This paper reviews the current research work on Homogeneous Charge Compression Ignition (HCCI) concept for diesel engines to meet future tightened emission norms. Heavy duty diesel engines are facing conflict between the goal of emission reduction and optimization of fuel consumption. In response to social demands and progressively strengthened emission regulations, diesel engines have been made cleaner through various means such as the combustion chamber, high pressure fuel injection, and turbocharger. In recent years, high pressure fuel injection has been considered as an effective method to reduce Particulate Matter (PM) by improving atomization and better air utilization, however, resulting in an increased Nitric Oxides (NOx) formation due to high temperature combustion. To fulfill future tightened emission norms, further developments on diesel engine technology and combustion improvements are required for simultaneous reduction of NOx and PM emissions as opposed to a trade-off.
Many of the researchers have tried HCCI combustion concept as a potential solution to reduce NOx and PM emissions simultaneously. In HCCI, charge (mixture of fuel, air and residual gases) is homogeneous as in the case of Spark Ignition (SI) engine and the charge is compressed to auto-ignition temperature as in the case of Compression Ignition (CI) engine. Upon compression, combustion initiates at several locations. The HCCI engine generally runs on a lean, diluted mixture of fuel, air and combustion products. Chemical kinetics plays a major role in the auto-ignition and fuel oxidation process that characterize the HCCI combustion, unlike SI and CI engines. Currently, HCCI is limited due to the difficulty in controlling the combustion phase of the mixture. Numerical modeling shows that at higher equivalence ratios, HCCI operation was unable to reduce NOx satisfactorily as compared to conventional CI engine.
Two possible approaches can be used to overcome this problem. The first approach is to use ultra lean operation at higher loads and the second approach is to use “dual mode” engine operation for full range of operation. Either approach will help in commercializing the HCCI technology.