Comparative computational study of hydrogen and natural gas in high-pressure direct-injection (HPDI) compression-ignition engines: Combustion characteristics, thermal efficiency, and local pollutant and greenhouse gas emissions

Abstract

• The study uses 1D and 3D computational fluid dynamics (CFD) models. • Natural gas (methane) and hydrogen are compared in a compression-ignition engine. • Hydrogen exhibited superior combustion properties in high-pressure direct injection. • Hydrogen showed 10–20% higher indicated and 3–8% higher brake thermal efficiency. • Lower GHG emissions with hydrogen require low-carbon grid or over 60% carbon capture. Hydrogen internal combustion engines offer a promising path for rapid decarbonization of heavy-duty transport. However, hydrogen engines using spark ignition typically show lower efficiency and power density than diesel compression-ignition (CI) engines. This study focuses on the high-pressure direct-injection (HPDI) approach with CI “diffusion” combustion to address these limitations. Given the commercial feasibility of HPDI engines fueled by methane (or natural gas), this research aims to implement hydrogen in HPDI engines and identify key differences in combustion characteristics, efficiency, and pollutant emissions to guide the development of efficient low-carbon engine technologies. The analysis integrates both concepts into the heavy-duty Volvo D13 engine using conditions relevant for double compression-expansion engine to evaluate overall powertrain brake thermal efficiency. Simulations use Converge CFD and GT-Suite software at air–fuel equivalence ratios (lambda) of 1.2 and 1.6, a peak motoring pressure of 150 bar, and ∼ 20 bar IMEP. Results indicate that hydrogen CI engines require significantly smaller pilot injections and lower TDC temperatures for ignition compared to methane. The dominant free-jet mixing characteristic of hydrogen, which results in a heat release rate nearly twice that of methane, underpins its greater potential for optimization. Although NOx emissions are similar for both fuels, hydrogen provides a superior efficiency-NOx tradeoff with higher EGR rates. Under the evaluated conditions, hydrogen CI engines achieve 10–20% higher indicated efficiency than methane engines due to larger net-positive molar expansion work, faster burn rates, and less unburned fuel owing to the substantially lower sensitivity to oxygen availability. However, significantly increased wall heat transfer losses raise necessity for increased piston cooling and lead to more modest improvements in powertrain brake thermal efficiency of 3–8%. A well-to-wheels (WTW) analysis indicates that CI hydrogen engines may demonstrate lower WTW efficiency and higher GHG emissions than methane engines when hydrogen is derived from common fossil-based production methods, as well-to-tank emissions of hydrogen production dominate. Nevertheless, with carbon capture rates exceeding 60% or hydrogen produced from low-carbon electricity grids, CI hydrogen engines demonstrate strong potential for large GHG emission reductions. The findings highlight hydrogen’s advantages and trade-offs as a carbon-free alternative to methane in HPDI engines.

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