Energies 2023, 16, 896. https://doi.org/10.3390/en16020896 www.mdpi.com/journal/energies Article Definition of the Thermodynamic Cycle of a Biomass-Fueled Internal Combustion Engine Gonzalo Suanes 1,2 , David Bolonio 1, * and Antonio Cantero 3 1 Department of Energy and Fuels, E.T.S. Ingenieros de Minas y Energía, Universidad Politécnica de Madrid, Ríos Rosas 21, 28003 Madrid, Spain 2 Research and PhD Sub-Directorate, Escuela Politécnica Superior del Ejército, Joaquín Costa 6, 28006 Madrid, Spain 3 Department of Weapon Systems Industrial Technologies, Research and PhD Sub-Directorate, Escuela Politécnica Superior del Ejército, Joaquín Costa 6, 28006 Madrid, Spain * Correspondence: david.bolonio@upm.es; Tel.: +34-91-067-6378 Abstract: Climate change and the depletion of fossil fuels make it urgent to find an alternative to oil-based fuels, especially in machines powered by internal combustion engines. Biomass is cur- rently a poorly used source of energy and meets the necessary conditions to replace a large part of oil-based fuels. However, current engines cannot burn solid biomass and a specific engine needs to be developed. This work proposes the thermodynamic cycle of a biomass-fueled internal combus- tion engine. The cycle is significantly different from the Otto cycle, since compression and heat ab- sorption occur simultaneously, in a single stage. Since it is not possible to find a function that relates pressure to volume at this stage, an approximate method is proposed to solve the cycle without resorting to numerical methods. The results show that the maximum pressure and temperature of a biomass-fueled engine cycle are somewhat higher than those of the equivalent Otto cycle. How- ever, more significantly, the cycle efficiency does not increase continuously with the compression ratio. There is an optimum compression ratio value for which the cycle efficiency is at its maximum. This fact will condition the design of the motors. Keywords: biomass; alternative fuels; combustion engine; thermodynamic cycle 1. Introduction Fossil fuels are currently a fundamental pillar of society. Food production, mining and transportation depend on energy obtained from internal combustion engines that use oil-based fuels. The substitution of these fuels is an essential task since: (1) As the latest IPCC (Intergovernmental Panel on Climate Change) report warns, the emission of green- house gases causes climate change that is seriously affecting society [1]; (2) The location of oil reserves generates serious geopolitical problems [2]; and (3) Oil is a finite energy source that, at current consumption rates, will be depleted before the end of the century [3]. Numerous studies address this problem, on the one hand, by evaluating alternative fuels [4], on the other hand, through the electrification of motor machines [5]. However, the alternatives proposed so far have only succeeded in replacing fossil fuels in specific applications and present strong drawbacks for large-scale implementation. The funda- mental reason is that oil is a source of energy and can only be replaced by another source of energy. The problem must, therefore, be approached from an energy point of view. In the case of ethanol, despite its widespread use in mixture with gasoline [6], several studies indicate that the classical process of alcoholic fermentation is not energetically vi- able. Producing ethanol requires 29% more energy than ethanol itself contains as fuel [7]. Citation: Suanes, G.; Bolonio, D.; Cantero, A. Definition of the Thermodynamic Cycle of a Biomass-Fueled Internal Combustion Engine. Energies 2023, 16, 896. https://doi.org/10.3390/ en16020896 Academic Editors: Vincenzo Motola, Reeta Goel and Neeta Sharma Received: 10 November 2022 Revised: 23 December 2022 Accepted: 28 December 2022 Published: 12 January 2023 Copyright: © 2023 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).