The application of Thermodynamics to engineering systems such as power generation, refrigeration and air-conditioning are grouped together to form this textbook. This book is written as a text for the subject "Thermal Engineering" under DOTE Syllabus.This book is written entirely in S.I. system of units. The subject treatment requires a basic knowledge of thermodynamics. The subject matter is methodically developed introducing the topics systematically. Additional information's outside the coverage of syllabus have been provided wherever necessary.The notable feature of this book is the introduction of the concept of energy. The concept of conservation and effective utilization of energy has lead to cogeneration systems which are also discussed. Many solved problems are available in each chapter. Unsolved problems with answers are provided in each chapter for the reader to practise. Thermodynamic data in S.I. units have been provided in the Appendix.