This paper presents two closed-loop control methods for monitoring and improving the combustion behavior and the combustion noise on two 4-cylinder diesel engines, in which an in-cylinder pressure and an accelerometer transducer are used to monitor and control them. Combustion processes are developed to satisfy the stricter and stricter regulations on emissions and fuel consumption. These combustion processes are influenced by the factors such as engine durability, driving conditions, environmental influences and fuel properties. Combustion noise could be increased by these factors and is detrimental to interior sound quality. Therefore, it is necessary to develop robust combustion behaviors and combustion noise. For this situation, we have developed two closed-loop control methods. Firstly, a method using in-cylinder pressure data was developed for monitoring and improving the combustion noise of a 1.7L engine. A new index using the values calculated from the data was proposed. The closed-loop control based on the new index produced the desired combustion noise by using some injection parameters. Secondly, a method for predicting the combustion behavior of a 1.6L engine was developed by analyzing the vibration signals obtained from the engine cylinder block. The closed-loop control based on a combustion factor, indicates combustion behavior that we wanted, produced the desired combustion behavior.