Using Simulation to Model Closed-Cycle Argon Hydrogen Engines

Evaluating Renewable Hydrogen Fuel

With the increasing demands for fuel-efficient and low-to zero-emissions technologies in the automotive industry, renewable hydrogen fuel is regarded as a promising energy storage form for vehicles. Pure hydrogen combustion emits no greenhouse gas CO2, and noble gases can eliminate environmental pollutants such as NO_x by replacing nitrogen. As a result, hydrogen combustion in a noble gas is expected to eliminate both carbon emissions and other pollutant emissions.
Additionally, the higher ratio of specific heats, can increase the theoretical thermal efficiency of an Otto cycle, k = Cp/Cv, as  can be determined from the equation below:

For which CR is the compression ratio.

Therefore, the use of a monoatomic working gas with a high specific heat ratio such as argon would ideally achieve much higher thermal efficiency than conventional internal combustion engine using air (nitrogen) as the working gas. Figure 1 shows the relationship of theoretical thermal efficiency and the specific heat ratio of working gas [1]. For a compression ratio of 10, the thermal efficiency drastically improves by about 30% on a relative basis, or 18% on an absolute basis from k = 1.4  to k = 1.67.

Figure 1 Relationship between Specific Heat Ratio of Working Gas and Otto Thermal Efficiency [1]

Simulating Hydrogen Engines with GT-SUITE

To demonstrate the possibility that GT-SUITE can model a closed-cycle argon-hydrogen engine, Gamma Technologies has built and included a model of such a system within the recently released v2022 build 1. In the example model ‘Ar-H2_ClosedCycle_Engine’, several advanced modeling concepts are applied, including condensation of combustion products, removal of water from the system and a semi-predictive condenser. Argon is recirculated in the system and oxygen is supplied via an injector object; hydrogen, as the only fuel present in this system, is injected directly into the engine cylinder. Upon the completion of combustion, the exhaust gas passes through the condenser to convert the water vapor to liquid water, which is later removed from the system.

A quite critical aspect for the closed-cycle simulation is to ensure the steady solution, which requires a strict balance of the system mass. Namely, it is required to maintain a balance between the mass entering the system and the mass exiting the system. Otherwise, the constantly changing system mass would prevent simulations from converging on a steady result. Consequently, the oxygen supply and hydrogen injection need to be carefully controlled to maintain the stable simulation.

Figure 2 Ar-H2 Closed Cycle Engine Model Configuration in GT-SUITE

The example model simulates at different levels of argon fractions in the working gas, namely the ratio of argon gas in the argon and oxygen mixture. The trends of thermal efficiency and specific heat ratio of the in-cylinder gas are observed to vary with the argon fraction as shown in Figure 3, which are consistent with the theory described in the Introduction section. The slopes of efficiency and specific heat ratio shown in Figure 3 are dependent on the condenser design parameters in the example model. Engine efficiency and specific heat ratio should be more sensitive to the change in argon fraction with a condenser of better performance.

Figure 3 Indicated Efficiency and Specific Heat Ratio Varying with Argon Mass Fraction

Benefits of Hydrogen/Noble Gas Engines Simulation

The discussed Ar-H2 closed cycle engine simulation in this blog is demonstrated with a non-predictive combustion model in GT-SUITE. It allows the user to perform a similar proof-of-concept hydrogen/noble gas engine simulation and serves as a reference/example for closed-cycle engine simulations. The research and development work on this topic can be extended to combine with a predictive combustion model in the future, such as the SI Turbulent Flame Combustion Model which has been widely used for gasoline combustion engine simulations.

References

[1] Kuroki, R., Kato, A., Kamiyama, E., and Sawada, D., “Study of High Efficiency Zero-Emission Argon Circulated Hydrogen Engine,” SAE Technical Paper 2010-01-0581, 2010, https://doi.org/10.4271/2010-01-0581.