1. Introduction

The main goal of studying internal combustion (IC) engines is to improve fuel conversion efficiency in the face of increasingly severe energy issues and global warming. The increasing competition among automotive OEMs together with the worsening of environmental pollution has led to the development of complex engine systems. Innovative control strategies are required to simplify and improve the engine management system (EMS), moving toward energy saving and complying with the stringent emission legislation. In this scenario, researchers are focusing on improving conventional combustion engines as well as the development of alternative combustion modes with the utilization of conventional as well as alternative fuels. Present chapter provides a brief introduction of conventional engines (spark ignition and compression ignition) as well as advanced low-temperature combustion engines. To improve the comprehension of engine performance and its combustion reactions, development of the comprehensive performance measurement technique, in-cylinder visualization technique, and numerical simulations is essential and strongly demanded. This chapter describes the need of combustion diagnostics and provides a brief overview of combustion diagnostics using in-cylinder pressure measurement and analysis. Additionally, the chapter presents the summary of the cylinder pressure signal processing methods for calculating combustion parameters and information extracted about engine combustion, performance, and emissions that can be used for further development, optimization, and calibration of modern engines.