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A Computational Comparison of the Predicted Thermal Efficiency of Hydrogen, Gasoline, and Ethanol in a Simplified Atkinson-Cycle Model
Author: Emma Wix Montes
Which fuel produces the greatest calculated ideal-cycle thermal efficiency under the selected fuel-specific combustion assumptions?
Abstract
The Atkinson cycle engine is typically used in hybrid vehicles, and how much fuel an engine uses has a direct impact on the environment. This investigation compares the thermal efficiency of an Atkinson cycle engine powered by hydrogen, gasoline or ethanol. A simulation was created in MATLAB that calculates the temperature, pressure and volume at each state of an ideal air-standard Atkinson cycle and obtains the thermal efficiency from the heat added and rejected per kilogram of air. The fuel enters the model only through its peak combustion temperature (~2700 K for hydrogen, ~2500 K for gasoline and ~2200 K for ethanol), and a Monte Carlo analysis with 100 scenarios per fuel includes the uncertainty in the inputs. The three fuels give almost identical efficiencies of about 64%, with hydrogen marginally highest and ethanol marginally lowest, but the differences are less than 0.1 percentage points, so the model does not show a meaningful efficiency advantage for any fuel. In an ideal cycle, efficiency depends mainly on the compression and expansion ratios. Real Atkinson engines are only around 40-41% efficient, and the choice of fuel is decided by practical factors such as cost, infrastructure, safety and emissions.
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