energies Review A Review of Hydrogen as a Fuel in Internal Combustion Engines Behdad Shadidi 1, * , Gholamhassan Najafi 2 and Talal Yusaf 3   Citation: Shadidi, B.; Najafi, G.; Yusaf, T. A Review of Hydrogen as a Fuel in Internal Combustion Engines. Energies 2021, 14, 6209. https:// doi.org/10.3390/en14196209 Academic Editor: Adam Revesz Received: 27 August 2021 Accepted: 26 September 2021 Published: 29 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Biosystem Engineering, Bu-Ali Sina University, Hamedan 6517838695, Iran 2 Tarbiat Modares University, Tehran 1497713111, Iran; g.najafi@modares.ac.ir 3 School of Engineering and Technology, Central Queensland University, Queensland 4009, QLD, Australia; t.yusaf@cqu.edu.au * Correspondence: b.shadidi@basu.ac.ir; Tel.: +98-8138381601 Abstract: The demand for fossil fuels is increasing because of globalization and rising energy demands. As a result, many nations are exploring alternative energy sources, and hydrogen is an efficient and practical alternative fuel. In the transportation industry, the development of hydrogen-powered cars aims to maximize fuel efficiency and significantly reduce exhaust gas emission and concentration. The impact of using hydrogen as a supplementary fuel for spark ignition (SI) and compression ignition (CI) engines on engine performance and gas emissions was investigated in this study. By adding hydrogen as a fuel in internal combustion engines, the torque, power, and brake thermal efficiency of the engines decrease, while their brake-specific fuel consumption increase. This study suggests that using hydrogen will reduce the emissions of CO, UHC, CO 2 , and soot; however, NO x emission is expected to increase. Due to the reduction of environmental pollutants for most engines and the related environmental benefits, hydrogen fuel is a clean and sustainable energy source, and its use should be expanded. Keywords: hydrogen fuel; renewable energy 1. Introduction The rise in the world population and the excessive use of fossil fuels have resulted in a significant increase in the global demand for energy production in the twenty-first century [1]. Total global oil consumption was 3928 million tons in 2008 and is expected to reach 5300 million tons by 2021 [2,3]. However, oil is a finite resource that is becoming increasingly scarce and expensive [4]. Because of their availability, fossil fuels now meet around 65% of the world’s energy demand [5]. Transportation vehicles and machinery that require fossil fuels to operate use a significant proportion of this energy. In 2018, 1.42 billion automobiles and light trucks were on the road globally [6]. By 2050, the number of automobiles is anticipated to exceed 2 billion [7]. The decline in fossil fuels availability has led to an energy (fuel) crisis. IC engines, on the other hand, will continue to dominate the transportation and power industries due to their flexibility. Pollutants emitted by internal combustion engines affect the atmospheric conditions and cause serious problems such as global warming, air pollution, acidic rains, and respiratory problems [8]. So far, many researchers have tried to create a suitable combustion system for the complete combustion of hydrocarbons [9,10]. However, the issue is still challenging. Increasing fuel quality and creating better fuel combustion in engines are among the methods that have been considered to solve this problem. In these methods, there is no need to make changes in the engine structure. The use of biofuels, water, nanoparticles, and other additives is among the solutions that have been studied to increase combustion and fuel quality [8]. To ensure the long-term safety and viability of IC engines, “clean-burning” fuels derived from renewable sources must be developed as soon as feasible [11]. In this respect, research and investments in the use of renewable Energies 2021, 14, 6209. https://doi.org/10.3390/en14196209 https://www.mdpi.com/journal/energies