Hydrogen with water addition: an exergy analysis of the internal combustion engine

Dardan N. RrustemiaLionel C. GanippaColin J. Axon^*

• Department of Mechanical Engineering, College of Engineering, Design and Physical Sciences, Brunel University London, Uxbridge, United Kingdom

The internal combustion engine is likely to be used for on- and off -road vehicles for many years yet, but the push to cease using fossil fuels is strong. Hydrogen is a possible alternative fuel with both advantages and disadvantages, so understanding and quantifying the efficiency losses of burning hydrogen is important. The limits to efficiency and the compromises to be reconciled for reducing losses can be investigated using exergy analysis. This analysis of a boosted lean-burn neat hydrogen spark ignition engine investigates exergetic processes under real-world engine operating conditions. Using a two-zone combustion model to study in-cylinder processes, the results suggest exergy transfer to work improves with increasing air dilution by diverting exhaust exergy to reversible work. Injecting water could potentially control emissions through in-cylinder thermo-physical property changes. For an equivalence ratio of 0.45 with 5% water addition, the exergy transfer to heat and work decreases by 12% and 7%, respectively. Conversely, the exergy transfer to combustion-related irreversibility and exhaust rises by 2% and 81%, respectively. But, it was shown that increasing manifold air pressures and compression ratios increases the quantity of exergy directed to work and heat, whilst reducing exergy expelled to exhaust. This exergy analysis of a hydrogen-fuelled spark ignition engine operating under real-world parameters shows the need to optimize water injection as the trade-off between engine performance, and emission reductions. Understanding the fundamentals of the thermodynamic mechanisms of work-loss may inform engineering improvements to minimize exergy losses, increasing efficiency and work output.