There has been strong public demand for reduced hazardous exhaust gas emissions and improved fuel economy for automobile engines. In recent years, a number of innovative solutions that lead to a reduction in fuel consumption rate have been developed, including in-cylinder direct injection and lean burn combustion technologies, as well as an engine utilizing a large volume of exhaust gas recirculation (EGR). Furthermore, a homogeneous charge compression ignition (HCCI) engine is under development for actual application. However, one of the issues common to these technologies is less stable combustion, which causes difficulty in engine management. Additionally, it is now mandatory to provide an onboard diagnosis (OBD) system. This requires manufacturers to develop a technology that allows onboard monitoring and control of the combustion state. This paper reports on an innovative combustion diagnostic method using an in-cylinder pressure sensor.As a key to the monitoring and controlling system, a new in-cylinder pressure sensor was developed that is integrated with the spark plug. This type of sensing device features advantages in durability and reliability, which are two fundamental requirements for incorporation with the OBD. As a piezoelectric element, a single-crystal lithium niobate (LiNbO3), which features superb pyroelectric properties, was utilized in addition to the conventional poly-crystal materials of lead titanate (PT; PbTiO3) and lead zirconate titanate (PZT; Pb(Zr,Ti)O3). The calculating processor is high-speed and highly reliable. As a combined result, the device diagnoses various combustion phenomena, including misfiring and abnormal combustion, occurring in each cylinder on a real-time basis, and thus allows optimum control of the combustion process.The control technique covers a variety of items, including; (1) knock control, (2) misfire detection, (3) torque-on-demand control, (4) minimum spark advance for the best torque (MBT) control, (5) lean burn/EGR control, (6) NOX restriction, (7) noise/vibration restriction, (8) valve timing detection, (9) cylinder detection and combustion-at-start judgment and, (10) Reid vapor pressure (RVP) detection and calibration.