Combustion characteristics of diesel HCCI engine: An experimental investigation using external mixture formation technique

Abstract

In compression ignition engines, soot–NOx paradox is an extremely challenging unresolved issue. Homogeneous charge compression ignition (HCCI) is one of the most promising solution that combines the advantages of both SI and CI combustion modes. It gives high thermal efficiency similar to compression ignition engines and resolve the associated issues of high levels of NOx and PM simultaneously. In HCCI combustion, homogeneous mixture of air and fuel burns spontaneously throughout the combustion chamber, which reduces the total combustion duration due to very high rate of heat release. Determination of precise control parameters for controlling the ‘rate of heat release’ and ‘start of combustion’ are major research challenges in the development and deployment of this technology. In the present research, experiments were performed in a two cylinder engine, in which one cylinder is modified to operate in HCCI mode, while other cylinder operate in conventional CI mode. Homogeneous mixture preparation is the most challenging part for achieving diesel HCCI combustion. Low diesel volatility remains the main obstacle in preparing the homogenous fuel–air mixture therefore a dedicated device called ‘diesel vaporizer’ was developed. Exhaust gas recirculation (0%, 10% and 20%) was used for controlling the rate of heat release. To study the combustion behavior, experiments were performed at three different relative air–fuel ratios (λ=4.95, 3.70 and 2.56). Enrichment of fuel–air mixture enhances the rate of heat release and the location of peak of in-cylinder pressure shift towards BTDC side due to earlier start of combustion. This was effectively controlled by EGR for leaner HCCI combustion conditions. Exhaust gases diluted the homogeneous charge and presence of non-reactive species reduce the rate of combustion. It controls the peak in-cylinder temperature, which is a responsible for extremely low NOx formation. For richer fuel–air mixtures, EGR was relatively less effective due to dominance of ‘rate of heat release’, which was significantly high.