Thermodynamic simulation and exergy analysis of a hydrogen SI engine

Abstract

Concerns of limited fossil fuel reserves, environmental pollution from their extraction, processing, and use, and health effects from localised tailpipe emissions are leading to a transition of the transportation sector towards low-carbon and carbon-free alternative fuels for internal combustion engines. As an alternative fuel hydrogen has the potential to significantly reduce tailpipe greenhouse gas emissions, and offers advantages in terms of its combustion properties. To understand hydrogen combustion in an internal combustion engine and the upper limit of efficiency, single- and two-zone combustion models exploiting the second law of thermodynamics are developed to assess the origins of the exergy losses. The single-zone model provides detailed analysis of the boosted operation strategy, showing that thermal efficiency increases significantly for lean-burn hydrogen mixtures. Operating hydrogen engines at high loads presents challenges arising from combustion abnormalities as increasing intake air pressure raises in-cylinder temperature, significantly increasing knock occurrence and nitric oxide emissions. However, low-temperature combustion through lean-burn and water injection has potential to mitigate combustion abnormalities and reduce nitric oxide emissions; the addition of water modulates the rate at which combustion occurs. A newly developed laminar flame speed correlation for hydrogen-air combustion accounts for water addition under engine-relevant conditions. The applicability of this new correlation is demonstrated by incorporating this empirical correlation into a two-zone combustion model to predict engine performance, combustion abnormalities and nitric oxide emissions. The simulation of a water-diluted hydrogen engine indicates that emission control and knock mitigation are achievable, but requires careful optimization to avoid significant reducing thermal efficiency. The simulations allow the production of a hydrogen operational map based on the indicated specific fuel consumption, nitric oxide, thermal efficiency, equivalence ratio, and water addition. A comprehensive exergy analysis of a hydrogen engine evaluates efficiency, irreversibility, and emissions, quantifying losses for each engine condition: intake manifold air pressure, fuel mixture, compression ratio, water addition, and spark timing. This enables a discussion of the compromises for designing and managing hydrogen-fuelled SI engines at various operating conditions, including equivalence ratios, spark timings, compression ratio, and boosted manifold air pressure.