The central process of combustion engines is the ignition and burning of the fuel mixed with air in the working chamber of the engine. The chemical energy created during this process is then transformed into mechanical energy through pistons and subsequently powers a crankshaft that sets the vehicle’s gears in motion. There are two types of combustion engines: Internal combustion engines and external combustion engines. Due to the fact that most vehicles with combustion engines rely on internal combustion, we will focus on this form of combustion. 
These engines are generally either spark ignition gasoline engines – informally known as Otto engine and based on petrol as fuel - or compression ignition diesel engines – also called Diesel engines and hence using diesel as fuel. As the name of both combustion engines already indicates, one’s fuel-air-mixture is ignited by spark and one uses compression of hot air which is sprayed with fuel, causing it to ignite.  
Important to note is that internal combustion engines vary by many different factors: The combustion process itself, the type of fuel used, the formation of the mixture, the ignition system, the cooling system and many other factors. As mentioned above, petrol and gasoline are two very common types of fuels used in combustion engines. Other possible fuels that can be combusted are: Gaseous fuels like methane, propane, biogas or hydrogen (to some extent), liquid fuels like kerosene or benzene or heavy liquid fuels like “biodiesel”.  
[Cover Image] Photo by lee attwood on Unsplash.
  Basshuysen, R. & Schäfer, F. (2016). Internal Combustion Engine Handbook - Basics, Components, System, and Perspectives. SAE International. pp. 9 – 14.
 Mishra, P. (2017). Difference between SI Engine and CI Engine. In Mechanical Booster. https://www.mechanicalbooster.com/2017/03/difference-between-si-engine-and-ci-engine.html.
  Vehicle Technologies Office. (2013, November 22). Internal Combustion Engine Basics. In Office of Energy Efficiency & Renewable Energy. https://www.energy.gov/eere/vehicles/articles/internal-combustion-engine-basics