ABSTRACT

Compression-ignition engines operate with a heterogeneous air-fuel mixture. A heterogeneous mixture means the air-fuel ratio or equivalence ratio in a spatial distribution of mixture is not the same in all parts of the combustion chamber. The diffusivity of high-cetane-number liquid fuels (diesel, biodiesel, F-T diesel, etc.) is less than that of gaseous fuels so these fuels have to be atomized using a high-injection, pressure-based injection system and subsequently the atomized fuel is mixed with the surrounding air, which is at a high temperature with intensified swirl and squish air motion. Then, the mixture starts to vaporize and is in an almost completely vapor state. A mixture prepared during the ignition delay period is called a premixed charge, whereas a mixture preparation at the end of the injection period is called a heterogeneous mixture. The available time duration for a premixed charge is relatively greater than in the later stage of preparation. The ignition delay of advanced engines is much less due to a better mixture formation with advanced technologies such as CRDI, even though the degree of homogeneous charge improves or the degree of heterogeneous charge decreases compared to conventional diesel injection technology. However, the degree of homogenous charge is the lowest with direct fuel injection based compression ignition engines compared to port injection or manifold injection based spark ignition engines because the time available to prepare mixture is lower with the direct fuel injection than manifold/port injection methods. For example, the mixture preparation time for a typical CI engine (injection duration from 10 CA to 20 CA at an engine speed of 3000 rpm) is in the range of 0.56 ms to 1.1 ms out of a cycle time of 40 ms. However, in the case of spark-ignition engines, the mixture preparation time will be nearly the half of a cycle time. The degree of homogenous charge can be increased by pulse injection or split injection because the fuel, which can be divided to any number of discrete quantities, is directly injected into the cylinder during the beginning of a compression stroke, and the time available for this type of injection is theoretically almost 180 CA. The injected fuel can mix properly due to more available time. However, control of autoignition (CAI) and combustion with knock with pulse injection are major technical challenges in a conventional compression-ignition engine. Note that pollutant formation in a CI engine is mainly due to the degree of charge homogeneity. PM forms less with a homogeneous mixture and more with a heterogeneous mixture. Even though advanced technology can drastically decrease the emission, it still will not meet the stringent emissions norms. It is 202a fact that the mass of PM is much less or negligible compared to the mass of fuel injected per cycle. The mass of PM is approximately less than 0.1% of fuel mass, but in terms of a particle’s size and the number of counts of PM, it is high and unable to meet to present or future emissions norms. In addition, the localized temperature in a combustion chamber during combustion is higher with a heterogeneous mixture, resulting in a higher level of NOx. If the in-cylinder temperature is high, PM emission is lower but NOx emission will be high, and vice versa. Hence, a simultaneous reduction of NOx and PM is still challenge in compression-ignition engines. These emissions, along with specific fuel consumption, can be significantly reduced with a homogeneous charge. Because liquid fuels such as diesel, F-T diesel, and biodiesel are less volatile, an external mixture preparation in an engine’s intake manifold during a suction stroke is not a desirable option. It is seen from the discussion that combustion with heterogeneous air-fuel in conventional diesel engines would lead to high NOx and smoke emissions. An LTC strategy with a premixed charge could reduce NOx and PM emissions because NOx formation needs high activation energy (Turns, 1999). In addition, soot emission with LTC will be reduced due to better mixing of a charge with a long ignition delay. However, the limitation with LTC is a less specific power output compared to conventional combustion in compression-ignition engines. Many researchers reported that NOx and smoke emissions in compression-ignition engines decreased simultaneously under HCCI and premixed charge compression ignition (PCCI) mode (Hardy et al., 2006; Opat et al., 2007).