In this course first we will understand what heat engine is and then we will see the difference between External Combustion (EC) engine and Internal Combustion (IC) engine. Since the internal combustion (IC) engine has been the dominant prime mover in our society it is very important to know about the internal combustion engine. The term internal combustion engine usually refers to an engine in which combustion is intermittent. We will also study the basic principal of combustion in engines. This course deals with the basics of Internal Combustion engines, the combustion phenomenon in both spark ignition and compression ignition engines. This course is basically divided into three sections. The first section is on basics of Internal Combustion engines and covers the classification of heat engine, various components of engines and terminologies of engine. Second section includes working of two stroke and four stroke engines. After this section you will be able to understand the working principal of 2 stroke petrol or diesel engine. Also the working principal of 4 stroke petrol or diesel engine. Third section includes the difference between 2 stroke and 4 stroke engine. Finally we will understand the difference between 2 strokes and four stroke engines and the drawback of 2 stroke engine with the need of introduction of 4 stroke engine.