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Thermodynamic characteristics of premixed compression ignition combustions were clarified quantitatively by heat balance estimation. Heat balance was calculated from temperature, mole fractions of intake and exhaust gases, mass and properties of fuels. Heat balance estimation was conducted for three types of combustion; a conventional diesel combustion, a homogeneous charge compression ignition (HCCI) combustion; fuel is provided and mixed with air in an intake pipe in this case, and an extremely early injection type PREmixed lean DIesel Combustion (PREDIC). The results show that EGR should be applied for premixed compression ignition combustion to complete combustion at lower load conditions and to control ignition timing at higher load conditions. With an application of EGR, both HCCI and PREDIC showed low heat loss characteristics at lower load conditions up to 1/2 load.

One of the problems in HCCI combustion is a knocking in higher load conditions. It governs the high load limit, and it is suggested that the knock increases heat loss[1], because it breaks the thermal boundary layer. But it is not clear how much knock affects on heat loss in the HCCI combustion in various conditions, such as ignition timing and load. The motivation of this study is to clarify the ratio of heat loss caused by knock in HCCI engines. The heat loss from zero-dimensional calculations with modified heat transfer coefficient, which is considering the effect of knock by adding a term of cylinder pressure rising rate dp/dt, agreed well with the results from the thermodynamic analysis in various conditions. And the results show that it is possible to avoid heat loss by knock by controlling the ignition timing at appropriate timing after T.D.C. and it will be possible to expand the load range if knock can be avoided.

Premixed diesel combustion was performed and various characteristics examined with fuel injection timing near top dead center (TDC). A lean and uniform fuel-air mixture was found to during 25° C.A. with a narrow injection angle (27.5° with respect to horizontal), shallow dish combustion chamber, and low cetane number fuel (CN=19). These conditions enabled low NOx combustion in no exhaust gas re-circulation (EGR), despite fuel injection timing around 25° BTDC. Furthermore, HC emissions were lower than with premixed diesel combustion of the early injection type. Because fuel injection timing was near TDC, the volume of the mixture dispersed to a squish area was decreased. This combustion mode was also achieved with a high-cetane fuel (conventional diesel fuel) and high EGR rate conditions. However, in this case, it was difficult to adjust the ignition timing near top dead center. This combustion system also showed good performance in conventional diesel combustion mode.

The mixture formation process in a premixed compression ignition engine was numerically analyzed. This study aimed to find out effective injection conditions for lean mixture formation with high homogeneity, since the NOx and soot emissions in the engine are closely related to the mixture homogeneity. To calculate fuel spray behavior, a practical computer code GTT (Generalized Tank and Tube) was employed. In a model for the premixed compression ignition engine, the effects of injection parameters, such as injection timing, initial droplet size, spray angle, injection velocity, nozzle type (pintle and hole) and injection position / direction, on the mixture homogeneity near ignition timing (or TDC) were investigated. To evaluate the homogeneity of the mixture, an index was defined based on the spatial distribution of fuel mass fraction. The fuel vapor mass fractions as well as the homogeneity indices, obtained as a function of time, were compared under various boundary conditions.