I. Introduction
The inherently high fuel economy of diesel combustion, along with recent technological advances enabling to meet the present and future legislative emission requirements, makes diesel engines an attractive option for the automotive industry. However, diesel engines traditionally tend to be noisier compared with their gasoline counterparts. Therefore, noise has been considered as an extra performance metric during engine system development and calibration in addition to emissions, fuel economy, and drivability. This has been improved greatly with the advent of high-pressure common rail injection systems allowing small pilot fuel injections for noise management. However, CN control for diesel engines is still an important area, since it affects the acoustic comfort, which plays a significant part in customer purchasing decision. Recently, advanced combustion modes (HCCI and PCCI) have been proposed for diesel engines to meet the stringent emission requirements. These advanced combustion modes are characterized by high EGR levels and earlier injection timings, which results in high rates of pressure rise and accompanying noise as load increases. Among other benefits, the burned EGR gas increases the ignition delay (time between fuel injection and start of combustion) allowing more time for fuel/air to premix homogeneously. Following this prolonged premixing period, fuel is burned more rapidly as dictated by the thermodynamic conditions of the in-cylinder mixture. In order to compensate for the increased ignition delay, fuel is injected earlier in the cycle (advanced timing) in multiple pulses. With high dilution and advanced injection timings, premixed CN can reach unacceptable levels in the presence of potential in-cylinder dilution and injection inaccuracy. Although, significant emission reduction (simultaneous NOx and smoke) has been achieved, the sensitivity of these combustion modes requires precise control of mixture and injection timing to make these advanced combustion engines viable in production. In accordance with this need, different forms of closed-loop combustion control, such as cylinder pressure-based combustion controls, are becoming integral part of the overall engine control system for applications running with premixed combustion modes.