The integration of hydrogen into diesel engines as a dual-fuel system presents a promising pathway to reduce emissions in heavy-duty transport. Heavy-duty vehicles, particularly those relying on diesel engines, are significant contributors to particulate matter (PM) and carbon dioxide (CO2) emissions. The combustion of diesel fuel inherently produces high levels of these pollutants, which have adverse environmental and health impacts. By supplementing diesel with hydrogen, dual-fuel engines can mitigate these emissions while maintaining performance. However, the trade-offs between PM and CO2 reduction, along with the optimization of injection timing, require careful evaluation.

Diesel engines operate under compression ignition, where fuel is injected directly into the combustion chamber. The high carbon content of diesel leads to substantial CO2 emissions, while incomplete combustion generates PM. Hydrogen, as a carbon-free fuel, offers a solution to these challenges. When hydrogen is introduced into the combustion process, it partially replaces diesel, reducing the overall carbon input. Hydrogen’s high flammability and wide flammability range enable leaner combustion, which can lower peak temperatures and reduce nitrogen oxide (NOx) formation. However, the interaction between hydrogen and diesel must be optimized to avoid unintended consequences, such as increased NOx emissions or unstable combustion.

One of the most significant benefits of hydrogen-diesel dual-fuel systems is the reduction in PM emissions. Particulate matter is primarily composed of soot, which forms during the pyrolysis of hydrocarbon fuels in oxygen-deficient zones of the combustion chamber. Hydrogen’s absence of carbon means it does not contribute to soot formation. Studies have shown that substituting even small amounts of diesel with hydrogen can lead to substantial PM reductions. For example, research indicates that a 30% hydrogen energy share can reduce PM emissions by up to 50% compared to pure diesel combustion. Higher hydrogen ratios further decrease PM, but practical limits exist due to engine design and combustion stability.

CO2 emissions are also reduced in hydrogen-diesel dual-fuel systems, though the extent depends on the hydrogen production method. If hydrogen is produced via steam methane reforming (SMR) without carbon capture, the upstream CO2 emissions may offset some of the tailpipe reductions. However, when green hydrogen—produced via electrolysis using renewable energy—is used, the net CO2 reduction can be significant. In heavy-duty applications, a 20% hydrogen energy substitution can reduce CO2 emissions by approximately 15%, while higher substitutions (50% or more) can achieve reductions exceeding 30%. These figures assume optimal combustion conditions and highlight the importance of system efficiency.

A critical factor in maximizing emission reductions is the optimization of injection timing. In conventional diesel engines, injection timing controls the start of combustion, influencing temperature, pressure, and emission formation. In dual-fuel systems, hydrogen is typically introduced into the intake air, while diesel is directly injected. The timing of diesel injection becomes even more crucial, as it ignites the hydrogen-air mixture. Advanced injection timing can improve combustion efficiency but may increase NOx emissions due to higher peak temperatures. Retarded injection timing, on the other hand, can lower NOx but may lead to incomplete combustion and higher PM or unburned hydrocarbons.

Research suggests that moderate injection timing adjustments—neither too advanced nor too retarded—strike the best balance for dual-fuel engines. For instance, injecting diesel slightly earlier than in conventional engines can ensure reliable hydrogen ignition while minimizing NOx spikes. Pilot injection strategies, where a small amount of diesel is injected before the main injection event, can further enhance combustion stability and emission control. Experimental data from heavy-duty engine tests show that optimized injection timing can reduce PM by 40-60% and CO2 by 10-20% without significant NOx penalties.

The trade-offs between PM, CO2, and NOx emissions necessitate a systems approach to dual-fuel engine design. While hydrogen reduces PM and CO2, its high combustion temperatures can elevate NOx if not managed properly. Exhaust gas recirculation (EGR) can mitigate NOx by diluting the intake charge and lowering combustion temperatures, but excessive EGR may increase PM. Similarly, turbocharging can improve efficiency but must be calibrated to avoid adverse emission interactions. The optimal configuration depends on the specific application, operating conditions, and emission regulations.

Heavy-duty transport operators must also consider practical challenges, such as hydrogen storage and refueling infrastructure. Hydrogen’s low energy density by volume requires high-pressure or cryogenic storage, which adds weight and complexity to vehicles. However, advancements in storage materials and compact tank designs are gradually addressing these limitations. Fleet-based applications, where vehicles operate on fixed routes with centralized refueling, are particularly well-suited for early adoption of hydrogen-diesel dual-fuel systems.

In summary, hydrogen-diesel dual-fuel engines offer a viable solution for reducing PM and CO2 emissions in heavy-duty transport. The extent of emission reductions depends on the hydrogen substitution ratio, production method, and combustion optimization. Injection timing plays a pivotal role in balancing PM, CO2, and NOx emissions, with moderate adjustments yielding the best results. While challenges remain in infrastructure and system integration, the potential environmental benefits make dual-fuel technology a compelling option for decarbonizing heavy-duty vehicles. Continued research and real-world testing will be essential to refine these systems and unlock their full potential.