workhorses of low-temperature oxidation chemistry, and their consumption by hydrogen starves the diesel fuel’s cool-flame chemistry of the radicals it needs to proceed.

The cool flame is a subtle, low-temperature pre-ignition reaction zone that diesel fuel undergoes before the main heat release. It is the chemical foundation on which the staged, controlled combustion of RCCI is built. The kinetics analysis showed that at high hydrogen energy ratios, the drain of OH radicals into the hydrogen oxidation pathway suppresses this cool flame chemistry, and the consequences are dramatic. When the hydrogen energy ratio reached 41 percent, the cool flame was inhibited to the point that the engine misfired—combustion simply failed to initiate reliably, and the operating point was lost. This misfire boundary, the study shows, is not a mechanical or hardware limitation but a fundamentally chemical one, rooted in the competition between hydrogen and diesel for the same radical pool.

Having identified the mechanism, the researchers then used the model to explore ways around it. The first lever they pulled was the charge temperature—the temperature of the reactant mixture as it enters the cylinder. Higher charge temperatures accelerate the low-temperature chemistry, providing the thermal push needed to sustain the cool flame even when OH radicals are being scavenged by hydrogen. The simulations demonstrated that raising the charge temperature to 340 Kelvin was sufficient to resolve the critical misfire at the 41 percent hydrogen energy ratio, restoring stable ignition and allowing the engine to operate at hydrogen fractions that would otherwise be unreachable.

The second lever was more elegant: rather than heating the intake charge, the team restructured the diesel injection itself. Using a narrow-angle direct injection configuration, they implemented a twin-pulse injection strategy, splitting the diesel delivery into two separate pulses rather than a single event. This reshaping of the fuel delivery profile alters the local reactivity distribution inside the cylinder and the timing of radical pool development, effectively re-orchestrating the chemical sequence so that the cool flame chemistry can proceed despite the hydrogen’s radical consumption. The twin-pulse approach achieved the same restoration of stable combustion as the temperature increase, but without the efficiency penalty that can accompany heated intake air.

Both optimization routes did more than merely rescue the engine from misfire. The simulations showed that stable combustion was re-established with enhanced indicated thermal efficiency, meaning more of the fuel’s energy was converted to useful work. Critically, the low-emission character of the RCCI concept was preserved. At the optimized conditions, the study reports approximately 92 percent lower soot compared to the baseline, alongside low NOx emissions. This combination—high efficiency, near-elimination of soot, and controlled NOx—is precisely the profile that engine designers have been chasing, and achieving it at extended hydrogen fractions represents a meaningful expansion of the hydrogen-diesel RCCI operating envelope.

The broader significance of the work lies in the methodology as much as the results. By coupling a detailed chemical kinetics model to the engine simulation, the researchers were able to diagnose the failure mode at the level of individual elementary reactions rather than treating misfire as an empirical black box. This allowed them to target interventions—temperature and injection strategy—at the actual chemical bottleneck, rather than searching blindly through a parameter space. The approach provides a template for how hydrogen dual-fuel engines can be systematically optimized as the transportation sector seeks practical pathways to decarbonize heavy-duty applications where battery-electric solutions remain challenging.

The findings collectively define what the authors describe as a viable and extended operational map for highly efficient, low-NOx hydrogen-diesel RCCI operation. As hydrogen infrastructure matures and the pressure to decarbonize freight, marine, and off-highway sectors intensifies, strategies that allow existing diesel-engine architectures to burn increasing fractions of hydrogen cleanly and efficiently will be at a premium. This study demonstrates that the barriers to high hydrogen fractions are chemically identifiable and, with the right combination of thermal management and injection design, chemically surmountable.

Subject of Research: Hydrogen-diesel reactivity-controlled compression ignition combustion optimization using coupled chemical kinetics modeling

Article Title: Extending the operational range of hydrogen diesel RCCI mode through charge temperature and injection strategy optimization guided by coupled chemical kinetics model

Article References: Gopinathan, R. L., Sathishkumar, S., & Mohamed Ibrahim, M. (2026). Extending the operational range of hydrogen diesel RCCI mode through charge temperature and injection strategy optimization guided by coupled chemical kinetics model. Discover Sustainability. https://doi.org/10.1007/s43621-026-04973-1

Image Credits: AI Generated

DOI: 10.1007/s43621-026-04973-1

Keywords: hydrogen fuel, RCCI, chemical kinetics, twin-pulse injection, charge temperature, misfire limit, soot emissions, NOx, compression ignition, combustion modeling, indicated thermal efficiency, decarbonization