Citation: Szwaja, S.; Gruca, M.; Pyrc, M.; Jukneleviˇ cius, R. Glycerol as an Anti-Knock Additive and Secondary Fuel as a Substitute for Gasoline-Based Fuels for the IC Engine. Energies 2023, 16, 4940. https://doi.org/10.3390/en16134940 Academic Editors: Gabriele Di Blasio, Andrzej Teodorczyk, Enhua Wang and Jie Liu Received: 24 January 2023 Revised: 24 March 2023 Accepted: 7 April 2023 Published: 25 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Glycerol as an Anti-Knock Additive and Secondary Fuel as a Substitute for Gasoline-Based Fuels for the IC Engine Stanislaw Szwaja 1, * , Michal Gruca 1 , Michal Pyrc 1 and Romualdas Jukneleviˇ cius 2 1 Department of Thermal Machinery, Faculty of Mechanical Engineering and Computer Science, Czestochowa University of Technology, Dabrowskiego 69, 42-200 Czestochowa, Poland 2 Faculty of Mechanics, Vilnius Gediminas Technical University, Plytin˙ es g. 25, LT-10105 Vilnius, Lithuania * Correspondence: stanislaw.szwaja@pcz.pl; Tel.: +48-885-840-483 Abstract: The article discusses the possibility of using glycerol as an additive to the engine fuel in order to reduce the tendency of combustion knock, and thus to increase the octane number of a given fuel. Experimental tests were carried out on the UIT-85 research engine with a variable compression ratio from eight to eleven to test the intensity of the knock. The completely renewable fuel—the blend of glycerol with butanol in the ratio of 25 and 75%, respectively—was tested. A comparative analysis of the knock intensity was conducted with gasoline 95 and N-butanol tested as reference fuels. The developed method for knock analysis using the proposed knock indicator was also presented. The experimental results proved the proposed blend of N-butanol and glycerol reduces the knock intensity by more than 50% in the spark-ignition engine at a compression ratio of 10, maintaining engine performance at a similar level as it was for a gasoline-fueled engine. The results confirmed the thesis on the reduction of knock intensity when adding glycerol to N-butanol. Keywords: glycerol; combustion knock; comparative analysis; internal combustion engine 1. Introduction The topics raised in the article deal with fuel testing and knock analysis. Hence, the literature review was divided into two parts on these two topics. As the literature databases are rich in research works focused on the knock phenomenon and alcohol combustion in internal combustion (IC) engines, this introduction only presents exemplary references related to the key topics; however, they are not strictly correlated with the essence of the article, and hence, prove a gap in knowledge. The tendency of IC engines to knock is one of the most important reasons limiting the thermal efficiency of engines. It prevents the achievement of an efficient combustion process, and limits the possibility of increasing the compression ratio. The knock results from the noise caused by the self-ignition of a certain part of the air-fuel mixture before the spreading flame front [1]. This phenomenon is inherent in the operation of IC engines and has been intensively researched for decades. There are several terms to describe this phenomenon as follows: combustion knock, engine knocking, or knocking combustion. High-frequency pressure oscillations occur in the combustion chamber during knocking combustion. The knocking combustion is harmful to the engine by causing damage mainly to the piston rings and piston crowns by melting and puncturing them. The results from knocking tests lead to a deeper understanding of combustion, and ultimately to improving engine construction, increasing durability, decreasing fuel consumption, and reducing toxic exhaust emissions and noise. The phenomenon consists of two characteristic phases. The first one is the flame propagation phase, which lasts from the ignition of the spark to the beginning of the pressure oscillation. In this phase, the temperature of the unburned mixture increases as a result of compression, caused by the exhaust gas and the piston motion, and heating because of the spreading flame. In the second phase, the pressure Energies 2023, 16, 4940. https://doi.org/10.3390/en16134940 https://www.mdpi.com/journal/energies